Pyridine photocycloaddition

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... 

Reports of [ 2 + 2] cycloaddition of nitrogen containing heterocycles to alkenes are so numerous that attention can be drawn here to only a few. Recent examples include the acetone-sensitized photoaddition of 4-oxazolin-2-one (248) to ethylene to give the cis-adduct 249,203 the photocycloadditions of N-substituted imidazoles to acrylonitrile204 and of N-methyl-4-hydroxy-quinol-2-one to cyclohexene,205 and the photoaddition of pentafluoro-pyridine to ethylene to give the 1 1- and 1 2-adducts 250 and 251,... 

High facial diastereoselectivity has been reported in the [2+2] photocycloaddition of aromatic aldehydes with a chiral enamide to give the ds-2,3-disubstituted oxetanes 11 and 12 with only minor amounts of the trans-oxetanes <99TL9003>. The same group of workers have investigated the photocycloaddition of m-substituted benzaldehydes to 3,4-dihydro-l.H-pyridin-2-ones to give mainly 13 <99JA10650>. 

Photocycloaddition of cyclopentene (116) to the trimethylsilyl ether (117) gave 2 stereoisomers (118) and (119). When (118) was reduced with LiAlH4 in Et20, followed by treatment will MsCl, pyridine and dimethylaminopyridine (DMAP), the silyl ether (120) was obtained. Finally, (120) was allowed ro react with KF in the presence of [18]-crown-6 in CH2C12, as a result, hexahydroazulenone (121) was isolated41 >. On the other hand, the cyclobutane (119) could also be converted similarly to (121) in an overall yield of 47 % 41K... 

Heteroaromatic compounds do not undergo the same variety of photocycloadditions with alkenes as do their carbocyclic counterparts. There are very few reports of this type of reaction for six-membered ring compounds such as pyridines, but five-membered ring systems such as furans do give 1,2-cycloadducts with a range of alkenes (e.g. 357). 

Pentafluoropyridine is known to undergo photocycloaddition with ethylene [89,90] and cycloalkenes [91] at the 3,4-position of the pyridine ring. Recently, Sakamoto et al. reported that metacrylonitrile adds to the 2,3-position of 2-alkoxy-3-cyano-4,6-dimethylpyridines (24a,b) to give 7-azabicyclo[4.2.0]octa-2,7-di-enes (25a,b) [92] (Scheme 10). This work was developed to the photocycloaddition of furan to pyridine ring as a first example (Scheme 10) [93],... 

The intramolecular 2 + 2-photocycloaddition of the dimeric vinyl pyridine 2,2-[l,2-ethanediylbis(oxy)]bis(5-ethenylpyridine) (10) formed the corresponding syn- and anti-pyridinophanes [(H)—(13)] via a singlet-excited species (Scheme 3).13... 

In enantioselective photocycloaddition reactions, 4-alkoxyquinolones perform in superior fashion to l,5-dihydropyrrol-2-ones and 5,6-dihydro-lff-pyridin-2-ones. Both, intermolecular and intramolecular reactions were performed with excellent enantioselectivity in the presence of the chiral template 115, or of its enantiomer ent-115 [147, 148], The well-established photocycloaddition reactions [149, 150] enabled access to a variety of chiral dihydroquinolones. 4-Methoxyquinolone (157) produced, upon direct irradiation in the presence of allyl acetate, the formal HT product 158 in 80% yield and with 92% ee (Scheme 6.56) [151]. 

Photocycloaddition reactions of alkyl and aryl 2-thioxo-3//-benzoxazole-3-carboxylates 142 to alkenes afforded stable isolable spirocyclic aminothietanes 143 <02HCA2383> similar reactions with both electron-poor and electron-rich alkenes were also performed on 2-methyloxazolo[5,4-h]pyridine <02EJO4211>. 

Silyl-tethered stilbazole derivatives 349 were synthesized and subjected to intramolecular photocycloaddition in benzene at room temperature to give compounds of type 350 with stereochemistry cis-trans-ris and cis-trans-trans (Equation 66). It was shown that complexation of pyridine-containing stilbazoles 349 with dicarboxylic acid or catechol enhanced both the efficiency and stereoselectivity of the photocycloaddition <2006TL7865>. 

Alkenes, perfluoro-cycloadditions to benzyl azide, 60, 35 pyridine imines and ylids, 60, 36 Alkenes, perfluoro-, as heterocyclic precursors, 59, 10 Alkylation, free-radical, of 1,3-dimethyluracil, 55, 227 A-Alkylation, [ 1,2,4]triazolo[ 1,5-u)-pyrimidines, 57, 110 Alkynes, photocycloaddition to uracils, rearrangement with HCNO elimination, 55, 149 Alkynes, ethoxy-, cyclaoddition to hexafluoroacetone azine, 60, 32 Alkynes, perfluoro-, as heterocyclic precursors, 59, 10 Allenes... 

The presence of both electron-donor (alkoxy) and electron-acceptor (cyano or carbomethoxy) substituents at the 2- and 3-positions respectively of the pyridine ring, have unlocked its photocycloaddition chemistry, and in recent years several such reactions of these heteroarenes have been described. Previously reported additions of ethenes involved acrylonitrile derivatives, but similar processes are now described with vinyl ethers as the addend. In these systems, however, it seems that the regiochemistry of addition is very sensitive to steric influences. Thus, although the dihydroazocine (48) is formed selectively from the 3-cyano-... 

The photocycloaddition chemistry of pyridines substituted with electron-donor and electron-acceptor groups at the 2- and 3- positions continues to be exploited. The results of irradiation of such pyridines in the presence of 2-cyanofuran have now been described. The yields of the (47r+47r) cycloadducts (29) and (30), the pyridine dimer (31) and the transposition isomer (32) are dependent on the level of methyl substitution on the heteroarene and are given in Scheme 2. Other photocycloadditions to heteroarenes reported within the year include the reactions of benzodithiophene (33) with butadiyne derivatives and dimethyl acetylene dicarboxylate, giving low yields of (34) and (35) respectively, the latter from photorearrangement of the primary adduct (36). The (271+471) photocycloaddition of indoles (37) to cyclohexa-1,3-dienes (38) is sensitized by the aromatic ketones (39), and yields (14-46%) of the exo and endo isomers of the adduct (40) in ratios which are dependent on the substituents on the addends. 

Irradiation of the pyridone (182) with a tethered enyne afforded the intramolecular [2 + 2] cycloadducts (183), whereas the compound (184) led to intramolecular [4 + 4] cycloadduct (185). The resulting (185) was unstable and underwent subsequent photochemical [2 + 2] dimerization. An intermolecular [4 + 4] photocycloaddition of a pyridine with an enyne was also reported. " ... 

When the 1,3-dienes are largely or exclusively in the s-trans-conformation, such as with diene 83, pyridone 18 leads to three products, 87, 88, and 89." The two cyclobutane products 87 and 88 derive from Cope rearrangement of the highly strained [4-1-4]-photocycloaddition adducts 84 and 85, a mechanism precedented by photocycloadditions of 1,3-dienes with other aromatic species. Pyridine 89 may be formed from oxetane 86. When the 1,3-diene is delivered intramolecularly (90), the only photoproduct is 91, presumably "via an intermediate relating to 85. In contrast with the chemistry of the cis-pyridone dimers (Scheme 3), cyclobutane 91 thermally rearranges to the cyclooctadiene 92. ... 

Nagano, X, Hirobe, M., and Okamoto, X, [4Jls -I- 47ts]-Photocycloadditions of 2-methyl-s-triaz-olo[l,5-a]pyridines to pyridones, Tetrahedron Lett., 3891-3894, 1977. 

Fujii, H., Shiba, K., and Kaneko, C., Photocycloadditions of 4-methoxy-2-pyridone to olefins and synthesis of l,2-dihydrocyclobuta[c]pyridin-3(4H)-ones,/. Chem. Soc., Chem. Commun., 537-538, 1980. 

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