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The catalyst-free [2 + 2] photocycloaddition between benzils and simple olefins is reported. The adoption of visible light proved essential for the transformation, as shorter wavelengths led to uncontrolled decomposition. When cyclic olefins were used, the reaction occurred smoothly to afford the expected oxetanes regio- and stereoselectively after 24 h of irradiation. In contrast, in the case of acyclic olefins, longer reaction times were typically required and small amounts (ca. 20%) of [4 + 2] photocycloadducts and by-products deriving from competitive hydrogen atom abstraction were observed. The selectivity of the transformation could be consistently improved by decreasing the reaction temperature, thus restoring the desired [2 + 2] reactivity. An overall mechanistic picture is also offered based on the chemical and photophysical quenching experiments and the stereochemical output is rationalized based on Griesbeck models. [ABSTRACT FROM AUTHOR]

This chapter focuses on well-defined metal complexes that serve as homogeneous catalysts for the production of polycarbonates from epoxides or oxetanes and carbon dioxide. Emphasis is placed on the use of salen metal complexes, mainly derived from the transition metals chromium and cobalt, in the presence of onium salts as catalysts for the coupling of carbon dioxide with these cyclic ethers. Special considerations are given to the mechanistic pathways involved in these processes for the production of these important polymeric materials. [ABSTRACT FROM AUTHOR]

The photocycloaddition of aldehydes and α-ketoesters to 2,5-dimethyl-4-isobutyloxazole leads to bicyclic oxetanes with high to moderate ( exo) diastereoselectivity that can be easily ring-opened to give α-amino-β-hydroxyketones. [ABSTRACT FROM AUTHOR]

Oxidation of methyl esters of ursolic and 3-acetoxyursolic acids by ozone formed products with a 12-oxo11S,13 R-oxetane moiety on ring C. [ABSTRACT FROM AUTHOR]