An easy ABD→ABCD strategy to indolo[2,3-a]quinolizin-4-one.: Synthesis of deplancheine and yohimbane

The efficient synthesis of indolo[2,3-alpha]quinolizin-4-ones 2 is described in two steps via formal [3 + 3] cycloaddition reaction of alpha-sulfonyl tryptaminylacetamide 4 with various alpha, beta-unsaturated esters 5 and the regioselective reduction of the resulting glutarimides 3 with sodium borohydride then sequent further dehydrated cyclization in the presence of boron trifluoride etherate. The useful building block is applied to synthesize deplancheine (1a) and yohimbane (1b). (C) 2004 Elsevier Ltd. All rights reserved.