Preparation and structural elucidation of (-)-tetrahydroberberine-(+)-2,3-di(p-toluyl) tartaric acid complex

Article abstract of DOI:10.1016/j.molstruc.2008.06.013

A new (2:1) complex of (-)-13aS-tetrahydroberberine [(-)-13aS-THB] with (+)-2,3-di(p-toluyl) tartaric acid (DTTA), i.e. 5,8,13,13a-tetrahydro-9,10-dimethoxy-2,3-methylenedioxy-6H dibenzo[a,g] quinolizine·2,3-di(p-toluyl) tartaric acid [2C₂₀H₂₀NO₄·C₂₀H₁₈O₈], as well as its optical active component (-)-THB, has been obtained from the resolution process of (±)-THB in methanol. The structures of this complex and an enantiomer (-)-13aS-THB have been characterized by CD, IR and NMR spectroscopy as well as by X-ray single crystal diffraction.

Full text of DOI:10.1016/j.molstruc.2008.06.013

Journal of Molecular Structure 892 (2008) 466–469
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Preparation and structural elucidation of (ꢀ)-tetrahydroberberine-(+)-2,
3-di(p-toluyl) tartaric acid complex
a
a
a
c,
d
Jin-Ming Gao a,b, , Wei-Tao Liu , Man-Lin Li , Han-Wei Liu , Xing-Chang Zhang , Zong-Xiao Li
*
*
a Research Centre for Natural Medicinal Chemistry, College of Science, Northwest A & F University, Yangling, Shaanxi 712100, PR China
b National Phytochemistry Engineering Research Center in West China, Yangling, Shaanxi 712100, PR China
c Institute of Soil & Water Conservation, Chinese Academy of Science, Yangling, Shaanxi 712100, PR China
d Department of Chemistry and Chemical Engineering, Baoji College of Arts and Sciences, Baoji 721007, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 12 May 2008
Received in revised form 10 June 2008
Accepted 16 June 2008
Available online 24 June 2008
A new (2:1) complex of (ꢀ)-13aS-tetrahydroberberine [(ꢀ)-13aS-THB] with (+)-2,3-di(p-toluyl) tartaric
acid (DTTA), i.e. 5,8,13,13a-tetrahydro-9,10-dimethoxy-2,3-methylenedioxy-6H dibenzo[a,g] quinoli-
zineꢁ2,3-di(p-toluyl) tartaric acid [2C20H20NO4ꢁC20H18O8], as well as its optical active component (ꢀ)-
THB, has been obtained from the resolution process of ( )-THB in methanol. The structures of this com-
plex and an enantiomer (ꢀ)-13aS-THB have been characterized by CD, IR and NMR spectroscopy as well
as by X-ray single crystal diffraction.
Keywords:
Ó 2008 Elsevier B.V. All rights reserved.
(ꢀ)-13aS-Tetrahydroberberine
(+)-2,3-Di(p-toluyl) tartaric acid
Complex
Crystal structure
Protoberberine
1. Introduction
( )-THB, few work was concerned about its optical isomers. In
the last few years chiral compounds have aroused much interest
The protoberberines represent a widely distributed class of sec-
ondary metabolites belonging to the large group of alkaloids con-
stitutionally derived from isoquinoline [1]. Tetrahydroberberine
(THB, 1), also known as canadine, and tetrahydropalmatine (THP)
are two of the major active constituents isolated from Corydalis
yanhusuo W.T. Wang, well known as a traditional Chinese herbal
medicine. THB and THP belong to the isoquinoline alkaloid family
with tetrahydroprotoberberine backbone (Fig. 1). ( )-THB and ( )-
THP are highly abundant in C. yanhusuo and a variety of Corydalis
plants [2–5].
Several studies have shown that ( )-THP possesses significant
biological activities such as analgesic, sedative, tranquilizing, hyp-
notic, antihypertensive and hypo-locomotion activity [6–10] and
that some mechanisms of action of ( )-THP are characterized
[11–17]. THB, an analog of THP, has pharmacological effects similar
to those of THP, e.g. analgesia, sedation, and muscle relaxation
[9,10], as well as protecting neurons from apoptosis induced by
b-amyloid peptides [18,19].
in pharmaceutical research and development, and the production
of enantiomerically pure drugs is one of the most important tasks
in today’s pharmaceutical industry. Single optical isomers are of
special significance in bioscience applications since the desired
activity generally resides in only one of a pair of enantiomers.
In continuation of our earlier work in search for pharmaceutical
compounds from natural products, by using (+)-2,3-di(p-toluyl)
tartaric acid (DTTA) as a resolving agent we performed resolution
of ( )-THB derived from the NaBH4 reduction of berberine chloride.
Unexpectedly, a new 2:1 complex (3) of (ꢀ)-THB with DTTA (Fig. 1)
and its optical active alkaloid (ꢀ)-THB (2) have been obtained from
the resolution. In this paper, we report the simple resolution of ( )-
THB and structural elucidation of both 2 and 3 which have been
characterized by means of IR, NMR and CD spectroscopy and
X-ray diffraction techniques.
2. Experimental
However, the survey of literatures revealed that much pharma-
cological work has been done on the basis of the racemic form of
2.1. General experimental procedures
(+)-2,3-Di(p-toluyl) tartaric acid (DTTA) with specific rotation
* Corresponding author. Address: Research Centre for Natural Medicinal Chem-
istry, College of Science, Northwest A & F University, Yangling, Shaanxi 712100, PR
China, Tel./fax: +86 29 87092335.
of ½a 2D0
ꢂ
+ 134° (EtOH) and sodium borohydride were supplied by
Sigma Aldrich, Inc. (St. Louis, MO, USA). Berberine chloride was
E-mail address: jinminggaocn@yahoo.com.cn (J.-M. Gao).
purchased from Baoji Yongjia Phytochemistry Company (Baoji,
0022-2860/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2008.06.013

J.-M. Gao et al. / Journal of Molecular Structure 892 (2008) 466–469
467
(1H, brs, H-8), 3.60 (2H, brs, H-8, and 13a-H), 2.71-3.31 (6H, m,
H-13, H-6, and H-5); 13C NMR (125 MHz, CDCl3): d 150.1 (C-10),
147.4 (C-2), 147.3 (C-3), 144.9 (C-9), 129.5 (C-13b), 128.5 (C-
12a), 127.6 (C-8a), 126.6 (C-4a), 123.7 (C-12), 111.2 (C-4), 110.8
(C-11), 108.5 (C-1), 100.6 (-OCH2O-), 59.1 (C-13a), 60.0 (9-OCH3),
55.6 (10-OCH3), 53.8 (C-8), 51.3 (C-6), 36.1(C-13), and 28.9 (C-5).
4
5
3
2
4a
O
O
O
O
6
13b
N
N
8
13a
13
1
H
8a
OMe
OMe
OMe
OMe
9
12a
12
10
2.3. Preparation of complex [(ꢀ)-THBꢁDTTAꢁ(ꢀ)-THB] (3)
11
1
2
A solution of DTTA (11.3 g, 0.029 mol) in methanol (22 mL) was
added to a solution of ( )-THB (10.0 g, 0.029 mol) in methanol
(455 mL). The light yellow mixture obtained was stirred for 5 min
and kept at 28 °C. After 30 days, the light yellow prismatic crystals
of complex 3 (11.2 g) were collected by filtration. Yield 36.3%; mp
CH3
OMe
D
O
O
MeO
158–159 °C (methanol); ½a D20
ꢂ
+ 60° (c 0.50, MeOH); IR (KBr cmꢀ1):
A
3425, 2992, 2944, 2909, 2836, 1713, 1612, 1498, 1460, 1433,
1387, 1338, 1277, 1227, 1177, 1109, 1082, 1040, 989, 936, 903,
860, 808, 751, 671, 590 cmꢀ1. Anal. Calcd. for C60H58N2O16: C,
67.79; H, 5.50; N, 2.64. Found: C, 67.84; H, 5.60; N, 2.46.
H
O
O
H
O
C
A
H
D
B
N
HOOC
COOH
N
B
O
H
C
2.4. Preparation of (ꢀ)-THB (2)
OMe
O
The crystals 3 (7.5 g) were dissolved with concentrated aqueous
ammonia (25–28%), and stirred strongly, then filtered, and the
resulting solid was washed with concentrated aqueous ammonia
three times, and the solid was then recrystallized from methanol
and (ꢀ)-THB (2) was obtained (3.8 g). Yield 80%. White crystal,
OMe
O
CH3
3
Fig. 1. Chemical structures of ( )-THB (1), (ꢀ)-THB (2) and complex (3).
mp 137–138 °C (methanol); ½a D20
ꢂ
ꢀ372° (c 0.45, MeOH); 1H and
13C NMR data of ( )-THB described in Section 2.2; CDkmax (MeOH)
China). All solvents used were purchased commercially and used
without further purification.
nm 205.6 (De-5.21); UV (MeOH) kmax 208.0, 285.8 nm.
Melting points were taken in open capillary and were uncor-
rected. The NMR spectra were recorded in dilute chloroform-d3
solutions at 303 K using a Bruker Avance DRX 500 FT-NMR 500
spectrometer at 500 and 125 MHz, respectively, and TMS as inter-
nal standard, J in Hz. Chemical shifts are reported in d (ppm) val-
ues. The UV spectra were collected on a Perkin-Elmer Lambda
900 UV–vis spectrophotometer. Elemental analyses (C, H, and N)
were performed on a Perkin-Elmer 2400 CHN elemental analyzer.
Mass spectrometric measurements were run using a Micromass
Waters Q-TOF Global mass spectrometer with electrospray ioniza-
tion (ESI) working on a positive mode. The specific rotation of the
resolved enantiomer of THB and the complex, dissolved in metha-
nol, were taken on a Perkin-Elmer model 241MC polarimeter
equipped with a sodium lamp operated at a wavelength of
589 nm. Circular dichroism (CD) spectra were run at ambient tem-
perature on a Jasco J-815 spectrometer equipped with a quartz cell
(1 mm path length) containing 0.1 mg/mL of THB solutions in
methanol. Fourier-transform IR spectra were recorded in the
4000–400 cmꢀ1 region using KBr pellets on an Alpha Centauri FT-
IR spectrophotometer with 4 cmꢀ1 spectral resolution. Wavenum-
bers of strong/sharp bands are accurate to 1 cmꢀ1 or better, and
all spectra were recorded in triplicate for test samples.
2.5. X-ray diffraction data
Crystals of the 2:1 complex of (ꢀ)-THB with DTTA for diffraction
measurements were grown from methanol. Crystallographic data
for specimen were collected on a Nonius Kappa CCD diffractometer
with graphite monochromatic MoK
a radiation (k = 0.71073 Å) at
298(2) K. The structure was solved by direct methods and refined
using full-matrix least-squares calculations with anisotropic dis-
placement parameters for all non-hydrogen atoms. All hydrogen
atoms were geometrically fixed at the calculated positions at-
tached to their parent atoms, and treated as riding atoms. All cal-
culations were performed using SHELXL-97 program [20].
Crystallographic data and experimental details for structure are
listed in Table 1. CCDC 689342 contains the supplementary crystal-
lographic data for the compound. These data can be obtained free
of charge at www.ccdc.cam.ac.uk/conts/retrieving.html or from the
Cambridge Crystallographic Data Centre, 12, Union Road, Cam-
bridge CB2 1EZ, UK. [deposit@ccdc.cam.ac.uk].
3. Results and discussion
In the present work, we firstly were to obtain ( )-THB 1 by
reduction of berberine chloride using NaBH4 at refluxing tempera-
ture. The structure of this racemate THB was determined by com-
parison of its 13C and 1H NMR spectroscopic data with those
reported in the literature [21]. For instance, the 1H NMR reso-
nances at d 3.87 (3H) and 3.85(3H) were assigned to the protons
of OCH3 and OCH3 groups. The doublets of the aromatic H-11 at
d 6.82 and H-12 at d 6.88 were readily distinguished from the sing-
lets of the H-1 at d 6.72 and H-4 at d 6.63 atoms; and a singlet sig-
nal was observed at d 5.92, attributable to the OCH2O moiety. The
13C NMR resonances are observed at d 60.0 and 55.6 were attrib-
uted to the carbons of OCH3 and OCH3 groups; and signals at d
108.5–150.1 regions and at d 100.6 can be ascribed to the aromatic
carbons and OCH2O group.
2.2. Preparation of racemic THB (1)
To a hot solution of berberine chloride (37.2 g, 0.10 mol) dis-
solved in methanol (600 mL) NaBH4 (4.6 g, 0.12 mol) was added
in portion, and the reaction mixture was stirred for 4 h at room
temperature. After the filtration, the resulting solid was washed
three times with distilled water and recrystallized from methanol
to afford ( )-THB 1 as a pale yellow crystal (27 g). Yield 79.6%; m.p.
169–170 °C. ESI-MS (positive mode): m/z 340.1533 [M+H]+; 1H
NMR (500 MHz, CDCl3): d 6.88 (1H, d, J = 8.3 Hz, H-12), 6.82 (1H,
d, J = 8.3 Hz, H-11), 6.72 (1H, s, H-1), 6.63 (1H, s, H-4), 5.92 (2H,
s, -OCH2O-), 3.87 (3H, s, 9-OCH3), 3.85 (3H, s, 10-OCH3), 4.25

468
J.-M. Gao et al. / Journal of Molecular Structure 892 (2008) 466–469
Table 1
All bond lengths and angles in the title complex 3 are within
Crystal data and refinement parameters for compound 3
normal ranges. The THB conformation in this complex is similar
to the THP conformation in related structures [22,23] and escholi-
dine [24]. In the case of THB moiety, the sum of the three valence
angles around nitrogen is 329.0(3)°, indicating that sp3 hybridiza-
tion of the N-atom. The ring B of the quinolizine moiety adopts a
twisted half chair conformation while the ring C conformation
resembles a half-chair, as indicated by the torsion angles of
C(27)–C(28)–C(33)–C(34) of ꢀ5.15°, C27–N2–C35–C34 of 59.20°,
C25–C26–N2–C35 of ꢀ45.21° and C26–C25–C24–C36 of 19.22°.
The results were in agreement with those reported in literature
[25–27]. The two methoxy groups in 3 are oriented in similar man-
ner as in 8-hydroxydihydroberberine [28]. Specifically, one MeO
group attached to C29 atom is almost perpendicular to the plane
D, while the other MeO group is nearly in-plane. The dioxolane
five-membered ring C22–C38–O6–C21–O5 is reasonably planar.
The aromatic rings A and D are twisted to form a dihedral angle
of 31.5 (1)° as compared to 33.8 (1)° in isocorypalmine [29],
36.7(1)° in corydaline [30], 32.6 (4)° in (ꢀ)-tetrahydropalmatine
[22,23].
In crystals of 3 there exist a large number of close intermolecu-
lar contacts observed between the H- and the O-atoms of THB and
DTTA molecules that are within the range of the Van der Waals
interactions. In addition, the crystal packing of this complex is sta-
bilized by the weak interactions. The molecule of 3 are packed in
columns along the x-axis, interestingly, no hydrogen bonds are
found between THB and DTTA molecules within the crystal struc-
ture, presumably because of steric hindrance with the N-atom [23],
molecules are held together only by Van der Waals forces. There-
fore, we conjecture that these interactions are able to assist in
the successful separation of the racemic THB using the DTTA.
Additionally, in order to determine the core composition struc-
ture of the complex, treatment of the crystalline substances 3 with
concentrated aqueous ammonia, which disrupted the weak inter-
action of THB with DTTA based on its high polarity and basicity,
followed by recrystallization from methanol afforded a pure enan-
tiomer (ꢀ)-THB 2, whose structure was further confirmed by high
resolution ESI-MS, CD spectra and specific rotation, as well as 1H
and 13C NMR data. The ESI-MS of 2 showed a molecular ion peak
at m/z 340.1533 [M+H]+, corresponding to the molecular formula
C21H26NO4. By circular dichroism spectrum and specific rotation,
the absolute configuration of THB was determined to be an S form
at the asymmetric C13a atom. This finding is compatible with that
of (ꢀ)-THP assigned by Luger et al. [23]. The CD spectrum showed a
Empirical formula
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system
Space group
a (Å)
C60H58N2O16
1063.08
298(2)
0.71073
Triclinic
P1
8.206(7)
12.147(9)
14.457(11)
105.296(12)
105.248(12)
92.646(13)
1330.7(11)
1
1.327
0.097
560
b (Å)
c (Å)
a
(°)
b (°)
c
(°)
Volume (Å3)
Z
Density calculated (mg/m3)
Absorption coefficient (mmꢀ1
F(000)
)
Crystal size (mm)
h Range (°)
Index range, h
Index range, k
0.40 ꢃ 0.15 ꢃ 0.10
2.59–28.09
ꢀ9 ? 9
ꢀ14 ? 14
ꢀ17 ? 8
Index range, l
Reflections collected/unique
Data/restraints/parameters
Refinement method
Goodness-of-fit on F2
6857/4648
704/5/464
Full-matrix least-squares on F2
1.043
Final R/ wR2 [I > 2
Final R/ wR2 (all data)
r
(I)]
0.0459/0.1372
0.0629/0.1204
Next, using (+)-2,3-di(p-toluyl) tartaric acid as a resolution
agent, and the resolution of the racemic THB was performed in
methanol. Fortunately, this resolution afforded a crystal complex
3 suitable for single-crystal X-ray diffraction. The details of crystal
data collection and structure refinement of 3 are summarized in
Table 1. The exact structure of the complex 3 was characterized
unambiguously by means of elemental analysis, specific rotation,
IR, and X-ray crystal diffraction. The X-ray analysis reveals that
the compound has a THB-to-DTTA ratio of 2:1, as shown in
Fig. 2, namely [(C20H20NO4)2. . .C20H18O8,ꢁ(THB)2 (DTTA)], and crys-
tallizes in a triclinic system with a P1 space group (Table 1). This
formula was also supported by elemental analysis.
negative Cotton effect at 206 nm (
of the aromatic system. Also, these data were supported by a neg-
D
e
= ꢀ5.21) due to the presence
ative ½a 2D0
ꢂ
ꢀ372° (c 0.45, MeOH) [lit: ½a D17
ꢀ252° (c 0.192, MeOH);
ꢂ
½
a 2D5
ꢂ
ꢀ303° (CHCl3)] [31,21] and the UV spectrum, which exhibited
bands of absorption at 208 nm. Also, 1H and 13C NMR data of (ꢀ)-
THB are in full accordance with those described above for ( )-THB.
Moreover, it is worthwhile to note that recrystallization of (ꢀ)-THB
with MeOH produced a pure enantiomer with constant specific
rotation value of ½a D20
ꢀ372°. In addition, the complex itself is dex-
ꢂ
trorotatory (½a D20
ꢂ
+ 60°), consequently, both alkaloid molecules in
the complex are levorotatory.
In FT-IR spectra of the title complex 3 investigated, in the high
wavenumber region, this compound shows at high wavenumbers a
strong and broad absorption at 3425 cmꢀ1 assignable to the
m (OH)
of the complex as it is shown in the IR spectrum (3443 cmꢀ1) of the
DTTA. This frequency shift difference is due to the interaction of
THB with DTTA. The medium intense bands with the peaks at
2992, 2944, 2909, and 2836 cmꢀ1 are attributed to CH3 mode
stretching of the DTTA and to CH2 and CH species of alkaloid por-
tion. The bands observed at 1612 and 1498 cmꢀ1 are clearly pres-
ent in the spectrum of phenyl rings, and they can be assigned as
double bond stretching modes. The most intense band, that at
Fig. 2. Molecule structure and ORTEP drawing of the complex crystal (3), showing
the atomic numbering.

J.-M. Gao et al. / Journal of Molecular Structure 892 (2008) 466–469
469
1498 cmꢀ1, can be assigned to a coupled C'O and C'C stretching
vibration, as well some contribution from the CH2 bending mode of
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