2 Pyrones, photocycloaddition

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... 

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... 

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]. 

Keywords double [4+2]photocycloaddition, 4-acyloxy-2-pyrone, maleimide... 

Obata. T. Shimo, T. Yasutake. M. Shinmyozu, T. Kawaminami, M. Yoshida. R. Somekawa, K. Remarkable interaction effects of molecular packing on site- and stereoselectivity in photocycloaddition of 2-pyrones with maleimide in the solid state. Tetrahedron 2001, 57. 1531 -1541. 

Photocycloaddition between di- or tri-2-pyrones and benzophenone in the solid-state gives the corresponding oxetanes (1 1 and/or 1 2 adducts)... 

Obata, X, Shimo, T., Yoshimoto, S., Somekawa, K., and Kawaminami, M., Peri-, site- and stereo-controUed photocycloaddition of 4-methoxy-6-methyl-2-pyrone with maleimide induced by the hydrogen bond and CT stacking in the sohd state, Chem. Lett., 181—182,1999. 

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... 

The peri-, site-, and regioselectivities in the photocycloadditions of the triplet 2-pyrones with alkenes are confirmed to be mainly controlled by the interaction between frontier molecular orbitals (FMOs), calculated using PM3-CI level (Figure 82.1 and Table 82.2). The regioselective [4+2]-cycloadducts 16 and site-selective [2+2]-cycloadducts 17 across the C5-C6 double bonds of the 2-pyrones with electron-poor alkenes are deduced to be formed by the preferred HSOMO-LUMO interactions and via more stable biradicals by C6-CP bonding (Scheme 13). On the other hand, the other kinds of regioselective [4+2]-adduct 21 and the [2+2]-adducts 18 across the C3-C4 double bonds of the 2-pyrones with electron-rich alkenes are inferred to arise from LSOMO-HOMO interactions and via biradicals by C3-CP bondings. Selectivities between 16 and 17, and 18 and 21 are also proposed to be influenced by the SOMOs of the biradicals. 

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... 

Figure 82.5 was reprinted from Tetrahedron, 58, T. Shimo, T. Uezono, T. Obata, M. Yasutake, T. Shinmyozu, and K. Somekawa, x-ray and MO analysis of highly stereoselective solid-state photocycloadditions of 2-pyrones with maleimide, p. 6114, 2002, with permission from Elsevier Science. 

Shimo, T., Yamasaki, S., Date, K., Uemura, H. and Somekawa, K., Site-selective photocycloadditions of 2-pyrones with electron-poor olefins and the derivation from the cycloadducts, /. Heterocyclic Chem., 30, 419, 1993. 

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