Neurosteroid analogues: Synthesis of 6-aza-allopregnanolone

Article abstract of DOI:10.1016/j.tet.2005.01.055

An efficient synthesis of 6-azapregnane derivatives and their biological activity is described. The nitrogen was introduced into the B ring using Beckmann rearrangement of the (E)-oxime of 6-oxo-B-nor-5α-pregnane derivatives. The required 3α-hydroxyl was produced either by solvolysis of the corresponding 3β-mesyloxy group or by the Meerwein-Ponndorf-Verley reduction of the 3-oxo group; this reduction could be carried out selectively with an unprotected 3,20-dioxo derivative. The binding of the 6-aza-steroids to the γ-aminobutyric acid receptor (GABA_A) was measured using [³⁵S]-tert-butyl-bicyclo[2.2.2]phosphorothionate (TBPS) and [ ³H]flunitrazepam. The only analogue to be slightly active was that lacking any oxygen function in position 3.

Full text of DOI:10.1016/j.tet.2005.01.055

Tetrahedron 61 (2005) 2269–2278
Neurosteroid analogues: synthesis of 6-aza-allopregnanolone
*
Alexander Kasal, Libor Matyas and Milos Budesınsky
´ˇ
ˇ
ˇˇ´
´
Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic, CZ166 10 Prague 6, Czech Republic
Received 13 October 2004; revised 20 December 2004; accepted 14 January 2005
Available online 29 January 2005
Abstract—An efficient synthesis of 6-azapregnane derivatives and their biological activity is described. The nitrogen was introduced into the
B ring using Beckmann rearrangement of the (E)-oxime of 6-oxo-B-nor-5a-pregnane derivatives. The required 3a-hydroxyl was produced
either by solvolysis of the corresponding 3b-mesyloxy group or by the Meerwein–Ponndorf–Verley reduction of the 3-oxo group; this
reduction could be carried out selectively with an unprotected 3,20-dioxo derivative. The binding of the 6-aza-steroids to the g-aminobutyric
acid receptor (GABAA) was measured using [35S]-tert-butyl-bicyclo[2.2.2]phosphorothionate (TBPS) and [3H]flunitrazepam. The only
analogue to be slightly active was that lacking any oxygen function in position 3.
q 2005 Elsevier Ltd. All rights reserved.
1. Introduction
some quaternary ammonium salts (e.g., compounds 6 and
7)15 functioned well on isolated receptors, they were
inactive in a living organism.
In the search for biologically active analogues of natural
hormones, substitution of a heteroatom for a carbon atom
has often been successful. Total1 or partial syntheses2–4
have led to a number of steroids with a nitrogen atom in the
skeleton. The substitution can modify the chemical nature of
functionalities present in the molecule: for example, oxo
derivatives are thus converted into lactams5,6 (e.g., 1, see
Fig. 1) or their vinylogues7 (e.g., 2). The analogues could
mimic an original hormone but not fulfil its functions8,9 or
behave as the original.10–12 Recently, 6-aza-5a-cholestan-
3b-ol (3) and its pregnane derivative 4 were found to be
specific phosphatidylinositol phospholipase C inhibitors
with antitumour activity.13 Further, analogues with a
heteroatom in the skeleton can also be useful for the study
of interactions14 between a ligand and its receptor.
Several methods are known for the preparation of
6-azasteroids, all based on oxidation of 5-unsaturated
steroids into 5-oxo-7-oic acids or their derivatives. This
process13 is less efficient in 3-substituted seco-steroids
which easily lose the 3-substituents. The loss could be
prevented by the use of 3-silylated intermediates17 or
exploited in a different route described by Sharp.18 Here we
present an alternative synthesis of 6-azapregnane
derivatives (3a-hydroxy-6-aza-5a-pregnan-20-one, 8, 3a-
hydroxy-6-aza-5a-pregnane-7,20-dione, 9): the loss of one
carbon atom, required for the synthesis of the piperidine B
ring, takes place during the preparation of the starting
material—a 7-norsteroid derivative.
Functional groups essential for neuronal activity of
allopregnanolone (3a-hydroxy-5a-pregnan-20-one, 5) com-
prise the 3a-hydroxy and 20-oxo groups. Since the B ring
seems to be distant enough from these critical points, we
envisaged that the introduction of a nitrogen atom into
position 6 of allopregnanolone (5) would have no
detrimental effect on its biological activity. In contrast, we
hoped to obtain products with increased solubility, since
low solubility of our earlier analogues in body liquids15,16
often marred their biological activity and the usual ways of
making compounds more soluble did not help: even though
2. Results and discussion
2.1. Synthesis
The starting material, (20R)-7-norpregn-5-ene-3b,20-diyl
acetate benzoate19 (10, see Scheme 1) was converted into
6a-bromo-5b-alcohol 11 and then epoxide 12. The 5b,6b-
configuration of the epoxide was apparent from its 1H NMR
spectrum (a narrow multiplet of H-3a, typical20 of 3b-
hydroxy-5b-steroids). Lewis acid treatment of the epoxide
12 yielded the desired 6-ketone 13. Although 6-oxosteroids
of the normal series give a single oxime only, ketone 13
reacted with hydroxylamine to give two oximes (14, 15).
Their 1H NMR spectra21 were conspicuously different with
Keywords: 6-Azasteroids; Beckmann rearrangement; Henbest reaction;
Conformation of oximes; GABAA receptor; NMR spectroscopy.
* Corresponding author. Tel.: C420 220 183 314; fax: C420 220 183 578;
e-mail: kasal@uochb.cas.cz
0040–4020/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2005.01.055

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A. Kasal et al. / Tetrahedron 61 (2005) 2269–2278
Figure 1.
the presence or absence of a signal at d 3.04. To distinguish
between the E- and Z-configuration of the oximes, the
complete structural assignment of all proton and carbon
rearrangement, induced with mesyl chloride in pyridine.
The 1H NMR spectrum of the resulting 3b-mesyloxy lactam
16 confirmed the above assignment of configuration: the
proton next to the nitrogen atom (i.e., the H-5) interacted
strongly with H-4 hydrogens (JZ12.6, 3.2 Hz). The
solvolysis of the mesylate 16 in the presence of potassium
nitrite in DMSO produced alcohol 17 with the required
3a-configuration. Before the subsequent transformations, its
hydroxy group was protected by etherification: the
20-benzoyloxy group in the methoxyethoxymethoxy
(MEM) ether 18 was hydrolysed, and oxidised; deprotection
afforded the 6-aza analogue of 7-oxo-allopregnanolone 9.
Equally, the MEM-ether 18 was reduced with lithium
aluminium hydride, oxidised and deprotected to yield the
desired 6-aza analogue of allopregnanolone 8 (Table 2).
1
signals in their H and 13C NMR spectra was carried out
using homonuclear and heteronuclear 2D-COSY experi-
ments (for data—see Table 1). The stereochemical assign-
ment of methylene protons (in a- and b-position) was
derived from 2D-ROESY spectra using mainly the NOE
contacts of b-protons with the 18- and/or 19-Me group. The
comparison of proton chemical shifts in both isomers shows
a significant downfield shift (0.95 ppm) for H-4a in the
minor and more lipophilic oxime 14, while the major and
less lipophilic oxime 15 shows a smaller downfield shift
(0.37 ppm) for H-15a. The inspection of models indicates
that H-4a is sterically closer to the oxime oxygen atom in
the isomer with the Z-configuration and the H-15a appears
very close to the oxime oxygen atom in the isomer with the
The low yield of the above solvolysis, however, failed to
justify the orthogonal protection of both hydroxy groups in
the starting material. Therefore, the hydroxy epoxide 12 was
first acetylated to 3-acetate 19 and only then treated with
boron trifluoride etherate. 3b-Acetoxy ketone 20 also
yielded a mixture of two oximes (21, 22), whose respective
structures were assigned analogously to the earlier men-
tioned oximes 14 and 15. The major and more polar oxime
22 was rearranged upon action of mesyl chloride in pyridine
yielding cleanly the 3b-acetoxy lactam sought (23).
˚
E-configuration (H/O distances are ca. 2.4 and 2.6 A,
respectively); the van der Waals deshielding effect is
presumably responsible for the observed downfield shifts.
Analogous downfield shifts were observed also for the
corresponding carbon atoms (1.99 ppm for C-4 and
1.59 ppm for C-15). Therefore, the Z-configuration could
be assigned to oxime 14 and E-configuration to oxime 15.
The major oxime 15 was submitted to the Beckman

A. Kasal et al. / Tetrahedron 61 (2005) 2269–2278
2271
Scheme 1. Reagents and conditions: (a) BF3$Et2O, 64% or 94%; (b) NH2OH$HCl, KHCO3, MeOH; 52%; (c) MsCl, py, 0 8C, 4 h; 97%; (d) KNO2, DMSO,
115 8C; 26%; (e) KOH, then PCC, then HCl; 72%; (f) LAH, then CrO3, then HCl, 20%; (g) CrO3, 57%; (h) H2IrCl6, H3PO3, (Me)2CHOH; 57 or 83%.
Table 1. Proton and carbon-13 chemical shifts of oximes 14, 15, 21 and 22 in CDCl3
Proton
14
15
21
22
Carbon
14
15
21
22
1a
1b
2a
2b
3
4a
4b
5
1.13
1.65
1.87
1.57
3.71
3.05
1.65
2.13
2.35
1.00
1.45
1.28
1.25
1.90
1.53
1.35
1.81
1.37
1.95
1.79
0.69
0.82
5.15
1.28
7.89
—
1.17
1.68
1.88
1.54
3.69
2.10
1.35
2.05
2.46
1.11
1.45
1.28
1.26
1.94
1.59
1.74
1.82
1.31
1.78
1.82
0.76
0.68
5.17
1.27
7.59
—
1.17
1.67
1.89
1.60
4.80
3.03
1.74
2.15
2.34
1.02
1.45
1.29
1.27
1.92
1.54
1.38
1.97
1.36
1.81
1.81
0.69
0.84
5.15
1.28
7.00
2.03
8.05
7.44
7.55
1.21
1.70
1.90
1.60
4.77
2.11
1.43
2.09
2.46
1.13
1.46
1.27
1.31
1.94
1.61
1.75
1.84
1.32
1.77
1.82
0.76
0.69
5.17
1.28
7.32
2.04
8.06
7.44
7.56
1
2
3
4
5
6
8
9
10
11
12
13
14
15
16
17
18
34.28
30.49
71.97
33.07
53.86
163.84
44.88
57.00
40.67
21.01
38.89
44.76
53.36
25.81
25.07
54.38
13.07
14.01
73.17
20.04
165.68
—
34.52
30.88
71.60
31.08
53.46
163.76
44.01
58.92
40.48
21.16
39.70
45.28
54.24
27.39
25.50
54.57
13.40
12.62
73.36
19.98
165.71
—
34.13
26.63
73.46
29.36
53.53
163.54
44.77
56.94
40.57
20.99
38.89
44.73
53.38
25.11
25.77
54.42
13.09
13.89
73.18
20.04
165.69
170.47
21.34
130.80
129.64
128.35
132.74
34.38
26.88
73.33
27.35
53.12
163.40
43.88
58.90
40.37
21.13
39.67
45.28
54.19
27.37
25.50
54.59
13.40
12.49
73.33
19.97
165.71
170.52
21.32
130.82
129.64
128.34
132.72
8
9
11a
11b
12a
12b
14
15a
15b
16a
16b
17
18-Me
19-Me
20
21-Me
N-OH
OAc
C6H5: o-
C6H5: m-
C6H5: p-
19
20
21
C]O
Ac: C]O
CH3
C6H5: i-
C6H5: o-
C6H5: m-
C6H5: p-
—
—
130.75
129.63
128.34
132.74
130.79
129.63
128.34
132.73
8.05
7.44
7.56
8.05
7.45
7.56

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A. Kasal et al. / Tetrahedron 61 (2005) 2269–2278
Table 2. Proton and carbon-13 chemical shifts of aza-derivatives 8, 9, 30, 32 and 34 in CDCl3
Proton
8
9
30
32
34
Carbon
8
9
30
32
34
1a
1b
2a
2b
3a
3b
4a
4b
5
7a
7b
8
1.34
1.50
w1.65
w1.65
—
w1.46
w1.46
w1.76
w1.70
—
4.19
w1.60
w1.60
3.59
—
0.99
1.65
1.56
1.52
1.31
1.76
1.37
1.42
3.01
—
0.89
1.66
1.46
1.39
1.29
1.71
w1.36
w1.36
2.18
2.30
2.99
1.57
0.78
1.50
1.28
1.24
2.05
1.03
1.65
1.16
1.09
1.54
1.32
0.76
0.88
3.72
1.13
0.90
1.66
1.48
1.42
1.31
1.73
w1.39
w1.39
2.20
2.32
3.01
1.57
0.81
1.60
1.31
1.42
2.02
1.16
1.62
1.20
1.63
2.15
2.51
0.62
0.88
—
1
2
3
4
5
7
8
9
10
11
12
13
14
15
16
17
18
30.32
28.56
66.73
35.67
58.66
52.44
35.96
53.67
36.36
20.41
38.91
44.34
53.95
23.87
22.97
63.50
13.50
10.94
209.48
31.52
—
28.89
28.46
65.76
34.16
55.04
173.52
42.35
51.55
35.33
20.69
38.44
45.66
52.24
26.25
23.51
62.59
13.48
9.42
34.45
21.01
24.48
27.34
61.72
173.43
42.32
51.60
35.36
20.73
38.92
44.27
52.02
25.74
26.05
54.13
12.68
10.35
72.95
19.92
170.38
21.54
36.53
21.53
25.64
28.70
65.67
52.64
35.83
54.16
36.42
20.29
40.02
42.68
53.24
25.76
23.93
58.34
12.62
12.08
70.55
23.65
—
36.50
21.47
25.56
28.53
65.59
52.36
35.83
53.98
36.40
20.41
38.94
44.34
53.98
23.86
22.95
63.54
13.49
12.03
209.50
31.50
—
4.15
w1.58
w1.58
2.69
2.36
3.02
1.57
0.90
1.62
1.31
1.43
2.03
1.17
w1.63
1.20
1.66
2.17
2.52
0.62
0.86
—
—
—
2.20
1.40
1.67
1.32
1.39
2.06
1.58
2.21
1.75
1.73
2.14
2.49
0.69
0.87
—
2.18
1.49
1.57
1.25
1.20
1.84
1.28
1.73
1.26
1.67
2.15
1.59
0.68
0.86
4.86
1.16
2.02
5.22
9
11a
11b
12a
12b
14
15a
15b
16a
16b
17
18-Me
19-Me
20
21-Me
OAc
NH
19
20
209.77
31.55
—
21
OAc: C]O
CH3
—
—
—
—
2.12
—
2.12
—
5.38
2.12
—
—
a
a
a
a Position of NH signal was not determined.
Hydrolysis and oxidation converted the lactam 23 into
3b,20-dihydroxy- and 3,20-dioxo lactams 24 and 25. The
Henbest22 reaction of the latter (i.e., partial reduction of the
3-oxo group with 2-propanol catalysed with hexachloro-
iridic acid in the presence of phosphorous acids) produced a
monohydroxy ketone identical with the above 3a-hydroxy
lactam 9.
ketone identical with the above 3a-hydroxy-6-aza-5a-
pregnan-20-one (8).
For comparison, a few 3-deoxy analogues were prepared
from (20R)-7-oxopregn-5-ene-3b,20-diyl diacetate23 (28,
see Scheme 2): oxidation according to a literature protocol24
yielded crude 5,7-seco-6-nor acid 29 which was treated with
1
ammonia, and hydrogenated: 13C and H NMR spectra of
Analogously, the lactam 23 was reduced with lithium
aluminium hydride in dioxane yielding dihydroxy amine 26.
Oxidation with chromic acid afforded dioxo amine 27, the
Henbest reduction of which produced a monohydroxy
lactam 30 proved the loss of the substituent in position 3.
The lactams 30 and 31 were converted to lactam 33 and
amines 32 and 34 by routine reactions (see Scheme 2). Since
the last compound was not found identical with recently
Scheme 2. (a) HIO4, KMnO4, K2CO3, (CH3)3COH, 45 8C, 6 h; then MeOH, CH2N2; (b) NH3, MeOH, 55 8C, 20 h, then H2/Pt; 14% of 30 from compound 28.

A. Kasal et al. / Tetrahedron 61 (2005) 2269–2278
2273
published13 ‘6-aza-pregnan-20-one’ (35), additional 1H
NMR experiments were carried out to establish the C-5
configuration. The NOE contacts observed in 2D-H,H-
ROESY spectrum allowed us to distinguish protons on the
a- and b-side of the steroid skeleton: the absence of NOE
contact between H-5 proton at d 2.20 and 19-methyl protons
on one side and the observed NOE contact of H-5 with axial
protons H-1a, H-3a, H-7a and H-9a on the other side
indicate unequivocally the 5a-configuration for our 6-aza-
pregnan-20-one 34. Thus the earlier produced ‘6-aza-
pregnan-20-one’ should be given the 5b-configuration (35).
50 8C /100 Pa. Optical rotations were measured in chloro-
form using an Autopol IV (Rudolf Research Analytical,
Flanders, USA, [a]D values are given in 10K1 deg cm2 gK1).
IR spectra were recorded on a Bruker IFS 88 spectrometer in
chloroform solutions, wave-numbers are given in cmK1
.
Detailed NMR study of selected compounds was performed
on Bruker AVANCE-500 instruments (1H at 500.13 MHz;
13C at 125.77 MHz). Proton NMR spectra of other
compounds were measured on Varian UNITY-200 (at
200 MHz) and/or Bruker AVANCE-400 spectrometer (at
400 MHz) in CDCl3 with tetramethylsilane as internal
reference. Chemical shifts are given in ppm (d-scale) and
coupling constants in Hz. Unless otherwise stated, the data
were interpreted as the first-order spectra. Thin-layer
chromatography (TLC) was performed on silica gel (ICN
Biochemicals). Preparative TLC (PLC) was carried out on
200!200 mm plates coated with a 0.7-mm thick layer of
the same material. For column chromatography, 60–120 m
silica gel was used. Whenever aqueous solutions of
hydrochloric acid, potassium hydrogencarbonate or carbon-
ate were used, their concentration was 5%. Solvents were
evaporated on a rotary evaporator in vacuo (0.2 kPa, bath
temperature 40 8C).
2.2. Activity evaluation
Neuronal activity of 6-aza-allopregnanolone (8) and its
7-oxo derivative 9 was routinely checked by in vitro tests
using [3H]muscimol and [35S]tert-butylbicyclo[2.2.2]-phos-
porothionate (TBPS) and [3H]flunitrazepam as radiolabelled
ligands to g-aminobutyric acid receptors (GABAA). The
former two tests utilised receptors isolated from male rat
brain and binding of the ligands was measured in the
absence and presence of the tested compounds. For the last
test, primary neuronal culture, obtained from young rat
brains, was used and the binding of [3H]flunitrazepam in
neurones was measured in the presence of the varying
concentration of the tested compounds. Allopregnanolone
was used as a standard to check the viability of the methods.
Preliminary results revealed that neither amine 8 nor lactam
9 exerted any binding in the three tests. Surprisingly, low
activity only was found in a 3-deoxy analogue 34. Complete
biological results will be published in a separate paper
dealing primarily with the testing methods.
The [3H]flunitrazepam test of binding of the products was
carried out by using neurones in culture.26 The TBPS and
muscimol test was done with GABAA receptors.15
4.1.1. (20R)-6a-Bromo-5-hydroxy-7-nor-5b-pregnane-
3b,20-diyl acetate benzoate (11). A solution of olefin 10
(3.9 g, 8.66 mmol) in dioxane (40 mL) was treated with
perchloric acid (10%, 2 mL) and N-bromo acetamide (1.8 g,
13.8 mmol) at 15 8C. After 1 h, the mixture was poured into
a cold solution of potassium hydrogen sulfite (7%, 100 mL).
The precipitate formed was filtered off, the product was
dissolved in chloroform (100 mL) and washed with water
(30 mL). The solution was dried over sodium sulfate and
filtered through a layer of silica gel (10 g) and the solvent
was evaporated in vacuo to give the title compound 11
(4.7 g, 99%) as a colourless oil. A small sample was purified
by thin layer chromatography; [found: C, 63.1; H, 7.4; Br,
14.1. C29H39BrO5 requires C, 63.32; H, 7.18; Br, 14.61%].
dH (200 MHz, CDCl3) 8.04 (2H, m, ortho-ArH), 7.60 (1H,
m, meta-ArH), 7.45 (2H, t, para-ArH), 5.35–5.10 (1H, m,
H-3), 5.21–5.10 (1H, m, H-20), 4.34 (1H, d, JZ6.2 Hz,
H-6), 2.06 (3H, s, MeCO), 1.28 (3H, d, JZ6.0 Hz, H-21),
0.90 (3H, s, H-19), 0.72 (3H, s, H-18).
3. Conclusions
While the synthesis proceeded according to expectation, no
neuronal activity mediated through the g-aminobutyric acid
receptor was found in allopregnanolone analogues 8 and 9.
Nevertheless, even these results can have their value: they
may demonstrate that the steroid binding site of the GABAA
receptor requires a steroid compound having no electro-
negative substitution in the B ring. The earlier report on
inactivity of 6-oxa-allopregnanolone25 points to the same
conclusion. Thus the structure–activity consideration should
concentrate not only on the presence of polar groups in
position 3 and 20, but should also reflect the lipophilic
region at the B ring. Hydrophobic interactions between the
steroid and the GABAA receptor apparently have a much
greater role than hitherto taken for granted and any
replacement of the hydrophobic B ring may lead to the
loss of activity. The low activity of the 3-deoxy amine 34
may be explained by the presence of a different binding site
within the receptor.
4.1.2. (20R)-5,6b-Epoxy-3b-hydroxy-7-nor-5b-pregnan-
20-yl benzoate (12). A solution of bromohydrin 11 (2.5 g,
4.57 mmol) in methanol (40 mL) was treated with the
solution of potassium carbonate (1.5 g, 10.9 mmol) in water
(10 mL) under stirring at laboratory temperature. After 20 h,
the solution was concentrated in vacuo to a quarter of its
volume and the product was precipitated on addition of
brine (50 mL). The product was filtered off, dissolved in
methylene chloride (10 mL), washed with water (2!
40 mL) and dried. The solvent was evaporated in vacuo to
give the title compound as a white amorphous solid 12
(1.88 g, 97%), mp 88–90 8C; [found: C, 76.2; H, 8.6.
C27H36O4 requires C, 76.38; H, 8.55%]; [a]DZK37.1 (c
0.3, CHCl3); nmax (CHCl3) 3609, 1728, 1707, 1451, 1284,
1050, 714; dH (200 MHz, CDCl3) 8.04 (2H, m, ortho-ArH),
4. Experimental
4.1. General methods and equipment
Melting points were determined on a Koefler melting point
micro apparatus Boetius (Germany) and are uncorrected.
Analytical samples were dried over phosphorus pentoxide at

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A. Kasal et al. / Tetrahedron 61 (2005) 2269–2278
7.60 (1H, m, meta-ArH), 7.45 (2H, t, para-ArH), 5.22–5.04
(1H, m, H-20), 4.0–3.84 (1H, m, H-3), 3.19 (1H, s, H-6),
2.37–2.25 (1H, m, H-16), 1.27 (3H, d, JZ6.0 Hz, H-21),
0.84 (3H, s, H-19), 0.64 (3H, s, H-18).
was destroyed with crushed ice (10 g) and the precipitate
was filtered off. The product was dissolved in methylene
chloride (40 mL) and washed with the solution of
hydrochloric acid (10 mL), water (5 mL) and potassium
hydrogen carbonate (10 mL). The extract was dried over
sodium sulfate and the solvent was evaporated to give the
title compound 16 (160 mg, 97%) mp 201–202 8C (CH2Cl2
and ether);[found: C, 65.3; H, 7.3; N, 3.0. C28H39NO6S
requires C, 64.96; H, 7.59; N, 2.71%]; [a]DZK17.2 (c 0.3,
CHCl3); nmax (CHCl3) 3390, 1706, 1657, 1603, 1585, 1344,
1358, 1175, 714, 533; dH (200 MHz, CDCl3) 8.05 (2H, m,
ortho-ArH), 7.56 (1H, m, meta-ArH), 7.44 (2H, t, para-
ArH), 5.34 (1H, s, H–N), 5.24–5.09 (1H, m, H-20), 4.77–
4.56 (1H, m, H-3), 3.03 (3H, s, MeOSO2), 1.28 (3H, d, JZ
6.0 Hz, H-21), 0.90 (3H, s, H-19), 0.72 (3H, s, H-18).
4.1.3. (20R)-3b-Hydroxy-6-oxo-7-nor-5a-pregnan-20-yl
benzoate (13). To a solution of epoxide 12 (1.880 g,
4.23 mmol) in tetrahydrofuran (50 mL) was added a
solution of boron trifluoride etherate (0.3 mL, 2.37 mmol)
in ether (50 mL) under stirring at laboratory temperature.
After 20 h, the solution was diluted with ether (150 mL),
washed with the solution of potassium hydrogen carbonate
(50 mL) and brine (50 mL). The solvents were removed in
vacuo and the product was purified by chromatography on
silica (80 g, toluene–ether 5:1). The major component
(1.21 g, 64%) consisted of the title compound 13. Mp
174–176 8C (acetone–heptane); [found: C, 76.1; H, 8.6.
C27H36O4 requires C, 76.38; H, 8.55%]; [a]DZC47.9 (c
0.3, CHCl3); nmax (CHCl3) 3609, 1729; 1707, 1284, 1274,
1050, 960, 714; dH (200 MHz, CDCl3) 8.06 (2H, m, ortho-
ArH), 7.57 (1H, m, meta-ArH), 7.45 (2H, t, para-ArH),
5.24–5.08 (1H, m, H-20), 3.71–3.54 (1H, m, H-3), 1.28 (3H,
d, JZ6.0 Hz, H-21), 0.84 (3H, s, H-19), 0.64 (3H, s, H-18).
4.1.7. (20R)-3a-Hydroxy-6-aza-7-oxo-5a-pregnan-20-yl
benzoate (17). A solution of mesylate 16 (80 mg,
0.15 mmol) in DMSO (2 mL) was stirred at 115 8C with
potassium nitrite (250 mg, 2.94 mmol) under nitrogen.
Brine (5 mL) was added and the mixture cooled in a
refrigerator. The precipitate was filtered off, washed with
water (25 mL) and dried. The product purified by PLC
(ethyl acetate). The most polar component was identified the
title compound 17 (18 mg, 26%), mp 153–154 and then
233–235 8C (acetone–heptane); [found: C, 73.6; H, 8.4; N,
3.0. C27H37NO4 requires: C, 73.77; H, 8.48; N, 3.19% N];
[a]DZK10.0 (c 0.3, CHCl3); nmax (CHCl3) 3614, 3392,
1706, 1651, 1603, 1281, 1586, 1006, 714; dH (200 MHz,
CDCl3) 8.05 (2H, m, ortho-ArH), 7.56 (1H, m, meta-ArH),
7.44 (2H, t, para-ArH), 5.25–5.08 (1H, m, H-20), 4.78–4.68
(2H, m, OCH2O), 4.26–4.16 (1H, m, H-3), 3.58 (1H, dd, JZ
4.6, 11.6 Hz, H-5), 2.19 (1H, t, JZ10.6 Hz, H-16), 1.28
(3H, d, JZ6.0 Hz, H-21), 0.82 (3H, s, H-19), 0.72 (3H, s,
H-18).
4.1.4. (Z,20R)-3b-Hydroxy-6-oximino-7-nor-5a-preg-
nan-20-yl benzoate (14). A solution of ketone 13
(200 mg, 0.47 mmol) in methanol (10 mL) was stirred
with potassium hydrogen carbonate (280 mg, 2.8 mmol)
and hydroxylamine hydrochloride (200 mg, 1.76 mmol) at
reflux temperature. After 5 h, the mixture was diluted with
brine (60 mL) and cooled in a refrigerator. The precipitate
was dissolved in methylene chloride (50 mL) and washed
with the potassium hydrogen carbonate solution (2!
20 mL). The solvent was removed in vacuo and the product
applied on a column of silica (50 mL). A mixture of ethyl
acetate and toluene (3:1) eluted the title compound 14
(62 mg, 29%) as a colourless solid. Mp 164–167 8C
(toluene); [found: C, 73.8; H, 8.4; N, 2.8. C27H37NO4
requires C, 73.77; H, 8.48; 3.19% N]; [a]DZC38.2 (c 0.2,
CHCl3); nmax (CHCl3) 3604, 3297, 3169, 1707, 1666, 1452,
1282, 1052, 965, 941, 714; dH (200 MHz, CDCl3) 8.06 (2H,
m, ortho-ArH), 7.56 (1H, m, meta-ArH), 7.44 (2H, t, para-
ArH), 5.25–5.07 (1H, m, H-20), 3.82–3.61 (1H, m, H-3),
3.05 (1H, bd, JZ12.0 Hz, H-4a), 1.28 (3H, d, JZ6.0 Hz,
H-21), 0.82 (3H, s, H-19), 0.69 (3H, s, H-18).
4.1.8. (20R)-3a-(20-Methoxyethoxy)methoxy-6-aza-7-
oxo-5a-pregnan-20-yl benzoate (18). A solution of
3-alcohol 17 (260 mg, 0.59 mmol) in dichloromethane
(3 mL) and N,N-diisopropylethylamine (0.5 mL,
3.1 mmol) was treated with (2-methoxyethoxy)methyl
chloride (0.3 mL, 2.6 mmol) at laboratory temperature.
After 18 h, the mixture was diluted with chloroform
(30 mL), washed with an aqueous solution of citric acid
(5%, 10 mL) and water (10 mL), and dried. The solvent was
evaporated in vacuo to yield the title compound 18 (312 mg,
100%) as colourless oil. [a]DZK10.3 (c 0.3, CHCl3). nmax
(CHCl3) 3392, 2824, 1707, 1651, 1282, 1177, 989, 714; dH
(400 MHz, CDCl3) 8.05 (2H, m, ortho-ArH), 7.56 (1H, m,
meta-ArH), 7.44 (2H, t, para-ArH), 5.20–5.11 (1H, m,
H-20), 3.97–3.92 (1H, m, H-3), 3.50 (1H, dd, JZ11.8,
4.8 Hz, H-5), 3.40 (3H, s, OMe), 2.22 (1H, t, JZ10.6 Hz,
H-16), 1.27 (3H, d, JZ6.0 Hz, H-21), 0.83 (3H, s, H-19),
0.72 (3H, s, H-18).
4.1.5. (E,20R)-3b-Hydroxy-6-oximino-7-nor-5a-preg-
nan-20-yl benzoate (15). The more polar eluate of the
above chromatography yielded the title compound 15
(107 mg, 52%) as white crystals, mp 155–157 8C (metha-
nol–ether); [found: C, 73.8; H, 8.4; N, 2.9. C27H37NO4
requires C, 73.77; H, 8.48; N, 3.19%]; [a]DZC27.8 (c 0.1,
CHCl3); nmax (CHCl3) 3610, 3589, 3295, 1706, 1667, 1603,
1586, 1277, 1049, 969; dH (200 MHz, CDCl3) 8.06 (2H, m,
ortho-ArH), 7.56 (1H, m, meta-ArH), 7.44 (2H, t, para-
ArH), 5.26–5.08 (1H, m, H-20), 3.79–3.60 (1H, m, H-3),
2.47 (1H, t, JZ10.2 Hz, H-8), 1.27 (3H, d, JZ6.0 Hz,
H-21), 0.77 (3H, s, H-19), 0.68 (3H, s, H-18).
4.1.9. 3a-Hydroxy-6-aza-5a-pregnane-7,20-dione (9). (a)
By modification of the side chain. Benzoate 18 (200 mg,
0.38 mmol) was dissolved in methanol (50 mL) and heated
to boiling point with a solution of potassium hydroxide
(500 mg, 8.9 mmol) in water (15 drops). After 8 h, the
solution was concentrated in vacuo to a quarter of its
volume, diluted with brine (50 mL) and placed in a
refrigerator. The precipitate formed was filtered off, washed
4.1.6. (20R)-6-Aza-7-oxo-5a-pregnane-3b,20-diyl mesyl-
ate benzoate (16). Mesyl chloride (0.2 mL, 2.6 mmol) was
dripped into a solution of oxime 15 (140 mg, 0.32 mmol) in
pyridine (1 mL) at 0 8C under stirring. After 4 h, the reagent

A. Kasal et al. / Tetrahedron 61 (2005) 2269–2278
2275
with water (15 mL), dried with sodium sulfate, and
concentrated in vacuo to yield (20R)-20-hydroxy-3a-
(20-methoxyethoxy)methoxy-6-aza-5a-pregnan-7-one
(152 mg, 95%)—as a colourless solid. This crude material
was dissolved in methylene chloride (2 mL) and stirred with
pyridinium chlorochromate (500 mg, 2.3 mmol) and the
suspension of potassium acetate (350 mg, 3.6 mmol) in
methylene chloride (3 mL). After 18 h, the mixture was
filtered through a column of Celite (5 mL), which was then
washed with additional methylene chloride. The combined
filtrate and eluate was evaporated in vacuo. The product was
deprotected using hydrochloric acid (0.2 mL) in tetra-
hydrofuran (4 mL). After 18 h, the mixture was diluted
with toluene (10 mL), partly evaporated and purified by
TLC (ethyl acetate). Elution of the major zone with a
mixture of acetone in chloroform (1:6) and evaporation
afforded the title compound 9 (91 mg, 72%) as white
crystals; mp 231–233 8C (acetone); [found: C, 70.2; H, 8.9;
N, 3.8. C20H31NO3$0.5H2O requires: C, 70.14; H, 9.42; N,
4.09%]; [a]DZ C39.7 (c 0.2, CHCl3). nmax (CHCl3) 3614,
3392, 1652, 1359, 1007, 1701; dH (400 MHz, CDCl3) 5.45–
5.39 (1H, m, H–N), 4.22–4.15 (1H, m, H-3), 3.59 (1H, dd,
JZ12.4, 4.0 Hz, H-5a), 2.50 (1H, t, JZ9.3 Hz, H-17), 2.13
(3H, s, MeCO), 0.86 (3H, s, H-19), 0.69 (3H, s, H-18). (b)
From the diketone 25. The reagent was prepared
from hydrogen hexachloroiridate (50 mg, 0.12 mmol),
phosphorous acid (400 mg, 4.9 mmol), 2-propanol (10 mL,
130.5 mmol) and water (2 mL). Part of this solution (1 mL)
was added to a test tube containing 3,20-diketone 25
(40 mg, 0.12 mmol). The test tube was sealed and kept in a
bath at 85 8C for 18 h. After cooling, the mixture was diluted
with ethyl acetate (15 mL), the solution was washed with
the solution of potassium hydrogen carbonate and water,
dried over sodium sulfate and concentrated in vacuo. The
product was purified by TLC (chloroform, acetone 6:1) to
yield the title compound 9 (23 mg, 57%), mp 231–233 8C
(acetone), identical with the sample prepared above.
(36 mg, 20%), mp 253–255 8C (aqueous methanol); [found:
C, 74.9; H, 10.4; N, 4.3. C20H33NO2 requires: C, 75.19; H,
10.41; N, 4.38%]; [a]DZC56.7 (c 0.18, CHCl3). nmax
(CHCl3) 3615, 3320, 1699, 1386, 1358, 1005; dH (400 MHz,
CDCl3) 4.17–4.11 (1H, m, H-3), 3.02 (1H, dd, JZ11.7,
4.4 Hz, H-7b), 2.69 (1H, dd, JZ12.3, 4.3 Hz, H-5a), 2.52
(1H, t, JZ8.8 Hz, H-17), 2.36 (1H, t, JZ11.7 Hz, H-7a),
2.12 (3H, s, MeCO), 0.86 (3H, s, H-19), 0.63 (3H, s, H-18);
m/z (EI) 319 (MC, 82), 304 (37), 276 (10), 246 (100%). (b)
From diketone 27. Compound 27 (160 mg, 0.50 mmol),
hydrogen hexachloroiridate (31 mg, 0.08 mmol), and phos-
phorous acid (220 mg, 2.68 mmol) were put into a test tube
and 2-propanol (3.5 mL, 45.7 mmol) and water (0.7 mL)
were added. The tube was sealed and heated at 95 8C. The
black mixture turned into a pale solution within the first half
an hour. After 18 h, the mixture was diluted with ethyl
acetate (50 mL) and transferred into a flask. Potassium
carbonate (370 mg, 2.68 mmol) was added and organic
solvents were evaporated. The content of the flask was made
alkaline with ammonia (5 mL) and steroid products were
extracted with chloroform (3!20 mL). The extract was
washed with water (10 mL), dried and concentrated in
vacuo. The remainder was purified by chromatography on a
column of silica gel (25 g). Ammoniacal chloroform with
2% of methanol eluted the title compound 8 (133 mg, 83%)
identical with the above sample.
4.1.11. (20R)-5,6b-Epoxy-7-nor-5b-pregnane-3b,20-diyl
acetate benzoate (19). Epoxide 12 (450 mg, 1.1 mmol) was
acetylated with acetic anhydride (0.4 mL) in pyridine
(1 mL) at laboratory temperature. After 20 h, the mixture
was poured into brine (10 mL), the precipitate formed was
extracted with dichloromethane (3!20 mL), washed with
the solution of potassium hydrogencarbonate (2!10 mL)
and water, and dried. Evaporation of solvents in vacuo was
repeated after dilution with toluene (10 mL) in order to
remove pyridine from the title compound 19 (450 mg,
91%); the colourless oil failed to crystallise from usual
solvents;[found: C, 74.4; H, 8.3. C29H38O5 requires: C,
74.65; 8.21% H]; [a]D K4.38 (c 0.3, CHCl3). nmax (CHCl3)
1728, 1709, 1255, 1071, 1037, 1027, 961, 714; dH
(200 MHz, CDCl3) 8.04 (2H, m, ortho-ArH), 7.55 (1H, m,
meta-ArH), 7.43 (2H, t, para-ArH), 5.23–5.07 (1H, m,
H-20), 5.04–4.88 (1H, m, H-3), 3.19 (1H, s, H-6), 2.04 (3H,
s, MeCO), 1.28 (3H, d, JZ6.0 Hz, H-21), 0.84 (3H, s,
H-19), 0.63 (3H, s, H-18); m/z (EI) 406 (16), 344 (6), 284
(13), 209 (14), 149 (14), 105 (46), 91 (100%).
4.1.10. 3a-Hydroxy-6-aza-5a-pregnan-20-one (8). (a)
From the 3a-alkoxy derivative 18. A solution of compound
18 (300 mg, 0.57 mmol) and lithium aluminium hydride (ca.
200 mg, 5.2 mmol) in dioxane (10 mL) was heated to
boiling point under argon. After 5 h, the excess of the
reagent was destroyed with wet ether (about 20 mL) and
then an aqueous solution of Na2SO4 (about 5 mL). The
mixture was saturated with anhydrous Na2SO4, inorganic
material was filtered off and washed with ethyl acetate
(60 mL). The filtrate was concentrated in vacuo. The
remainder (230 mg, 99%, 0.56 mmol) was dissolved in
methylene chloride (3 mL) and added to a suspension of
pyridinium chlorochromate (700 mg, 3.25 mmol) and
potassium acetate (350 mg, 3.56 mmol) in methylene
chloride (4 mL). The mixture was stirred for 18 h at room
temperature and then filtered through a layer of Celite
(5 mL). The combined filtrate and eluate was evaporated
and the remainder was dissolved in tetrahydrofuran (6 mL)
containing hydrochloric acid (0.3 mL). After 4 h, the
solution was made alkaline with ammonia, partly concen-
trated in vacuo, and extracted with chloroform (40 mL). The
solution was dried over Na2SO4, and evaporated. The
remainder (110 mg) was purified by chromatography on a
column of silica gel (10 mL) in ammoniacal chloroform.
Elution of the major zone afforded the title compound 8
4.1.12. (20R)-7-Nor-6-oxo-5a-pregnane-3b,20-diyl acet-
ate benzoate (20). In analogy with the preparation of ketone
13, epoxide 19 (17.8 g, 38.1 mmol) was treated with a
solution of boron trifluoride etherate (3.0 mL, 23.67 mmol)
in a mixture of ether (900 mL) and tetrahydrofuran
(400 mL). After 20 h, the reaction mixture was worked up
as above and the product was purified by chromatography a
column of silica (400 g). Ether in toluene (1:30) eluted the
title compound 20 (16.8 g, 94%), mp 145–146 8C (ether–
heptane); [found: C, 74.7; H, 8.5. C29H38O5 requires C,
74.65; H, 8.21% H]; [a]DZC48.1 (c 0.28, CHCl3); nmax
(CHCl3) 1730, 1709, 1451, 1283, 1271, 1259, 1044, 994; dH
(200 MHz, CDCl3) 8.06 (2H, m, ortho-ArH), 7.60 (1H, m,
meta-ArH), 7.45 (2H, t, para-ArH), 5.25–5.09 (1H, m,
H-20), 4.70–4.60 (1H, m, H-3), 2.23 (1H, bd, JZ10.8 Hz,

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A. Kasal et al. / Tetrahedron 61 (2005) 2269–2278
H-5), 2.04 (3H, s, MeCO), 1.28 (3H, d, JZ6.0 Hz, H-21),
0.85 (3H, s, H-19), 0.66 (3H, s, H-18).
of methanol). After 8 h, the solution was concentrated in
vacuo, diluted with brine (5 mL) and placed in a
refrigerator. The title compound 23 (29 mg, 100%) formed
precipitate, which was filtered off and washed with water
(15 mL); mp 274–275 8C (acetone); [found: C, 69.7; H, 9.9;
N, 3.9. C20H33NO3$0.5H2O requires C, 69.73; H, 9.95; N,
4.07%]; [a]DZK10.1 (c 0.35, CHCl3). nmax (CHCl3) 3609,
3393, 1653, 1042; dH (200 MHz, CDCl3) 5.27–5.19 (1H, m,
H–N), 3.82–3.62 (2H, m, H-3, H-20), 3.05 (1H, dd, JZ12.2,
3.6 Hz, H-5), 1.15 (3H, d, JZ6.1 Hz, H-21), 0.92 (3H, s,
H-19), 0.80 (3H, s, H-18).
4.1.13. (Z,20R)-6-Oximino-7-nor-5a-pregnane-3b,20-
diyl acetate benzoate (21). A solution of ketone 20
(750 mg, 1.6 mmol) in methanol (10 mL) was stirred with
potassium hydrogen carbonate (1 g, 9.99 mmol) and hydro-
xylamine hydrochloride (750 mg, 10.8 mmol) at reflux
temperature. After 5 h, the mixture was worked up as in
the preparation of oxime 14 and the product was applied on
a column of silica (80 g). Ether in toluene (1:10) eluted the
title compound 21 (176 mg, 23%), mp 193–195 8C (ether–
heptane); [found: C, 72.3; H, 8.4; 2.8. C29H39NO5 requires:
C, 72.32; H, 8.16; N, 2.91%]; [a]DZC40.2 (c 0.2, CHCl3);
nmax (CHCl3) 3587, 3 298, 1729, 1708, 1452, 1281, 1271,
1262, 1246, 1047, 1033, 965, 941, 714. dH (200 MHz,
CDCl3) 8.05 (2H, m, ortho-ArH), 7.56 (1H, m, meta-ArH),
7.44 (2H, t, para-ArH), 5.23–5.07 (1H, m, H-20), 4.90–4.70
(1H, m, H-3), 3.03 (1H, bd, JZ10.8 Hz, H-4a), 2.03 (3H, s,
MeCO), 1.28 (3H, d, JZ6.0 Hz, H-21), 0.84 (3H, s, H-19),
0.69 (3H, s, H-18).
4.1.17. (20R)-6-Aza-5a-pregnane-3b,20-diol (26). Dry
lactam 23 (147 mg, 0.31 mmol) was put in a dripping
funnel placed between a reflux condenser and a flask with a
boiling solution of lithium aluminium hydride (ca. 200 mg,
5.27 mmol) in dioxane (10 mL). The substrate was
gradually dissolved in the solvent condensed and washed
into the solution. After 6 h, the solution was cooled, the
excess of reagent was destroyed with ethyl acetate and a
saturated, aqueous solution of sodium sulfate. Anhydrous
sodium sulfate was added and the solution was filtered over
sodium sulfate. The filter cake was washed with hot
chloroform (3!30 mL). The solvent was evaporated in
vacuo to yield the title compound 26 (69 mg, 70%), mp
189–192 8C (chloroform); [found: C, 67.2; H, 10.7; N, 3.5.
C20H35NO2.2H2O requires C, 67.19; H, 10.99; N, 3.92%];
[a]DZK7.8 (c 0.13, CHCl3) nmax (CHCl3) 3614, 3394,
1045; dH (200 MHz, CDCl3) 3.81–3.52 (2H, m, H-3, H-20),
3.13 (1H, dd, JZ12.4, 4.8 Hz, H-5), 1.14 (3H, d, JZ6.2 Hz,
H-21), 0.90 (3H, s, H-19), 0.76 (3H, s, H-18).
4.1.14. (E,20R)-6-Oximino-7-nor-5a-pregnane-3b,20-
diyl acetate benzoate (22). A more polar fractions from
the above chromatography yielded the title compound 22
(384 mg, 51%) as white crystals, mp 203–205 8C (ether–
heptane); [found: C, 72.2; H, 8.3; N, 2.7. C29H39NO5
requires C, 72.32; H, 8.16; N, 2.91%]; [a]DZC30.2 (c 0.2,
CHCl3); nmax (CHCl3) 3586, 3 296, 1728, 1708, 1667, 1451,
1282, 1274, 1259, 1049, 1033, 957, 924, 965, 714; dH
(200 MHz, CDCl3) 8.06 (2H, m, ortho-ArH), 7.56 (1H, m,
meta-ArH), 7.44 (2H, t, para-ArH), 5.26–5.09 (1H, m,
H-20), 4.86–4.68 (1H, m, H-3), 2.46 (1H, bt, JZ9.9 Hz,
H-8b), 2.03 (3H, s, MeCO), 1.28 (3H, d, JZ6.0 Hz, H-21),
0.76 (3H, s, H-19), 0.69 (3H, s, H-18).
4.1.18. 6-Aza-5a-pregnane-3,7,20-trione (25). Diol 24
(135 mg, 0.38 mmol) was dissolved in acetone (20 mL) and
treated with Jones reagent at laboratory temperature. After
15 min, the reagent was decomposed with methanol (1 mL),
the solution diluted with chloroform (100 mL) and its
volume was reduced in vacuo to quarter of its volume. The
mixture was washed with a solution of potassium hydrogen
carbonate and brine. The mixture was dried over sodium
sulfate and the solvent was evaporated. Purification of the
crude product by PLC (ammoniacal chloroform with 10% of
acetone) yielded the title compound 25 (77 mg, 58%), mp
249–250 8C (acetone–heptane); [found: C, 70.4; H, 8.6; N,
4.0. C20H29NO3$0.5H2O requires C, 70.56; H, 8.88; N,
4.11%]; [a]DZC37.4 (c 0.36, CHCl3); nmax (CHCl3) 3390,
1714, 1704, 1660, 1418, 1356, 1322; dH (200 MHz, CDCl3)
5.34–5.27 (1H, s, H–N), 3.42 (1H, dd, JZ12.8, 5.5 Hz,
H-5), 2.14 (3H, s, H-21), 1.10 (3H, s, H-19), 0.72 (3H, s,
H-18); m/z (EI) 317 (MC, 6), 302 (5), 279 (3), 246 (100%).
4.1.15. (20R)-6-Aza-7-oxo-5a-pregnane-3b,20-diyl acet-
ate benzoate (23). Oxime 22 (360 mg, 0.75 mmol) was
dissolved in pyridine (1.0 mL) and cooled to 0 8C under
stirring. Mesyl chloride (0.3 mL, 3.9 mmol) was dripped
into the solution. After 2 h, the reagent was destroyed with
crushed ice (10 g) and the precipitate was filtered off. The
product was dissolved in methylene chloride (20 mL) and
washed with the solution of hydrochloric acid, water and the
potassium hydrogen carbonate solution. The extract was
dried over sodium sulfate and the solvent was evaporated to
yield the title compound 23 (360 mg, 100%) as white
crystals. Mp 211–214 8C (278 mg, 77%, acetone–heptane);
[found: C, 71.9; H, 8.3; N 2.80. C29H39NO5 requires C,
72.32; H, 8.16; N, 2.91%]; [a]DZK19.7 (c 0.2, CHCl3);
nmax (CHCl3) 3392, 1731, 1709, 1654, 1273, 1252, 1036,
1027, 964, 714; dH (200 MHz, CDCl3) 8.04 (2H, m, ortho-
ArH), 7.57 (1H, m, meta-ArH), 7.44 (2H, t, para-ArH),
5.24–5.09 (1H, m, H-20), 4.85–4.65 (1H, m, H-3), 4.16–
3.62 (1H, m, H–N), 3.21 (1H, bd, JZ12.0 Hz, H-5), 2.03
(3H, s, MeCO), 1.28 (3H, d, JZ6.0 Hz, H-21), 0.88 (3H, s,
H-19), 0.74 (3H, s, H-18); m/z (EI) 481 (MC, 27), 359 (25),
344 (14), 224 (8), 105 (40), 65 (44), 43 (100%).
4.1.19. 6-Aza-5a-pregnane-3,20-dione (27). A solution of
chromium trioxide (230 mg, 2.3 mmol) in water (8 drops)
was added to a solution of diol 26 (286 mg, 0.89 mmol) in
acetic acid (13 mL) under stirring at room temperature.
After 24 h, the mixture was cooled with ice and made
alkaline with ammonia (30 mL). The resulting precipitate
was extracted with ether, the extract washed with water and
dried. Chromatography on a silica column (16 g) in
ammoniacal chloroform yielded the title compound 27
(162 mg, 56%), mp 202–204 8C (toluene–heptane);
[a]DZC79.5 (c 0.3, CHCl3); dH (200 MHz, CDCl3) 3.05
(1H, dd, JZ11.7, 4.3 Hz, H-5), 2.52 (1H, t, JZ8.7 Hz,
4.1.16. (20R)-3b,20-Dihydroxy-6-aza-5a-pregnan-7-one
(24). Diester 23 (43 mg, 0.09 mmol) was treated with a
boiling solution of potassium hydroxide (104 mg,
1.85 mmol) in aqueous methanol (2 drops of water, 10 mL

A. Kasal et al. / Tetrahedron 61 (2005) 2269–2278
2277
H-17), 2.12 (3H, s, H-21), 1.07 (3H, s, H-19), 0.66 (3H, s,
H-18); HRMS (EI) MC found 317.23539. C20H31NO2
requires 317.23548.%H, 10.46%, 4.16% N.
yield the title compound 33 (88 mg, 89%); mp 247–251 8C
(toluene); [found: C, 75.2; H, 9.8; N, 4.2. C20H31NO2
requires C, 75.67; H, 9.84; N, 4.41%]; [a]DZC48.5 (c 0.1,
CHCl3). nmax (CHCl3) 3391, 1701, 1652, 1357; dH
(200 MHz, CDCl3) 5.37–5.27 (1H, m, H–N), 3.02 (1H,
dd, JZ10.5, 4.9 Hz, H-5), 2.13 (3H, s, H-21), 0.88 (3H, s,
H-19), 0.69 (3H, s, H-18).
4.1.20. (20R)-6-Aza-7-oxo-5a-pregnan-20-yl acetate (30).
To a solution of (20R)-7-oxopregn-5-ene-3b,20-diyl
diacetate (28, 8.0 g, 19.2 mmol) in 2-methyl-2-propanol
(480 mL) were added24 aqueous solutions of potassium
carbonate (5.0 g, 36.2 mmol in 140 mL of water), potassium
permanganate (70 mg, 0.44 mmol in 9 mL of water), and
sodium periodate (7 g, 32.7 mmol in 80 mL of water). The
mixture was stirred for 30 min at laboratory temperature and
then an additional solution of sodium periodate (28.0 g,
130.9 mmol, in 350 mL of water) and several crystals of
potassium permanganate were added. The solution was
stirred for a further 6 h at 45 8C, then potassium hydrogen
pyrosulfite was added until the solution became colourless.
The solution was partially evaporated, sulfuric acid (5 mL,
93.1 mmol) was added and the organic substance was taken
up into diethyl ether. The extract was washed with brine and
concentrated in vacuo. Crude seco acid was dissolved in
methanol (70 mL) and esterified with diazomethane
(140 mL of ether solution). On evaporation, ester 29 was
dissolved in methanol (16 mL), transferred into an auto-
clave and cooled to K60 8C. Liquid ammonia (ca. 20 mL)
was added and the autoclave was sealed and heated to 55 8C
for 20 h. The mixture was concentrated in vacuo and
hydrogenated in acetic acid (50 mL) on Adam’s catalyst
(240 mg). Flash chromatography on a column of silica gel
(toluene–ethyl acetate, 10:1) yielded the title compound 30
(940 mg, 14%); mp 280–282 8C (acetone); [found: C, 72.7;
H, 10.2; N, 3.9. requires C, 73.09; H, 9.76; N, 3.87%];
[a]DZC25.2 (c 0.4, CHCl3). nmax (CHCl3) 3392, 1723,
1651, 1258, 1050; dH (200 MHz, CDCl3) 5.43–5.33 (1H, bs,
H–N), 4.94–4.76 (1H, s, H-20), 3.01 (1H, dd, JZ10.0,
5.0 Hz, H-5), 2.02 (3H, s, MeCO), 1.16 (1H, d, JZ6.2 Hz,
H-20), 0.87 (3H, s, H-19), 0.68 (3H, s, H-18); m/z (EI): 361
(MC, 100), 333 (12), 318 (7), 302 (36), 286 (24), 206 (22),
192 (35%).
4.1.23. (20R)-6-Aza-5a-pregnan-20-ol (32). Lactam 31
(430 mg, 1.35 mmol) was reduced with lithium aluminium
hydride as in Section 4.1.17. The chloroform extract was
evaporated in vacuo to yield the title compound 32 (365 mg,
88%); mp 160–162 8C (acetone–heptane); [found: C, 78.4;
H, 11.7; N, 4.4. C20H35NO requires C, 78.63; H, 11.55; N,
4.58%]; [a]DZK10.2 (c 0.2, CHCl3). dH (200 MHz,
CDCl3) 3.81–3.63 (1H, m, H-20), 1.14 (3H, d, JZ6.1 Hz,
H-21), 0.88 (3H, s, H-19), 0.76 (3H, s, H-18).
4.1.24. 6-Aza-5a-pregnan-20-one (34). Alcohol 32
(100 mg, 0.33 mmol) was oxidised as in Section 4.1.19.
The solvent evaporated and the product was purified by PLC
(ammoniacal CHCl3 and 5% MeOH) to yield the title
compound 34 (58 mg, 58%); mp 129–131 8C (ether);
[found: C, 78.8; H, 11.1; N, 4.54. C20H33NO requires C,
79.15; H, 10.96; N, 4.62%]; [a]DZC81.0 (c 0.1, CHCl3).
dH (200 MHz, CDCl3) 3.02 (1H, dd, JZ11.6, 4.4 Hz, H-7),
2.51 (1H, t, JZ8.4 Hz, H-17), 2.11 (3H, s, H-21), 0.87 (3H,
s, H-19), 0.62 (3H, s, H-18).
Acknowledgements
This work was carried out within the frame of co-operation
between the Academy of Sciences of the Czech Republic
´
and Consejo Superior de Investigaciones Cientıficas (Spain)
and was supported by grants S5011007 and Z4 055 0506.
4.1.21. (20R)-20-Hydroxy-6-aza-5a-pregnan-7-one (31).
Acetate 30 (860 mg, 2.38 mmol) was hydrolysed in a
solution of hydrochloric acid (8.0 mL, 97.4 mmol) in
chloroform (15 mL) and methanol (100 mL) at 50 8C for
44 h. A mixture was concentrated in vacuo to a quarter of its
volume. After addition of brine (50 mL), the title compound
31 (585 mg, 77%) precipitated; mp 264–265 8C (toluene);
[found: C, 75.3; H, 10.6; N,4.3. C20H33NO2 requires C,
75.19; H, 10.41; 4.38%N]; [a]DZK16.1 (c 0.3, CHCl3).
nmax (CHCl3) 3609, 3392, 1648, 1102, 1093; Circular
dichroism: D3221 K1.2, D3244 C0.3; dH (200 MHz, CDCl3)
5.37–5.27 (1H, m, H–N), 3.84–3.66 (1H, m, H-20), 3.01
(1H, dd, JZ10.5, 4.9 Hz, H-5), 1.15 (3H, d, JZ6.1 Hz,
H-21), 0.87 (3H, s, H-19), 0.80 (3H, s, H-18).
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