Pyridones, cycloadditions photochemical

Hoomaert has studied Diels-Alder reactions of pyridine oquinodimethane analogs generated from functionalized o-bis(chloromethyl)pyridines <96T(52)11889>. The photochemical cycloaddition of 2-alkoxy-3-cyano-4,6-dimethylpyridine with methacrylonitrile gives a bicyclic azetine, 6-alkoxy-3,5-dicyano-2,5,8-trimethyl-7-azabicyclo[4.2.0]octa-2,7-diene, in moderate yield <96CC1349>. Regiospecific hydroxylation of 3-(methylaminomethyl)pyridine to 5-(methylaminomethyl)-2-(17/)-pyridone by Arthrobacter ureafaciens has been reported <96MI173>. 

H-stacking interactions have also been exploited to orientate olefinic moieties in a geometry suitable for photochemical cycloaddition reactions, and have been invoked by Coates et al. to explain the photodimerization and photopolymerization of mono- and diolefins carrying phenyl and perfiuorophenyl groups [43]. Matsumoto et al. reported the photodimerization of 2-pyridone in co-crystals with naphthalene-substituted monocarboxyhc acids, where the stacking of the naphthalene rings provides carbon-carbon distances appropriate for [4+4] cycloaddition [44]. 

FMO calculations using PM3-C1 were used to investigate the regioselectivities obtained by the photochemical reactions between 2-pyridone and pcnta-2,4-dienoate.46 The hard and soft acid-base principle has been successfully used to predict product formation in Patemo-Buchi reactions.47 The 2 + 2-photo-cycloaddition of homobenz-valene with methyl phenylglyoxylate, benzyl, benzophenone, and 1,4-benzoquinone produced the corresponding Patemo-Buchi products.48 The photo-cycloaddition of acrylonitrile to 5-substituted adamantan-2-ones produces anti- and svn-oxetanes in similar ratios irrespective of the nature of the 5-substituent49... 

A phase-selective photochemical reaction of 2-pyridones is observed. Irradiation of 225 in benzene gives mainly rearrangement products 226, whereas, in the solid state, [4+4] photocycloaddition to the photodimer 227 occurred in quantitative yield (Scheme 39) <20040L683>. The stereochemistry of the photodimer was exclusively the trans- /+configuration, as shown. This is presumably due to Jt-rt-stacking and dipole-dipole interactions between the pyridones. Intermolecular photocycloaddition of 2-pyridone mixtures can be selective and lead to useful quantities of [4+4] cycloaddition cross-products <1999JOC950>. 

Intramolecular photodimerization of 2-pyridones provides a series of primary photoproducts, the structures of which were a function of the chain length comecting the two reactive centers." The initially formed dimers were characterized as being derived from a photochemically allowed [2 + 2] cycloaddition process in each case examined. A particularly interesting example from this study in terms of [4 + 4] cycloaddition chemistry is the conversion of dipyridone (114) into Ae net [4 + 4] dimer (115) via a facile thermal rearrangement. " Further irradiation of the [4 + 4] species (115) produced a second [2 + 2] adduct that was isomeric with the initially formed metastable species. 

Nitrogen-containing heterocyclic enone systems also reacted with allene to give (2 + 2)-cycloadducts, as was shown in alkaloid synthesis.178 As the keystep in the synthesis of an annotinine derivative, allene was added to 157 and the adduct 158 was obtained in quantitative yield. Various uracils have been modified by photochemical (2 + 2)-cycloaddition with olefins, e.g., vinylene carbonate,17,18° vinyl ethers, vinyl acetates, and (cetene acetals yielded 159.181 Very recently the photochemical addition of cyanoethylenes to 2-pyridones has been observed to yield mixtures of tetrahydroazocin-2-ones (160) and (2 + 2)-cycloadducts (161).182... 

Of special interest are the photochemical intramolecular (2 + 2)-cycloadditions of 2-pyrones and 2-pyridones because the resulting 2-oxa- or 2-azabicyclo[2.2.0]hex-5-en-3-ones are precursors of cyclobutadiene and stable complexes of cyclobutadiene.12,238-261 In 1964 Corey and Streith238 reported the photochemical conversion of 2-pyrone (222) and N-methyl-2-pyridone (223) into the corresponding bicyclic isomers (224 and 225) at —20°. More recent studies performed at a temperature as low as 8°K with 2-pyrones in a matrix of argon or nitrogen showed that the intramolecular (2 + 2)-cycloaddition is only a minor photochemical pathway.12,260-262 The major reaction proved to be... 

Photoreactions of Pyridones - Irradiation of 1-benzyl-1,4-dihydronicotina-mide (161) with the malonate derivative (162) affords a variety of products resulting from debromination and dimerisation. The dihydropyridine derivatives (163) are photochemically reactive in the solid phase. The formation of the products by irradiation has been shown to be a two step process affording the (2+2)-cycloaddition product (164) in the first step. Secondary irradiation of (164) then gives the cage compounds (165) in yields greater than 90%. 

Photosubstrate 69 was an intriguing and optimistic approach with all but two carbons of the tetracyclic taxol target. Unfortunately, this bis-2-pyridone did not photocyclize to 70 but only slowly decomposed photochemically. We speculated that the obstacle to this cycloaddition was the requirement that four fiilly substituted sp carbons be created in a single step. Molecular modeling reinforced this view. ... 

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