In agreement with theoretical predictions, the "normal' preference for formation of 5-substituted isoxazolines in 1,3-dipolar ~~cloadditions of nitrones to substituted alkenes changes to a preference for the 4substituted adduct as the ionization potential of the nitrone decreases, or that of the alkene increases.The exclusive formation of 5-substituted isoxazolidine and isoxazoline adducts in the reactions of both electron-rich and electron-deficient alkenes or alkynes with nitrones was long an unexplained phenomenon.'In 1973, we proposed that nitrones would give "reversed" (4-substituted) adducts with very electrondeficient alkenes and alkynes,3 and this prediction was soon proven experimentally.'We now wish to report that the extent of "reversal" of regioselectivity in cycloadditions of nitrones regularly increases as the nitrone ionization potential (IP)
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Houk et al. (1977) studied this question.