Water → (4S,5R)-3-t-butoxycarbonyl-2-(2,4-dimethoxyphenyl)-4-phenyl-5-methoxycarbonyl-1,3-oxazolidine

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A solution of 0.497 g of methyl (2R,3S)-3-t-butoxycarbonylamino-2-hydroxy-3-phenylpropionate, 0.012 g of pyridinum b-toluenesulphonate and 0.295 g of 2,4-dimethoxybenzaldehyde in 20 cm3 of anhydrous toluene is heated to reflux for 24 hours. The water formed during the reaction is removed by means of a Dean and Stark apparatus. After cooling to a temperature in the region of 20° C., the solution is washed with 37% (w/w) aqueous sodium hydrogen sulphite solution and then with saturated aqueous sodium bicarbonate solution. After concentration of the organic phase under reduced pressure, 0.700 g of (4S,5R)-3-t-butoxycarbonyl-2-(2,4-dimethoxyphenyl)-4-phenyl-5-methoxycarbonyl-1,3-oxazolidine is obtained in an 80% yield in the form of a virtually equimolecular mixture of the diastereo-isomeric forms A and B, the characteristics of which are as follows:

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