Flexible molecules with defined shape .3. Conformational analysis of bis(tetrahydropyran-2-yl)methanes

(R,R)-Bis(tetrahydropyran-2-yl)methane 4 along with its racemate have been synthesized. MM3 calculations suggest that the conformer 4a should be populated to ca. 85% in the conformer equilibrium. Analysis of the H-1 NMR coupling constants show that one conformer predominates by about 9:1. That this is the conformer 4a is shown by various NMR techniques, as well as by comparison of calculated with measured CD spectroscopic data. The study is extended to the methyl-substituted bis(tetrahydropyranyl)methanes 21 and 23 which show, as predicted from MM3 calculations, essentially mono-conformational behaviour.