Monoclinic polymorph of 2,5-dideoxy-2,5-epithio-1,3:4,6-bis-O-[(R)-phenylmethylene]-l-iditol1

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 C 2 molecular 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 −37.2 (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.

The title compound C 20 H 20 O 4 S, is polymorphic. In the tetragonal form, the molecule lies on a crystallographic twofold axis, while the monoclinic form has only approximate C 2 molecular 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 À37.2 (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.

Comment
The tetragonal form of the title compound (I), C 20 H 20 O 4 S, was determined at 295 K (Rao et al., 1993, hereinafter P4 295 ).
We re-examined the tetragonal form at 100 K in our laboratory (hereinafter P4 100 ) and determined the absolute configuration. These data are deposited at the Cambridge Crystallographic Data Centre (Allen, 2002), CCDC 851380 (Gibson et al., 2011). There is a 2.8% volume decrease upon lowering the temperature from 295 K to 100 K but the molecular symmetry is C 2 at both temperatures and there is no significant change in molecular structure. The best molecular fit (Gould et al., 1988) of all non-H atoms in P4 295 and P4 100 yields δ r.m.s. = 0.022 Å, while excluding the phenyl groups yields δ r.m.s. = 0.011 Å.
In the monoclinic form at 100 K (P2 100 ) the molecular symmetry is C 1 . The molecular volume of P2 100 (435.2 (3) Å 3 /molecule) is greater than P4 100 (433.6 (1) Å 3 /molecule). The thirteen non-H atoms of the three central rings in P2 100 and P4 100 are arranged similarly with best molecular fit δ r.m.s. = 0.066 Å. The most pronounced structural difference between P2 100 and P4 100 is in the orientation of the phenyl rings. In P4 100 , two equivalent O-C-C-C torsions of -14.5 (3)° become +32.4 (2)° and -51.7 (2)° in P2 100 . The global minimum conformation of this molecule has been calculated using MM2 (Winn & Goodman, 2001) and confirmed by us (Cambridgesoft, 2010). Like the tetragonal form, the minimum conformation has C 2 symmetry, but the two equivalent O-C-C-C torsions representing the orientation of the phenyl rings are -56°.

Experimental
The title compound was prepared by Dr. Ronald Voll according to the scheme reported by Rao et al. (1988). A suitable single-crystal was obtained by recrystallization from ethanol.