Diels–Alder reactivity of pyrano[3,4-b]indol-3-ones. Part 2. Steric and electronic effects in the addition to alkynes

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Abstract
The pyrano[3,4-b]indol-3-ones (1a), prepared from ethyl 2-acetoxymethylindol-3-ylacetate (2), and (1b)–(1h), prepared from indol-3-ylacetic acid, undergo Diels–Alder reaction with dimethyl acetylenedicarboxylate to give, with concomitant loss of carbon dioxide, carbazole 2,3-diesters (7). The reactions of (1) with the unsymmetrical acetylene, ethyl propiolate, give mixtures of the carbazole 3-ester (8) and carbazole 2-ester (9), with the former predominating. The regioselectivity of the Diels–Alder reaction increases with increasing steric bulk at the 1-position of the dienes (1), although in the case of the N-ethoxycarbonyl derivative (10) regioselectivity is reversed. Reaction of the pyrone (1b) with a variety of unsymmetrical acetylenes gives mixtures of carbazoles (13) and (14), the regioselectivity of the Diels–Alder reaction being dependent on the acetylene substituents, and in some cases being regiospecific.

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