DIMETHYLDIOXIRANE OXIDATION OF 3-ARYLIDENECHROMANONES

Oxidation of the (E) and (Z) isomers of the 3-arylidenechromanones 5 by isolated dimethyldioxirane (as acetone solution) at ambient temperature resulted in the formation of the spiro epoxides trans- and cis-6, respectively, in moderate yields (23-51%), together with considerable amounts (16-40%) of the 3-aroylchromones 7 from both (E)- and (Z)-5 isomers. Minor products, namely 3-(alpha-hydroxy-4-methoxybenzyl)-4H-1-benzopyran-4-one (8b), 1a,7a-dihydro-7a-(alpha-hydroxy-4-methylbenzyl)-7H-oxireno[b] [1]benzopyran-7-one (9d), and the 1a,7a-dihydro-7a-aroyl-7H-oxireno[b][1]benzopyran-4-ones 10a-c have also been isolated and characterized. Presumably, the formation of 3-aroylchromones 7 can be attributed to competitive allylic oxidation with dimethyldioxirane to afford first the 3-(alpha-hydroxyarylmethyl)-chromones 8, whose secondary alcohol functionality is subsequently oxidized by dimethyldioxirane to give the 3-aroylchromones 7. Further oxidation of the latter by dioxirane affords the epoxides 10, as established by a control experiment. Alternatively, first oxidation of 8 to yield epoxide 9 and subsequent oxidation of the alcohol functionality to afford the overoxidized product 10 cannot be disposed of on grounds of our product data. Nonetheless, despite the moderate yields of the desirable chromanone epoxides 6, the stereoselective oxidation directly of the chromanones by dimethyldioxirane offers a convenient preparation. Relative configuration and stereochemistry of compounds 6-10 were elucidated by NMR spectroscopy and AM1 calculations.