Preparation of Alkyl-Substituted Indoles in the Benzene Portion. Part 12. Enantiospecific Synthesis of Hapalindole O.

Abstract

An important chiral ketone derivative, (3S, 4R)-3-methyl-4-pivaloyloxy-3-vinylcyclohexan-1-one (16) was prepared from (R)-(-)-carvone (10) using a stereo-controlled conjugate addition of the vinyl group to (R)-3-methyl-6-(1-methylethylidene)-4-pivaloyloxy-2-cyclohexen-1-one (13). The first enantiospecific total synthesis of a terrestrial blue-green alga constituent, hapalindole O (1) was accomplished by condensation of this ketone 16 with α, α-dimethyl-1-(p-toluenesulfonyl)-1H-indole-4-methanol (6) to construct the fundamental carbon framework of the hapalindole 21, followed by introduction of the nitrogen function, stereoselective reduction of the tetra-substituted double bond with lithium aluminum hydride, and subsequent isothiocyanate formation.