P. Ferraboschi et al. / Steroids 104 (2015) 137–144
143
Table 4
Summary of the puckering [26] parameters.
Ring
3 b
2
QT (Å)
# (°)
/ (°)
QT (Å)
# (°)
/ (°)
A:C1/C2/C3/C4/C5/C10
B:C5/C6/C7/C8/C9/C10/
C:C8/C9/C11/C12/C13/C14
D:C13/C14/C15/C16/C17
0.447(3)[0.452(3)]
0.489(3)[0.486(2)]
0.555(3)[0.554(2)]
/
52.41(4)[49.4(3)]
130.6(4)[131.4(4)]
7.83(3)[8.9(2)]
/
À150.0(5)[À147.3(4)]
À34.6(5)[À35(5)]
0.436(2)
0.485(2)
0.588(2)
/
50.4(2)
130.6(2)
5.1(1)
/
À105.7(3)
À84.3(2)
À55(2)
176.5(2)[À116.7(1)]
À154.8(6)[158.3(2)]
À153.1(3)
Table 5
Selected torsion angles of the tetracycle.
Torsion angle (°)
Compound
3
a
3 b
2
C10–C1–C2–C3
C5–C6–C7–C8
À44
14
À45
14
À54
1
C9–C11–C12–C13
C14–C13–C17–C20
À55
À54
85
159
159
159
Fig. 6. Overlay between 2 (red) and 3 (‘‘a” molecule in green) obtained through the
best rms fit of C1 and C16 atoms.
5
compound 3 was dissolved in d -pyridine in order to avoid the fast
degradation of the enol ether moiety, observed in chloroform.
Unambiguous assignments of protons and carbons of compounds
compound 2. In particular, the same relative configuration of 17-
(Tables 2 and 3) were recognized by 1D NMR spectra, as well as
CH
NOE correlations. Indeed, when the signal at 1.03 ppm (17-CH
was irradiated NOE was observed on H-15 , H-16 and H-14a
3
of the latter was determined also in precursor 3, based on its
2
D NMR homocorrelation (COSY, TOCSY and NOESY) and hetero-
3
)
correlation (HSQC and HMBC) spectra were employed for complete
structural assignments. For overlapped signals of hydrogen atoms
in the final product 2, the 1D HOHAHA [15] technique was used
to obtain the chemical shifts and coupling constants.
a
a
showing that these protons were on the same side of the D ring.
Most of the proton assignments were accomplished using
general knowledge of chemical shift dispersion with the aid of
the proton-proton coupling pattern ( H NMR spectra), the gs-COSY
3.3. X-ray analysis
1
We started the X-ray analysis examining the molecular struc-
ture of the commercially available precursor 3 (see Scheme 2) in
order to assess the maintenance of the proper stereochemistry at
C17 during the synthetic route leading to 2.
Compound 3 crystallized with two independent molecules in
the asymmetric unit, that are shown in Fig. 3 as a ORTEP [25] draw-
ing, while the solid state conformation of medrogestone (2) is
shown in Fig. 4.
The crystallographic data allowed the complete stereochemical
assignment of all the stereogenic carbons for both compounds and
their absolute configuration was assigned according to the known
S configurations of C9, C10 and C13, which were unaffected in the
reaction pathway. Consequently, it was possible to unambiguously
establish for both compounds the stereochemistry S at C17.
Their overall molecular structure is characterized by the com-
mon tetracyclic skeleton formed by three condensed hexatomic
rings fused with a pentacyclic ring, all in trans configuration.
The two entities of 3 have a very similar geometry, except for a
slightly different orientation of the lateral chain linked to C3
and gs-NOESY experiments. In ambiguous cases, the gs-HSQC and
gs-HMBC spectra were used as a final and unequivocal tool to
make specific assignment.
In particular, in medrogestone 2 1H NMR chemical shifts of
H-2a and H-2b (2.59 and 2.45 ppm) and H-16b (2.70 ppm) protons
can be assigned to the influence of the anisotropy of the neighbor-
ing carbonyl group. Starting from the characteristic resonances of
this three protons and the two olefinic protons H-4 (5.87 ppm)
and H-7 (5.98 ppm) it was possible to assign the resonances of
all the other protons.
NOE experiments were performed to confirm the configuration
of the 17-CH
observed when methyl protons at 1.15 ppm were irradiated
Fig. 2). The NOE correlations of 17-CH (1.15 ppm) with H-15
H-16 and H-14 indicated that all these protons were on the
same face of the ring system. On the other hand, the NOE correla-
tions of CH -21 (2.15 ppm) with CH -18, H-16b and H-12b showed
that these protons were on the opposite face of the ring system
demonstrating unequivocally that the 17-CH was in position.
When the CH -18 (0.77 ppm) was irradiated, NOE enhancement
was observed on H-8b, H-11b, H-15b, H-16b, and CH -21; when
the CH in C-6 (1.85 ppm) was irradiated, NOE enhancement was
3
. The following significant enhancements were
(
3
a,
a
a
3
3
3
a
(see Fig. 5), as indicated by the torsion angles
1
s C16–C17–C20–
3
O2 of 6(1)[À13(1)]°,
s
2
C4–C3–O1–C23 of 5(1)[2(1)]° and of s
3
3
C2–C3–O1–C23–174(1)[À179(1)]°. The values in the square brack-
3
ets refer to the ‘‘b” labeled molecules.
observed on H-4 and H-7. Finally NOE were observed on H-2b,
H-8b and on proton at 2.02 ppm, assigned to the H-1b, when the
The two molecules of the precursor and the final product have a
similar rings conformation and the main deviations are related to
the / angles values of the six-membered rings, indicating their dif-
ferent puckering (Table 4) [26]. The A and B rings exhibit a ‘‘twisted
envelope” conformation, but in 3 the ring A is less distorted with
respect to 2. In the precursor, atom C10 of ring A has the higher dis-
tance from the mean plane C1/C2/C3/C4/C5, being 0.618(1)[0.580
(1)] Å, while in 2, the out-of-plane atom is C1 by 0.593(1) Å. In ring
B, the distances of C8 and C9 calculated from the best mean plane
of the remaining ring atoms are 0.324[0.318] and 0.423[0.426] Å in
3 and they are 0.251(1) and 0.787(1) Å for 2, respectively. The ring
CH
3
-19 (1.10 ppm) was irradiated.
The assignments of carbon atoms of CH, CH
2
3
and CH groups
were confirmed by the gs-HSQC experiment. The quaternary
carbon atoms were assigned unambiguously using the information
1
13
obtained from H/ C gsHMBC experiment.
1
13
H NMR and C NMR spectra of compounds 3, 8 and 9 showed
similar chemical shifts and coupling constants to those of 2 except
for C/H-6, C/H-7 and C/H-8, due to different double bond position,
putting in evidence that these compounds closely resemble