Monoclinic modification of N-[(1,1-dimethylethoxy)carbonyl]-3-[(R)-prop-2-en-1-ylsulfinyl]-(R)-alanine ethyl ester at 200 (1) K

In the monoclinic polymorph of the title compound, C13H23NO5S, intermolecular N—H⋯O hydrogen bonds link molecules into one-dimensional chains along [100]. The atoms of the terminal propenyl group are disordered over two sets of sites with refined occupancies of 0.69 (2) and 0.31 (2).

In the monoclinic polymorph of the title compound, C 13 H 23 NO 5 S, intermolecular N-HÁ Á ÁO hydrogen bonds link molecules into one-dimensional chains along [100]. The atoms of the terminal propenyl group are disordered over two sets of sites with refined occupancies of 0.69 (2) and 0.31 (2).

Related literature
For the crystal structure of the triclinic modification of the title compound at 120 (1) K see the paper which follows: Singh et al. (2009). For background information on chiral sulfoxides, see: Rose et al. (2005);Fernandez & Khiar, (2003); Olbe et al., 2003. For synthetic details, see: O'Donnell & Schwan (2003. For related crystal structures see: Allain et al. (1980); Nakamura et al. (1996). For temperature-dependent phase transition in cysteine, see: Paukov et al. (2007), Kolesov et al.   (Rose et al., 2005). Owing to the existence of pharmacological and toxicological differences between stereoisomers, chiral recognition has now become an integral part of drug research and development. Therefore, there has been a great interest in the synthesis of optically pure sulfoxides in recent years (Fernandez & Khiar, 2003). For example, the chiral switch drug esomeprazole [(S) isomer of omeprazole], the first single-optical-isomer gastric proton pump inhibitor (PPI), generally provides better acid control than current racemic PPIs and has a favourable pharmacokinetic profile relative to omeprazole (Olbe et al., 2003). However, only few examples of crystal structures of chirally pure cysteinyl sulfoxides have been reported in the literature (Nakamura et al., 1996). Moreover, temperature dependent phase trasitions in the crystal structures of cysteinyl sulfoxides are unknown till date (Paukov et al., 2007 andKolesov et al., 2008).

Experimental
The N-protected amino acid derivative 1 (Fig. 3) was synthesized following a reported procedure. N-Boc-Alliin-OEt 2 was also synthesized by following the described procedure for its benzyl analog (O'Donnell and Schwan, 2003). Rerystallization from ethyl acetate and hexanes gave major diastereomer (R C , R S ) of 2 as white solid, which was then dissolved in ethyl acetate and hexanes (9:1) and the solvent was allowed to evaporate slowly for several days to give white crystals of the major diastereomer (R C ,R S ) of 2.

Refinement
Hydrogen atoms were placed in calculated positions with C-H = 0.95-0.99; N-H = 0.88 Å and refined as riding with U iso (H) = 1.2U eq (C, N) or 1.5U eq (methyl C). The atoms of the terminal propenyl group are disordered over two sites with refined occupancies of 0.69 (2) and 0.31 (2); the bond lengths in the minor component were restrained to be equal to those of the major component.

Figures
Fig . 1. The molecular structure of (I): displacement elllipsoids are drawn at the 30% probabilty level. The minor disorder component is not shown.