Synthesis of diverse classes of thiahelicenes by transition metal-catalysed cross coupling reactions

Tetrathia[7]helicenes (7-THs) are an attractive class of polyconjugated ortho-fused heteroaromatic compounds, endowed with inherent chirality due to the helical shape of their π-conjugated system.1 Their unique structural and chiroptical properties have stimulated manifold studies in optoelectronics,2 catalysis,3 and biology.4 In our ongoing studies on the synthesis and functionalization of 7-TH systems, we have recently developed an innovative diversity-oriented synthesis of 7-THs exploiting transition metal-catalysed cross coupling reactions as key steps (Figure 1).In this communication we report the synthesis of novel classes of thiahelicenes 1ac starting from the key intermediates 2, from which we can obtain: i) helicenes 1a through Sonogashira coupling with terminal alkynes, followed by In- or Pt-catalysed intramolecular hydroarylation of alkynes 3; ii) helicenes 1b through palladium-catalysed annulation of 2 with internal alkynes; iii) helicenes 1c through Suzuki coupling with (hetero)aryl boronic esters, followed by oxidative photochemical cyclization of intermediates 4.