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

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Abstract
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; 1d is not converted into 2d under the conditions of the oxidation reaction.Extended heating of compounds 1 slowly converts them into 2 and 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 9 on brief heating or in polar solvents. In the case of the benzoyl compound 8a the isomerization to 9a is reversible in benzene at 80 °C, favouring the latter isomer by a factor of about 9:1.

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