Sequential Catalytic Asymmetric Heck-Iminium Ion Cyclization: Enantioselective Total Synthesis of the Strychnos Alkaloid Minfiensine

摘要

A concise catalytic asymmetric chemical synthesis entry to alkaloids containing the 1,2,3,4-tetrahydro-9a,4a-(imino-ethano)-9H-carbazole (1) moiety is reported. The central step in this sequence is a sequential catalytic asymmetric Heck-N-acyliminium ion cyclization of dienyl carbamate triflate 10 to deliver enantiopure 3,4-dihydro-9a,4a-(iminoethano)-9Hcarbazole (12). This intermediate and related intermediates containing a 2,3 double bond should be versatile precursors for constructing a variety of pentacyclic indole alkaloids containing the (hydroiminoethano)-carbazole fragment. The enantioselective total synthesis of (+)-minfiensine (4) reported herein provides the first verification of this total synthesis strategy.