Synthesis and characterization of new fluorescent boro-β-carboline dyes

Article abstract of DOI:10.1039/d1ra02132j

The first representatives of the new fluorescent boro-β-carboline family were synthesized by the insertion of the difluoroboranyl group into the oxaza or diaza core. The resulting compounds showed good photophysical properties with fine Stokes-shifts in the range of 38-85 nm with blue and green emission. The energetics of the excitation states and molecular orbitals of two members were investigated by quantum chemical computations suggesting effects for the improved properties of diazaborinino-carbolines over oxazaborolo-carbolines. These properties nominated this chemotype as a new fluorophore for the development of fluorescent probes. As an example, diazaborinino-carbolines were used for the specific labeling of anti-Her2 antibody trastuzumab. The fluorescent conjugate showed a high fluorophore-antibody ratio and was confirmed as a useful tool for labeling and confocal microscopy imaging of tumour cellsin vitrotogether with theex vivotwo-photon microscopy imaging of tumour slices.

Full text of DOI:10.1039/d1ra02132j

RSC Advances
View Article Online
View Journal | View Issue
PAPER
Synthesis and characterization of new fluorescent
boro-b-carboline dyes†
Cite this: RSC Adv., 2021, 11, 12802
c
Denes Szepesi Kovacs,ab Imre Hajdu,ab Gergely Meszaros,cd Lucia Wittner,
´
´
´ ´
ce
cd
f
f
}
Zita Hegedus, Ivan Ranđelovic,
´
Domokos Meszena,
Estilla Zsofia Toth,
´
´
´
g
g
Jozsef Tovari,f Tımea Szabo,
´ ´
Bence Szilagyi,b Matyas Milen,
´ ´
´
´
´
´
ab
ab
*
*
}
Gyorgy Miklos Keseru
and Peter A´branyi-Balogh
¨
´
´ ´
The first representatives of the new fluorescent boro-b-carboline family were synthesized by the insertion
of the difluoroboranyl group into the oxaza or diaza core. The resulting compounds showed good
photophysical properties with fine Stokes-shifts in the range of 38–85 nm with blue and green emission.
The energetics of the excitation states and molecular orbitals of two members were investigated by
quantum chemical computations suggesting effects for the improved properties of diazaborinino-
carbolines over oxazaborolo-carbolines. These properties nominated this chemotype as
a new
fluorophore for the development of fluorescent probes. As an example, diazaborinino-carbolines were
used for the specific labeling of anti-Her2 antibody trastuzumab. The fluorescent conjugate showed
a high fluorophore-antibody ratio and was confirmed as a useful tool for labeling and confocal
microscopy imaging of tumour cells in vitro together with the ex vivo two-photon microscopy imaging
of tumour slices.
Received 17th March 2021
Accepted 24th March 2021
DOI: 10.1039/d1ra02132j
rsc.li/rsc-advances
the harmala alkaloids4 found in a high number of plants.
Harmaline (1), its several tryptamine analogue derivatives (2)
Introduction
and other serotonin (3) receptor agonists show uorescence
properties.5,6 Particularly, b-carboline (4) and methyl-b-
carboline-3-carboxylate (5) are responsible for the blue uores-
cent cuticle of two scorpion species. The latter (5) and its 3,4-
dihydro derivative (6) can also be found in the human grey
cataract.7 In addition, as close relatives, melatonin (7)
analogues equipped with a diuoroboranyl group (8) were
identied as uorescent melatonin receptor agonists,8 showing
a structural similarity to the widely used BODIPY (9).9
In continuation of our research on new b-carboline derivatives10–13
and new uorescent agents,14 we aimed to synthesize b-carbolines
coordinating the BF2 group either by two nitrogen atoms [like in
BODIPY (9)] or a nitrogen and an oxygen [like boroisoquino-
lines(10)15,16] (Fig. 1). Finding a possible application for this new
family of uorophores we aimed to equip them with functionalities
to label antibodies for tumour diagnostics and to investigate the
conjugates in confocal and two-photon microscopy (Fig. 1).14,17
Fluorophores or uorescent probes are widely used tools in
chemical biology for tagging proteins, cells or subcellular
compartments in order to make the visualization of these
species available. Although the development of synthetic uo-
rophores is important, Nature also gave us light absorbing and
emitting tool compounds.1,2 A merged direction is the modi-
cation of natural compounds or their derivatives by turning
them into efficient uorophores. The b-carboline core is widely
present in natural and synthetic biologically active compounds
(Fig. 1).3 One of the most well-known families is represented by
aResearch Centre for Natural Sciences, Medicinal Chemistry Research Group, POB 286,
1519 Budapest, Hungary. E-mail: abranyi-balogh.peter@ttk.hu; keseru.gyorgy@ttk.
hu; Tel: +36-13826961
bDepartment of Organic Chemistry and Technology, Budapest University of Technology
and Economics, 1521 Budapest, Hungary
cResearch Centre for Natural Sciences, Comparative Psychophysiology Research Group,
POB 286, 1519 Budapest, Hungary
d
´
´
´
Faculty of Information Technology and Bionics, Pazmany Peter Catholic University,
POB 278, 1444 Budapest, Hungary
Results and discussion
e
´
´
Janos Szentagothai Doctoral School of Neurosciences, Semmelweis University, 1085,
Aer the successful synthesis of different boroisoquinolines15,16
we turned our attention to present a new uorescent family based
on the b-carboline core. Our design strategy was introducing the
diuoroboranyl group between two heteroatoms (two nitrogens or
a nitrogen and an oxygen) that was realized by the pyridoindole
structure containing two nitrogens in rings B and C. Thus, we
Budapest, Hungary
fNational Institute of Oncology, Department of Experimental Pharmacology, POB 21,
1525 Budapest, Hungary
gEgis Pharmaceuticals Plc., Directorate of Drug Substance Development, POB 100,
1475 Budapest, Hungary
† Electronic supplementary information (ESI) available: For experimental
procedures and compound characterization. See DOI: 10.1039/d1ra02132j
12802 | RSC Adv., 2021, 11, 12802–12807
© 2021 The Author(s). Published by the Royal Society of Chemistry

View Article Online
Paper
RSC Advances
Table 1 Results of the synthesis of 1-(ethyl-1-ol)-b-carbolines (14), 15
ketones and 11 boro-b-carbolines
decided to synthesize members of two families, the oxazaborolo-
carbolines (11) and the diazaborinino-carbolines (12) (Fig. 1).
Synthesis of oxazaborolo-carbolines (11)
For the synthesis of oxazaborolo-carbolines we used the natural
product 1-formyl-b-carboline (13) (Kumujian C) synthesized in 5
steps using literature methods with good yields (61–95%),
starting from tryptamine (2).21 Then Kumujian C has been
transformed to 1-(ethyl-1-ol)-b-carbolines by Grignard reaction
with methyl iodide, ethyl iodide, benzyl chloride and 4-methoxy
benzyl-chloride resulting in 14a–d with various yields (35–77%)
(Table 1). The next step was the oxidation of 14 alcohols to
ketones (15) using activated manganese(IV)-oxide (MnO2) or
Dess–Martin periodinane (DMP).
In some cases, MnO2 led to signicant degradation, but
using DMP in milder conditions resulted in higher yields.
Finally, we equipped the 15 keto-b-carbolines with the diuor-
oboranyl group using boron triuoride diethyletherate in the
presence of diisopropylethylamine (Scheme 1). These reactions
showed good to excellent yields for the alkyl (methyl and ethyl,
Entry Compound R1
R2
14 [%]a 15 [%]a 11 [%]a
1
2
3
4
5
6
a
b
c
d
e
f
H
H
H
H
H
CH3
Ph
77
66
73
70
57
46
82
55
—
—
79
92
4-MeO-C6H4 35
H
52
CH3
CH3 CH3
74
72
46b
50b
a
Isolated yields. b Yields with Dess–Martin periodinane.
15a and 15b) ketones, but in the case of benzyl derivatives
(benzyl and 4-methoxybenzyl, 15c and 15d) no product was
detected by HPLC-MS. Under harsher conditions (higher equiva-
lent of BF3$OEt2, reux) still the starting material was recovered
that can be explained by the electron withdrawing character of the
aromatic moiety abolishing the reactivity of the nitrogen. Next, we
planned to increase the electron density for better uorescence by
the introduction of an electron donating group to the nitrogen
atom of the pyrrole ring. Kumujian C (13) was methylated with MeI
in the presence of sodium hydride with good yield (76%). The next
borylating step leading to 11e and 11f oxazaborolo-carbolines have
been performed similar to 11a and 11b and led to the products in
79% and 92% yields, respectively (Table 1).
Synthesis of diazaborinino-carbolines (12)
The synthesis of diazaboranino-carbolines (12) started from
tryptamine (2a) or tryptophan methyl ester (2b) (Scheme 2). In the
case of 16a,b the crude product of the rst Pictet–Spengler reaction
was taken directly to dehydrogenation (Method A). The b-carboline
core was formed with moderate yields. In the case of 16c,d the
Pictet–Spengler reaction and the dehydrogenation was performed
in one-pot reaction at 140 ꢀC with a slightly longer reaction time,
following the method of Singh et al. with modications (Method
B).22 Finally, the diuoroboranil group was incorporated to the
structure by BF3$OEt2 in the presence of Na2CO3 at 85 ꢀC leading
to the desired heterocycles in acceptable to high yields (up to 95%)
(Table 2). Notably, the imidazole-derivative (12a) gave the lowest
yield that could be rationalized by its instability due to the
decreased donor effect of the smaller aromatic ring to the N–B
bond formation. In the nal step, it was crucial to work under dry
conditions. The work-up could be performed by a simple ltration
through celite followed by chromatography.
Photophysical properties
Aer the synthesis of the borocarbolines (11a,b,e,f and 12a–d),
we have investigated the absorption, excitation and emission
Fig. 1 Biologically active and fluorescent compounds together with
the aims of this work.18–20
© 2021 The Author(s). Published by the Royal Society of Chemistry
RSC Adv., 2021, 11, 12802–12807 | 12803

View Article Online
RSC Advances
Paper
Scheme 1 Synthesis of oxazaborolo-carbolines (11).
Scheme 2 Synthesis of diazaborinino-carbolines (12a–d).
spectra, molar absorption coefficients, quantum yields and nm). We hypothesize that the lower exibility of the core and
brightness. Specic wavelengths are given in Table 3. The less enhanced push–pull system are behind the weaker photo-
absorbance maxima are usually located in the range of near UV physical properties of oxazaborolo compounds.
(363–396 nm for compounds 11a,b,e,f and 12a), but three
derivatives goes up to low-vis (405–414 nm for compounds 12b–
d). The emission maxima are found from blue light to the
Computed properties
The best members of each family were chosen for quantum
chemical investigations. The optimal geometry conformers of
compounds 11f and 12d were taken in S0 and S1 states, and
transition energies were calculated. These calculations showed
only small difference between the excitation and relaxation
energy values (Fig. 2), just like in the energy of the band gap
between the HOMO and LUMO orbitals (5.72 eV for 11f and
5.48 eV for 12d). The trends were the same, the less energy was
in correspondence with the lower energy gap. Studying the
change in the HOMO and LUMO energies upon excitation, it
could be seen that for both compounds the LUMO level
decreased similarly by 30–31 kJ molꢁ1, while the HOMO
increased by 11f with 31.9 kJ molꢁ1, and by 12d with
16.1 kJ molꢁ1. These data suggest that the better uores-
cence properties of 12d might be caused by the lower energy
excited state, the lower HOMO–LUMO band gap and the
lower excited state HOMO energy difference. Visualizing the
molecular orbitals, many similar patterns could be observed
between the HOMO and LUMO of 11f and 12d because of the
common carboline core (Fig. 2). Interestingly, however, the
participation of BF2 in the HOMO was larger in the structure
of 12d while quite different conjugations could be observed
in HOMO-1 orbital. This suggests that the vinylene group
next to the oxazaborolidine ring has different effect than the
beginning of green light of the visible spectra (417–489 nm).
The oxazaborolo-carbolines (11a,b,e,f) are shied hyp-
sochromic direction if we compare it with the diazaborinino-
carbolines (12a–d). The Stokes shis are usually in a range of
60 to 85 nm that can be considered as relatively large compared
to the widely used BODIPY dyes (6–32 nm).
The molar absorbance coefficient is generally lower for
oxazaborolo-carbolines than the diazaborinino carbolines, and
the same phenomenon can be observed regarding the quantum
yields resulting in usually moderate brightness values.
Outstanding compound is 12d borocarboline with good
brightness (B ¼ 6360 Mꢁ1 cmꢁ1) and the largest Stokes shi (85
Table 2 Results of the synthesis of compounds 16a–d and 12a–d
Entry
Compound
R1
R2
16 [%]a
12 [%]a
1
2
3
4
a
b
c
H
H
Imb
49
27
95
63
74
Bimc
Bimc
Pyd
33e
67e
15e
COOMe
COOMe
d
a
b
c
d
Isolated yields. Imidazole-2-yl. Benzimidazole-2y-l. Pyridine-2-yl.
Method B was applied.
e
12804 | RSC Adv., 2021, 11, 12802–12807
© 2021 The Author(s). Published by the Royal Society of Chemistry

View Article Online
Paper
RSC Advances
Table 3 Photophysical properties of oxazaborolo- and diazaborinino carbolines
Dl
[nm]
B
3 (labsmax) [Mꢁ1 cmꢁ1
]
FF [ꢁ]
[Mꢁ1 cmꢁ1
]
max
#
Compound
R1
R2
labsmax [nm]
lexcmax [nm]
lem
[nm]
1
2
3
4
5
6
7
8
11a
11b
11e
11f
12a
12b
12c
12d
H
H
Me
Me
H
H
H
Me
H
Me
Ima
Bimb
Bimb
Pyc
375
379
377
363
396
414
405
407
378
376
377
378
380
398
393
404
450
417
445
435
472
484
465
489
75
38
68
72
76
70
60
85
506
11 700
5150
11 700
3750
8860
9320
23 500
0.49
0.21
0.0062
0.27
0.40
0.45
0.47
0.27
248
2460
32
3160
1500
3990
4380
6360
COOMe
COOMe
a
b
Imidazole-2-yl. Benzimidazole-2y-l. c Pyridine-2-yl.
pyridine ring connected to the carboline core. The vinylene
group participates in the highest occupied orbitals and it is
more distant from the BF2 group, while the pyiridine is
directly connected participating only in HOMO. The atomic
charges and the electron density was also investigated
(Fig. 2). The partial negative charges are lower around BF2 of
12d, the electron density is more homogenous than that for
11f. In the latter the vinylene group acts as a borderline
between the negatively and positively charged parts of 11f.
Effects of solvents and pH
Next, the performance of the best of each uorescent carboline
families (11f and 12d) were investigated in different solvents.
This solvent screen showed that the wavelength and intensity of
emission of 11f shis in batochromic direction with the
increasing polarity of the solvent (e.g. in acetonitrile, EtOH or
water, Fig. 3a). The intensity is the greatest in acetonitrile, but in
water we still have decent emission, and the emission
maximum is above 450 nm. In the case of 12d the intensity
decreased as we headed towards more polar solvents, but except
of water the emission maximum did not change signicantly
(Fig. 3b). Notably, in water, the emission maxima shied above
500 nm. The uorescence of 12d did not change signicantly in
the pH range of 4.4–8.4 (Fig. S9†).
Stability
Finally, to prove the ability for biological application, we have
investigated the photostability of 11f and 12d in acetonitrile, excited
with UV light (8 W, 366 nm). The half of the original uorescence
intensity was reached aer 1 h irradiation by 11f, but on the
contrary, that did not decrease signicantly aer 1 h for 12d (Fig. 4).
We also followed the changes in 10% serum containing DMEM cell
media and found 12d stabile over 24 hours (Fig. S10†). In addition,
12d did not show cytotoxicity and neither anti-proliferative activity
by the treatment of MRC-5 human lung broblast cell line for 24 h
and 72 h, respectively (Tables S1 and S2, Fig. S11 and S12†).
These results show that both compounds are promising
candidates for chemical biology applications, considering that the
excitation during imaging procedures usually does not take more
Fig. 2 Schematic representation of the excitation and fluorescence
with computed energetics together with the visualized molecular
orbitals, atomic charges and electrostatic potential (ESP) surface for 11f
and 12d. On the ESP surface charges change from red (negative) to
blue (positive). The atomic charges change from red (negative) to
green (positive).
Fig. 3 Solvent effect on excitation and emission spectra of 11f oxazaborolo-carboline (a) and 12d diazaborinino-carboline (b).
© 2021 The Author(s). Published by the Royal Society of Chemistry
RSC Adv., 2021, 11, 12802–12807 | 12805

View Article Online
RSC Advances
Paper
Fig. 4 Photostability of 11f and 12d carbolines.
than dozens of minutes. Due to its easy coupling and benecial
photophysical properties we selected 12d diazaborinino-carboline
for antibody labelling experiments. Since antibodies are target
cancer cells selectively, and deliver their uorescent23 or cytotoxic
Fig. 5 (A) Confocal images showing the BK-bound trastuzumab
antibody (18) on MDA-MB-231 (left), OVCAR-8 (middle) and SKOV-3
payloads24 specically, we have conjugated 12d to the human IgG (right) tumour sections. Note that only the cell membrane is marked,
therefore cells appear as circles (white arrows). Scale bar: 100 mm. (B)
Two-photon microscopy images of living tumour tissue incubated
with the BK-bound trastuzumab antibody. Infrared image (IR, left)
trastuzumab creating a potential diagnostical tool for imaging
Her2 positive tumour cells (Scheme 3a).25
shows the tumour tissue, fluorescent image (middle) demonstrates
Synthesis and biological evaluation of trastuzumab conjugate
18
that cells of the SKOV-3 tumours bound the antibody. Note that the
fluorescent image is coloured artificially. Connective tissue
surrounding the groups of tumour cells are autofluorescent. Scale bar:
50 mm.
We hydrolysed 16d ester with sodium hydroxide, followed by the
incorporation of the diuoroboranil group. Then carboxylic acid
12e was transformed to 12f NHS ester. Reverse-phase chromatog-
raphy was necessary for purication that decreased the isolated
yields unexpectedly aer full conversion. The NHS-ester (12f) was
able to acylate the free lysines on the antibody (Scheme 3a) leading
to a conjugate (18).26 The conjugate was analysed by SDS-PAGE to
conrm the attachment of the uorophores (Scheme 3b), and the
uorophore-antibody ratio (FAR) was determined by UV-VIS
absorbance measurement to be FAR ¼ 18.
cell line slightly positive for Her2, tumour cells emitted a slight
signal (Fig. 5A middle), whereas SKOV-3 an ovarian cell line
expressing high amounts of Her2 showed a stronger signal (Fig. 5A
right). For the two photon imaging tumours were resected from the
anaesthetised animals, and 200 mm thick slices were cut. The living
ꢀ
tumour tissue was incubated with 18 at 35 C for 1 h. The uo-
rescent image demonstrated that 18 antibody was bound to the
cells of the SKOV-3 tumours (Fig. 5B).
Having the antibody-dye conjugate in hand, three types of
tumour cell lines were treated (Fig. 5A). The animals were anaes-
thetized, and the tumours were resected and xed. The 100 mm
thick slices were treated with 18 for 24 h. No specic signal could
be detected in cells of the epithelial human breast cancer cell line
Ethical statement
MDA-MB-231 negative for Her2 by confocal microscopy (Fig. 5A All animal experiments will be conducted following standards
le). The cell membrane of the OVCAR-8, an ovarian carcinoma and procedures approved by the Animal Care and Use
Scheme 3 (a) Synthesis of 12f dye with NHS ester group and antibody-fluorophore conjugate 18; (b) SDS-PAGE in non-reducing gel: (I)
molecular weight protein marker (kDa), (II) trastuzumab, (III) 18, (IV) 18 under 366 nm light.
12806 | RSC Adv., 2021, 11, 12802–12807
© 2021 The Author(s). Published by the Royal Society of Chemistry

View Article Online
Paper
RSC Advances
´
˜
Committee of the National Institute of Oncology, Budapest
(license number: PEI/001/2574-6/2015).
6 J. Hidalgo, C. Carmona, M. Balon and M. A. Munoz, Pharm.
Weekbl., Sci. Ed., 1990, 12, 142–144.
7 S. J. Stachel, S. A. Stockwell and D. L. Van Vranken, Chem.
Biol., 1999, 6, 531–539.
8 J. Thireau, J. Marteaux, P. Delagrange, F. Lefoulon,
L. Dufourny, G. Guillaumet and F. Suzenet, ACS Med.
Chem. Lett., 2014, 5, 158–161.
9 A. Loudet and K. Burgess, Chem. Rev., 2007, 107, 4891–4932.
10 M. Milen, P. Abranyi-Balogh, Z. Mucsi, A. Dancso, D. Frigyes,
L. Pongo and G. Keglevich, Curr. Org. Chem., 2013, 17, 1894–
1902.
11 M. Milen, P. Abranyi-Balogh, Z. Mucsi, A. Dancso,
T. Kortvelyesi and G. Keglevich, Curr. Org. Chem., 2011, 15,
Conclusions
We have synthesized new families of boro-b-carbolines
including oxazaborolo-carbolines and the diazaborinino-
carbolines. Their photophysical analysis showed that the
compounds can be excited in the 370–405 nm range, and
emitting up to 489 nm. Stokes-shis are greater than those of
the BODIPY derivatives indicating their potential in chemical
biology applications. Brightness data are acceptable and pho-
tostability was impressive by the best member of the
diazaborinino-carbolines. The energetics of excitation states
and molecular orbitals of two representatives were investigated
by quantum chemical computations suggesting effects for the
improved properties of diazaborinino-carbolines over
oxazaborolo-carbolines. The best diazaborinino-carboline (12d)
was further functionalized to an NHS-ester (12f) that was
successfully used for antibody labelling and visualizing Her2
positive tumour cells in confocal and two-photon microscopy.
1811–1825.
´
´
¨
¨
12 P. Abranyi-Balogh, T. Foldesi, A. Grun, B. Volk, G. Keglevich
and M. Milen, Tetrahedron Lett., 2016, 57, 1953–1957.
´
13 M. Milen, P. Slegel, P. Keglevich, G. Keglevich, G. Simig and
B. Volk, Tetrahedron Lett., 2015, 56, 5697–5700.
´
´
¨
¨
14 L. Petri, P. A. Szijj, A. Kelemen, T. Imre, A. Gomory,
´
´
}
M. T. W. Lee, K. Hegedus, P. Abranyi-Balogh,
}
V. Chudasama and G. M. Keseru, RSC Adv., 2020, 10,
14928–14936.
´ ´
´
}
15 D. Sovari, A. Kormos, O. Demeter, A. Dancso, G. M. Keseru,
Conflicts of interest
´
´
M. Milen and P. Abranyi-Balogh, RSC Adv., 2018, 8, 38598–
38605.
There are no conicts to declare.
´
´ ´
´
}
16 D. Sovari, G. M. Keseru and P. Abranyi-Balogh, Materials,
2020, 13, 199.
Acknowledgements
17 J. M. Warram, E. de Boer, A. G. Sorace, T. K. Chung, H. Kim,
R. G. Pleijhuis, G. M. van Dam and E. L. Rosenthal, Cancer
Metastasis Rev., 2014, 33, 809–822.
18 A. Chattopadhyay, R. Rukmini and S. Mukherjee, Biophys. J.,
1996, 71, 1952–1960.
Supported by the 2018-1.3.1-VKE-2018-00032 and Hungarian
Thematic Excellence Programme TKP2020-NKA-26 grant of the
National Office of Science, Innovation and Technology (NKFIH).
´
We are grateful for the support of NKFIH PD124598 and UNKP-
19-3-I-BME-408 New National Excellence Program of the
Ministry for Innovation and Technology. The authors
˜
19 D. Reyman, M. J. Tapia, C. Carcedo and M. H. Vinas, Biophys.
Chem., 2003, 104, 683–696.
20 A. Mallick, P. Das and N. Chattopadhyay, J. Photochem.
Photobiol., C, 2010, 11, 62–72.
´
acknowledge the supportive work of Krisztina Nemeth.
´
Notes and references
21 T. Szabo, V. Hazai, B. Volk, G. Simig and M. Milen,
Tetrahedron Lett., 2019, 60, 1471–1475.
22 V. Kumar, D. Singh, A. K. Paul, R. Shrivastava and V. Singh,
New J. Chem., 2019, 43, 18304–18315.
23 J. M. Warram, E. de Boer, A. G. Sorace, T. K. Chung, H. Kim,
R. G. Pleijhuis, G. M. van Dam and E. L. Rosenthal, Cancer
Metastasis Rev., 2014, 33, 809–822.
24 P. Khongorzul, C. J. Ling, F. U. Khan, A. U. Ihsan and
J. Zhang, Mol. Cancer Res., 2020, 18, 3–19.
25 T. A. D. Smith, Br. J. Radiol., 2010, 83, 638–644.
26 C. Martin, G. Brachet, C. Colas, E. Allard-Vannier, C. Kizlik-
Masson, C. Esnault, R. Respaud, C. Denevault-Sabourin,
I. Chourpa, V. Gouilleux-Gruart, M.-C. Viaud-Massuard and
N. Joubert, Pharmaceuticals, 2019, 12, 176.
1 A. J. Singh, A. P. Gorka, H. R. Bokesch, A. Wamiru,
B. R. O'Keefe, M. J. Schnermann and K. R. Gustafson, J.
Nat. Prod., 2018, 81, 2750–2755.
2 R. Duval and C. Duplais, Nat. Prod. Rep., 2017, 34, 161–193.
3 E. V. Costa, M. L. B. Pinheiro, C. M. Xavier, J. R. A. Silva,
A. C. F. Amaral, A. D. L. Souza, A. Barison, F. R. Campos,
A. G. Ferreira, G. M. C. Machado and L. L. P. Leon, J. Nat.
Prod., 2006, 69, 292–294.
4 J. S. Glasby, Encyclopedia of the Alkaloids, Springer US,
Boston, MA, 1983.
5 R. A. Glennon, M. Dukat, B. Grella, S.-S. Hong, L. Costantino,
M. Teitler, C. Smith, C. Egan, K. Davis and M. V. Mattson,
Drug Alcohol Depend., 2000, 60, 121–132.
© 2021 The Author(s). Published by the Royal Society of Chemistry
RSC Adv., 2021, 11, 12802–12807 | 12807