Highly efficient and concise synthesis of both antipodes of SB204900, clausenamide, neoclausenamide, homoclausenamide and ζ-clausenamide. Implication of biosynthetic pathways of clausena alkaloids

The synthesis of both antipodes of N-methyl-N-[(Z)-styryl]-3-phenyloxirane-2-carboxamide (SB204900), clausenamide, neoclausenamide, homoclausenamide and ζ-clausenamide have been accomplished using (2S,3R)- and (2R,3S)-3-phenyloxirane-2-carboxamides as the starting materials, and SB204900 was found to be a common precursor to other N-heterocyclic clausena alkaloids. Mediated by Bronsted acids under different conditions, for example, SB204900 underwent efficient and diverse alkene-epoxide cyclization, enamide-epoxide cyclization and arene-epoxide cyclization reactions to produce the five-membered N-heterocyclic neoclausenamide, its 6-epimer, the six-membered N-heterocyclic homoclausenamide and the eight-membered N-heterocyclic ζ-clausenamide, respectively, in good to excellent yields. Regiospecific oxidation of neoclausenamide and its 6-epimer afforded neoclausenamidone. Enolization of neoclausenamidone in the presence of LiOH and the subsequent protonation under kinetic conditions at −78 °C led to the epimerization of neoclausenamidone into clausenamidone. Reduction of clausenamidone using NaBH4 furnished clausenamide in high yield.