Abstract: Ethylmethylmalonic acid reacted with l-menthol to form the diastereo-meric l-menthyl hydrogen ethylmethylmalonates (Ⅰ).From(Ⅰ)the diaster-eomeric l-menthyl ethylmethylmalonamides(Ⅲ) were obtained in nearly quantitative yield. To saponify the amides(Ⅲ) with alkali, the partially optically active ethylmethylmalonanic acid(Ⅳ) were produced in 70% yield. (Ⅲ) were degraded to partially optically active a-amino-methylethylacetic acid (Ⅴ) (V,8.8%e.e S - (+) -isomer)by the Hoffmann degradaton.In DMFthe monobrucine salts of ethylmethylmalonic acid reacted with iodoethane and diazomethane to produce the optically inactive mono-ethyl ester (Ⅵ) and monomethyl ester (Ⅴ) respectively.