Photocycloaddition reactions stereoselectivity

Stereoselective intermolecular [2- -2]-photocycloaddition reactions of unsaturated heterocycles with formation of fused systems 98S683. 

Since then, the photocycloaddition reaction has been extensively studied and has become a powerful tool for the construction of complex polycyclic molecules. High stereoselectivities are observed in some cases. The configuration of the diradical intermediate determines the stereochemistry of the adduct [33], Typical examples... 

Photocycloaddition proceeds between allylsilane and A-methylphthalimide to yield a mixture of [2 + 2] and [4 + 2] adducts along with the allylated product291. Intramolecular cycloadditions of the vinylsilanes with the cyclopentenone moieties in 200 furnish good yields of cyclic products stereoselectively (equation 164)292. In the presence of 1,4-dicyanonaphthalene, diallylsilane 201 undergoes an intramolecular photocycloaddition reaction in an aromatic solvent to give a four-membered ring product (equation 165)293. 

Demonstration of the unique synthetic utility of the [2 + 2] photocycloaddition reaction of enones to alkenes and the success in controlling the stereoselectivity, to some extent, in the intermolecular additions (discussed above) prompted further studies and development of new synthetic applications in the intramolecular photoadditions during the last decade. In most cases that have been studied, the alkene was tethered to the cyclic enone by three carbon units or two carbons and one heteroatom. 

Photocycloaddition reactions have been used as key transformations in many organic syntheses to construct organic compounds having unique structures that are hardly accessible by other methods. However, their stereoselectivity is not necessarily high, and many efforts have been done to attain the highly regioselective and stereoselective photocycloadditions. They are discussed in terms of the electronic nature of substrates, the steric repulsions between substituents, and the conformational restrictions of intramolecular reactions. 

Intermolecular and intramolecular photocycloaddition and photoaddition to aromatic rings in the electron-donor and electron-acceptor systems were discussed in this chapter. The highly stereoselective and regioselective photocycloaddition is a synthetically useful method for the construction of polycyclic carbon-skeleton compounds, including natural products. New aspects for the stereoselective intermolecular and intramolecular photocycloaddition reactions via exciplexes in less... 

A promising tool for controlling the regio- and stereoselectivity of photocycloaddition reaction (PCA) as well as the efficiency may be provided by assembling alkenes into a supramolecular structure with pre-organization of reactants such that the spatial arrangement of molecules would be favorable for the formation of only one cyclobutane isomer in a high yield. 

Thus, the CESD are capable to form supramolecular complexes containing two dye molecules and metal cations. The self-assembly of the dimeric complexes was shown to be a unique tool for controlling regio- and stereoselectivity of [2+2]-photocycloaddition reaction. The variation of the structure of CESD would make it possible to change the supramolecular spatial structure of the dimer in a desired direction and thus to control the efficiency of interaction and stereochemistry of the final product of PC A. 

Bach, T. (2002) Stereoselective total synthesis of the tricyclic sesquiterpene (i)-kelsoene by an intramolecular Cu(I)-catalyzed [2 + 2]-photocycloaddition reaction. Synlett, 1305-1307. 

Substrates A3 (Q = O) have been employed not only as starting materials for fragmentation reactions but also to probe novel stereoselectivity concepts. The photochemical transformation of axial chirality into central chirality was achieved by Carreira et al., who employed chiral, enantiomerically pure allenes in intramolecular [2 + 2]-photocycloaddition reactions (Scheme 6.27) [79]. The reaction of enantiomerically pure (99% ee) cyclohexenone 71, for example, yielded the two diastereomeric products 72a and 72b, which differed only in the double bond configuration. Apparently, the chiral control element directs the attack at the allene to its re face. The double bond isomerization is due to the known configurational liability of the vinyl radical formed as intermediate after the first C—Cbond formation step (see Scheme 6.2, intermediate C). 

Termont, D., De Keukeleire, D., and Vandewalle, M. (1977) Regio- and stereoselectivity in [ 2 + 2] photocycloaddition reactions between cyclopent-2-enone and electron-rich alkenes. Journal of the Chemical Society, Perkin Transactions 1, 2349-2353. 

Bach, T., Bergmann, H., Grosch, B., and Harms, K. (2002) Highly enantioselective intra- and intermolecular [2 + 2] photocycloaddition reactions of 2-quinolones mediated by a chiral lactam host host-guest interactions, product configuration, and the origin of the stereoselectivity in solution. Journal of the American Chemical Society, 124, 7982-7990. 

In contrast to photocycloaddition reactions of carbonyl compounds to electron-rich alkenes (which proceed with a low degree of stereoselectivity in the case of triplet excited carbonyls), reactions with electron-deficient alkenes, such as cyanoalkenes, are, although rather inefficient, but highly stereoselective [101]. Kinetic analysis showed that these reactions involve the... 

Silylenolethers 127 were also investigated by Bach et al. in the last decade [126]. A series of photocycloaddition reactions of benzaldehyde with these substrates showed a trend in stereoselectivity which at first sight was in contradiciton to the rules described above, i.e., the thermodynamically more stable Z diastereoisomers 128 (with respect to the substituent at C-2 and C-3) were formed preferentially and the ZjE ratio increases with increasing size of the C-3 substituent (Sch. 42). 

The stereochemical outcome of a photochemical [2 + 2] photocycloaddition depends highly on the approach of the two reactants as well as the sterochemical features of the product(s). In this context stereoselective [2 + 2]-photocycloaddition reactions have emerged as powerful C-C bond forming transformations leading to useful and versatile building blocks [59,60]. Some examples of such reactions are presented below. 

Brandes S, Selig P, Bach T (2004) Stereoselective intra- and intermolecular [2+2]-photocycloaddition reactions of 4-(2,-aminoethyl)quinolones. Syn-lett 2588-2590... 

An exciplex A "] is formed by interaction of an electronically excited molecule A (or D ) with a ground state molecule D (or A). Exciplex formation is favored in nonpolar solvents. In general, an exciplex has a relatively rigid and oriented structure. Highly regio- and stereoselective photocycloaddition reactions between A and D can often be achieved via this intermediate. 

A further report of the photocycloaddition reactions of benzothiophenes has been made. The two photoproducts (199) and (200) from the benzophenone-sensitized cycloaddition of bromoethene to the 2,3-dichloro-derivative (201a) have been demonstrated to have the stereochemistry shown.The photoaddition of cis-dichloroethene to (201a) is stereoselective and affords predominantly the adduct (202). When 2,3-dibromobenzothiophene (201b) is used with bromoethane, a different reaction path is followed, yeilding (203). Linear products are also obtained with dichloroethene and (201b). [2 + 2] Photocycloaddition of furan or thiophene to the iso-oxazolines (204) affords the adducts... 

Bach, T. Stereoselective intermolecular [2+2] photocycloaddition reactions and their application in synthesis. Synthesis 1998, 683-703. 

Buhr, S., Griesbeck, A. G., Lex, J., Mattay, J., Schroeer, J. Electronic control of stereoselectivity in photocycloaddition reactions. 7. Stereoselectivity in the Paterno-Biichi reaction of 2,2-diisopropyl-1,3-dioxole with methyl trimethylpyruvate. Tetrahedron Lett. 1996, 37, 1195-1196. 

A series of 5-(2-methoxyphenyl)pent-l-enes (65), substituted at the a-, P-, or y- positions with either an OH or OSiMe3 group, undergo an intramolecular meta photocycloaddition reaction to give a product (66) having endo stereochemistry at the substituted carbon atom. The diastereoselectivity is a consequence of minimisation of steric interactions between the side chain substituent and the ortho-MQO group of the arene unit. A molecular mechanics study of the regio- and stereoselectivity of cation-dependent [2 + 2]-photo-... 

Griesbeck, A. G., Mauder, H., Stadtmuller, S., Intersystem Crossing in Triplet 1,4 Biradicals Conformational Memory Effects on the Stereoselectivity of Photocycloaddition Reactions, Acc. Chem. Res. 1994, 27, 70 75. 

The Patemo-Buchi reaction is one of the more predictable photocycloaddition reactions. Regiocontrol of the photoproduced oxetane is a function of the stepwise addition of the carbonyl chromophore to the alkene [30]. In the case of electron-rich alkenes, excitation of the carbonyl group produces a triplet species that adds to the alkene. The product regioselectivity is a result of addition that generates the most stable biradical, and the triplet lifetime of the intermediate biradical allows for substantial stereoselectivity prior to closing (see Scheme 2). Electron poor alkenes are more likely to undergo cycloaddition with carbonyl groups directly from an exciplex [31]. 

The excellent stereoselectivity of the above photocycloaddition reaction is explained by the preferred approach of the vinylogous amide from the a-face of the indole 45. The approach of the amide from the p-face 46, (Scheme 10) is disfavored because of the steric interaction of the indole aromatic ring and the R substituent alpha to the vinylogous amide. 

Reviews of the Patemo-Buchi photocycloaddition reaction for the synthesis of mono- and bi-cyclic oxetanes <00SL1699> and the stereoselective synthesis of oxetanones are available... 

Oxygen atoms in the tetrahydrofuran rings embedded in inositol-based tris(spirotetrahydrofuranyl)ionophores were useful as chelation sites towards selective alkali metal ion binding and as a result rodlike supramolecular ionic polymer frameworks were prepared and analyzed in this study <01JA4974>. The temperature and viscosity dependence of the spin-directed stereoselectivity of the Patemo-Buchi photocycloaddition reactions between 2,3-dihydrofuran and benzaldehyde were studied <01 AG(E)4684>. 

Griesbeck AG, MauderH, StadtmiillerS (1994) Intersystem crossing in triplet 1,4-biradicals conformational memmy effects on the stereoselectivity of photocycloaddition reactions. Acc Chem Res 27 70-75... 

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