Ene reactions of indane-1,2,3-trione (a super-enophile) and related vicinal tricarbonyl systems

Abstract

Indane-1,2,3-trione 1 is conveniently prepared in quantitative yield by the azeotropic drying of ninhydrin 2 using chlorobenzene as solvent. The central CO group of the trione is extremely electrophilic and ene addition occurs at this site with a wide range of alkenes and with terminal alkynes in aprotic solvents at moderate temperatures (70–130 °C). The reactivity of trione 1 is somewhat attenuated by the solvent, and the ene additions are consistently faster in chloroform than in tetrahydrofuran. Stereoselectivity, when relevant, appears largely to be dictated by steric factors. Regiochemical control can be exercised if the ene contains two reaction sites. Isoprene and 2,4-dimethylpenta-1,3-diene, however, react by Diels–Alder rather than by ene addition; the adducts are the expected regioisomers 18 and 20, respectively. Attempts to catalyse the ene reactions with Lewis acids were unsuccessful. The analogous ene reactions of 4,4,5,5-tetramethyl-cyclo-pentane-1,2,3-trione 44, 4,4,6,6-tetramethylcyclohexane-1,2,3-trione 45, and 2,2-dimethyl-1,3-dioxane-4,5,6-trione (‘oxo-Meldrum's acid’)46 were also briefly investigated.