Concerning selectivity in the oxidation of peptides by dioxiranes. Further insight into the effect of carbamate protetcting groups

With use of methyl(trifluoromethyl)dioxirane (TFDO), the oxidn. of some tripeptide esters protected at the N-terminus with carbamate or amide groups could be achieved efficiently under mild conditions with no loss of configuration at the chiral centers. Expanding on preliminary investigations, it is found that, while peptides protected with amide groups (PG = Ac, Tfa, Piv) undergo exclusive hydroxylation at the side chain, their analogs bearing a carbamate group [PG = Cbz, Moc, Boc, TcBoc; Moc = COOMe; TcBoc = COOC(Me)2CCl3] give competitive and/or concurrent hydroxylation at the terminal NH moiety. Valuable nitro derivs. are also formed as a result of oxidative deprotection of the carbamate group with excess dioxirane. A rationale is proposed to explain the dependence of the selectivity upon the nature of the protecting group.