Heterodiene syntheses. Part 20. 4-Arylidene-5-pyrazolones and ynamines : a [2 + 2]cycloaddition followed by electrocyclic ring opening, in competition with a [4 + 2]cycloaddition; the influence of the substituents on the intermediate

Abstract

4-Arylidene-5-pyrazolones (1) react with NN-diethylaminomethylacetylene to give a mixture of pyrano[2,3-c]pyrazoles (3), in accordance with a [4 + 2]cycloaddition, and 4-(3-aryl-1-diethylamino-2-methylpropenylidene)pyrazol-5-ones (5) which are formed by electrocyclic ring opening of the [2 + 2]cycloadducts (4). The configuration of the open-chain compounds (5) permits the deduction of that of the originally formed cyclobutene adducts. The reason for the choice between a [4 + 2] and a [2 + 2] cycloaddition can be rationalized in terms of the effect of both the substituents and the configuration of the substrate.