(E)-(25S)-23-Acetyl-5β-furost-22-ene-3β,26-diol

The title steroid, C29H46O4, is a furostene derivative with a C=C double-bond length of 1.353 (3) Å and an E configuration. The side chain is oriented toward the α face of the A–E steroidal nucleus and presents a disordered terminal CH2—OH group [occupancies for resolved sites are 0.591 (9) and 0.409 (9)]. The methyl group at C20 attached to ring E is also oriented toward the α face, avoiding steric hindrance with the carbonyl O atom of the acetyl group. The furostene and acetyl functionalities form an α,β-unsaturated ketone system, with an s-cis configuration. All hydroxy and carbonyl groups are involved in weak intermolecular hydrogen bonds. The absolute configuration was assigned from the synthesis.

The title steroid, C 29 H 46 O 4 , is a furostene derivative with a C C double-bond length of 1.353 (3) Å and an E configuration. The side chain is oriented toward the face of the A-E steroidal nucleus and presents a disordered terminal CH 2 -OH group [occupancies for resolved sites are 0.591 (9) and 0.409 (9)]. The methyl group at C20 attached to ring E is also oriented toward the face, avoiding steric hindrance with the carbonyl O atom of the acetyl group. The furostene and acetyl functionalities form an ,-unsaturated ketone system, with an s-cis configuration. All hydroxy and carbonyl groups are involved in weak intermolecular hydrogen bonds. The absolute configuration was assigned from the synthesis.

Comment
We are interested in the preparation of new steroidal derivatives, through the cleavage of the F ring in sarsasapogenin (Sandoval-Ramírez et al., 2003;Meza-Reyes et al., 2004). Such reactions are valuable entries to furostenes; the title compound is a new representative of this family.
The molecular conformation of the title compound compares well with that previously observed for the corresponding diacetate (Sandoval-Ramírez et al., 2003). The functionality C22═C23 has a bond length of 1.353 (3) Å [1.355 (3) Å for the diacetate] and is E configured. The side chain C24/C25/C26/O26 is oriented toward the α face of the A-E steroidal nucleus (Fig. 1). The terminal group CH 2 -OH is disordered over two positions (denoted a and b), and has a poorly defined geometry (see Experimental). The acetyl group substituting the furostenic atom C23 forms a s-cis α,β-unsaturated ketone system with the E configuration at the C22═C23 double bond. The methyl group C21 is placed on the α steroidal face, in agreement with a general rule followed by furostenes: the bulkier group at C20 avoids steric hindrance with groups substituting C23 (Meza-Reyes et al., 2004). The solid-state conformation of the title compound is retained in solution, as confirmed by NMR data.

Refinement
The final part of the lateral chain is badly disordered. Atoms C26 and O26 were modeled over two sites, and occupancies refined with the sum for a single atom constrained to 1. Site occupation factors converged to 0.591 (9) (C26a and O26a) and 0.409 (9) (C26b and O26b). Bond length C25-C26a was restrained to 1.50 (1) Å, while other dimensions were refined freely. The poor geometry for this part probably indicates that the actual disorder is more complex than a two-sites model.
C-bonded H atoms were placed in idealized positions and refined as riding to their parent atoms. C-H bond lengths were fixed to 0.98 (methine CH), 0.97 (methylene CH 2 ) or 0.96 Å (methyl CH 3 ) and isotropic displacement parameters calculated as U iso (H) = xU eq (carrier C) with x = 1.5 (CH 3 ) or x = 1.2 (CH 2 and CH). Disordered hydroxyl H atoms H26E and H26F were placed in idealized positions and refined fixing O-H bond lengths to 0.82 Å and U iso (H) = 1.5U eq (carrier O). Finally, hydroxyl H atom H3' was found in a difference map and refined with this as-found position and U iso (H3') = 1.5U eq (O3').