Oxetan-3-ones, photochemistry

The first examples of exclusive oxetane formation upon olefin photoaddition to the cyclohexen-l,4-dione 4-oxoisophorone 45, leading to two novel 2-substituted l-oxaspiro[3,5]non-5-en-7-ones 46 and 47, respectively, was reported by Catalani and coworkers (Sch. 12) [42]. They demonstrated that the chemoselectivity of the olefin-enone photochemistry... 

The photochemistry of more complex and highly substituted alkenic partners has been studied. In 1978, Hartmann and coworkers reported the photocycloaddition of 4-oxazolin-2-one with acetone, used as a photosensitizer in the reaction of 4-oxazolin-2-one with alkenyl and alkynyl partners, to form oxe-tane (44). Recently, Scharf has described the photochemistry of 3-acetyl-2,3-dihydio-2,2-dimcthyloxa-zole (45). Irradiation of (45) in the presence of acetophenone produced the oxetane (46) with the phenyl group endo (17%), in addition to 21% of a ring-opened derivative. The stereoselectivity is in agreement with the high exo carboxyl selectivity observed in the photocycloaddition of methyl phe-nylglyoxylate with 2,2-dimethyl-1,3-dioxole to produce oxetane (47). 

In the presence electron-rich alkenes such as 2,3-dimethylbut-2-ene, irradiation of CA gives the allylethers 59 and 60, whereas with BQ, a substantial amount of the spiro-oxetane is also formed.The product distribution of the allyl ethers is rationalized by steric effects on the H+ abstraction and on the recombination of radicals as well as spin densities. The crucial role of solvent polarity in CA photochemistry is well illustrated by the results of a study into the reaction between the quinone and cyclohexanone enol trimethylsilyl ether 61 using time-resolved (ps) spectroscopy. The influence of the solvent occurs following the formation of the radical ion pair (CA - 61+-). The CA- species is short lived in nonpolar solvents and cyclohex-2-en-l-one and 62 are the reaction products, whereas in acetonitrile, the lifetime is much longer, which allows diffuse separation of the radical ion pair and transference of the TMS to the solvent. The resulting ketyl radical couples to CA - yielding 63. 

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