Regioselective Grignard reagent additions to 3-methoxy-maleimides and subsequent diastereoselective reductive dehydroxylation of the resulting N,O-acetals were studied. On the basis of these studies, a flexible and highly regio- and diastereoselective approach to methyl 5-alkyltetramate derivatives was disclosed. The method is the first direct and flexible asymmetric cationic synthon-based approach, and allows for the synthesis of various methyl (5S)-5-alkyltetramate derivatives that are otherwise inaccessible by the commonly used methods based on alpha-amino acids. ; NSFC [20572088, 20602028, 20832005]; Fujian Province