Divergent Enantioselective Total Synthesis of (−)-Ajmalicine, (+)-Mayumbine, and (−)-Roxburghine C

We report herein a divergent enantioselective total synthesis of (−)-ajmalicine, (+)-mayumbine, and (−)-roxburghine C. The synthesis employs Franzén's organocatalytic reaction between N-acetoacetyl tryptamine and (E)-5-hydroxypent-2-enal to generate a functionalized pentacyclic compound with high diastereo- and enantioselectivity. This intermediate serves as a versatile platform for accessing the three heteroyohimbine alkaloids. Notably, a diastereoselective intramolecular Pictet–Spengler reaction of methyl ketone and chemoselective reduction of β-amidoester to β-enaminoester were exploited for the synthesis of (−)-roxburghine C.