合成路线1

Synthesis of the second synthon: L-Leucinol (V) was converted into the protected derivative (VI) by treatment with tert-butoxycarbonyl anhydride in methylene chloride. Oxidation of compound (VI) with NaOCl and TEMPO in the presence of sodium bromide yielded the aldehyde (VII). This aldehyde (VII), when treated with sodium bisulfite overnight at -5 to 0 C, produced the sodium bisulfite salt derivative (VIII), which was treated with KCN to obtain the nitrile derivative (IX). This nitrile derivative (IX) was heated under reflux with concentrated HCl, and after several crystallizations, afforded the desired amino acid (2R,3S)-3-amino-2-hydroxy-5-methylhexanoic acid (X). Amino acid (X) was dissolved in a mixture of water/dioxane and then treated with tert-butoxycarbonyl anhydride in the presence of TEA to yield the Boc derivative (XI), which was converted into the mixture of methyl esters (XII) in a conventional manner. Compound (XII) was heated with 2,4-dimethoxybenzaldehyde dimethyl acetal in THF in the presence of pyridinium p-toluensulfonate as catalyst to obtain the acetal (XIII). When hydrolyzed with potassium carbonate in aqueous methanol, acetal (XIII) yielded (4S,5R)-3-(tert-butoxycarbonyl)-2-(2,4-dimethoxyphenyl)-4-isobutyloxazolidine-5-carboxylic acid (XIV).