Transition-State Mimetics for HIV Protease Inhibitors:  Stereocontrolled Synthesis of Hydroxyethylene and Hydroxyethylamine Isosteres by Ester-Derived Titanium Enolate Syn and Anti-Aldol Reactions

Stereocontrolled syntheses of hydroxyethylene dipeptide isostere and aminoalkyl epoxides for hydroxyethylamine isosteres are described. The stereochemistry of both stereogenic centers of the aminoalkyl epoxides 10 and 15 as well as the gamma-lactone 17 was assembled by our recently developed highly selective ester-derived titanium enolate aldol reactions. The Ti-enolate of 6 reacted with (benzyloxy)acetaldehyde and cinnamaldehyde to provide the syn-aldol product 7 and anti-aldol product 12, respectively. Removal of the chiral template followed by Curtius rearrangement of the resulting acid provided the desired amine functionality. The present syntheses represent practical and enantioselective entry to a range of other dipeptide isosteres, which are not limited to amino acid derived substituents.