Oxetanes heterocycles addition

Abe, M. Photochemical oxetane formation addition to heterocycles. CRC Handbook of Organic Photochemistry and Photobiology (2nd... 

Rivas, C., Oxetane formation addition to heterocycles, in Organic Photochemistry and Photobiology, Horspool, W. M. and Song, P.-S., Eds., CRC Press, Boca Raton, 1994, 536. 

Four-membered heterocycles are easily formed via [2-I-2] cycloaddition reac tions [65] These cycloaddmon reactions normally represent multistep processes with dipolar or biradical intermediates The fact that heterocumulenes, like isocyanates, react with electron-deficient C=X systems is well-known [116] Via this route, (1 lactones are formed on addition of ketene derivatives to hexafluoroacetone [117, 118] The presence of a trifluoromethyl group adjacent to the C=N bond in quinoxalines, 1,4-benzoxazin-2-ones, l,2,4-triazm-5-ones, and l,2,4-tnazin-3,5-diones accelerates [2-I-2] photocycloaddition processes with ketenes and allenes [106] to yield the corresponding azetidine derivatives Starting from olefins, fluonnaied oxetanes are formed thermally and photochemically [119, 120] The reaction of 5//-l,2-azaphospholes with fluonnated ketones leads to [2-i-2j cycloadducts [121] (equation 27)... 

The synthesis of nitrogen containing heterocyclic systems by photocylo-addition processes is virtually limited to examples involving [ 2 + 2] cycloaddition of imines, nitriles, and azo compounds. Successful additions are few in number and the requirements for success uncertain. The reactions do not proceed with the facility with which carbonyl containing compounds undergo photocycloaddition to alkenes to give oxetans, and various explanations have been advanced to account for this observed lack of reactivity.226... 

In addition to furan, other heterocycles have been examined." Thiophene undergoes efficient photocycloaddition with benzaldehyde to afford a single exo photoproduct (183) in 60% yield. As reported by Jones and coworkers,the photolysis of IV-methylpyrrole in the presence of aldehydes or ketones yields the corresponding 3-hydroxyalkyl derivative (184), even when the reaction mixture is free from any trace of acid. In order to use the pyrrole nucleus for stereoselective alkaloid synthesis (cf. caesalpinine, 185) in the fashion developed with the furan nucleus, pyrrole substituents that stabilize the presumed intermediate bicyclic oxetane must be discovered. 

As can be seen from the foregoing, phthalimide photocyclizations have provided useful synthetic routes to a variety of heterocyclic products. Machida et al. have used the reaction to yield the spiro compounds (325) from the irradiation (in methanol) of the phthalimide derivatives (326). The products are presumed to be formed via bond formation in the biradical produced by the addition of methanol to the radical cation/r ulical anion pair. The indoles (327) undergo photochemical (2-f-2)-addition with A methyl phthalimide to afford the oxetan adducts (328). This work has also been the subject of a patent application. " ... 

Some of the reactions included have been discussed in other review articles on the photochemistry of heterocyclic compounds,1,7 dimerization reactions,l9,118,119 photochemical oxetane synthesis,6 and photochemical addition reactions.120... 

The mechanisms of two particular classes of photochemical (2 + 2)-cycloaddition reactions have been studied extensively, viz. (i) of reactions in which either the heterocyclic or the substrate contains an enone moiety,1231,24 which are believed to occur via the excited triplet state of the enone, and (ii) of oxetane formation, which is thought to proceed via n — 7T excitation of the carbonyl group, followed by intersystem crossing to the triplet state and addition to the carbon-carbon double bond in its ground state6 in some cases, however, an oxetane has been reported to be formed from a ground-state ketone and an excited-state olefin (Section III,C,2). 

Aluminum porphyrins are versatile initiators which are applicable to controlled ring-opening polymerization of various heterocyclic monomers (Table 1) such as epoxides (11)," oxetanes (12) lactones with four-, six-, and seven-membered rings (13-15), lactide (16), six-membered cyclic carbonates (17) and cyclic siloxanes (19).-" They are also excellent initiators for the controlled addition polymerization of unsaturated monomers such as acrylates (20), methacrylates (21)-- and methacrylonitrile (22).- - ... 

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