| 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. 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 the first part of this dissertation research program, α,β-unsaturated iminium ions are applied as surrogates for enones in intramolecular [2+2]-photocycloaddition reactions. Conjugated eniminium salts were expected to cycloadd with olefins following concerted pathways and consequently, the photocycloaddition is anticipated to proceed with the retention of both enone and olefin stereochemistry.; Naturally occurring and synthetic cyclic peptides and their analogs have been the subjects of efforts aimed at exploring conformationally defined and hydrolytically more stable polypeptide mimetics. Owing to the chemical and biological significance of these cyclic peptides, synthetic methods, which can be applied to the preparation of new targets, are still in demand. SET-Promoted photocyclization of α-silylamine-, -sulfamide-, -ether-, -thioether-tethered phthalimides has been used in the efficient synthesis of macrocyclic polyether, polythioether, polysulfonamide. The efficiency of these processes is a consequence of highly regioselective desilylation reactions of the key cation radical intermediates. In the second part of this dissertation program, the scope of this methodology is extended by its application to the synthesis of cyclic peptide analogs which will be conducted by the SET-promoted photocyclization of a group of substrates that contain N-terminal phthalimide groups as light absorbing acceptors and C-terminal α-amidosilane and α-amidocarboxylate moieties as reactive cation radical sites. |
