(2S,3S,5R)-2-[6-Benzyloxy-3-(tert-butoxycarbonyl)amino-2-hydroxy-5-methylhexyl]-N-(butyl)acrylamide (A) and (2R,3S,5R)-2-[6-benzyloxy-3-(tert-butoxycarbonyl)amino-2-hydroxy-5-methylhexyl]-N-(butyl)acrylamide (B): In a manner analogous to that described in Example 1h), there is obtained first, by reduction of (2S,4R)-5-benzyloxy-2-(tert-butoxycarbonyl)amino-4-methylpentanoic acid methyl ester (9.51 g) in the presence of a 1.2M diisobutylaluminium hydride solution in toluene, (2S,4R)-5-benzyloxy-2-(tert-butoxycarbonyl)amino-4-methyl-pentanal (8.2 g of crude product) in the form of a pale-yellow oil. The crude aldehyde is reacted in a manner analogous to that described in Example 1g) without being purified further. The crude product (17.1 g) so obtained comprises an approximately 1:1.45 mixture of the two diastereoisomers A and B. Purification by column chromatography with separation of the two stereoisomers on silica gel (0.6 kg, eluant gradient from C to E) yields title compound A (2.05 g) in the form of a waxy solid, Rf (E)=0.43, MS(FAB) m/e 463 (M+ +1), and title compound B (3.01 g) in the form of a light-yellow oil, Rf (E)=0.35, MS(FAB) m/e 463 (M+ +1).
