Cyclophanes. 14. Synthesis, Structure Assignment, and Conformational Properties of (2,5)Oxazolo- and Thiazolophanes

Article abstract of DOI:10.1021/ja00380a022

The first synthesis of cyclophanes containing aromatic nuclei with two heteroatoms is described.A Hoffman pyrolytic route was used.Two isomeric <2.2>(2,5)oxazolophanes (6a and 6b) and two isomeric <2.2>(2,5)thiazolophanes (13a and 13b) were isolated.In both cases the isomers were found to have the anti-anti and the anti-syn structure.The assignments were made by spectral analysis and variable-temperature nuclear magnetic resonance spectroscopy in the oxazolophane case and on the basis of NMR spectral comparisons between normal (13a and 13b) and deuterated derivatives (14a and 14b) in the thiazolophane case.While the aromatic nuclei in the thiazolophanes were found to be conformationally rigid on the NMR time scale, those in the oxazolophanes were found to be conformationally mobile with a rotational barrier of 17.8 kcal/mol.