Olefins photocycloadditions

The literature of diene and polyene photochemistry provides many cases of synthetically useful reactions. As a result, certain arbitrary decisions have been made regarding what is covered in this chapter. For example, intramolecular [2 + 2]-photocycloaddition reactions of a, >-dienes can be formally included under the general rubric of diene photochemistry. However, we have chosen to restrict our discussion to dienes and polyenes which constitute a self-contained chromophore, viz. conjugated, cross-conjugated and 1,4-diene systems. Likewise, arene-olefin photocycloadditions will not be considered. These two broad classes of photoreactions have been applied extensively in synthesis, and have been the subject of recent reviews3,4. 

A schematic representation of the possible processes in arene-olefin photocycloaddition according to Leismann et al. [182] is given in Scheme 35. Increasing charge separation in the exciplex, caused by decreasing ionization potentials of the olefins, can lead to zwitterionic intermediates, which are able to form ortho adducts. Increasing solvent polarity can reduce the formation of adducts by propagating the formation of radical ion pairs. The possibility that the ortho adduct may be formed directly or via a different short-lived intermediate was not incorporated in the scheme. 

In general, the stereoelectronic influence of substituents in [2 + 2]-photocycload-dition reactions is minor, and the preferred ground-state conformation often accounts for the formation of the major diastereoisomer. Inspection of molecular models and force field calculations provide a good picture of possible transition states leading via 1,4-biradicals to cyclobutane products. The total synthesis of (+)-guanacastepene represents another recent example for the use of stereoselective intramolecular cyclopentenone-olefin photocycloadditions in natural products synthesis [32]. 

Crimmins, M.T. (1988), Synthetic Applications of Intramolecular Enone-Olefin Photocycloadditions. Chem. Rev. 88. 1453. 

Nuss, J.M., Chinn, J.P., and Murphy, M.M., Substituent control of excited state reactivity. The intramolecular ortho arene-olefin photocycloaddition, 7. Am. Chem. Soc., 117, 6801, 1995. 

Nuss, J. M., Chinn, J. P., Murphy, M. M., Substituent Control of Excited state Reactivity. The Intramolecular ortho Arene Olefin Photocycloaddition, J. Am. Chem. Soc. 1995, 117, 6801 6802. 

We took advantage of this valence isomerization method we prepared the precursor 8b efficiently by using enone-olefin photocycloaddition and subsequent ring contraction. Thermal isomerization of 8b proceeded quantitatively, giving methoxycarbonyl-substituted cyclophane lb (Scheme 3) [5]. Bis(methoxycarbonyl) derivative Ic [6] and diketo derivative le (Structures 2)... 

In the meantime, Corey, Eaton, de Mayo, and others examined intermolecular enone-olefin photocycloadditions. In 1962, Eaton reported... 

Participation of alkenyl boronates in intermolecular [2+2] enone-olefin photocycloadditions has been demonstrated with cycloalkenones. The major photoadducts with cyclopentenone resulted from head-to-head addition (Scheme 9.48) [101]. 

A fascinating new route to ( )-hirsutene involves an arene-olefin photocycloaddition of the acetate (105) to form the alcohol (106), albeit in low yield, as a mixture of three isomers. Acid-induced cleavage of a cyclopropane ring then... 

Crimmins MT (1988) Synthetic applications of intramolecular enone-olefin photocycloadditions. Chem Rev 88 1453-1473... 

Crimmins, M. T. and Watson, P. S., Stereoselective intramolecular enone-olefin photocycloadditions of 1,7-dienes model studies on the synthesis of lycopodium alkaloids, Tetrahedron Lett., 34, 199-202,1993. 

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