Total Synthesis of Dehydroaltenuene A. Revision of the Structure and Total Synthesis of Dihydroaltenuene B

Total synthesis of alternaria toxins starting from previously synthesized altenuene (3) and isoaltenuene (4) is described. Dihydroaltenuene B (9) was prepared by hydrogenation of 3, and the non-natural epimer 3-epi-dihydroaltenuene A was obtained analogously from 4. Inspection of the spectroscopic data for 9 revealed that the originally proposed structure was in error. A revised structure (11), unambiguously proven by total synthesis, is reported herein. Oxidation of 4 with oxygen in the presence of palladium(II) acetate as catalyst led to the formation of dehydroaltenuene A (8), while oxidation of 3 using identical conditions yielded ent-dehydroaltenuene B (ent-9). Oxidation of 4 with manganese(IV) oxide furnished dehydroaltenusin (12), although only impure material was obtained in low yield.