Formation and reactions of C-nitrosoformate esters, a new class of transient dienophiles

Oxidation of N-hydroxycarbamic esters, ROCONHOH, with tetraethylammonium or sodium periodate in the presence of conjugated dienes gave N-alkoxycarbonyl-3,6-dihydro-2H-1,2-oxazines, formed apparently by cycloaddition of transient C-nitrosoformate esters, ROCONO, with the dienes. Cleavage of various N-alkoxycarbonyl derivatives under mild conditions is exemplified. The cycloadduct (10a) of benzyl nitrosoformate and 9,10-dimethylanthracene decomposed in benzene at 80 °C in the presence of thebaine (3) to give the corresponding adduct (4a) of thebaine, together with 9,10-dimethylanthracene. Similarly, the adduct (6b) of 2,2,2-trichloroethyl nitrosoformate and cyclopentadiene, when heated with ergosteryl acetate, gave the corresponding adduct (8b) of the steroid. Benzyl and t-butyl azidoformates decomposed in dimethyl sulphoxide at 115–130 °C in the presence of thebaine to give the adducts (4a) and (4c) of benzyl and t-butyl nitrosoformate and the alkaloid. The other, major products were the sulphoximides (20a) and (20c). Alkoxycarbonylnitrenes, therefore, attack dimethyl sulphoxide either on sulphur, to give sulphoximides, or on oxygen, to give nitrosoformates. Benzyl nitrosoformate, generated thermally from either the adduct (10a) or (6a), reacted with triphenylphosphine to give, apparently, benzyloxycarbonylnitrene which attacked the solvent, benzene, to form N-benzyloxycarbonylazepine (24a). The adduct (6c) of t-butyl nitrosoformate and cyclopentadiene behaved likewise to give the azepine (24c). The reaction of (6c) with triphenylphosphine in benzene or dichloromethane gave a small quantity of 5,5-dimethyloxazolidin-2-one (26), a known cyclisation product of t-butyloxy-carbonylnitrene.