Benzene → (4S,5R)-N-Benzoyl-2-(2,4-dimethoxyphenyl)-4-phenyl-5-oxazolidinecarboxylic acid methyl ester

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Reaction Steps

N-Benzoyl-βphenyl isoserine methyl ester (3A, 0.5 g, 1.67 mM) is dissolved in dry THF (10 mL) and benzene (10 mL) and the solution treated with 2,4-dimethoxy benzaldehyde dimethyl acetal (4, 0.420 g, 1.98 mM) and pyridinium p-toluenesulfonate (12 mg) and the solution warmed to reflux. After 30 minutes the reaction is cooled to RT and allowed to stand overnight. It is then again warmed to slowly distill off ½ of the solvent over 1 hr. TLC shows the reaction to be finished at this point. The reaction is concentrated in vacuo and the residue chromatographed over 50 g silica gel packed in (5-95) methanol-toluene. The column is eluted with methanol-toluene (5-95). Fractions of 12 mL are collected. The product elutes as a mixture. Thus, fractions containing both 5Aa & 5Ab are recombined and evaporated. The residue (0.90 g) is rechromatographed over silica gel (100 g) The column is eluted with ethyl acetate-toluene (500 mL of 15-85 and 500 mL of 20-80). Fractions of 20 mL are collected and analyzed by TLC. The fractions containing each 5Aa & 5Ab are combined and evaporated under vacuum.

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