Origin of regioselectivity in the intramolecular [2+2] photoreaction of α, β-unsaturated furanones to a terminal alkene

The intramolecular [2+2] photoreaction of α,β-unsaturated furanone to a terminal alkene presents a remarkable change in regioselectivity, with variation of the chain length of the terminal alkene. Head-to-tail (ht) regioselectivity was exclusively produced through one-carbon atom connection (furanone-1n), whereas it was completely inverted when connection of two-carbon atoms takes place (furanone-2n). The origin of this remarkable change in regioselectivity was elucidated by means of B3LYP, UHF and PM5 levels of calculation. The transition state (TS) analysis was used to rationalize this change in regioselectivity. The results indicate that a TS on the lowest-lying triplet potential energy surface may play an important role in controlling the regioselectivity. The role of the biradical intermediates and, in particular, the relative energies of the closure TS and the retro-cleavage TS leading to the original furanones were also studied. We observed that the behaviour of the biradicals that are appropriate for ring closure do not follow the so-called 'rule of five'.