THE PHOTOCHEMISTRY OF ISOBENZOFURAN .1. STRUCTURE OF THE DIMERS RESULTING FROM ULTRAVIOLET-IRRADIATION OF ISOBENZOFURAN IN ACETONE AND ETHER SOLUTION

Irradiation of isobenzofuran (1) in acetone solution yields a symmetri cal [pi8s+pi8s] dimer (24) involving bonding at the peri-position of t he furan moiety in each molecule. The anti-stereochemistry of this dim er was established by a novel application of lanthanide induced shift spectroscopy. In contrast, irradiation of isobenzofuran (1) in ether s olution yielded the unsymmetrical dimer (34) as the major product, tog ether with lesser amounts of the symmetrical dimer (24), and small amo unts of a new dimer (50) resulting from [pi8s+pi4s] cycloaddition, whe re the 8pi system of the isobenzofuran of one molecule reacts with the carbocyclic 4pi diene of the other. The structure of the unsymmetrica l dimer (34) was confirmed by synthesis. No evidence for the Dewar for m (7e) of isobenzofuran could be obtained in these reactions conducted at -60-degrees and monitored by H-1 n.m.r. spectroscopy. However, the small, but persistent, production of o-phthalaldehyde may implicate a n intermediate derived from (7e).