Abstract trans-head-to-head Coumarin dimer cleaves into coumarin in chloroform by irradiation with the light of wavelength 277 nm in high yield. When a ring opened dimer, trans-3,cis-4-bis(2-hydroxyphenyl)-l,trans-2-cyclobutanedicarboxylic acid in aqueous solution is irradiated, a mixture of trans-2,2′-dihydroxystilbene and fumaric acid is obtained in high yield.
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Hasegawa et al. (1972) studied this question.