Stereoselective Reactions of N-(9-Phenylfluoren-9-yl)-4-oxoproline Enolates. An Expedient Route for the Preparation of Conformationally Restricted Amino Acid Analogues.

Methodology for the stereoselective preparation of 3-alkylprolines from N-(9-phenylfluoren-9-yl)-4-oxoproline is presented. The enolate of a N-(9-phenylfluoren-9-yl)-4-oxoproline ester was shown to give stereoselective aldol condensations with aromatic and aliphatic aldehydes. The enolate was preferentially approached by the electrophile through the Re face, and high threo selectivity was also observed and provided a route to trans 3-substituted prolines. The reactions are kinetically controlled except for the case of electron-rich or sterically hindered aromatic aldehydes. The ease of the equilibration between the erythro- and threo-aldolates is dictated by the electronic nature of the group at C-4 in substituted benzaldehydes, and the steric compression of the aldehyde group increases the rate of erythro/threo equilibration. Very high stereoselection was also observed in the reductions of the keto group in 3-substituted 4-oxoproline esters. Alkylation or Michael additions of the same enolate were poorly stereoselective, probably due to equilibration of the initially formed products. Kinetic protonation of enolates of 3-alkyl-4-oxoprolines proceeded with high diastereoselection to provide the corresponding cis-3-alkylprolines.