Oxetanes intermolecular additions

Griesbeck, A. G., Bondock, S. Oxetane formation intermolecular additions. CRC Handbook of Organic Photochemistry and Photobiology... 

Intramolecular photoaddition in cis-5,5,6-trimethylhepta-3,6-dien-2-one (284) takes a different course,296 yielding not the oxetane but the two dihydropyrans (285 and 286). This is in contrast to the inter-molecular cycloaddition of a, jS-unsaturated aldehydes to alkenes which affords only oxetanes, and has been accounted for in terms of diradical intermediates (287 and 288) formed from the s-cis conformation (284) of the dienone. The intermolecular equivalent is thought to occur by addition to the s-trans conformation. 

This photochemical umpolung is useful from the perspective of chemoselectivity in synthetic transformations. In addition, oxetane-containing products, prepared regioselectively, can be valuable synthetic intermediates. Both oxetane and vinyloxetane derivatives react with nucleophiles in intermolecular and intramolecular condensations and with transition metals and carbenoids. A second feature of oxetane-containing photoproducts, which has important implications, is that they can act as vehicles for subsequent manipulations and stereocontrolled synthesis. For example, the oxetane photoproduct (6)... 

The intramolecular attack of an excited carbonyl on an alkene can occur to provide oxetane products, even in cases when the corresponding intermolecular reaction is imsuccessful. Thus the intramolecular reaction surely benefits from favorable entropic considerations. Jones and Carless have summarized the scope and utility of intramolecular Patemo-BUchi photocycloadditions. There is general agreement that successful implementation of an intramolecular reaction requires that the Norrish type II photoreactionsand other hydrogen abstraction processes be overcome. In addition, the intramolecular reaction provides access to polyoxygenated ring systems that can exhibit remarkable properties because of their strain. 

Mill and Montorsi [38], in a more detailed kinetic study, showed that, not only was intramolecular abstraction the dominant process, but the ratio of rates of intra- and intermolecular abstraction was almost unchanged with temperature indicating little (<1 kcal mole-1), if any, difference in activation energy between the two steps. Moreover, at very low oxygen concentrations, some oxetane formed by ring closure of I in competition with (the much faster) addition of oxygen, viz. 

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). 

It is also worthwhile comparing the intramolecular photochemical cycloaddition reactions of ethylenic aldehydes and ketones with free radical intramolecular additions. For instance, irradiation of 5-hexen-2-one (470) (Scheme 161) in the gas phase gives the oxetane 471 as only cyclized product, as expected from the known photochemical intermolecular reaction between olefins and ketones. If the irradiation is conducted in solution 470 gives 471 (26%) and 472 (18%). With other y,< -unsaturated ketones, the bicyclic compound analogous to 472 may become the major product. With 2-allylcyclanones such as 473 (Scheme 161) bicyclic compounds are obtained (80% yield) as a mixture of 474 and 475, with 475 being the major product, but such compounds are difficult to isolate. " In the same manner, selective irradiation of the carbonyl group of 2-acyl-2,3-dihydro-4/f-pyrans (476) leads exclusively (23% yield) to exo-brevicomin (477) (a sex attractant), neither oxetane formation nor Norrish type II reaction being observed. The formation of the compounds 472, 475, and 477 which was considered as unexpected... 

Another intermolecular reaction of n excited ketones is their addition to olefins to form oxetanes, a very nice example being the photochemical addition of acetone to trans- (54) and cis- (61) 1-methoxy-l-butene, "... 

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