Total syntheses of mitragynine, paynantheine and speciogynine via an enantioselective thiourea-catalysed Pictet-Spengler reaction

The pharmacologically interesting indole alkaloids (-)-mitragynine, (+)-paynantheine and (+)-speciogynine were synthesised in nine steps from 4-methoxytryptamine by a route featuring (i) an enantioselective thiourea-catalysed Pictet-Spengler reaction, providing the tetrahydro-beta-carboline ring and (ii) a Pd-catalysed Tsuji-Trost allylic alkylation, closing the D-ring.