Tetronic intramolecular photocycloaddition

The photochemical reactivity of P-ketoesters is different form that of P-diketones. Irradiation of a P-ketoester in the presence of an alkene produces oxetane via the ketone carbonyl instead of the desired cyclobutane ring system. Therefore, it is necessary to covalently lock the ketoesters as the enol tautomers. To this end, silyl enol ethers, 129 and 132a, and enol acetates, 130 and 132b, were prepared, but these substrates still fail to undergo the desired intramolecular [2 + 2] photocycloaddition with olefins. The only new products observed in these reactions result from the photo-Fries rearrangement of the cyclic enol acetate (130 to 131) and cis-trans isomerization of both acyclic substrates 132a/b. However, tetronates are appropriate substrates for both intermolecular and intramolecular photocycloadditions with olefins. In addition, enol acetates and silyl enol ethers of p-keto esters are known to undergo [2 + 2] photoaddition with cyclic enones (vide infra). 

The C-4 substituted tetronates 159 also undergo intramolecular photocycloaddition to give the tricyclic adducts 160 with the heterocyclic ring directly anellated to the cyclobutane moiety. In consistency with the rule of fives, these photocycloadditions proceed exclusively to furnish the straight adducts. 

Basler, B., Schuster, O., and Bach, T. (2005) Conformationally constrained p-amino acid derivatives by intramolecular [2 + 2]-photocycloaddition of a tetronic acid amide and subsequent lactone ring opening. Journal of Organic Chemistry, 70, 9798-9808. 

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