REGIOSELECTIVITY OF CYCLOADDITIONS OF NITRILE OXIDES AND NITRONES TO 4-METHYLENETETRAHYDROTHIOPYRANE

The cycloaddition of nitrile oxides and nitrones to 4-methylene-tetrah ydrothiopyrane proceeds regioselectively under the formation of spiro- substituted isoxazole derivatives 4 and 9. Semiempirical calculations (AM1) were used to analyze the electronic structure of reactants, ener gies of products, and activation barriers leading to these products in order to rationalize this exclusive regioselectivity. It was shown th at the main factor responsible for the high stereoselectivity of this reaction is not frontier orbital control, but mainly electrostatic and steric interactions. The spiro compounds 4 were cleaved by hydrogenol ysis to gamma-amino-alcohols 11, which were recyclized to spiro-oxazin es 12 and 13.4 and 9 as well as 12 and 13 are derivatives of novel het erocylic systems.