Heterodiene syntheses. Part 21. 1-Acetyl-2-oxoindolin-3-ylideneacetophenones and ethoxyethyne : spirobicyclic intermediates in competition with [2 + 2]- and [4 + 2]-cycloadditions

Abstract

1-Acetyl-2-oxo-1H-indolin-3-ylideneacetophenones (1) react with ethoxyethyne (2) to give colourless pyrano[2,3-b]indoles (3), in accordance with a (1,4) cycloaddition, and coloured 3-propenylideneindolin-2-ones. The latter are formed by a (1,2) cycloaddition followed by electrocyclic ring opening of the spirocyclobutene intermediate to the unsaturated ketone (5) or by rearrangement of two spirobicyclic intermediates which undergo ring opening to give the unsaturated esters (6) and (7). Adducts (3), (6), and (7) are stereochemically correlated through a stable isomer (8) whose (E,E)-configuration was confirmed by X-ray analysis. The mechanism of formation of the various adducts is discussed.