Thermal Rearrangement of Alkynyl Three-Membered Rings. Evidence for an Oxacycloheptatriene Intermediate

Article abstract of DOI:10.1021/jo01291a010

The substituted ethynylvinyloxiranes 9a-e were obtained by condensation of vinylsulfonium ylides with acetylenic carbonyl compounds.Thermolysis of the cis isomers of 9 was investigated in both the gas phase and the liquid phase.The first procedure afforded only cyclopropanecarboxaldehydes 17a-e, the stereochemistry of which depended on the nature and position of the substituents and on the experimental conditions.In the liquid phase 9a-e rearranged to yield, besides 17a-e, dihydrooxepins 20 and 21c-e or phenol 19a, these products also being obtained from 17a-e.Moreover, thermolysis of 21c,d led to the corresponding phenols 19c,d.Compounds 19 are believed to arise from arene oxides in equilibrium with substituted oxepin intermediates.All these findings are consistent with the initial formation of an oxacycloheptatriene (22) by a Cope reaction from 9 or a retro-Claisen reaction from 17.The observed stereoselectivity of the reaction is explicable in terms of conformational preferences.