A new asymmetric route to bridged indole alkaloids: formal syntheses of (–)-suaveoline, (–)-raumacline and (–)-Nb-methylraumacline

Abstract

When the homologated nitrile 11 derived from _L-tryptophan undergoes a modified Pictet–Spengler reaction with methyl propynoate, the resulting cis-tetrahydro-β-carboline 12a is ideally functionalised for cyclisation to the bridged ketonitrile 14; simple functional group modification via the nitrile 15(structure confirmed by X-ray crystallography) allow convergence with the tetracyclic α,β-unsaturated aldehyde 10, which is an advanced intermediate for the synthesis of a range of bridged indole alkaloids.