On the Origin of Diastereoselectivity in [2 + 2 + 2] Cycloisomerization of Chiral Triynes:  Controlling Helicity of Helicene-like Compounds by Thermodynamic Factors

Diastereoselective Co^I-mediated [2 + 2 + 2] cycloisomerization of CH₃O-substituted optically pure aromatic triynes to obtain nonracemic functionalized helicene-like compounds (comprising a penta-, hexa-, and heptacyclic helical scaffold) was studied. The stereochemical outcome of the reaction at 140 °C using CpCo(CO)₂ was controlled by thermodynamic factors yielding diastereomeric ratios up to 91:9. Using CpCo(ethylene)₂ at room temperature, a kinetic control took place leading to the loss of stereoselectivity. Barriers to epimerization for selected helicene-like compounds were measured indicating their lower configurational stability in comparison to the parent carbohelicenes. Free energy differences between corresponding pairs of diastereomers (calculated at the DFT B3LYP/TZV+P level) were in excellent agreement with the experimental data and allowed for the prediction of the stereochemical outcome of the reaction. An optically pure hexacyclic helicene-like alcohol was prepared on a multigram scale. Its X-ray structure confirmed the previous helicity assignments being based on ¹H−¹H correlations in ROESY ¹H NMR spectra.