Structure of Equilenin at 100 K: an estrone-related steroid

The structure of the estrone related steroid, Equilenin 1, has been determined at 100 K. It is of great interest to investigate what the structural and conformational consequences are on the C and D rings of the steroid framework of 1 by having fully unsaturated A and B rings.


Chemical context
The title compound, Equilenin 1, is one member of a series of three estrogenic steroids, the other members being Equilin 2 and 17-estrone 3, that are components of the hormone replacement therapy medication, 'Premarin', a mixture of natural estrogens isolated from the urine of pregnant equine mares. It can be seen from the scheme that on going from 17estrone 3 through to the title compound Equilenin 1, there is a progressive aromatization of the B ring of the steroid framework where in 1 rings A and B comprise a fully aromatic naphthalene core. The structure of Equilin 2, was determined by Sawicki et al. (1999b), who demonstrated that the presence of the unsaturated C7-C8 bond in the B ring rotates the C and D rings of the steroid such that the 17-keto oxygen atom, O17, is translated by 0.73 Å with respect to the analogous oxygen atom of 3 when an overlay of the two structures was performed based on the superposition of the A rings. The translation of the oxygen atom was implicated in the increased anti-human estrogenic 17-hydroxysteroid dehydrogenase 1 (17-HSD1) inhibitory behaviour of 2 with respect to 17estone 3 (Sawicki et al., 1999a). The impact of the inhibitory behaviour of 2 is that it causes a reduction of the active estrogen, 17-estradiol, which is present in elevated concentrations in human breast tumour tissues and responsible for the accelerated growth of the tumour tissue. It is therefore of great interest to investigate what the structural and conformational consequences are on the C and D rings of the steroid framework of 1 by having fully unsaturated A and B rings. Although the unit-cell parameters of 1 at room temperature have been previously reported by Ohrt et al. (1967), no threedimensional structure analysis of this important estrone ISSN 2056-9890 steroid has been determined. Herein, we report on the crystal structure of this final member of the estrone series of steroids, Equilenin 1, at 100 K.
Compounds 1 and 2, possibly owing to increased conformational constraint in the B ring, have lower oestrongenic activity than 17-estrone itself, which has the B ring as the principal point of molecular flexibility (Duax et al., 1976;Busetta et al., 1973). Interestingly this reduction in activity (Marshall, 1970) does not directly relate to the crystallographically determined degree to which the A and B rings of the steroid are constrained to coplanarity, since 1, possessing an essentially planar naphthalene core, is about five times more estrogenic than 2 which features only approximate coplanarity of its A and B rings with an r.m.s. deviation of the fitted atoms of 0.102 Å , and a total puckering amplitude of 0.270 (2) Å (Sawicki et al., 1999b). An overlay of structures 1 (red), 2 (blue) and 3 (green) is shown in Fig. 2. The overlay was performed by a superposition of the atoms in the A ring only. From this overlay it can be calculated that the keto oxygen atom is translated by 0.78 and 0.69 Å , respectively, for compounds 1 and 2 from its position on 3. Perhaps more significant is the degree of translation of the methyl group C18 which is translated by 0.79 and 1.40 Å , respectively, for compounds 1 and 2 from its position on 3 which may account for the increased estrogenic activity of 1 over 2. The stereochemistry assignments at C13 and C14 are S, S; confirmed by resonant scattering through the Flack x parameter value of À0.05 (4).

Supramolecular features
In the crystal, the Equilenin 1 molecules are linked head-totail by a single O-HÁ Á ÁO i hydrogen bond (Table 1), to form chains propagating along the c-axis direction. A view along the b-axis of the crystal packing of the title compound is shown in Fig. 3.

Synthesis and crystallization
In common with Equilin 2 the title compound, 1, was isolated from the urine of a pregnant mare (Girard et al., 1932;Fieser & Fieser, 1959). The sample used for the X-ray data collection was gifted to us from the J. W. Cook collection of the University of Glasgow. Suitable crystals were obtained as needles from ethanol solution, m.p. 531-532 K (evacuated sealed capillary).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. The O-bound H atom was located from a difference-Fourier map and freely refined. All remaining H atoms were placed geometrically in idealized positions and refined using a riding model (including free rotation about the methyl C-C bond): C-H = 0.95-0.99 Å with U iso (H) = 1.5U eq (C-methyl) and 1.2U eq (C) for other H atoms. The absolute stereochemistry of 1, was confirmed through the Flack x parameter value of À0.05 (4). This was View of the structure overlay of compounds 1 (red), 2 (blue) and 3 (green). The overlay was performed by a superposition of the atoms in the A ring only.  (17) 157 (3) Symmetry code: (i) Àx þ 1 2 ; Ày þ 1; z À 1 2 .

Special details
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.