N-monoalkylation of alpha-amino acid esters under solid-liquid PTC conditions

N-(2-Nitrophenylsulfonyl)- (o-NBS-AA-OMe, 4) and N- (4-Nitrophenylsulfonyl)-alpha-amino acid methyl esters (p-NBS-AA-OMe, 5) were N-alkylated with a variety of alkyl halides 6 under solid-liquid phase-transfer catalysis (SL-PTC) conditions, affording the alkylated products o-NBS-N-R-2-AA-OMe 7 and p-NBS-N-R-2-AA-OMe 8 in excellent yields without any detectable racemization.