C-nitrosoformamides, a new class of transient dienophiles formed by oxidation of N-hydroxyureas
Oxidation of hydroxyurea with sodium or tetraethylammonium periodate in the presence of cyclopentadiene gave a cycloadduct (3a) formed, apparently, by capture of the transient dienophile, C-nitrosoformamide. N-Methyl-, N-phenyl-, N,N-dimethyl-, and N,N-diphenyl-C-nitrosoformamide were likewise trapped as their cycloadducts with cyclopentadiene. Cycloadducts of 2,3-dimethylbuta-1,3-diene, thebaine, ergosteryl acetate, and 9,10-dimethylanthracene (DMA) were prepared similarly. The cyclopentadiene adducts (3) dissociated at 80 °C in the presence of 2,3-dimethylbuta-1,3-diene to give the corresponding adducts (4) of the nitrosoformamides and dimethylbutadiene. Unexpectedly, the 1,4-adducts (4) were accompanied by substantial amounts of hydroxamic acids (5) arising from ‘ene’ reactions of the nitrosoformamides and dimethylbutadiene. The cyclopentadiene adducts of N,N-dimethyl- and N,N-diphenyl-C-nitrosoformamide, when heated alone, decomposed to give the corresponding carbamic anhydrides. The cycloadduct (15) of DMA and C-nitrosoformamide dissociated at 40 °C in the presence of thebaine (11) to give the thebaine adduct (12; R = H) and DMA.