Aziridine−Allylsilane-Mediated Total Synthesis of (−)-Yohimbane

Abstract
A total asymmetric synthesis of (−)-yohimbane and ent-alloyohimbane is reported. The synthesis utilizes a novel aziridine−allylsilane cyclization reaction as a key step in the synthesis. Treatment of optically pure aziridine−allylsilane 16 with BF3·OEt2 provided a mixture of aminomethyl substituted carbocycles trans-20a and cis-20b in excellent yield and modest diastereoselectivity (trans/cis 3:1). Alkylation of the tosylamide followed by oxidation of the olefin in 20 provided the lactam 38, which was converted to (−)-yohimbane and ent-alloyohimbane by a Bischler−Napieralski reaction. The synthesis provided (−)-yohimbane in eight steps and 24% overall yield (from 16).

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