2-Pyridones photoreactions

Pyridone photochemistry has been extensively studied because of the important and interesting P-lactam and cyclooctadiene products (see Scheme 2), the dominance of a short-lived singlet excited state in most of the photochemistry, interesting substituent effects, and an absence of solvent effects. A number of reviews have included aspects of pyridone photochemistry - but no comprehensive discussion has been published. The breadth of 2-pyridone photoreactivity stands in contrast to its thermal stability, reluctant partners in Diels-Alder reactions, for example. " The 4-pyridones have provided less useful photochemical reactivity and, consequently, have been the subject of fewer studies. 

Since photochemical reactions in inclusion compounds have been described in one chapter of the previous book [1], enantioselective (3-lactam formation reactions are summarized in this present chapter as a typical application of the inclusion technique for enantioselective photosynthesis. In addition, as a representative enantioselective single-crystal-to-single-crystal photoreaction, the photodimerization reactions of coumarin and thiocoumarin in their inclusion compound with a chiral host are also described. Furthermore, a host-catalyzed photodimerization reaction of chalcone and 2-pyridone in the solid state is also added to this chapter as a unique example of the application of inclusion techniques to selective photoreaction. 

Photodimerization of 2-pyridone (46) in the presence of the 2,2/-biphenyldi-carboxylic acid host (45) also proceeded via a catalytic process. First, irradiation of the 1 2 inclusion complex of 46 and 45 in the solid state gave the trans-anti dimer (47) in 92 % yield [27], The mechanism of this stereoselective photoreaction was investigated through X-ray analysis of this complex. In the complex, two 46 molecules are arranged in ideal positions for yielding 47 by dimerization [27], Secondly, a catalytic dimerization reaction of 46 was carried out. Photoirradiation for 20 h of a 1 4 mixture of powdered 45 and 46 under occasional mixing in the solid state gave 47 in 81 % yield. These data clearly show that molecules of... 

Photoreaction of Tropolone Alkyl Ether, Cycloocta-2,4-dien-l-one and Pyridone... 

Photoreaction of a-pyridones give [2 + 2] dimers. The photolysis of N-methyl-a-pyridone in a dilute solution gives a bicyclic isomer, which has a mass spectrum showing m/e 52, perhaps due to cyclobutadiene (Scheme 30).64... 

Several accounts of (4ti + 4tt) intramolecular photodimerisation of tethered pyridones have appeared in recent years and the product from (20), having five stereogenic centres, is reported to have application towards a synthesis of taxanes (Sieburth et al.). A number of examples of intramolecular (2ti + 2k) photocycloadditions involving pyrimidone units have been described within the year. The bichromophore (21) is reported to photoreact with chemo-, stereo- and regio-... 

There are many kinds of six-membered heterocyclic compounds but only few valence-bond isomers have been isolated. Thus, we will first describe the photoreactions of pyrones and pyridones and subsequently those of the usual heteroaromatic compounds with increasing number of hetero atom s. 

The photoreaction of concentrated pyridone solutions usually gives dimers whereas in dilute solution, an isomer of the Dewar type is formed (130)139>. 

Photoreactions of Pyridones - The pyridone derivative (122) undergoes ready (2 + 2) head-to-head photochemical addition when irradiated in acetone. The reaction appears to be very facile and requires only 16 minutes irradiation at 5°C to give a 79% yield of the adduct (123) which has been used as the starting material in a total synthesis of (-)-perhydrohistrionico-toxin (124). 

Photoreactions of Pyridones. - Enantioselective cyclizations have been carried out using the chiral molecule (91). Formation of inclusion compounds of this with pyridones (92), followed by irradiation, afforded the optically active lactams (93) with an ee of 91-99%. Irradiation of the pyridones (94) in water leads to a racemic mixture of the cyclized product (95). However, irradiation of... 

An example of efficient chiral induction in a photoreaction is provided by the irradiation of inclusion compounds of pyridones (11) with the chiral host (10) (Tanaka et al, Chapter 2). This process afforded optically active p-lactams (12) with ee in the range 91-99%. The intramolecular 2 4-2 alkene photocycloaddition of (13) catalysed by Cu(I) afforded the tricyclic compound (14) in 89% yield,... 

It is therefore interesting to note that the photoreaction of pyridone 234, which can be regarded either as an electrocyclic process involving 47t... 

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%. 

Figure 3. Chloropyridone 7 undergoes an unusual photoreaction for 2-pyridones. ...

Photoreaction of 6-chloro-2-pyridone 7 in methanol is the unique example of an apparent six-electron electrocyclic rearrangement that produces an intermediate ketene/chloroimine 8 which undergoes loss of HCl and addition of methanol to yield a mixture of three isomeric unsaturated nitrile esters 9 and 10. °... 

Photo-[4+4] cycloaddition of 2-pyridone is not restricted to reaction with other pyridones. Sato, Ikeda, and Kanaoka found that cyclic and acyclic-1,3-dienes photoreact with 2-pyridones (Figure 8), ° reactions similar to the [4+4] photocycloaddition of 1,3-dienes with other aromatic molecules. Cyclopentadiene is abetter substrate than cyclohexadiene, as it is for the Diels-Alder reaction. A mixture of trans (24) and cis (25) isomers is formed in all cases. Use of acyclic... 

With the possible exception of Sharp and Hanunond, who isolated but did not identify other dimeric products from the photoreaction of 2-pyridones in 1972, it was not until 1978 that the first attempt was made to isolate and identify isomers other than the heretofore discussed traits head-to-tail photodimers like 3. In a detailed study, Nakamura investigated the chromatographically isolable products and their yields as a function of solvent and concentration effects. These results, partially summarized in Figures 2 and 10, were the first to show the significant levels of the cis head-to-tail isomer 28 that was formed in all solvents. This study remains the only instance where regiochemical fidelity has been violated and head-to-head products 29 and 30 have been isolated. This occurs only in aqueous solution. It is noteworthy that the three very different solvents all give a similar mixture of cis and trans head-to-tail 3 and 28. 

During nearly forty years of study, the photocycloaddition of 2-pyri-dones has consistently exhibited versatile and reliable [4+4] reactivity. When dimerization is not desired, intramolecular reactions efficiently steer two pyridones to react. Alternatively, intermolecular cross reaction with an excess of another four-electron reactant can be extended to other heterocycles and to simple 1,3-dienes. It is perhaps surprising how few photoreactions of 2-pyridone fail to yield [4+4] products. Failure of the [4+4] reaction can result from a tethering unit that prevents the normal head-to-tail reactivity and introduces strain (Figures 14 and 22) or from steric hindrance caused by excessive substitution at the reacting carbons (Figure 23). A 4-alkoxy substrate shuts down the normally dominant photodimerization reaction and provides opportunities for other [4+-2] or [4+4] reactions (Figures 6 and 7). 

SCHEME 15 Photoreaction of 2-pyridone with aliphatic and heterocyclic amines/ ... 

Photoreaction of 2-pyridone with aliphatic and aromatic amines leads to addition of the amines at C4 and C6 to give mixtures of dihydropyridone products (Scheme 15). All of these reactions appear to derive from a single electron transfer process. Tertiary aliphatic amines such as triethylamine 150 yield pyridones 151 and 152 (2 1) plus the reductively coupled pyridone dimer 153. Pyrrole 154 leads to a similar mixture of 4- and 6-substituted dihydropyridones 155 and 156 (1 1). Dimethylpyrrole 157 and indole 158 lead to analogous products of 4- and 6-substition of the pyridone (at C3 of the pyrrole 157). N-Methyl pyrrole 159, however, does not yield photoproducts. When the N-methyl pyrrole is alkyl substituted at C2 (160), addition to the pyridone yields 161 and 162 (1 1). ... 

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