New supramolecular host systems. 1. A structural and conformational study of the 1,3,5,7-tetraoxadecalin core system

This is the first in a series of studies of new host systems based on the 1,3,5,7-tetraoxadecalin (TOD) core moiety. The parent molecule (2) and its 2,6-di(trans-styryl) derivative (5) were studied in the crystal (low temperature X-ray diffraction, electron density, geometry) and in solution (static and dynamic NMR). The relative energies from the solution studies and the structural parameters from the X-ray analysis were compared with those from a ring-fragment analysis and with structural data of 1,3-dioxanes and 1,3,5,7-tetraoxadecalins which were retrieved from the Cambridge Structural Database. Molecular mechanics calculations using MM2 and MM3 performed poorly in reproducing relative energies and structural parameters. This is attributed to stereoelectronics in the 1,3,5,7-TOD system.