di-pimethane rearrangement

noun

A photochemical reaction of a molecular entity comprising two p-systems, separated by a saturated carbon atom (a 1,4-diene or an allyl-substituted aromatic analog), to form an ene- (or aryl-) substituted cyclopropane. The rearrangement formally amounts to a 1,2 shift of one ene group (in the diene) or the aryl group (in the allyl-aromatic analog) and ‘bond formation’ between the lateral carbons of the non-migrating moiety.