Bimanes. 10. Photochemical Rearrangement of 1,5-Diazabicyclo[3.3.0]octa-3,7-diene-2,6-diones (9,10-Dioxa-anti-bimanes)

1,5-Diazabicyclo[3.3.0]octa-3,7-diene-2,6-diones (9, 10-dioxa-anti-bimanes) rearrange quantitatively on irradiation in the 320-nm absorption band to isomeric lactones, one from symmetrical bimanes and two from unsymmetrical bimanes (“mixed” bimanes). The quantum yield of lactone is often high and depends upon bimane substitution and solvent viscosity. Fast intersystem crossing (Huppert et al.⁴), oxygen inhibition of lactone formation, and efficient formation of lactone via a triplet sensitizer implicate the triplet as a key intermediate. The suggested mechanism of lactone formation involves a twisted π-π* triplet.