Aziridine-allylsilane-mediated total synthesis of (-)-yohimbane

A total asymmetric synthesis of(-)-yohimbane and ent-alloyohimbane is repor ted. The synthesis utilizes a novel aziridine-allylsilane cyclization react ion as a key step in the synthesis. Treatment of optically pure aziridine-a llylsilane 16 with BF3. OEt2 provided a mixture of aminomethyl substituted carbocycles trans-20a and cis-20b in excellent yield and modest diastereose lectivity (trans/cis 3:1). Alkylation of the tosylamide followed by oxidati on of the olefin in 20 provided the lactam 38, which was converted to (-)-y ohimbane and ent-alloyohimbane by a Bischler-Napieralski reaction. The synt hesis provided (-)-yohimbane in eight steps and 24% overall yield (from 16) .