Total synthesis of stevastelins: Structure confirmation of stevastelins B and B3, and structure revision of stevastelin C3

The total syntheses of stevastelins B, B3, C3, and the 5-deoxy derivative of stevastelin C3, novel cyclic depsipeptides starting from L-quebrachitol, and amino acids are described. Stereoselective introduction of two methyl groups into L-quebrachitol, followed by regioselective cleavage of the cyclohexane ring by way of the Baeyer-Villiger reaction effectively afforded the fatty acid moiety of stevastelins. Introduction of the peptide and subsequent macrolactamization gave stevastelin B. Stevastelins C3 and B3 were also synthesized by a similar way. The direct comparison of synthetic stevastelins with natural compounds revealed that the synthetic stevastelins B and B3 are identical to the natural products, confirming the proposed structures. However, the synthetic stevastelin C3 was found to not be identical with the natural product. To elucidate the structure of stevastelin C3, degradation of the natural product was carried out to show the possibility that the natural product could be a 5-deoxy derivative of the proposed structure. Thus, the 5-deoxy derivative of the fatty acid moiety was prepared and transformed into a macrocycle. The synthetic 5-deoxy compound was fully identical to natural stevastelin C3. Based on these studies, it was shown that the structure of stevastelin C3 should be revised.