Summary.


        The oxidative and/or reductive cycloreversion (CR) of oxetanes has not been systematically investigated, in spite of its cosiderable biological and synthetical interest.
        As regards the photooxidative pathway, taking into account the already published theoretical calculations and experimental results on its mechanistic nature, it appeared interesting to perform additional experimental studies in order to gain further insight into the problem. In this context, trans,trans-2-cyclopropyl-3-phenyl-4-methyloxetane (1) has been chosen to clarify some mechanistic aspects of the oxidative CR of oxetanes. Splitting of 1 with initial cleavage of C2-C3 bond leads back to the substrates employed in the Paterno-Büchi photocycloaddition to synthesize the oxetane. By contrast, initial cleavage of the O-C2 bond leads to en intermediate which is trapped by acetonitrile in an intermolecular nucleophilic reaction, to give oxazine-like adducts. This is in better agreement with a stepwise mechanism for the oxidative CR of oxetanes.
        Concerning the photoreductive pathway, cycloreversion of 2-(p-cyanophenyl)-4-methyl-3-phenyloxetane (4) has been achieved using 1-methoxynaphthalene as electron transfer photosensitizer. The experimental results are consistent with the reaction taking place from the singlet excited state of the sensitizer.  Ring splitting of the radical anion 4? - occurs with cleavage of O-C2 and C3-C4 bonds, leading to products (acetaldehyde and p-cyanostilbene) different from the reagents used in Paterno-Büchi synthesis of 4. The olefin radical anion involved in the electron-transfer process has been detected by means of laser flash photolysis (?max = 500 nm). 
        Moreover, intramolecular photoinduced electron transfer (PET) cycloreversion of (2R,3S,4S)-[2-(4-cyanophenyl)-3-phenyloxetan-4-yl]methyl (2S)-2-(6-methoxynaphthalen-2-yl)propanoate (8) and (2S,3R,4R)-[2-(4-cyanophenyl)-3-phenyloxetan-4-yl]methyl (2S)-2-(6-methoxynaphthalen-2-yl)propanoate (9) has been achieved in acetonitrile and chloroform as solvents. Interestingly, a higher photoreactivity has been found in acetonitrile, while a significant stereodifferentiation has been found in chloroform. This stereodifferentiation can be attributed to the folded conformation which predominates in 9, with the naphthalene ring directed towards the oxetane region, allowing for the intramolecular electron transfer. Accordingly, intramolecular fluorescence quenching is also more efficient in acetonitrile, whereas stereodifferentiation is markedly higher in chloroform. Thus, a good correlation can be established between the results from steady-state irradiations and fluorescence measurements. 
        The oxetanes 2,6-dioxa-3-phenylbicyclico [3,2,0] heptane and 3-(4-cyanophenyl)-2,6-dioxabicyclico [3,2,0] heptane, resulting from photocycloaddition of aromatic aldehydes to 2,3-dihydrofuran, have been efficiently cleaved by means of electron-transfer reduction, photoinduced by the electronically excited reductants 1-methoxynaphthalene and 2,7-dimethoxynaphthalene in acetonitrile. Fluorescence, as well as triplet, quenching rates of 1-methoxynaphthalene and 2,7-dimethoxynaphthalene  by both oxetanes have been determined, showing a marked dependence on the substitution at the phenyl group. The product analysis has been allowed us to establish a “photo-photo metathesis”, where both cycloaddition and cycloreversion processes are induced by photochemical processes.