1 H and 13 C NMR of synthetic steroid sapogenins. Part II. ♣ C-23 Substituted derivatives of (25 S )-spirostanes

The main 1 H and 13 C NMR characteristics of synthetic (25 S )-5 β -spirostantes are described. Changes on chemical shifts due to substitution at C-23 are briefly analyzed


Introduction
Steroidal sapogenins are closely connected with the history of organic chemistry in Mexico.1a,b In general, this family of compounds are widespread in both natural and synthetic domains.During the mid part of the last century this kind of compound attracted the attention of natural product chemists due to the special reactivity of the heterocyclic side chain.1c Some of the members of this class have served as starting materials in the synthesis of sex hormones 2 and corticosteroids. 3More recently, steroid sapogenin chemistry has experienced a revival; new reactions 4 and new synthetic applications 5 are being reported.
Although the NMR characteristics of the spiroketal side chain have been described and reviewed, 6 to the best of our knowledge, only one report has analyzed the changes on 1 H and 13 C chemical shifts derived from common substitution at C-23 of (25R)-spirostanes.6e We now present an analysis of the 1 H and 13 C spectra of a number of synthetic (25S)-spirostanes bearing oxygenated or non-common functionality at C-23, which as a part of our program on bioactive steroidal derivatives, are being examined as synthetic precursors.Main changes on the side chain chemical shifts derived from this substitution are described and rationalized.

Results and Discussion
Satisfactory single-crystals were obtained by slow evaporation for compounds 4, 6 and 7. Structure refinements revealed the expected geometries for the A-F steroidal nucleus, including cis A/B junctions.The PM3 optimized geometries are in good agreement with their corresponding X-ray structures.Bond lengths and angles for C23 unambiguously stand for sp 2 hybridized atoms in the case of 4 and 6, while a sp 3 hybridization is observed in the case of 7.For instance, C23-N32 and C23-C23' bond lengths in 4 and 6, respectively, are 1.269(3) and 1.345(6) Å [1.323(6) Å for the second independent molecule], vs. 1.506 (7) Å for the C23-C23' bond length in 7.As a consequence, one conformation should be observed in the solid-state for groups bonded to C23 in 4 and 6, while a free rotation for the acetyt group at C23 may be expected in the case of 7. The observed conformations for the oxiimino and ethyliden groups in 4 and 6 are anti, respect to C22 bonded.In both molecules, this conformation generates close contacts between H20 and N32 or C23' atoms; corresponding observed distances for H20•••N32 in 4 is 2.49 Å, and H20•••C23' in 6 is 2.73 Å [2.72 Å in the second independent molecule].The acetyl group at C23 in 7 is placed on an equatorial position of the F cycle, giving also a short contact, H20•••C23' = 2.65 Å, similar to that observed in 6. Figure 1     As expected, substitution at C-23 does not exert significant influence on1 H and 13 C chemical shifts at the steroidal A, B, C and D rings, which are in good agreement with the previously reported shielding data, 6d but significant differences for chemical shifts of H and C atoms in E and F rings were found as discussed below.
Introduction of the new functionality at C-23 produces the expected downfield shifts in both axial and equatorial H-24 signals.Table 1 shows the main 1 H chemical shifts observed for compounds 1 to 7. substituted compounds when compared with those of the C-23 non-substituted 1.This fact could be rationalized in terms of van der Waals compression of H-20 due to the proximity of the substituent attached to C-23 in compounds 5 and 7, and a combination of such effect with the magnetic anisotropy caused by the double bond in compounds 2 -4 and 6.
Downfield shifts of axial H-26 signals of 23-nitroimino 2 and 23-keto 3 may be rationalized in terms of a combination ef electric field and magnetic anisotropy of the nitroimino and carbonyl functions.Additionally, the presence of a 23-axial hydroxyl group in compound 5 results on δ-syn interaction which produce a downfield shift of the signal corresponding to 27-methyl group.

C NMR
Besides the expected downfield shifts on C-23 and C-24 associated to the introduction of the new functionality, the presence of a subsistent attached to C-23 exerts different effects at carbon nuclei of the side chain.Table 2 shows the assignments of chemical of the 13 C chemical shifts observed for compounds 1 to 7. The most salient feature is the shielding of C-20 in compounds 2 -4, 6 and 7, which is in consonance with the van der Waals compression of H-20 due the γ-gauche interaction between C-20 and X-23.See Figures 1 and 2. Downfield shifts of C-27 signals of compounds 2 -4 and 6 (sp 2 C-23) may be attributed to the lost of a shielding γ-gauche interaction present in compound 1, (sp 3 C-23).Deshielding of C-27 in compound 5 may be interpreted on term of 1,3 repulsive diaxial interaction with the axial hydroxyl group, which compress C-27.This respulsive interaction also produces a deformation of F-ring which results on weaker γ-shielding effects over C-17.Additionally, spatial proximity of the axial 23-OH to C-21 in compound 5 (See figure 2) produces a δ-deshielding effect on C-21 signal.Table 3 shows the effects of the substitution at C-23 on the 13 C shifts of the (25S) spirostanic side chain.

Experimental Section
Spectroscopy.NMR spectra were measured in CDCl 3 on a Varian Mercury spectrometer at 400 MHz for 1 H or 100 MHz for 13 C.Chemical shift are expressed on ppm downfield from TMS and were assigned with the aid of DEPT, HETCOR and COSY spectra. 7Single-crystal X-ray analyses were carried out on a Bruker P4 diffractometer, at room-temperature, using standard procedures for data collection as for structure solutions and refinements.In the case of 4 and 6, H atoms for the hydroxyl groups (including a water molecule for 6) were found on difference maps.Non-H atoms were refined anisotropically in final least-squares cycles, with a riding model for all H atoms.It should be mentioned that for 6, the asymmetric unit contains two independent molecules with very similar geometries.

Figure 2
Figure2shows the optimized geometries9 of the fragments corresponding to the side chains of componds 1 to 3 and 5. Spatial proximity between the group attached to C-23 and H-20 can be appreciated in compounds 2 and 3.

Figure 2 .
Figure 2. PM3 optimized geometries 10 of the side chains of compounds 1 to 3 and 5.