Organocatalytic Enantioselective Pictet–Spengler Reaction of α‐Ketoesters: Development and Application to the Total Synthesis of (+)‐Alstratine A

Abstract

AbstractWe report herein an asymmetric Pictet–Spengler reaction of α‐ketoesters. In the presence of a catalytic amount of simple alanine‐derived squaramide and p‐nitrobenzoic acid, reaction of tryptamines with methyl 2‐oxoalkanoates afforded the corresponding 1‐alkyl‐1‐methoxycarbonyl tetrahydro‐β‐carbolines (THBCs) in high yields and ee values. A primary kinetic isotope effect (KIE=4.5) using C2‐deteurium‐labelled tryptamine indicates that rearomatization through deprotonation of the pentahydro‐β‐carbolinium ion could be the rate‐ and enantioselectivity‐determining step. A concise enantioselective total synthesis of (+)‐alstratine A, a hexacyclic cagelike monoterpene indole alkaloid, featuring this reaction as a key step, was subsequently accomplished. Remeasurement of the [a]D value of the natural product indicates that natural alstratine A is dextrorotatory rather than levorotatory as it was initially reported in the isolation paper.