title: On the origin of diastereoselectivity in [2 + 2 + 2] cycloisomerization of chiral triynes: controlling helicity of helicene-like compounds by thermodynamic factors.
abstract: Diastereoselective CoI-mediated [2 + 2 + 2] cycloisomerization of CH(3)O-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 degrees 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 (1)H-(1)H correlations in ROESY (1)H NMR spectra.
authors.affiliation: Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic, Flemingovo nAm. 2, 166 10 Prague 6, Czech Republic.
authors.forename: Petr
authors.initials: P
authors.lastname: Sehnal
authors.forename: Zuzana
authors.initials: Z
authors.lastname: Krausova
authors.forename: Filip
authors.initials: F
authors.lastname: Teplý
authors.forename: Irena G
authors.initials: IG
authors.lastname: Stara
authors.forename: Ivo
authors.initials: I
authors.lastname: Starý
authors.forename: Lubomír
authors.initials: L
authors.lastname: Rulísek
authors.forename: David
authors.initials: D
authors.lastname: Saman
authors.forename: Ivana
authors.initials: I
authors.lastname: Císarova