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 Franzen'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 beta-amidoester to beta-enaminoester were exploited for the synthesis of (-)-roxburghine C.