N-Benzoyl-1,5-benzothiazepine and Its S-Oxide as Vasopressin Receptor Ligands: Insight into the Active Stereochemistry around the Seven-Membered Ring

Article abstract of DOI:10.1021/acs.jmedchem.5b00289

The stereochemistry of N-benzoyl-1,5-benzothiazepine and its S-oxide derivatives as vasopressin receptor ligands was examined in detail by freezing the conformation with a methyl group at the C6 or C9 of 1,5-benzothiazepine. It was revealed that the active forms recognized by the receptors are (cis,aS) for 1,5-benzothiazepine (5-7)-II and (cis,1S,aS) (syn) for its S-oxide (8-10)-II. The C9-methyl derivative of 1,5-benzothiazepine S-oxide (10-II) was designed and synthesized, achieving the putative active syn-isomer. (Chemical Equation Presented).

Full text of DOI:10.1021/acs.jmedchem.5b00289

Brief Article
pubs.acs.org/jmc
N‑Benzoyl-1,5-benzothiazepine and Its S‑Oxide as Vasopressin
Receptor Ligands: Insight into the Active Stereochemistry around
the Seven-Membered Ring
Tetsuya Yoneda,† Hidetsugu Tabata,† Tomohiko Tasaka,‡ Tetsuta Oshitari,† Hideyo Takahashi,†
and Hideaki Natsugari*,†
†Faculty of Pharma Sciences, Teikyo University, 2-11-1 Kaga, Itabashi-ku, Tokyo 173-8605, Japan
‡Affinity Science Corporation, 4-1-1 Akasaka, Minato-ku, Tokyo 107-0052, Japan
S
* Supporting Information
ABSTRACT: The stereochemistry of N-benzoyl-1,5-benzothiazepine and its
S-oxide derivatives as vasopressin receptor ligands was examined in detail by
freezing the conformation with a methyl group at the C6 or C9 of 1,5-
benzothiazepine. It was revealed that the active forms recognized by the
receptors are (cis,aS) for 1,5-benzothiazepine (5−7)-II and (cis,1S,aS) (syn)
for its S-oxide (8−10)-II. The C9-methyl derivative of 1,5-benzothiazepine S-
oxide (10-II) was designed and synthesized, achieving the putative active syn-
isomer.
relation to the biological activity for receptors (e.g., NK1,7a
INTRODUCTION
■
GABAA,7b VP7c) or enzymes (γ-secretase,7d ACAT7e). In our
Since the first discovery of a non-peptide arginine vasopressin
previous communication,7c atropisomerism in N-benzoyl-1,5-
benzodiazepines (for 1-I/II) as VP receptor ligands was
described (Figure 2), in which the cis/trans-amide rotamers8
(AVP) V2 receptor antagonist (mozavaptan1) (Figure 1),
Figure 1. Non-peptide VP receptor antagonists.
extensive research to find new ligands (antagonists and
agonists) has been carried out.2 To date, the “vaptan” class of
ligands (e.g., tolvaptan,3 conivaptan,4 and lixivaptan5) (Figure
1) has been developed as agents for the treatment of
hyponatremia, congestive heart failure, etc. Interest is still
growing in the search for new ligands, their new indications,
and the biological role of the V1a and V1b subtype receptors. A
topic of recent interest is the approval of tolvaptan as the first
drug for autosomal-dominant polycystic kidney disease
(ADPKD).6 Many of the vaptan class of drugs contain a
preserved scaffold, i.e., a benzo-fused seven-membered-ring
nitrogen heterocycle (e.g., 1-benzazepine, 1,4-benzodiazepine)
linked through N-1 to a substituted p-amidobenzoyl group
(Figure 1). Thus far, we have investigated the conformation of
several benzo-fused seven-membered-ring nitrogen hetero-
cycles (e.g., 1-benzazepine, 1,5-benzodiazepine, and 1,5-
benzothiazepine) to reveal the atropisomeric properties in
Figure 2. Four stereoisomers in 1 (I, II) and affinity at the VP
receptors (Ki) for 1-II.7c
around the N−C(O) bond and (aS)/(aR)-axial isomers9
based on the Ar−N(CO) (sp2−sp2) axis were considered
(Figure 2). The molecules with a substituent at the ortho-
position (C9) on the benzene ring (e.g., CH3, 1-I/II) were
conformationally “frozen” and could be separated into the (aS)-
and (aR)-axial isomers using chiral HPLC, although the
conformational change in the molecules without the substituent
Received: February 18, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/acs.jmedchem.5b00289
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Journal of Medicinal Chemistry
Brief Article
was too rapid for isolation of the isomers (i.e., achiral form) at
room temperature (rt). That study led to the finding of the
importance of axial chirality in exerting the biological activity of
the vaptan class of AVP receptor ligands, exhibiting the active
structure with the (cis,aS)-form of the type 1-II (Figure 2).7c
Recently, we have also become interested in the 1,5-
benzothiazepin-4-one S-oxide,10 which may exist as the major
metabolite of the biologically active sulfide derivatives, to reveal
the unique stereochemical properties originating in the
stereogenic elements at the sulfur atom and axis.
In this context, we synthesized the N-benzoyl-1,5-benzothia-
zepine11a,b and its S-oxide derivatives as VP receptor ligands
and examined the stereochemistry (conformation and config-
uration) in detail by freezing the conformation with a methyl
group at the C6 or C9 of 1,5-benzothiazepine to identify the
active molecular form recognized by the receptor. The results
are presented here.
Figure 3. (aS)/(aR) axial chirality in cis-N-benzoyl-1,5-benzothiaze-
pines (5−7) (I, II). As for the cis/trans amide rotamer, the trans-
1
isomer shown in brackets was negligible in the H NMR spectrum.
CH3) was prepared for freezing the ring conformation of the S-
oxide derivatives 10 (I, II) (described below). S-Oxidation of
the 1,5-benzothiazepines (5−7) (I, II) was performed with m-
chloroperoxybenzoic acid (mCPBA) (1.05 equiv) at −78 °C for
1 h in CH2Cl2 to afford the sulfoxides 8−10 (I, II) (61−
81%).14 Theoretically, the sulfoxides 8−10 exist as a mixture of
diastereomers (anti/syn-isomers) originating in the two stereo-
genic elements at the sulfur atom and axis; the description
“anti/syn” is used for the relative arrangement of the S-oxygen
atom and the N-benzoyl group, i.e., anti and syn denote the
opposite (1S*,aR*) and the same side (1S*,aS*) arrangement,
respectively (Figure 4). The anti and syn forms possess the S-
CHEMISTRY
■
Preparation of N-Benzoyl-1,5-benzothiazepine and
Its S-Oxide Derivatives. N-Benzoyl-1,5-benzothiazepine (5−
7) and its S-oxide derivatives (8−10) were prepared using
conventional methods starting from 1,5-benzothiazepines (2−
4)12 (Scheme 1). N-Benzoylation of 2−4 using benzoyl
Scheme 1. Preparation of N-Benzoyl-1,5-benzothiazepine
and Its S-Oxide
Figure 4. The anti/syn diastereomers in cis-N-benzoyl-1,5-benzothia-
zepine S-oxides (8−10) (I, II).
chloride and p-(2-methylbenzamido)benzoyl chloride gave 5-
I−7-I and 5-II−7-II, respectively. The I series of compounds
(Y = H) were prepared to examine the basic stereochemistry
and the II series of compounds [Y = p-(2-methylbenzamido)-
amino] were used for evaluation of the biological activity, both
of which were shown to have similar structural features from
the analytical data. As for the cis/trans-amide rotamers around
the N−C(O) bond, 5−10 all existed predominantly in the
spectrum (Figure 3).13 In the 1H NMR spectrum (CDCl3) of 5
(I, II), all the methylene protons (6H) of the thiazepine ring
were observed as separated peaks, which indicates that the
protons are diastereotopic and suggests the presence of axial
chirality caused by the Ar−N(CO) axis. The atropisomers,
however, could not be separated at rt using chiral HPLC, which
implies that the energy barrier between the atropisomers of 5
(I, II) is low, and thus 5 exists as an achiral compound at rt. On
the other hand, in 6 (I, II), which has a CH3 group at C6 (R1 =
CH3), the three CH2 protons of 6 appeared as six sharp
separated peaks, suggesting that the rotation around the Ar−
N(CO) axis is restricted to form stable atropisomers, and 6
exists as the racemates of separable (aS)- and (aR)-
atropisomers at rt. 7 (I, II) with a methyl group at C9 (R2 =
oxygen disposed in a ps.eq. and a ps.ax. orientation,
respectively, in a chairlike conformation. Compounds 8 and
10 each existed as single isomer as analyzed by nonchiral
HPLC, whereas 9 existed as the anti/syn mixture (∼97:3), and
only the major isomer was isolated. Further stereochemical
characterization of the sulfoxides 8−10 in relationship with the
activity is described in detail in Results and Discussion.
Separation of the Racemate into the Enantiomers [6,
8−10] and Their Physicochemical Properties (Table 1).
Separation of the N-benzoyl derivatives in the racemic form
into the enantiomers with HPLC using a chiral column was
attempted for the sulfide (6-I/II) and the S-oxides (8-I/II, 9-I,
and 10-I/II). The (aR)/(aS)-atropisomers of 6-I/II, the (1R)/
(1S)-enatiomers of 8-I/II and 10-I/II, and the enantiomers of
9-I (only for the major diastereomer) were separated and
successfully isolated using preparative HPLC. The physico-
chemical properties ([α]D and ΔG⧧ values) are shown in Table
1. The absolute stereochemistry of the separated enantiomers
was unambiguously determined by the X-ray structure analysis
of (−)-6-I, (−)- and (+)-8-I, (−)-9-I (major isomer), and
1
cis-form, and the trans-isomer was negligible in the H NMR
B
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Journal of Medicinal Chemistry
Brief Article
Table 1. Physicochemical Properties of the Separated
Enantiomers of N-Benzoyl-1,5-benzothiazepines (6-I/II)
and the S-Oxides (8-I/II, 9-I, 10-I/II)
a
ΔG⧧,
R1
R2
Y
[α]D (CH3OH)
kJ/mol
23
X
b
6-I
S
S
CH3
H
H
(aS): +214.0
(aR): −220.7
(aS): +226.1
(aR): −246.7
(1S): −184.1
(1R): +184.0
(1S): −188.5
(1R): +187.0
(1S,aR): −202.8
99
b
6-II
8-I
CH3
H
H
Ar
H
99
c
SO
SO
SO
SO
SO
H
64
d
8-II
H
H
Ar
H
NT
e
d
9-I
CH3
H
H
NT
f
(1R,aS): +164.5
d
10-I
CH3
CH3
H
(1S): +424.8
(1R): −425.4
(1S): +456.5
NT
d
10-II
H
Ar
NT
Figure 5. X-ray crystal structures of the optically active compounds
(−)-(aR)-6-I, (−)-(1S)-8-I, (+)-(1R)-8-I, (−)-(1S,aR)-9-I, and
(+)-(1S)-10-I and the racemate (1R*)-8-I. The crystal structure of
(1R*)-8-I was extracted from the CIF data of X-ray analysis of the
racemate 8-I.
(1R): −455.5
a
b
Activation free energy barrier to rotation of axis. Racemized in
toluene at 37 °C after 3 h. Estimated by VT NMR. Not tested.
Major diastereomer [(1S*,aR*)(=anti)] of 9-I; the minor isomer
c
d
e
[(1S*,aS*)(= syn)] could not be isolated because of the instability.
∼90% ee as analyzed by chiral HPLC.
f
Table 2. In Vitro Affinity for Human VP V1a and V2
Receptors of 5−10 (II)
(+)-10-I as shown in Figure 5.15 The X-ray structures also
showed that all the compounds possess the cis and chair forms
and that the anti and syn diastereomers of 8−10 have the S-
oxygen disposed in a ps.eq. and a ps.ax. orientation,
respectively. The absolute stereochemistry of the II-series of
compounds (6, 8, 10) was determined from the (+)/(−) angle
of the optical rotation α of the corresponding I-series of
compounds. The stereochemical stability of the separated
atropisomers of 6-I/II and conformational (diastereomeric)
isomers of 8-I was next examined. The activation free energy
barrier to rotation (ΔG⧧)16 and the conditions required for
racemization of the enantiomers of 6-I/II are shown in Table 1.
The enantiomers of 6-I/II had the ΔG⧧ value of 99 kJ/mol (24
kcal/mol) and racemized after ∼3 h in toluene at 37 °C. The
stability of the conformational (diastereomeric) isomers of 8-I
a
Ki (μM)
hV1a
0.068
hV2
0.021
5-II
6-II
(achiral)
racemate
0.037
0.360
(+)-(aS)
0.019
0.184
(−)-(aR)
(achiral)
0.147
1.800
7-II
8-II
0.048
0.880
racemate
0.910
0.870
(−)-(1S)
(+)-(1R)
racemate (1S*,aR*)
racemate
0.550
0.820
1
was next estimated using variable-temperature H NMR (VT
b
b
57% inh
55% inh
0.870
1.240
NMR)17 in DMSO-d6 (Table 1), in which the signals of one of
the diastereomeric C4-methylene protons (δ 4.7 and 4.9 ppm
with a ratio of ∼10:1) of the thiazepine ring coalesced at
around 323 K (50 °C) with a calculated ΔG⧧ value of 64 kJ/
mol (15 kcal/mol).18
c
b
b
b
9-II
22% inh
34% inh
48% inh
10-II
d
(+)-(1S)
0.660
e
b
b
(−)-(1R)
53% inh
0% inh
a
b
Ki values shown are the mean of duplicate measurements. Inhibition
(%) at 10 μM. Major diastereomer. Axial chirality is confined to
c
d
RESULTS AND DISCUSSION
e
■
(aS). Axial chirality is confined to (aR).
Affinity at the V1a and V2 Receptors (Table 2). The in
vitro affinities at the human VP V1a and V2 receptors of the II
series of compounds [Y = (2-methylbenzoyl)amino] are shown
in Table 2. First, the achiral compounds (5-II, 7-II) and
compounds in the racemic form (6-II, 8-II, 9-II, 10-II) were
evaluated. The benzothiazepines (5-II−7-II) exhibited good
potency (Ki) at the 10 nanomolar level for the V1a receptor. It is
interesting to note that the compounds bearing a methyl group
on the benzene ring (6-II and 7-II) have a tendency to bind
selectively to V1a rather than to V2 receptors. Among the S-
oxide derivatives (8-II−10-II), which could be potential
metabolites of the sulfides (5-II−7-II), 8-II, and 10-II showed
almost equal, moderate activity for the V1a receptor. However,
C
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Journal of Medicinal Chemistry
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the potency of the S-oxide 9-II (the major diastereomer) was
considerably diminished.
because the minor isomer readily isomerized to the major one,
resulting in a mixture of isomers. The structure of the major
isomer was determined to be anti[=(1S*,aR*)] form by the X-
ray structure analysis of the separated optically active (−)-9-I.
Although determination of the relative anti/syn stereochemistry
of the S-oxides 8-I and 9-I in solution was not easy, it could be
finally assigned after obtaining the pure syn[=(1S*,aS*)] isomer
Next, the separated enantiomers of 6-II, 8-II, and 10-II were
subjected to the binding assay to examine the difference in
potency between the enantiomers. As shown in Table 2, the
enantiomers exhibited clear differences in the affinities between
the isomers with median potency in the corresponding
racemate. As for the (aS)/(aR)-atropisomers of 6-II, the
(aS)-isomer exhibited higher potency than the (aR)-isomer. As
for the (1S)/(1R)-enantiomer of 8-II and 10-II, both of the
(1S)-isomers had greater potency than the (1R)-isomers.
Active Molecular Forms Recognized by the VP
Receptors. The clear difference in potency between the
enantiomers of 6-II, 8-II, and 10-II indicates that the
stereochemistry (conformation and configuration) at the
scaffold region is recognized by the receptors. As for the
thiazepine 6-II, the VP receptors recognize the (cis,aS)-form,
which is in accordance with the VP ligands of the 1,5-
benzodiazepine series 1-II reported in our previous communi-
cation.7c This may hold true for the achiral thiazepines 5-II and
7-II; the binding form is assumed to have the (cis,aS)-form,
although a pair of the (aS)/(aR)-axial isomers is rapidly
interconverting and inseparable. For the S-oxides 8-II−10-II,
the situation is rather complex because the anti/syn
diastereomers due to the two stereogenic centers at the sulfur
atom and axis as described in Figure 4 are present in 8−10 (I,
II). Table 3 shows the ratio of the diastereomers as analyzed by
1H NMR.
1
(10-I) (described below): i.e., the H NMR spectrum of 10-I
accorded with that of the minor isomer of 8-I and 9-I (see
Figure S4 in the Supporting Information).
Since the S-oxide 9-II (the major diastereomer) exhibited
diminished activity, the presumption of the active form of the S-
oxide derivatives led to confusion. Considering that (−)-(1S)-
8-II was the active enantiomer, the relative stereochemistry of
the active form of 8-II was tentatively presumed to be (1S,aR),
1
which is the major anti-diastereomer observed in the H NMR
spectrum (Table 3). The (aR)-stereochemistry at the axis,
however, is inconsistent with the active (aS)-form of the
thiazepines 5-II−7-II. Thus, we speculated that the actual
active form of (−)-(1S)-8-II is the minor form [(1S,aS)(=syn)]
in the equilibrium state (Table 3). The energy barrier between
the two conformers (diastereomers) seems fairly surmountable
under ambient conditions as suggested by the X-ray structure
analysis of 8-I (Figure 5). As shown in Figure 5, the X-ray
structure analysis of the chiral (−)-(1S)-8-I and (+)-(1R)-8-I
exhibited diastereomeric structures with a chairlike form, i.e.,
(−)-(1S)-8-I has an anti-form, and (+)-(1R)-8-I a syn-form,
whereas interestingly the racemate 8-I (1R*) exhibited anti-
stereochemistry. The appearance of the diastereomeric syn and
antistructures in the crystalline enantiomers and racemate of 8-I
could be a result of a fortuitous crystal packing arrangement
and implies that the energy barrier between the two conformers
(diastereomers) is fairly low.18 Thus, it may not be discrepant
to assume that the active stereochemistry of the S-oxide
derivatives is that of the minor isomer [(1S,aS)(=syn)] (Figure
6). The weak potency of the S-oxide 9-II (major isomer)
Table 3. Equilibrium Ratio for Stereoisomers (anti, syn) in
cis-N-Benzoyl-1,5-benzothiazepine S-Oxides (8-I, 9-I, and
1
10-I) Estimated by H NMR
R1
R2
anti:syn
a
b
8-I
H
H
100:0 in CDCl3
87:13 in CD3OD
91:9 in DMSO-d6
97:3 (by HPLC)
9-I
CH3
H
H
a
10-I
CH3
0:100 in CDCl3, CD3OD
a
For 8-I and 10-I, the ratio (anti/syn) at the equilibrium state in
solution at rt was estimated by 1H NMR. Both compounds each
existed as single isomer as analyzed by nonchiral HPLC. For 9-I, a
b
mixture of anti/syn isomers was obtained in the S-oxidation. The ratio
1
was estimated by nonchiral HPLC and H NMR.
Figure 6. Active stereochemistry of the thiazepines (5−7)-II and the
S-oxide derivative (8-II).
Compound 8-I (R1 = R2 = H) exhibited an interesting
conformational property in the 1H NMR spectrum; i.e.,
although at 295 K (22 °C) in CDCl3, only a single isomer
(presumed to be anti) was observed, in polar solvents another
minor isomer (presumed to be syn) appeared with a ratio of
∼91:9 (in DMSO-d6) or ∼87:13 (in CD3OD) (Table 3). The
sulfoxide 9-I (R1 = CH3, R2 = H) was revealed to be a mixture
of diastereomers with a ratio of ∼97:3 by HPLC and 1H NMR
analyses, from which only the major isomer could be isolated.
Attempted isolation of the minor isomer in the pure form failed
(Table 2) may also be ascribed to the (1S*,aR*)(=anti)-
stereochemistry, and the active isomer would be the minor
diastereomer with (1S*,aS*)(=syn)-stereochemistry, which
could not be isolated.
After obtaining various information on the active stereo-
chemistry of the S-oxide derivatives, we designed the
benzothiazepine S-oxide with a C9-methyl group (10) to
obtain the putative active syn[=(1S*,aS*)]-stereochemistry
(Figure 7). Because of the steric effect of the C9 methyl
D
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Journal of Medicinal Chemistry
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mL) at 0 °C under argon was added sodium hydride (60% in oil) (41
mg, 1.03 mmol). The mixture was stirred at 25 °C for 30 min, cooled
to 0 °C, and treated with benzoyl chloride (0.24 mL, 2.05 mmol).
After it was stirred at 25 °C for 1 h, the mixture was treated with H2O
and extracted with ethyl acetate. The extract was washed with brine,
dried, and concentrated. The concentrate was purified by column
chromatography (silica gel, ethyl acetate/hexane = 1:5) to afford 5-l21
as colorless crystals (170 mg, 0.63 mmol, 93%).
Similarly, 6-methyl (6-I) and 9-methyl (7-I) derivatives were
prepared from the corresponding 1,5-benzothiazepines (3, 4)12
according to a similar procedure described for the preparation of 5-I
from 2. Also, the II-series of compounds (5-II, 6-II, and 7-II) were
similarly prepared from the corresponding 1,5-benzothiazepines by
reaction with p-(2-methylbenzamido)benzoyl chloride. The physico-
chemical data of 5−7 (I, II) are shown in the Supporting Information.
S-Oxidation of N-Benzoyl-2,3,4,5-tetrahydro-1,5-benzothia-
zepines (5−7/I, II): Typical Example Described for Preparation
of 8-I. To a solution of 5-l (50 mg, 0.19 mmol) in CH2Cl2 (1 mL) at
−78 °C under argon was added dropwise a solution of mCPBA (49
mg, 0.19 mmol) in CH2Cl2 (1.7 mL) with stirring. After being stirred
at −78 °C for 1 h, the mixture was treated with sat. aq NaHCO3 and
extracted with ethyl acetate. The extract was washed with brine, dried,
and concentrated. The concentrate was purified by column
chromatography (silica gel, ethyl acetate/hexane = 2:1) to afford
sulfoxide as colorless crystals 8-l (39 mg, 0.14 mmol, 72%) and sulfone
(12 mg, 0.04 mmol, 21%).
Compounds 8-II, 9 (I, II), and 10 (I, II) were prepared from the
corresponding 1,5-benzothiazepines according to a similar procedure
described for the preparation of 8-I from 5-I. The physicochemical
data of 8−10 (I, II) are shown in the Supporting Information.
Separation of the N-Benzoyl-1,5-benzothiazepines (6-I/II)
and Their S-Oxides (8-I/II, 9-I, and 10-I/II) into the
Enantiomers. The racemic compounds, 6-I/II, 8-I/II, 9-I, and 10-
I/II were separated using chiral HPLC (CHIRALPAK IA for 6-I, 10-I,
and 10-II, CHIRALPAK IB for 6-ll, CHIRALPAK OJ-H for 8-l and 9-
I, CHIRALPAK IC for 8-lI). The separation conditions and
physicochemical properties of the enantiomers are shown in the
Supporting Information.
Figure 7. (1S)-5-Benzoyl-9-methyl-1,5-benzothiazepine S-oxide
[(+)-(1S)-10-I)] with the putative active stereochemistry [(1S,aS) =
syn] at the scaffold region in which the S-oxygen occupies a
pseudoaxial position in a chairlike form.
group, the S-oxide group would be confined to adopt a
sterically less hindered ps.ax. orientation in a chairlike form.
Gratifyingly, as expected, 10 existed only in the desired
syn[=(1S*,aS*)]-form in the solid [X-ray crystal analysis for
(+)-(1S)-10] (Figure 5) and solution (1H NMR) states, and
the activity of 10-II resided in the (1S)-enantiomer with the
(aS)-axis as shown in Table 2. These results are consistent with
our presumption of the active stereochemistry.
These prospects for the active molecular forms further
suggest the active stereochemistry of the vaptan class of VP
ligands (e.g., mozavaptan, tolvaptan) developed as racemates.
Considering that the eutomer (more potent enantiomer) of
these drugs has (5S)-stereochemistry,19 although rotation
around the Ar−N(CO) axis is rapidly occurring and the
diastereomers are inseparable, the receptors in the body are
presumed to recognize the stereochemistry of the (cis,5S,aS)-
(syn)-form with C5-substituent in the ps.ax. orientation as
described in this study (Figure 8).
Single-Crystal X-ray Analysis. The crystal structures of the
optically active compounds (−)-(aR)-6-I, (−)-(1S)-8-I, (+)-(1R)-8-I,
(−)-(1S,aR)-9-I, and (+)-(1S)-10-I and the racemate (1R*)-8-I were
obtained by single-crystal X-ray analysis (Figure 5). Typical crystal
data and CIF files are in the Supporting Information.
Figure 8. Plausible active form of vaptan class of VP ligands with a C5-
substituent (e.g., mozavaptan, tolvaptan). For the exact structures of
mozavaptan and tolvaptan, see Figure 1.
Stereochemical Stability of Atropisomers of 6 (I, II) and
Diastereomer of 8-I. The stereochemical (thermodynamic) stability
of (aR/aS)-atropisomers of 6 (I, II) and diastereomer of 8-I was
determined as previously described.7c,e The details including the
figures of conversion profiles are shown in the Supporting
Information.
CONCLUSION
■
In Vitro Binding Assay to hV1a and hV2. Binding of [3H]
(Arg8)-vasopressin to human VP V1a and V2 receptors was measured
by conventional methods. The procedure is shown in the Supporting
Information. All test compounds (shown in Table 2) had a purity of
≥95% as determined by HPLC analyses.
By freezing the conformation with a methyl group at the C6 or
C9 of 1,5-benzothiazepine, we elucidated the stereochemistry
(conformation and configuration) of N-benzoyl-1,5-benzothia-
zepine and its S-oxide and gained insight into the actual active
stereochemistry of the scaffold region of the VP ligands. On the
basis of these structural insights, the C9-methyl derivative of
1,5-benzothiazepine S-oxide (10) was designed and synthe-
sized, achieving stereoselective formation of the putative active
syn-isomer. When exerting biological activity by binding with
ligands, the target receptors or enzymes must recognize the
actual active structure caused by the labile conformational
change of the seven-membered ring, where chirality may also
exist in the latent form. We hope that this study will contribute
to future drug design and development.
ASSOCIATED CONTENT
■
S
* Supporting Information
General experimental procedure, 1H, 13C, and 2D NMR spectra
and physicochemical properties for new compounds, VT NMR
for 8-I, figures of thermal isomerization rate of atropisomers for
(aR)/(aS)-6-I and (aR)/(aS)-6-II, X-ray crystal data (CIF) for
(aR)-6-I, (1R)-8-I, (1S)-8-I, (1R*)-8-I, (1S,aR)-9-I, and (1S)-
10-I. This material is available free of charge via the Internet at
http://pubs.acs.org.
EXPERIMENTAL SECTION20
N-Benzoylation of 2,3,4,5-Tetrahydro-1,5-benzothiazepines
(2−4) Using Benzoyl Chloride and p-(2-Methylbenzamido)-
benzoyl Chloride: Typical Example Described for Preparation
of 5-I. To a stirred solution of 212 (113 mg, 0.68 mmol) in THF (7
■
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: natsu@pharm.teikyo-u.ac.jp. Phone: +81-3-3964-8096.
E
DOI: 10.1021/acs.jmedchem.5b00289
J. Med. Chem. XXXX, XXX, XXX−XXX

Journal of Medicinal Chemistry
Brief Article
one nucleus. Org. Lett. 2008, 10, 4871−4874. (e) Tabata, H.; Wada,
N.; Takada, Y.; Nakagomi, J.; Miike, T.; Shirahase, H.; Oshitari, T.;
Takahashi, H.; Natsugari, H. Active conformation of seven-membered-
ring benzolactams as new ACAT inhibitors: latent chirality at N5 in
the 1,5-benzodiazepin-2-one nucleus. Chem.Eur. J. 2012, 18, 1572−
1576.
(8) The description “cis/trans” is used for the relative arrangement of
the two benzene rings.
(9) The terms aS and aR (chiral axis nomenclature) correspond to P
and M (helix nomenclature), respectively.
(10) Tabata, H.; Yoneda, T.; Oshitari, T.; Takahashi, H.; Natsugari,
H. Stereochemistry of 1,5-benzothiazepin-4-one S-oxide: insight into
the stereogenic elements at the sulfur atom and axis. J. Org. Chem.
2013, 78, 6264−6270.
(11) (a) N-Benzoyl-1,5-benzothiazepines with several substituents at
the 2 position as VP receptor antagonists have been reported:
Urbanski, M. J.; Chen, R. H.; Demarest, K. T.; Gunnet, J.; Look, R.;
Ericson, E.; Murray, W. V.; Rybczynski, P. J.; Zhang, X. 2,5-
Disubstituted 3,4-dihydro-2H-benzo[b][1,5]thiazepines as potent
and selective V2 arginine vasopressin receptor antagonists. Bioorg.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported in part by a Grant-in-Aid for Scientific
Research (C) (25460154) and a Grant-in-Aid for Young
Scientists (B) (25860091) from the Japan Society for the
Promotion of Science. H.T. extends thanks for financial support
by Astellas Foundation for Research on Metabolic Disorders
(2013) and MEXT-Supported Program for the Strategic
Research Foundation at Private Universities (2013−2017).
ABBREVIATIONS USED
■
ADPKD, autosomal-dominant polycystic kidney disease; AVP,
arginine vasopressin; VP, vasopressin; NK, neurokinin; ACAT,
acyl-coenzyme A/cholesterol acyltransferase; GABA, γ-amino-
butyric acid; mCPBA, m-chloroperoxybenzoic acid; VT NMR,
variable-temperature 1H NMR; ps.ax., pseudoaxial; ps.eq.,
pseudoequatorial
Med. Chem. Lett. 2003, 13, 4031−4034. (b)
N-Benzoyl-1,5-
benzothiazepine (5-II) and the S-oxide (8-II) (racemate) were briefly
described in a patent application: Ogawa, H.; Miyamoto, H.; Kondo,
K.; Yamashita, H.; Nakaya, K.; Komatsu, H.; Tanaka, M.; Takara, S.;
Tominaga, M.; Yabuchi, Y. Preparation of N-benzoyl benzo-fused
heterocyclic compounds as vasopressin antagonists. Jpn. Kokai Tokkyo
Koho, JP 04321669, 1992.
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DOI: 10.1021/acs.jmedchem.5b00289
J. Med. Chem. XXXX, XXX, XXX−XXX