Photocycloadditions of substituted oxazoles with isoquinoline-1,3,4-trione--chemo-, regio-, diastereoselectivities and transformation of the photocycloadducts.

Photoreactions of isoquinoline-1,3,4-triones and oxazoles with different substituents were found to give different chemo-, regio- and diastereoselectivities. The substituent at the C5 on the oxazole ring showed great influence on the chemoselectivity of the photoreaction as well as on the transformation of the photocycloadducts. The 2-methyl-5-methoxyoxazoles reacted with isoquinoline-1,3,4-triones rapidly and gave spirooxetanes with high regio- and diastereo-selectivity. Diastereoselectivity in the reaction of 2-phenyl-5-methoxyoxazoles with isoquinoline-1,3,4-triones was relevant to the substituent on the 4-position on the oxazole ring. Replacement of the 5-methoxy group with 5-methyl or 5-phenyl resulted in significant decrease on the reactivity of the oxazole as well as change on the diastereoselectivity in photocycloaddition with isoquinoline-1,3,4-triones. Acid-mediated transformations of the photocycloadduct spirooxetanes was found to give different type of products including β-hydroxy-α-aminocarbonyl compounds and spiroisoquinolineoxazolines under different reaction conditions. Substituents on the spirooxetanes as well as the type and amount of acid used in the reaction played important roles in determining the type and diastereoselectivity of the products in the transformations.