1,3-dipolar cycloaddition nitroso dienophiles

A unified picture of the mechanism of this cycloaddition for various structural types of nitroso dienophiles is not available. Depending upon the electronic and steric nature of the particular nitroso compound, the reaction apparently may vary from a concerted pericyclic process to one involving discrete dipolar intermediates. The net result of this mechanistic diversity is a few anomalous regiochemi-cal results in the cycloaddition. 

Arylnitroso compounds of various types react with both electron-rich and electron-deficient dienes under mild conditions (usually 0-l(X)°C) to produce dihydrooxazines. As mentioned in the preceding section, Kresze s group has looked in considerable detail at orientational preferences and kinetics of [4 + 2] cycloadditions with various phenylnitroso compounds and substituted 1,3-dienes. Table 3-1 contains some of the results of cycloadditions of aromatic nitroso dienophiles with representative unsymmetrical 1- and 2-substituted dienes." Scheme 3-1 shows the subtle regiochemical trends that can be observed in a series of these reactions. These results were rationalized based on consideration of putative dipolar and nonpolar transition states and their relative stabilization by aromatic electron-donating and -withdrawing substituent groups. 

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