POLYLITHIATED BETA-PEPTIDES - LIKE-SELECTIVE C-TERMINAL ALKYLATION OFBOC-BETA-HVAL-BETA-HALA-BETA-HLEU-OME

A series of N-Boc-protected beta-tripeptide derivatives with or withou t N-methyl groups and with free or Me-ester-protected C terminus has b een prepared (8-14, 16, 17). As with alpha-peptides (--> A), the beta- peptide derivatives can be polylithiated (--> B, C). No epimerization of stereogenic centers and no beta elimination (exception 17 --> 24) i s observed upon treatment with bases as strong as tBuLi. The C termina l ester Li-enolate moiety of tetralithio beta-tripeptides (cf. C) can be selectively alkylated with methyl, benzyl and allyl halides, and wi th tert-butyl bromoacetate in yields ranging from 35-80% (8 --> 18, 14 --> 20-23).