Conformational Analysis of Sulfur-Containing Heterocycles, Part I. Synthesis and Structural Determination of Diastereomerically Pure 4,6-Bis-(phenoxymethyl)-1,2,5-trithiepanes

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Some diastereomerically pure 4,6-bis-(phenoxymethyl)-l,2,5-trithiepanes were synthesized and unambiguously assigned. Their conformational properties and dynamic behavior were investigated by various NMR spectroscopic methods and quantum-chemical calculations at the HF/6-31G* level. The ground states of these compounds proved to be twist-chairs. A ring interconversion can occur in the meso-isomers as well as in the (±)-isomers. This interconversion can be described as a simultaneous inversion of the disulfide bridge. In the case of the meso-isomers, both ground states are mirror images of each other and the transition state is a highly symmetrical chair. The barrier heights of interconversion were determined to be in the range of 50 kJ/mol by variable-temperature NMR measurements. The ground states as well as the transition state of the (±)-isomers were found to be nonsymmetrical. However, those dynamic processes that are fast with respect to the NMR time scale lead to averaged NMR spectra at room temperature. A further dynamic process found through the quantum-chemical calculations is a “flapping” of the meth-ylene groups of the rings. The energy barrier of this flapping was calculated to be very small (< 20 kJ/mol) and could not be observed by low-temperature NMR measurements.

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