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-a]quinolizin-4-ones is described in two steps via formal [3+3] cycloaddition reaction of α-sulfonyl tryptaminylacetamide with various α,β-unsaturated esters and the regioselective reduction of the resulting glutarimides with sodium borohydride then sequent further dehydrated cyclization in the presence of boron trifluoride etherate. The useful building block is applied to synthesize deplancheine () and yohimbane ().