(20S,2′′S)-20-[4′-(3′′-Hydroxy-2′′-methylpropyl)-3′-methylisoxazol-5-yl]-5β-pregnan-3β,16β-diol

The title steroidal compound, C29H47NO4, was prepared in a one-pot reaction starting from a sarsasapogenin derivative of known configuration. The isoxazole heterocycle is oriented towards the α face of the steroid nucleus and, although fully functionalized on C atoms, does not provoke steric hindrance with the adjacent D ring. The absolute configuration observed for chiral centers is as expected, and shows that no epimerization occurred in the precursors. In the crystal, the three OH groups serve as donors for hydrogen bonding with O and N atoms. The isoxazole N atom is involved in O—H⋯N hydrogen bonds, forming chains along [100]. These chains are further connected via O—H⋯O and weak C—H⋯O contacts, giving rise to a three-dimensional supramolecular network.

The title steroidal compound, C 29 H 47 NO 4 , was prepared in a one-pot reaction starting from a sarsasapogenin derivative of known configuration. The isoxazole heterocycle is oriented towards the face of the steroid nucleus and, although fully functionalized on C atoms, does not provoke steric hindrance with the adjacent D ring. The absolute configuration observed for chiral centers is as expected, and shows that no epimerization occurred in the precursors. In the crystal, the three OH groups serve as donors for hydrogen bonding with O and N atoms. The isoxazole N atom is involved in O-HÁ Á ÁN hydrogen bonds, forming chains along [100]. These chains are further connected via O-HÁ Á ÁO and weak C-HÁ Á ÁO contacts, giving rise to a three-dimensional supramolecular network.

Comment
There is a continuous interest for new synthetic routes affording steroids functionalized with heteroatoms and heterocycles, since these groups modify the biological activity of related molecules (Banday et al., 2008;Pathak & Jindal, 1998). For example, the synthesis of suitable precursors for steroids analogs to brassinosteroids has been reported (Litvinovskaya et al., 1998). Functionalization with an isoxazol heterocycle has been limited to date to few examples, where the heterocycle is fused with the A ring of the steroid. An example of a molecule belonging to this family is danazol (Gupta et al., 1999), a derivative of ethisterone, which has numerous medicinal applications.
In relation with this general goal, we have developed several reagents for the direct one-step functionalization of steroids on remote positions. The title compound was synthesized readily starting from 23-acetylsarsasapogenin (Meza-Reyes et al., 2005) in a one-pot reaction carried out in dry media (see Experimental). This new route improves known procedures, which make necessary the isolation of an oxime intermediate, prior to the heterocyclization in acid conditions (Beam et al., 2000). Full details about the involved chemistry and mechanistic aspects of this unprecedented reaction will be reported elsewhere. It was however essential to X-ray characterize the product, in order to determine if any epimerization occurred during the cleavage of rings E and F.
The title molecule displays the expected cis-fused A/B ring system, characteristic of sarsasapogenin derivatives ( Fig.   1). Rings A,B and C have the expected chair conformation, while the 5-membered ring D is twisted on C13-C14. The spiroketal E/F system was cleaved during the reaction, affording a C 21 -pregnane nucleus substituted at C20 by an isoxazol heterocycle. Positions for O and N atoms in the heterocycle were unambiguously determined from X-ray data, and are consistent with the positions for double bonds, C22=C26 and C25=N24. The isoxazol ring is oriented towards the α face, and its plane approximately bisects the mean plane of the A···D steroidal nucleus. This conformation avoids any hindrance with the methyl group, C21, and OH group at C16. The observed absolute configuration indicates that the E/F rings cleavage occurred without epimerization, despite use of the strongly acidic medium used for the reaction. C20 is retained as S, and the chiral C atom C29 in the lateral chain has the S configuration.
In the crystal, molecules are associated via O-H···N hydrogen bonds involving the isoxalic N atom as an acceptor, forming chains running along the [100] direction (Fig. 2). The hydroxyl groups at O32 and O34, form O-H···O contacts between chains. The resulting three-dimensional supramolecular network also includes weak C-H···O hydrogen bond interactions involving the O23 and O24 atoms as acceptors.

Experimental
A mixture of 23-acetylsarsasapogenin (2 mmol), hydroxylamine hydrochloride (4 mmol) and a previously prepared P 2 O 5 / SiO 2 reagent (1 g) were grounded thoroughly in a mortar. An immediate color change was observed. The mortar was covered with a watch glass and put inside a microwave device (2450 MHz, 1200 W). The mixture was irradiated for 3 min, allowing the reaction to complete (TLC). The mixture was then cooled to room temperature, and 10 ml of 5% aqueous HCl was supplementary materials sup-2 added. The resulting solution was extracted with CH 2 Cl 2 (2×10 ml) and dried over CaCl 2 . Evaporation of solvent under reduced pressure gave the pure title compound, in 81% yield. Anal. found (calc. for C 29 H 47 NO 4 ): C 73.52 (75.53), H 9.98 (9.99), N 2.95 (2.95%). Single crystals were obtained by slow evaporation of an acetone solution.

Refinement
H atoms for hydroxyl groups, H32, H33 and H34, were found in a difference map and refined freely. C-bonded H atoms were placed in idealized positions and refined using a riding approximation, with C-H bond lengths fixed to 0.96 (methyl), 0.97 (methylene) or 0.98 Å (methine). Methyl groups were allowed to rotate about their C-C bonds. Isotropic displacement parameters for H atoms were computed from displacement of carrier atoms: U iso (H) = 1.5U eq (carrier atom) for methyl and hydroxyl groups, and U iso (H) = 1.2U eq (carrier C) for other H atoms. Fig. 1. The title molecule with displacement ellipsoids for non-H atoms shown at the 30% probability level.