Ethyl Acetate → 2-[(4R)-4-Acetylthio-2-oxoazetidin-1-yl]-2-hydroxyacetic acid p-nitrobenzyl ester

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200 mg of 2-ethoxy-2-hydroxyacetic acid p-nitrobenzyl ester and 2 g of molecular sieves A4 are added to a solution of 54.5 mg of (3S,4R)-4-acetylthio-3-methoxy-2-oxoazetidine in a mixture of 4 ml of toluene and 1 ml of dimethylformamide, and the mixture is stirred for 2 hours at 50°. The molecular sieves are filtered off and the filtrate is concentrated in vacuo. The residue is chromatographed over silica gel, and by elution with toluene/ethyl acetate (9:1) the title compound, contaminated with some glyoxylate, is obtained. TLC: Rf =0.48 (ethyl acetate); IR spectrum (in CH2Cl2): absorption bands at 2.89 (broad), 5.64, 5.73, 5.91, 6.24, 6.56, 7.44, 7.62, 7.83-8.15 (broad, sh), 8.26, 8.39 and 8.80-9.30 (broad) μ.

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