Synthesis and stereochemical studies on the reductions of some pyrrole derivatives

Abstract

Total syntheses of the unique pyrrole derivatives (2a–d) and the steroid analogue (1) are reported. Reductions of the above imines with sodium borohydride or by a catalytic procedure have been found to be stereospecific, yielding only the trans-amine in each case. Metal–ammonia reductions of the benzoxe-pino-(2a) and benzocyclohepteno-pyrrole (2d) provided a 2:1 mixture of the corresponding trans- and the cis-amine respectively. Reductions of the indole derivative (2e) with lithium aluminium hydride, sodium in liquid ammonia, or by a catalytic method furnished in each case the cis-amine (22b). The rate of reduction of (2e) with sodium borohydride was much slower, and under special conditions it afforded a 3:1 mixture of the cis-(22b) and the trans-amtne (22a) respectively. The reasons for the completely different stereochemical results observed in the reductions of the imines (2a–d) and (2e) have been briefly discussed.