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<A NAME="RS07703ST-10">10</A> For a general overview for the synthesis of phosphono- and phosphinopeptides,
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<A NAME="RS07703ST-12">12</A> Inverse addition (cesium salt of dialkylphosphite to CS2) was also tried and did not result in a higher yield. Also, formation of secondary
products resulting from the reaction between dialkylphosphite salts with the in situ
generated cesium phosphonodithiocarboxylate leading to the expected desulfurization
forming methylene diphosphonates did not arise as previously described for sodium
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General Experimental Procedure: To a solution of diethyl phosphite 4 (0.12 g, 0.85 mmol, 1 equiv) in anhyd DMF (5 mL) was added Cs2CO3 (0.83 g, 2.55 mmol, 3 equiv) and TBAI (0.94 g, 2.55 mmol, 3 equiv) with vigorous
stirring for 10 min at r.t. under a N2 atmosphere. CS2 (0.15 mL, 2.55 mmol, 3 equiv) was added and the fuchsia colored mixture was stirred
for 1 h. After this time period, benzyl bromide (0.30 mL, 2.55 mmol, 3 equiv) was
added and stirred for an additional 24 h. The resultant yellow reaction suspension
was then poured into water (30 mL) and extracted with EtOAc (3 × 30 mL). The organic
layer was washed with water (2 × 30 mL), brine (30 mL), and dried over anhyd Na2SO4. Evaporation of the solvent followed by flash chromatography (hexanes-EtOAc, 9:1)
afforded benzyl diethoxyphosphoryldithioformate(5) as a dark red oil (0.25 g, 97%).
1H NMR (270 MHz, CDCl3): δ = 1.36 (t, J
1,2 = 7.6 Hz, 6 H), 4.26 (m, 4 H), 4.46 (s, 2 H), 7.30 (s, 5 H). 13C NMR (100 MHz, CDCl3): δ = 16.20 (d, J
CP = 6.34 Hz), 40.63 (d, J
CP = 2.72 Hz), 64.70 (d, J
CP = 6.94 Hz), 128.00 (s), 128.77 (s), 129.26 (s), 133.53 (s), 228.16 [d, J
CP = 174.54 PC(S)S]; 31P NMR (85 MHz, CDCl3)δ from 30% H3PO4-H2O: -4.57. MS: m/z = 91, 121, 182, 248, 276, 304 (M+). Anal. Calcd for C12H17O3PS2: C, 47.35; H, 5.63. Found: C, 47.42; H, 5.64.
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Vogtle F. In
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<A NAME="RS07703ST-15">15</A> Formation of ‘naked anions’ by solvation of cesium ions has been previously postulated
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For examples of efficient cesium-promoted alkylations, see:
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TBAI is strongly believed to act as a phase-transfer catalyst in the reaction, therefore,
facilitating alkylations producing high product yields. For other phase-transfer catalyzed
phosphorus alkylations, see:
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Kem KM.
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<A NAME="RS07703ST-17D">17d</A> However, we cannot rule out an internal Finkelstein-type reaction for the in-situ
generation of alkyl iodides from bromides and chlorides, hence improving yields. Although,
the use of alkyl iodides directly, without TBAI gave lower product yields. Therefore,
we propose TBAI minimizes or prohibits direct alkylation of the phosphite with an
alkyl halide presumably enhancing the rate of CS2 incorporation/and or stabilizing the phosphoryl dithioformate anion through conjugation
with the tetrabutylammonium cation. Whereas, the cesium ion tends to weakly coordinate
to the conjugate anions, making them more nucleophilic. Prior to addition of the
halide, the phosphite and CS2 were reacted to pre-form the incipient dithioformate anion, which is belived to suppress
direct alkylation of the phosphite. For a similar example, see ref. for a mechanistic
interpretation and for the use of TBABr and other onium salts in the formation of
urethanes:
Yoshida M.
Hara N.
Okuyama S.
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Since dithioesters are known to be very good thioacylating agents for amines due to
the high electrophilicity of the C=S which is activated by the phosphono-substituent
(an electron-withdrawing group), amino bromides 13 and 15 were used as the corresponding hydrobromide salts, since a spontaneous intermolecular
reaction to give thioamides or cyclization to the phosphonothiazoline can readily
occur. Neither the aforementioned thioacylation reaction (which usually occurs with
the alkyl group in the phosphono-moiety) or cyclization was seen. Therefore, we wish
to strongly emphasize product structures for aminodithioesters from 13 and 15 were isolated and assigned without ambiguity from the exact mass (MS), 1H, 13C, and 31P NMR spectroscopy.
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Nagle AS.
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1H NMR, 13C NMR, 31P NMR and 2D NMR analysis indicate the product as a single diastereomer.
For new biologically active phosphonates, see:
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Hildebrand R.
The Role of Phosphonates in Living Systems
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