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

Abstract

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%).

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

“…Initially, the reaction was probed for its possible involvement of radical species, employing 2.0 equiv. of 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO) 24 or 2,6-di-terbutyl-4-methylphenol (BHT) 25 as radical scavengers under the established conditions. The formation of product 3a from both experiments reveals the absence of radical participation (Scheme 2).…”