Conformationnal equilibrium of glucides on the level of liaisons sp 2 -sp 3 C-C. IV. Hybrid furans sp 2 in C(3)

The conformation of 34 branched-chain unsaturated sugars, prepared by reacting different Wittig reagents with a series of 1,2-O-isopropylidene-furannosul-3-oses, has been studied by PMR spectroscopy. The position of each compound in the flexible conformational cycle has been established by using the known stereo-dependence of the allylic coupling constants and checked with the other parameters of the spectra. An ‘isopropylidenic’ ⁴J_2,4 coupling constant, present in all compounds whose H—C(4) is endo and absent in their C(4)-epimers has proved useful for configurational assignment at C(4).