S. Li et al. / Chinese Chemical Letters 27 (2016) 635–639
637
2.4. Insecticidal activity assay
Insecticidal test of APIs was done with cowpea aphids (Aphis
craccivora). The plant leaves of horsebean with about 50 apterous
adults were dipped in corresponding APIs solutions containing
Triton X-100 (0.1 mg/L) for 5 s and the excess solution was sucked
out with a filter paper. Then burgeons were positioned in the
conditioned room (25 Æ 1 8C). Water with Triton X-100 (0.1 mg/L)
was used as control. Twenty-four hours after treatment, the mortality
rate was measured. Each treatment had three repetitions and the data
were subjected to probit analysis. Insecticidal test for house fly
(Musca domestica). House fly adults were anesthetized with carbon
dioxide for 10 min and then were treated with the APIs dissolved in
distilled water or 10% DMSO aqueous solution by intrathoracic
injection (0.4 mL for each fly). Twenty flies were treated for each
dosage with duplicate samples. The mortality rate was measured 24 h
after treatment. Each treatment had three repetitions and the data
were subjected to probit analysis.
Scheme 2. Synthesis of API 5.
(petroleum ether/ethyl acetate = 20:1, silica gel) afforded the pure
product as an orange solid. Compound 4a: Yield 73%, 1H NMR
(400 MHz, CDCl3): d 8.74 (d, 1H, J = 2.0 Hz), 8.08–8.01 (m, 2H), 7.94
(dd, 1H, J = 8.2, 2.3 Hz), 7.82 (d, 1H, J = 8.2 Hz), 7.57–7.52 (m, 3H),
4.55 (s, 2H).
3. Results and discussion
Synthesis of N-(1-((6-((E)-(2-bromophenyl)diazenyl)pyridin-
3-yl)methyl)imidazolidin-2-ylidene)nitramide (API 1): N-nitroi-
minoimidazolidine (14 mmol, 1.4 equiv.) and K2CO3 (14 mmol,
1.4 equiv.) were added to DMF (5 mL) and the mixture was stirred
for 10 min, then compound 4a (10 mmol, 1.0 equiv.) was added to
this solution. The resulting mixture was stirred at 60 8C under Ar
for 8 h. The precipitate was separated by filtration. The solvent
was removed in vacuo. Purification by chromatography (dichloro-
methane/ethyl acetate = 3:1, silica gel) yielded API 1 as an orange
solid. Yield 56%, m.p. 153.0–154.3 8C; 1H NMR (400 MHz,
Previously, we merged the IMI with azobenzene to successfully
generate photoswitchable insecticidal molecules. Here, a similar
strategy was employed using azopyridine as a replacement.
Imidacloprid was a successful neonicotinoid in the past decades
with annual sales approaching to one billion US dollars [21,22]. The
SAR study indicated that the pyridine is an indispensable
pharmacophore by interacting with the nicotinic acetylcholine
receptors through cation–p interactions [23]. Thus, we incorpo-
rated the azopyridine into the IMI by sharing a common pyridine or
into the imidazoline ring of IMI to generate the photoswitchable
azopyridine-imidacloprid (API) derivatives.
The isomerization of the API analogues was investigated in
detail using 1H NMR spectroscopy, UV–vis spectroscopy and HPLC
analysis. The trans-isomers showed a typical azobenzene absorp-
DMSO-d6):
d 9.00 (s, 1H), 8.67 (d, 1H, J = 1.9 Hz), 8.03–7.92
(m, 3H), 7.75 (d, 1H, J = 8.2 Hz), 7.67–7.62 (m, 3H), 4.60 (s, 2H),
3.71–3.62 (m, 2H), 3.59–3.52 (m, 2H); 13C NMR (101 MHz,
DMSO-d6):
d 162.27, 160.39, 151.83, 148.75, 138.24, 134.16,
132.49, 129.60, 122.94, 113.35, 45.16, 44.99, 41.61, HRMS (ESI):
m/z calcd. for C15H15N7O2 [M+Na]+ 324.1185, found 348.1186.
tion at around 325 nm (319–330 nm) belonging to the
p–p*
transition band. Irradiation with 365 nm UV light isomerizes the
2.3. Photoisomerization
azopyridine part to its metastable cis-form with the appearance of
absorption at around 430 nm corresponding to n–p* transition
Absorption spectra were recorded on a Lambda 25 UV/vis
spectrometer (PerkinElmer, Shanghai). Target compounds were
dissolved in CH3CN using a microcuvette. Photochromism is
band, indicating the initiation of trans-to-cis photoisomerization
(Fig. 2). The trans/cis content was determined by HPLC analysis at
the photostationary state. The trans-to-cis isomerization showed
high transformation rate of photoisomerization ranging from
24:76 to 10:90. The cis-to-trans change can be achieved upon
irradiation at 430 nm and the half-life of the thermal relaxation of
the cis-isomer was more than 120 h, which guaranteed the stability
of cis-isomers for biological test.
Then the water solubility of the compounds was studied. All
the compounds except API 5 have higher solubility than the
azobenzene analogue ABI 1. API 5 had very poor water solubility as
a trans–isomer, indicating that appending azopyridine in such a
way was unfavourable to the solubility. Interestingly, almost a
2-fold solubility increase was observed for compound API 1-API 5
irradiated by
a hand-held MQK-WFH-204B ultraviolet lamp
(365 nm, 10 mW /cm2, MQK, Shanghai). UV–vis absorption spec-
troscopy was used to detect the change of absorption profiles of
target compounds upon 365 nm irradiation. The isomerization of
the API 1 and API 4 analogues was also investigated in detail by 1H
NMR spectroscopy. The corresponding UV–vis and 1H NMR spectra
were provided in supporting information. The proportions of trans/
cis isomer at the photostationary states before and after irradiation
were determined by HPLC analysis. The trans/cis ratios were
calculated at the isosbestic points. The half-lives of thermal
relaxations under dark condition are shown in Table 1.
Table 1
Maximum absorbance wavelength (nm), ratio of trans and cis isomers, the rate of thermal relaxation and the water solubility of APIs.
Compd.
p–
p*
Nonirradiated (trans:cis)
n–p
*
Irradiated (trans:cis)
t1/2 (h)
Water solubility
(mg/L)
trans
cis
API 1
API 2
API 3
API 4
API 5
ABI 1
320
324
320
320
327
321
96:4
78:22
83:17
98:2
432
435
445
435
441
432
16:84
25:75
16:84
24:76
10:90
17:83
158.2
140.0
127.6
155.5
183.5
342.1
12
12
17
16
5
27
26
37
31
11
11
96:4
95:5
6