Novel transformations of two kinds of chlorinated photo [2 + 2] cycloadducts of 2-pyrone-5-carboxylate

Novel reactions of 7,7-dichloro- and 7,7,8-trichloro-3-oxo-2-oxabicyclo4.2.0]oct-4-ene-6-carboxylates 5 with 1,8-diazabicyclo5.4.0]undec-7-ene (DBU) in alcohol gave pyrano4,3-b]pyran-2,5-diones 8 via (Z)-(2H-pyran-2-on-3-yl)butenoates 7. On the other hand, the same treatment of 7,7,8-trichloro-2-oxo-3-oxabicyclo-4.2.0]oct-4-ene-5-carboxylate 6b afforded 2-oxo-3-oxabicyclo4.2.0]oct-4,7-diene-5-carboxylate 14 via cyclobutene formation and S_N2′ displacement by attack of the alkoxy anion.