Pyridones 4+4 photocycloaddition

Figure 23. Application of pyridone photocycloaddition to taxol and photosubstrates 69 and 52. °...

The literature of mechanistic aromatic photochemistry has produced a number of examples of [4 + 4]-photocycloadditions. The photodimerization of anthracene and its derivatives is one of the earliest known photochemical reactions of any type97. More recently, naphthalenes98, 2-pyridones" and 2-aminopyridinium salts100 have all been shown to undergo analogous head-to-tail [4 + 4]-photodimerization. Moreover, crossed [4+4]-photocycloaddition products can be obtained in some cases101. Acyclic 1,3-dienes, cyclohexadienes and furan can form [4 + 4]-cycloadducts 211-214 with a variety of aromatic partners (Scheme 48). 

Pyridone on irradiation in concentrated solution gives the dimer (437) 2-aminopyridine behaves similarly. The acridizinium ion (see Scheme 39) like anthracene undergoes [4 + 4] photocycloaddition... 

S. M. Sieburth in Advances in Gycloaddition. The Inter- and Intra-Molecular [4+4]-Photocycloaddition of 2-Pyridones and Its Applications to Natural Product Synthesis , M. Harmata, Ed. Jai Press, Greenwich, CT, 1999, vol. 5, p. 85. 

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>. 

An intramolecular [4+4] photocycloaddition of a 2-pyridone with a furan ring yields the complex 1,5-cyclooctadiene 231 <20060L3367>. The proposed transition state conformation leading to the realized (and desired) f vsy -product is shown (Equation 110). The isopropyl group on the cyclopentane of the pyridone demonstrated stereocontrol and the... 

CONTENTS The Synthesis of Seven-Membered-Rings General Strategies and the Design and Development of a New Class of Cycloaddition Reactions, Paul A. Wender and Jennifer A. Love. Recent Advances in Diels -Alder Cycloadditions of 2-Pyrones, Benjamin T. Woodard and Gary H. Posner. The Inter- and Intramolecular [4 + 4] Photocycloaddition of 2-Pyridones and Its Application to Natural Product Synthesis, Scott McN. Sieburth. 3 + 4 Annulations Between Rhodi- J s... 

The natural propensity of 2-pyridone photodimerization to favor the head-to-tail [4+4] adducts can be reversed by the use of a tether (Sch. 42). For head-to-head 189, photocycloaddition leads to a 1 1 mixture of the cis and trans adducts [122]. For tail-to-tail isomer 192, a high proportion of the cis adduct 194 is generated, although product mixtures for both reactions change under extended irradiation times, favoring the cis isomers [123]. 

Sieburth S McN. The inter- and intramolecular [4+4] photocycloaddition of 2-pyridones and its application to natural product synthesis. In Advances of Cycloaddition. Harmata M, ed. Greenwich, CT JAI, 1999 85-118. 

Recently, many investigations have been carried out on the [4 + 4] photocycloaddition of 2-pyridones and its application to organic synthesis. Although the studied reactions involved only racemic materials, high diastereoselectivities have been obtained with chiral substrates. This makes this type of cycloaddition very promising for asymmetric synthesis [93]. 

A novel [2+2] photocycloaddition reaction was reported for the bis-pyridone 79 to afford the diazocine 24 possessing the ds yn,air-configuration (Equation 10) (see Section 14.06.3.1) <1993TL5555>. 

The photoaddition of methyl acrylate to pyridones (49) gave the three [2 + 2] cycloaddition products (50), (51), and (52).Similar observations have been reported previously.A patent has been lodged which deals with the photoad-dition of alkenes (RCH=CH2 R = H, CN, or MeC02CH2) to the pyridones (53) to yield the adducts (54) and (55). The photocycloaddition of vinyl acetate to the pyridinones (56) affords the head-to-tail adducts (57) preferentially. ... 

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-... 

Pyrex-filtered irradiation of methanol solutions of the pyridone (125) results in the formation of the (2 + 2) cycloadduct (126). This photoisomer is not, however, the primary photochemical product and the route to (126) is thought to involve (4 + 4)-photocycloaddition to yield the adduct (127). This adduct is thermally unstable and undergoes a facile Cope rearrangement to yield (126). 

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