Formation of Benzofuranols

The photocyclization of o-alkoxy phenyl ketones to yield benzofuranols (57 and 58) represents one of the earliest example of 8-H-abstraction from the lowest n, n triplet Wagner et al. [18] have provided detailed photokinetic data studying the photocyclization of a variety of o-alkoxyphenyl ketones 56, and have revealed that quantum efficiency for cyclization for 56d was the lowest (0.023) and that for 56f the highest (1.00). The diastereoselectivity for cyclization of 56 was found to be higher in benzene and lower in polar solvents. From the estimated kH values (0.6-25 x 106 s 1), it was inferred that the low rate constant for 56e (8 x 106 s ) compared to that for 56g (25 x 106s 1) i s due to the alkyl chain in the alkoxy groups that points away from the o-carbonyl moiety in the most populated equilibrium conformations (Table 8.1). 

Horaguchi and coworkers [19, 20] have studied extensively the effect of solvents and the groups attached to the 8-carbon, and have concluded that polar solvents lower both photocyclization yield and diastereoselectivity. Studies with different substrates have revealed that the development of diastereoselectivity is the result of conformational preferences of the biradical rather than of the steric interactions between the two radicals. The same research group has also studied the photocyclization of ethyl oc-(o-benzoylphenoxy)carboxylates (59), and reported that the product yields and Z/E ratios (60a 60b) in benzene and acetonitrile are, interestingly, much the same as for other ketones in which R is not an electron-withdrawing group [20] (Table 8.2). 

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