2 Pyrones, photocycloaddition reactions

CONTENTS Preface, Mark Lautens. Photocyclization and Photocycloaddition Reactions of 4- and 2-Pyrones, Frederick G. West. Intramolecular [4+3] Cycloaddition Reactions, Michael Harmata. Lewis Acid Catalyzed [2+2] Cycloaddition Reactions of Vinyl Sulfides and Their Analogues Catalytic Asymmetric [2+2] Cycloaddition Reactions, Koichi Narasaka and Yujiro Hayashi. Vinylboranes as Diels-Alder Dienophiles, Daniel A. Singleton. Preparation and Exo-Selective [4+2] Cycloaddition Reactions of Cobaloxime-Substituted 1,3-Dienes, P... 

Recently, pyrone [2 + 2]-photocycloaddition reactions were used to construct macrocyclic compounds. A dipyrone was irradiated in the presences of a,(0-diolefins yielding 18- to 25-membered rings by a sequential intermolecular and intramolecular cycloaddition [146]. 

Intermolecular photocycloaddition reactions of 2-pyrones with alkenes have been much less extensively studied than those of coumarins. A typical reaction of coumarin 11 with an alkene is the formation of 12 (Scheme 4). The photocycloaddition reaction of dehydroacetic acid 13 with cyclohexene was first reported by Takeshita et al. in 1973 (Scheme 5). Irradiation of a mixture of 13 and cyclohexene leads to the formation of a diastereomeric mixture of [2+2]-cycloadducts 14, together with the usual dimer of the 2-pyrone. 

Photocycloaddition reactions of the 2-pyrones 15 with alkenes are expected to lead to the [4+2]-cycloadduct 16, the 5, 6-[2+2]-cycloadduct 17 (formed by addition to the C5-C6 double bond), and/or... 

A psoralen photoadduct to adenine was also isolated and characterized upon irradiation of a mixture of adenine and psoralen derivative, however, the latter photoadduct does not appear to be generated in cellular DNA. The psoralen adducts within nuclear DNA are formed primarily by a [2 -i- 2] photocycloaddition reaction between the 4, 5 -furan double bond of the psoralen molecule and the 5,6-double bond of thymine. Monoadducts involving the 3,4-pyrone double bond of the psoralen and the pyrimidine 5,6-double bond are also formed, but to a lesser extent (Figure 142.4). Photoisomerization can lead to 4, 5 -monoadducts being converted into 3,4-monoadducts, at least in isolated DNA. Interstrand crosslinks can be generated as the result of photoexcitation of the 4, 5 -monoadduct and its subsequent cycloaddition with the pyrone 3,4 double bond. The structures of the monoadducts and cross-links are shown in Figure 142.4. The cross-linking process depends on the structure of the psoralen derivative. Linear psoralens form cross-links efficiently with a yield, which may reach up to 50% of the overall adducts formed. 

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... 

Intramolecular [2+2]-photocycloadditions of cycUc a,P-unsaturated enones with remote double bonds have been extensively used to synthesize a variety of interesting compounds, including natural products. An analysis of the mechanism of the additions has also been carried out. 2-Pyrones having pendant enes and dienes undergo synthetically useful photocycloaddition processes to give tricyclic lactones and lactone-bridged cyclooctadienes by [2+2]-and [4+4]-cydoadditions, respectively. The photochemical reactions... 

I- 2] Photocycloadditions are the most common psoralen photoaddition reactions. Ciamician and Silber first proposed a cyclobutane structure for a photodimer of coumarin. Von Wessely and Dinjaski and von Wessely and Plaichinger found that the same reaction occurs with UV-irradiated psoralens. They obtained dimers in which the pyrone rings of both monomers are joined through a cyclobutane ring (pyrone-pyrone dimer). 

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