Photocycloaddition and ortho-hydrogen abstraction reactions of methyl arylglyoxylates:: structure dependent reactivities

In photocycloaddition reactions between methyl arylglyoxylates and certain cyclo-1,3-dienes, the diastereochemical outcome of the photoproducts is shown to depend on the steric demand of the aryl group in the glyoxylate. Exclusive endo-aryloxetanes were produced with bulky arylglyoxylates while no significant stereoselectivity was observed with glyoxylates containing less demanding aryl groups. This observation is rationalized by considering the stability of possible conformers of the intermediate 1,4-biradical at the instance of intersystem crossing. When these ortho-substituted phenylglyoxylates were irradiated in benzene, different reaction patterns were observed with different substituents on the aryl rings. The rates of ortho-hydrogen abstractions vary significantly among the individual compounds. Photoproducts thus resulting are dependent on the substrate structures.