Bis(1,3-dioxan-5-yl)ethanes 4, 30, 32, and 33 were synthesized in order to verify the high conformational preferences of their flexible backbones predicted by force-field calculations. Conformationalanalysis based on 3JH,H coupling constants gave results different from expectations based on the force-field calculations. The MM3* force-field of MACROMODEL was apparently not parametrized suitably to