Thermal ene reactions of alloxan and 1,3-dimethylalloxan

Abstract

Both alloxan 1 and 1,3-dimethylalloxan 2 are conveniently prepared from their hydrates, respectively 3 and 4, by either chemical removal of the water (acetic anhydride in boiling acetic acid) or by azeotropic drying using chlorobenzene as solvent. The central CO groups of these triones are extremely electrophilic and ene addition occurs at these sites with a wide range of alkenes and with terminal alkynes in aprotic solvents at moderate temperatures (100–130°C). The ene adducts are 5-allyl-5-hydroxybarbituric acids or their N,N-dimethyl analogues. Stereoselectivity, when relevant, appears largely to be dictated by steric factors. Isoprene, however, reacted by Diels-Alder rather than by ene addition; the adducts are the expected regio-isomers 24.