5 years ago

Synthesis of Long Oxahelicenes by Polycyclization in a Flow Reactor

Synthesis of Long Oxahelicenes by Polycyclization in a Flow Reactor
Miloš Buděšínský, David Šaman, Michal Šámal, Jiří Rybáček, Jana Vacek Chocholoušová, Maria Neumeier, Ladislav Sieger, Miroslava Tobrmanová, Lucie Bednárová, Jaroslav Vacek, Florence Szydlo, Ivo Starý, Jindřich Nejedlý, Christophe Coudret, Irena G. Stará
A series of oxahelicenes composed of ortho/meta-annulated benzene/pyridine and 2H-pyran rings were synthesized on the basis of the cobalt(I)-mediated (or rhodium(I)- or nickel(0)-mediated) double, triple, or quadruple [2+2+2] cycloisomerization of branched aromatic hexa-, nona-, or dodecaynes, thus allowing the construction of 6, 9, or 12 rings in a single operation. The use of a flow reactor was found to be beneficial for the multicyclization reactions. The stereogenic centers present in some of the oligoynes steered the helical folding in such a way that the final oxa[9]-, [13]-, [17]- and [19]helicenes were obtained in both enantiomerically and diastereomerically pure form. Specifically, the oxa[19]helicenes beat the current record in the length of a helicene backbone. Single-molecule conductivity was studied by the mechanically controllable break-junction method with a pyridooxa[9]helicene. Think big: Oxahelicenes comprising up to 19 fused rings (see example) were synthesized by multiple cobalt(I)-mediated cycloisomerization reactions of oligoynes, most efficiently in a flow reactor. The stereogenic centers in enantiomerically pure substrates steered the diastereoselective polycyclization of the oligoynes. Single-molecule conductivity was studied in a pyridooxa[9]helicene by the break-junction method.

Publisher URL: http://onlinelibrary.wiley.com/resolve/doi

DOI: 10.1002/anie.201700341

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