A new benzylisoquinoline alkaloid from Argemone mexicana

Article abstract of DOI:10.1080/14786410902800723

A new benzylisoquinoline alkaloid, argemexirine, together with two known protoberberine alkaloids, dl-tetrahydrocoptisine and dihydrocoptisine, have been isolated from the methanolic extract of the whole plant of Argemone mexicana L. The compounds were identified by spectral and chemical evidence. This is the first report of these alkaloids in this plant species.

Full text of DOI:10.1080/14786410902800723

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Natural Product Research: Formerly
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A new benzylisoquinoline alkaloid from
Argemone mexicana
Sarita Singh a , T.D. Singh a , V.P. Singh a & V.B. Pandey a
a Department of Medicinal Chemistry , Institute of Medical
Sciences, Banaras Hindu University , Varanasi–221005, UP, India
Published online: 09 Dec 2009.
To cite this article: Sarita Singh , T.D. Singh , V.P. Singh & V.B. Pandey (2010) A new
benzylisoquinoline alkaloid from Argemone mexicana , Natural Product Research: Formerly Natural
Product Letters, 24:1, 63-67, DOI: 10.1080/14786410902800723
To link to this article: http://dx.doi.org/10.1080/14786410902800723
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Natural Product Research
Vol. 24, No. 1, 10 January 2010, 63–67
A new benzylisoquinoline alkaloid from Argemone mexicana
*
Sarita Singh, T.D. Singh, V.P. Singh and V.B. Pandey
Department of Medicinal Chemistry, Institute of Medical Sciences, Banaras Hindu University,
Varanasi – 221005, UP, India
(Received 11 July 2008; final version received 2 February 2009)
A new benzylisoquinoline alkaloid, argemexirine, together with two known
protoberberine alkaloids, dl-tetrahydrocoptisine and dihydrocoptisine, have been
isolated from the methanolic extract of the whole plant of Argemone mexicana L.
The compounds were identified by spectral and chemical evidence. This is the first
report of these alkaloids in this plant species.
Keywords: Argemone mexicana; Papaveraceae; alkaloids; argemexirine; dl-tetra-
hydrocoptisine; dihydrocoptisine
1. Introduction
Argemone mexicana L. (Papaveraceae) is a spiny herbaceous seasonal plant distributed
throughout India. It generally grows in agricultural wastelands in the months of January–
April. The plant is bitter, diuretic and purgative. It is used to destroy worms, and as cures
for itching, leprosy, inflammations, bilious fevers, diarrhoea, dysentery and various skin
diseases (Asolkar, Kakkar, & Chakre, 1992; CSIR, 1976). A number of protopine,
berberine, benzophenanthridine and benzylisoquinoline alkaloids have been reported from
this plant before (Chang et al., 2003; Chang, Chang, Khalil, Hsieh, & Wu, 2003; Doepke,
Hess & Jimenez, 1976; Haisova, & Slavik, 1975; Hussain, Nakkady, Khan, & Shamma,
1983; Mishra, Bhakuni, Sharma, & Kaul, 1961; Nakkady & Sharma, 1988; Slavikova &
Slavik, 1955). Here, we report the isolation of two known protoberberine alkaloids,
dl-tetrahydrocoptisine (1) and dihydro-coptisine (2), and a new benzylisoquinoline
alkaloid (3) from the methanolic extract of the whole plant of A. mexicana. The new
alkaloid was designated with the trivial name argemexirine (3).
2. Results and discussion
Chromatographic resolution of the crude tertiary base fraction of the methanolic extract
of the whole plant of A. mexicana furnished dl-tetrahydrocoptisine (1) (Pandey, Ray, &
Dasgupta, 1974), dihydrocoptisine (2) (Tripathi & Pandey, 1987) and argemexirine (3).
Compound (3) was obtained as colourless amorphous powder. The HRMS
data (Obsd. m/z 285.1368 for [Mþ]) revealed the molecular formula to be C17H19NO3.
The compound gave a positive Dragendorff reaction for alkaloids. The appearance of a
*Corresponding author. Emails: pandeyvb@sify.com; pandeyvb@gmail.com
ISSN 1478–6419 print/ISSN 1029–2349 online
ß 2010 Taylor & Francis
DOI: 10.1080/14786410902800723
http://www.informaworld.com

64
S. Singh et al.
Table 1. 1H NMR data of compound 3 and 5 and 13C NMR data of 3 (CDCl3 þ 20%CD3OD).
Compound 3
Compound 3
Compound 5
1H NMR, ꢁH
(multi., J in Hz)
1H NMR, ꢁH
(multi., J in Hz)
Position
13C NMR, ꢁC
1
2
4.45 (1H, dd, 3.10)
2.75 (1H, m)
57.0
4.34 (1H, dd, 3, 10)
2.54 (3H, s)
3
4
4a
5
6
7
8
8a
2.75 (1H, m) 3.35 (1H, m)
2.95 (2H, m)
37.5
28.6
2.66 (1H, m) 3.24 (1H, m)
2.86 (2H, m)
–
125.0
119.5
144.0
142.3
109.5
128.0
37.5
132.0
130.5
114.0
158.0
114.0
130.5
56.0
–
6.80 (1H, s)
6.76 (1H, s)
–
–
–
–
6.64 (1H, s)
–
2.86 (1H, m) 3.24 (1H, m)
–
7.20 (1H, d, 8)
6.89 (1H, d, 8)
–
6.89 (1H, d, 8)
7.20 (1H, d, 8)
3.88 (3H, s)
3.70 (3H, s)
3.82 (3H, s)
6.75 (1H, s)
–
2.95 (1H, m) 3.35 (1H, m)
–
7.30 (1H, d, 8.5)
7.00 (1H, d, 8.5)
–
7.00 (1H, d, 8.5)
7.30 (1H, d, 8.5)
3.85 (3H, s)
–
ꢂ
10
20
30
40
50
60
40-OMe
6-OMe
7-OMe
–
–
–
blue colour with phosphomolybdic acid and ammonia vapour revealed the phenolic nature
of the compound. Its UV spectrum showed absorption bands at ꢀmax 228 and 275 nm,
characteristic of benzylisoquinoline alkaloids (Shamma, 1972). It showed an absorption
band in its IR spectrum at 3300 cmꢀ1 for hydroxyl and at 2820 cmꢀ1 for methoxyl groups.
Its 1H NMR spectrum (Table 1) exhibited six proton multiplets (ꢁ 2.75, 2.95 and 3.35) for
three methylene protons, one –NH, two aromatic proton singlets (ꢁ 6.75 and 6.80), four
ortho–meta (ꢁ 7.00 and 7.30) coupled aromatic protons, and one aromatic methoxyl group
(ꢁ 3.85), together with one proton double doublet (ꢁ 4.45), which favoured the structure
3 as an alkaloid. The carbon signals of C-6 (144.0), C-7 (142.3) and C-40 (158.0) in the
13C NMR spectrum, which appeared downfield, indicated the attachments at these
positions. The base peak at m/z 164 (ion a) and a strong peak at 121 (ion b) appeared in the
mass spectrum due to the cleavage of C-1 to a C-ꢂ bond, which is characteristic of
benzylisoquinoline alkaloids (Shamma, 1972) and suggested the presence of two hydroxy
groups in ring A and one methoxy group in ring C. The structure 3 was thus proposed,
and to which the trivial name argemexirine was designated. The chemical structure of
argemexirine was further supported by methylation of 3 with HCHO and NaBH4, which
gave N-methylated product 4. Compound 4, upon methylation with CH2N2, furnished
compound 5, identical to O-methylarmepavine (Deulofeu, Comin, & Vernengo, 1968).
Furthermore, argemexirine (3) on methylation with CH2N2 gave N-nor-O-methyl-
armepavine (6) (Deulofeu et al., 1968), which on N-methylation furnished the compound
5 identical to O-methylarmepavine (Figure 1).
In 1H–1H COSY NMR, the chemical shift of protons of the aromatic region indicated
that the proton of C-20 couples with the protons of C-30 and C-60, whereas the C-30 proton

Natural Product Research
65
Figure 1. The chemical structures of compounds 1–6 and fragment ions a and b.
couples with the protons of C-20 and C-50. The chemical shift of protons of the aliphatic
region indicated that protons of C-3 and C-4 couple with each other and the C-1 proton
1
1
couples with two protons of C-ꢂ. H and DEPT 13C NMR (Table 1) and H–1H COSY
data fits well with the structure 3 for argemexirine, a new benzylisoquinoline alkaloid.
Three benzylisoquinoline alkaloids, namely argenaxine, higenamine and reticuline,
have earlier been reported from A. mexicana (Chang et al., 2003).
3. Experimental
3.1. General experimental procedures
Melting points are uncorrected. IR spectra, UV spectra and optical rotations were
measured with a Perkin–Elmer spectrophotometer, a Carry-14 spectrophotometer and a
1
Perkin–Elmer polarimeter 141, respectively. H and 13C NMR were run on a 500 MHz
Bruker Hx-90 in CDCl3 þ 20% CD3OD with TMS as an internal standard. MS were
measured with a Kratos MS-50 mass spectrometer which was operated at 70 eV with
evaporation of the sample in the ion source at 200ꢁC. CC: silica gel column (BDH, 60–120
mesh); TLC: silica gel G (Merck); solvents for TLC: CHCl3 : MeOH (9 : 1). Analytical
samples were dried routinely over P2O5 for 24 h in vacuo.

66
S. Singh et al.
3.2. Plant material
The whole plant material of A. mexicana was collected in the first week of April 2006
from the Varanasi district, UP, India, and was identified by Prof. N.K. Dubey,
Department of Botany, Banaras Hindu University, Varanasi, India. Specimen sample
no. 221 is kept in the department.
3.3. Extraction and isolation
The air-dried, powdered, whole plant material of A. mexicana (3 kg) was extracted with
MeOH in a Soxhlet extractor, and on evaporation of the solvent gave a brown coloured
semi-solid (320 g). The MeOH extract was extracted with 7% aqueous citric acid and the
acidic solution on basification and extraction with CHCl3 furnished the crude base
fraction (15.0 g). It was chromatographed over SiO2 gel column eluting with solvents of
increasing polarity. The eluants collected from C6H6 : CHCl3 (1 : 1), (1 : 2) and (1 : 20) were
crystallised separately from methanol, which furnished respectively the alkaloids
dl-tetrahydrocoptisine (1) (25 mg), dihydrocoptisine (2) (18 mg) and argemexirine (3)
(16 mg).
3.3.1. Argemexirine (3)
25
Compound 3 was crystallised from MeOH as colourless powder, m.p. 221–23ꢁC, ½ꢂꢂD ꢃ 0ꢁ
(c, 0.45, CHCl3) Rf 0.42 (CHCl3 : MeOH, 9 : 1). It exhibited UV (MeOH) ꢀmax (log "): 228
(3.82), 275 (3.80) nm; IR (KBr) ꢃmax: 3300 (ꢀOH), 2820 (ꢀOMe), 1610 and 1560 (Ar ꢀ
double bond) cmꢀ1; for 1H and 13C NMR data see Table 1; HRMS, m/z (relative
intensity%): 285.1368 ([M]þ, C17H19NO3, Calcd for 285.1365) (4), 164 (100), 163 (26), 121
(27), 106 (18), 67 (25).
3.3.2. Methylation of argemexirine (3)
Compound 3 (15 mg) was dissolved in MeOH and stirred magnetically with formaldehyde
(10 drops) and a small amount of sodium borohydride for 8 h. The reaction mixture was
poured over crushed ice, stirred for 1 h and then extracted with CHCl3. The CHCl3 extract
was crystallised with MeOH, which gave compound 4 as amorphous solid. The mass
spectrum of compound 4 exhibited a molecular ion peak at m/z 299.1525 ([M]þ,
C18H21NO3, Calcd for 299.1521) and other significant peaks at m/z 178 (base peak), 177,
121, 106. Compound 4 was dissolved in MeOH and treated with diazomethane in ether
solution. The reaction mixture was allowed to stand overnight at room temperature, which
furnished a colourless solid. It was crystallised from MeOH, which gave colourless
granules of compound 5, m.p. 91–92ꢁC, ½ꢂꢂD ꢃ 0ꢁ (c, 0.40, CHCl3). For 1H NMR
25
see Table 1. It exhibited a molecular ion peak at m/z 327.1838 ([M]þ, C20H25NO3 Calcd
for 327.1834) and other fragment ion peaks at m/z 206 (base peak), 191, 176, 121, 106.
1H NMR, MS and m.p. were identical to O-methylarmepavine.
Compound 3 (14 mg) was dissolved in MeOH, treated with diazomethane in ether
solution, and the reaction mixture was allowed to stand for overnight. On evaporation, it
gave a colourless solid, which on crystallisation from MeOH furnished the amorphous
compound 6. It exhibited a molecular ion peak at m/z 313.1685 ([M]þ C19H23NO3, Calcd
for 313.1677) and other fragment ion peaks at m/z 192 (base peak), 177, 162, 121, 106,
which were identical to N-nor-O-methylarmepavine. Compound 6 on methylation with

Natural Product Research
67
formaldehyde and sodium borohydride, as mentioned above, furnished O-methylarme-
pavine (5). m.p. ¼ 91–92ꢁC, C20H25NO3 ([M]þ, m/z 327.1839, Calcd for 327.1834).
Acknowledgements
The authors are thankful to Prof. G. Rucker, Pharmazeutisches Institut der Universitat Bonn,
¨
Germany for spectral analysis and S. Singh is thankful to UGC for financial assistance.
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