Accurate stereochemistry for two related 22,26-epiminocholestene derivatives

Regioselective opening of ring E of solasodine under various conditions afforded (25R)-22,26-epiminocholesta-5,22(N)-diene-3β,16β-diyl diacetate (previously known as 3,16-diacetyl pseudosolasodine B), C31H47NO4, or (22S,25R)-16β-hydroxy-22,26-epiminocholesta-5-en-3β-yl acetate (a derivative of the naturally occurring alkaloid oblonginine), C29H47NO3. In both cases, the reactions are carried out with retention of chirality at the C16, C20 and C25 stereogenic centers, which are found to be S, S and R, respectively. Although pseudosolasodine was synthesized 50 years ago, these accurate assignments clarify some controversial points about the actual stereochemistry for these alkaloids. This is of particular importance in the case of oblonginine, since this compound is currently under consideration for the treatment of aphasia arising from apoplexy; the present study defines a diastereoisomerically pure compound for pharmacological studies.

Regioselective opening of ring E of solasodine under various conditions afforded (25R)-22,26-epiminocholesta-5,22(N)-diene-3,16-diyl diacetate (previously known as 3,16-diacetyl pseudosolasodine B), C 31 H 47 NO 4 , or (22S,25R)-16-hydroxy-22,26-epiminocholesta-5-en-3-yl acetate (a derivative of the naturally occurring alkaloid oblonginine), C 29 H 47 NO 3 . In both cases, the reactions are carried out with retention of chirality at the C16, C20 and C25 stereogenic centers, which are found to be S, S and R, respectively. Although pseudosolasodine was synthesized 50 years ago, these accurate assignments clarify some controversial points about the actual stereochemistry for these alkaloids. This is of particular importance in the case of oblonginine, since this compound is currently under consideration for the treatment of aphasia arising from apoplexy; the present study defines a diastereoisomerically pure compound for pharmacological studies.

Comment
Many plants of the Solanaceae family accumulate steroidal alkaloids based on the C 27 cholestane skeleton, e.g. solasodine, (1), and tomatidine, which are N-analogues of sapogenins (Friedman & McDonald, 1997). For a long time, (1) has been an essential starting material for the partial synthesis of pregnane derivatives (Sato et al., 1957). 22,26-Epiminocholestenes are also accessible from (1), through a selective E-ring cleavage under reductive conditions (Bird et al., 1979) or by acidic acetolysis. For instance, Sato et al. (1957) performed a selective E-ring opening of (1) by treatment with Ac 2 O/AcOH and ZnCl 2 at 298 K over a long time period, affording the 22,26-epiminocholestadiene framework (I), also known as 3,16-diacetyl pseudosolasodine B.
During our work on sapogenin acetolysis, we probed a variety of Lewis acids with the hope of optimizing both the ability to regioselectively open ring E and the reaction rates. We have found that Et 2 OÁBF 3 gives excellent results. In this way, (I) may be prepared starting from (1) in quantitative yield in remarkably short reaction times (see scheme). All spectroscopic and physical data for the product fit well with those of an authentic sample of pseudosolasodine B diacetate. However, an X-ray study was necessary in order to assess the absolute configuration at atom C20. This aspect should not be seen as a trivial matter, since Yang et al. (2004) claimed that epimerization at C20 occurs if an acid harder than ZnCl 2 is used. We, however, have never detected such an epimerization in the case of sapogenins treated under similar conditions (Sandoval-Ramírez et al., 1999, 2003. It is also known that the reaction course may be complicated by nucleophilic attacks at atom C16 (Iglesias-Arteaga et al., 2004) and epimerization at atom C25 (LaCour et al., 1999).
A similar controversy about stereochemistry appeared in the case of oblonginine, a naturally occurring 22,26-epiminocholestene. The molecule was first assigned a stereochemistry of 22R,25S (Kadota et al., 1995). However, a re-examination using X-ray crystallography and high-resolution NMR spectroscopy revealed that oblonginine is (22S,25R)-22,26epiminocholest-5-en-3-ol (Lowe et al., 1998). The X-ray structure of oblonginine monohydrate reported in that paper was unfortunately not deposited with the Cambridge Structural Database (Version 5.29; Allen, 2002) (see refcode CADNIE). We can now confirm this assignment on the basis of the X-ray structure of a C16-functionalized oblonginine monoacetate, (II), synthesized from (1) following a published procedure (Kusano et al., 1970; see scheme).
The A-D steroidal nucleus of (I) exhibits the expected geometry, identical to that of solasodine (Vega-Baez et al., 2006). The cleavage of the C22-O bond in ring E of solasodine occurs without inversion, and the stereogenic centers remain as 16S and 20S. The six-membered ring F includes a organic compounds o214 # 2008 International Union of Crystallography double bond (C22 N27; Table 1) and exhibits a half-chair conformation [the puckering parameters are = 127.1 (7) and ' = 31.7 (9) ; the puckering amplitude is 0.466 (6) Å ; the atom sequence defining ring F is N27/C22-C26 (Cremer & Pople, 1975)]. As mentioned by Sato et al. (1957), (I) should equilibrate only under constraint to its tautomeric form including a C20 N22 double bond. In the solid state, the C20-C22 bond length clearly shows that this bond is a single bond without participation of tautomeric forms. The absolute configuration for atom C25 is unchanged compared with the starting material, viz. 25R, with methyl atom C28 occupying an equatorial position in ring F (Fig. 1).
The geometric features for the A-D nucleus in (II) are similar to those found in (I). Cleavage of the C22-O bond of solasodine occurs with retention of configuration for atoms C16, C20 and C25, as for (I). The 22R configuration of the spiro C atom of solasodine changes to 22S in (II), owing to the formation of a C-H bond at atom C22. The main difference between (I) and (II) is thus the conformation of ring F (Fig. 2 and Table 2), which now exhibits a chair form [the puckering parameters are = 177.6 (4) and ' = 93 (8) ; atom sequence as for (I)]. The relative positions of the NH group in ring F and the hydroxy substituent at atom C16 in ring D allow the formation of a rather strong intramolecular O-HÁ Á ÁN hydrogen bond [DÁ Á ÁA = 2.777 (4) Å , HÁ Á ÁA = 1.93 (4) Å and D-HÁ Á ÁA = 157 (5) ]. This stabilizing interaction explains why the OH group at atom C3 in solasodine is acetylated, while the OH group formed during E-ring opening is retained as a hydroxy group, at least when the conditions of Kusano et al. (1970) are applied. Such a protection of the C16-OH functionality is not possible for diacetate (I).

Experimental
For the synthesis of (I), a mixture of solasodine (300 mg, 0.72 mmol), Ac 2 O (3.0 ml, 32 mmol), AcOH (1.0 ml, 17 mmol) and Et 2 OÁBF 3 (0.7 ml, 5.5 mmol) was stirred for 20 s at 298 K. The reaction mixture was poured into iced water and shaken vigorously. Concentrated NH 4 OH was added until a basic pH was obtained, and the product was extracted with CH 2 Cl 2 (3 Â 10 ml), washed with brine and water, dried over anhydrous Mg 2 SO 4 , and finally concentrated to dryness under reduced pressure. The crude product was chromatographed over silica gel using hexane/EtOAc (7:3), affording (I) in quantitative yield. Suitable single crystals were obtained by slow evaporation of an AcOEt solution. Spectroscopic data are in full agreement with the crystal structure (see archived CIF). Compound (II) was prepared following a literature procedure (Kusano et al., 1970). Suitable single crystals of (II) were obtained by slow evaporation of an AcOEt solution. Spectroscopic data are in full agreement with the crystal structure (see archived CIF).  The structure of (I), with displacement ellipsoids at the 30% probability level for non-H atoms.

Figure 2
The structure of (II), with displacement ellipsoids at the 30% probability level for non-H atoms. Atoms O31B and C32B, disordered with O31A and C32A, have been omitted for clarity. The intramolecular hydrogen bond is shown as a dashed line.

Data collection
Bruker P4  Carbonyl atom O31 and methyl group C32 in (II) are disordered over two sites, for which occupancies were refined and converged to 0.47 (4) and 0.53 (4) for O31A/C32A and O31B/C32B, respectively. In order to obtain a sensible geometry for this acetate group, bond lengths involving disordered sites were restrained [C30-O31 = 1.20 (1) Å (two restraints) and C30-C32 = 1.53 (1) Å (two restraints)]. All C-bonded H atoms in (II) were placed in idealized positions and constrained to ride on their parent atoms, with C-H bond lengths fixed at 0.93 (H6A), 0.96 (methyl CH 3 ), 0.97 (methylene CH 2 ) and 0.98 Å (methine CH). U iso (H) values were calculated at 1.3U eq (C) for methyl groups and 1.2U eq (C) otherwise. Rigid methyl groups were allowed to rotate about their C-C bonds in order to obtain accurate torsion angles. H atoms bonded to heteroatoms N27 and O33 were found in a difference map and refined with free coordinates [U iso (H) = 1.3U eq (N,O)]. For (I), the acetate group at atom C3 is almost certainly disordered as in (II), as reflected in the displacement parameters for atoms O31 and C32. However, we were unable to model disordered sites satisfactorily. H atoms were placed in idealized positions, with C-H bond lengths fixed as for (II) and with U iso (H) values of 1.5U eq (C) for methyl groups and 1.2U eq (C) otherwise. In both structures, Friedel pairs [663 for (I) and 394 for (II)] were merged and the stereochemistry assumed from the synthesis.
SB is grateful to Benemé rita Universidad Autó noma de Puebla (BUAP) for diffractometer time. The authors thank VIEP-BUAP for grant No. 9/I/NAT/05. Supplementary data for this paper are available from the IUCr electronic archives (Reference: GD3197). Services for accessing these data are described at the back of the journal.   Table 1 Selected geometric parameters (Å , ) for (I).