Photodimerization pyridines

Unusual heterocyclic systems can be obtained by photodimerizations and for five-membered heterocycles with two or more heteroatoms such dimerizations need be effected on their ring-fused derivatives. Cyclobutanes are usually obtained as in the photodimerization of the s-triazolo[4,3-a]pyridine (540) to the head-to-head dimer (541). These thermally labile photodimers were formed by dimerization of the 5,6-double bond in one molecule with the 7,8-double bond in another (77T1247). Irradiation of the bis( 1,2,4-triazolo[4,3-a]pyridyl)ethane (542) at 300 nm gave the CK0ifused cyclobutane dimer (543). At 254 nm the cage-like structure (544) was formed (77T1253). 

Cycloadditions are in general an effective way of constructing cyclobutane rings. A wide variety of heterocyclic systems dimerize in this way. 1,3-Diacetylindole, for example, affords the head-to-tail dimer 242 on irradiation in ethanol.185 Ethyl 2-ethoxy-l,2-dihydroquinoline-l-carboxy-late is similarly converted in diethyl ether into the trans head-to-head dimer.186 Notable among many analogous photodimerizations are those reported in 1,4-dihydropyridines,187 in furo[3,2-b]pyridin-2(4//)-ones,188 in 8-methyl-s-triazolo[4,3-a]pyridine,189 and in 2H-2-benzazepine-1,3-diones.190 The [ 2 + 2] dimerization of amidopyrine is the first reported example of a photocycloaddition in a 4-pyrazolin-3-one.191... 

Marubayashi et al. <1997J(P2)1309> have also shown that solid-state dimerization is possible and propose that there is a buffer zone in the crystal structure of 1,4-dihydropyridines that governs the solid-state photodimerization process. This is exemplified by the fact that dimethyl l,4-dihydro-2,6-dimethyl-4-(3-nitrophenyl)pyridine-3,5-dicar-boxylate 83 cannot undergo solid-state photodimerization (Equation 21), whereas the structurally related (4/ 3, l / 3 )-methyl-l-phenyl-2-piperidinoethyl-l,4-dihydro-2,6-dimethyl-4-(2-thienyl)pyridine-3,5-dicarboxylate 84 affords a single product 5 (Scheme 2). Interestingly, when the photodimerization conditions are applied to the corresponding solution-phase reaction, the sole product is that of aromatization giving product 85. 

Ultraviolet irradiation of the methiodide or hydrochloride of 2-styryl-pyridine (XCIII) in the solid state results in transformation of the trans-isomers to the corresponding dimers (XCVa, b) on the other hand irradiation in benzene solution gives both isomerization and dimerization.309 Dimer (XCVa) was produced in low yield on irradiation of XCIII in the powdered form in the presence of air.308 This is in contrast to the reported stability of XCIII toward photodimerization.109 Similar dimerizations have been reported in the case of 2,4-dichloro-3-cyano-6-styrylpyridine,164 2-styrylquinaldine (XCV) and frans-4,4 -diguanyl-stilbene bis(2-hydroxyethane sulfonate) (stilbamidine) (XCIV).82,83... 

The photodimerization of 2-pyridones is an efficient, regiospecific, and stereoselective [4+ 4]-cycloaddition [56] that converts two achiral aromatics into a highly functionalized tricyclic cyclooctadiene with four stereogenic centers (Scheme 9.34). For tethered pyridones, the trans isomer is usually the major product when one or both pyridine nitrogens are methylated. By contrast, in the unsubstituted systems,... 

Definitive evidence for the alkenic nature of the pyridine dienic unit arose when several [l,2,4]triazolo[4,3-a]pyridines were found to undergo photodimerization (77T1247). The products from this process resulted from dimerization between the 5,6-double bond in one molecule with the 7,8-double bond in another to give (113). 

Durr et al. have described some novel complex cinnoline derivatives which show photochromic properties. Albini et al. have provided some new evidence on the mechanism for the photoisomerization of heterocyclic N-oxides. Simple pyridine N-oxides are exceptional. Thus irradiation of pyridine N -oxide in aqueous base affords the ring-opened product (23). Aoyama et al. have described the unprecedented photocyclization of the amide (24) to the lactam (25). The first examples of C2a+2Tr] photoreactions of a three-membered ring and an azo-compound have been described by Hunig and Schmitt. Nicolaou et al. have prepared the first stable example of a 1,2-dithiethane the procedure involves (2i +2Tr) photodimerization of C=S groups. 

Cycloaddition of thioparabanates to a variety of electron-donor and electron-acceptor alkenes, giving the appropriate thietans in high yield, has been reported.200 With ethoxyethylene, for example, the adduct (270) is obtained in 88% yield. The use of light with > 400 nm is essential as it provides selective excitation to the state on which the success of the addition depends. The formation of thietans by the addition of thiophosgene201 and aromatic thionecarbonates202 to alkenes has also been described. An unusual photodimerization of 1-hydroxy-pyridine-2-thione (271) has been reported 208 the dimer (272) is further converted into 1,6-diazathianthrene (273) on heating at 100 °C. 

Cross-photodimerization of ( )-2-(2-phenylethenyl)benzoxazole and ( )-4-[2-(4-chlorophenyl)ethenyl]-pyridine in sulfuric acid solution results in the formation of a product identified as (7 )-cw,traw, raw -l-(2-benzoxazolyl)-2-(4-chlorophenyl)-4-phenyl-3-(4-pyridinyl)cyclobutane. " ... 

Lee et al. [169] reported the preparation of new soluble and intrinsically photosensitive poly(amide-co-imide)s containing p-phenylenediacryloyl moiety. The copolymers were formed from p-phenylenediacryloyl chloride, aromatic dianhydrides, and two equivalents of aromatic diamines. The products were subsequently imidized by reactions with the poly(amide-co-amic acid), acetic anhydride, and pyridine. The polymers were stable up to 350°C, showed good solubility in polar aprotic solvents, and became insoluble after the irradiation due to the photodimerization of phenylenediacryloyl moiety. The photo-reactivity increases with the irradiation temperature [169]. 

Photodimerizations with charged compounds have been studied in several styryl dyes wherein alkylation of an V-heterocycle (e.g., pyridine, benzothiazole) generates a salt." Gromov and coworkers have also shown that when a styryl dye and a crown ether (e.g., 18-C-6 Scheme 9) are covalently connected, stereospecific [2- -2] reactions are facilitated by inclusion of the cation within the ethereal host." In addition, Gromov also showed that counteranion... 

Photodimerizations of the isomeric 1,2,4-5-triazolo[4,3-a]pyridines, e.g. (99), have also been reported. " The 3-methyl derivative yields a thermally labile... 

Nagano, T, Hirobe, M., Itoh, M., and Okamoto, X, Reversible photodimerization of 2-methyl-s-triazolo[l,5-a]pyridines. Tetrahedron Lett., 3815-3818,1975. 

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