A Flexible Asymmetric Approach to Methyl 5-Alkyltetramates and Its Application in the Synthesis of Cytotoxic Marine Natural Product Belamide A

By using a methyl tetramate derivative (R)- or (S)-9 as a novel chiral building block, a direct, flexible, and highly enantioselective approach to methyl (R)- or (S)-5-alkyltetramates (2) is disclosed. Among the synthesized methyl 5-alkyltetramates 2, methyl 5-methyltetramate (2a) is found in cytotoxic mirabimide E (4) and dysideapyrrolidone (5), and methyl 5-benzyltetramate (2g) is a substructure in the potent antineoplastic dolastatin 15 (3). On the basis of this method, the first asymmetric synthesis of the antimitotic tetrapeptide belamide A (7) has been achieved in seven steps from (S)-9, with an overall yield of 23.8%. Not only have the structure and absolute configuration of (+)-belamide A (7) been confirmed, but also the solvent used for recording the C-13 NMR spectrum, the C-13 NMR spectrum data correlation, and optical rotation data of natural belamide A (7) have been revised.