Two polymorphs of 20-desmethyl-beta-carotene.

Two polymorphs of 20-desmethyl-beta-carotene (systematic name: 20-nor-beta,beta-carotene), C(39)H(54), in monoclinic and triclinic space groups, were formed in the same vial by recrystallization from pyridine and water. Each polymorph crystallizes with the complete molecule as the asymmetric unit, and the two polymorphs show differing patterns of disorder. The beta end rings of both polymorphs have the 6-s-cis conformation, and are twisted out of the plane of the polyene chain by angles of -53.2 (8) and 47.3 (8) degrees for the monoclinic polymorph, and -43.6 (3) and 56.1 (3) degrees for the triclinic polymorph. The cyclohexene end groups are in the half-chair conformation, but the triclinic polymorph shows disorder of one ring. Overlay of the molecules shows that they differ in the degree of nonplanarity of the polyene chains and the angles of twist of the end rings. The packing arrangements of the two polymorphs are quite different, with the monoclinic polymorph showing short intermolecular contacts of the disordered methyl groups with adjacent polyene chain atoms, and the triclinic polymorph showing pi-pi stacking interactions of the almost parallel polyene chains. The determination of the crystal structures of the two title polymorphs of 20-desmethyl-beta-carotene allows information to be gained regarding the structural effects on the polyene chain, as well as on the end groups, versus that of the parent compound beta-carotene. The absence of the methyl group is known to have an impact on various functions of the title compound.