Steric Buttressing Changes Torquospecificity in Thermal Cyclo­butenone Rearrangements, Providing New Opportunities for 5H-Furanone Synthesis

Abstract

Thermally induced rearrangements of 4-hydroxycyclo­butenones are known to provide clean and reliable access to an array of useful carbocyclic and fused heterocyclic ring systems. Rarely, such reactions have been diverted to an alternative pathway leading to furanone formation. Herein, we show that these switches in the course of the rearrangement occur when a substrate bears a bulky substituent and are due to adverse steric buttressing as the transition state for electrocyclisation is approached. We also show how the reaction provides new opportunities for furanone synthesis and how bulky proton and halogen surrogates can be used to divert classical rearrangement pathways toward furanone formation. Additionally, we show that classical rearrangement pathways can be promoted by the simple expedient of alcohol protection.

Key words

cyclobutenones - rearrangement - thermolysis - flow chemistry - furanones - steric buttressing - torquoselectivity

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