Photoreactions of cyclic benzylidene acetals with ketones in the presence and absence of dissolved oxygen.

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Photoreactions of 2-phenyl-1,3-dioxane (1) and 4,6-O-benzylidene-1,2,3-tri-O-acetylglucopyranose (10) in the presence of benzophenone, acetophenone, or acetone have been studied in both deoxygenated and aerated benzene solutions at room temperature. Photolysis of 1 with ketone in deoxygenated solution gave a dimeric product of 1 (8), cross adduct, and pinacol, while in aerated solution an oxidative ring cleavage product, i.e., trimethyleneglycol monobenzoate (9), was obtained instead of 8, besides smaller amounts of pinacol and cross adduct. The yields of pinacol and cross adduct decreased with the change in ketone in the order benzophenone>acetophenone>acetone, while the yield of 8 or 9 was in the reverse order. Photolysis of 10 with ketone led to results parallel to those of 1: photolysis under nitrogen atomosphere gave pinacol, cross adduct, and a dimeric product of 10 (14), while under bubbling air 4-O-monobenzoate and 6-O-monobenzoate derivatives of 10 were obtained instead of 14, besides pinacol and cross adduct. The effects of temperature, oxygen, and structure on the course of the reaction, to produce dimeric or ring opening products, have been discussed.
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