SYNTHESIS OF ALPHA-BENZYL GAMMA-LACTAM, ALPHA-BENZYL DELTA-LACTAM, AND ALPHA-BENZYLPROLINE DERIVATIVES AS CONFORMATIONALLY RESTRICTED ANALOGS OF PHENYLALANINAMIDE

The ready availability of N-(trifluoroacetyl)-alpha-allylphenylalaninamide (4) via a dehydration/hetero-Cope rearrangement/ammonolysis sequence starting with N-(trifluoroacetyl)phenylalanine allyl ester made it an attractive intermediate for elaboration into C-alpha to N- or C-alpha to N'bridged products as conformationally restricted phenylalaninamide analogues. Oxidative one-carbon degradation of the side-chain olefin followed by acid-catalyzed silane reduction afforded C-alpha to N'-bridged gamma-lactam. Hydroboration/oxidation of the side-chain olefin provided an intermediate that could be cyclized selectively either to a delta-lactam or a proline analogue depending on choice of dehydrating conditions. For preparation of a target dipeptide containing the alpha-substituted proline moiety, a preferred route involved N-deprotection of 4 and coupling to Boc-Asp(OBn)-OH to give a dipeptide intermediate, which similarly could be elaborated selectively to either the alpha-benzyl delta-lactam analogue or the alpha-benzylproline analogue.