A synthesis of optically active α-quaternary α-amino acids and esters by assembling three components, ketones, (R)-chloromethyl p-tolyl sulfoxide, and sodium azide, via sulfinyloxiranes

Treatment of lithium α-sulfinyl carbanion of chloromethyl p-tolyl sulfoxides with ketones at low temperature afforded adducts in almost quantitative yields, which were exposed to t-BuOK to give sulfinyloxiranes in high yields. The sulfinyloxirane was reacted with benzylamine to give α-amino aldehyde, which was oxidized with iodine in methanol to afford α-amino carboxylic ester in moderate yield. The sulfinyloxiranes were treated with sodium azide to afford α-azido aldehydes in good yields. Oxidation with NaClO₂ followed by catalytic hydrogenation of the azido group of the α-azido aldehydes gave α-quaternary α-amino acids in good overall yields. The oxidation of the azido aldehydes with iodine in methanol in the presence of KOH followed by the catalytic hydrogenation resulted in α-quaternary α-amino acid methyl esters in good yields. When these reactions were carried out starting from unsymmetrical ketones and optically pure (R)-chloromethyl p-tolyl sulfoxide, a new method for a synthesis of optically active α-quaternary α-amino acids and esters in good overall yields was realized.