Synthesis of (E,E)-Thiacyclodeca-4,7-diene and of Its 3-Methyl Derivative from D-Mannitol. Stereochemical and Conformational Behavior

Article abstract of DOI:10.1021/jo00139a031

(E,E)-1,4-Cycloalkadienes are conformationally chiral species.The title compound, 1, a ten-membered (E,E)-1,4-diene, has been synthesized by way of a reaction sequence where the configurations of C₃ and C₄ of D-mannitol, an enantiomerically pure starting material, were carried intact until the penultimate step.The last step was a Whitham cycloelimination of a 2-phenyldioxolane derivative, a reaction known to be stereospecific, which ought to have yielded optically active 1.The material, however, had no appreciable optical activity, indicating a fast enantiomerization process is taking place.A dynamic 13C NMR study of the 3-methyl derivative, 14, indicates the barrier for epimerization is 11.0 kcal/mol.The enantiomerization of 1 must then have a barrier of approximately this height, too low for optical activity to be observable.The conformational motions that may bring about enantiomerization have been considered, along with another motion responsible for 13C NMR signal averaging.Low-temperature experiments indicate ΔG(excit.) is less than or equal to 6 kcal/mol for this latter process.