Cholesteryl 2,4-dichlorobenzoate

The cholesteryl 2,4-dichlorobenzoate (C34H48O2Cl2) has been crystallized in two polymorphic forms. Both forms are orthorhombic, space group P212121, with form (1a) crystallizing from dichloromethane and form (1b) from hexane solution. The bond distances and angles of two molecules are constistent with those found in other cholesterol derivatives. There are considerable differences in the conformations of molecules 1a and 1b but not in the tetracyclic system. The major difference in the overall shape of the molecules can be attributed to the twist of the ester chains about the C3-O3 bond and the C28-C29 bond. The torsion angles of C2-C3-O3C28 and O3-C28-C29-C30 are 88.2(7) and 160.8(6) in molecule 1a and 156.7(7) and -10.7(11) in molecule 1b, respectively. The crystal structures are quite different from each other, although both forms crystallize in P212121, and their corresponding unit cell distances are in good agreement within the 1 A. The polymorphism could modify molecular geometries 1a and 1b, which are the conformational isomers of each other. These lead to different molecular packings. No solvent molecules of crystallization are observed in both crystals. The crystal structure of 1a is remarkable in forming layer structures in which the central region of the layers, composed largely of semi-rigid cholesteryl groups, is closely packed, and the packing of dichlorobenzoate and the isoprenoid tail of the cholesterol forms the interface region between layers. The molecules are aligned antiparellel to form layers alternately. These layers are piled up in a herring-bone arrangements. Crystals of 1a melt to give an isotropic liquid (132.5 C) with an intermediate cholesteric phase (130.8 C). Such behaviour seems consistent with the nature of the layer packing arrangement, which is dominated by strong cholesteryl-cholesteryl interactions[1].

[1] Park, Young Ja. “Two Polymorphic Crystal Structures of Cholesteryl 2,4-Dichlorobenzoate.” Bulletin of The Korean Chemical Society 19 1 (2004): 751–753.