Regioselective and Stereoselective [2 + 2] Photocycloadditions

This chapter deals with tethered systems, intermediate stabilization, and steric template methodologies. Significant work has also been published in the area of regioselectivity and/or stereoselectivity control via crystal lattice constraints. Control of solution photochemistry has been demonstrated with use of templates, tethering of reagents, and substituent stabilization to affect the regioselectivity or stereoselectivity. Photocycloaddition between an excited state enone and a ground state enone can have different selectivity than the cycloaddition between the singlet excited state alkene and a ground state alkene. Cycloaddition with enone reactants has been studied using cyclopentenone or cyclohexenone derivatives in order to avoid side reactions resulting from cistrans isomerization of the alkene. Reaction conditions such as solvent, reaction temperature, and concentration of reagents may influence regioselectivity of the enone-alkene cycloaddition. Numerous studies have been conducted on the intramolecular photoaddition of cyclic enones with tethered alkenes connected at different positions of the cyclic systems.