A general strategy for absolute stereochemical control in enone-olefin 2+2-photocycloaddition reactions

Enone-olefin 2+2-photocycloaddition reactions are among the most widely used photochemical processes in synthetic organic chemistry. Both inter- and intramolecular versions of these cycloadditions result in the generation of cyclobutyl ketones whose ring strain often is the driving force for a variety of secondary C-C bond cleavage and migration reactions. The mechanism of enone-olefin photocycloadditions has been thoroughly studied and the process generally follows non-concerted pathways via 1,4-biradical intermediates. As a result, both enone and olefin stereochemistry is often not preserved in these reactions. In addition, a generally applicable method is not yet available to control the absolute stereochemistry of 2+2-photocycloadditions.; In this dissertation, α,β-unsaturated iminium ions as surrogates for enones will be applied in intramolecular 2+2-photocycloaddition reactions. Conjugated eniminium salts are expected to cycloadd with olefins following concerted pathways. In addition, C₂-chiral auxiliaries will be incorporated into eniminium salts for asymmetric 2+2-photocycloadditions. The degree of facial selectivity offered by the C₂-chiral groups will be evaluated and the enantiomeric excess of cyclobutyl photoproducts obtained will be an important factor that determines the potential synthetic application of this methodology.