Regiochemistry photocycloaddition

Excellent regioselectivity and stereoselectivity has been achieved in each photocycloaddition mode [45 48], Regiochemistry and stereochemistry in the meta process is decided by the orientation of the addends in the exciplex and by stabilization of biradical intermediates having a change transfer (CT) character (6) by the substituents on the arene. Intermolecular meta cycloaddition of arenes with cycloalkenes proceeds with endo selectivity (7) (Scheme 5). In the ortho-process, selectivities can be controlled mainly by the substituents on the reactants. 

Al-Jalal reported that the photocycloaddition of l-cyano-4-methoxynaph-thalene with alkenes commonly occurs at the 1,2 bonds of the naphthalene ring [126,131], However, when 2-cyano-6-methoxynaphthalene (52) was treated with 2-chloroacrylonitrile, ( )-ethoxyacrylonitrile, or ( )-cinnamonitrile, the (2 + 2) cycloadducts showed a different regiochemistry [132,133] (Scheme 21). Irradiation of 52 in the presence of ethyl vinyl ether or ( )-dichloroethene did not give a cross-adduct, but gave a photodimer of 52. 

To clarify the effects of linking chains, Gilbert et al. investigated the photocycloaddition of 163-180, which produce various types of adducts [231,234-237] (Scheme 53). In some cases, not only the meta addition but also ortho and para addition and ene reaction took place. The observed regiochemistry was rationalized in terms of the stabilization of zwitterionic intermediates and the steric constraints. 

Numerous examples of [ 2 -I- 2] photocycloaddition of five-membered oxygen-containing heterocycles to alkenes have been reported, many of which have useful synthetic applications. The regiochemistry and stereochemistry of each adduct has not always been fully established. The lactone (311) undergoes photoaddition to cyclopentene to afford the cis,anti,cis adduct (312), whereas three stereoisomeric adducts were obtained with cyclohexene. Analogous additions of substituted lactones to alkenes have been employed in the syntheses of ( )-dihydrofomannosin acetate and cyclobutane analog of chrysanthemumic acid. The photoaddition of... 

The effect of substituents on the regiochemistry of the intramolecular meta photocycloaddition of the l-(3-butenyl)indanes (49)-(54) has been examined by Keese and coworkers.In the methoxy-substituted compounds (49)-(51) meta addition of the side chain alkene occurs across the 1,5-positions of the benzene ring while for (52)-(54) addition occurs across the 1,3-positions. 

The presence of both electron-donor (alkoxy) and electron-acceptor (cyano or carbomethoxy) substituents at the 2- and 3-positions respectively of the pyridine ring, have unlocked its photocycloaddition chemistry, and in recent years several such reactions of these heteroarenes have been described. Previously reported additions of ethenes involved acrylonitrile derivatives, but similar processes are now described with vinyl ethers as the addend. In these systems, however, it seems that the regiochemistry of addition is very sensitive to steric influences. Thus, although the dihydroazocine (48) is formed selectively from the 3-cyano-... 

Stereochemistry of enone photocycloaddition is more complicated than regiochemistry. Considering the stereochemistry at the four carbons of the newly formed cyclobutane ring, eight stereomers can result from each regiomer. However, not all of the possible products are usually obtained. Structure I represents a typical photoadduct from a cyclic enone and an acyclic alkene5. 

He found that this is also true in the [2 + 2] photocycloaddition between 1,1-disubstituted silyl enol ethers (e.g., 75a) [83a] and benzaldehyde, as in Scheme 43. The regiochemistry is largely determined on the basis of the most stable biradical, which, before closing, arranges itself so that the bulkiest substituents are trans to each other. Hence, in the cases of 1,1-disubstituted and trisubstituted silyl enol ethers, sterics is the determining factor in the stereoselectivity of the Patem6-Buchi reaction. 

The intermediate 37 can be viewed as arising from interaction of the triplet excited state of the indole with the alkene partner so as to form the most stable triplet 1,4-biradical intermediate possible. Alternatively, 37 may arise from attack of the 2-position of the triplet-excited indole on the less-substituted, and hence less sterically hindered terminus of the alkene. Either way, the preferential formation of 37 means that with monosubstituted alkenes, the photocycloaddition reaction with AT-acylin-doles is regioselective, while with unsymmetrically 1,2-disubstituted alkenes the reaction will be nonregioselective, as was seen for the reaction between N-benzoylindole and methyl 3-methyl-2-butenoate shown in Scheme 19. Should it be desired, the opposite regiochemistry to that normally obtained with monosubstituted alkenes can be induced by tethering the alkene to the indole [52] the reaction of 1,2-disubstituted alkenes can be made regioselective in the same way [53]. Examples of this are shown in Schemes 20 and 21. 

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