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<A NAME="RG26303ST-2B">2b</A> 2,3-Dibromofuran-5-carboxylate:
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<A NAME="RG26303ST-2F">2f</A> 3,4-Dibromo- and 2,3-dichloro-2(5H)-furanone:
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<A NAME="RG26303ST-3A">3a</A> Dibromomethylenation:
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<A NAME="RG26303ST-20">20</A>
5-(Dibromomethylene)-2(5
H
)-furanone (
2): 3,5-Dibromolevulinic acid (6; 4.77 g, 17.4 mmol), oleum (18 mL, 65% SO3), and concentrated H2SO4 (9 mL) were stirred at 50-60 °C for 6 min. The solution was poured on ice and extracted
with CH2Cl2 (3 × 30 mL). The combined organic extracts were dried (Na2SO4) and evaporated under reduced pressure. After flash chromatography on silica gel
[22]
(cyclohexane:EtOAc 15:1Æ5:1) compound 2 (1.22 g, 28%) was obtained as a slightly yellow solid (mp 132-134 °C). 1H NMR (300 MHz, CDCl3): δ = 6.41 (d, J
3,4 = 5.6 Hz, 3-H), 7.67 (d, J
4,3 = 5.6 Hz, 4-H).
<A NAME="RG26303ST-21">21</A>
(
Z
)-4-Bromo-5(bromomethylene)-2(5
H
)-furanone (
Z-
5): 3,5-Dibromolevulinic acid (6; 912 mg, 3.33 mmol) and concentrated H2SO4 (20 mL) were stirred at r.t. for 20 min and at 85 °C for 30 min. The solution was
poured on ice and extracted with CH2Cl2 (3 × 30 mL). The combined organic extracts were dried (Na2SO4) and evaporated under reduced pressure. After flash chromatography on silica gel
[22]
(cyclohexane:EtOAc 15:1Æ5:1) compound Z-5 (346 mg, 41%) was obtained as a slightly yellow solid (mp 96-97 °C). 1H NMR (300 MHz, CDCl3): δ = 6.41 (s, 1′-H), 6.50 (s, 3-H).
<A NAME="RG26303ST-22">22</A>
Still WC.
Kahn M.
Mitra A.
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1978,
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2923
<A NAME="RG26303ST-23">23</A>
Labadie JW.
Tueting D.
Stille JK.
J. Org. Chem.
1983,
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4634
<A NAME="RG26303ST-24">24</A> Method:
Gilman H.
Rosenberg SD.
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1953,
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Entwistle DA.
Jordan SI.
Montgomery J.
Pattenden G.
Synthesis
1998,
603
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Amatore C.
Bahsoun AA.
Jutand A.
Meyer G.
Ntepe AN.
Ricard L.
J. Am. Chem. Soc.
2003,
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4212
<A NAME="RG26303ST-26">26</A>
The stereochemical problem was clarified by the (albeit small) NOE observed at 2′-H
(δ = 7.28 ppm in CDCl3) while irradiating 4-H (δ = 8.02 ppm) of the isomer labeled E-7. The regiochemistry problem was solved by the occurrence of two vicinal Csp2
-H/Csp2
-H couplings (J = 11.7 Hz, 15.8 Hz) for the olefinic protons of the isomer designated Z-13 as contrasting with only one such coupling (J = 16.1 Hz) in the isomer designated Z-17. The same answer came from the occurrence of one olefinic 1H NMR singlet in Z-13 (δ = 6.21 ppm) as opposed to two such singlets in Z-17 (δ = 6.28 and 6.31 ppm).
<A NAME="RG26303ST-27">27</A>
The differentiation of compound Z-9 from stereoisomer E-9 followed from irradiating 4-H (δ = 7.87 ppm in CDCl3) and the resulting increase of absorption by 2′-H (δ = 7.07 ppm). Butenolide Z-14 was distinguished from the isomeric structure iso-14 by the occurrence of two olefinic 3-bond H,H couplings (J
1
′
,2
′ = 11.4 Hz, J
3
′
,2
′ = 15.8 Hz) rather than one. Monocoupling product Z-16 was differentiated from regioisomer iso-16 by an NOE experiment: irradiation of ortho-H (δ = 7.78-7.83 ppm in CDCl3) enhanced only the absorption by 1′-H (δ = 6.36 ppm) and not by 1′-H and 3-H as expected
for iso-16.
<A NAME="RG26303ST-28">28</A>
(
Z
)-4-Bromo-5-(
trans
-3-phenyl-2-propenylidene)-2(5
H
)-furanone (
Z
-14): 1H NMR (300 MHz, CDCl3): δ = 6.30 (d, J
1
′
,2
′ = 11.4 Hz, 1′-H), 6.37 (s, 3-H), 6.93 (d, J
3
′
,2
′ = 15.8 Hz, 3′-H), 7.25-7.41 (m, 2′-H, 2 × meta-H, para-H), 7.50-7.53 (m, 2 × ortho-H). 13C NMR (125.7 MHz, CDCl3): δ = 114.51 (C-1′), 119.11 (C-3), 121.01 (C-2′), 127.40 (2 × C
ortho
), 128.92 (2 × C
meta
), 129.41 (C
para
), 136.11 and 136.19 (C-4, C
ipso
), 140.13 (C-3′), 147.23 (C-5), 166.64 (C-2). Anal. Calcd for C13H9BrO2 (276.5): C, 56.34; H, 3.27. Found: C, 56.36; H, 3.19.
<A NAME="RG26303ST-29">29</A>
(
Z
)-5-(1-Bromo-1-phenylmethylene)-2(5
H
)-furanone (
Z
-11): 1H NMR (300 MHz, CDCl3): δ = 6.31 (d, J
3,4 = 5.6 Hz, 3-H), 7.41-7.49 (m, C6H5 and 4-H). 13C NMR (75.4 MHz, CDCl3): δ = 110.88 (C-1′), 120.80 (C-3), 128.75 and 130.11 (2 × C
ortho
, 2 × C
meta
), 130.23 (C
para
), 135.39 (C
ipso
), 140.88 (C-4), 149.10 (C-5), 168.46 (C-2). Anal. Calcd for C11H7BrO2 (250.5): C, 52.62; H, 2.81. Found: C, 52.66; H, 2.70.
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While we debrominated 1,1-dibromoolefin 2 using Pd(PPh3)4 and HSnBu3 (ÆZ-10, Scheme
[4]
) the same reagents
[30]
or variations thereof [Pd(PPh3)4/HSiEt3 or HSi(SiMe3)3 or NH4HCO2; Pd(dba)2 or NiCl2(PPh3)2/HSnBu3] failed to reduce 1,3-dibromodiene Z-5 due to inertness (15 was not formed, Scheme
[5]
).
Method, albeit without sonication:
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The C1
′ = Cγ configuration of debromination product E-8 followed from the NOE’s induced by irradiating, in CDCl3 solution, 4-H (δ = 7.84 ppm): The absorption of 1′-H (δ = 6.50 ppm) remained unaltered
while the absorption of 2′-H (δ = 7.02 ppm) increased. In Z-8 an analogous assignment was impossible because the resonances of 4-H and 2′-H overlapped.
The C1
′ = Cγ configurations of the debromination products E-12 and Z-12 were established by the absence(presence) of an NOE at 1′-H (δ = 6.81 ppm and δ =
6.03 ppm, respectively, in CDCl3) when irradiating 4-H (δ = 7.82 ppm and δ = 7.49 ppm, respectively).
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