Key Words
heparanase - mimetic - pentosan polysulfate (PPS) - trichloroacetimidate - xylose
Heparan sulfates (HSs) are glycosaminoglycan polysaccharides and represent fundamental
components of the extracellular matrix (ECM) and the cell surface glycocalyx. They
play, as their conjugates with core proteins, pivotal roles in cell–cell interactions,
cellular differentiation, cell proliferation and cell migration.[1] The endoglycosidase heparanase-1 (HSPE-1, EC 3.2.1.166)[2]
[3] acts on HSs so as to cleave them and thus facilitating the degradation and remodelling
of the ECM. Since the overexpression of this enzyme is observed in tumours and is
also associated with various other pathological conditions (notably inflammatory ones),
inhibitors of it are of great interest. Mimetics of the natural substrates (viz. the
HSs) are obvious starting points for the design and assembly of HPSE inhibitors (as
well for creating probes for defining structure–activity relationships), but the structural
complexities of the HSs means that such an approach presents significant synthetic
challenges. These are being addressed, with some notable success, in a range of different
ways, including by using both chemical and chemoenzymatic techniques.[4,5]
Pentosan polysulfate (PPS), also known as sodium pentosan polysulfate or NaPPS,[6] and marketed under the brand name ELMIRON®,[7] is a synthetically derived, heparin-like compound (or mimetic) that is deployed
clinically for alleviating the pain associated with interstitial cystitis (bladder
pain) as well as being used to treat osteoarthritis in dogs and horses. Furthermore,
it has been shown to inhibit heparin-binding growth factors (HBGFs) released from
tumour cells and so blocking the proliferation of these in animal models.[8]
Figure 1 The structure, 1, of the smallest repeat unit of PPS containing a glucuronic acid residue
The preparation of PPS starts with the isolation, from beech tree bark, of the hemi-cellulose-type
polysaccharide xylan, which is largely comprised of xylose residues. Sulfation, oxidative
deploymerization, salt-forming and membrane-based nano-filtration steps then follow
and so delivering a heterogeneous product within the 3,000 to 10,000 amu range.[9] The associated backbones of the constituent xylo-oligosaccharides are of varying
length and some, but not all, bear a branching glucuronic acid unit. To a first approximation,
then, the smallest repeating unit associated with these constituents could be considered
to be the pentasaccharide 1 (Figure [1]), bearing eleven sodium sulfate residues with two of these being ‘capping groups’
attached to a hydroxyl group at each end of the tetraxylose backbone.[10]
Our ongoing interest[10]
[11] in developing chemical techniques for the preparation of HS mimetics and related
species for probing the mechanisms of actions of HPSE, and so thereby developing new
therapeutic agents for treating a range of disease states, prompted us to consider
methods for the chemical synthesis of compound 1 and related systems. In so doing, we anticipated that this target could be attained
by preparing (and then coupling) the suitably activated forms, 2–5 (Figure [2]), of the component disaccharide, keystone, branching and capping fragments, respectively,
of target 1.[12]
Figure 2 The carbohydrates 2–5 sought as possible precursors to the target pentasaccharide 1
A key consideration in defining the activating functionalities associated with fragments
2–5 was the need to establish β-configured anomeric linkages between the backbone xylose
residues and an α-configured one between the keystone xylose residue (3) and the appended/branching glucuronic acid residue (4). Amongst the various methods available for effecting glycosylations, the trichloroacetimidate
method developed by Schmidt[13] appeared the most attractive in the present setting because of its mildness and
the predictable stereochemical outcomes that can be realized depending upon the precise
conditions used.
The initial focus of our efforts was on the preparation of fragment 2 by the route shown in Scheme [1]. To that end a commercial sample of the xylo-oligosaccharide mixture derived from
corn cobs was purchased and, on subjecting this to low-resolution electrospray ionization
(positive mode) mass spectrometry, this material was confirmed to contain xylobiose
(6) as an admixture with its tri- and tetra-meric counterparts. Accordingly, this mixture
was subject to exhaustive acetylation under standard conditions and the desired per-acetylated
product 7 was readily separated, by flash chromatography, from the corresponding and accompanying
derivatives of the higher-order oligomers. By such means, compound 7
[10] was obtained as a near 1:1 mixture of α- and β-anomers and in 42% yield (on a w/w
basis from the original xylo-oligosaccharide mixture). The anomeric acetyl group associated
with compound 7 could be selectively cleaved on exposure of it to 3-(dimethylamino)-1-propylamine
(8) in THF at 22 °C for 5 h and so affording compound 9
[10] in 71% yield as a ca. 3:1 mixture of α- and β-anomers. Finally, treatment of a dichloromethane
solution of penta-acetate 9 with trichloroacetonitrile in the presence of 1,8-diazabicylo[5.4.0]undec-7-ene (DBU)
at –5 °C afforded the targeted and previously reported[10] trichloroacetimidate 2 in 86% yield. As determined using standard NMR spectroscopic techniques, the α-anomeric
form of compound 2 predominated.
Scheme 1 Synthesis of the xylobiose-derived trichloroacetimidate 2
The synthesis of the previously unreported keystone fragment 3 was achieved by the route shown in Scheme [2]. So, in the opening step, and by following an established procedure,[14]
d-xylose (10) was treated with allyl alcohol in the presence of acetyl chloride and so affording
ether 11
[14] as a ca. 6:1 mixture of α- and β-anomers in 49% combined yield. Fractional recrystallization
of this mixture from ethanol afforded a nearly pure sample of the α-anomer and so
facilitating spectral analysis. On reacting a DMF solution of compound 11 with 2-methoxypropene in the presence of a trace of HCl over the temperature range
0 to 22 °C then a chromatographically separable mixture of the isomeric, previously
unreported ketals 12 (38%) and 13 (17%) was obtained. HMBC and 1H–1H COSY experiments were carried out (necessarily in C6D6 rather than CDCl3 due to their high acid sensitivities) to differentiate between these regioisomeric
products and key outcomes are shown in the Supporting Information. Protection of the
single free hydroxyl group within compound 12 as the corresponding TBS ether was achieved under standard conditions and so affording
compound 14 in 96% yield. Cleavage of the ketal residue associated with this last compound proved
less straightforward and after examining a range of conditions, the most favourable
outcome was achieved by briefly treating substrate 14 with oxalic acid and cerium trichloride heptahydrate in acetonitrile.[15] By such means, the required diol 15 was obtained in 87% yield.
Scheme 2 Synthesis of xylose derivative 3
Efforts to selectively protect the C3-hydroxyl group within compound 15 proved challenging, as evidenced by the preferential C2-benzoylation of this substrate
under standard conditions and so affording ester 16 in 69% yield. This outcome is consistent with that reported by Kondo[16] for the highly selective C2-tosylation of methyl α-xylopyranoside. Ultimately, it
was found that by treating a dichloromethane solution of compound 15 and pyridine maintained at –78 °C with ca. one equivalent of α-chloroacetyl chloride
followed by two equivalents of acetyl chloride then the mixed di-ester 17 could be formed in 75% yield. Co-production of the bis-α-chloroacetate 18 was essentially avoided under the optimized reaction conditions eventually established
for the conversion of 15 into 17. Interestingly, single-crystal X-ray analysis of a crystalline product sample obtained
from an unoptimized esterification revealed, as shown in Figure [1] of the Supporting Information, that a solid comprised of an 85:15 mixture of compounds
17 and 18 was formed in one instance.
A range of conditions was then explored so as to affect the selective cleavage of
the α-chloroacetate residue within compound 17, and the most efficient means for doing so was achieved by treating this substrate
with a combination of hydrazine dithiocarbonate (HDTC), 2,6-lutidine and acetic acid
in ethanol/water[17] and so finally providing the target xylose derivative 3 as a single diastereoisomer in 90% yield. A series of HMBC and 1H–1H COSY NMR experiments (see the Supporting Information, Figure 3) provided support
for the assigned structure of compound 3 but definitive confirmation of this came from a single-crystal X-ray analysis of
a derivative (see below).
The synthesis of the target ‘capping’ fragment 5 was very straightforward and simply involved, as shown in Scheme [3], converting diol 15 into the corresponding diacetate 19 (86%) under standard conditions and then treating the latter compound with aqueous
fluorosilicic acid in acetonitrile (so as to affect cleavage of the TBS-ether). By
such means compound 5 was obtained in 84% yield.
Scheme 3 Synthesis of xylose 5
Scheme 4 Synthesis of glucuronic acid derivative 4 (R = Me)
The synthesis of the glucuronic acid fragment 4 (R = Bn or Me) was more involved, with the ultimately successful route to this being
shown in Scheme [4] and inspired by the work of Kosma et al.[18] To begin, d-glucose (20) was treated sequentially with allyl alcohol in the presence of triflic acid and
then with benzaldehyde dimethyl acetal and p-toluenesulfonic acid monohydrate in DMF at 40 °C. As a result, the benzylidene acetal
21 was obtained in 25% yield and as a ca. 7:1 mixture of α- and β-anomers after chromatographic
purification. Bis-O-benzylation of diol 21 under standard conditions then provided the fully protected glucose derivative 22 in 80% yield and again as a ca. 7:1 mixture of anomers. Hydrolytic cleavage of the
benzylidene acetal residue associated with this last compound was achieved under standard
conditions and so affording the new diol 23 (94%) and the 1°-hydroxyl group of which was selectively protected, again under standard
conditions, as the corresponding trityl ether 24 (74%). O-Methylation of the single free hydroxyl group within compound 24 was affected using methyl iodide and sodium hydride and so delivering the anticipated
product 25 (87%) as a ca. 7:1 mixture of α- and β-anomers. The trityl group associated with
per-ether 25 was then selectively cleaved using p-toluenesulfonic acid monohydrate in methanol and the 1°-alcohol so revealed, oxidized
to the corresponding carboxylic acid using Jones’ reagent. This acid was then reacted,
without purification, with either benzyl bromide or methyl iodide in the presence
of potassium carbonate to afford the corresponding ester 27 (57%) or 28 (69%), respectively. Selective cleavage of the allyl protecting group at the anomeric
centre within compound 27 proved challenging. When this ester was treated, as defined by Ogawa et al.,[18]
[19] with 3.7 mole equivalents of PdCl2 in the presence of aqueous acetic acid and sodium acetate, then target alcohol 29 was only obtained in ca. 35% yield and accompanied by a range of difficult-to-separate
impurities. By drastically reducing the amount of PdCl2 used (to 0.2 equiv) as well as employing methanol as solvent, then compound 29 could be obtained in 63% yield.
Scheme 5 Synthesis of coupling product 34
While all the spectral data acquired on this product matched those recorded in the
literature,[18] it was contaminated with varying quantities of the corresponding methyl ester 30 that arises through a transesterification reaction. In an attempt to circumvent this
process, the deprotection reaction was run using benzyl alcohol as solvent, but little
of the target product 29 was formed even after using extended reaction times and elevated temperatures. Furthermore,
it proved very difficult to separate compound 29 from residual benzyl alcohol. The deprotection of compound 28 proved much more straightforward, and product 30 was obtained in 80% yield under the best of the conditions defined above. Finally,
treatment of compound 30 with trichloracetonitrile, in the presence of potassium carbonate,[20] afforded the targeted trichloroacetimidate 4 (R = Me) in 81% yield and as a ca. 1:3 mixture of α- and β-anomers. All of the spectra
data acquired on this compound were in accord with the assigned structure, with the
1H NMR spectrum of the latter (predominant) anomer including a doublet at δ = 5.85
ppm that is assigned to H-1 and with the associated vicinal coupling to H-2 of 7.3
Hz being indicative of the α-configuration of the oxymethine hydrogen at C-1. In the
α-anomer the analogous signal appeared as a doublet at δ = 6.43 ppm and the coupling
constant was 3.5 Hz.
With the targeted building blocks 2, 3, 4 (R = Me) and 5 to hand, an initial study of their capacities to serve as coupling partners was undertaken.
To such ends, and as shown in Scheme [5, a] TMSOTf-mediated glycosylation reaction[13] was carried out at ambient temperatures using acceptor 3 and donor 4 and by such means a chromatographically separable mixture of the diastereoisomeric
adducts 31 (12%) and 32 (48%) was obtained. While ESI mass spectrometric analyses of these compounds served
to identify them as coupling products, their 1H and HMBC NMR spectroscopic features, details of which are provided in the Supporting
Information (Figure 5), allowed for the determination of the illustrated stereochemistries
about the newly established glycosidic bonds. Treatment of disaccharide 32 with H2SiF6 in acetonitrile at ambient temperature[21] affected a smooth desilylation reaction and so delivering the alcohol 33 (82%) that was now coupled, at –20 °C, with an excess of donor 2 in the presence of TMSOTf. This conversion proved messy and extensive chromatographic
operations were required to obtain the target product 34 in pure form and then only in 7% yield. Once again, 1H and HMBC NMR spectroscopic experiments, as detailed in the Supporting Information
(Figure 6), served to establish the illustrated stereochemical features, most particularly
the β-configurations at highlighted anomeric centres (see red arrows) and α-configurations
at the remaining two, including the one associated with the glucuronic acid residue.
Scheme 6 The stereocontrolled synthesis of the glycosylated xylose 40
In a complementary study allowing for the synthesis of a coupling product incorporating
a β-configured glucoronic acid residue, commercially available glucuronic acid 35 (Scheme [6]) was peracetylated using acetic anhydride and molecular iodine and so affording
anhydride 36
[22] that, upon reaction with methanol, afforded methyl ester 37
[23] (26% from free acid). On reacting this last compound with a slight excess of tri-n-butyltin methoxide then hydrolysis took place at the anomeric centre to afford compound
38
[24] (49%) which was obtained as a ca. 5:1 mixture of α- and β-anomers. Treating a dichloromethane
solution of acetal 38 with trichloroacetonitrile in the presence of DBU at –5 °C then gave trichloroacetimidate
39
[25] which was obtained in 95% yield and exclusively in the α-anomeric form. The reaction
of this last compound with the keystone building block 3 in the presence of TMSOTf at –20 °C afforded, after extensive chromatographic purification
of a complex product mixture, the crystalline coupling product 40, albeit it in just 7% yield. The anticipated β-configured stereochemistry at the
anomeric centre of the gluconic acid residue was evident from the derived NMR spectral
data and confirmed by single-crystal X-ray analysis (see the Supporting Information).
The studies detailed here provide a means for assembling the key components of PPS
so, when considered in conjunction with our recently developed methods for the O-sulfation of oligosaccharides,[10] the stereocontrolled synthesis of compound 1 and various of its congeners now seems possible. Work directed toward such ends is
underway in our laboratories and results will be reported in due course.
Allyl 2,3-O-Isopropylidene-α-d-xylopyranoside (12) and Allyl 3,4-O-Isopropylidene-α-d-xylopyranoside (13)
Allyl 2,3-O-Isopropylidene-α-d-xylopyranoside (12) and Allyl 3,4-O-Isopropylidene-α-d-xylopyranoside (13)
Following a modification of a procedure reported by Stick et al.,[26] a magnetically stirred solution of allyl α-d-xylopyranoside (11)[14] (954 mg, 5.02 mmol) in DMF (8.0 mL) was treated with HCl in dioxane (40 μL of a
4 M aq. solution, 0.16 mmol). The ensuing mixture was cooled to –10 °C then treated
with 2-methoxypropene (1.30 mL, 13.6 mmol) and stirring continued for 1 h. Thereafter,
the cooling bath was removed and the reaction mixture stirred for a further 2 h at
22 °C before being diluted with water (10 mL) and extracted with chloroform (3 × 10
mL). The combined organic layers were washed sequentially with NaHCO3 (5 mL of a sat. aq. solution) and brine (5 mL) before being dried (Na2SO4), filtered and concentrated under reduced pressure. Subjection of the residue thus
obtained to flash column chromatography (silica, 40:1 v/v dichloromethane/methanol
elution) afforded two fractions, A and B.
Concentration of fraction A (Rf
= 0.2 in 20:1 v/v dichloromethane/methanol) afforded compound 12 (440 mg, 38%) as a white, crystalline solid.
Mp 73–75 °C; [α]
D
+126 (c = 0.80, CHCl
3
).
1H NMR (400 MHz, C6D6): δ = 5.72 (m, 1 H), 5.23 (app. dq, J = 17.2, 1.7 Hz, 1 H), 5.03 (d, J = 3.0 Hz, 1 H), 4.99 (app dq, J = 10.5, 1.7 Hz, 1 H), 4.16 (app. t, J = 9.4 Hz, 1 H), 4.01 (m, 1 H), 3.86–3.74 (complex m, 2 H), 3.66 (dd, J = 11.2, 5.5 Hz, 1 H), 3.52–3.34 (complex m, 2 H), 2.40 (br s, 1 H), 1.42 (s, 3 H),
1.38 (s, 3 H).
13C NMR (101 MHz, C6D6): δ = 134.3, 116.8, 110.5, 96.5, 77.9, 76.5, 70.6, 68.6, 63.2, 27.2, 26.7.
IR: 3437, 2986, 2934, 1372, 1225, 1103 cm–1.
HRMS (ESI, +ve): m/z [M + Na]+ calcd for C11H18O5·Na: 253.1052; found: 253.1046.
Concentration of fraction B (Rf
= 0.4 in 20:1 v/v dichloromethane/methanol) afforded compound 13 (197 mg, 17%) as a clear, colourless oil.
[α]
D
+57 (c = 0.40, CHCl
3
).
1H NMR (400 MHz, C6D6): δ = 5.65 (m, 1 H), 5.10 (app dq, J = 16.8, 1.5 Hz, 1 H), 4.97 (app. dq, J = 10.4, 1.5 Hz, 1 H), 4.76 (d, J = 2.8 Hz, 1 H), 3.94 (m, 1 H), 3.85–3.78 (complex m, 2 H), 3.76 (dd, J = 9.6, 4.6 Hz, 1 H), 3.70 (m, 1 H), 3.63 (app. t, J = 10.1 Hz, 1 H), 3.33 (m, 1 H), 2.52 (d, J = 7.5 Hz, 1 H), 1.39 (s, 3 H), 1.38 (s, 3 H).
13C NMR (101 MHz, C6D6): δ = 134.3, 117.4, 110.5, 98.5, 80.0, 74.4, 72.5, 68.8, 61.8, 27.1, 26.7.
IR: 3459, 2986, 1372, 1228, 1022 cm–1.
HRMS (ESI, +ve): m/z [M + Na]+ calcd for C11H18O5·Na: 253.1052; found: 253.1053.
Allyl 2,3-O-Isopropylidene-4-O-tert-butyl-dimethylsilyl-α-d-xylopyranoside (14)
Allyl 2,3-O-Isopropylidene-4-O-tert-butyl-dimethylsilyl-α-d-xylopyranoside (14)
A magnetically stirred solution of alcohol 12 (240 mg, 1.04 mmol) in DMF (1.0 mL) was treated with imidazole (153 mg, 2.24 mmol)
then cooled to 0 °C (ice-bath) before TBS-Cl (279 mg, 1.85 mmol) was added. Thereafter,
the cooling bath was removed and the reaction mixture was stirred at 22 °C for 16
h then diluted with NaHCO3 (3 mL of a sat. aq. solution) and extracted with diethyl ether (2 × 10 mL). The combined
organic layers were washed with brine (10 mL) then dried (Na2SO4), filtered, and concentrated under reduced pressure. The residual oil was subjected
to flash column chromatography (silica, 25:1 v/v pet. spirit/EtOAc elution) to afford,
after concentration of the appropriate fractions (Rf
= 0.2) compound 14 (344 mg, 96%) as a clear, colourless oil.
[α]
D
+100 (c = 0.81, CHCl
3
).
1H NMR (400 MHz, CDCl3): δ = 5.94 (m, 1 H), 5.33 (app. dq, J = 17.2, 1.7 Hz, 1 H), 5.19 (app. dq, J = 10.6, 1.7 Hz, 1 H), 5.09 (d, J = 3.0 Hz, 1 H), 4.22 (app. ddt, J = 13.1, 5.2, 1.6 Hz, 1 H), 4.07 (app. ddt, J = 13.1, 5.8, 1.6 Hz, 1 H), 3.96–3.84 (complex m, 2 H), 3.65–3.56 (complex m, 1 H),
3.43–3.27 (complex m, 2 H), 1.44 (s, 3 H), 1.41 (s, 3 H), 0.89 (s, 9 H), 0.11 (s,
3 H), 0.10 (s, 3 H).
13C NMR (101 MHz, CDCl3): δ = 134.0, 117.3, 110.1, 96.2, 77.3, 75.9, 71.2, 68.8, 63.6, 27.1, 26.5, 25.9,
18.3, –4.5, –4.8.
IR: 2931, 2858, 1094, 1022, 935, 778 cm–1.
HRMS (ESI, +ve): m/z [M + H]+ calcd for C17H32O5Si·H: 345.2092; found: 345.2091.
Allyl 4-O-tert-Butyldimethylsilyl-α-d-xylopyranoside (15)
Allyl 4-O-tert-Butyldimethylsilyl-α-d-xylopyranoside (15)
Method A: Following a modification of a procedure reported by Xiao and Bai,[15] a magnetically stirred solution of acetonide 14 (3.47 g, 10.1 mmol) in acetonitrile (55.0 mL) was cooled to 0 °C then treated with
CeCl3·7H2O (7.50 g, 20.1 mmol). The resulting mixture was stirred for 5 min at 0 °C then oxalic
acid (66.1 mg, 0.73 mmol) was added. After a further 10 min at 0 °C and then 10 min
at 22 °C, the reaction was quenched with NaHCO3 (5 mL of a sat. aq. solution) before concentrating the mixture under reduced pressure.
The colourless residue so formed was diluted with water (20 mL) then extracted with
EtOAc (3 × 20 mL) and the combined organic phases were then dried (Na2SO4), filtered and concentrated under reduced pressure. The oil thus obtained was subjected
to flash column chromatography (silica, 10:1 v/v pet. spirit/EtOAc elution) to afford,
after concentration of the appropriate fractions (Rf
= 0.5), diol 15 (2.65 g, 87%) as a clear, colourless oil.
[α]
D
+99 (c = 0.80, CHCl3).
1H NMR (400 MHz, CDCl3): δ = 5.92 (m, 1 H), 5.30 (app. dq, J = 17.0, 1.5 Hz, 1 H), 5.21 (app. dq, J = 10.4, 1.5 Hz, 1 H), 4.85 (d, J = 3.8 Hz, 1 H), 4.21 (m, 1 H), 4.00 (app. ddt, J = 12.8, 6.2, 1.5 Hz, 1 H), 3.69–3.54 (complex m, 2 H), 3.54–3.44 (complex m, 3 H),
2.53 (d, J = 2.2 Hz, 1 H), 2.25 (d, J = 9.6 Hz, 1 H), 0.89 (s, 9 H), 0.11 (s, 3 H), 0.09 (s, 3 H).
13C NMR (101 MHz, CDCl3): δ = 133.8, 117.9, 97.5, 75.5, 72.4, 71.3, 68.6, 62.5, 25.9, 18.2, –4.47, –4.53.
IR: 3400, 2929, 2857, 1251, 1055, 835, 776 cm–1.
HRMS (ESI, +ve): m/z [M + Na]+ calcd for C14H38O5Si·Na: 327.1598; found: 327.1599.
Method B: Following minor modifications of a procedure reported by Reissig et al.,[27] a magnetically stirred solution of acetonide 14 (172 mg, 0.50 mmol) and InCl3 (224 mg, 1.01 mmol) in acetonitrile (8.0 mL) containing a trace of water (40 μL)
was stirred at 22 °C for 2 h. Conventional extractive work-up followed by flash chromatography
then afforded compound 15 (90.1 mg, 60%) that was identical in all respects with that obtained by Method A.
Allyl 3-O-Benzoyl-4-O-tert-butyldimethylsilyl-α-d-xylopyranoside (16)
Allyl 3-O-Benzoyl-4-O-tert-butyldimethylsilyl-α-d-xylopyranoside (16)
Following a procedure analogous to that reported by Hutchinson et al.,[28] a magnetically stirred solution of diol 15 (51.9 mg, 0.17 mmol) in pyridine (1.0 mL) was cooled to –10 °C then treated with
benzoyl chloride (20 μL, 0.17 mmol). The ensuing mixture was warmed to 22 °C and stirred
for 24 h before being diluted with dichloromethane (5 mL) and washed with HCl (3 mL
of a 1 M aq. solution) followed by NaHCO3 (3 mL of a sat. aq. solution). The combined aqueous phases were extracted with dichloromethane
(3 × 5 mL) and the combined organic layers washed with ammonium chloride (3 mL of
a sat. aq. solution) then CuSO4 (3 mL of a 5% w/w aq. solution) before being dried (Na2SO4), filtered and concentrated under reduced pressure. The residue so obtained was subjected
to flash column chromatography (silica, 4:1 v/v pet. spirit/EtOAc elution) to afford,
after concentration of the appropriate fractions (Rf
= 0.5), alcohol 16 (48.1 mg, 69%) as a clear, colourless oil.
[α]
D
+82 (c = 0.20, CHCl
3
).
1H NMR (400 MHz, CDCl3): δ = 8.09 (m, 2 H), 7.59 (m, 1 H), 7.45 (m, 2 H), 5.84 (m, 1 H), 5.28 (app. dq,
J = 17.2, 1.7 Hz, 1 H), 5.13 (app. dq, J = 10.6, 1.7 Hz, 1 H), 5.10 (d, J = 3.7 Hz, 1 H), 4.94 (dd, J = 10.0, 3.7 Hz, 1 H), 4.19 (m, 1 H), 4.07 (dd, J = 10.0, 8.5 Hz, 1 H), 3.97 (m, 1 H), 3.74 (m, 1 H), 3.63–3.55 (complex m, 2 H), 2.28
(br s, 1 H), 0.90 (s, 9 H), 0.13 (s, 3 H), 0.12 (s, 3 H).
13C NMR (101 MHz, CDCl3): δ = 166.4, 133.9, 133.4, 130.0, 129.9, 128.5, 117.4, 95.7, 73.6, 72.6, 72.0, 68.5,
62.2, 25.9, 18.2, –4.4, –4.5.
IR: 3515, 2953, 2859, 1723, 1276, 1103, 1040, 838 cm–1.
HRMS (ESI, +ve): m/z [M + Na]+ calcd for C21H32O6Si·Na: 431.1866; found: 431.1865.
Allyl 3-O-Acetyl-2-O-chloroacetyl-4-O-tert-butyldimethylsilyl-α-d-xylopyranoside (17)
Allyl 3-O-Acetyl-2-O-chloroacetyl-4-O-tert-butyldimethylsilyl-α-d-xylopyranoside (17)
A magnetically stirred solution of diol 15 (2.18 g, 7.16 mmol) in anhydrous dichloromethane (40.0 mL) was treated with pyridine
(690 μL, 8.59 mmol) then cooled to –78 °C before being treated, dropwise, with α-chloroacetyl
chloride (630 μL, 7.87 mmol). After 1.5 h pyridine (1.30 mL, 16.5 mmol) then acetyl
chloride (1.00 mL, 14.3 mmol) were added to the reaction mixture that was then stirred
at 22 °C for a further 3 h. Thereafter, the reaction mixture was poured into NaHCO3 (10 mL of a sat. aq. solution) containing ice, then extracted with dichloromethane
(3 × 10 mL). The combined organic phases were washed with ammonium chloride (15 mL
of a sat. aq. solution) then CuSO4 (15 mL of a 5% w/w aq. solution). The combined organic phases were then filtered
through a short pad of Celite® contained in a sintered glass funnel. The pad was then washed with dichloromethane
(10 mL) and the combined filtrates washed with brine (10 mL) before being dried (Na2SO4), filtered and concentrated under reduced pressure. The yellow oil thus obtained
was subjected to flash column chromatography (silica, 7:1 v/v pet. spirit/EtOAc elution)
to afford, after concentration of the appropriate fractions (Rf
= 0.6), compound 17 (2.46 g, 81%) as a clear, colourless oil.
[α]
D
+84 (c = 0.88, CHCl
3
).
1H NMR (400 MHz, CDCl3): δ = 5.86 (m, 1 H), 5.37 (m, 1 H), 5.29 (app. dq, J = 16.8, 1.6 Hz, 1 H), 5.20 (m, 1 H), 4.99 (d, J = 3.7 Hz, 1 H), 4.78 (dd, J = 10.2, 3.7 Hz, 1 H), 4.18 (m, 1 H), 4.03 (s, 2 H), 3.96 (m, 1 H), 3.78 (ddd, J = 9.9, 8.9, 6.4 Hz, 1 H), 3.66–3.52 (complex m, 2 H), 2.03 (s, 3 H), 0.84 (s, 9 H),
0.05 (s, 3 H), 0.04 (s, 3 H).
13C NMR (101 MHz, CDCl3): δ = 169.8, 167.1, 133.5, 118.0, 94.8, 73.0, 72.5, 69.5, 68.5, 62.2, 40.7, 25.6,
21.1, 18.0, –4.6, –4.8.
IR: 2931, 2859, 1755, 1223, 1045, 837 cm–1.
HRMS (ESI, +ve): m/z [M + Na]+ calcd for C18H31
35ClO7Si·Na: 445.1425; found: 445.1422.
Chromatographic fractions of compound 17 containing congener 18 were allowed to evaporate at 22 °C and so affording colourless crystals (mp 139–142
°C). This material was subjected to single-crystal X-ray analysis, details of which
are provided in the Supporting Information.
Allyl 3-O-Acetyl-4-O-tert-butyldimethylsilyl-α-d-xylopyranoside (3)
Allyl 3-O-Acetyl-4-O-tert-butyldimethylsilyl-α-d-xylopyranoside (3)
Method A: Using a minor modification of a procedure reported by van Boeckel and Beetz,[17] a magnetically stirred solution of hydrazine hydrate (730 μL, 15.0 mmol) in ethanol/water
(30.0 of a 2:1 v/v mixture) maintained at 0 °C was treated with a solution of CS2 (700 μL, 11.6 mmol) in 1,4-dioxane (6.0 mL). Di-isopropylethylamine (2.60 mL, 14.9
mmol) was then added dropwise and the ensuing mixture was stirred for 0.5 h and so
providing a stock solution of HDTC. HDTC (3.40 mL of the stock solution, 0.985 mmol)
was added to a magnetically stirred and ice-cold solution of chloroacetate 17 (133 mg, 0.314 mmol) and 2,6-lutidine (3.00 mL, 25.9 mmol) in acetic acid (1.00 mL,
17.5 mmol). The cooling-bath was then removed and the reaction mixture stirred at
22 °C for 1 h before being diluted with dichloromethane (10 mL) then washed with water
(2 × 10 mL), CuSO4 (20 mL of a 5% w/w aq. solution) and brine (10 mL). The separated organic layer was
then dried (Na2SO4), filtered and concentrated under reduced pressure to afford a dark-brown oil that
was subjected to flash column chromatography (silica, 7:3 v/v pet. spirit/EtOAc elution).
Concentration of the appropriate fractions (Rf
= 0.6) then gave compound 3 (98.3 mg, 90%) as a clear, colourless oil.
[α]
D
+106 (c = 0.27, CHCl3).
1H NMR (400 MHz, CDCl3): δ = 5.92 (m, 1 H), 5.31 (app. dq, J = 17.2, 1.6 Hz, 1 H), 5.23 (app. dq, J = 10.3, 1.6 Hz, 1 H), 5.08 (app. t, J = 9.4 Hz, 1 H), 4.84 (d, J = 3.8 Hz, 1 H), 4.23 (m, 1 H), 4.01 (m, 1 H), 3.72 (m, 1 H), 3.60–3.43 (complex m,
3 H), 2.11 (s, 3 H), 2.08 (d, J = 11.6 Hz, 1 H), 0.85 (s, 9 H), 0.06 (s, 3 H), 0.06 (s, 3 H).
13C NMR (101 MHz, CDCl3): δ = 171.3, 133.6, 118.1, 97.8, 76.3, 71.4, 69.0, 68.8, 62.7, 25.7, 21.4, 18.0,
–4.6, –4.8.
IR: 3450, 2930, 2858, 1743, 1232, 1063, 1039, 836 cm–1.
HRMS (ESI, +ve): m/z [M + Na]+ calcd for C16H30O6Si·Na: 369.1709; found: 369.1705.
Method B: Following a procedure analogous to that reported by Banwell et al.,[29] a solution of chloroacetate 17 (2.42 g, 5.72 mmol) in MeOH (50.0 mL) maintained at 22 °C was treated with Zn(OAc)2·2H2O (1.28 g, 5.83 mmol). After 4 h, the reaction mixture was worked-up and the residue
subjected to chromatographic purification and so affording alcohol 3 (1.33 g, 67%) that was identical in all respects with that obtained by Method A.
Allyl 2,3-Di-O-acetyl-4-O-tert-butyldimethylsilyl-α-d-xylopyranoside (19)
Allyl 2,3-Di-O-acetyl-4-O-tert-butyldimethylsilyl-α-d-xylopyranoside (19)
A magnetically stirred solution of diol 15 (676 mg, 2.22 mmol) in dichloromethane (10 mL) was cooled to –10 °C then treated
with pyridine (720 μL, 8.94 mmol) and acetyl chloride (480 μL, 6.75 mmol). Thereafter,
the reaction mixture was warmed to 22 °C and after 3 h it was poured into ice-cold
NaHCO3 (10 mL of a sat. aq. solution) and extracted with dichloromethane (3 × 10 mL). The
combined organic phases were then washed with brine (10 mL) before being dried (Na2SO4), filtered and concentrated under reduced pressure. The yellow oil thus obtained
was subjected to flash column chromatography (silica, 8:1 v/v pet. spirit/EtOAc elution)
to afford, after concentration of the appropriate fractions (Rf
= 0.4), compound 19 (741 mg, 86%) as a clear colourless oil.
[α]
D
+84 (c = 0.84, CHCl
3
).
1H NMR (400 MHz, CDCl3): δ = 5.87 (m, 1 H), 5.39–5.26 (complex m, 2 H), 5.19 (app. dq, J = 10.4, 1.4 Hz, 1 H), 4.96 (d, J = 3.6 Hz, 1 H), 4.74 (dd, J = 10.3, 3.6 Hz, 1 H), 4.18 (m, 1 H), 3.97 (m, 1 H), 3.77 (ddd, J = 10.3, 8.9, 6.1 Hz, 1 H), 3.65–3.50 (complex m, 2 H), 2.04 (s, 3 H), 2.03 (s, 3
H), 0.84 (s, 9 H), 0.05 (s, 3 H), 0.04 (s, 3 H).
13C NMR (101 MHz, CDCl3): δ = 170.6, 169.8, 133.7, 117.7, 95.1, 72.7, 71.4, 69.6, 68.4, 62.2, 25.6, 21.1,
20.9, 18.0, –4.5, –4.8.
IR: 2931, 2858, 1752, 1219, 1050, 837 cm–1.
HRMS (ESI, +ve): m/z [M + Na]+ calcd for C18H32O7Si·Na: 411.1815; found: 411.1817.
Allyl 2,3-Di-O-acetyl-α-d-xylopyranoside (5)
Allyl 2,3-Di-O-acetyl-α-d-xylopyranoside (5)
Following a procedure due to Pilcher and DeShong,[21] a magnetically stirred solution of compound 19 (337 mg, 0.867 mmol) in acetonitrile (10 mL), and maintained at 22 °C in a falcon
tube, was treated with fluorosilicic acid (610 μL of a 25% w/w aq. solution, 1.29
mmol). After 6 h the reaction mixture was treated with CaCO3 (2 mL of a sat. aq. solution) and NaHCO3 (2 mL of a sat. aq. solution) and after a further 0.25 h the reaction mixture was
diluted with brine (10 mL) then extracted with EtOAc (2 × 15 mL). The combined organic
phases were then dried (Na2SO4), filtered and concentrated under reduced pressure. The residue thus obtained was
subjected to flash column chromatography (silica, 1:1 v/v pet. spirit/EtOAc elution)
to afford, after concentration of the appropriate fractions (Rf
= 0.4), compound 5 (199 mg, 84%) as a clear, colourless oil.
[α]
D
+101 (c = 0.97, CHCl
3
).
1H NMR (400 MHz, CDCl3): δ = 5.86 (m, 1 H), 5.29 (app. dq, J = 17.3, 1.6 Hz, 1 H), 5.21 (m, 2 H), 4.97 (d, J = 3.7 Hz, 1 H), 4.79 (dd, J = 10.0, 3.7 Hz, 1 H), 4.18 (app. ddt, J = 13.2, 5.0, 1.6 Hz, 1 H), 3.97 (app. ddt, J = 13.2, 6.1, 1.6 Hz, 1 H), 3.82–3.68 (complex m, 2 H), 3.61 (m, 1 H), 2.80 (br s,
1 H), 2.09 (s, 3 H), 2.06 (s, 3 H).
13C NMR (101 MHz, CDCl3): δ = 172.1, 170.4, 133.5, 117.8, 94.9, 74.0, 70.7, 69.6, 68.4, 61.8, 21.0, 20.9.
IR: 3456, 2942, 2887, 1750, 1370, 1230, 1042 cm–1.
HRMS (ESI, +ve): m/z [M + Na]+ calcd for C12H18O7·Na: 297.0950; found: 297.0950.
Allyl 2,3-Di-O-benzyl-4,6-O-benzylidene-d-glucopyranoside (22)
Allyl 2,3-Di-O-benzyl-4,6-O-benzylidene-d-glucopyranoside (22)
Following a minor modification of a procedure reported by Kosma et al.,[18] a magnetically stirred solution of diol 21
[30] (5.55 g, 18.0 mmol) in anhydrous DMF (70 mL) was cooled to 0 °C then treated with
NaH (2.18 g of a 60% dispersion in oil, 54.5 mmol). After 1 h benzyl chloride (4.56
mL, 40.0 mmol) was added, dropwise, to the reaction mixture and this was followed
by the portion-wise addition of TBAI (702 mg, 2.18 mmol). The ensuing mixture was
allowed to warm to 22 °C and after a further 4 h poured into ice-water (500 mL). Diethyl
ether (200 mL) was then added and the organic phase separated. The aqueous phase was
extracted with diethyl ether (3 × 200 mL) and combined organic phases then dried (Na2SO4), filtered and concentrated under reduced pressure. The yellow oil so obtained was
subjected to flash column chromatography (silica, 9:1 v/v pet. spirit/EtOAc elution)
to afford, after concentration of the appropriate fractions (Rf
= 0.2), compound 22 (7.01 g, 80%) as a white solid and a ca. 7:1 mixture of α- and β-anomers. The NMR
spectral data recorded on this material match those reported in the literature.[18]
[31]
Benzyl (Allyl 2,3-di-O-benzyl-4-O-methyl-α-d-glucopyranosid)urinate (27)
Benzyl (Allyl 2,3-di-O-benzyl-4-O-methyl-α-d-glucopyranosid)urinate (27)
In a minor modification of a procedure reported by Kosma et al.,[18] a magnetically stirred solution of compound 26 (950 mg, 2.29 mmol) in acetone (20 mL) maintained at 0 °C was treated, dropwise,
with Jones’ reagent (2.10 mL of a 3 M solution in aq. H2SO4, 6.30 mmol). The ensuing mixture was heated to 50 °C for 3 h then cooled before being
poured into ice-water (100 mL) and extracted with chloroform (3 × 50 mL). The combined
organic phases were washed with water until the washings were neutral and then dried
(Na2SO4), filtered and concentrated under reduced pressure. The viscous oil thus obtained
was dissolved in DMF (30 mL) and the resulting, magnetically stirred solution was
cooled to 0 °C (ice-bath) then treated with KHCO3 (1.66 g, 16.5 mmol). After 0.5 h the reaction mixture was treated with benzyl bromide
(550 μL, 4.63 mmol) then warmed to 22 °C. After a further 3 h the reaction mixture
was poured into ice-water (100 mL) and extracted with dichloromethane (3 × 50 mL).
The combined organic phases were washed with NaHCO3 (50 mL of sat. aq. solution) and water (50 mL) before being dried (Na2SO4), filtered and concentrated under reduced pressure. The residue so obtained was subjected
to flash column chromatography (silica gel, 6:1 v/v pet. spirit/EtOAc elution) to
afford, after concentration of the relevant fractions (Rf
= 0.2), compound 27 (678 mg, 57%) as a clear, colourless oil.
1H NMR (400 MHz, CDCl3): δ = 7.41–7.26 (complex m, 15 H), 5.92 (m, 1 H), 5.32 (app. dq, J = 17.1, 1.6 Hz, 1 H), 5.27–5.19 (complex m, 3 H), 4.92 (d, J = 10.8 Hz, 1 H), 4.84–4.74 (complex m, 3 H), 4.62 (d, J = 12.1 Hz, 1 H), 4.22–4.14 (complex m, 2 H), 4.02 (m, 1 H), 3.91 (m, 1 H), 3.54 (dd,
J = 9.6, 3.6 Hz, 1 H), 3.43 (dd, J = 10.0, 9.0 Hz, 1 H), 3.35 (s, 3 H).
13C NMR (101 MHz, CDCl3): δ = 169.7, 138.8, 138.1, 135.4, 133.4, 128.7, 128.5(8), 128.5(6), 128.5(4), 128.4(9),
128.2, 128.0(9), 128.0(7), 127.8, 118.8, 96.2, 81.6, 81.5, 79.2, 75.9, 73.6, 70.5,
68.8, 67.3, 60.8.
IR: 3032, 2931, 1748, 1455, 1089 cm–1.
HRMS (ESI, +ve): m/z [M + Na]+ calcd for C31H34O7·Na: 541.2202; found: 541.2208.
All the NMR, IR and MS data recorded on this material matched those reported previously.[18]
Methyl (Allyl 2,3-Di-O-benzyl-4-O-methyl-d-glucopyranosid)urinate (28)
Methyl (Allyl 2,3-Di-O-benzyl-4-O-methyl-d-glucopyranosid)urinate (28)
Following the procedure detailed immediately above, a magnetically stirred solution
of compound 26
[18] (2.79 g, 6.73 mmol) in acetone (56.0 mL) was cooled to 0 °C then treated, dropwise,
with Jones’ reagent (5.80 mL of a 3 M solution in aq. H2SO4, 17.4 mmol). The ensuing mixture was then heated at 50 °C for 3 h before being cooled,
poured into ice-water (200 mL) then extracted with chloroform (3 × 100 mL). Further
work-up in the manner detailed above afforded a clear, colourless oil that was dissolved
in DMF (70 mL) with the resulting solution being reacted with KHCO3 (4.53 g, 45.2 mmol) and then MeI (830 μL, 13.3 mmol). Work-up in the manner detailed
above gave an oil that was subject to flash column chromatography (silica, 7:1 v/v
pet. spirit/EtOAc elution) to afford, after concentration of the appropriate fractions
(Rf
= 0.15), compound 28 (2.04 g, 69%) as a clear, colourless oil and a 6:1 mixture of α- and β-anomers.
[α]
D
+38 (c = 0.29, CHCl3).
1H NMR (400 MHz, CDCl3): δ (α-anomer) = 7.42–7.25 (complex m, 10 H), 5.92 (m, 1 H), 5.33 (app. dq, J = 17.1, 1.6 Hz, 1 H), 5.24 (app. dq, J = 10.3, 1.6 Hz, 1 H), 4.93 (d, J = 10.9 Hz, 1 H), 4.85–4.73 (complex m, 3 H), 4.62 (d, J = 12.0 Hz, 1 H), 4.19 (m, 1 H), 4.15 (d, J = 10.0 Hz, 1 H), 4.01 (m, 1 H), 3.91 (app. t, J = 9.3 Hz, 1 H), 3.80 (s, 3 H), 3.54 (m, 1 H), 3.50 (s, 3 H), 3.44 (dd, J = 10.0, 9.0 Hz, 1 H).
13C NMR (101 MHz, CDCl3): δ (α-anomer) = 170.4, 138.8, 138.1, 133.4, 128.6, 128.5(0), 128.4(9), 128.2, 128.1,
127.8, 118.8, 96.3, 81.6, 81.4, 79.2, 75.9, 73.6, 70.3, 68.7, 60.9, 52.7.
IR: 3031, 2930, 1749, 1078, 1027, 736, 697 cm–1.
HRMS (ESI, +ve): m/z [M + Na]+ calcd for C25H30O7·Na: 465.1889; found: 465.1885.
Benzyl (2,3-Di-O-Benzyl-4-O-methyl-d-glucopyranosyl)urinate (29)
Benzyl (2,3-Di-O-Benzyl-4-O-methyl-d-glucopyranosyl)urinate (29)
Following a procedure due to Potter et al.,[32] a magnetically stirred solution of compound 27 (289 mg, 0.56 mmol) in anhydrous MeOH (10.0 mL) and protected from moisture using
a CaCl2 drying tube was cooled to 0 °C then treated with PdCl2 (21.3 mg, 0.120 mmol). The ensuing mixture was allowed to warm, over 2 h, to 22 °C
then filtered through a short pad of pad of Celite® contained in a sintered-glass funnel. The filtrate was concentrated under reduced
pressure and the residue so-obtained subjected to flash column chromatography (silica,
3:1 v/v pet. spirit/EtOAc elution) to afford, after concentration of the relevant fractions
(Rf
= 0.3) compound 29 (167 mg, 63%) as a white gum and a 3:1 mixture of α- and β-anomers.
[α]D +13 (c = 0.27, CHCl3).
1H NMR (400 MHz, CDCl3): δ (α-anomer) = 7.40–7.29 (complex m, 15 H), 5.21 (m, 3 H), 4.85 (d, J = 10.9 Hz, 1 H), 4.79 (d, J = 11.1 Hz, 1 H), 4.74 (m, 1 H), 4.65 (d, J = 11.8 Hz, 1 H), 4.41 (d, J = 9.6 Hz, 1 H), 3.94–3.83 (complex m, 1 H), 3.59–3.52 (complex m, 1 H), 3.49–3.40
(complex m, 1 H), 3.36 (s 3 H), 2.98 (br s, 1 H).
13C NMR (101 MHz, CDCl3): δ (α-anomer) = 169.7, 138.5, 137.7, 135.3, 128.8, 128.7(2), 128.6(9), 128.6, 128.5,
128.2, 128.1, 127.9, 91.8, 81.3, 80.7, 79.0, 75.7, 73.7, 70.7, 67.4, 60.6 (one signal
obscured or overlapping).
IR: 3420, 2937, 1744, 1454, 1076, 697 cm–1.
HRMS (ESI, +ve): m/z [M + Na]+ calcd for C28H30O7·Na: 501.1889; found 501.1897.
All the NMR, IR and MS data recorded on this material matched those reported previously.[18]
Methyl (2,3-Di-O-benzyl-4-O-methyl-d-glucopyranosyl)urinate (30)
Methyl (2,3-Di-O-benzyl-4-O-methyl-d-glucopyranosyl)urinate (30)
A magnetically stirred solution of compound 28 (2.02 g, 4.57 mmol) in anhydrous MeOH (20 mL) and maintained at 0 °C while being
protected from moisture, was treated with PdCl2 (136 mg, 0.770 mmol). The ensuing mixture was allowed to warm, over 2 h, to 22 °C
then filtered through a short pad of pad of Celite® contained in a sintered-glass funnel. The filtrate was concentrated under reduced
pressure and the residue so-obtained subjected to flash column chromatography (silica,
5:2 v/v pet. ether/EtOAc elution) to afford, after concentration of the relevant fractions
(Rf
= 0.2), compound 30 (1.47 g, 80%) as a tan solid and a ca. 8:1 mixture of α- and β-anomers.
Mp 92–95 °C; [α]
D
+29 (c = 0.33, CHCl
3
).
1H NMR (400 MHz, CDCl3): δ (α-anomer) = 7.42–7.27 (complex m, 10 H), 5.20 (app. t, J = 3.2 Hz, 1 H), 4.89–4.81 (complex m, 2 H), 4.77 (d, J = 11.8 Hz, 1 H), 4.66 (d, J = 11.8 Hz, 1 H), 4.38 (d, J = 9.6 Hz, 1 H), 3.87 (app. t, J = 9.0 Hz, 1 H), 3.79 (s, 3 H), 3.56 (m, 1 H), 3.50 (s, 3 H), 3.46 (m, 1 H), 3.02
(br s, 1 H).
13C NMR (101 MHz, CDCl3): δ (α-anomer) = 170.3, 138.5, 137.7, 128.7, 128.6, 128.3, 128.1, 127.9, 91.8, 81.2,
80.6, 79.0, 75.7, 73.7, 70.5, 60.7, 52.7 (one signal obscured or overlapping).
IR: 3406, 2946, 1741, 1084, 1071, 734, 694 cm–1.
HRMS (ESI, +ve): m/z [M + Na]+ calcd for C22H26O7·Na: 425.1576; found: 425.1574.
Methyl (2,3-Di-O-benzyl-4-O-methyl-d-glucopyranosyltrichloroacetimidate)urinate [4 (R = Me)]
Methyl (2,3-Di-O-benzyl-4-O-methyl-d-glucopyranosyltrichloroacetimidate)urinate [4 (R = Me)]
Following a procedure due to Das and Mukhopadhyay,[20] a magnetically stirred solution of acetal 30 (305 mg, 0.757 mmol) in dichloromethane (5.0 mL) maintained at 22 °C was treated
with K2CO3 (523 mg, 3.79 mmol). After 0.5 h the reaction mixture was cooled to 0 °C then treated,
dropwise, with trichloroacetonitrile (910 μL, 9.09 mmol). The mixture was then allowed
to warm to 22 °C and after a further 16 h it was filtered through a pad of Celite® contained in a sintered-glass funnel and the filtrate concentrated under reduced
pressure. The yellow residue so formed was subjected to flash column chromatography
(silica, 3:1 v/v pet. spirit/EtOAc elution) to afford, after concentration of the
appropriate fractions (Rf
= 0.3), compound 4 (R = Me) (336 mg, 81%) as a white, crystalline solid and a ca. 1:3 mixture of α-
and β-anomers. Further chromatographic purification of a 10 mg sample of this mixture
under the same conditions afforded an essentially pure sample of the β-anomer suitable
for characterisation purposes.
Mp 84–86 °C; [α]
D
+18 (c = 0.38, CHCl3).
1H NMR (400 MHz, CDCl3): δ (β-anomer) = 8.73 (s, 1 H), 7.36–7.27 (complex m, 10 H), 5.85 (d, J = 7.3 Hz, 1 H), 4.99–4.69 (complex m, 4 H), 4.01 (d, J = 9.1 Hz, 1 H), 3.81 (s, 3 H), 3.78–3.61 (complex m, 3 H), 3.51 (s, 3 H).
13C NMR (101 MHz, CDCl3): δ (β-anomer) = 168.9, 161.1, 138.3, 137.9, 128.5(4), 128.5(2), 128.1, 128.0, 127.9,
98.2, 90.8, 83.6, 80.9, 80.4, 75.6, 75.0, 74.8, 60.8, 52.8 (one signal obscured or
overlapping).
IR: 3337, 3032, 2930, 1751, 1288, 1053, 797, 697 cm–1.
HRMS (ESI, +ve): m/z [M + Na]+ calcd for C24H26
35Cl3O7N·Na: 568.0673; found: 568.0675.
Allyl 3-O-Acetyl-4-O-tert-butyl-dimethylsilyl-α-d-xylopyranosyl-(1→2)-methyl-(2,3-di-O-benzyl-4-O-methyl-β-d-glucopyranosyl) (31) and Allyl 3-O-Acetyl-4-O-tert-butyl-dimethylsilyl-α-d-xylopyranosyl-(1→2)-methyl-(2,3-di-O-benzyl-4-O-methyl-α-d-glucopyranosyl)uronate (32)
Allyl 3-O-Acetyl-4-O-tert-butyl-dimethylsilyl-α-d-xylopyranosyl-(1→2)-methyl-(2,3-di-O-benzyl-4-O-methyl-β-d-glucopyranosyl) (31) and Allyl 3-O-Acetyl-4-O-tert-butyl-dimethylsilyl-α-d-xylopyranosyl-(1→2)-methyl-(2,3-di-O-benzyl-4-O-methyl-α-d-glucopyranosyl)uronate (32)
Trichloroacetimidate 4 (R = Me) (336 mg, 0.61 mmol) and alcohol 3 (141 mg, 0.41 mmol) were placed in separate round-bottom flasks, each fitted with
a stirrer bar and charged with activated 4 Å molecular sieves (100 mg). After each
flask was sealed with a rubber septum, they were placed under high vacuum for 1 h
and thereafter anhydrous dichloromethane (3.0 mL) was added to each flask and the
resulting mixtures stirred at 22 °C for 2 h under an argon atmosphere. A solution
of TMSOTf (24 μL in 220 μL of anhydrous dichloromethane, 0.13 mmol) was prepared,
dried over activated 4 Å molecular sieves then added to the reaction mixture containing
alcohol 3. The solution containing the trichloroacetimidate 4 (R = Me) was then slowly added, via cannula, to the solution of the alcohol and the resulting yellow solution stirred
at 22 °C for 1 h. Thereafter, the solution was quenched with triethylamine (3 drops)
resulting in a colour change to pink. The reaction mixture was then concentrated under
reduced pressure and the residue so-formed subjected to flash column chromatography
(silica, 9:1 v/v dichloromethane/diethyl ether then 6:1 v/v pet. spirit/EtOAc elution)
to give two fractions, A and B.
Concentration of fraction A (
Rf
= 0.3 in 5:1 v/v pet. spirit/EtOAc) afforded compound 32 (142 mg, 48%) as a clear, yellow oil.
[α]
D
+70 (c = 0.70, CHCl3).
1H NMR (400 MHz, CDCl3): δ = 7.41–7.27 (complex m, 10 H), 5.88 (m, 1 H), 5.38 (dd, J = 10.0, 8.8 Hz, 1 H), 5.30 (app. dq, J = 16.9, 1.6 Hz, 1 H), 5.14 (app. dq, J = 10.4, 1.6 Hz, 1 H), 4.93 (d, J = 3.5 Hz, 1 H), 4.87 (m, 2 H), 4.79 (d, J = 11.0 Hz, 1 H), 4.71 (d, J = 11.9 Hz, 1 H), 4.63 (d, J = 11.9 Hz, 1 H), 4.17 (m, 2 H), 3.96 (m, 1 H), 3.85 (d, J = 9.3 Hz, 1 H), 3.81 (s, 3 H), 3.70 (m, 1 H), 3.60 (m, 2 H), 3.50 (m, 2 H), 3.44
(s, 3 H), 3.37 (m, 1 H), 2.07 (s, 3 H), 0.85 (s, 9 H), 0.05 (s, 3 H), 0.04 (s, 3 H).
13C NMR (101 MHz, CDCl3): δ = 170.6, 169.9, 138.7, 138.3, 134.0, 128.5(4), 128.4(8), 128.2, 128.0, 127.9,
127.7, 117.9, 97.2, 95.2, 81.8, 80.9, 78.6, 76.3, 75.7, 73.7, 73.1, 70.7, 69.9, 68.7,
62.3, 60.7, 52.7, 25.7, 21.3, 18.0, –4.5, –4.8.
IR: 2931, 2858, 1751, 1227, 1097, 1064, 1043 cm–1.
HRMS (ESI, +ve): m/z [M + Na]+ calcd for C38H54O12Si·Na: 753.3282; found: 753.3287.
Concentration of fraction B (Rf
= 0.4 in 5:1 v/v pet. spirit/EtOAc) afforded compound 31 (35.0 mg, 12%) as a clear, yellow oil.
[α]
D
+48 (c = 0.70, CHCl3).
1H NMR (400 MHz, CDCl3): δ = 7.40–7.27 (complex m, 10 H), 5.95 (m, 1 H), 5.47 (dd, J = 10.2, 8.4 Hz, 1 H), 5.36 (app. dq, J = 17.2, 1.7 Hz, 1 H), 5.20 (app. dq, J = 10.7, 1.7 Hz, 1 H), 4.97 (d, J = 3.6 Hz, 1 H), 4.86 (dd, J = 11.3, 3.6 Hz, 2 H), 4.76 (d, J = 10.9 Hz, 1 H), 4.65 (d, J = 11.7 Hz, 1 H), 4.50 (d, J = 7.6 Hz, 1 H), 4.21 (m, 1 H), 4.06 (m, 1 H), 3.81 (s, 3 H), 3.77 (dd, J = 6.8, 2.7 Hz, 1 H), 3.72 (m, 1 H), 3.65 (app. td, J = 10.3, 3.3 Hz, 2 H), 3.56–3.50 (complex m, 3 H), 3.48 (s, 3 H), 3.43 (m, 1 H), 1.86
(s, 3 H), 0.86 (s, 9 H), 0.06 (s, 3 H), 0.04 (s, 3 H).
13C NMR (101 MHz, CDCl3): δ = 169.9, 169.0, 138.5, 134.2, 128.4, 128.3, 128.2, 127.9, 127.7, 127.5, 117.0,
104.9, 98.0, 83.9, 81.1, 80.8, 77.7, 75.7, 74.6, 74.2, 73.9, 70.3, 68.9, 62.3, 60.8,
52.6, 25.7, 21.2, 18.0, –4.5, –4.9 (one signal obscured or overlapping).
IR: 2931, 2858, 1751, 1227, 1071, 1044, 1027, 838 cm–1.
HRMS (ESI, +ve): m/z [M + Na]+ calcd for C38H54O12Si·Na: 753.3282; found: 753.3287.
Allyl 3-O-Acetyl-α-d-xylopyranosyl-(1→2)-methyl-(2,3-di-O-benzyl-4-O-methyl-α-d-glucopyranosyl)urinate (33)
Allyl 3-O-Acetyl-α-d-xylopyranosyl-(1→2)-methyl-(2,3-di-O-benzyl-4-O-methyl-α-d-glucopyranosyl)urinate (33)
A magnetically stirred solution of disaccharide 32 (120 mg, 0.16 mmol) in acetonitrile (3.5 mL) and maintained at 22 °C in a falcon
tube was treated with fluorosilicic acid (116 μL of a 25 wt % in H2O solution, 0.25 mmol). After 6 h the reaction mixture was treated with CaCO3 (2 mL of a sat. aq. solution) and NaHCO3 (2 mL of a sat. aq. solution) and after a further 0.25 h the reaction mixture was
diluted with brine (10 mL) then extracted with EtOAc (2 × 15 mL). The combined organic
phases were then dried (Na2SO4), filtered and concentrated under reduced pressure. The residue thus obtained was
subjected to flash column chromatography (silica, 1:2 v/v pet. spirit/EtOAc elution)
to afford, after concentration of the appropriate fractions (Rf
= 0.6), compound 33 (83.1 mg, 82%) as a clear, colourless oil.
[α]
D
+63 (c = 0.80, CHCl3).
1H NMR (400 MHz, CDCl3): δ = 7.42–7.26 (complex m, 10 H), 5.88 (m, 1 H), 5.30 (app. dq, J = 16.9, 1.6 Hz, 1 H), 5.18 (m, 2 H), 4.94 (dd, J = 7.1, 3.4 Hz, 2 H), 4.89 (d, J = 10.9 Hz, 1 H), 4.79 (d, J = 10.9 Hz, 1 H), 4.69 (m, 2 H), 4.30 (d, J = 10.0 Hz, 1 H), 4.20 (m, 1 H), 3.98 (m, 1 H), 3.87 (app. t, J = 9.3 Hz, 1 H), 3.81 (s, 3 H), 3.79–3.59 (complex m, 4 H), 3.52 (dd, J = 9.7, 3.5 Hz, 1 H), 3.46 (s, 3 H), 3.41 (dd, J = 10.0, 9.0 Hz, 1 H), 2.91 (br s, 1 H), 2.17 (s, 3 H).
13C NMR (101 MHz, CDCl3): δ = 172.9, 170.4, 138.7, 138.1, 133.7, 128.6, 128.5, 128.1(0), 128.0(8), 128.0,
127.8, 118.3, 96.3, 95.0, 81.6, 81.0, 78.5, 75.7, 75.5, 73.8, 73.2, 70.6, 70.0, 68.8,
62.4, 60.9, 52.7, 21.2.
IR: 3460, 2928, 1747, 1230, 1077, 1044, 744 cm–1.
HRMS (ESI, +ve): m/z [M + Na]+ calcd for C32H40O12·Na: 639.2417; found: 639.2397.
2,3,2′,3′,4′-Penta-O-acetyl-β-d-xylobiosyl-(1→4)-allyl-3-O-acetyl-α-d-xylopyranosyl-(1→2)-methyl-(2,3-di-O-benzyl-4-O-methyl-α-d-glucopyranosyl)urinate (34)
2,3,2′,3′,4′-Penta-O-acetyl-β-d-xylobiosyl-(1→4)-allyl-3-O-acetyl-α-d-xylopyranosyl-(1→2)-methyl-(2,3-di-O-benzyl-4-O-methyl-α-d-glucopyranosyl)urinate (34)
A magnetically stirred mixture of trichloroacetimidate 2 (154 mg, 0.24 mmol) and activated 4 Å molecular sieves (100 mg), maintained under
an atmosphere of argon, was treated with anhydrous dichloromethane (2.0 mL). In a
separate flask, a magnetically stirred mixture of alcohol 33 (74.1 mg, 0.12 mmol) and activated 4 Å molecular sieves (100 mg) was also treated
with anhydrous dichloromethane (2.0 mL). Both mixtures were then cooled to –20 °C
[water/ethanol/CO2(s)] and stirring continued for 2 h. TMSOTf (60 μL of a 0.62 M solution in dichloromethane,
0.036 mmol) was added to the reaction mixture containing alcohol 33 and then the solution of trichloroacetimidate 2 was added, slowly and via cannula, to the reaction mixture containing compound 33 and TMSOTf. Stirring of the mixture was continued at –20 °C for 1 h then the reaction
was quenched by the addition of triethylamine (2 drops), this resulting in a colour
change from yellow to pink. After warming to 22 °C, the reaction mixture was concentrated
under reduced pressure and the residue thus obtained subjected to successive flash
column chromatographic purifications (silica, 1:2 v/v pet. spirit/EtOAc elution) to afford, after concentration of the relevant fractions
(Rf
= 0.2), compound 34 (8.8 mg, 7%) as a clear, colourless oil.
[α]
D
+13 (c = 0.77, CHCl3).
1H NMR (600 MHz, CDCl3): δ = 7.34–7.27 (complex m, 10 H), 5.85 (m, 1 H), 5.41 (app. t, J = 9.1 Hz, 1 H), 5.29 (br dd, J = 16.5, 1.7 Hz, 1 H), 5.14 (br dd, J = 11.0, 1.7 Hz, 1 H), 5.09 (app. t, J = 7.8 Hz, 1 H), 5.05 (app. t, J = 8.1 Hz, 1 H), 4.92 (d, J = 3.5 Hz, 1 H), 4.89 (m, 1 H), 4.86 (d, J = 10.9 Hz, 1 H), 4.83 (d, J = 3.4 Hz, 1 H), 4.81 (m, 1 H), 4.78 (d, J = 10.9 Hz, 1 H), 4.71 (m, 2 H), 4.62 (d, J = 12.0 Hz, 1 H), 4.56 (d, J = 6.0 Hz, 1 H), 4.48 (d, J = 6.0 Hz, 1 H), 4.17 (d, J = 10.1 Hz, 1 H), 4.14 (br. dd, J = 13.1, 5.7 Hz, 1 H), 4.09 (dd, J = 12.0, 4.7 Hz, 1 H), 3.95 (m, 2 H), 3.84 (app. t, J = 9.4 Hz, 1 H), 3.80 (s, 3 H), 3.78 (d, J = 8.1 Hz, 1 H), 3.69 (app. q, J = 9.1 Hz, 1 H), 3.60 (m, 3 H), 3.48 (m, 1 H), 3.43 (s, 3 H), 3.41–3.31 (complex m,
3 H), 2.06 (s, 3 H), 2.05(4) (s, 3 H), 2.05(1) (s, 3 H), 2.03(4) (s, 3 H), 2.03(2)
(s, 3 H), 2.02(9) (s, 3 H).
13C NMR (151 MHz, CDCl3): δ = 170.5, 170.1, 169.9, 169.8, 169.5, 169.3, 138.8, 138.3, 133.8, 128.6, 128.5,
128.1, 127.9(8), 127.9(7), 127.7, 118.0, 100.6, 99.6, 97.3, 95.1, 81.8, 80.9, 78.7,
76.8, 76.1, 75.7, 74.3, 73.3, 71.9, 71.4, 71.2, 70.8, 70.7, 70.6, 68.8, 68.5, 62.4,
61.7, 60.7, 59.4, 52.6, 21.2, 20.9(3), 20.8(8), 20.8(1), 20.8(0), 20.7(8) (one signal
obscured or overlapping).
IR: 2926, 2855, 1747, 1370, 1220, 1072, 1043, 752 cm–1.
HRMS (ESI, +ve): m/z [M + Na]+ calcd for C52H66O25·Na: 1113.3791; found: 1113.3766.
Allyl 3-O-Acetyl-4-O-tert-butyl-dimethylsilyl-α-d-xylopyranosyl-(1→2)-methyl-2,3,4-tri-O-acetyl-β-d-glucopyranuronate (40)
Allyl 3-O-Acetyl-4-O-tert-butyl-dimethylsilyl-α-d-xylopyranosyl-(1→2)-methyl-2,3,4-tri-O-acetyl-β-d-glucopyranuronate (40)
Trichloroacetimidate 39
[25] (282 mg, 0.59 mmol) and alcohol 3 (137 mg, 0.39 mmol) were placed in a round-bottom flask charged with a stirrer bar
and activated 4 Å molecular sieves (100 mg). After being sealed with a rubber septum,
the flask was placed under high vacuum for 1 h then refilled with argon before anhydrous
dichloromethane (5 mL) was added. The resulting mixture was stirred at 22 °C for 2
h before being cooled to –20 °C then treated with a solution of TMSOTf in anhydrous
dichloromethane (1.00 mL of a 0.12 M solution, 0.12 mmol) maintained at –20 °C under
an atmosphere of argon. The resulting yellow solution was stirred at –20 °C for 1
h then quenched with triethylamine (3 drops) and so resulting in a colour change to
pink. The reaction mixture was warmed to 22 °C then concentrated under reduced pressure.
The residue thus obtained was subjected to flash column chromatography (silica, 2:1
v/v pet. spirit/EtOAc elution then, separately, employing 9:1 v/v dichloromethane/diethyl
ether elution and 5:2 v/v pet. spirit/EtOAc elution) to afford, after concentration
of the relevant fractions (Rf
= 0.2 in 5:2 v/v pet. spirit/EtOAc), compound 40 (26.3 mg, 10%) as a white solid. Slow evaporation of a solution of this material
in diethyl ether at 22 °C afforded a crystalline solid suitable for single-crystal
X-ray analysis.
Mp 140–142; [α]
D
+17 (c = 0.38, CHCl3).
1H NMR (400 MHz, CDCl3): δ = 5.92 (m, 1 H), 5.37–5.29 (complex m, 2 H), 5.27–5.15 (complex m, 3 H), 4.98
(m, 1 H), 4.91 (d, J = 3.6 Hz, 1 H), 4.66 (d, J = 7.8 Hz, 1 H), 4.17 (m, 1 H), 4.05 (m, 1 H), 4.00 (d, J = 9.5 Hz, 1 H), 3.74 (s, 3 H), 3.68 (m, 1 H), 3.59 (m, 2 H), 3.50 (dd, J = 10.4, 5.6 Hz, 1 H), 2.09 (s, 3 H), 2.03 (s, 3 H), 2.01 (s, 3 H), 2.00 (s, 3 H),
0.84 (s, 9 H), 0.04 (s, 3 H), 0.03 (s, 3 H).
13C NMR (101 MHz, CDCl3): δ = 170.4, 169.5, 169.4, 169.3, 167.1, 134.0, 117.3, 101.2, 97.6, 77.4, 74.2, 72.5,
72.4, 71.1, 70.0, 69.5, 69.1, 62.2, 53.0, 25.6, 21.4, 20.8, 20.6, 20.5, 18.0, –4.5,
–4.9.
IR: 2933, 2858, 1748, 1374, 1217, 1042 cm–1.
HRMS (ESI, +ve): m/z [M + Na]+ calcd for C29H46O15Si·Na: 685.2504; found: 685.2508.
