Monoclinic polymorph of 2,5-dide­oxy-2,5-epithio-1,3:4,6-bis-O-[(R)-phenyl­methyl­ene]-L-iditol

The title compound C₂₀H₂₀O₄S, is polymorphic. In the tetra­gonal form, the mol­ecule lies on a crystallographic twofold axis, while the monoclinic form has only approximate C₂ mol­ecular symmetry. The greatest excursion from C₂ symmetry is in the orientation of the two phenyl rings; at 100 K, one of the rings is rotated -37.2 (3)° and the other by 46.9 (3)° from their symmetric (tetra­gonal) positions. There are only minor differences in the three-ring nucleus; the best mol­ecular fit of the tetra­gonal and monoclinic forms, both at 100 K and excluding phenyl rings and H atoms, shows an r.m.s. deviation of 0.066 Å. Both forms have the same absolute configuration.