A Convenient Synthesis of (16S,20S)-3β-Hydroxy-5α-pregnane-20,16-carbolactam and Its N-alkyl Derivatives

Article abstract of DOI:10.3390/molecules25102377

A concise synthesis of (16S,20S)-3β-hydroxy-5α-pregnane-20,16-carbolactam from tigogenin via the corresponding lactone is described. The most efficient synthetic route consisted of the lactone ring-opening with aminoalane reagent followed by PDC or Dess-Martin oxidation. The oxo-amide obtained was subjected to cyclization with Et3SiH/TFA or Et3SiH/Bi(TfO)3. Alternately, the lactone was converted first to the oxo-acid, which was then subjected to the microwave-assisted reductive amination. N-Alkyl derivatives were also obtained in a similar way.

Full text of DOI:10.3390/molecules25102377

Article
A Convenient Synthesis of (16S,20S)‐3β‐Hydroxy‐5α‐
pregnane‐20,16‐carbolactam and Its N‐alkyl
Derivatives
Agnieszka Wojtkielewicz *, Damian Pawelski, Przemysław Bazydło, Aneta Baj,
Stanisław Witkowski and Jacek W. Morzycki
Faculty of Chemistry, University of Białystok, K. Ciołkowskiego 1K, 15‐245 Białystok, Poland;
d.pawelski@uwb.edu.pl (D.P.); bazydlo_przemyslaw@wp.pl (P.B.); aneta.baj@uwb.edu.pl (A.B.);
wit@uwb.edu.pl (S.W.); morzycki@uwb.edu.pl (J.W.M.)
*
Correspondence: a.wojtkielewicz@uwb.edu.pl; Tel.: +48‐85‐738‐8043
Academic Editor: Rafal R. Sicinski
Received: 3 May 2020; Accepted: 19 May 2020; Published: 20 May 2020
Abstract: A concise synthesis of (16S,20S)‐3β‐hydroxy‐5α‐pregnane‐20,16‐carbolactam from
tigogenin via the corresponding lactone is described. The most efficient synthetic route consisted of
the lactone ring‐opening with aminoalane reagent followed by PDC or Dess‐Martin oxidation. The
oxo‐amide obtained was subjected to cyclization with Et
3
SiH/TFA or Et
3
SiH/Bi(TfO) . Alternately,
3
the lactone was converted first to the oxo‐acid, which was then subjected to the microwave‐assisted
reductive amination. N‐Alkyl derivatives were also obtained in a similar way.
Keywords: lactams; reductive amination; spirostane degradation; steroids
1
. Introduction
In recent years, much attention has been paid to the preparation and biological activity
evaluation of 23,24‐bisnorcholano‐22,16‐lactones (bisnorcholanic lactones) [1]. One of them,
16S,20S)‐3β‐hydroxy‐pregn‐5‐ene‐20,16‐carbolactone (vespertilin; Figure 1), is a natural product
(
isolated from Solanum vespertilio [2]. Furthermore, other bisnorcholanic lactones have been isolated
from different plants [3–5]. The lactones usually occur in plants as glycosides with sugar linked to
the oxygen atom of aglycone at C3. Vespertilin shows a variety of biological activities including
anticancer, antifungal, and bactericidal [6].
O
O
O
NH
H
H
H
H
H
H
H
H
HO
HO
H
vespertilin
(16S,20S)-3-hydroxy-5-pregnane-20,16-carbolactam
2
1
2
5
2
6
H
N
O
20
1
8
1
2
27
N
H
1
7
22 24
23
1
1
1
9
O
O
1
3
H
1
9
H
14
H
H
H
H
1
6
2
1
0
8
7
15
5
H
6
H
H
H
H
H
HO
3
HO
HO
4
H
H
H
tigogenin
tomatidine
demissidine
Figure 1. Vespertilin, the target lactam, tigogenin, and steroidal alkaloids.
Molecules 2020, 25, 2377; doi:10.3390/molecules25102377
www.mdpi.com/journal/molecules

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The availability of vespertilin and other bisnorcholanic lactones from natural sources is rather
limited. However, they are readily available by degradation of steroidal sapogenins and alkaloids
(spirosolanes, Figure 1). Alternately, bisnorcholanic lactones can be obtained from steroidal 17‐
ketones but the synthesis is long and expensive [7]. More conveniently, the lactones can be prepared
by oxidation of steroidal spirostanes or spirosolanes either directly or via intermediate 23‐oxo
derivatives. The latter are readily available from sapogenins using the Barton’s procedure (NaNO
2
in
the presence of BF ∙Et O in glacial acetic acid) [8,9]. Further oxidation of 23‐oxo‐sapogenins to
3
2
bisnorcholanic lactones can be easily carried out using different oxidizing agents, e.g.,
+
MCPBA/BF
3∙Et
2
O [10–12], H
2O
2
/H [6], (PhSe)
2
/PhIO [13], or even may proceed with TMSOTf in the
2
absence of an oxidant [14]. However, direct transformation of sapogenins or spirosolanes to
bisnorcholanic lactones is advantageous over two‐step procedures. Though spirosolanes (e.g.,
solasodine, tomatidine) have been recently reported [15,16] to directly yield lactones when treated
with different oxidizing agents, they are more expensive than sapogenins. A practical and direct
degradation method of tigogenin to the corresponding bisnorcholanic lactone has been patented by
Chinese chemists [17,18]. The method consists of peroxyacid/I oxidation in acid medium. We have
2
slightly improved the method by using more concentrated hydrogen peroxide/trifluoroacetic acid,
and the new protocol is described in Supplementary Materials.
In contrast to the well‐known bisnorcholanic lactones, the corresponding lactams, to the best of
our knowledge, have not been reported in the literature yet. Many biologically active lactams have
been isolated from natural sources and some of them are used, along with different synthetic lactam
compounds, in contemporary medicine [19–26]. Herein, we report the results of our study on the first
synthesis of (16S,20S)‐3β‐hydroxy‐5α‐pregnane‐20,16‐carbolactam and its N‐alkyl derivatives
(bisnorcholanic lactams).
2
. Results and Discussion
The first three‐step strategy for the synthesis of the target lactam from bisnorcholanic lactone
involves lactone ammonolysis to corresponding hydroxy‐amide, its oxidation to oxo‐derivative,
followed by reductive cyclization (Scheme 1).
Our study began from an improvement of Chinese method of the lactone synthesis from
tigogenin [17,18]. The use of a 60% solution of hydrogen peroxide and a catalytic amount of
trifluoroacetic acid increased the yield to 91% on the multigram scale (see Supplementary Materials).
With a sufficient amount of protected lactone 1b in hand, we tried the ammonolysis reaction (Scheme
1
). The classical lactone ammonolysis with gaseous NH
3
or ammonia aqueous solution in MeOH/THF
[27], as well as a microwave assisted reaction [28], failed in the case of the bisnorcholanic lactone 1b.
Therefore, we employed aminoalane reagent prepared in situ from DIBAlH and ammonium chloride.
In our previous studies, we discovered that it is a very efficient reagent for amide and nitrile synthesis
from acids and their derivatives [29]. The reaction of lactone 1b with 20 equivalents of aluminum
amide in THF at reflux provided the desired hydroxy‐amide 2b in high yield (Scheme 1). The product
appeared to be unstable, when left neat or in solution in a freezer for 24 h, it underwent lactonization
spontaneously. For that reason, crude 2b, without purification, was subjected to oxidation. The
oxidation of hydroxy‐amide 2b with various oxidizing reagents (Dess‐Martin periodinane,
PCC/AcONa, PDC, Swern reagent) was studied. The best yield (62% after two steps) of the desired
oxo‐derivative 3b was obtained with PDC in dichloromethane (Scheme 1). The key step of the lactam
synthesis was the cyclization of the obtained oxo‐amide 3b to the desired γ‐lactam. Few methods of
amide/lactam preparation by condensation of ketone and amide have been reported in the literature.
Most of them are based on a two‐step protocol comprising of inter‐ or intramolecular reactions of
ketone and amide under acidic conditions, followed by enamide hydrogenation in the presence of
rhodium or ruthenium catalysts [30–33]. Among the reported procedures, there is also a reductive
cyclization method leading directly to lactam. Frequently triethylsilane in presence of acid was used
for the transformation of oxo‐amide into lactam [34,35]. Another convenient reagent for this
transformation is sodium triacetoxyborohydride in acetic acid [36]. After reviewing the literature, we
decided to subject the oxo‐amide 3b to simultaneous cyclization and ionic hydrogenation with EtSiH

Molecules 2020, 25, 2377
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and TFA. The reaction of compound 3b with 20 equivalents of EtSiH and 10 equivalents of TFA in
refluxing DCE resulted in the desired lactam in high yield. However, the partial deprotection of TBS
ether was observed under the employed conditions. Therefore, the 3‐TBS protected lactam 4b was
isolated in 73% yield in addition to the corresponding desilylated lactam 4a (14%). Decreasing either
the amount of acid used or the reaction temperature did not prevent the partial 3‐TBS hydrolysis.
However, when bismuth triflate or TBS triflate were employed instead of TFA for the reductive
cyclization, only the deprotected product 4a was isolated quantitatively.
The configurations at C16, C17, and C20 stereogenic centers remained intact during the lactam
synthesis as proved by NOE and ROESY correlation experiments (Figure 2). The differential NOE as
well as ROESY spectra obtained for compound 4a showed a strong correlation between 17‐H at 1.82
ppm (dd) with 16‐H at 4.02 ppm (m) and 21‐CH
3
protons at 1.21 ppm (d), as well as a correlation
between 18‐methyl protons at 0.75 ppm (s) and 20‐H at 2.34 (d), and no correlation between 18‐CH
3
and 17‐H at 1.82 ppm (dd). This implied 16α, 17α, and 20β proton configurations. These configuration
assignments were confirmed by correlations of 16‐H at 4.02 ppm (m) with 21‐methyl protons at 1.21
ppm (d) and 14α‐H at 1.05 ppm (m) (Figure 2) observed in ROESY spectrum.
O
O
NH
O
H
H
CONH
CONH
2
2
DIBAlH/NH
PhCH /THF, reflux
6h
4
Cl
PDC
Et
SiH
3
H
H
H
OH
H
O
H
3
DCM, rt, 4h
acid
3
1
RO
H
H
H
H
for acid = TFA: 4b (R = TBS, 73%)
and 4a (R = H, 14%)
for acid = Bi(OTf) : 4a ( R = H, 100%)
3
1
a (R = H)
2b (R = TBS)
3b (R = TBS, 62%)
1b (R = TBS)
DIBAlH/MeNH
or DIBAlH/BnNH
PhCH /THF, rt, 16h
2
xHCl
2
3
O
CONHR'
OH
CONHR'
O
Et SiH
Bi(OTf)3
NR'
3
H
H
PDC
DCM, rt, 16h
H
H
DCE/MeCN
reflux, 16h
H
H
H
H
RO
H
2
2
c (R = TBS, R' = Me)
d (R = TBS, R' = Bn)
3c (R = TBS, R' = Me, 61%)
3d (R = TBS, R' = Bn, 86%)
4c (R = H, R' = Me, 100%)
4d (R = H, R' = Bn, 100%)
Scheme 1. Synthesis of bisnorcholanic lactam derivatives via oxo‐amide intermediates.
Figure 2. NOE and ROESY correlations diagram for compounds 4a.
The described methodology for bisnorcholanic lactam synthesis was successfully applied to
preparation of its N‐alkyl derivatives (Scheme 1, lower row). In the case of N‐substituted lactams, the
first step included lactone 1b aminolysis with aminoalane reagent prepared from DIBAlH and an
appropriate amine or its hydrochloride affording secondary hydroxy‐amides. The reactions of N‐
alkylaminoalanes with lactone 1b proceeded smoothly at room temperature. Employing the reported
procedure, N‐methyl and N‐benzyl derivatives 2c and 2d were prepared. The further syntheses of N‐
alkyl‐lactams from the secondary amides 2c and 2d were carried out analogously to their N‐
unsubstituted congener. Although the secondary amides (2c, 2d) appeared to be less prone to
lactonization than the analogous primary hydroxy‐amide 2b, they underwent spontaneous
intramolecular alcoholysis when allowed to stand for a longer period of time. Therefore, they were

Molecules 2020, 25, 2377
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oxidized without purification with PDC to provide the desired oxo‐derivatives 3c and 3d in high
yields. The reductive cyclization with EtSiH/Bi(OTf) produced target N‐alkylated lactams 4c and 4d
3
quantitatively (61% and 86% total yields, respectively, for the three‐step synthesis from lactone).
Taking into account the economic aspect of bisnorcholanic lactam synthesis, we also tested other
synthetic strategies. The second approach involved the saponification of the tigogenin derived
lactone, followed by oxidation of the obtained hydroxy‐acid to oxo‐acid and reductive amination. It
was expected that the γ‐amino acid should spontaneously cyclize to the corresponding γ‐lactam
under neutral conditions. A base induced ring‐opening of lactone 1b resulted in the formation of the
corresponding hydroxy‐acid, which rapidly cyclized in solution at room temperature back to the
lactone. To avoid lactonization, the crude hydroxy‐acid was immediately oxidized to oxo‐acid 5b
(Scheme 2). From several tested oxidizing agents (PDC/DCM, Dess Martin, CrO3) the best result was
obtained using CrO in pyridine (70% after two steps). Although the literature describes a cyclic form
3
of oxo‐acid (16α‐hydroxy‐5α‐pregnane‐20,16‐carbolactone) [37], we cannot confirm the reported
equilibrium of oxo‐acid 5b with its cyclic form. In addition to the expected product 5b, lactone 1b
was recovered in small amounts (15–20%). In order to avoid partial lactonization, basic conditions
were employed for oxidation of the hydroxy‐carboxylate initially formed during hydrolysis. In the
first attempt, a one‐pot protocol was employed for hydrolysis and oxidation. The obtained hydrolysis
product, γ‐hydroxy‐carboxylate, was treated in situ with an aqueous solution of sodium hypochlorite
(NaOCl). After a two‐day reaction at room temperature, only a minor conversion of γ‐hydroxy‐
carboxylate to the oxo derivative was observed. After changing of the oxidizing agent to RuO /KIO
2
4
in water‐MeCN‐DCM biphasic system, [38] the desired oxo‐acid 5b was obtained in 85% yield. This
intermediate was transformed directly into lactam by reductive amination followed by spontaneous
cyclization. A large number of reducing hydride reagents have been studied for direct reductive
amination, including NaBH
isopropoxide [42]. In our study, the classical Leuckart reaction [NH
amination with ammonium acetate [NH OAc (10–15 equiv.), NaBH
3
CN [39], NaBH(OAc)
3
[40], pyridine–BH
OCHO] as well as the reductive
CN (1–5 equiv.) in MeOH or
3
[41], and NaBH
4
/titanium(IV)
4
4
3
MeOH/THF] [43] proved unsuccessful even at elevated temperature. The addition of acid resulted in
a complex mixture of products with lactone 1b in the largest amount [44]. Finally, the application of
microwaves (MW) [45] for reductive amination/cyclization generated the expected lactam 4b in 61%
yield from oxo‐acid 5b (52% total yield from lactone 1b). The use of a decreased amount of NaBH
1.2–3 equiv.) prevented the formation of lactone 1b as a by‐product. The elaborated alternative
3CN
(
method, like the previously described one, proved also be a convenient way for preparation N‐
alkylated derivatives of lactam 4b. We examined the reductive amination of oxo‐acid 5b under MW
irradiation in the presence of amines such as BnNH
were obtained in a one‐pot two‐step synthesis. The reducing agent was added to the reaction mixture
when the imine formation was completed. An increase of NaBH CN excess from 1 to 3 equiv.
2
and pMeOBnNH2. In this case, the best results
3
provided higher product yield. The microwave assisted reductive amination/cyclization generated
the N‐substituted lactams from oxo‐acid 5b in a short reaction time (30–40 min at 140 °C) and
satisfactory yields (70–73%). Target lactams 4e and 4f were obtained from lactone 1b in 63% and 60%
total yield, respectively.

Molecules 2020, 25, 2377
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Scheme 2. Synthesis of bisnorcholanic lactam derivatives via an oxo‐acid intermediate.
. Materials and Methods
.1. General
3
3
NMR spectra were recorded with Bruker Avance II 400 spectrometer operating at 400 MHz,
1
using CDCl
3
solutions with TMS as the internal standard (only selected signals in the H NMR spectra
are reported). Coupling constants (J) are given in Hz. The FTIR spectra were obtained using Nicolet™
700 spectrometer (Thermo Scientific, Waltham, MA, USA). The spectra were recorded in the range
6
−1
−1
between 4000 and 500 cm with a resolution of 4 cm and 32 scans using Attenuated Total Reflectance
ATR) techniques. ESI and ESI‐HRMS spectra were obtained on the Agilent 6530 Accurate‐Mass Q‐
TOF ESI and LC/MS system. Melting points were determined using MP70 Melting Point System
Mettler Toledo, Greifensee, Switzerland). Microwave reactions were performed in a Discover SP
(
(
microwave synthesizer (CEM Corp., Matthews, NC, USA) in a closed vessel with maximum power
input of 300 W. Thin‐layer chromatography (TLC) was performed on aluminum plates coated with
silica gel 60 F254 (Merck, Darmstadt, Germany), by spraying with ceric ammonium molybdate
(
CAM) solution, followed by heating. The reaction products were isolated by column
chromatography, performed using 70–230 mesh silica gel (J.T. Baker, Center Valley, PA, USA).
Tigogenin was obtained by hydrogenation of commercial diosgenin (Sigma‐Aldrich) in presence of
Pd/C catalyst [46].
3
.2. Chemical Synthesis
3
.2.1. Synthesis of Bisnorcholanic Lactam Derivatives via Oxo‐Amide Intermediates
Procedure for Lactam 4a and 4b Synthesis
Preparation of the Aminoalane Reagent from DIBALH and NH Cl
4
A solution of DIBAlH in toluene (1 M, 2.17 mL, 2.17 mmol, 20 equiv. relative to lactone 1b) was
added to a cooled (0–5 °C) suspension of NH Cl (0.122 g, 2.28 mmol, 21 equiv.) in anhydrous THF (10
4
mL) under argon. The reaction was stirred for 15 min in an ice bath and then 1.5 h at room
temperature. After this time, the obtained reagent solution was used directly for amide synthesis.
Synthesis of Oxo‐Amide 3b
The solution of aminoalane reagent (prepared from 20 equiv. of DIBAlH) was added dropwise
to a solution of lactone 1b (0.05 g, 0.1087 mmol, 1 equiv.) in anhydrous THF (ca 6 mL) at room
temperature. Stirring was continued for 16 h at reflux. After this time, the reaction mixture was
cooled, quenched with aqueous solution of KHSO and the product was extracted with ether. The
4

Molecules 2020, 25, 2377
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extract was washed with water, dried over anhydrous sodium sulfate, and the solvent was
1
evaporated. The crude product 2b ( H NMR (CDCl
3, 400 MHz) δ 5.73 (bs, 1H), 5.31 (bs, 1H), 4.29 (m,
1
3
0
H), 3.55 (m, 1H), 2.96 (m, 1H), 2.83 (m, 1H), 2.22 (m, 1H), 1.90 (m, 1H), 1.26 (d, J = 6.8, 3H), 0.91 (s,
H), 0.89 (s, 9H), 0.82 (s, 3H), 0.06 (s, 6H)) was immediately oxidized. PDC (3 equiv., 0.336 mmol,
.123 g) was added to the solution of crude hydroxyamide 2b in dry DCM. The reaction was stirred
for 3 h at room temperature. The solvent was evaporated and the crude product was purified by silica
gel column chromatography with MeOH/DCM (1:99) elution. Product 3b was obtained in 62% yield
(after two steps).
(
°
20S)‐3β‐t‐butyldimetylsilyloxy‐16‐oxo‐5α‐pregnane‐20‐carboxyamide (3b): white solid m.p. 210–212
1
C (DCM/MeOH). H NMR (CDCl
3, 400 MHz) δ 5.81 (bs, 1H), 5.50 (bs, 1H), 3.56 (m, 1H), 2.48 (d, J =
9
3
5
.9, 1H), 2.34 (m, 1H), 2.22 (m, 1H), 2.01 (m, 1H), 1.22 (d, J = 7.0, 3H), 0.88 (s, 9H), 0.81 (s, 3H), 0.76 (s,
H), 0.04 (s, 6H). 13C NMR (CDCl
, 100 MHz) δ, 218.4 (C), 179.0 (C), 72.0 (CH), 65.3 (CH), 54.1 (CH),
0.8 (CH), 44.9 (CH), 42.4 (C), 38.84 (CH), 38.82 (CH ), 38.5 (CH ), 37.7 (CH ), 36.8 (CH ), 35.6 (C), 34.4
), 31.8 (CH ), 28.5 (CH ), 25.9 (3CH ), 20.7 (CH ), 18.3 (C), 17.1 (CH ), 13.1 (CH ), 12.3
). IR (ATR): νmax (cm ): 3400, 3194, 1733, 1700, 1635, 1459, 1250, 1097, 1085. ESI‐MS
3
2
2
2
2
(CH), 32.1 (CH
2
2
2
3
2
3
3
−
1
(CH
3
), −4.6 (2CH
3
+
+
+
4
98 [M + Na] , 974 [2M + Na] . HRMS calcd. for C28
H
49NO SiNa 498.3374 (M + Na) , found 498.3364.
3
Synthesis of Lactam 4b with EtSiH/TFA
TFA (0.048 mL, 0.63 mmol, 10 equiv.) and Et SiH (0.2 mL, 1.26 mmol, 20 equiv.) were added to
3
the solution of oxo‐amide 3b (30 mg, 0.063 mmol) in dry DCE (4 mL). The reaction mixture was stirred
for 16 h at reflux. Then it was poured into water and product was extracted by DCM. The extract was
dried over anhydrous sodium sulfate, and the solvent was evaporated. Silica gel column
chromatography afforded two products: 3‐TBS lactam 4b (73%) eluted with 0.7% MeOH/DCM and
3
‐hydroxy‐lactam 4a (14%) eluted with 3% MeOH/DCM.
(
16S,20S)‐3β‐t‐butyldimetylsilyloxy‐5α‐pregnane‐20,16‐carbolactam (4b): white crystals, m.p. 275–
1
2
1
77 °C (DCM/MeOH). H NMR (CDCl3, 400 MHz) δ 5.72 (bs, 1H), 4.05 (m, 1H), 3.55 (m, 1H), 2.37 (m,
H), 2.09 (m, 1H), 1.24 (d, J = 7.4, 3H), 0.89 (s, 9H), 0.81 (s, 3H), 0.78 (s, 3H), 0.05 (s, 6H). 13C NMR
, 100 MHz) δ, 181.7 (C), 72.0 (CH), 58.9 (CH), 55.4 (CH), 55.3 (CH), 54.6 (CH), 45.0 (CH), 42.0
C), 38.8 (CH ), 38.6 (CH ), 37.2 (CH ), 36.7 (CH), 35.6 (C), 35.0 (CH), 33.9 (CH ), 32.3 (CH ), 31.9 (CH ),
), 25.9 (3CH ), 20.6 (CH ), 18.7 (C), 18.2 (CH ). IR (ATR):
(CDCl
3
(
2
2
2
2
2
2
2
8.6 (CH
2
3
2
3
), 14.3 (CH
3
), 12.4 (CH
3
), ‐4.6 (2CH
3
−
1
+
+
ν
max (cm ): 3173, 3082, 1701, 1655, 1454, 1249, 1102. ESI−MS 460 [M + H] , 941 [2M + Na] . HRMS calcd.
+
for C28
H50NO
2
Si 460.3605 (M + H) , found 460.3598.
Synthesis of Lactam 4a with EtSiH/Bi(OTf)
Bi(OTf) (14 mg, 0.5 equiv.) and Et SiH (0.012 mL, 2 equiv.) were added to the solution of oxo‐
amide 3b (20 mg, 0.04 mmol) in dry DCE (2 mL)/MeCN (2 mL). The reaction mixture was stirred for
6 h at reflux. After cooling the reaction mixture was poured into water and product was extracted
3
3
3
1
with chloroform. The extract was dried over anhydrous sodium sulfate, and the solvent was
evaporated. Silica gel column chromatography yield quantitatively product 4a eluted with 3%
MeOH/DCM.
(
16S,20S)‐3β‐hydroxy‐5α‐pregnane‐20,16‐carbolactam (4a): white crystals, m.p. 210–212 °C
1
(
2
DCM/MeOH). H NMR (CDCl
3
, 400 MHz) δ 6.53 (bs, 1H), 4.02 (m, 1H), 3.56 (m, 1H), 2.34 (m, 1H),
, 100 MHz)
δ, 182.0 (C), 71.0 (CH), 58.7 (CH), 55.6 (CH), 55.1 (CH), 54.4 (CH), 44.7 (CH), 41.9 (C), 38.7 (CH ), 38.0
), 20.6
.23 (bs, 1H), 2.06 (m, 1H), 1.21 (d, J = 7.5, 3H), 0.80 (s, 3H), 0.75 (s, 3H). 13C NMR (CDCl
3
2
(
(
CH
CH
2
), 36.9 (CH
), 18.6 (CH
2
), 36.8 (CH), 35.5 (C), 34.9 (CH), 33.7 (CH
2
), 32.1 (CH
2
), 31.93 (CH
2
), 28.5 (CH
2
−1
2
3), 14.2 (CH
3), 12.3 (CH
3). IR (ATR): νmax (cm ): 3267, 1685, 1450, 1044. ESI−MS 346 [M
+
+
+
+
H] , 713 [2M + Na] . HRMS calcd. for C22
H36NO
2
346.2741 (M + H) , found 346.2742.
Procedure for N‐Alkyl‐Lactam 4c and 4d Synthesis
Preparation of aminoalane reagent from DIBALH and BnNH
2
or MeNH xHCl
2

Molecules 2020, 25, 2377
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The desired aminoalane reagents were prepared from solution of DIBALH in toluene (1 M, 10
equiv. with reference to lactone 1b, 2.17 mmol, 2.17 mL) and MeNH xHCl (10.3 equiv. 2.2 mmol, 0.15
g) or BnNH (10.3 equiv. 2.2 mmol, 0.24 mL), analogously to aminoalane preparation from DIBALH
and NH Cl.
2
2
4
Synthesis of Hydroxy‐Amides 2c and 2d
The solution of aminoalane reagent (prepared from 10 equiv. of DIBAlH) was added dropwise
to a solution of lactone 1b (0.1 g, 0.217 mmol, 1 equiv.) in anhydrous THF (ca 4 mL) at room
temperature. Stirring was continued for 16 h at rt. After this time, the reaction mixture was cooled,
quenched with aqueous solution of KHSO and the product was extracted with ether. The extract was
4
washed with water, dried over anhydrous sodium sulfate, and the solvent was evaporated. The crude
product, without purification, was used in the next step.
(
20S)‐N‐methyl‐3β‐t‐butyldimetylsilyloxy‐16β‐hydroxy‐5α‐pregnane‐20‐carboxyamide (2c): white
1
solid, H NMR (CDCl
3, 400 MHz) δ 5.72 (d, J = 4.5, 1H), 1H), 4.21 (m, 1H), 3.55 (m, 1H), 3.42 (bs, 1H),
2
3
5
.83 (d, J = 4.8, 3H), 2.19 (m, 1H), 1.89 (m, 1H), 1.23 (d, J = 7.0, 3H), 0.90 (s, 3H), 0.89 (s, 9H), 0.81 (s,
13
H), 0.05 (s, 6H). C NMR (CDCl
4.2 (CH), 45.0 (CH), 42.7 (C), 40.2 (CH
), 31.9 (CH ), 28.6 (CH ), 26.4 (CH
), −4.6 (2CH ). ESI−MS 492 [M + H] . HRMS calcd. for C29
3
, 100 MHz) δ, 178.5 (C), 72.5 (CH), 72.1 (CH), 59.7 (CH), 54.4 (CH),
), 38.8 (CH), 38.6 (CH ), 37.1 (CH ), 35.8 (CH ), 35.5 (C), 35.1
), 25.9 (3CH ), 20.9 (CH ), 18.3 (C), 16.6 (CH
2
2
2
2
(CH), 32.0 (CH
2
2
2
3
3
2
3
), 13.2
+
+
(CH ), 12.4 (CH
3
3
3
H
54NO
3
Si 492.3873 (M + H) ,
found 492.3888.
(
20S)‐N‐benzyl‐3β‐t‐butyldimetylsilyloxy‐16β‐hydroxy‐5α‐pregnane‐20‐carboxyamide (2d): white
1
solid, H NMR (CDCl
3
, 400 MHz) δ 7.32 (m, 3H), 7.28 (m, 2H), 6.09 (t, J = 5.7, 1H), 4.45 (d, J = 5.7, 2H),
4
0
1
.22 (m, 1H), 3.55 (m, 1H), 2.83 (m, 1H), 2.19 (m, 1H), 1.89 (m, 1H), 1.25 (d, J = 7.1, 3H), 0.89 (s, 12H),
13
.81 (s, 3H), 0.06 (s, 6H). C NMR (CDCl
3
, 100 MHz) δ, 177.6 (C), 138.4 (C), 128.7 (2CH), 127.7 (2CH),
), 42.7 (C), 40.1
), 31.9 (CH ), 28.6
), −4.6 (2CH ). ESI−MS 568 [M
Si 568.4186 (M + H) , found 568.4197.
27.5 (CH), 72.4 (CH), 72.1 (CH), 59.4 (CH), 54.5 (CH), 54.1 (CH), 44.9 (CH), 43.5 (CH
2
(
(
CH
CH
2), 38.9 (CH), 38.6 (CH
2), 37.1 (CH2), 36.0 (CH
2), 35.5 (C), 35.1 (CH), 32.0 (CH
2
2
2
), 25.9 (3CH ), 20.9 (CH
3
2
), 18.2 (C), 16.7 (CH
3
), 13.2 (CH ), 12.3 (CH
3
3
3
3
+
+
+
+
H] , 1157 [2M + Na] . HRMS calcd. for C35
H58NO
Synthesis of Oxo‐Amides 3c and 3d
PDC (4 equiv.) was added to the solution of crude hydroxy‐amide 2c or 2d in dry DCM. The
reaction was stirred for 16 h at room temperature. The solvent was evaporated and the crude product
was purified by silica gel column chromatography. Product 3c was obtained in 61% yield (after two
steps) with hexane/AcOEt (3:7) elution. Product 3d was obtained in 86% yield (after two steps) with
hexane/AcOEt (75:25) elution.
(
20S)‐N‐methyl‐3β‐t‐butyldimetylsilyloxy‐16‐oxo‐5α‐pregnane‐20‐carboxyamide (3c): white crystals,
1
m.p. 203–204 °C (hexane/EtOAc). H NMR (CDCl
3
, 400 MHz) δ 5.63 (bd, J = 4.8, 1H), 3.55 (m, 1H), 2.84
= 2.8, J = 9.3, 1H), 1.21 (d, J = 7.0, 3H), 0.89
s, 9H), 0.83 (s, 3H), 0.76 (s, 3H), 0.06 (s, 6H). C NMR (CDCl , 100 MHz) δ, 218.5 (C), 177.2 (C), 72.0
CH), 65.2 (CH), 54.0 (CH), 50.7 (CH), 44.8 (CH), 42.4 (C), 39.6 (CH), 38.8 (CH
(
(
(
d, J = 4.8, 3H), 2.51 (d, J = 9.8, 1H), 2.23 (m, 2H), 2.02 (dt, J
1
2
13
3
2
), 38.5 (CH
2
), 37.8 (CH
2
),
),
3
1
1
4
6.8 (CH
8.3 (C), 17.1 (CH
461, 1250, 1099. ESI−MS 490 [M + H] . HRMS calcd. for C29
2
), 35.6 (C), 34.4 (CH), 32.1 (CH
2
), 31.8 (CH
2
), 28.4 (CH
3
2
), 26.4 (CH
3
), 25.9 (3CH
3
), 20.7 (CH
2
−
1
3
), 13.1 (CH ), 12.3 (CH
3
3), −4.6 (2CH
). IR (ATR): νmax (cm ): 3294, 1739, 1643, 1558,
+
+
H
52NO
3
Si 490.3716 (M + H) , found
90.3725.
(
20S)‐N‐benzyl‐3β‐t‐butyldimetylsilyloxy‐16‐oxo‐5α‐pregnane‐20‐carboxyamide (3d): white crystals,
1
m.p. 199–200 °C (hexane/EtOAc). H NMR (CDCl
3
, 400 MHz) δ 7.36 (m, 4H), 7.28 (m, 1H), 5.87 (t, J =
= 5.5, J = 14.8, 1H), 3.57 (m, 1H), 2.58 (d, J = 10.0,
H), 2.25 (m, 2H), 2.03 (m, 1H), 1.24 (d, J = 7.0, 3H), 0.90 (s, 9H), 0.83 (s, 3H), 0.76 (s, 3H), 0.07 (s, 6H).
5
1
.5, 1H), 4.64 (dd, J
1
= 5.5, J
2
= 14.8, 1H), 4.44 (dd, J
1
2
13
C NMR (CDCl
3, 100 MHz) δ, 218.3 (C), 176.3 (C), 138.6 (C), 128.6 (2CH), 127.8 (2CH), 127.3 (CH),
7
2.0 (CH), 65.2 (CH), 54.0 (CH), 54.5 (CH), 50.7 (CH), 44.9 (CH), 43.7 (CH
2), 42.4 (C), 39.8 (CH), 38.9

Molecules 2020, 25, 2377
8 of 13
(
CH
2
), 38.5 (CH
2
), 37.7 (CH
2
), 36.8 (CH
2
), 35.6 (C), 34.4 (CH), 32.1 (CH
2
), 31.8 (CH
3
2
), 28.5 (CH
2
), 25.9
−1
(3CH
3), 20.7 (CH
2), 18.2 (C), 17.2 (CH
3), 13.1 (CH
3), 12.3 (CH
3), −4.6 (2CH ). IR (ATR): νmax (cm ): 3280,
+
+
1
5
737, 1640, 1557, 1451, 1250, 1100. ESI−MS 566 [M + H] , 1153 [2M + Na] . HRMS calcd. for C35
66.4029 (M + H) , found 566.4046.
H56NO Si
3
+
Procedure for Lactam 4c and 4d Synthesis
Bi(OTf) (0.5 equiv.) and Et SiH (2 equiv.) were added to the solution of oxo‐amide 3c or 3d (10
3
3
mg) in mixture of dry DCE (2 mL) and MeCN (2 mL). The reaction mixture was stirred for 16 h at
reflux. Then the reaction mixture was poured into water and product was extracted by DCM. The
extract was dried over anhydrous sodium sulfate, and the solvent was evaporated. The crude product
was purified by silica gel column chromatography. Product 4c was eluted with hexane/AcOEt (2:8).
Product 4d was eluted with hexane/AcOEt (1:1).
(
16S,20S)‐N‐methyl‐3β‐hydroxy‐5α‐pregnane‐20,16‐carbolactam (4c): obtained in quantitative yield
1
as an amorphous solid. H NMR (CDCl
3
, 400 MHz) δ 3.92 (m, 1H), 3.55 (m, 1H), 2.76 (s, 3H), 2.42 (q,
= 12.0, 1H), 1.20 (d, J = 7.4, 3H), 0.82 (s, 3H), 0.68 (s, 3H). C
13
J = 7.4, 1H), 2.01 (ddd, J
NMR (CDCl , 100 MHz) δ, 178.1 (C), 71.2 (CH), 61.9 (CH), 55.9 (CH), 54.7 (CH), 54.4 (CH), 44.8 (CH),
1.9 (C), 38.8 (CH ), 38.1 (CH ), 37.04 (CH), 36.98 (CH ), 35.6 (C), 35.0 (CH), 32.2 (CH ), 31.5 (CH ),
1.1 (CH ), 28.5 (CH ), 27.6 (CH ), 20.6 (CH ), 18.8 (CH
1
= 5.9, J
2
= 7.8, J
3
3
4
3
3
+
2
2
2
2
2
−
1
2
2
3
2
3
), 14.2 (CH
3
), 12.3 (CH
3
). IR (ATR): νmax (cm ):
+
+
329, 1661, 1448, 1051. ESI−MS 360 [M + H] , 741 [M + Na] . HRMS calcd. for: C23
H) , found 360.2906.
H
38NO
2
360.2903 (M
+
(
16S,20S)‐N‐benzyl‐3β‐hydroxy‐5α‐pregnane‐20,16‐carbolactam (4d): obtained in quantitative yield
1
as an amorphous solid. H NMR (CDCl
3, 400 MHz) δ 7.32 (m, 3H), 7.22 (m, 2H), 5.02 (d, J = 14.5, 1H),
3
0
7
.81 (d, J = 14.5, 1H), 3.75 (m, 1H), 3.59 (m, 1H), 2.50 (q, J = 7.4, 1H), 1.22 (d, J = 7.4, 3H), 0.81 (s, 3H),
13
.72 (s, 3H). C NMR (CDCl
1.2 (CH), 58.8 (CH), 55.6 (CH), 54.8 (CH), 54.4 (CH), 44.8 (CH), 44.3 (CH
3
, 100 MHz) δ, 178.2 (C), 136.6 (C), 128.6 (2CH), 128.2 (2CH), 127.5 (CH),
), 42.0 (C), 38.8 (CH ), 38.1
), 31.1 (CH ), 28.5 (CH ), 20.6
2
2
(
(
CH
CH
2), 37.1 (CH), 37.0 (CH
2), 35.6 (C), 35.0 (CH), 32.2 (CH
2
), 31.5 (CH
2
2
2
−
1
2
), 19.0 (CH
3
), 14.3 (CH
3), 12.3 (CH
3). IR (ATR): νmax (cm ): 3359, 1659, 1441, 1046. ESI−MS 436 [M
+
+
+
+
H] , 893 [2M + Na] . HRMS calcd. C29
H42NO
2
436.3216 (M + H) , found 436.3224.
3
.2.2. Synthesis of Bisnorcholanic Lactam Derivatives via Oxo‐Acid 5b
Procedure for Oxo‐Acid 5b Synthesis
Method I. To a solution of lactone 1b (1.0 g, 2.17 mmol, 1.0 equiv.), in THF (75 mL) a solution of
KOH (0.608 g, 10.85 mmol, 5.0 equiv.) in water (30 mL) was added. The resulting suspension was
stirred vigorously at 50 °C until a conversion of 100% was reached. The progress of the reaction was
monitored by TLC (hexane/AcOEt, 7:3). After cooling to rt 1M hydrochloric acid was added until pH
6
‐7 was reached and the product was extracted with Et
were washed with brine, dried over anhydrous sodium sulfate, and concentrated under reduced
pressure. Crude hydroxy‐acid in dry pyridine was added to an ice‐cold solution of CrO (0.428 g, 4.28
2
O (3 × 50 mL). The combined organic layers
3
mmol, 2 equiv.) in pyridine (20 mL) and stirred at rt overnight. Then the mixture was decanted from
the tarry residue, which was washed with three portions of diethyl ether. The combined organic
solution was washed with 5% hydrochloric acid, aqueous 5% sodium hydrogen carbonate and brine.
The obtained solution was dried over anhydrous sodium sulfate and the solvent was evaporated
under reduced pressure. Purification by silica gel column chromatography gave lactone 1b (20%)
eluted with 70% hexane/AcOEt and oxo‐acid 5b (70%) eluted with 50% hexane/AcOEt.
Method II. To a solution of lactone 1b (5.0 g, 0.011 mol, 1.0 equiv.), in THF/H
NBr (0.07 g, 2 mol%) and LiOH (1.3 g, 0.054 mol, 5.0 equiv.) were added. The resulting suspension
2
O (200 mL, 5/2) n‐
Bu
4
was stirred overnight at room temperature. The progress of the reaction was monitored by TLC. After
the hydrolysis was completed the solvent was evaporated under reduced pressure (50). The residue
was dissolved in the MeCN (120 mL) and DCM (120 mL) mixture, then RuO
mol%) and distilled water (160 mL) were added to the solution. The aqueous layer was acidified with
2
(0.03 g, 0.2 mmol, 2

Molecules 2020, 25, 2377
9 of 13
1
0% aqueous HCl solution, to pH 8, and then solid KIO
4
(2.5 g, 0.011 mol, 1.0 equiv.) was added in
five portions. The reaction mixture was stirred vigorously at room temperature for 2 days. The
progress of the reaction was monitored by TLC in the MeOH/DCM (1/9) mixture. Then isopropyl
alcohol (20 mL) was added to quench the reaction and stirring was continued until complete
precipitation of RuO was observed. The reaction mixture was diluted with water (200 mL) and
2
acidified with 10% HCl solution to pH 6. The product was extracted with DCM (3 × 30 mL). The
combined organic layers were dried over anhydrous sodium sulfate, and the solvent was evaporated.
The residue was filtered through a short pad of silica gel with hexane/AcOEt (1:1) elution. Crude
product was purified by crystallization (hexane/AcOEt, 1:1) to give 5b (4.401 g, 85%) as a white
crystalline material.
(
20S)‐3β‐t‐butyldimetylsilyloxy‐16‐oxo‐5α‐pregnane‐20‐carboxylic acid (5b): white crystals, m.p.
1
2
2
34–236 °C (hexane/AcOEt). H NMR (CDCl
3
, 400 MHz) δ 9.64 (bs, 1H), 3.57 (m, 1H), 2.52 (m, 1H),
= 18.4, 1H), 2.02 (m, 1H), 1.26 (d, J = 7.0, 3H), 0.89 (s, 9H), 0.83
, 100 MHz) δ, 217.2 (C), 181.9 (C), 72.0 (CH), 65.0
CH), 54.1 (CH), 50.8 (CH), 44.9 (CH), 42.2 (C), 39.0 (CH ), 38.5 (CH ), 37.6 (CH), 37.5 (CH ), 36.8 (CH ),
), 25.9 (3CH ), 20.7 (CH ), 18.2 (C), 16.4 (CH ), 13.1
.39 (d, J = 10.5, 1H), 2.25 (dd, J
1
= 6.9, J
2
(
(
s, 3H), 0.77 (s, 3H), 0.06 (s, 6H). 13C NMR (CDCl
3
2
2
2
2
3
5.6 (C), 34.4 (CH), 32.1 (CH
CH ), 12.3 (CH ), −4.6 (2CH
087. ESI‐MS 475 [M − H] . HRMS calcd. for C28
2
), 31.8 (CH
2
), 28.4 (CH
2
3
2
3
−1
(
1
3
3
3
). IR (ATR): νmax (cm ): 3300‐2500 (broad), 1743, 1721, 1459, 1250, 1097,
−
−
H
47
O Si 475.3249 (M − H) , found 475.3248.
4
Reductive Amination/Cyclization of Oxo‐Acid (5b) with Ammonium Acetate/Sodium Borohydride
Oxo‐acid 5b (0.15 g, 0.32 mmol, 1 equiv.), NH OAc (0.365 g, 4.74 mmol, 15 equiv.), NaBH CN
0.060 g, 0.91 mmol, 3.0 equiv.) and MeOH (4 mL) were mixed in the 10 mL‐tube sealed with a
4
3
(
stopper. The resulting suspension was stirred for 40 min at 120 °C under microwave irradiation. Then
the reaction mixture was poured to 5% citric acid aqueous solution (15 mL) and product was
extracted with DCM (3 × 15 mL). The combined organic layers were washed with brine (1 × 15 mL),
dried over anhydrous sodium sulfate and the solvent was evaporated. Purification by silica gel
column chromatography gave product 4b (61% yield) eluted with 1% MeOH/DCM.
Procedure for N‐Alkyl‐Lactam 4e, 4f ‐ Synthesis from Oxo‐Acid 5b
Oxo‐acid 5b (0.15 g, 0.32 mmol, 1 equiv.), amine (BnNH
2
, p‐MeOBnNH , 0.91 mmol, 3 equiv.)
2
and THF (3 mL) were added to the 10 mL‐tube sealed with a stopper. The resulting suspension was
stirred for 20 min at 140 °C under microwave irradiation (a clear yellow solution was formed). Then,
NaBH
was heated under the same conditions for another 20 min. Then the reaction mixture was poured into
0 mL of a 5% citric acid aqueous solution and product was extracted with DCM (3 × 20 mL). The
3CN (0.060 g, 0.91 mmol, 3.0 equiv.) and MeOH (3 mL) were added to the tube. The solution
2
combined organic layers were washed with saturated NaCl solution (1 × 20 mL), dried over
anhydrous sodium sulfate, and the solvent was evaporated. The crude product was purified by
column chromatography on silica gel.
(
16S,20S)‐N‐benzyl‐3β‐t‐butyldimetylsilyloxy‐5α‐pregnane‐20,16‐carbolactam (4e): eluted with
1
hexane/AcOEt/DCM (6:2:2) in 73% yield (0.126 g), as an amorphous solid. H NMR (CDCl
3, 400 MHz)
δ 7.31 (m, 3H), 7.21 (m, 2H), 5.01 (d, J = 14.5, 1H), 3.81 (d, J = 14.5, 1H), 3.74 (m, 1H), 3.53 (m, 1H), 2.50
(
6
5
m, 1H), 1.87 (m, 1H), 1.77 (m, 1H), 1.21 (d, J = 7.5, 3H), 0.89 (s, 9H), 0.80 (s, 3H), 0.71 (s, 3H), 0.04 (s,
13
H). C NMR (CDCl
8.8 (CH), 55.6 (CH), 54.8 (CH), 54.5 (CH), 44.9 (CH), 44.3 (CH
), 35.6 (C), 35.0 (CH), 32.3 (CH ), 31.9 (CH ), 31.1 (CH
), 18.2 (C), 14.3 (CH ), 12.3 (CH ), −4.6 (2CH
50 [M + H] , 572 [M + Na] . HRMS calcd. for C35
3
, 100 MHz) δ, 178.2 (C), 136.6 (C), 128.6 (CH), 128.2 (2CH), 127.4 (CH), 72.0 (CH),
), 42.0 (C), 38.8 (CH ), 38.6 (CH ), 37.1
), 28.6 (CH ), 25.9 (3CH ), 20.6 (CH ),
2
2
2
(CH, CH
2
2
2
2
2
3
2
−1
1
5
8.9 (CH
3
3
3
3
). IR (ATR): νmax (cm ): 1670, 1249, 1090. ESI−MS
+
+
+
H
56NO
2Si 550.4075 (M + H) , found 550.4078.
(
16S,20S)‐N‐(p‐methoxybenzyl)‐3β‐t‐butyldimetylsilyloxy‐5α‐pregnan‐20,16‐carbolactam
(4f):
, 400
1
eluted with hexane/AcOEt/DCM (6:2:2) in 75% yield as an amorphous solid. H NMR (CDCl
3
MHz) δ 7.14 (d, J = 8.6, 2H), 6.85 (d, J = 8.6, 2H), 4.95 (d, J = 14.4, 1H), 3.80 (s, 3H), 3.75 (d, J = 14.4, 1H),

Molecules 2020, 25, 2377
10 of 13
3
3
.72 (m, 1H), 3.54 (m, 1H), 2.48 (q, J = 7.4, 1H), 1.86 (m, 1H), 1.20 (d, J = 7.4, 3H), 0.89 (s, 9H), 0.80 (s,
H), 0.70 (s, 3H), 0.05 (s, 6H). 13C NMR (CDCl
, 100 MHz) δ, 178.1 (C), 158.9 (C), 129.5 (2CH), 128.7
), 54.8 (CH), 54.5 (CH), 44.9 (CH), 43.7
), 35.6 (C), 35.0 (CH), 32.3 (CH ), 31.9 (CH ), 31.1
), 18.2 (C), 14.3 (CH ), 12.4 (CH ), −4.6 (2CH ),. IR
3
(
(
(
(
C), 113.9 (2CH), 72.4 (CH), 58.7 (CH), 55.6 (CH), 55.2 (CH
3
CH
CH
2
), 42.0 (C), 38.8 (CH
2
), 38.6 (CH
2
), 37.2 (CH, CH
2
2
2
2), 28.6 (CH ), 25.9 (3CH
2
3), 20.6 (CH
2
), 18.9 (CH
3
3
3
3
−1
ATR): νmax (cm ): 1682, 1612, 1512, 1453, 1244, 1090. ESI‐MS 580 [M + Na]+, 602 [M + Na]+. HRMS
+
calcd. for C36
H
58NO
3
Si 580.4180 (M + H) , found 580.4186.
4
. Conclusions
To summarize, two alternative procedures for the synthesis of bisnorcholanic lactams, hitherto
unknown aza‐analogs of naturally occurring vespertilin lactone, were reported. Both elaborated
methods provided short and efficient routes to the target lactam as well as to its N‐alkyl derivatives.
A convenient substrate employed in these syntheses was bisnorcholanic lactone, readily available by
tigogenin degradation. A key‐intermediate in the first strategy was 16‐oxo‐5α‐pregnane‐20‐
carboxyamide obtained in two consecutive reactions: ammonolysis of bisnorcholanic lactone with
prepared in situ aminoalane and the 16β‐hydroxyl group oxidation. The cyclization of the oxo‐amide
with a simultaneous deprotection of the 3β‐hydroxy group proceeded smoothly under reductive
conditions (Et
3
SiH/Bi(TfO) ) and provided the desired lactam in a quantitative yield. When the
3
lactone ring‐opening was carried out using the aluminum amide reagent prepared from primary
amine or its hydrochloride (instead of ammonium chloride), different N‐alkyl lactams were obtained
by employing an analogous protocol. An alternative strategy for the lactam preparation comprised
of lactone hydrolysis, oxidation of the obtained in situ hydroxy‐acid to oxo‐acid, and a microwave‐
assisted reductive amination. In addition, this procedure can be successfully adapted to the synthesis
of N‐alkylated derivatives of bisnorcholanic lactam by using various amines for the reductive
amination step. Of the two elaborated synthetic strategies, the first approach is advantageous as is
more efficient and completely stereoselective. This method benefited from the intramolecular
reductive amination of the 16‐carbonyl group that occurs from the β‐side only, contrary to
intermolecular reductive amination used in the second protocol, in which the lactam precursor, 16β‐
amino‐5α‐pregnanecarboxylic acid, was accompanied by minor amounts of its 16α‐amino isomer,
which is unable to lactamize due to steric reasons. Further study on the application of bisnorcholanic
lactam in the synthesis of steroidal alkaloids and the evaluation of biological activity of different
lactam congeners is currently in progress.
Supplementary Materials: Experimental procedure for synthesis of lactone 1b and H‐NMR, 13C‐NMR spectra
1
of compounds 2b–5b, NOE and ROSEY spectra for compound 4a, respectively.
Author Contributions: conceptualization, A.W.; investigation, A.W., P.B., D.P., A.B.; methodology, A.W., A.B.,
D.P., S.W.; formal analysis, A.W.; A.B.; writing—original draft preparation, A.W., A.B.; writing—review, editing
and supervising, J.W.M.; funding acquisition, J.W.M. All authors have read and agreed to the published version
of the manuscript.
Funding: This research was funded by the POLISH NATIONAL SCIENCE CENTRE, grant number
2015/17/B/ST5/02892.
Conflicts of Interest: The authors declare no conflicts of interest.
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Sample Availability: Samples of the compounds are not available from the authors.
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