Experimental and Theoretical Investigation of the Synchronicity of Ambident Silyloxypyrone-Based (5 + 2) Cycloadditions

The reaction pathway of silyloxypyrone-based (5 + 2) cycloadditions was determined to be extremely dependent on the nature of the dipolarophile. Neutral alkenes were the least reactive, whereas both electron-deficient and electron-rich dipolarophiles were more reactive, thus providing evidence for ambident oxidopyrylium intermediates. Qualitative rate studies, Hammett linear free energy relationships, and theoretical calculations combined to provide evidence for a spectrum of reactivity that passes through the borderlands of concerted and stepwise.