Design, synthesis and biological activity evaluation of benzoate compounds as local anesthetics

Article abstract of DOI:10.1039/c9ra00476a

Tetracaine and pramocaine were used as the lead compounds to design benzoate compounds. The combination principle was used to design the target molecule, and the target molecule was modified by bioisostere formation and modification with alkyl groups. In this research, a total of 16 compounds were designed and synthesized. In the process of synthesis, we selected a route with high total yields, mild conditions and simple operation. Three steps were used in the synthesis of the new target compounds, namely, alkylation, esterification and alkylation. The newly designed target compounds were evaluated via surface anesthesia, infiltration anesthesia, block anesthesia and acute toxicity tests. The results of biological activity experiments showed that compounds 4d, 4g, 4j, 4k, 4n, and 4o had a good local anesthetic effect, and the results of acute toxicity tests showed that the target compounds had low toxicity.

Full text of DOI:10.1039/c9ra00476a

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Design, synthesis and biological activity evaluation
of benzoate compounds as local anesthetics
Cite this: RSC Adv., 2019, 9, 6627
ab
ab
Huiying Zou,ab Guangying Chen
and Shiyang Zhou
*
*
Tetracaine and pramocaine were used as the lead compounds to design benzoate compounds. The
combination principle was used to design the target molecule, and the target molecule was modified by
bioisostere formation and modification with alkyl groups. In this research, a total of 16 compounds were
designed and synthesized. In the process of synthesis, we selected a route with high total yields, mild
conditions and simple operation. Three steps were used in the synthesis of the new target compounds,
namely, alkylation, esterification and alkylation. The newly designed target compounds were evaluated
via surface anesthesia, infiltration anesthesia, block anesthesia and acute toxicity tests. The results of
biological activity experiments showed that compounds 4d, 4g, 4j, 4k, 4n, and 4o had a good local
anesthetic effect, and the results of acute toxicity tests showed that the target compounds had low toxicity.
Received 19th January 2019
Accepted 11th February 2019
DOI: 10.1039/c9ra00476a
rsc.li/rsc-advances
cause addiction and some other toxic effects, such as allergic
reactivity, tissue stimulation and instability in aqueous solu-
1. Introduction
tions. Hence, the clinical application of cocaine was limited. As
a result, structural studies and modications of cocaine have
been carried out in the search for better local anesthetics. From
studies of the structure of cocaine, it was gradually realized that
the removal of the methyl carboxylate group in the structure of
cocaine could eliminate addiction. The local anesthetic effect of
ethyl-4-aminobenzenecarboxylate (benzocaine) was rst
conrmed in 1890, and procaine was successfully developed in
1904. The discovery of procaine made people realize the
important role of the 4-aminobenzoate structure in local anes-
thetic drugs and started the study of the 4-aminobenzoate
structure22–24 (tetracaine) and the development of local anes-
thetic drugs with amide25,26 (lidocaine), aminoketone27 (dyclo-
nine) and aminoether28,29 (pramocaine) structures (Fig. 1).
In this research, tetracaine and pramocaine were used as the
lead compounds to design the target molecules. The lead
compounds were integrated into an organic molecule by the
combination principle. Then the integrated organic molecule
was modied by bioisostere formation and modication with
alkyl groups (Fig. 2). A series of novel target compounds with
local anesthetic effects were designed. In terms of the structure–
activity relationship (SAR) of the drugs, the designed
compounds were composed of three basic skeleton structures,
namely, a lipophilic section, a link section and a hydrophilic
section (Fig. 3). According to the SAR, the target compounds
should have a local anesthetic effect. Structural modication of
the integrated molecules was performed in order to ensure that
the onset time was short, the duration of action was long and
the toxic effect was slight. The newly designed benzoate
compounds, with 4-aminobenzoic acid (1a) and 4-hydrox-
ybenzoic acid (1b) as starting materials, were synthesized by the
An anesthetic is a drug that can act on the nervous system and
inhibit it and is thus a drug that produces anesthesia.1,2 Local
anesthetics act on nerve endings and nerve trunks and can
reversibly block the conduction of nerve impulses. This results
in a loss of local sensation without affecting consciousness,
which is convenient for local surgery and treatment.3–5 Local
anesthetics are commonly used in stomatology, ophthalmology,
gynecology and minor surgical operations to temporarily relieve
pain.6–9 Local anesthetics bind to specic parts of the sodium
ion (Na+) channel on the nerve membrane and affect the
membrane potential by reducing the passage of sodium ions
through the sodium ion channel, and thereby block the
conduction of nerve impulses. Local anesthetics can reduce the
excitability of the membrane and have no effect on the resting
potential.10–14
The rst local anesthetic that was used in clinical practice
comprised active ingredients derived from coca leaves from
South America. As early as the 16th century, Peruvians were
known to chew coca leaves to relieve pain. In the 1860s Nie-
mann extracted crystals of an alkaloid from coca leaves and
named it as cocaine.15–17 In 1868 Maiz was the rst to suggest
that cocaine might be a local anesthetic, and in 1884 Koller (an
Austrian ophthalmologist) was the rst to use cocaine for clin-
ical surgery, which led to the discovery of new anesthesia
techniques and the rapid development of local anesthetic
drugs.18–21 However, in the process of use, cocaine was found to
aCollege of Chemistry and Chemical Engineering, Hainan Normal University, Haikou
571158, China. E-mail: chgying123@163.com; zhoushiyang520@126.com
bKey Laboratory of Tropical Medicinal Plant Chemistry of Ministry of Education,
Hainan Normal University, Haikou 571158, China
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Fig. 1 Chemical structures of local anesthetics.
Fig. 2 Design of target compounds.
purication. Nuclear magnetic resonance (NMR) spectroscopy
was performed using a Bruker AMX-400 spectrometer (IS as
TMS). Mass spectroscopy was performed with an Agilent 6460
system. HPLC analysis of all nal biologically tested
compounds was carried out using an Agilent 1260 series HPLC
system. The purity was determined by reversed-phase HPLC and
was $99% for all biologically tested compounds.
Fig. 3 Basic skeleton structures of target compounds.
2.1.1. General method for synthesis of compounds 2a and
2b. Firstly, 4-aminobenzoic acid (1a, 13.71 g, 0.10 mol) and 1-
chloropropane (8.8 mL, 0.10 mol) were put into a 500 mL round-
bottom ask, and then 200 mL N,N-dimethylformamide (DMF)
as a solvent, anhydrous sodium carbonate (5.30 g, 0.05 mol) as
a deacidifying reagent and 0.1% of 4-dimethylaminopyridine
(DMAP), which was used as a catalyst for the reaction, were
added. Aer the reactants were added, the reaction lasted for 12
hours under reuxing. When the reux reaction was completed,
the mixture was ltered at a high temperature, the ltrate was
collected, cooled, and le to stand for 24 hours, and white
crystals were precipitated. The crystals were ltered and dried to
obtain the crude product, namely, 4-(propylamino)benzoic acid
three steps of alkylation, esterication and alkylation to give the
target compounds (Scheme 1). The synthetic route was charac-
terized by simple operation, high total yields, and mild reaction
conditions.
2. Materials and methods
2.1. Chemistry section
Chemistry materials and general methods. The reagents
(chemicals) were purchased and used without further
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Scheme 1 Synthetic route to target compounds.
(2a). The crude product was recrystallised from toluene, ltered, the reux reaction was completed, the mixture was ltered at
and dried to give the pure product 4-(propylamino)benzoic acid a high temperature, the ltrate was collected, cooled, and le to
(2a) as white crystals. The same general method was used to stand for 12 hours, and white crystals were precipitated. The
synthesize compound 2b as white crystals.
crystals were ltered and vacuum-dried to obtain the crude
namely, 2-(piperidin-1-yl)ethyl-4-(propylamino)
2.1.2. General method for synthesis of compounds 3a to product,
3d. Firstly, N,N0-carbonyldiimidazole (CDI, 16.22 g, 0.10 mol) benzoate (4a). The crude product was recrystallised from
was put into 50 mL tetrahydrofuran (THF). Aer CDI was acetone, ltered, and vacuum-dried to give the pure product 2-
completely dissolved, 4-(propylamino)benzoic acid (2a, 17.92 g, (piperidin-1-yl)ethyl-4-(propylamino)benzoate (4a) as white
0.10 mol) was added and stirring was continued for 1 hour to crystals. The same general method was used to synthesize
form a mixture. The mixture was put into a 500 mL round- compounds 4b to 4p.
bottom ask, and 150 mL THF was added. Under reaction
2-(Piperidin-1-yl)ethyl-4-(propylamino)benzoate (4a). White
conditions of 20 ꢀC, 2-bromoethanamine (8.7 mL, 0.10 mol) was crystals, yield 82.1%, mp 141–143 ꢀC; 1H NMR (300 MHz,
added dropwise. The reaction lasted for 8 hours under reux- DMSO) d: 0.90 (3H, t, J ¼ 7.1 Hz, –CH3), 1.38 (2H, dtt, J ¼ 10.8,
ing. The mixture was cooled and le to stand for 12 hours, and 3.3, 2.4 Hz, –CH2–), 1.50 (4H, ddd, J ¼ 10.8, 3.3, 2.4 Hz, –CH2–),
white crystals were precipitated. The crystals were ltered and 1.57 (2H, tq, J ¼ 7.4, 7.1 Hz, –CH2–), 2.42 (4H, ddd, J ¼ 12.5, 2.9,
vacuum-dried to obtain the crude product, namely, 2- 2.6 Hz, –CH2–), 3.15 (2H, t, J ¼ 7.1 Hz, –CH2–), 3.36 (2H, t, J ¼
bromoethyl-4-(propylamino)benzoate (3a). The crude product 7.4 Hz, –CH2–), 4.29 (2H, t, J ¼ 7.1 Hz, –CH2–), 6.79 (2H, ddd, J ¼
was recrystallised from acetone, ltered, and vacuum-dried to 8.6, 1.2, 0.5 Hz, Ph-H), 7.69 (2H, ddd, J ¼ 8.6, 1.7, 0.5 Hz, Ph-H);
give the pure product 2-bromoethyl-4-(propylamino)benzoate 13C NMR (75 MHz, DMSO) d: 11.4, 23.2, 24.6, 25.9, 47.1, 54.6,
(3a) as white crystals. The same general method was used to 56.7, 63.1, 112.1, 118.4, 130.6, 151.8, 165.9; HR-ESI-MS m/z:
synthesize compounds 3b to 3d.
calcd for C17H26N2O2 {[M + H]+} 290.4073, found 290.1998; anal.
2.1.3. General method for synthesis of compounds 4a to calcd for C17H26N2O2: C, 70.31; H, 9.02; N, 9.65; O, 11.02; found:
4p. Firstly, 2-bromoethyl-4-(propylamino)benzoate (3a, 28.62 g, C, 70.33; H, 9.01; N, 9.64; O, 11.02%.
0.10 mol) and piperidine (11.4 mL, 0.10 mol) were put into
2-(Piperazin-1-yl)ethyl-4-(propylamino)benzoate (4b). White
a 500 mL round-bottom ask, and then 200 mL toluene as crystals, yield 86.3%, mp 180–182 ꢀC; 1H NMR (300 MHz, DMSO)
a solvent and anhydrous potassium carbonate (6.90 g, 0.05 mol) d: 0.90 (3H, t, J ¼ 7.1 Hz, –CH3), 1.59 (2H, tq, J ¼ 7.4, 7.1 Hz,
as a deacidifying reagent were added. Aer the reactants were –CH2–), 2.33 (4H, ddd, J ¼ 14.1, 10.2, 2.5 Hz, –CH2–), 2.66 (4H,
added, the reaction lasted for 8 hours under reuxing. When ddd, J ¼ 10.2, 9.4, 2.5 Hz, –CH2–), 3.16 (4H, t, J ¼ 7.1 Hz, –CH2–),
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3.34 (2H, t, J ¼ 7.4 Hz, –CH2–), 4.31 (2H, t, J ¼ 7.1 Hz, –CH2–), 6.77
(2H, ddd, J ¼ 8.6, 1.2, 0.5 Hz, Ph-H), 7.69 (2H, ddd, J ¼ 8.6, 1.7,
0.5 Hz, Ph-H); 13C NMR (75 MHz, DMSO) d: 11.4, 23.2, 46.3, 47.3,
54.3, 57.4, 63.3, 112.0, 118.4, 130.6, 151.8, 165.8; HR-ESI-MS m/z:
calcd for C16H25N3O2 {[M + H]+} 291.3954, found 291.1945; anal.
calcd for C16H25N3O2: C, 65.95; H, 8.65; N, 14.42; O, 10.98; found:
C, 65.96; H, 8.64; N, 14.43; O, 10.99%.
2-Morpholinoethyl-4-(propylamino)benzoate (4c). White crys-
tals, yield 90.4%, mp 140–142 ꢀC; 1H NMR (300 MHz, DMSO) d:
0.90 (3H, t, J ¼ 7.1 Hz, –CH3), 1.57 (2H, tq, J ¼ 7.4, 7.1 Hz, –CH2–
), 2.50 (4H, ddd, J ¼ 9.4, 3.1, 2.5 Hz, –CH2–), 3.14 (2H, t, J ¼
7.1 Hz, –CH2–), 3.34 (2H, t, J ¼ 7.4 Hz, –CH2–), 3.56 (4H, ddd, J ¼
11.8, 10.2, 2.5 Hz, –CH2–), 4.30 (2H, t, J ¼ 7.1 Hz, –CH2–), 6.77
(2H, ddd, J ¼ 8.6, 1.2, 0.5 Hz, Ph-H), 7.67 (2H, ddd, J ¼ 8.6, 1.7,
0.5 Hz, Ph-H); 13C NMR (75 MHz, DMSO) d: 11.4, 23.2, 47.3, 54.4,
55.7, 63.3, 66.7, 112.0, 118.4, 130.6, 151.8, 165.8; HR-ESI-MS m/
z: calcd for C16H24N2O3 {[M + H]+} 292.3793, found 292.1789;
anal. calcd for C16H24N2O3: C, 65.73; H, 8.27; N, 9.58; O, 16.42;
found: C, 65.74; H, 8.26; N, 9.59; O, 16.41%.
2-Thiomorpholinoethyl-4-(propylamino)benzoate (4d). White
crystals, yield 85.3%, mp 172–174 ꢀC; 1H NMR (300 MHz,
DMSO) d: 0.90 (3H, t, J ¼ 7.1 Hz, –CH3), 1.57 (2H, tq, J ¼ 7.4,
7.1 Hz, –CH2–), 2.55 (4H, ddd, J ¼ 10.9, 10.2, 2.3 Hz, –CH2–),
2.74 (4H, ddd, J ¼ 8.7, 3.4, 2.3 Hz, –CH2–), 3.14 (2H, t, J ¼ 7.1 Hz,
–CH2–), 3.34 (2H, t, J ¼ 7.4 Hz, –CH2–), 4.30 (2H, t, J ¼ 7.1 Hz,
–CH2–), 6.77 (2H, ddd, J ¼ 8.6, 1.2, 0.5 Hz, Ph-H), 7.67 (2H, ddd,
J ¼ 8.6, 1.7, 0.5 Hz, Ph-H); 13C NMR (75 MHz, DMSO) d: 11.4,
23.2, 28.4, 47.3, 53.7, 60.4, 63.3, 112.0, 118.4, 130.6, 151.8, 165.8;
HR-ESI-MS m/z: calcd for C16H24N2O2S {[M + H]+} 308.4403,
found 308.1554; anal. calcd for C16H24N2O2S: C, 62.31; H,
7.84; N, 9.08; O, 10.37; S, 10.39; found: C, 62.33; H, 7.83; N, 9.07;
O, 10.38; S, 10.38%.
2-Morpholinoethyl-4-propoxybenzoate (4g). White crystals,
yield 88.7%, mp 113–115 ꢀC; 1H NMR (300 MHz, DMSO) d: 0.98
(3H, t, J ¼ 7.5 Hz, –CH3), 1.74 (2H, t, J ¼ 7.5 Hz, –CH2–), 2.50 (4H,
ddd, J ¼ 9.4, 3.1, 2.5 Hz, –CH2–), 3.14 (2H, t, J ¼ 7.1 Hz, –CH2–),
3.56 (4H, ddd, J ¼ 11.8, 10.2, 2.5 Hz, –CH2–), 4.02 (2H, t, J ¼
7.5 Hz, –CH2–), 4.30 (2H, t, J ¼ 7.1 Hz, –CH2–), 7.06 (2H, ddd, J ¼
8.6, 1.2, 0.4 Hz, Ph-H), 7.95 (2H, ddd, J ¼ 8.6, 1.4, 0.4 Hz, Ph-H);
13C NMR (75 MHz, DMSO) d: 10.5, 22.5, 54.4, 55.7, 63.3, 66.7,
69.4, 114.4, 121.6, 130.4, 163.6, 165.8; HR-ESI-MS m/z: calcd for
C
C
16H23NO4 {[M + H]+} 293.3631, found 293.1629; anal. calcd for
16H23NO4: C, 65.51; H, 7.90; N, 4.77; O, 21.81; found: C, 65.52;
H, 7.91; N, 4.76; O, 21.80%.
2-Thiomorpholinoethyl-4-propoxybenzoate (4h). White crystals,
yield 83.1%, mp 150–152 ꢀC; 1H NMR (300 MHz, DMSO) d: 0.98
(3H, t, J ¼ 7.5 Hz, –CH3), 1.74 (2H, t, J ¼ 7.5 Hz, –CH2–), 2.55 (4H,
ddd, J ¼ 10.9, 3.4, 2.2 Hz, –CH2–), 2.74 (4H, ddd, J ¼ 10.2, 8.7,
2.2 Hz, –CH2–), 3.14 (2H, t, J ¼ 7.1 Hz, –CH2–), 4.02 (2H, t, J ¼
7.5 Hz, –CH2–), 4.30 (2H, t, J ¼ 7.1 Hz, –CH2–), 7.06 (2H, ddd, J ¼
8.6, 1.2, 0.4 Hz, Ph-H), 7.95 (2H, ddd, J ¼ 8.6, 1.4, 0.4 Hz, Ph-H);
13C NMR (75 MHz, DMSO) d: 10.5, 22.5, 28.4, 53.7, 60.2, 63.3,
69.4, 114.4, 121.6, 130.4, 163.6, 165.8; HR-ESI-MS m/z: calcd for
C
C
16H23NO3S {[M + H]+} 309.4243, found 309.1401; anal. calcd for
16H23NO3S: C, 62.11; H, 7.49; N, 4.53; O, 15.51; S, 10.36; found:
C, 62.13; H, 7.48; N, 4.52; O, 15.52; S, 10.35%.
2-(Piperidin-1-yl)ethyl-4-(butylamino)benzoate (4i). White crys-
tals, yield 76.3%, mp 148–150 ꢀC; 1H NMR (300 MHz, DMSO) d:
0.88 (3H, t, J ¼ 7.1 Hz, –CH3), 1.33 (2H, tq, J ¼ 7.4, 7.1 Hz, –CH2–
), 1.36 (4H, dtt, J ¼ 10.8, 3.3, 2.4 Hz, –CH2–), 1.46 (2H, t, J ¼
7.4 Hz, –CH2–), 1.48 (4H, ddd, J ¼ 10.8, 3.3, 2.6 Hz), 2.43 (4H,
ddd, J ¼ 12.5, 2.9, 2.6 Hz, –CH2–), 3.14 (2H, t, J ¼ 7.1 Hz, –CH2–),
3.30 (2H, t, J ¼ 7.4 Hz, –CH2–), 4.30 (2H, t, J ¼ 7.1 Hz, –CH2–),
6.77 (2H, ddd, J ¼ 8.6, 1.2, 0.4 Hz, Ph-H), 7.67 (2H, ddd, J ¼ 8.6,
1.4, 0.4 Hz, Ph-H); 13C NMR (75 MHz, DMSO) d: 13.7, 20.1, 24.4,
25.8, 26.0, 43.5, 54.4, 56.7, 63.3, 112.0, 118.4, 130.6, 151.8, 165.8;
HR-ESI-MS m/z: calcd for C18H28N2O2 {[M + H]+} 304.4341,
found 304.2153; anal. calcd for C18H28N2O2: C, 71.02; H, 9.27; N,
9.20; O, 10.51; found: C, 71.01; H, 9.28; N, 9.21; O, 10.50%.
2-(Piperazin-1-yl)ethyl-4-(butylamino)benzoate (4j). White crys-
tals, yield 80.4%, mp 193–195 ꢀC; 1H NMR (300 MHz, DMSO) d:
0.88 (3H, t, J ¼ 7.1 Hz, –CH3), 1.33 (2H, tq, J ¼ 7.4, 7.1 Hz, –CH2–
), 1.46 (2H, t, J ¼ 7.4 Hz, –CH2–), 2.35 (4H, ddd, J ¼ 14.1, 10.2,
2.5 Hz, –CH2–), 2.64 (4H, ddd, J ¼ 10.2, 9.4, 2.5 Hz, –CH2–), 3.14
(2H, t, J ¼ 7.1 Hz, –CH2–), 3.30 (2H, t, J ¼ 7.4 Hz, –CH2–), 4.30
(2H, t, J ¼ 7.1 Hz, –CH2–), 6.77 (2H, ddd, J ¼ 8.6, 1.2, 0.4 Hz, Ph-
H), 7.67 (2H, ddd, J ¼ 8.6, 1.4, 0.4 Hz, Ph-H); 13C NMR (75 MHz,
DMSO) d: 13.7, 20.1, 26.0, 43.5, 46.3, 54.3, 57.4, 63.3, 112.0,
118.4, 130.6, 151.8, 165.8; HR-ESI-MS m/z: calcd for C17H27N3O2
2-(Piperidin-1-yl)ethyl-4-propoxybenzoate (4e). White crystals,
yield 80.5%, mp 108–110 ꢀC; 1H NMR (300 MHz, DMSO) d: 0.98
(3H, t, J ¼ 7.5 Hz, –CH3), 1.36 (2H, dtt, J ¼ 10.8, 3.3, 2.4 Hz,
–CH2–), 1.48 (4H, ddd, J ¼ 10.8, 3.3, 2.6 Hz, –CH2–), 1.74 (2H, t, J
¼ 7.5 Hz, –CH2–), 2.43 (4H, ddd, J ¼ 12.5, 2.9, 2.6 Hz, –CH2–),
3.14 (2H, t, J ¼ 7.1 Hz, –CH2–), 4.02 (2H, t, J ¼ 7.5 Hz, –CH2–),
4.30 (2H, t, J ¼ 7.1 Hz, –CH2–), 7.06 (2H, ddd, J ¼ 8.6, 1.2, 0.4 Hz,
Ph-H), 7.95 (2H, ddd, J ¼ 8.6, 1.4, 0.4 Hz, Ph-H); 13C NMR (75
MHz, DMSO) d: 10.5, 22.5, 24.4, 25.8, 54.4, 56.7, 63.3, 69.4,
114.4, 121.6, 130.4, 163.6, 165.8; HR-ESI-MS m/z: calcd for
C
C
17H25NO3 {[M + H]+} 291.3912, found 291.1836; anal. calcd for
17H25NO3: C, 70.07; H, 8.65; N, 4.81; O, 16.47; found: C, 70.06;
H, 8.66; N, 4.82; O, 16.46%.
2-(Piperazin-1-yl)ethyl-4-propoxybenzoate (4f). White crystals,
yield 84.2%, mp 170–172 ꢀC; 1H NMR (300 MHz, DMSO) d: 0.98
(3H, t, J ¼ 7.5 Hz, –CH3), 1.74 (2H, h, J ¼ 7.5 Hz), 2.35 (4H, ddd, J
¼ 14.1, 10.2, 2.5 Hz, –CH2–), 2.64 (4H, ddd, J ¼ 10.2, 9.4, 2.5 Hz,
–CH2–), 3.14 (2H, t, J ¼ 7.1 Hz, –CH2–), 4.02 (2H, t, J ¼ 7.5 Hz,
–CH2–), 4.30 (2H, t, J ¼ 7.1 Hz, –CH2–), 7.06 (2H, ddd, J ¼ 8.6,
{[M
17H27N3O2: C, 66.85; H, 8.91; N, 13.76; O, 10.48; found: C,
+
H]+} 305.4223, found 305.2105; anal. calcd for
C
66.86; H, 8.90; N, 13.75; O, 10.49%.
2-Morpholinoethyl-4-(butylamino)benzoate (4k). White crystals,
yield 86.7%, mp 152–154 ꢀC; 1H NMR (300 MHz, DMSO) d: 0.88
(3H, t, J ¼ 7.1 Hz, –CH3), 1.33 (2H, tq, J ¼ 7.4, 7.1 Hz, –CH2–),
1.46 (2H, t, J ¼ 7.4 Hz, –CH2–), 2.50 (4H, ddd, J ¼ 9.4, 3.1, 2.5 Hz,
–CH2–), 3.14 (2H, t, J ¼ 7.1 Hz, –CH2–), 3.30 (2H, t, J ¼ 7.4 Hz,
–CH2–), 3.56 (4H, ddd, J ¼ 11.8, 3.1, 2.5 Hz, –CH2–), 4.30 (2H, t, J
1.2, 0.4 Hz, Ph-H), 7.95 (2H, ddd, J ¼ 8.6, 1.4, 0.4 Hz, Ph-H); 13
C
NMR (75 MHz, DMSO) d: 10.5, 22.5, 46.3, 54.3, 57.4, 63.3, 69.4,
114.4, 121.6, 130.4, 163.6, 165.8; HR-ESI-MS m/z: calcd for
C
C
16H24N2O3 {[M + H]+} 292.3793, found 292.1788; anal. calcd for
16H24N2O3: C, 65.73; H, 8.27; N, 9.58; O, 16.42; found: C, 65.74;
H, 8.28; N, 9.57; O, 16.41%.
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¼ 7.1 Hz, –CH2–), 6.77 (2H, ddd, J ¼ 8.6, 1.2, 0.4 Hz, Ph-H), 7.67 307.3902, found 307.1787; anal. calcd for C17H25NO4: C, 66.43;
(2H, ddd, J ¼ 8.6, 1.4, 0.4 Hz, Ph-H); 13C NMR (75 MHz, DMSO) H, 8.20; N, 4.56; O, 20.82; found: C, 66.41; H, 8.20; N, 4.57; O,
d: 13.7, 20.1, 26.0, 43.5, 54.4, 55.7, 63.3, 66.7, 112.0, 118.4, 130.6, 20.83%.
151.8, 165.8; HR-ESI-MS m/z: calcd for C17H26N2O3 {[M + H]+}
2-Thiomorpholinoethyl-4-butoxybenzoate (4p). White crystals,
306.4062, found 306.1944; anal. calcd for C17H26N2O3: C, 66.64; yield 80.1%, mp 162–164 ꢀC; 1H NMR (300 MHz, DMSO) d: 0.95
H, 8.55; N, 9.14; O, 15.66; found: C, 66.63; H, 8.54; N, 9.15; O, (3H, t, J ¼ 7.1 Hz, –CH3), 1.46 (2H, tq, J ¼ 7.4, 7.1 Hz, –CH2–),
15.67%.
1.74 (2H, t, J ¼ 7.4 Hz, –CH2–), 2.55 (4H, ddd, J ¼ 10.9, 3.4,
2-Thiomorpholinoethyl-4-(butylamino)benzoate (4l). White 2.2 Hz, –CH2–), 2.74 (4H, ddd, J ¼ 8.7, 3.4, 2.3 Hz, –CH2–), 3.14
crystals, yield 79.2%, mp 184–186 ꢀC; 1H NMR (300 MHz, (2H, t, J ¼ 7.1 Hz, –CH2–), 4.00 (2H, t, J ¼ 7.4 Hz, –CH2–), 4.30
DMSO) d: 0.88 (3H, t, J ¼ 7.1 Hz, –CH3), 1.33 (2H, tq, J ¼ 7.4, (2H, t, J ¼ 7.1 Hz, –CH2–), 7.06 (2H, ddd, J ¼ 8.6, 1.2, 0.4 Hz, Ph-
7.1 Hz, –CH2–), 1.46 (2H, t, J ¼ 7.4 Hz, –CH2–), 2.55 (4H, ddd, J ¼ H), 7.95 (2H, ddd, J ¼ 8.6, 1.4, 0.4 Hz, Ph-H); 13C NMR (75 MHz,
10.9, 3.4, 2.2 Hz, –CH2–), 2.74 (4H, ddd, J ¼ 8.7, 3.4, 2.3 Hz, DMSO) d: 14.2, 19.0, 28.4, 31.7, 53.7, 60.4, 63.3, 68.5, 114.4,
–CH2–), 3.14 (2H, t, J ¼ 7.1 Hz, –CH2–), 3.30 (2H, t, J ¼ 7.4 Hz, 121.6, 130.4, 163.6, 165.8; HR-ESI-MS m/z: calcd for C17H25NO3S
–CH2–), 4.30 (2H, t, J ¼ 7.1 Hz, –CH2–), 6.77 (2H, ddd, J ¼ 8.6, {[M
+
H]+} 323.4513, found 323.1558; anal. calcd for
C17H25NO3S: C, 63.13; H, 7.79; N, 4.33; O, 14.84; S, 9.91; found:
1.2, 0.4 Hz, Ph-H), 7.67 (2H, ddd, J ¼ 8.6, 1.4, 0.4 Hz, Ph-H); 13
C
NMR (75 MHz, DMSO) d: 13.7, 20.1, 26.0, 28.4, 43.5, 53.7, 60.4, C, 63.15; H, 7.78; N, 4.32; O, 14.85; S, 9.90%.
63.3, 112.0, 118.4, 130.6, 151.8, 165.8; HR-ESI-MS m/z: calcd for
C
17H26N2O2S {[M + H]+} 322.4671, found 306.1944; anal. calcd
2.2. Biological section
for C17H26N2O2S: C, 63.32; H, 8.13; N, 8.69; O, 9.92; S, 9.94;
found: C, 63.34; H, 8.12; N, 8.68; O, 9.93; S, 9.93%.
2.2.1. Surface anesthesia and invasive anesthesia. The
2-(Piperidin-1-yl)ethyl-4-butoxybenzoate (4m). White crystals, surface anesthetic effects of the target compounds were inves-
yield 78.2%, mp 115–117 ꢀC; 1H NMR (300 MHz, DMSO) d: 0.95 tigated by corneal reex experiments in rabbits, and the inl-
(3H, t, J ¼ 7.1 Hz, –CH3), 1.37 (4H, dtt, J ¼ 10.8, 3.3, 2.4 Hz, tration anesthetic effects of the target compounds were
–CH2–), 1.46 (2H, tq, J ¼ 7.4, 7.1 Hz, –CH2–), 1.49 (4H, ddd, J ¼ investigated by abdominal acupuncture experiments in mice.
10.8, 10.2, 3.3, 2.6 Hz, –CH2–), 1.75 (2H, t, J ¼ 7.4 Hz, –CH2–),
Surface anesthesia. 190 healthy rabbits without eye diseases
2.43 (4H, ddd, J ¼ 12.5, 2.9, 2.6 Hz, –CH2–), 3.14 (2H, t, J ¼ were taken and the eyelashes were cut off, and the rabbits were
7.1 Hz, –CH2–), 4.00 (2H, t, J ¼ 7.4 Hz, –CH2–), 4.30 (2H, t, J ¼ then randomly divided into an average of 19 groups. For each
7.1 Hz, –CH2–), 7.06 (2H, ddd, J ¼ 8.6, 1.2, 0.4 Hz, Ph-H), 7.95 target compound, the rabbits were divided into one experimental
(2H, ddd, J ¼ 8.6, 1.4, 0.4 Hz, Ph-H); 13C NMR (75 MHz, DMSO) group, two positive control groups and one blank control group.
d: 14.2, 19.0, 24.4, 25.8, 31.7, 54.4, 56.7, 63.3, 68.5, 114.4, 121.6, Each rabbit's lower eyelid was pulled into a cup shape with the
130.4, 163.6, 165.8; HR-ESI-MS m/z: calcd for C18H27NO3 {[M + thumb and forenger, the middle nger was pressed against the
H]+} 305.4183, found 305.1992; anal. calcd for C18H27NO3: C, nasolacrimal duct to prevent the liquid from owing into the
70.79; H, 8.91; N, 4.59; O, 15.72; found: C, 70.78; H, 8.92; N, 4.58; nasolacrimal duct, and a solution of the corresponding drugs
O, 15.73%.
(1% target compound and 1% positive control substance) was
2-(Piperazin-1-yl)ethyl-4-butoxybenzoate (4n). White crystals, dropped, with two drops in each eye. The lower eyelid was gently
yield 80.6%, mp 183–185 ꢀC; 1H NMR (300 MHz, DMSO) d: 0.95 rubbed to allow the drug solution to fully contact the cornea, and
(3H, t, J ¼ 7.1 Hz, –CH3), 1.46 (2H, tq, J ¼ 7.4, 7.1 Hz, –CH2–), it was allowed to overow aer 2 min. Tetracaine and pramo-
1.75 (2H, t, J ¼ 7.4 Hz, –CH2), 2.35 (4H, ddd, J ¼ 14.1, 10.2, caine were used as the positive controls and NS as the blank
2.5 Hz, –CH2), 2.64 (ddd, J ¼ 10.2, 9.4, 2.5 Hz, –CH2), 3.14 (2H, t, control. The effects of surface anesthesia were evaluated by
J ¼ 7.1 Hz, –CH2), 4.00 (2H, t, J ¼ 7.4 Hz, –CH2), 4.30 (2H, t, J ¼ a number of corneal stimulations. Corneal stimulation was used
7.1 Hz, –CH2), 7.06 (2H, ddd, J ¼ 8.6, 1.2, 0.4 Hz, Ph-H), 7.95 to gently touch the cornea at around 5 points in the upper, lower,
(2H, ddd, J ¼ 8.6, 1.4, 0.4 Hz, Ph-H); 13C NMR (75 MHz, DMSO) middle, le and right parts. Aer stimulation had been per-
d: 14.2, 19.0, 31.7, 46.3, 54.3, 57.4, 63.3, 68.5, 114.4, 121.6, 130.4, formed 100 times, the blink reex occurred less than 5 times, and
163.6, 165.8; HR-ESI-MS m/z: calcd for C17H26N2O3 {[M + H]+} the anesthetic effect was considered to be effective.
306.4065, found 306.1947; anal. calcd for C17H26N2O3: C, 66.64;
H, 8.55; N, 9.14; O, 15.66; found: C, 66.65; H, 8.54; N, 9.13; O, the completion of the infusion and every 20 min for 1 hour
15.67%. aerward, and the tests were continued until the blink reex
The corneal reex was tested every 10 min for 1 hour before
2-Morpholinoethyl-4-butoxybenzoate (4o). White crystals, yield occurred more than 5 times. Blinks were compared between
85.5%, mp 120–122 ꢀC; 1H NMR (300 MHz, DMSO) d: 0.95 (3H, t, groups and in terms of how they changed over time.
J ¼ 7.1 Hz, –CH3), 1.46 (2H, tq, J ¼ 7.4, 7.1 Hz, –CH2–), 1.75 (2H,
Invasive anesthesia. 90 healthy mice were taken and randomly
t, J ¼ 7.4 Hz, –CH2–), 2.50 (4H, ddd, J ¼ 9.4, 3.1, 2.5 Hz, –CH2–), divided into 9 groups. For each target compound, the mice were
3.14 (2H, t, J ¼ 7.1 Hz, –CH2–), 3.56 (4H, ddd, J ¼ 11.8, 3.1, divided into one experimental group, two positive control
2.5 Hz, –CH2–), 4.00 (2H, t, J ¼ 7.4 Hz, –CH2–), 4.30 (2H, t, J ¼ groups and one blank control group. The hair on the abdominal
7.1 Hz, –CH2–), 7.06 (2H, ddd, J ¼ 8.6, 1.2, 0.4 Hz, Ph-H), 7.95 skin of the mice was shaved in an area with a diameter of about
(2H, ddd, J ¼ 8.6, 1.4, 0.4 Hz, Ph-H); 13C NMR (75 MHz, DMSO) 2.5 cm. A circle with a diameter of 2 cm was drawn in this area
d: 14.2, 19.0, 31.7, 54.4, 55.6, 63.3, 66.7, 68.5, 114.4, 121.6, 130.4, with a marker, and a solution of the corresponding drugs (1%
163.6, 165.8; HR-ESI-MS m/z: calcd for C17H25NO4 {[M + H]+} target compound and 1% positive control substance) was
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injected, of which 1 mL was injected subcutaneously to form many deciencies; for example, tetracaine causes adverse
a pichu. Tetracaine and pramocaine were used as the positive reactions (toxicity), and an excessive dosage of drugs is likely to
controls and NS as the blank control. A home-made test needle cause toxic reactions in the central nervous system (CNS) and
was used to test the response of the skin to the pinprick aer the cardiovascular system. In addition, pramocaine has an anes-
injection. In the beginning, it was tested every 15 min 10 times thetic effect of limited duration. In terms of their chemical
for each acupuncture and 1.5 hours aer the injection, and the structure, these local anesthetic drugs all contain a benzene
tests were performed every 0.5 hours until the mice had ring, which is the parent structure (pharmacophore) of this kind
completely recovered to a state without anesthesia. The skin of drug. In this research, the target compounds were designed
contraction reaction of the mice during acupuncture was posi- with tetracaine and pramocaine as the lead compounds, and
tive (no pain inhibition). The changes in the pain inhibition rate a new organic molecule was formed by the combination prin-
over time were compared in each group.
ciple (Fig. 2). On this basis, the structure of the new organic
2.2.2. Block anesthesia and acute toxicity tests. The nerve molecule was modied, and a series of novel benzeneamide
block anesthetic effects of the compounds were evaluated by compounds were designed by the use of bioisostere formation
a rat model of a tail nerve block, and the acute toxicity was and modication of the compound with alkyl groups (Fig. 2).
evaluated by oral administration to mice.
The structure of the new organic molecule was determined, the
Block anesthesia. A certain number of rats were taken and substituent R on the benzene ring was modied, and two
subjected to thermal irradiation, and the normal tail ick substituents, namely, –C3H7 and –C4H9, were selected in the
reaction time of the rats was routinely measured. Eighty rats designed process. For X1, N and O were selected as the con-
with a tail length of 2–4 cm were randomly divided into 8 necting atoms. In addition, in the process of modication of the
groups. The radiation intensity of the pain-inducing tail ick six-membered heterocyclic ring, for X2 C, N, O and S atoms were
instrument was adjusted to 75, and then the rats were xed and selected. From the perspective of the structure–activity rela-
only the rats' tails were exposed. Five points were selected in tionship (SAR), the newly designed series of compounds had
each tail, and a solution of the corresponding drugs (1% target three basic skeleton structures, namely, a lipophilic section,
compound and 1% positive control substance) was injected, of a link section and a hydrophilic section (Fig. 3). The purpose of
which 0.2 mL was injected at each point (no injection into the the modication of R and X1 was to design target compounds
blood vessels was allowed). Tetracaine and pramocaine were with rapid onset and long duration of anesthesia (lipophilic
used as the positive controls and NS as the blank control. Aer section), whereas the modication of X2 was performed to
the tail nerve block operation was completed, the length of tail obtain compounds with low toxicity (hydrophilic section). In the
icks was measured every 2 min with the pain-inducing tail ick process of synthesizing the target compounds, we selected
instrument, and the onset of analgesia and duration of anal- a route with high total yields, mild conditions and simple
gesia were recorded.
operation. Three steps were used in the synthesis of the new
Acute toxicity tests. An orally administered Korbor test was target compounds, namely, alkylation, esterication and alkyl-
performed.
ation. The rst alkylation step used 4-(propylamino)benzoic
acid (2a), DMF as a solvent, anhydrous sodium carbonate as
a deacidifying reagent and DMAP as a catalyst, and the reux
reaction was completed in 12 hours. The esterication used 2-
bromoethyl-4-(propylamino)benzoate (3a), THF as a solvent,
and CDI as a dehydrating agent, and the reux reaction was
completed in 8 hours. The second alkylation step used 2-
(piperidin-1-yl)ethyl-4-(propylamino)benzoate (4a), toluene as
a solvent, and anhydrous potassium carbonate (K2CO3) as
a deacidifying reagent, and the reux reaction was completed in
2.3. log P and pKa tests
Firstly, an oil phase (octanol) and a water phase were saturated
with each other for 24 hours. Then an appropriate amount of
the respective compound was added to a test tube, followed by
1 mL of the pre-saturated two-phase solution. The mouth of the
test tube was closed and the test tube was oscillated for 72 hours
to make the distribution reach equilibrium. Aer the distribu-
tion was balanced, centrifugal separation was carried out to
measure the sample concentrations in the two respective pha-
ses, and then log P was calculated. The compound was also
prepared as a dilute solution with a certain concentration, and
half of the equivalent sodium hydroxide solution was added.
Then, aer the solution had been shaken well, the pH of the
solution was directly measured with a pH meter, and then the
pKa was calculated.
1
10 hours. The products were analyzed by H NMR, 13C NMR,
MS, and elemental analysis. Moreover, some physical and
chemical properties (log P and pKa) of the compounds were
studied (Table 1). From Table 1, we can nd that these target
compounds had high lipid solubility (log P ranged from 1.57 to
3.73) and were easily absorbed.
3.2. Biological activity
Local anesthetics act on nerve endings and nerve trunks and
play a role in local anesthesia. Local anesthetics are commonly
used in stomatology, ophthalmology, gynecology and minor
3. Results and discussion
3.1. Synthesis
At present, many types of local anesthetic drugs are used in surgical operations. The mechanism of action of a local anes-
clinical practice, such as tetracaine, lidocaine, dyclonine and thetic is that the drug binds to the receptor of the sodium ion
pramocaine (Fig. 1). However, these local anesthetic drugs have (Na+) channel on the nerve membrane, reduces the passage of
6632 | RSC Adv., 2019, 9, 6627–6635
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Na+ through the ion channel and blocks the conduction of nerve
impulses so as to achieve the purpose of local anesthesia.
People expect that local anesthetics should have the character-
istics of rapid onset, long duration and low toxicity. At present,
the commonly used local anesthetics have some disadvantages,
such as slow onset, short duration and high toxicity, and hence
it is necessary to design new local anesthetics. The newly
designed target compounds were evaluated via surface anes-
thesia, inltration anesthesia, block anesthesia and acute
toxicity tests to select target compounds with good biological
activity. The mechanism of action of these compounds was
studied. The results of the biological activity tests showed that
these compounds had good local anesthetic effects (Tables 2–4).
In particular, compounds 4d, 4g, 4j, 4k, 4n, and 4o had excellent
effects in terms of surface anesthesia, inltration anesthesia
and block anesthesia. In the process of the local anesthesia
experiment, it was found that the anesthetic effects of
Table 1 Physical and chemical properties of the target compounds
Compound
R
X1
X2
log P
pKa
4a
4b
4c
4d
4e
4f
4g
4h
4i
4j
4k
4l
4m
4n
4o
4p
C3H7–
C3H7–
C3H7–
C3H7–
C3H7–
C3H7–
C3H7–
C3H7–
C4H9–
C4H9–
C4H9–
C4H9–
C4H9–
C4H9–
C4H9–
C4H9–
NH
NH
NH
NH
O
O
O
O
NH
NH
NH
NH
O
O
O
O
C
N
O
S
C
N
O
S
C
N
O
S
C
N
O
S
2.93
1.57
1.78
2.54
3.29
1.96
2.18
2.90
3.26
1.98
2.21
2.94
3.73
2.36
2.59
3.31
8.79
7.90
7.76
8.00
8.77
7.86
7.73
7.92
8.81
7.93
7.76
7.98
8.76
7.85
7.75
7.95
Table 2 Surface anesthetic effects of the target compounds
Blink reexes (n ¼ 10)
Compound
10 min
20 min
30 min
40 min
50 min
60 min
80 min
100 min
120 min
140 min
4a
4b
4c
4d
4e
4f
4g
4h
4i
4j
4k
4l
4m
4n
4o
4p
7 ꢁ 0.3
7 ꢁ 0.4
6 ꢁ 0.3
4 ꢁ 0.3
6 ꢁ 0.4
7 ꢁ 0.5
4 ꢁ 0.2
6 ꢁ 0.3
7 ꢁ 0.5
5 ꢁ 0.3
4 ꢁ 0.3
6 ꢁ 0.6
6 ꢁ 0.5
3 ꢁ 0.2
4 ꢁ 0.2
8 ꢁ 0.7
6 ꢁ 0.4
6 ꢁ 0.5
100
7 ꢁ 0.2
6 ꢁ 0.6
6 ꢁ 0.7
3 ꢁ 0.1
6 ꢁ 0.6
6 ꢁ 0.5
3 ꢁ 0.2
6 ꢁ 0.5
6 ꢁ 0.7
4 ꢁ 0.3
3 ꢁ 0.2
5 ꢁ 0.5
5 ꢁ 0.6
2 ꢁ 0.1
3 ꢁ 0.2
7 ꢁ 0.9
5 ꢁ 0.6
5 ꢁ 0.8
100
6 ꢁ 0.2
5 ꢁ 0.3
4 ꢁ 0.2
3 ꢁ 0.2
5 ꢁ 0.3
5 ꢁ 0.5
2 ꢁ 0.1
5 ꢁ 0.6
5 ꢁ 0.6
3 ꢁ 0.2
2 ꢁ 0.1
4 ꢁ 0.3
4 ꢁ 0.3
1 ꢁ 0.1
2 ꢁ 0.1
6 ꢁ 0.8
4 ꢁ 0.4
3 ꢁ 0.3
100
5 ꢁ 0.2
4 ꢁ 0.3
3 ꢁ 0.2
2 ꢁ 0.1
4 ꢁ 0.7
4 ꢁ 0.8
1 ꢁ 0.1
4 ꢁ 0.5
4 ꢁ 0.5
3 ꢁ 0.2
1 ꢁ 0.1
3 ꢁ 0.2
3 ꢁ 0.3
1 ꢁ 0.1
2 ꢁ 0.1
5 ꢁ 0.7
3 ꢁ 0.2
2 ꢁ 0.1
100
4 ꢁ 0.2
3 ꢁ 0.2
3 ꢁ 0.2
2 ꢁ 0.1
4 ꢁ 0.5
4 ꢁ 0.5
1 ꢁ 0.1
3 ꢁ 0.2
4 ꢁ 0.5
2 ꢁ 0.1
1 ꢁ 0.1
3 ꢁ 0.5
3 ꢁ 0.3
1 ꢁ 0.1
1 ꢁ 0.1
4 ꢁ 0.6
2 ꢁ 0.1
2 ꢁ 0.1
100
3 ꢁ 0.1
3 ꢁ 0.3
4 ꢁ 0.5
2 ꢁ 0.1
6 ꢁ 0.8
5 ꢁ 0.4
2 ꢁ 0.1
3 ꢁ 0.3
3 ꢁ 0.3
2 ꢁ 0.1
1 ꢁ 0.1
4 ꢁ 0.4
4 ꢁ 0.5
2 ꢁ 0.1
2 ꢁ 0.1
4 ꢁ 0.5
2 ꢁ 0.1
2 ꢁ 0.1
100
6 ꢁ 0.6
5 ꢁ 0.5
6 ꢁ 0.6
4 ꢁ 0.5
8 ꢁ 0.9
9 ꢁ 1.1
4 ꢁ 0.7
5 ꢁ 0.7
6 ꢁ 0.8
5 ꢁ 0.6
3 ꢁ 0.2
6 ꢁ 0.7
5 ꢁ 0.6
2 ꢁ 0.1
2 ꢁ 0.1
7 ꢁ 0.9
5 ꢁ 0.6
6 ꢁ 0.8
100
25 ꢁ 2.3
21 ꢁ 2.1
27 ꢁ 2.5
20 ꢁ 1.9
30 ꢁ 3.2
32 ꢁ 3.1
21 ꢁ 2.0
23 ꢁ 2.1
26 ꢁ 2.2
24 ꢁ 2.3
14 ꢁ 1.3
28 ꢁ 2.7
21 ꢁ 2.0
10 ꢁ 1.2
8 ꢁ 0.9
44 ꢁ 4.1
40 ꢁ 4.2
49 ꢁ 3.9
39 ꢁ 4.0
53 ꢁ 4.7
57 ꢁ 5.1
38 ꢁ 2.6
41 ꢁ 4.0
46 ꢁ 4.3
48 ꢁ 4.1
33 ꢁ 2.8
50 ꢁ 4.7
40 ꢁ 4.1
38 ꢁ 3.5
24 ꢁ 1.9
50 ꢁ 4.1
38 ꢁ 3.0
44 ꢁ 4.0
100
72 ꢁ 6.3
70 ꢁ 6.2
89 ꢁ 7.3
71 ꢁ 7.3
92 ꢁ 8.2
94 ꢁ 8.3
82 ꢁ 7.5
83 ꢁ 7.6
86 ꢁ 8.3
90 ꢁ 8.5
69 ꢁ 5.1
93 ꢁ 8.9
86 ꢁ 8.0
80 ꢁ 7.3
65 ꢁ 5.3
96 ꢁ 9.5
79 ꢁ 6.4
89 ꢁ 7.8
100
29 ꢁ 3.0
18 ꢁ 1.3
23 ꢁ 2.1
100
Tetracaine
Pramocaine
NS
Table 3 Infiltration anesthetic effects of the target compounds
Pain inhibition (n ¼ 10, %)
Compound 15 min
30 min 45 min 60 min 75 min 90 min 2 h 2.5 h
3 h
3.5 h
4 h
4.5 h
5 h
4d
4g
4j
4k
4n
4o
100
100
100
100
100
100
100
100
100
100
100
100
0
100
100
100
100
100
100
100
100
0
100
100
100
100
100
100
100
100
0
100
100
100
100
100
100
100
100
0
100 100
100 100
100 100
100 100
100 100
100 100
100 100
100
100
100
100
100
100
100
100
100
100
100
100
93.2 ꢁ 0.5 85.2 ꢁ 1.3 76.5 ꢁ 1,5
98.9 ꢁ 0.2 86.7 ꢁ 1.2 73.8 ꢁ 1.4
91.1 ꢁ 0.6 78.9 ꢁ 2.1 56.4 ꢁ 2.1
98.4 ꢁ 0.3 100
100
100
100
100
100
100
100
100
100
99.7 ꢁ 0.2 90.8 ꢁ 0.9
100
100
94.3 ꢁ 0.5
92.0 ꢁ 0.6
Tetracaine 98.7 ꢁ 0.2 100
94.1 ꢁ 0.5 89.6 ꢁ 0.7 70.3 ꢁ 1.0 46.3 ꢁ 3.1 32.1 ꢁ 3.2
Pramocaine 99.1 ꢁ 0.2 100
100 98.5 ꢁ 0.4 90.3 ꢁ 0.8 81.5 ꢁ 0.6 63.9 ꢁ 2.1 40.2 ꢁ 3.0 25.5 ꢁ 3.4
NS
0
0
0
0
0
0
0
0
0
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Table 4 Block anesthetic effects of the target compounds and results of acute toxicity tests
Onset of analgesia
(min)
Duration of analgesia
(min)
Compound
LD50 (mg kgꢂ1
)
4d
4g
4j
4k
4n
4o
8.8 ꢁ 0.4
8.3 ꢁ 0.4
12.5 ꢁ 0.7
7.2 ꢁ 0.3
6.4 ꢁ 0.2
6.6 ꢁ 0.2
12.3 ꢁ 0.9
10.4 ꢁ 0.8
230.7 ꢁ 3.2
254.6 ꢁ 3.6
221.8 ꢁ 3.0
279.9 ꢁ 3.2
293.5 ꢁ 3.4
286.4 ꢁ 3.3
172.3 ꢁ 2.6
166.5 ꢁ 2.3
543.7 ꢁ 2.3
854.6 ꢁ 3.7
735.1 ꢁ 3.2
907.2 ꢁ 4.3
976.5 ꢁ 4.5
928.9 ꢁ 4.9
435.4 ꢁ 3.3
472.1 ꢁ 3.6
Tetracaine
Pramocaine
Fig. 4 Binding of the target compounds to the receptor.
compounds 4k, 4n and 4o had the characteristics of rapid onset compounds via a route with high total yields, mild conditions
and long duration. In addition, the results of the acute toxicity and simple operation.
test showed that the median lethal dose (LD50) of compounds
The biological activity was evaluated by surface anesthesia,
4k, 4n and 4o was higher and indicated that the target inltration anesthesia, block anesthesia and acute toxicity tests,
compounds had low toxicity. In the study of the mechanism of and the experimental results showed that compounds 4d, 4g, 4j,
local anesthetic action, we found that the Na+ channels bound 4k, 4n, and 4o had a good local anesthetic effect. The results of
to by this kind of drug bind to the receptor, and thus the acute toxicity tests showed that the target compounds had low
passage of Na+ was affected. The binding of these compounds to toxicity.
the receptor involves multiple bonds (Fig. 4), including van der
Waals forces, dipole–dipole interactions, electrostatic attrac-
tion, and hydrophobic action.
Conflicts of interest
The authors declare that they have no conict of interest.
4. Conclusion
Acknowledgements
Tetracaine and pramocaine were used as the lead compounds, The Project was sponsored by the Innovation Team Project of
and a total of 16 target compounds were designed and synthe- Universities in Chongqing (No. CXTDX201601018). The Project
sized. The combination principle was used to design the target was also supported by the Chongqing Scientic and Techno-
molecule, which was modied by bioisostere formation and logical Innovation Special Project of Social Undertakings and
modication with alkyl groups to design benzoate compounds. People’s Livelihood Guarantee (No. cstc2015shmszx80060),
We selected three steps for the synthesis of the target Chongqing University Students' Training Project of Innovation
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