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2-Phenyl-3-aryl-6-methoxycarbonyl-3,6-dihydro-1,2-oxazines (1) and, generally in lower yields, 2-phenyl-6-aryl-3-methoxycarbonyl-dihydro-oxazines (2) are formed by reaction of nitrosobenzene with 1-methoxycarbonyl-4-arylbutadienes. Chromatography on basic alumina yields substituted pyrroles 3 from 1, and substituted α-pyrones 4 from 2. A general reaction scheme explains the complicated influence of substituents on isomer ratio and rates; the formation of isomer 2 with negative substituents is similar to an “inverse” diene synthesis.

The kinetics of Diels-Alder reactions with p -substituted nitroso benzenes and p -substituted 1-phenyl butadienes gives Hammett-reaction constants, clearly depending on the nature of the substituents. From this dependence a reaction scheme for diene synthesis with nitroso compounds is derived, describing the transition state of the reaction as an intermediate (quantitatively: as linear combination) between unpolar and zwitterionic structures. In our reactions it is an intermediate structure (depending on the substituents) between the transition states of a cyclo addition (concerted reaction) and an electrophilic addition.