Divergent Asymmetric Total Synthesis of (−)‐Voacafricines A and B

A divergent asymmetric total synthesis of voacafricines A and B, hexacyclic monoterpene indole alkaloids, has been accomplished featuring the following key steps: a)a catalyst-controlled asymmetric Pictet– Spengler reaction of 6-methoxytryptamine with a chiral α-ketoester affording a 1,1-disubstituted tetrahydro-β- carboline in excellent yield and diastereoselectivity; b) oxidative cleavage of a 3,5-disubstituted cyclopentene furnishing a dialdehyde intermediate, which was effec- tively differentiated through spontaneous cyclization with the neighboring hydroxy and secondary amine functions; c) intramolecular nucleophilic addition of a tertiary amino nitrogen atom to the in situ generated oxonium species generating stereoselectively an unpre- cedented 8-alkyl octahydro-2H-5,8-methanofuro[2,3- b]azepin-8-ium motif bearing five contiguous stereo- centers. The synthesis confirmed the absolute configu- ration of these two natural products.