Asymmetric construction of a quaternary carbon center by tandem [4 + 2]/[3 + 2] cycloaddition of a nitroalkene. The total synthesis of (-)-mesembrine

An efficient, total synthesis of the Sceletium alkaloid (-)-mesembrine is accomplished in seven steps and 19% yield from a functionalized nitroalkene (itself prepared in six steps and 34% yield from ethyl 3-bromopropionate). The construction of the octahydroindole framework of mesembrine features a tandem inter [4 + 2]/intra [3 + 2] cycloaddition of a 2,2-disubstituted 1-nitroalkene and a chiral vinyl ether derived from (1R,2S)-2-(1-methyl-1-phenylethyl)cyclohexanol as the central strategic element. The two stereogenic centers of the natural product, which include a benzylic, quaternary center, were established in 26/1 selectivity in the tandem process.