PHOTOCHEMISTRY OF ACYLAZIDES .8. DO ACYLNITRENES REACT LIKE 1,3-DIPOLES

The formation of three- ore five-membered heterocyclic rings by the reaction of acylnitrenes with olefins depends on the electron density at the double bond. The generally expected formation of aziridines by a cheletropic reaction was observed by photolysis of aroylazides 1 in the presence of 2,5-dihydrofuran 2. But with enolethers 3 and 4 oxazolines were directly formed. This [3 + 2] cycloaddition is regiospecific. The cycloadditon is modestly stereoselective by steric hindrance within the cyclic enolether 3b. Very small de-values were found with chiral substituents in the acylazide 13. The azide decomposition was also achieved by photoinduced electron transfer. The same cycloaddition products as obtained by direct photolysis of the azides were obtained via radical anions of the acylazide. Using Michler's ketone as electron donor in the triplet state the formation of isocyanate which diminishes the yield of cycloadducts can be avoided.