1-Phenylisoquinoline larvicidal activity against Culex quinquefasciatus

Article abstract of DOI:10.1080/14786419.2011.560846

The larvicidal potential of several dopamine 1-phenylisoquinoline derivatives against Culex quinquefasciatus third instar larvae was investigated in a rational search for insecticides. The results showed that these isoquinolines presented moderate larvicidal activity, that the presence of a substituent is needed on carbon 1 for such activity to be presented and that it may be possible to develop novel insecticidal compounds having potential use in controlling insects by appropriate structural modification of 1-phenylisoquinolines. This article presents a possible structure-larvicidal activity relationship for isoquinolinic compounds.

Full text of DOI:10.1080/14786419.2011.560846

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1-Phenylisoquinoline larvicidal activity
against Culex quinquefasciatus
Rodolfo Quevedo a , Edwin Baquero a & Martha L. Quiñones b
a Departamento de Química, Facultad de Ciencias, Universidad
Nacional de Colombia, Bogotá, Colombia
b Departamento de Salud Pública, Facultad de Medicina,
Universidad Nacional de Colombia, Bogotá, Colombia
Version of record first published: 01 Nov 2011.
To cite this article: Rodolfo Quevedo, Edwin Baquero & Martha L. Quiñones (2012): 1-
Phenylisoquinoline larvicidal activity against Culex quinquefasciatus , Natural Product Research:
Formerly Natural Product Letters, 26:12, 1094-1100
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Natural Product Research
Vol. 26, No. 12, June 2012, 1094–1100
1-Phenylisoquinoline larvicidal activity against Culex
quinquefasciatus
a
Rodolfo Quevedo , Edwin Baquero and Martha L. Quin
a
˜
onesb
*
a
´
Departamento de Quımica, Facultad de Ciencias, Universidad Nacional de Colombia,
Bogota, Colombia; Departamento de Salud Publica, Facultad de Medicina,
b
´
´
´
Universidad Nacional de Colombia, Bogota, Colombia
(Received 3 September 2010; final version received 3 February 2011)
The larvicidal potential of several dopamine 1-phenylisoquinoline deriva-
tives against Culex quinquefasciatus third instar larvae was investigated in a
rational search for insecticides. The results showed that these isoquinolines
presented moderate larvicidal activity, that the presence of a substituent is
needed on carbon 1 for such activity to be presented and that it may be
possible to develop novel insecticidal compounds having potential use in
controlling insects by appropriate structural modification of 1-phenyliso-
quinolines. This article presents a possible structure–larvicidal activity
relationship for isoquinolinic compounds.
Keywords: Pictet–Spengler; isoquinoline; alkaloid; Culex quinquefasciatus;
dopamine; Berberis
1. Introduction
The continued use of synthetic insecticides generating serious environmental
problems in water, air and soils and contamination of foods and animals, is
ecologically inacceptable because it produces adverse effects on beneficial organisms
and leads to the development of resistance in insects, fungi and bacteria. Developing
resistance consequently brings the application of ever-increasing doses, accompanied
by a greater risk of human intoxication. In addition to environmental problems,
public health becomes seriously affected by diseases such as filariasis, malaria,
dengue, yellow fever and leishmaniasis which are transmitted by insect vectors. These
types of disease cause a great economic and social impact, especially in regions with
insanitary conditions and deficient potable water supply and in overpopulated
regions such as Latin America and the Caribbean. The Culex quinquefasciatus
mosquito is amongst the vectors which are most frequently associated with urban
˜
and rural human habitat (Alvarez, Briceno, & Oviedo, 2006; Bisset, Rodriguez, Diaz,
& Soca, 1998).
Botanical insecticide-oriented studies have been carried out during the past few
years in the search for alternatives to synthetic insecticides. These are of great interest
because they are natural, biodegradable, have low toxicity and can be used as
*Corresponding author. Email: arquevedop@unal.edu.co
ISSN 1478–6419 print/ISSN 1478–6427 online
ß 2012 Taylor & Francis
http://dx.doi.org/10.1080/14786419.2011.560846
http://www.tandfonline.com

Natural Product Research
1095
OCH
3
3
H CO
3
N
H C
OCH
O
OCH
3
3
N
N
H C
OH
O
O
CH
3
3
OH
OCH
3
OCH
H C
3
3
OCH
3
N
HO
OCH
3
1
2
Figure 1. Compound 1 shows tabienine structure and 2 the tabienine b structure.
insecticides, larvicides, antifeedants, repellents or oviposition deterrents and growth
inhibitors (Isman, 2006; Mandal, 2010).
On the other hand, alkaloid isoquinolines have received considerable attention as
they are genetic precursors for a large variety of natural compounds arousing great
biological interest due to their potential usefulness in treating many diseases
(Fajardo, Podesta & Urzu´ a, 1986; Leet, Fajardo, Freyer, & Shamma, 1983) and
controlling insects (Baird, Taylor, & Brayden, 1997; Wright et al., 2000).
The Berberidaceae family is a recognised source of isoquinoline alkaloids having
novel structures. Studies concerning the chemical composition of an active fraction
(LC50 ¼ 8 mg mLꢀ1) against C. quinquefasciatus third instar larvae obtained from
Berberis tabiensis (Berberidaceae) stems (a native species from the Colombian Andes)
have led to the two bisbenzylisoquinoline alkaloids being obtained: tabienine 1 and
˜
tabienine b 2 (Figure 1) (Quevedo, Nu´ nez, & Moreno, 2011; Quevedo, Valderrama,
Moreno, Laverde, & Fajardo, 2008).
In spite of the promising larvicidal activity of the fraction where tabienine 1 and
tabienine b 2 were isolated, the low amount of compound which was isolated did not
permit bioassays to be carried out with pure compounds; it can only be suggested
that these two compounds having isoquinolinic nuclei are responsible for larvicidal
activity. Some simple isoquinolines were synthesised as their structure would lead to
the establishment of the possible structure–activity relationships which would orient
the search for isoquinolines having larvicidal activity so as to determine whether the
isoquinolinic nucleus was responsible for larvicidal activity and observe the influence
of the substituent on carbon 1 (Table 1).
2. Results and discussion
It is known that some plant and essential oil extracts have larvicidal activity against
different mosquito species; thus, the use of their active constituents in controlling
insects is promising due to their selectivity, biodegradability and low toxicity.
Chemical composition and bioactivity studies of alkaloidal extracts from
species from the Berberis genus which grow in the Colombian Andes led to the
proposal that their benzylisoquinoline-type constituents present potential usefulness
in controlling insect plagues (Moreno, Morgensztern, Luque, & Fajardo, 1995;

1096
R. Quevedo et al.
Table 1. Larvicidal activity of 1-phenylisoquinolines.
Larvicidal activity,
)
Larvicidal activity,
Compound
LC50 (mg mLꢀ1
Compound
LC50 (mg mLꢀ1
)
3
4
5
42000
42000
1035 ꢁ 89
460 ꢁ 39
450 ꢁ 29
7
8
9
10
179 ꢁ 23
612 ꢁ 71
479 ꢁ 49
1624 ꢁ 171
78
6
Caffeine
Tabienine fraction
HO
HO
HO
CH3COOH/CH3OH / RCHO
reflux
NH+3
–
Br
NH+
–
2 Br
HO
R
3 – 9
H
N
Cl
3
7
R=
R=
H–
4
8
R=
R=
5
R=
R=
6
R=
O
N
2
OH
HO
HO
HO
H CO
3
9
H CO
3
Scheme 1. Isoquinoline synthesis from dopamine.
Quevedo et al., 2011). A study of the larvicidal activity of non-natural
1-phenylisoquinoline compounds was thus begun due to the structural characteristics
of these alkaloids, their low percentage found in nature and the availability of less
knowledge regarding their potential use in controlling insects. Such compounds were
synthesised by means of Pictet–Spengler cyclisation between dopamine or
3,4-dimethoxyphenylethylamine and aldehydes using methanol as solvent and
acetic acid as catalyser (Schemes 1 and 2) (Cox & Cook, 1995; Quevedo, Baquero,
& Rodrıguez, 2010).
´
Dopamine- and formaldehyde-derived isoquinoline (3) was synthesised first to
establish whether the presence of the isoquinolinic nucleus itself only presented
larvicidal activity (Table 1). The activity assay gave an LC50 value greater than
2000 mg mLꢀ1, thereby indicating that the presence of a single isoquinolinic nucleus
was not sufficient for the molecule to be bioactive. A molecule possesses two
isoquinolinic units in its structure 4 by means of dopamine reaction with
terephthaldehyde; the obtained product was synthesised, giving an LC50 value
greater than 2000 mg mLꢀ1. This result showed that the number of isoquinolinic
nuclei was not a determinant factor in bioactivity.
After establishing that the number of isoquinolinic nuclei did not significantly
influence larvicidal activity, it was necessary to explore the influence of the

Natural Product Research
1097
10
Scheme 2. Isoquinoline synthesis from 3,4-dimethoxyphenylethylamine.
substituent on carbon 1. The compound 1-(3-nitrophenyl)-6,7-dihydroxytetrahy-
droisoquinoline (5) was thus synthesised; this compound had low larvicidal activity
(1035 mg mLꢀ1), but was higher than that for both previously assayed isoquinolines.
The following stage was to assay 1-(3-nitrophenyl)-6,7-dimethoxytetrahydroisoqui-
noline (10) to observe the influence of substituents in positions 6 and 7.
This compound presented lower larvicidal activity than that of 5 (1624 mg mLꢀ1).
This result led to the conclusion that the activity was greater when positions 6 and 7
were hydroxylated.
Finally, an attempt was made to ascertain the influence of substituents on the
aromatic ring located on carbon 1. The compound 1-(4-chlorophenyl)-6,7-
dihydroxytetrahydroisoquinoline 6 was thus synthesised; its activity (460 mg mLꢀ1
)
improved in the case of the isoquinolines which are not present in carbon 1 or in
rings having strong electroattractor substituents. This result suggested that the
substituents on this ring were the determinants for this type of molecule to present
larvicidal activity. The compound 1-(4-methoxy)-6,7-dihydroxytetrahydroisoquino-
line (7) was then evaluated; this compound presented the best LC50 value
(179 mg mLꢀ1), indicating that the presence of this electrodonor group favoured
larvicidal activity. LC50 rose to 612 mg mLꢀ1 when the methoxy group was replaced
by a hydroxy group as in 1-(4-hydroxyphenyl)-6,7-dihydroxytetrahydroisoquinoline
(8). Likewise, the presence of a hydroxyl group and a methoxyl group on ring 9
caused a reduction in bioactivity (479 mg mLꢀ1) when compared to compound 7. It
was interesting to see that compound 9 presented an intermediate LC50 value
compared to that for compounds 7 and 8 which have similar substituents. The
bioactivity observed for compounds 7, 8 and 9 did not present a lineal relationship
with the number of electrodonor groups present on the aromatic ring.
The results obtained led to the proposal of a preliminary structure–larvicidal
activity relationship for 1-phenylisoquinolin type compounds: (1) larvicidal activity
was independent of the number of isoquinoline units present in the molecule; (2)
larvicidal activity was lower when oxygen in positions 6 and 7 was methylated; (3)
the presence of a substituent on carbon 1 was a necessary condition for larvicidal
activity to be presented; and (4) the substituents in the aromatic ring (C ring) had a
direct effect on larvicidal activity.
Few studies are known to have been carried out regarding the larvicidal activity
of alkaloids. It has been reported that some isobutylamide alkaloids present potent
larvicidal activity against Culex pipiens pullens third instar larvae having LC50 values
ranging from 3.2 to 0.028 mg mLꢀ1 (Park, Lee, Shin, Park, & Ahn, 2002); aporphine
alkaloid (þ)-dicentrine has been shown to have a 30.2 mg mLꢀ1 LC50 value against
Aedes aegypti larvae.

1098
R. Quevedo et al.
The activity observed for synthesised 1-phenylisoquinoline alkaloids in all cases
was lower than that observed for the alkaloidal fraction where tabienine and
tabienine b were isolated and reported for isobutylamide alkaloids and (þ)-
dicentrine. This pattern indicated that some additional structural aspects to those
considered in this study are important in natural isoquinolines.
3. Experimental
3.1. General procedure for dopamine reaction with aldehydes
The 1-phenyltetrahydroisoquinolines studied were obtained by means of Pictet–
Spengler regioselective cyclisation between dopamine or 6,7-dimethoxyphenylethy-
lamine and aldehydes.
Acetic acid (0.1 mol) was added to a solution of dopamine hydrobromide
(X mol) and the respective aldehyde (X mol) in methanol; the mixture was refluxed
for 48 h. The precipitated product was filtered and washed with methanol
(Quevedo et al., 2010).
3.2. Procedure for 6,7-dimethoxyphenylethylamine reaction with 3-
nitrobenzaldehyde
A solution of 6,7-dimethoxyphenylethylamine (X mol) and 3-nitrobenzaldehyde
(X mol) in methanol was refluxed for 72 h. The solvent was evaporated and the crude
product treated with 37% HCl; the resulting mixture was refluxed for 1 h. The
precipitated product was filtered and washed with methanol (Quevedo et al., 2010).
3.3. Larvicidal activity bioassay
Culex quinquefasciatus third instar larvae were used for the bioassays; these came
from standardised breeding colonies kept in the Entomology laboratory of
Universidad Nacional de Colombia. The larvicidal activity assays followed the
methodology proposed by McLaughlin, Lingling, Rogers and Anderson (1998) using
multiwell plates (96 well), 250-mL unit working volume, five concentrations ranging
from 1000 to 10 mg mLꢀ1, at laboratory temperature (25ꢂC ꢁ 2) and one larva per
well. The pure compound solutions were prepared by serial dilution with
dechlorinated water from a stock solution of the sample in water containing 0.5%
dimethylsulphoxide. The larvicidal activity of each sample was determined at 1000,
500, 100, 50 and 10 mg mLꢀ1. Water was used as absolute control and caffeine as
positive control. No mortality was observed in any absolute control. All treatments
were replicated three times using 20 larvae per replicate. LC50 data represented the
mean values of each separate experiment (Table 1).
The effects were measured as larval mortality after 48 h of exposure. Larvae were
considered to be dead if appendages did not move when prodded with a wooden
dowel. Probit analysis was used to establish the lethal concentration for 50%
mortality (LC50) and 95% confidence intervalvalues for the respective compounds
(Finney & Stevens, 1948).

Natural Product Research
1099
4. Conclusions
It may thus be concluded that the larvicidal activity observed in some isoquinolinic
compounds did not depend on the number of isoquinolinic nuclei present in the
molecule, that activity was less when positions 6 and 7 had methoxyl groups and that
the presence of a substituent on carbon 1 was necessary for larvicidal activity,
activity being greater when the substituent on carbon 1 was an aromatic ring having
electrodonor substituents.
The activity observed in all cases was lower than that observed for the alkaloidal
fraction where tabienine and tabienine b were isolated. This pattern indicated that
some additional structural aspects to those considered in this study are important in
natural isoquinolines.
Acknowledgements
The authors gratefully wish to acknowledge the Universidad Nacional de Colombia for
financing this work, the Departamento de Quımica for providing the laboratory facilities and
´
the Medicine Faculty’s Entomology laboratory for rearing the mosquito larvae.
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