N.m.r. studies on 1,3:2,5:4,6-tri-O-methylene-D-mannitol (in thionyl chloride from ambient temperature down to –82°) and on 1,3:4,6-di-O-benzylidene-2,5-O-methylene-D-mannitol indicate that the 2,5-O-methylene protons are magnetically equivalent. This may be interpreted in two ways. Either the 1,3-dioxepan ring is undergoing a fast ring-inversion process or it exists predominantly in the conformationally stable twist-chair (TC) or twist-boat (TB) forms, in which the 2,5-O-methylene protons are magnetically equivalent on account of C2 symmetry. Conformational analysis predicts that the conformation containing the TC form is more stable than those containing the TB or chair (C) forms. It is also proposed that the TC form is the most stable for the 1,3-dioxepan ring of 1,3:4,6-di-O-methylene-2,5-O-isopropylidene-D-mannitol.
