The formation and interconversion of oxazines and dioxazines from the reaction of nitrosocarbonyl compounds with cyclopentadienes

The nitrosocarbonyl arenes or -alkanes, obtained by oxidation of hydroxamic acids, behave as dienophiles towards cyclopentadiene to give the bridged oxazines 1. In the case of pivalo-hydroxamic acid, however, the nitrosocarbonyl compound also reacts as a heterodiene, giving, as well as the oxazine 1d, the 1,4,2-dioxazine 2d; 1dis not converted into 2dunder the conditions of the oxidation reaction.Extended heating of compounds 1slowly converts them into 2and other more complex reaction products. This contrasts with the rapid and quantitative isomerization of the analogous azodicarbonyl adducts, and probably reflects the lesser crowding of the reacting NCOR group by an adjacent oxygen than by an adjacent NCOR group.1,4-Dimethyl-2,3-diphenylcyclopentadiene traps the nitroso compounds to form the labile oxazines 8, which rapidly isomerize to the dioxazines 9on brief heating or in polar solvents. In the case of the benzoyl compound 8athe isomerization to 9ais reversible in benzene at 80 °C, favouring the latter isomer by a factor of about 9:1.