Molecular structure of cis- and trans-tergurides

Article abstract of DOI:10.1135/cccc19980425

Four isomers of terguride, a semisynthetic ergot alkaloid derivative, have been prepared by catalytic hydrogenation of (5R.8S)- and (5S,8S)-lisuride [1,1-diethyl-3-(6-methyl-8-ergolenyl)urea]. Relative stereochemistry of the isomers is based on NMR and CD spectra. Absolute configuration of all the series has been confirmed by the X-ray crystal structure determination of (5R,8S,10S)-terguride 2-bromobenzoate [1,1-diethyl-3-(6-methyl-8-isoergolenyl)urea, cis-dihydrolisuride].

Full text of DOI:10.1135/cccc19980425

Molecular Structure of Terguride
425
MOLECULAR STRUCTURE OF cis- AND trans-TERGURIDES*
a1
a2
b1
b2
Michal HUSAK , Bohumil KRATOCHVIL , Petr SEDMERA , Vladimir HAVLICEK ,
c
d
d
e,
Hana VOTAVOVA , Ladislav CVAK , Petr BULEJ and Alexandr JEGOROV **
a
Department of Solid State Chemistry, Prague Institute of Chemical Technology, 166 28 Prague 6,
Czech Republic; e-mail: husakm@vscht.cz, kratochb@vscht.cz
Institute of Microbiology, 142 20 Prague 4, Czech Republic, e-mail: sedmera@biomed.cas.cz,
vlhavlic@biomed.cas.cz
Institute of Organic Chemistry and Biochemistry, 166 10 Prague 6, Czech Republic;
e-mail: hanka@marilyn.uochb.cas.cz
Galena Co., R.& D., 747 70 Opava-Komarov, Czech Republic
Galena Co., Research Unit, Branisovska 31, 370 05 Ceske Budejovice, Czech Republic;
e-mail: husakm@marvin.jcu.cz
1
2
b
c
1
2
d
e
Received January 2, 1998
Accepted January 11, 1998
Four isomers of terguride, a semisynthetic ergot alkaloid derivative, have been prepared by catalytic
hydrogenation of (5R,8S)- and (5S,8S)-lisuride [1,1-diethyl-3-(6-methyl-8-ergolenyl)urea]. Relative
stereochemistry of the isomers is based on NMR and CD spectra. Absolute configuration of all the
series has been confirmed by the X-ray crystal structure determination of (5R,8S,10S)-terguride
2
-bromobenzoate [1,1-diethyl-3-(6-methyl-8-isoergolenyl)urea, cis-dihydrolisuride].
Key words: Tergurides; Ergot alkaloids; Crystal structure determination; NMR spectroscopy; Abso-
lute configuration.
Terguride, a semisynthetic ergot alkaloid derivative, possesses three chiral centers at
2
C5, C8 and C10 atoms. Among eight possible isomers, the only (5R,8S,10R)-one has
been described as an active substance “terguride” in the manufacture of drug for the
®
treatment of Parkinsonism and for the inhibition of prolactin secretion (Mysalfon ).
2
–5
Until now, all terguride structures were reported as the (5R,8S,10R)-isomers, assum-
ing that the absolute configuration of the parent skeleton present in the starting natural
6
compounds was not changed during the chemical modifications . In order to study the
conformational flexibility/rigidity of ergoline skeleton, four terguride isomers have
been prepared. The (5R,8S,10R)-configuration is simply referred as trans-D-terguride
*
*
This paper is the 20th continuation-in-part of our systematic study of the molecular structure and
polymorphism of ergot alkaloids .
*The author to whom correspondence should be addressed.
1
Collect. Czech. Chem. Commun. (Vol. 63) (1998)

4
26
Husak et al.:
and the (5R,8S,10S)- as cis-D-terguride in this work, according to the relative positions
of H5 and H10 hydrogen atoms (Fig. 1).
EXPERIMENTAL
General Synthetic Route to Tergurides
Either (5R,8S)- or (5S,8R)-lisuride (20 g, Galena Co., Czech Republic) was dissolved in methanol
(400 ml) and hydrogenated over 5% Pd/C catalyst (8 g) in a 600 ml stainless-steel autoclave (65 °C, 6
MPa). The reaction mixture was filtered and evaporated to dryness. Hydrogenation of (5R,8S)-lisuride
afforded predominantly (5R,8S,10R)-terguride with (5R,8S,10S)-terguride as the minor product. Simi-
larly, (5S,8R,10R)-terguride and (5S,8R,10S)-terguride were obtained in the same way from (5S,8R)-
lisuride. The crude tergurides were purified by column chromatography on a silica gel (350 g, Merck
Kieselgel 60, 0.063–0.100 mm) in dichloromethane–methanol, with step gradient [1.0, 1.5, 2.0, 2.5,
3
.0, 3.5, 4.0, 5.0, and 7.0% of methanol (v/v), fractions 1 200 ml]. Fractions containing pure isomers
were pooled, evaporated and crystallized from methanol or acetone.
1
13
Compounds were characterized by their H and C NMR spectra (Varian VXR-400, CDCl ), 2D NMR
3
experiments (COSY, delay-COSY, NOESY, ROESY, HETCOR, HOM2 J) (Tables I and II), and CD
spectra (Jobin–Yvon 5, 190–350 nm, methanol) (Table III, Fig. 2). Following compounds were used
as models for CD spectra comparison: ergometrine, ergometrine maleate, dihydroergotamine mesylate,
α-bromokryptine, α-bromokryptinine, α-bromokryptine mesylate, α-ergokryptine, α-ergokryptinine,
ergotamine, ergotamine tartrate, pergolide, pergolide mesylate, and metergoline (all from Galena Co.,
H
NH CO NEt
N
H
NH CO NEt
N
H
NH CO NEt
N
2
2
2
8
5
8
5
H
1
0
CH
3
CH
3
CH
3
H
H
H
HN
HN
HN
(5R,8S)-lisuride
(5S,8R)-lisuride
(5R,8S,10R)-terguride
H
H
NH CO NEt
N
H
NH CO NEt
N
H
H
NH CO NEt
N
2
2
2
H
CH
3
CH
3
CH
3
H
H
H
HN
HN
(5S,8R,10S)-terguride
HN
(
5S,8R,10R)-terguride
(5R,8S,10S)-terguride
FIG. 1
Stereochemistry of tergurides
Collect. Czech. Chem. Commun. (Vol. 63) (1998)

Molecular Structure of Terguride
427
Czech Republic). EI-MS spectra (Finnigan MAT 90, ionizing energy 70 eV, emission current 1 mA,
accelerating voltage 5 kV, direct inlet).
Preparation of (5R,8S,10S)-Terguride 2-Bromobenzoate
(
5R,8S,10S)-Terguride (100 mg) was dissolved in ethanol (3 ml) and mixed with 2-bromobenzoic
acid (59 mg in 3 ml ethanol, molar ratio 1 : 1). Subsequently, water (0.5 ml) was added and the
solution was allowed slowly to evaporate.
Crystal Structure Determination
(
8
5R,8S,10S)-Terguride 2-bromobenzoate, C H N O⋅C H BrO (M = 541.49), space group P2 , a =
20 28 4 7 6 2 r 1
3
–3
.650(2), b = 14.018(3), c = 11.687(3) Å, = 111.15(2)°, V = 1 321.7(6) Å , Z = 2, D = 1.361 g cm ,
calc
–
1
µ(MoKα) = 1.57 mm , F(000) = 564.
TABLE I
1
3
1
C and H NMR spectra (400 and 100 MHz, CDCl , 25 °C) of (5R*,8S*,10S*)-tergurides (data
3
taken from spectra of (5R,8S,10S)-terguride)
Atom
δ(C), ppm
Multiplicity
δ(H), ppm
n H
Multiplicity
ddd
J, Hz
2
3
4
118.27
110.74
14.90
d
s
t
6.886
–
1
–
1
1
1
2
1
1
1
1
–
1
1
1
–
–
3
–
6
4
1
2.0, 1.0, 1.0
15.5, 4.1, 1.0
10.6, 4.1, 4.0
2.937
ddd
m
3.060
5
7
8
9
59.92
54.79
47.01
35.65
d
t
3.354
2.480
4.211
2.484
ddd
m
d
t
m
m
4.197
m
1
1
1
1
1
1
1
0
1
2
3
4
5
6
38.25
134.34
115.18
122.86
108.41
132.92
126.21
42.68
156.54
13.63
42.68
–
d
s
2.116
–
m
d
d
d
s
6.849
7.100
7.147
–
dd
dd
dd
6.7, 1.0
8.2, 6.7
8.2, 1.0
s
–
N–CH3
N–C=O
CH3
q
s
2.491
–
s
q
t
1.048
3.123
8.678
t
6.2
6.2
2.0
CH2
q
d
N–H
–
Collect. Czech. Chem. Commun. (Vol. 63) (1998)

4
28
Husak et al.:
The structure was solved by the heavy-atom technique and refined by full-matrix least-squares
based on F values. Anisotropic temperature factors were applied to bromine and oxygen atoms
whereas the remaining atoms could be refined isotropically only. As the terminal methyl group C25
was disordered, two carbon positions, designated as C251 and C252 were refined. Sum of their oc-
cupancy factors [0.56(2) and 0.44(2)] was held to be 1, and for both Uiso = 0.1 was taken. Since the
geometry of some molecular parts became unrealistic during refinement, the restraints procedure was
applied to set of the distances: N21–C22, N21–C24, C22–C23, C24–C251 and C24–C252. The
values 1.455(7) Å for N–C and 1.48(1) Å for the C–C bonds were taken for calculating of weights.
Hydrogen atoms were included in ideal positions, with fixed Uiso values equal to Uiso or Ueq ones of
the bond partners. The position of 2-bromobenzoate OH-hydrogen was not localized. Flack’s enantio-
7
2
2
2
pole parameter was calculated from the equation |F(h,x)| = (1 – x) |F(h)| + x |F(–h)| . Parameters
used for the crystal structure analysis are listed in Table IV.
TABLE II
1
3
1
C and H NMR spectra (400 and 100 MHz, CDCl , 25 °C) of (5R*,8S*,10R*)-tergurides (data
3
taken from spectra of (5R,8S,10R)-terguride)
Atom
δ(C), ppm Multiplicity δ(H), ppm
n H
Multiplicity
dd
J, Hz
1.8, 1.8
2
3
4
117.86
111.27
26.95
d
s
t
6.895
–
1
2.698
1
1
1
1
1
1
1
1
1
–
1
1
1
–
–
3
–
6
2
2
1
1
ddd
dd
14.6, 11.1, 1.8
14.6, 4.3
3
.414
5
7
67.57
61.80
d
t
2.211
2.484
ddd
dd
11.1, 9.7, 4.3
11.7, 2.6
2
.890
ddd
m
11.7, 2.5, 2.5
8
9
45.03
32.52
d
t
4.315
1.634
ddd
dddd
m
13.4, 12.8, 3.3
2
.813
13.4, 4.2, 2.6, 2.5
1
1
1
1
1
1
1
0
1
2
3
4
5
6
36.53
133.04
112.90
122.85
108.59
133.30
126.21
43.30
d
s
3.088
–
d
d
d
s
6.857
7.135
7.182
–
ddd
dd
6.9, 1.3, 1.1
8.2, 6.9
ddd
8.2, 1.1, 0.7
s
–
N–CH3
N–C=O
CH3
q
s
2.417
–
s
156.67
13.80
q
t
1.164
3.266
t
7.1
CH2
41.09
dq
dq
s
14.8, 7.1
14.8, 7.1
3
.362
N–H
N–H
–
–
–
–
8.726
5.645
d
8.2
Collect. Czech. Chem. Commun. (Vol. 63) (1998)

Molecular Structure of Terguride
429
RESULTS AND DISCUSSION
Relative Stereochemistry
All four compounds provided molecular ion-radical (m/z 340, C H N ) and follow-
2
0
28 40
ing ergine ions : m/z 154 (C H N, ABC-ring system), m/z 167 (C H N, increment of
1
1
8
12
9
CH), and m/z 225 (whole ABCD-ring system). The urea part was characterized by ion
+
m/z 100 [ CO–N(C H ) ]. However, no differences in the mass spectra allowing the
2
5 2
discrimination of individual stereoisomers or cis- and trans-tergurides were observed.
NMR spectra provide the relative stereochemistry at all chiral centers with
(
5R*,8S*,10S*)-tergurides, the magnitude of J(5,10) = 4.0 Hz combined with a strong
cross-peak between these protons in the NOESY spectrum indicate a cis-junction. The
TABLE III
CD spectra of tergurides and lisurides
trans-Tergurides
cis-Tergurides
∆εmax
Lisurides
∆εmax
∆
εmax
λ, nm
λ, nm
λ, nm
5
R,8S,10R 5S,8R,10S
5R,8S,10S 5S,8R,10R
5R,8S
5S,8R
2
96
0.9
1.5
2.7
1.8
–0.9
–1.5
–2.7
–1.8
322
5.5
3.0
3.4
–5.5
–3.0
–3.4
10
2
2
2
86 sh
70
295 sh
264
275
4.9
–4.9
40
250
–10
a
2
∆
ε
b
–2
FIG. 2
CD spectra of trans-tergurides: a 5R,8S,10R;
b 5S,8R,10S
2
50
λ, nm
300
Collect. Czech. Chem. Commun. (Vol. 63) (1998)

4
30
Husak et al.:
1
linewidth of H-8 (21 Hz) implies its equatorial position. Several signals in both H and
1
3
13,14
C NMR spectra are seriously broadened
, indicating a possible exchange process
involving conformational changes in C and D rings. In contrast, large vicinal constants
J(5,10) = 9.7, J(4a,5) = 11.1, J(9a,10) = 12.8 Hz require nearly antiperiplanar arrange-
ment of the protons in (5R*,8S*,10R*)-tergurides (trans-tergurides). That dictates also
a chair conformation of ring D. Small couplings of both H-7 protons to H-8 and also
small J(8,9a) and J(8,9e) indicate that H-8 is equatorial (the urea moiety is, therefore,
axial). Nuclear Overhauser effect (NOE) observed between H-9 and H-12 suggests a
short distance between these protons. A measurable effect between H-5 and H-7a re-
flects their 1,3-diaxial orientation. Finally, the NOE between N-methyl and H-4e and
H-7e is consistent with an equatorial orientation of the N-methyl group. However, en-
antiometric terguride pair gave identical NMR spectra and thus are undistinquishable
by this technique.
Although the CD spectra are considered to be rather obsolete and scarcely used no-
wadays, nevertheless, it is the only technique suitable for a quick differentiation of
TABLE IV
Data collection and refinement parameters for (5R,8S,10S)-terguride 2-bromobenzoate
Crystal dimensions
0.3 × 0.3 × 0.2 mm
Diffractometer and radiation used
Enraf–Nonius CAD4, MoKα,
λ = 0.71073 Å
Scan technique
Temperature
ω/2θ
293 K
Number and θ range of reflections for lattice parameter
20; 18–19°
refinement
Range of h, k and l
–9→9, –15→15, –12→12
Standard reflections monitored in interval; intensity fluctuation 120 min; –2%
Total number of reflections (Friedel pairs not merged)
Rint
2 809
0.037
Criterion for observed reflections
Function minimized
I ≥ 1.96σ(I)
w (| Fo| − | Fc| )2
∑
2
–1
Weighting scheme
w = [ σ (Fo)]
Parameters refined
166
Value of R, wR and S
0.105, 0.114 and 3.88
Ratio of max. least-squares shift to e.s.d. in the last cycle
Max. and min. heights in final ∆ρ map
Source of atomic scattering factors
0.08
–
3
1.77, –2.07 e Å
International Tables for X-Ray
Crystallography
8
9
10
Programs used
CRYSTALS (ref. ), SDP (ref. ),
1
1
12
SHELXS86 (ref. ), PARST (ref. )
Collect. Czech. Chem. Commun. (Vol. 63) (1998)

Molecular Structure of Terguride
431
enantiomers. In the ergot alkaloid series the first band in spectra corresponds to π–π*
1
1
14
L and L transitions in the UV region 270–300 nm (ref. ). For 9-ergolenes related to
b
a
D-lysergic acid, a pronounced positive Cotton effect was typical [e.g., ergometrine 322
5.3), α-ergokryptine 318 (4.9), α-bromokryptine 315 (7.0), ergotamine 318 (5.0), (nm,
ε)]. Epimerization at the C8 carbon (compounds belonging to D-isolysergic acid
series) did not change the sign of the Cotton effect [e.g., lisuride, Table III, α-bromo-
(
∆
1
6
kryptinine 322 (15), see also ref. ]. Ergot alkaloid salts exhibited in methanol spectra
identical with those of free bases. Corresponding transitions in ergoline derivatives af-
forded only a weak Cotton effects. For the series of compounds used in this study
derivatives corresponding to D-dihydrolysergic acid exhibited Cottot effect (∆ε) ap-
proximately –1 at 290 nm (dihydroergotamine, metergoline, pergolide), whereas in
(
5R,8S,10R)-terguride (corresponds to D-dihydroisolysergic acid) positive Cotton effect
was found (Table III). Although the exact structural parameters are available from the
2
–5
crystal structure determinations of some similar compounds (terguride , meter-
1
7
18
19,20
21
22,23
goline , pergolide , ergotamine
, dihydroergotamine , bromokryptine
, li-
2
4
25,26
suride , and ergometrine
), we did not found any simple parameter (like torsion
angles about the C/D ring junction) which would enable the prediction of their CD
spectra. Nevertheless, if the absolute configuration of the only one member of certain
series of isomers is known, CD spectra then make possible fast identification of the
remaining members.
Absolute Configuration of (5R,8S,10S)-Terguride
In order to obtain the absolute configuration, the crystal structure determination of
(
5R,8S,10S)-terguride with the heavy-atom-containing 2-bromobenzoate anion was
undertaken. The final positional and thermal parameters of the non-H atoms of
5R,8S,10S)-terguride 2-bromobenzoate are deposited in Cambridge Crystallographic
(
Database. Figure 3 shows cis-terguride molecule with the atom numbering. Higher
C22
O20
C23
C19
C8
N21
C24
C12
C10
N6
C9
C11
N18
C13
C14
C7
C252
C251
C16
C5
C4
C17
O2
C32
C15
N1
C3
C2
C27
C31
C28
FIG. 3
O1
C29
Ortep drawing of (5R,8S,10S)-ter-
guride 2-bromobenzoate
C26
Br
C30
Collect. Czech. Chem. Commun. (Vol. 63) (1998)

4
32
Husak et al.:
values of reliability factors are probably caused by a poor quality of the single crystal
available (repeated attempts of preparation of better crystals were not successful). The
correct assignment of the (5R,8S,10S)-stereoisomer was confirmed by Flack’s enantio-
6
pole parameter value of –0.02(3). The indole moiety, formed by A and B rings of
2
ergoline skeleton, is almost planar, as usual. The χ test value 1.06 for phenyl A ring
and 1.76 for pyrrole B ring, respectively, was obtained. Dihedral angle of A and B ring
planes is 1.4(6)°. Some conformation puckering was found for rings C
(
C5,C4,C3,C16,C11,C10) and D (N6,C5,C10,C9,C8,C7) (Table V). The C ring is en-
1
4
veloped ( E), while the ring D has a chair conformation C . The system of hydrogen bond
1
is formed by following contacts: N18–H…O2: 2.86(2) Å, 143(2)°, N1–H…O20(x, y, z – 1):
2
.86(3) Å, 164(2)° and N6–H…O1(x – 1, y, z): 2.67(2) Å, 156(1)°.
Until now, four (5R,8S,10R)-terguride crystal structures have been described in lit-
2
3
erature (terguride hydrogen maleate monohydrate , terguride two-third hydrate , ter-
4
5
guride monohydrate , and terguride methanol solvate ) including seven
symmetrically-independent (5R,8S,10R)-terguride molecules altogether. This enables
2
7
us to compare ring-puckering parameters of individual terguride isomers (Table V).
Significant difference was found for the ring D of (5R,8S,10S)-terguride (cis-terguride),
4
where the inversion chair conformation ( C ) is caused by a different configuration at
1
C10. On the other hand, this difference does not influence the conformation of cis-ter-
guride ring C. The effect of protonation at N6 on the geometry of terguride molecules
was not proved.
TABLE V
2
6
Comparison of ring-puckering parameters of C and D rings in tergurides. Estimated standard devia-
tions are approximately 0.005 Å and 0.5°, for φ < 10° are one order higher
Ring C
Ring D
Com-
pound
a
Confor-
mation
Confor-
mation
Q, Å
φ, °
θ, °
Q, Å
φ, °
θ, °
1
1
A, 1
A, 2
A, 3
B, 1
B, 2
M
0.456
0.429
0.437
0.466
0.408
0.475
0.458
0.426
–11.4
–6.6
–11.0
–7.1
3.7
49.0
54.8
52.6
51.0
49.5
52.3
51.4
51.9
E
0.597
0.600
0.578
0.580
0.590
0.566
0.588
0.548
51.9
4.9
7.7
C4
1
1
E
3.2
2.3
C4
1
1
E
–93.2
17.6
C4
1
1
E
1.3
C4
1
1
E
–59.4
–123.7
–39.4
117.7
10.0
1.6
C4
1
1
9.7
E
C4
1
1
G
–9.9
–5.6
E
1.9
C4
1
4
cis-T
E
174.9
C1
a
A terguride two-thirds hydrate; B terguride monohydrate; M terguride hydrogen maleate mono-
hydrate; G terguride methanol solvate; cis-T (5R,8S,10S)-terguride 2-bromobenzoate; numbers denote
the independent molecules in the crystal structure.
Collect. Czech. Chem. Commun. (Vol. 63) (1998)

Molecular Structure of Terguride
433
This work was supported by grants No. 203/94/0135 and No. 203/96/0111 of the Grant Agency of the
Czech Republic.
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