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Abstract
The Paternò-Büchi reaction is a photochemical reaction between a carbonyl compound and an alkene to give the corresponding oxetane. In this review the mechanism of the reaction is discussed. On this basis the described use in the reaction with electron rich alkenes (enolethers, enol esters, enol silyl ethers, enanines, heterocyclic compounds has been reported. The stereochemical behavior of the reaction is particularly stressed. We pointed out the reported applications of this reaction to the synthesis of naturally occuring compounds.
Highlights
Oxetanes are important component in the scaffold of compounds with relevant biological properties: an oxetane ring is present in the scaffold of Taxol® (1), an important drug used in the treatment of ovarian cancer [1], merrilactone A (2), a sesquiterpene dilactone with neurotrophic activity [2], and several antiviral oxetanes, such as 3, 4, and 5, have been described in the literature (Figure 1) [3,4,5]
Interaction is the main process observed where the half-occupied π* orbital of the carbonyl compound interacts with the unoccupied π* molecular orbital of an electron deficient alkene: the results of this interaction is the formation of a C,O-biradical (A in Scheme 2)
The irradiation in the presence of benzophenone of 2,4-dimethylthiazole gave the corresponding oxetane, while the same reaction failed when acetophenone was used as carbonyl compound [123]
Summary
Oxetanes are important component in the scaffold of compounds with relevant biological properties: an oxetane ring is present in the scaffold of Taxol® (1), an important drug used in the treatment of ovarian cancer [1], merrilactone A (2), a sesquiterpene dilactone with neurotrophic activity [2], and several antiviral oxetanes, such as 3, 4, and 5, have been described in the literature (Figure 1) [3,4,5]. HSOMO-LUMO interaction is the main process observed where the half-occupied π* orbital of the carbonyl compound interacts with the unoccupied π* molecular orbital of an electron deficient alkene: the results of this interaction is the formation of a C,O-biradical (A in Scheme 2). A theoretical study of the Paternò-Büchi reaction showed that there are two conical intersection points located near the C-C and C-O bonded biradical regions of the ground state These two conical intersections support a mechanism where the decay from the excited state is accompanied by a geometric rotation of the terminal group, in the case of C-O attack, and by an orbital rotation at the oxygen center, in the case of C-C attack. Evidence of monoelectron transfer processes is reported for electron-rich alkenes [42]
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