Activation of enamine by photoexcited organocatalyst assisted singlet oxygen: synthesis of oxazoles and quinoxalines.

Herein, a novel transition-metal-free thiol-based donor-acceptor organophotocatalyst-assisted, singlet-oxygen-mediated tandem oxidative cyclization for the synthesis of substituted oxazoles in moderate-to-good yields is described. The developed method demonstrates applicability for the synthesis of various substituted quinoxalines in good-to-excellent yields. The metal-free methodology shows a practical route for the synthesis of oxazole and quinoxaline derivatives, which are privileged moieties prevalent in various biologically active compounds and natural products. To the best of our knowledge, both the thiol photocatalyst and synthesis of oxazoles by visible-light irradiation are reported for the first time.

Visible Light Driven Metal-Free Photoredox Catalyzed α-benzylation and α-oxygenation of N-substituted Tetrahydroisoquinolines: Applications to Synthesis of Natural Products.

Herein, visible light mediated organophoto redox catalysed simple and convenient method for the α-benzylation and α-oxygenation of tertiary amines is demonstrated. Synthesis of novel thiophenol based donor acceptor organophotoredox catalysts 4 a-4 d were investigated along with commercial catalyst 4-CzIPN (4 e). A diverse biologically active α-benzylated tetrahydroisoquinolines and natural products such as (±)-Norlaudanosine, (±)-laudanosine and (±)-xylopinine have been synthesized under the optimized conditions in yields ranging from from 62-91%. Exploitation of synthesized α-benzylated compound using present phtoredox catalyzed conditions gave rise to dehydyrogenative benzylic oxidation product under oxygen atmosphere which is known to display biologically and structurally important properties. Also, various N-protected tertiary amines were found to be suitable for the α-oxygenation reactions using catalyst 4 e and resulted in good yields (61-85%).