Stereochemistry of hexachlorocyclopentadiene [4+2]-cycloaddition to 2-substituted 4,7-dihydro-1,3-dioxepins

Experimental investigation of hexachlorocyclopentadiene [4+2]-cycloaddition to 2-substituted 4,7-dihydro-1,3-dioxepins revealed an atypical stereochemical effect. Clear experimental evidence was obtained that more bulky C2 substituents favour the thermodynamically and sterically less favourable endo -isomers. The possible reasons for such behaviour are secondary interactions of the highest occupied and lowest unoccupied orbitals in the transition state for endo- cycloaddition, diastereotopic solvation and coordination of the attacking diene reagent to the acetal oxygens. The reaction stereoselectivity also depends on the solvent nature and reaction temperature. We have also found that microwave irradiation significantly influences the [4+2]-cycloaddition yields and stereochemistry, though the nature of the underlying effects remains unclear.