Synthesis of a cyclic isostere of α-methyl homoserine by a stereoselective acylation–alkylation sequence of a chiral γ-lactam

Starting from a chiral 4-hydroxymethyl pyrrolidin-2-one, an isostere of α-methyl homoserine tethered on a γ-lactam ring was prepared exploiting a stereoselective acylation–methylation sequence, followed by Curtius rearrangement, and structural assignment was confirmed by n.O.e. experiments. By reverting the sequence, the 3-carboxy-3-methyl derivative having the opposite configuration at C-3 was obtained with total stereoselection, but Curtius rearrangement invariably afforded only inseparable mixtures of decomposition products.