Molecular structures and conformational studies of triarylcyclopropyl and related nonsteroidal antiestrogens.

Molecular structures and conformational characteristics of a series of 1,1-dichloro-2,2,3-triarylcyclopropanes (DTACs), which were reported previously to be distinctly antiestrogenic and inhibitors of the estrogen-receptor-positive MCF-7 human breast cancer cells in culture, are reported. In addition, structural and conformational features of the DTACs were compared to the first-known nonsteroidal antiestrogen, MER25, and the clinically useful antiestrogen Tamoxifen. The molecular structures of four DTAC compounds were determined by X-ray diffraction. Crystallographic structures show that the DTAC molecules have nearly the same relative conformation for the three aryl rings which is designated as a "nonpropeller" conformation in contrast to the observed "propeller" conformation for the three rings in all known triarylethylenes. Systematic conformational searches were performed to find the conformational preferences of DTACs, MER25, and Tamoxifen using idealized model compounds built from their respective crystal structure. Energy-minimization and conformational-search studies demonstrated that all DTAC molecules have a common, single global minimum energy conformer for their central core containing the dichlorotriarylcyclopropyl system, which is similar to that found in their crystal structures. Conformational search of MER25 showed that the molecule can assume a number of low-energy conformers of which two, one anti (A1) and one gauche (G1A), have about the same energy. The anti conformation is similar to the one observed in its crystal structure and resembles the estrogenic E-isomer of Tamoxifen, while the lowest energy gauche conformer of MER25 resembles more closely the antiestrogenic Z-isomer of Tamoxifen. NMR spectroscopic analysis of MER25 showed that the molecule exists predominantly in the anti conformation in solution. A comparative review of the structural features and bioactivities of Tamoxifen, DTACs, and MER25 provides a possible explanation for their low estrogen receptor binding affinity which is common to these compounds together with their antiestrogenic activity.

Structure of 1-(4-[2-(diethylamino)ethoxy]phenyl)-2-(4-methoxyphenyl)-1-phenylethan- 1-ol, the non-steroidal antiestrogen MER25.

C27H33NO3, M(r) = 419.6, monoclinic, P2(1)/a, a = 22.833 (6), b = 9.370 (3), c = 11.434 (4) A, beta = 110.71 (8) degrees, V = 2288.2 A3, Z = 4, Dx = 1.22 g cm-3, lambda (Cu K alpha) = 1.54178 A, mu = 5.8 cm-1, F(000) = 904, T = 138 K, R = 0.049 for 3265 observed reflections. The molecule of MER25 assumes an extended conformation with rings alpha' and beta in an antiperiplanar (trans) conformation giving the solid-state conformer a closer resemblance to the estrogenic (E) isomer of tamoxifen than the antiestrogenic (Z) isomer. The geometrical features of the triarylethan-1-ol moiety are comparable to related structures but the orientations of the phenyl rings are different. The O-C-C-N segment in the (diethylamino)ethoxy side chain has the uncommon trans conformation instead of the more commonly observed gauche conformation seen in tamoxifen and many of its derivative structures. The amino N atom forms a hydrogen bond with the hydroxyl group of a neighboring molecule to form an infinite chain along the b axis.