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 CO 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.
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