Cycloaddition photosensitized

A-Substituted pyrroles, furans and dialkylthiophenes undergo photosensitized [2 + 2] cycloaddition reactions with carbonyl compounds to give oxetanes. This is illustrated by the addition of furan and benzophenone to give the oxetane (138). The photochemical reaction of pyrroles with aliphatic aldehydes and ketones results in the regiospecific formation of 3-(l-hydroxyalkyl)pyrroles (e.g. 139). The intermediate oxetane undergoes rearrangement under the reaction conditions (79JOC2949). 

When buta-1,3-diene 3 is irradiated in the presence of a photosensitizer (e.g. benzophenone), the isomeric divinylcyclobutanes 6 and 7 are formed via the intermediate diradical species 4 and 5 respectively in addition the [4 + 2] cycloaddition product 4-vinylcyclohexene (see Diels-Alder reaction) is obtained as a side product ... 

Irradiation of Z-but-2-ene 8 initiates a cyclodimerization reaction, even without a photosensitizer." This cycloaddition proceeds from a singlet state and is likely to be a concerted, one-step reaction. Bond formation occurs suprafacial with respect to both reactants, whereupon only the tetramethylcyclobutanes 9 and 10 can be formed ... 

A photosensitized dimerization of an isolated olefin, norbomene, has been reported by Scharf and Korte.<3) Irradiation in acetone or in the presence of acetophenone (Et = 74 kcal/mole) produced dimers (5) and (6) as major products. However, benzophenone (Et = 69 kcal/mole) failed to sensitize the reaction to (5) and (6), but in ether solution led to the quantitative formation of benzpinacol and in benzene to the oxetane (7) in 80% yield. Sensitizers of intermediate energy, such as xanthone (Et — 72 kcal/mole), demonstrated a competition between energy transfer to form triplet norbomene and cycloaddition to form the oxetane ... 

A third important group of reactions that may be discussed with the first two groups is not necessarily photosensitized. The dimerizations and additions of cyclic a,/S-unsaturated ketones can be initiated by direct n-n excitation of the ketone, followed by addition reactions. However, the reactions are efficiently photosensitized by triplet sensitizers, and it is reasonable to propose that the unsensitized cycloaddition reactions also proceed via triplet states. 8>63>94> Examples are given in Eqs. 28—... 

The antidepressant protriptyline (116) causes skin photosensitization in man. Jones and Sharpies irradiated an aqueous solution of the hydrochloride with a medium-pressure mercury lamp for 16 h and separated the products by preparative TLC. First formed was the epoxide (117) which photohydrated to the diol (118). Also isolated was the enol (119) [84], Earlier, Gasparro and Kochevar had shown that only the hydrochloride was photodegraded under nitrogen in water or ethanol. Three products were isolated and all lysed erythrocytes, but the structure of only one was suggested. This was a cyclobutyl dimer as shown by its mass spectrum and its photolysis back to protriptyline by light of 254 nm. Presumably, a [2 + 2] cycloaddition of the olefine bonds had occurred [85]. 

The photosensitized decomposition reduces the hydride shift, as may be expected for a triplet species. However, the stereochemical change in the cycloaddition is relatively small. Similieir results were obtained by Baer and Gutsche for the sensitized photolysis of o-n-butyl-phenyl-diazomethane 63... 

There has been considerable interest in various photocycloaddition reactions over the last years which not only broadened the number of useful photochemical applications but also revealed further mechanistic insight into these reactions [76,77]. Among these reactions, reports focusing on either the [2 -h 2] or the [4 -I- 2] cycloaddition, are numerous. Also the efforts toward the enantiodifferentiating photosensitization in photocyclization reactions have to be mentioned [78],... 

Blechert and co-workers successfully employed the [4 -I- 2] cycloaddition for the transformation of indole derivatives [79-81], For instance, using 2-vinyl-indole derivatives as heterodienes, (3-acceptor-substituted cyclic and acyclic ena-mines (dienophile), and triarylpyrylium tetrafluoroborate as the photosensitizer, the corresponding Diels-Alder adducts were formed in moderate to good yields with complete regiochemical and stereochemical control [79], Alternatively, good results could be obtained in the reaction of indoles and exocyclic 1,3-dienes, thus providing an easy excess to multifunctionalized carbazoles [80], Quantum... 

Phosphoryl-stabilized anions, 25, 2 Photochemical cycloadditions, 44, 2 Photocyclization of stilbenes, 30, 1 Photooxygenation of olefins, 20, 2 Photosensitizers, 20, 2 Pictet-Spengler reaction, 6, 3 Pig liver esterase, 37, 1 Polonovski reaction, 39, 2... 

Pyronene (255) undergoes photosensitized 1,4-cycloaddition of oxygen to yield the endoperoxide 256, according to equation 88. The structure of this compound and many... 

Another mode for catalyzed cycloaddition involves the generation of radical cations from electron-rich alkenes with single-electron oxidants such as tris(4-bromophenyl)amminium hexachloroantimonate (TBAH). An equivalent reaction involves the photosensitized electron transfer (PET) process (see Section 1.3.2.3.). These processes have been recently reviewed,9 and are limited to electron-rich alkenes capable of producing radical cations. Furthermore, some of the cyclobutanes themselves undergo secondary isomerization under the oxidative conditions, e.g. formation of 31-35.10-12... 

It is possible that some of these photochemical cycloadditions take place by a [,2S + 2S] mechanism (which is of course allowed by orbital symmetry) when and if they do, one of the molecules must be in the excited singlet state (St) and the other in the ground state.980 The nonphotosensitized dimerizations of cis- and trans-2-butene are stereospecific,981 making it likely that the [ 2S + 2S] mechanism is operating in these reactions. However, in most cases it is a triplet excited state that reacts with the ground-state molecule in these cases the diradical (or in certain cases, the diionic) mechanism is taking place. In one intramolecular case, the intermediate diradical has been trapped.982 Photosensitized 2ir + 2-rr cycloadditions almost always involve the triplet state and hence a diradical (or diionic) mechanism. 

With the exception of the photosensitized reaction of methyl acrylate with 1-benzoylindole to produce the four-membered ring adduct, with subsequent conversion into a benz[6]azepine, no other photoinduced cycloaddition reactions of indoles with alkenes have been reported (80 3403). 

Thermally unstable azetidines may be obtained by irradiation of furan in the presence of oxazolinones or 3-ethoxyisoindolone by [7r2s+ .2s] cycloaddition (Scheme 90) (75JA7298). Benzophenone photosensitized [ 2S+W2S] cycloaddition of furan to dimethylmaleic anhydride yields an adduct (34%). This reaction has been extended to 2,5-dimethylfuran and other maleic anhydrides and maleimides (76CA(84)120785). 

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