Intramolecular 1 + 2 and 3 + 2 Cycloaddition Reactions of Cyclopropenone Ketals

摘要

The first intramolecular thermal reactions of cyclopropenone ketals are reported enlisting substrates tethered to an electron-deficient olefin bearing a single electron-withdrawing substituent. Whereas the intermolecular variants of the reactions provide only the products of an endo selective [1 + 2] cycloaddition or a carbonyl addition reaction of a thermally generated π-delocalized singlet vinylcarbene, the intramolecular variants provide either [1 + 2] or [3 + 2] cycloadducts in reactions that depend on the reaction conditions, the alkene activating substituent, and the nature of the tethering. In addition to providing key mechanistic insights into the thermal [3 + 2] cycloaddition reaction for such substrates, they were also found to proceed under conditions that reflect the ease and regioselectivity of the cyclopropenone ketal cleavage for π-delocalized singlet vinylcarbene generation. The most effective combination of structural features that impact the reactivity was observed with substrates bearing an aldehyde or ketone substituted electron-deficient olefin and incorporating an aryl cyclopropenone ketal substituent built into the linking tether. Simply warming a solution of such substrates in toluene at 80–100 °C directly provided the [3 + 2] cycloadducts in excellent yield (60–88%) under mild thermal reaction conditions.