Allylation of Glyoxylates with allyltrimethylsilane; alpha hydroxy-ester

Dougal Ritson ([email protected]),
A contribution from the Cox Group, Bristol

Chemicals Used
p-nitrobenzyl glyoxylate (dried - Dean-Stark) Boron trifluoride diethyl etherate (Aldrich) Allyltrimethylsilane (Aldrich) Dichloromethane (anhydrous)

Procedure
p-nitrobenzyl glyoxylate (1.215g, 6 mmol) was dissolved in anhydrous DCM (15 mL) and the reaction vessel was cooled to 0°C. Boron trifluoride diethyl etherate (1.356g, 1.20 mL, 9.6 mmol) was then introduced and allyltrimethylsilane (1.043g, 1.45 mL, 9.1 mmol) was added to the solution 1/2 hr later. After 1 hr the reaction was warmed to room temp. and left to stir overnight. The reaction was washed with water ( 3 x 20 mL), dried (MgSO₄) and filtered and concentrated. The orange oil was purified by flash chromatography, eluting with 3:2 petrol-EtOAc, to give 2-hydroxy-pent-4-enoic acid 4-nitro benzyl ester (1.177g, 5.5 mmol, 78%) as an orange oil.

Author's Comments
The use of this reaction has not been tested on aldehydes remote i.e. not alpha to, an ester carbonyl. The proximity of the delta positve carbonyl carbon belonging to the ester will increase the electrophilicity of the aldehyde and may be a prerequisite for the reaction to work. A stronger Lewis acid may overcome this possible problem. The yields were found to differ initially but this was attributed to the quality of the starting material i.e. presence of the hydrated gem-diol. This problem was solved by drying the aldehyde more throroughly.

Data
IR (film)/ cm^-1 3492 (OH), 3090 (CH), 2950 (CH), 1738 (CO), 1518 (ONO), 1350 (ONO); H NMR 8.24 (2H, d, J 8.9, 2 x ArH), 7.57 (2H, d, J 8.9, 2 x ArH), 5.79 (1H, m, CH), 5.3 (2H, d, J 1.7, ArCH₂), 5.17 (1H, m, ChH), 5.12 (1H, m, CHh), 4.37 (1H, td, J 6.3, 4.6, COCH), 2.72 (1H, d, 6.3, OH), 2.55 (2H, m, ChCH₂Ch); C NMR 174.0 (CO), 147.6 (CNO₂), 142.2 (ArC-), 132.0 (gama-CH), 128.7 (2 x ArCH), 123.9 (2 x ArCH), 119.2 (delta-CH₂), 70.0 (alpha-CH), 65.8 (ArCH₂), 38.7 (beta-CH₂); m/z (CI) 252 [MH+, (24%)], 235 [MH+ - OH, (9)], 206 [MH+ - ONO, (45)]; HRMS (CI) calc [MH+] 252.0872 found 252.0874.

Lead Reference
Floris P.J.T. Rujtes, T. Martijn Kooistra, Henk Hiemstra and Hans E. Schoemaker, Synlett, 1998, 192.

Other References

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