Structural study of 8-azole derivatives of protoberberine alkaloids: Experimental and quantum chemical approach

Article abstract of DOI:10.1016/j.tet.2010.09.030

New derivatives of protoberberine alkaloids were prepared by nucleophilic addition of some azoles (differing in bulkiness) to the iminium functionality of the quaternary protoberberine alkaloids. Compounds were structurally characterized mainly by ^1<

Full text of DOI:10.1016/j.tet.2010.09.030

Tetrahedron 66 (2010) 9277e9285
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Structural study of 8-azole derivatives of protoberberine alkaloids: experimental
and quantum chemical approach
a
b
a
b
a,b,
ꢀ ꢀ a
ꢀ
ꢀ
*
ꢀ
ꢀ
ꢀ
ꢀ
Lukás Maier , Tomás Solomek , Matej Pipíska , Zdenek Kríz , Marek Necas , Radek Marek
a National Center for Biomolecular Research, Masaryk University, Kamenice 5/A4, CZ-62500 Brno, Czech Republic
b Department of Chemistry, Masaryk University, Kamenice 5, CZ-62500 Brno, Czech Republic
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 20 April 2010
Received in revised form 26 August 2010
Accepted 13 September 2010
Available online 17 September 2010
New derivatives of protoberberine alkaloids were prepared by nucleophilic addition of some azoles
(differing in bulkiness) to the iminium functionality of the quaternary protoberberine alkaloids. Com-
pounds were structurally characterized mainly by 1H and 13C NMR spectroscopy, and the structure of
8-carbazol-1-yl-7,8-dihydroberberine was determined using single-crystal X-ray diffraction. Addition-
ally, conformational behaviors of five derivatives varying in bulkiness of the azole moiety have been
investigated by low temperature NMR spectroscopy and quantum chemical calculations. Ring current
effects of pyrrole and carbazole moieties on selected 1H NMR resonances have been characterized,
visualized, and discussed.
Keywords:
Barrier to rotation
Berberine
Low-temperature NMR spectroscopy
Nucleus-independent chemical shift
Palmatine
Ó 2010 Elsevier Ltd. All rights reserved.
Theoretical calculations
1. Introduction
Quaternary protoberberine alkaloids can react as electrophiles
due to the presence of the iminium moiety.
Alkaloids represent a group of naturally occurring molecules
being frequently studied for their interesting pharmacological ef-
fects. The group of quaternary protoberberine alkaloids (QPAs)
belongs to the large family of isoquinolines. Compounds of the QPA
group are, together with some other isoquinoline alkaloids, widely
distributed in plant families Papaveraceae, Berberidaceae, Ruta-
ceae, Ranunculaceae, etc.1 The basic skeleton of QPA is 5,6-dihy-
drodibenzo[a,g]quinolizinium, which is usually stabilized by the
anion of an organic acid or chloride. Substituents of the basic
scaffold are limited mainly to hydroxy, methoxy, and methyl groups
at several positions. The 2,3,9,10-tetrasubstituted QPAs represent
the most abundant subgroup of QPAs. Berberine (1), palmatine (2),
and coptisine (3) are the main representatives of this subgroup
(Fig. 1). Various biological activities of natural QPAs and some
synthetic derivatives of QPAs have been described in several pa-
pers.2 It has been found that these compounds have substantial
antibacterial, antifungal, antiviral, and antimalarial properties.2
They may inhibit certain metabolic pathways and interact with
oligonucleotides and DNA.3 However, pharmacological applications
of these alkaloids are still restricted due to their high cytotoxicity.
* Corresponding author. Fax: þ420 549492556; e-mail address: rmarek@chemi.
Fig. 1. Structures of QPAs.
muni.cz (R. Marek).
0040-4020/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2010.09.030

9278
L. Maier et al. / Tetrahedron 66 (2010) 9277e9285
Their reactions with Grignard reagents, cyanide, trichloromethyl
Extending our research effort in this field we communicate here
an investigation into the addition of some bulky azoles, such as
indole or carbazole, and the study of barriers to rotation around
the newly created CeN bond. The conformational behavior is
investigated also for the recently published 8-pyrrol-1-yl-7,8-
dihydroberberine.5
Dynamic NMR spectroscopy has proven to be a powerful tool to
study various dynamic processes in molecules ranging from small
organics and organometallics to biological systems. Reversible
intramolecular dynamic processes, such as rotation around a single
bond, can be investigated experimentally by NMR techniques in
solution and solid state.6 The temperature dependent NMR exper-
iments and correlation spectroscopy (e.g., EXSY) can be com-
plemented by computational methods and molecular modeling.7
anion, and oxygen nucleophiles,4 yield 8-substituted dihydro-
protoberberines. The stability of 7,8-dihydroprotoberberberine is
highly dependent on the nature of the nucleophile. The stabilities of
adducts formed by the addition of C-nucleophiles are significantly
higher as compared to those containing O, N, or S linking atoms. The
latter represent good leaving groups so the adducts can undergo
additional chemical transformations towards the more thermody-
namically stable molecules. Chemical transformations of 8-hydroxy
derivatives of berberine and coptisine observed in solutions are good
examples of this behavior.4
Very recently, we reported the successful preparation of co-
valent adducts of berberine, palmatine, and coptisine with so-
dium salts of some simple azoles and methanethiolate.5
Fig. 2. The nucleophilic addition of azoles to the quaternary protoberberine skeleton.

L. Maier et al. / Tetrahedron 66 (2010) 9277e9285
9279
An approach combining NMR spectroscopy and theoretical calcu-
lations is employed in this contribution.
approximately 0.6 ppm. The topological positions of H-8 and 9-
OCH3 estimated from NMR data have been supported by the solid-
state molecular structure of compound 5e determined by X-ray
diffraction (Fig. 3) and quantum chemical calculations discussed
below. Several structural features of 8-carbazol-1-yl-7,8-dihy-
droberberine (5e) deserve more detailed discussion.
2. Results and discussion
Preparation of 8-substituted derivatives of naturally occurring
alkaloids berberine, coptisine, and palmatine is schematically depic-
ted in Fig. 2. The sodium salt of the corresponding azole was prepared
by reacting NaH with the azole in THF under an argon atmosphere. In
the following step, the nitrogen atom of the salt attacked the electron
deficient carbon of the iminium functionality resulting in C-8
substituted derivatives of 7,8-dihydroprotoberberine.
In order to investigate the influence of the substituents on this
reaction, 13-benzylberberine bromide (compound 4) was prepared
according to the published procedure via well-known 8-aceto-
nylberberine as a key intermediate2 and was subjected to nucleo-
philic addition with some azoles.
MS analysis of the products was complicated by an extraordi-
nary lability of the newly created CeN bond as reported very re-
cently.5 The reaction products were therefore characterized mainly
by using NMR spectroscopy. All resonances in the 1H and 13C NMR
spectra were completely assigned using COSY, NOESY, 1He13C
HMQC, 1He13C HMBC, and 1He13C GSQMBC experiments. Assign-
ments of 1H and 13C resonances for compounds 4e8 are summa-
rized in Tables 1 and 2, respectively.
Interatomic distance C8eN10 (1.476 Å) is comparable with those
obtained for several berberine adducts with azole rings.5 The
methoxy group at C-10 lies almost in the plane of aromatic ring D as
previously found in the crystals of the quaternary protoberberines
and QPA adducts with oxygen and nitrogen nucleophiles.1,4,5
The C9eOMe group is nearly perpendicular to the plane of ring
D, interestingly in the position close to the carbazole part of mol-
ecule. The same orientation has been reported for the crystal
structure of 5a and 8-pyrazol-1-yl-7,8-dihydroberberine.5 In con-
trast, the 9-OMe group is orientated reversely in the crystal struc-
ture of 8-trichlormethyl-7,8-dihydroberberine and 8-methoxy-7,
8-dihydroberberine.4c
Very broad resonances in the aromatic region were found in 1H
NMR spectrum of 8-carbazol-1-yl-7,8-dihydroberberine (5e)
Nucleophilic addition and the formation of 7,8-dihy-
droprotoberberine induce a substantial shift change for atoms H-8,
H-13, C-8, and C-13 due tothe loss of aromaticity of the C ring (Fig.1).
Similar tothe trendsobservedfor simple azoles,5 the resonance ofH-
8 is shifted from 9.91 ppm for 1 to 7.2e7.8 for compounds 5.13C NMR
resonances are affected in a similar way, for example, the chemical
shift of 145.37 ppm was observed for C-8 in berberine chloride,
whereas C-8 in adducts resonate in the range 66e72 ppm. However,
these values should be compared with caution because different
solvents were used to measure QPAs and 7,8-dihy-
droprotoberberines. Observation of two chemically nonequivalent
protons at positions C-5 and C-6 for compounds 5e8 correlates with
the induction of the stereogenic center at position C-8.
The covalent attachment of azole to the protoberberine skeleton
was supported by observing NOE interactions betweenprotons of the
protoberberine moiety and protons of the azole part. This was further
confirmedforseveralcompoundsbydetectingthelong-range1He13C
interactions between H-8 and some carbons of the azole part.
Resonances of C-2, C-3, C-9, and C-10 were assigned by inter-
preting the long-range 1He13C coupling constants. For example,
Fig. 3. The molecular structure of 8-(carbazol-1-yl)-7,8-dihydroberberine (5e) obtained
from single-crystal X-ray diffraction.
2
experimentally observed JH4,C3¼3.7 Hz for 7e nicely corresponds
3
to the DFT calculated value of 3.5 Hz. In contrast, JH4,C2 is ap-
2
proximately twice as large compared to the JH4,C3. The experi-
3
mental value of JH4,C2¼7.2 Hz is again in agreement with
theoretical prediction (7.3 Hz). A similar strategy based on the
2
3
differences between JH,C and JH,C was applied for assigning C-9
and C-10 resonances.
It is interesting to note the exceptionally large substituent-in-
duced shifts observed for H-8 and 9-OCH3 resonances in adducts 5e
and 6e and for H-8 in 7e. More comprehensive discussion on this
topic is provided in following sections.
Differences in selected 1H NMR chemical shifts for compounds
5e7 are summarized in Table S1. A formal replacement of the
pyrrole moiety by indole in compound 5 induces deshielding of H-8
by 0.61 ppm, whereas protons of the 9-OCH3 group are shielded by
0.25 ppm. Data comparing pyrrole and carbazole fragments in
compounds 5e7 indicate an even more pronounced influence of
the aromatic rings. The deshielding of H-8 by 1.12 ppm could be
rationalized by the position of H-8 in the plane of the carbazole
moiety. In contrast, position of 9-OCH3 group above the plane of
Fig. 4. A portion of the 1H NMR spectrum for compound 5e measured in CD2Cl2 at
several temperatures (some signals of protons corresponding to the free 1H-carbazole,
which was created after dissolving 5e in CD2Cl2, are marked by an asterisk).
aromatic
p-system of carbazole moiety gives rise to shielding of

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L. Maier et al. / Tetrahedron 66 (2010) 9277e9285
Table 1
1H NMR chemical shifts (
d
in ppm) for 13-benzylberberine bromide (4 in DMSO-d6) and 8-substituted 7,8-dihydroprotoberberines 5e8 (in CD2Cl2)
Atom
5b
5c
5d
5e
6e
7e
4
8a
303 K
303 K
303 K
223 Ka
223 Ka
233 Ka
303 K
303 K
H-1
H-4
H-5
7.23s
6.51s
2.55m
2.84m
2.84m
3.43m
7.29s
6.95d
6.99d
6.19s
5.92d
d
7.23s
6.51s
2.56m
2.84m
2.84m
3.41m
7.22s
6.94d
6.98d
6.16s
5.92d
d
7.22s
6.51s
2.53m
2.83m
2.83m
3.48m
7.53s
6.95d
6.98d
6.20s
5.91d
d
7.28s
6.47s
2.44m
2.82m
2.53m
3.23m
7.77s
6.87d
6.94d
6.17s
5.93d
d
7.26s
6.48s
2.44m
2.84m
2.58m
3.27m
7.80s
6.87d
6.94d
6.24s
3.91s
3.72s
2.66s
3.69s
7.34d
7.17t
d
7.28s
6.46s
2.39m
2.79m
2.44m
3.13m
7.67s
6.76d
6.88d
6.18s
5.93d
d
6.98s
7.17s
3.17t
d
6.85s
6.64s
2.62m
2.86m
3.24m
3.51m
6.80s
6.88d
6.95d
d
H-6
4.89t
d
H-8
H-11
H-12
H-13
10.04s
8.10d
7.79d
d
2,3-OCH2O/2-OCH3
3-OCH3
9,10-OCH2O/9-OCH3
6.08s
d
5.86d
d
3.23s
3.80s
7.00d
6.33d
d
3.22s
3.80s
6.73d
d
3.19s
3.79s
d
2.67s
3.69s
7.31d
7.16t
d
5.72d
d
4.13s
4.03s
d
3.46s
3.80s
6.67m
6.01m
d
10-OCH3
H-20
7.31d
7.18t
d
H-30
7.90s
d
d
H-30-CH3
H-40
2.14s
7.44d
7.04t
7.22t
7.71d
d
d
7.52d
7.05t
7.23t
7.76d
d
7.48d
7.25t
7.05t
7.63d
d
7.05t
7.96d
8.04d
7.24t
7.55t
7.99d
7.06t
7.97d
8.05d
7.24t
7.55t
8.00d
7. 09t
7.99d
8.05d
7.24t
7.51t
7.90d
d
6.01m
6.67m
–
H-50
d
H-60
d
H-70
d
d
H-80
d
d
H-90
d
d
d
d
d
a
Signals are resolved and no spectral overlap was observed at this temperature.
Table 2
13C NMR chemical shifts (
d
in ppm) for 13-benzylberberine bromide (4 in DMSO-d6) and 8-substituted 7,8-dihydroprotoberberines 5e8 (in CD2Cl2)
Atom
5b
5c
5d
5e
6e
7e
4
8a
303 K
303 K
303 K
263 Ka
253 Ka
253 Ka,b
303 K
303 K
C-1
C-2
C-3
C-4
C-4a
C-5
C-6
C-8
C-8a
C-9
C-10
C-11
C-12
C-12a
C-13
C-13a
104.38
146.97
147.72
107.77
127.70
29.80
46.51
66.50
120.74
145.50
150.16
114.10
120.69
128.06
94.71
137.45
125.05
101.39
d
104.88
147.43
148.17
108.26
128.18
30.33
46.98
66.50
121.42
145.94
150.61
114.43
120.00
128.67
95.09
137.94
125.60
101.89
d
104.42
146.92
147.66
107.72
124.46
29.79
46.10
71.54
119.15
145.88
150.01
114.27
119.35
129.19
94.47
137.66
125.01
101.35
d
104.50
146.70
147.54
107.68
129.11
29.41
44.89
66.72
118.52
145.99
149.74
113.37
119.00
128.29
92.19
137.75
124.48
101.43
d
107.01
147.78
149.20
110.27
127.50
29.03
104.49
146.71
147.57
107.68
129.04
29.29
44.52
66.17
107.31
144.90
144.60
109.25
116.33
128.89
92.37
137.79
124.34
101.49
d
108.09
146.36
149.19
108.44
129.98
27.24
56.97
145.42
121.22
144.24
150.18
126.17
121.61
132.75
134.02
137.13
119.97
102.00
d
108.81
146.13
147.78
108.24
128.13
31.85
49.22
69.69
122.70
145.10
150.91
113.73
119.01
133.37
104.64
136.74
126.03
101.83
d
45.10
66.96
118.63
146.13
149.82
113.47
118.95
128.59
91.71
137.91
122.74
56.05
55.92
59.20
55.76
140.87
110.90
125.83
d
C-13b
2,3-OCH2O/2-OCH3
3-OCH3
9,10-OCH2O/9OCH3
10-OCH3
C-1a0
60.01
56.05
d
60.54
56.55
d
59.90
55.99
d
59.06
55.62
140.70
112.82
125.61
d
101.73
d
62.02
56.91
d
60.53
56.36
d
140.66
112.54
125.98
d
C-20
C-30
126.79
102.56
d
124.46
112.28
9.80
129.72
119.25
119.25
d
d
d
d
133.18
d
d
d
C-30eCH3
C-3a0
d
d
129.22
119.59
119.42
d
128.45
110.77
126.28
d
d
d
d
d
d
C-40
119.22
119.86
122.58
123.39
120.16
119.22
d
119.39
120.10
122.89
123.52
120.38
119.39
d
119.97
120.21
122.51
123.48
120.34
119.45
d
d
d
C-50
d
d
C-5a0
d
d
C-5b0
d
d
d
d
d
C-60
121.82
110.04
135.87
d
122.27
110.17
136.71
d
120.54
120.75
138.54
d
d
d
C-70
d
d
C-7a0
d
d
C-80
125.80
109.45
138.02
126.02
109.66
138.16
125.98
108.95
137.73
d
d
C-90
d
d
d
d
d
C-9a0
d
d
d
d
d
a
Signals are resolved and no spectral overlap was observed at this temperature.
b
Assignments of signals C-2, C-3, C-9, and C-10 were accomplished by interpreting the 1He13C coupling constants.

L. Maier et al. / Tetrahedron 66 (2010) 9277e9285
9281
measured at 303 K (see Fig. 4). Resonance broadening indicated
a dynamic process occurring in the solution. Observation of this
dynamic behavior initiated our interest in the conformational study
of adducts in solution. The resonance broadening could be ratio-
nalized by slow hindered rotation of the carbazole moiety around
the newly created CeN bond. Experimental NMR study and theo-
retical calculations were performed in order to confirm an assumed
phenomenon of hindered rotation and to estimate the barriers to
rotation. A portion of the 1H NMR spectrum for 5e measured at
several low temperatures is shown in Fig. 4.
Temperature of coalescence for several of the 1H NMR signals
was estimated from these measurements. From chemical shifts for
individual atoms observed at low temperatures, exchange rates
and Gibbs activation energies (D
Gzobs) of barriers to rotation were
calculated by transition state theory.8 The experimental values for
5e, 6e, and 7e together with the calculated energies obtained by
density functional theory (DFT) calculations are summarized in
Table 3.
NOESY experiments were used as a supporting tool for the dy-
namic study and for the resonance assignment. Exchange signals
between pairs of protons observed in the spectrum of 5e at 263 K
(Fig. 5a) facilitated resonance assignment. It allowed unequivocal
assignments of protons of the carbazole moiety. Schematic repre-
sentation of observed interactions can be found in Supplementary
data (Fig. S1). In contrast, solely NOE interactions were observed
in the NOESY experiment recorded at 193 K without any signs of
exchange (Fig. 5b). This result clearly indicates that the rotation of
carbazole moiety around the berberine part of molecule is slow
enough at 193 K to obtain discrete resonances for all protons of the
carbazole part of the molecule. Moreover, observed H-8/H-90 NOE
interaction allowed 3-D assignments of resonances for the carba-
zole frame. Analogous dynamic behavior was observed for palma-
tine and coptisine derivatives 6e and 7e.
Despite relatively sharp resonances observed in the 1H NMR
spectrum of 8-indol-1-yl-7,8-dihydroberberine (5b) at 303 K, we
performed temperature dependent 1H NMR experiments (Fig. S2).
Since CeN rotation for compound 5b possesses lower symmetry
than those for 5a and 5e, NMR signals belonging to two rotamers
differing in energy might be detected at low temperatures. The
NMR signals at 303 K represent time-averaged and population-
weighted contributions of individual conformers. Signals are well
resolved still at 253 K, but upon cooling the sample, we observed
signal broadening for H-70 and H-20 at 223 K and 203 K, re-
spectively. H-70 and H-20 are clearly the protons most influenced by
Fig. 5. A portion of the NOESY spectrum (mixing time 700 ms) for compound 5e. (a)
T¼263 K with assignment of exchange signals for carbazole moiety (b) T¼193 K with
assignment of NOE signals.
The methyl group in the indole moiety has vanishingly small
effect on the dynamic behavior as indicated by comparative low-
temperature NMR study of 5c.
2.1. Theoretical study of barriers to rotation
Table 3
To get a deeper insight into the conformational behavior of se-
lected compounds, we employed quantum chemical calculations
to obtain the rotational barriers in compounds 8-pyrrol-1-yl-7,
8-dihydroberberine (5a),5 8-indol-1-yl-7,8-dihydroberberine (5b),
8-carbazol-1-yl-7,8-dihydroberberine (5e), and 8-carbazol-1-yl-
7,8-dihydrocoptisine (7e). The energy profile of the conformational
motion was simulated by performing a relaxed potential energy
Rate constants (kobs) determined experimentally by temperature dependent 1H
NMR spectroscopy and corresponding barriers to rotation obtained experimentally
(D D
Gzobs) and calculated by DFT ( Gzcalcd, 298 K)
Compound
kobs/sꢁ1
D
Gz (kcal/mol)
D
Gz
(kcal/mol)
calcd
obs
5a
5b
5e
6e
7e
da
da
da
14.56293K
14.83298K
14.43288K
4.66
da
10.66
14.96
db
86.47293K
85.11298K
30.67288K
surface scan by increasing the dihedral angle
q
¼N7eC8eN10eC20
with an increment of 10ꢀ over 180ꢀ in the case of 5a, 5e, 7e, and
13.78
360ꢀ for 5b.
a
Not determined experimentally.
Not calculated theoretically.
b
The energy minimum for 8-pyrrol-1-yl-7,8-dihydroberberine
(5a) has been found for the angle
most stable conformer is in very good agreement with that
obtained from the X-ray diffraction data (
q
¼115.0ꢀ. The structure of the
the rotation of the indole moiety around the CeN bond. Finally, we
observed relatively sharp signals again at 183 K. Unfortunately, we
were not able to identify the resonances of the minor component
probably due to its low population (for calculated energies, see
theoretical study) in solution and the overlapping signals in the
q
¼110.4ꢀ).5 We located
a transition state of the pyrrole moiety rotation with a rotational
barrier
D
Gz
(at 298 K) of 4.66 kcal molꢁ1 (Table 3).
calcd
The potential energy profile for 8-indol-1-yl-7,8-dihy-
droberberine (5b) is more complicated and as expected, four en-
ergy minima were found. The most stable conformer possesses the
crowded aromatic spectral region. Therefore,
missing from Table 3.
D
Gz
for 5b is
obs
angle
q
¼60.1ꢀ. The benzene ring of the indole moiety is directed

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L. Maier et al. / Tetrahedron 66 (2010) 9277e9285
Fig. 6. The most stable conformer of 8-indol-1-yl-7,8-dihydroberberine (5b).
outside the molecule while the 9-OMe group is orientated towards
the indole part as mentioned previously (Fig. 6).
Another significant conformation (1.16 kcal molꢁ1 higher in en-
ergy) shares almost the same orientation of the indole moiety
(
q
¼54.1ꢀ) but interestingly, the 9-OMe group is orientated the other
way around. The conformer with the reverse orientation of the in-
dole moiety (
q
¼ꢁ119.6ꢀ) and the 9-OMe group pointing towards the
indole and the last minor conformer (
q
¼ꢁ125.5ꢀ, 9-OMe group is
reversed) are higher in energy by 2.29 and 3.42 kcal molꢁ1, re-
spectively. The structures together with the corresponding energies
are given in Fig. S3. The overall barrier to rotation of 10.66 kcal molꢁ1
has been found for indol-1-yl-7,8-dihydroberberine (5b) (Table 3).
The most stable conformer of 8-carbazol-1-yl-7,8-dihy-
droberberine (5e) is defined by the angle
q
¼57.4ꢀ. Exactly the same
value has been measured in the X-ray diffraction experiment of 5e.
The C9eOMe is nearly perpendicular to the ring D and is orientated
towards the carbazole part the same way as observed in the crystal.
The overall barrier to rotation of 14.96 kcal molꢁ1 has been calcu-
lated for 5e (Table 3). This is in excellent agreement with the value
of 14.56 kcal molꢁ1 obtained from the experiment.
The energy minimum for 8-carbazol-1-yl-7,8-dihydrocoptisine
(7e) corresponds to the angle
q
¼51.9ꢀ and an overall barrier to
rotation of 13.78 kcal molꢁ1 has been calculated. This is also in very
good agreement with the experimentally observed value of
14.43 kcal molꢁ1
.
2.2. Interpretation of chemical shifts
Finally, we calculated 1H NMR chemical shifts with respect to
tetramethylsilane for 5a, 5b, and 5e in order to support our struc-
ture elucidation and to obtain additional information about
Table 4
Experimental and calculated 1H NMR chemical shifts for compounds 5
H-8
9-OMe
EXPa
10-OMe
EXPa
EXPa
CALCb
CALCb
CALCb
a303K
b303K
e300K
6.68c
7.29
7.80
0.61
0.51
1.12
6.55
7.20
7.76
0.65
0.56
1.21
3.48c
3.23
2.85
ꢁ0.25
ꢁ0.38
ꢁ0.63
3.31
3.05
2.82
ꢁ0.26
ꢁ0.23
ꢁ0.49
3.83c
3.80
3.73
ꢁ0.03
ꢁ0.07
ꢁ0.10
3.78
3.77
3.71
ꢁ0.01
ꢁ0.06
ꢁ0.07
D
D
D
(bꢁa)
(eꢁb)
(eꢁa)
Fig. 7. Visualization of the differential NICS matrix between carbazole and pyrrole in
coordination system of 5e. Blue area represents a shielding region whereas red surface
maps a deshielding sphere (cutoffꢂ0.28 ppm). (A) Total view; (B) shielding region
incorporating 9-OMe group; (C) deshielding region embedding H-8 atom.
a
Experimentally observed data.
b
B3LYP/6-31G(d,p), PCM solvent model//B3LYP/6-31G(d) geometry; referenced
to TMS.
c
Previously reported experimental data.5

L. Maier et al. / Tetrahedron 66 (2010) 9277e9285
9283
electronic distribution in these molecules. Selected 1H NMR
chemical shifts calculated using B3LYP are summarized in Table 4
and compared with those observed experimentally.
with the nebulizing and drying gas N2 (300 ꢀC) at a flow rate of
10 L minꢁ1, a nebulizer pressure of 80 psi, and a capillary voltage
3.5 kV. The full mass scan covered the range from m/z 200 to 1500.
IR spectra were measured by DRIFTIR method on Bruker VER-
TEX80v instrument. HR mass spectra were measured using APEX-
Ultra FTMS instrument equipped with a 9.4 T superconducting
magnet and a Dual II ESI/MALDI ion source (Bruker Daltonics, Bill-
erica MA) using nano-assisted laser desorption/ionization (NALDI).
Note that only the most stable conformer for each compound has
been considered andno averaging has beenattempted for simplicity.
This approach has been shown to give reasonable results in some
cases.9 Although a small basis set (3-21G(d)) was initially used for
the geometry optimization, the data computed for 5a, 5b, and 5e
correspond nicely totheexperimentallyobserved values of chemical
shifts with only slight deviations in some cases (see Table S2). Values
for H-8 and C10eOMe are in excellent agreement with experiment
while those computed for the C9eOMe group differ slightly.
To improve the agreement between the calculated chemical
shifts and the experimental data for the 9-OMe group in 5a, 5b, and
5e, a more accurate geometry optimized with 6-31G(d) basis set
had to be used. By employing a new geometry, the agreement be-
tween theoretical and experimental data for 9-OCH3 was signifi-
cantly improved (compare Table 4 and Table S2).
It is evident from Table 4 that substantial differences in the
chemical shifts for H-8 and 9-OMe are induced by the ‘ring current’
effects of pyrrole, indole, and carbazole. This ‘through-space’ effect
was investigated in detail theoretically by calculating the nucleus-
independent chemical shift (NICS)10 and also by visualizing the iso-
NICS surfaces.10b To investigate the differences between pyrrole (a)
and carbazole (e) derivatives as examples, we calculated differen-
tial matrix of the iso-NICS values between free carbazole and pyr-
role (see section Experimental). The resulting differential iso-NICS
surface in coordination system of 5e is shown in Fig. 7.
4.2. NMR spectroscopy
NMR spectra were recorded on a Bruker Avance 300 spectrometer
operating at frequencies of 300.13 MHz (1H) and 75.48 MHz (13C) and
a Bruker Avance 500 spectrometer operating at frequencies of 500.13
MHz (1H) and 125.77 (13C). The 1H and 13C NMR chemical shifts were
referenced tothesignalofCD2Cl2 [5.31 ppm (1H)and 53.80 ppm (13C)]
and DMSO-d6 [2.50 ppm (1H) and 39.50 ppm (13C)]. The temperature
was calibrated using methanol sample. Mixing time tm in NOESY ex-
periment11 was 700 ms, 500 ms, or 300 ms. 1He13C HMQC experi-
ments12 were adjusted to 1JH,C¼150 Hz. 1He13C HMBC13 and 1He13C
GSQMBC14 experiments were adjusted to nJH,C¼7.5 Hz or 4.2 Hz.
4.3. X-ray diffraction
Diffraction data were collected on a KUMA KM4CCD four-circle
diffractometer (Oxford Diffraction, Abingdon, UK) equipped with
a Cryostream low-temperature device (Oxford Cryosystems, Ox-
ford, UK). The crystallographic package ShelXTL15 was used to solve
and refine the structure16 of compound 5e.
Protons of the 9-OMe group are clearly embedded in the
shielding region (blue surface in Fig. 7), which results in the sig-
nificant shielding of these protons in 5e as compared to those in 5a
(experimental Dd¼ꢁ0.63 ppm, see Table 4). In contrast, H-8 is lo-
cated in the deshielding region (see Fig. 7c) with experimentally
measured Dd¼þ1.12 ppm.
4.4. Synthesis
4.4.1. 13-Benzylberberine bromide (4)2. A round-bottomed flask
(50 mL) was charged with 8-acetonylberberine (0.7970 g, 2 mmol)
dissolved in acetonitrile (10 mL). Benzylbromide (1.5 equiv, 0.513 g)
was added and the reaction mixture was refluxed for 6 h. The re-
action mixture was cooled down to room temperature and solvent
was evaporated under vacuum. The dark yellow residue was dis-
solved in CHCl3 and pre-adsorbed on alumina. Pure product was
obtained by column-chromatography using CHCl3 as eluent system.
Yield: 50% of yellow solid; mp: w200 ꢀC decomp.; ESI-MS: m/z: 426
[M]þ (100%), 411 (21%), 397 (5%); 1H and 13C NMR chemical shifts are
summarized in Tables 1 and 2 with exception for signals of the benzyl
moiety. 1H NMR: CH2eAr¼4.76 ppm, CH2eAr¼7.25e7.41 ppm; 13C
NMR: CH2eAr¼56.97 ppm, CH2eAr¼126.17, 126.76, 129.05 ppm.
3. Conclusion
Insummary, new derivatives of protoberberines were synthesized
by the nucleophilic addition of sodium salts of azoles to quaternary
protoberberine alkaloids. Detailed structuralstudy withestimation of
barriersto rotationaroundthe CeN bondhas been accomplished. The
experimentally observed data obtained by low-temperature NMR
spectroscopy and single-crystal X-ray diffraction analysis are nicely
supported by the quantum chemical calculations. Although partly
hindered, the reactive C]Nþ bond is accessible even for relatively
bulky groups like carbazole and probably also for the purine bases of
nucleic acids, which will be the subject of our future study.
4.5. General procedure for nucleophilic addition of nitrogen
heterocycles to a quaternary protoberberine skeleton
4. Experimental
4.1. Reagents and techniques
A pear-shaped flame-dried flask (25 mL) with freshly distilled
THF was charged with NaH (1.1 equiv) and nitrogen heterocycle
(1 equiv) under argon atmosphere. The reaction mixture was son-
icated for 15 min and after degassing, previously dried alkaloid
(1 equiv) was added. The reaction mixture was then vigorously
magnetically stirred at room temperature for several hours. When
the suspension settled down, the organic phase was taken carefully
through a syringe and in some cases filtered through HPLC filter
and concentrated under vacuum. Products were usually obtained
by crystallization in freezer.
Berberine chloride hydrate (Fluka), palmatine chloride hydrate
(SigmaeAldrich), and coptisine isolated from natural sources as
chloride5 were dried in oven (90 ꢀC, 4 h) before using. Nitrogen
heterocycles and other reagents were purchased from Sigma-
eAldrich and were used without further purification. THF was dis-
tilled from Na with benzophenone prior to use.
EI mass spectra were measured using TRIO 1000 quadrupole
mass spectrometer, Finnigan MAT (Fisons Instrument, San Jose,
CAN, USA). MS analyses were performed by electron ionization
technique (EI) with a direct insertion probe (DIP) used in EIþ ion-
ization mode, applying electron energy 30 eV. ESI-MS were col-
lected using an Agilent HP 1100 LC/MSD Trap VL Series in MeOH,
using direct infusion with a linear pump (kd Electronics) at a flow
4.5.1. 8-(Pyrrol-1-yl)-7,8-dihydroberberine (5a). Compound 5a was
prepared according to procedure described recently.5
4.5.2. 8-(Indol-1-yl)-7,8-dihydroberberine (5b). NaH (0.0220 g,
0.55 mmol), indole (0.0586 g, 0.5 mmol), and berberine chloride
rate of 300
m
L minꢁ1. The spectra were collected in positive mode,

9284
L. Maier et al. / Tetrahedron 66 (2010) 9277e9285
(0.1859 g, 0.5 mmol) were used. Reaction mixture was magnetically
stirred for 5 h.
(100); 1H and 13C NMR chemical shifts are summarized in Tables 1
and 2 with exception for signals of the benzyl moiety. 1H NMR:
CH2eAr¼4.22 ppm, CH2eAr¼7.21e7.38 ppm; 13C NMR:
CH2eAr¼35.80 ppm, CH2eAr¼126.62, 128.78, 129.34, 142.29 ppm;
IR (KBr, cmꢁ1) 3413, 2929, 1483, 1274, 1238, 1060, 1047, 756, 727,
632, 445; HRMS (NALDI) m/z calculated for C31H27N2O4 (MꢁHꢃ)
491.1965, found 491.1963.
Yield: 26% of brown solid; mp: w200 ꢀC decomp.; EIMS (30 eV):
m/z: 336 [1]þ (38%), 335 (42), 320 (61), 118 [indolþH]þ (100); 1H
and 13C NMR chemical shifts are summarized in Tables 1 and 2; IR
(KBr, cmꢁ1) 3448, 2939, 2839, 1596, 1508, 1458, 1290, 1047, 746,
723, 433, 395, 389; HRMS (NALDI) m/z calculated for C28H23N2O4
(MꢁHꢃ) 451.1652, found 451.1650.
4.6. Theoretical calculations
4.5.3. 8-(3-Methylindol-1-yl)-7,8-dihydroberberine(5c). NaH (0.0220 g,
0.55 mmol), 3-methylindole (0.0656 g, 0.5 mmol), and berberine
chloride (0.1859 g, 0.5 mmol) were used. Reaction mixture was mag-
netically stirred for 5 h.
All the calculations were performed with the GAUSSIAN 0317
package of programs. Geometries were optimized at the B3LYP18
level of theory with the 3-21G(d) or 6-31G(d) basis set when nec-
essary. In the case of a flat potential energy surface near the min-
imum, where the optimization procedure using the approximate
Hessian matrix was terminated prior to full convergence, force
constants were calculated analytically. Harmonic vibrational fre-
quencies were computed for all stationary points to provide ther-
mal correction to Gibbs free energy at 298 K and 1 atm. Finally,
single point energies for the B3LYP geometries were computed at
the B3LYP level of theory with the 6-311þG(2d,p) basis set. Tran-
sition structures (TS) on the potential energy surface were located
by using the facility of GAUSSIAN for the synchronous transit-
guided quasi-Newton method.19
Yield: 43% brown solid, mp: w200 ꢀC decomp.; EIMS (30 eV): m/z:
337 (47%), 335 (48), 320 (54), 130 [3-methylindolþH]þ (100); 1H and
13C NMR chemical shifts are summarized in Tables 1 and 2; IR (KBr,
cmꢁ1) 3716, 2939,1506,1350,1240, 757, 626, 513; HRMS (NALDI) m/z
calculated for C29H25N2O4 (MꢁHꢃ) 465.1809, found 465.1802.
4.5.4. 8-(Indazol-1-yl)-7,8-dihydroberberine (5d). NaH (0.0220 g,
0.55 mmol), 1H-indazole (0.0591 g, 0.5 mmol), and berberine
chloride (0.1859 g, 0.5 mmol) were used. Reaction mixture was
magnetically stirred for 3 h.
Yield: 29% ofbrownsolid, mp: w175 ꢀC decomp.; EIMS (30 eV): m/
z: 335 (53), 334 (53), 320 (49), 118 [indazolþH]þ (100); 1H and 13C
The NMR shielding tensors were calculated on the gas phase
geometries with the B3LYP/6-31G(d,p) level of theory using the
GIAO20 procedure. Self-consistent reaction field (SCRF) calculations
were performed to simulate the solvent effects in this case. The
polarized continuum model (PCM) of Tomasi and co-workers21
NMR chemical shifts are summarized inTables 1 and 2; IR (KBr, cmꢁ1
)
3552, 3197, 3058, 2960, 2918, 1506, 1284, 615, 433, 399; HRMS
(NALDI) m/z calculated for C27H22N3O4 (MꢁHꢃ) 452.1605, found
452.1601.
with CH2Cl2
(
e
¼8.93) as a solvent was used. The final chemical
4.5.5. 8-(Carbazol-1-yl)-7,8-dihydroberberine (5e). NaH (0.0118 g,
0.259 mmol), carbazole (0.0449 g, 0.269 mmol), and berberine
chloride (0.1859 g, 0.5 mmol) were used. Reaction mixture was
magnetically stirred for 6 h.
shifts were obtained with respect to tetramethylsilane.
Nucleus-independent chemical shift (NICS) calculations were
performed by applying GIAO method at HF/6-31G level of theory
in GAUSSIAN 03. Three-dimensional matrixes of dummy atoms in
Cartesian coordinates (with spacing of 0.1 Å) were created for the
iso-NICS surface analysis.10b Differential iso-NICS surface was cre-
ated as a difference between the iso-NICS surface for carbazole and
that for indole. Calculated differential iso-NICS surface was then
projected into the coordination system of compound 5e. Differen-
tial iso-NICS surfaces were visualized by using Molekel software.22
*
Yield: 29% of yellow crystals; mp: 284 ꢀC decomp.; EIMS (30 eV):
m/z: 486 (17%), 471 (11), 336 [1]þ (52), 320 (100), 319 (89); 1H and
13C NMR chemical shifts are summarized in Tables 1 and 2; IR (KBr,
cmꢁ1) 3695, 3573, 2900, 1521, 381, 376, 372; HRMS (NALDI) m/z
calculated for C32H25N2O4 (MꢁHꢃ) 501.1809, found 501.1807.
Crystal data16 for 5e: CCDC ref. No. 749835. Crystallized from
THF, C32H26N2O4, Mrel¼502.56, T¼120 K, triclinic, space group Pꢁ1,
a¼6.9873(3) Å, b¼10.4394(4) Å, c¼16.5719(8) Å,
a¼98.067(4),
Acknowledgements
b
¼92.521(4),
g
¼100.738(4), V¼1172.83 Å3, R¼0.035.
ꢀ
The authors would like to thank Professor Jirí Dostál for his gift
4.5.6. 8-(Carbazol-1-yl)-7,8-dihydropalmatine (6e). NaH (0.006 g,
0.151 mmol), carbazole (0.0229 g, 0.137 mmol), and palmatine
chloride (0.0532 g, 0.137 mmol) were used. Reaction mixture was
magnetically stirred for 5 h.
of plant material from Chelidonium majus (mixture of QPAs),
ꢀ
Dr. Lubomír Prokes for his help with measuring the EI mass
ꢀ
spectra and Dr. Karel Smejkal for measuring the ESI mass spectra.
The financial support of the Ministry of Education of the Czech
Republic (MSM0021622413 and LC06030 to L.M., M.P., Z.K., and
R.M.) is gratefully acknowledged. Gratitude is also expressed to
the Department of Chemistry of the University of Fribourg for use
of their computer facilities. The computational resources were
partially provided by MetaCentrum, Czech Republic (grant
MSM6383917201).
Yield: 17%, pale yellow solid, mp: 146e155 ꢀC decomp.; EIMS
(30 eV): m/z: 352 [3]þ (59%), 320 (40), 319 (49), 167 [carbazoleþH]þ
(100); 1H and 13C NMR chemical shifts are summarized in Tables 1
and 2.
4.5.7. 8-(Carbazol-1-yl)-7,8-dihydrocoptisine (7e). Yieþld: 38%; mp:
185 ꢀC decomp.; EIMS (30 eV): m/z: 321 [3þH] (54%), 167
e
[carbazol]þ (100); 1H and 13C NMR chemical shifts are summarized
in Tables 1 and 2; IR (KBr, cmꢁ1) 3311, 2900, 2852, 1506, 1488, 1282,
1029, 738, 696, 630, 474; HRMS (NALDI) m/z calculated for
C31H21N2O4 (MꢁHꢃ) 485.1496, found 485.1485.
Supplementary data
Supplementary data associated with this article can be found in
the online version at doi:10.1016/j.tet.2010.09.030. These data in-
clude MOL files and InChIKeys of the most important compounds
described in this article.
4.5.8. 8-(Pyrrol-1-yl)-13-benzyl-7,8-dihydroberberine
(8a). NaH
(0.0088 g, 0.22 mmol), pyrrole (13.8 L, 0.2 mmol), and 13-ben-
m
zylberberine bromide (0.1013 g, 0.2 mmol) were used. Reaction
mixture was magnetically stirred for 4 h.
References and notes
Yield: 47%, pale yellow solid, mp: 232.5 ꢀC; EIMS (30 eV): m/z:
427 [M-pyrrol]þ (54%), 336 [1]þ (42), 320 (59), 68 [pyrrolþH]þ
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