Tetramethylethylene, photocycloaddition with

Benzene undergoes photocycloaddition with simple olefins to produce 1,3, 1,2 and 1,4 adducts as shown below for tetramethylethylene ... 

Addition to six-membered oxygen heterocycles is also common. The photocycloaddition of 5,7-dimethoxycoumarin to tetramethylethylene has been described,269 and 4-hydroxycoumarin (326) undergoes facile addition to cyclohexene on direct irradiation to give the cyclobutane (327)270 analogous additions to a variety of other alkenes have been reported, and the cycloaddition of 4-methoxycoumarin to 2-methylpropene has been employed in a synthesis of l,2-dihydrocyclobuta[c]coumarin.271 Photoaddition of the 1,2-bisenol lactone (328) to tran.s-stilbene yields propellane (329),272 and [ 2 + 2] cycloaddition is observed along with other competing photoreactions on irradiation of chromone in the presence of alkenes.273... 

Electron-rich alkenes are often used as addends for this type of cycloaddition. As described by R. Neier a cyclic P-amido cyclohexenone (compare with the open-chain substrate before) undergoes clean [2+2] cycloaddition to tetramethylethylene. Additionally a useful method for the enhancement of diastereomeric purity is described. Many starting materials, which are used for [2+2] photocycloaddition reactions, are protected enols of... 

Another side route is photocycloaddition of stilbene to olefins as studied by Chapman and Lura [467], A quantum yield of 0.54 (313 nm in n-hexane) has been reported for 4 M olefin (e.g., tetramethylethylene). Cycloaddition may occur by reaction of the excited trons-stilbene with the olefin or from an excited ground state complex with electron-deficient alkenes (e.g., fumaronitrile) [468,469]. 

The photocycloaddition of thiobenzophenone to acrylonitrile at 366 nm is known to give the thietan (130). It has now been shown that the reactive state of the thione under these conditions is the second excited (tt, n ) singlet, and that the 1,3-dithian (131) is an intermediate in this reaction. " In contrast to earlier findings, irradiation at longer wavelengths (>500 nm) has been found also to result in product formation. The heterocyclic compounds (132) and (133) were the main products in the latter photolysis reaction, and their formation was interpreted in terms of an intermediate lowest triplet state of thiobenzophenone. The irradiation of thiobenzophenone in tetramethylethylene solution at -78 C afforded two 1 1 adducts, (134) and (135), respectively. Various thietans were the products in the photocycloaddition reactions of some aromatic thioketones with tetramethylallene, 2,4-dimethylpenta-1,3-diene, dimethyl fumarate, and dimethyl maleate, respectively. The nitrile function has also been found to be susceptible to addition by the excited thione group. Thus several alicyclic and aromatic thioketones, on irradiation in acetonitrile solution, have been converted into N-thioacylketimines (136), probably via an intermediate 1,3-thiazetine (137). Two recent papers by de Mayo and his... 

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