Monoclinic polymorph of 2,5-dide-oxy-2,5-epithio-1,3:4,6-bis-O-[(R)-phenyl-methyl-ene]-l-iditol.

The title compound C(20)H(20)O(4)S, is polymorphic. In the tetra-gonal form, the mol-ecule lies on a crystallographic twofold axis, while the monoclinic form has only approximate C(2) mol-ecular symmetry. The greatest excursion from C(2) symmetry is in the orientation of the two phenyl rings; at 100 K, one of the rings is rotated -37.