Efficient Asymmetric Synthesis of Pumiliotoxin Cvia Intramolecular [4+2] Cycloaddition

An efficient asymmetric synthesis in nine steps of natural (−)-pumiliotoxin C (1), a decahydroquinoline alkaloid found in the skin of Central American frog species, is presented. The enantiomerically pure starting material (S)-norvalinol (3), obtained from commercial (S)-norvaline, was cyclized in a one-pot procedure to the tosylated aziridine 5. Ring opening with propargylmagnesium bromide led to the acetylenic sulfonamide tosylamide 6, free of the allenic isomer. Compound 6 was methylated on the acetylenic C-atom, reduced, and deprotected with Na in liquid NH3 to give the (E)-configured unsaturated amine 8, which was condensed with crotonaldehyde to the imine 9 and N-acylated with isobutanoyl chloride to the key intermediate 2. Intramolecular Diels-Alder reaction furnished a diastereoisomeric mixture of N-protected octahydroquinolines 10. After catalytic hydrogenation and cleavage of the amide, natural 1 was obtained as the main product in 25% overall yield; 3.2% of its isomer 11 with the inverse configurations in position 4a, 5, and 8a was also isolated.