Redox N-alkylation of tosyl protected amino acid and peptide esters

Abstract

Condensation of Nx-tosylated amino acid and peptide esters with alcohols (MeOH, EtOH, iPrOH) in the presence of the triphenylphosphine-diethyl azodi-carboxylate adduct produced excellent yields of the corresponding Nx-alkylated derivatives. Optically pure Nx-alkylamino acids can only be obtained from methyl and benzyl esters using iodotrimethylsilane and hydrogenolysis, respectively, for the carboxy-deprotection and sodium in liquid ammonia for the amino-deprotection. That carboxy-deprotection of methyl esters by saponification is accompanied by racemization was established by high-performance liquid chromatography studies. Alkylation rates and yields for the reactions examined were found to depend only on the relative positions of the tosylamino and the carboxy functions. Removal of the carboxy group from the α-position resulted in longer reaction times and significant decreases in the yield of the desired N-alkylated derivatives. Accordingly, tosyl-protected lysyl and ornithyl side-chains of fully protected amino acids and peptides were selectively N-methylated in moderate yields in the presence of other amino functions bearing the terf-butoxycarbonyl (Boc) group which is commonly used for protection in peptide synthesis.