substantially higher than the total highest barrier leading to the desired 1H-2-benzo [c]oxocins a (+7.5 kcal mol -1 ) from this same intermediate (at 110 o C), thus explaining the predominant formation of products a over products d for kinetic reasons (Scheme 1). Meanwhile, the different outcome upon heating dihydro-4Hcyclobuta [c]isochromenes with alkyl or aryl groups can be readily explained by the much higher energy barrier needed for thermal ring-opening (e.g., R = Me in Scheme 1). For dihydro-4H-cyclobuta [c]isochromenes with aliphatic substituents, the barrier from b to Woodward-Hoffmann intermediate a' is too high to be overcome simply by heating (Scheme 1, 4.7 kcal mol -1 higher for R = Me than for R = Ph), which inhibits the formation of o-QDM intermediates needed in the follow-up steps. The difference in energy barrier of ring-opening for dihydro-4H-cyclobuta [c]isochromenes with alkyl or aryl groups can be explained by the strength of the breaking bond. Compared with the optimized geometries of b with methyl and phenyl group, the length of the breaking C-C bond (via TS1 to release a') is different: The bond length is slightly longer when R = Ph (R = Me: 1.592 Å; R = Ph: 1.606 Å), suggesting a weaker bond in the aryl analogue. The higher energy barrier for the ring-opening of dihydro-4Hcyclobuta [c]isochromenes with an alkyl group is likely to be additionally influenced by electronic effects. Aromatic substituents (e.g. R = Ph) at the enol ether position are in electronic conjugation with the -system of the forming conjugated diene moiety, allowing electronic delocalization in TS1 providing stabilization of the transition state leading to a lower energy barrier compared with dihydro-4H-cyclobuta [c]isochromenes substituted with an alkyl group.