Preparation and use of chloromethyl (−)‐menthyl ether in the synthesis of optically pure α‐branched α‐amino nitriles

The synthesis of optically pure chloromethyl (−)‐menthyl ether (2b) and its use in the synthesis of di‐(−)‐menthyl acetal (−)‐menthyl (+)‐menthyl acetals are described. The diastereomeric mixed acetals 7a, b and 8a, b are easily obtainable from the nitrones 5 and 6, KCN and 2b. The mixtures of diastereomers are separated into the pure components 7a, 7b and 8a, 8b by simple silica gel column chromatography. Hydrolysis of these products (H₂O₂/Na₂CO₃, ultrasound) followed by NO cleavage affords the heterocyclic α‐methyl‐α‐amino amides 11a, 11b and 12a, 12b. These are subsequently hydrolyzed to give the corresponding α‐methyl‐α‐amino acids with S and R configuration, respectively.