Regioselective enolization and alkylation of 4-oxo-N-(9-phenylfluoren-9-yl)proline: Synthesis of enantiopure proline-valine and hydroxyproline-valine chimeras

The regioselective enolization of 4-oxo-N-(9-phenylfluoren-9-yl)proline benzyl ester (5) followed by alkylation with different alkyl halides has been used to synthesize a variety of beta-alkylproline derivatives. In particular, enolization of 5 with 400 mol % of KN(SiMe(3))(2) and alkylation with iodomethane provided 3,3-dimethyl-4-oxo-N-(9-phenylfluoren-9-yl)proline benzyl ester (7a) in excellent yield. Subsequent hydride reduction of ketone 7a and protecting group exchange by hydrogenation in the presence of di-tert-butyl dicarbonate provided enantiopure (2S,4R)- and (2S,4S)-3,3-dimethyl-4-hydroxy-N-(BOC)prolines. 2. Hydroxyproline-valine chimeras (2S,4R)- and (2S,4S)-2 are each synthesized from hydroxyproline in six steps and 27% respective overall yield. Deoxygenation of 3,3-dimethyl-4-hydroxy-N-(9-phenylfluoren-9-yl)proline benzyl esters 9 via their conversion to xanthates 10 followed by tributylstannane-mediated reduction provided 3,3-dimethyl-N-(9-phenylfluoren-9-yl)proline benzyl ester (11) in excellent yield. Hydrogenation of 11 with Pearlman's catalyst in the presence of di-tert-butyl dicarbonate then furnished (2S)-3,3-dimethyl-N-(BOC)proline (1) in the last step of an eight-step synthesis (41% overall yield) from hydroxyproline. Both proline-valine and hydroxyproline-valine chimeras 1 and 2 were designed to serve as tools for studying the conformational requirements of biologically active peptides.