Total Synthesis of (−)-Hennoxazole A

An enantioselective, convergent total synthesis of the antiviral marine natural product (−)-hennoxazole A is completed in 14 steps (longest linear sequence) from commercially available 4-methyloxazole-2-carboxylic acid. Synthesis of the C₁−C₁₅ pyran/bisoxazole fragment takes advantage of an aldol-like coupling between a dimethyl acetal and an N-acetylthiazolidinethione for the direct, stereoselective installation of the C₈-methoxy-bearing stereocenter. A one-pot acetoacetate acylation/decarboxylation/cyclodehydration of another elaborate thiazolidinethione allows for rapid assembly of the pyran-based ring system. Synthesis of the C₁₅−C₂₅ skipped triene side chain fragment makes use of a [2,3]-Wittig−Still rearrangement for efficient installation of the trisubstituted Z-double bond. Key late-stage coupling of the two fragments is effected by deprotonation of the methyl group on the bisoxazole system using lithium diethylamide, followed by alkylation with an allylic bromide side chain segment to form the C₁₅−C₁₆ bond.