INTRODUCTION:

Elimination and prevention of pain syndrome is an urgent problem of medicine. One of the most important directions in its solution is the development and creation of medicines with a local anesthetic effect. In surgery, when general anesthesia is not the only possible method of anesthesia, it is advisable to use methods of infiltration and conduction anesthesia, because of their simplicity and safety. Recently, the proportion of local anesthesia has increased particularly, due to new ideas about the role of local anesthesia, as well as the emergence of new effective green local anesthetics [1, 2].

The modern period of development of organic chemistry demonstrates not only the potential of organic synthesis, but also its importance for the development of both chemistry in general and many related fields of science and practice, in particular, providing humanity with medicines.

Due to the high physiological activity of azacyclanes, in particular, piperidine derivatives, these studies acquire the status of one of the urgent problems of modern chemistry, biology and medicine [3,4].

This study focuses on the search and development of new drugs in the series of fluorine-containing piperidine derivatives.

The chemistry of organofluorine compounds has extremely important theoretical and practical value [5-9].

Numerous studies have confirmed the high biological activity of a number of fluorine-containing organic compounds, as a result of that were created and successfully used drugs such as fluoroquinolone antibiotics, risperidone, fluorophenazine, haloperidol and etc.

The number of publications and especially patents in this area is rapidly increasing, and a significant number of them belong to aromatic and heterocyclic compounds with fluorine-containing substituents [10].

This interest is caused by a significant improvement in the pharmacological properties of fluorinated compounds. Entering a fluorine atom into molecules of organic compounds increases their bioavailability, metabolic stability, lipophilicity, and also improves the ability of these substances to interact with target proteins [11].

In addition, fluorine can change the basicity of the compound. Like other electronegative atoms, fluorine has a strong effect on the acidity or basicity of functional groups. The change in pKa affects the pharmacokinetic properties of the molecule and their binding affinity. Highly basic groups may have a limited effect on bioavailability. The fluorine atom, introduced close to the main group, reduces its basicity, this leads to a better penetration of the compound into the membrane and, thus, to improved bioavailability.

In connection with the foregoing, the synthesis of N-substituted piperidols and their acyl derivatives, in particular fluorobenzoyloxy derivatives, was determined as the task of this work.

By reducing the amino ketones (1,2) with sodium borohydride in isopropanol, the corresponding secondary alcohols (3.4) were obtained in quantitative yields as oily products with R_f 0.35 and R_f 0.38.

To determine the effect of introducing a fluorine atom on the pharmacological activity of compounds, by acylation of 1-propyl-4-hydroxypiperidine (3) with 4-fluoro-benzoyl chloride, 3-fluoro-benzoyl chloride and 2-fluoro-benzoyl chlorides and by acylation of1-benzyl-4-hydroxypiperidine (4) with4-fluoro-benzoyl chloride, the corresponding hydrochlorides of esters (5-8) were obtained. The reaction was carried out in absolute dioxane at room temperature or under heating; the piperidol: acylating agent ratio was taken to be 1:1.5, 1:2.

The resulting corresponding hydrochlorides of amino esters (5-8) are white crystalline substances with a clear melting point.

Table 1 shows the yields, melting point, Rf and absorption bands of ester carbonyl as the most characteristic for esters.

Figure 1 - Synthesis of piperidine fluorobenzoates

Table 1 - Outputs and physico-chemical characteristics of the hydrochlorides of fluorobenzoate alcohols 1- (propyl-, benzyl-) piperidin-4-one (5-8)

Compound	Output, %	*R_f	M.p., °С	Found, % Calculated		IR spectrum, cm-1	Gross Formula
Compound	Output, %	*R_f	M.p., °С	С	Н	С═О ester (С=N)
5	50,3	0,84	147-149	59,71 59,70	7,15 7,01	1723,2	С₁₅Н₂₁NО₂ClF
6	47,2	0,87	154-156	60,01 59,70	7,09 7,01	1716,6	С₁₅Н₂₁NО₂ClF
7	40,5	0,85	131-133	59,83 59,70	7,31 7,01	1725,4	С₁₅Н₂₁NО₂ClF
8	73,7	0,89	202-204	65,39 65,23	6,15 6,05	1731	C₁₉H₂₁NO₂ClF
Note - * Eluent: benzene: dioxane = 4: 1

Table 2 - Values of chemical shifts of carbon atoms in ¹³C NMR spectra of 1- (propyl-, benzyl-) -piperidin-4-one fluorine benzoate hydrochlorides (d, ppm)

Compound	С_2,6	С_3,5	С₄	С=О	С₆H₄F	C-F	N-R
5	49,88; 47,45	28,05; 27,05	65,83	172,21	116,19-132,98	п-F 164,44	CH₂CH₂CH₃ 57,67;17,35;11,55
6	49,81; 47,41	27,97; 27,01	66,18	164,29	120,93-131,60	м-F 161,30	CH₂CH₂CH₃ 57,63;17,17;11,55
7	49,77; 47,50	27,92-26,94	66,18	162,93	118,67-135,92	о-F 160,43	CH₂CH₂CH₃ 57,75;17,07;11,52
8	47,16; 49,57	26,93; 27,83	65,85	159,66	116,41-132,93	п-F 164,41	CH₂C₆H₅ 59,19;129,20-132,03

Study of Biological Activity:

An experimental study of specific activity in the infiltration and conduction anesthesia of new compounds of the piperidine series under the laboratory ciphers MAB-199 - MAB-201 was conducted using the primary screening methods recommended by the Pharmacological Committee of the Republic of Kazakhstan and the Guidelines for the experimental (preclinical) study of pharmacological substances [12].

МАВ-199

МАВ-200

МАВ-201

Figure 2 - 1- propyl-4-hydroxypiperidine p-, m-, o-fluorobenzoates under the codes MAB-199 - MAB-201, studied for local anesthetic activity

It was established that all tested compounds showed a certain effect (Table 3). Among the compounds studied during infiltration anesthesia, the compound MAB-201 was very active, its anesthesia index approaches to trimecaine. The total insensitivity of the skin of guinea pigs with the introduction of the MAB-201, was 13.5, which is somewhat shorter than that of trimecaine and lidocaine and longer than that of novocaine. The total duration of the MAB-201 was 45.0 minutes. The initial sensitivity of guinea pigs was restored by testing trimekain after 38.3 minutes, approximately 1.18 faster than with the use of MAB-201.

In terms of the duration of action, MAB-201 was statistically significantly higher than that of lidocaine and novocaine by 1.46 and 1.5 times, respectively.

The anesthesia indices of MAB-199 and MAB-200 approached the corresponding indicator of trimecaine and slightly exceeded the corresponding parameters of lidocaine and novocaine. Complete anesthesia of MAB-199 approximately corresponded to that of the comparison preparations. MAB-200 in this parameter was inferior to trimecaineand lidocaine and corresponded to this indicator of novocaine. 0.25% solutions of MAB-199 and MAB-200 are the same for the duration of action with trimecaine.

In conduction anesthesia, all the tested compounds are approximately the same in duration of complete anesthesia. The compounds MAB-199, MAB-200, MAB-201, are approximately 1.42 times statistically significantly more active than trimecaine, equal to the effect of lidocaine and 1.9 times stronger than novocaine. The total duration of action of the compounds of MAB-199, MAB-201 is within 162 minutes, which is 2.85-2.89 times longer than that of trimecaine, lidocaine and novocaine.

MAB-200 has a duration of action greater than trimecaineby 2.12 times, corresponds to lidocaine and statistically significantly exceeds the activity of novocaine by 2.85 times.

Table 3 - Activity and duration of action of MAB-199 MAB-201

Compounds	Infiltration anesthesia			Conduction anesthesia
	0,25%			1,0 %
	Anesthesia index М±м	Duration full anesthesia, min	Continue actions, min	Duration full anesthesia, min	Continue actions, min
МАВ-199	26,16±3,95	14,16±3,26	38,33±7,29	67,50±3,36	162,5±13,7
МАВ-200	26,16±1,47	10,0±1,29	31,0±3,38	67,6±2,25	120,8±8,75
МАВ-201	29,5±1,79	13,5±4,59	45,0±3,88	65,8±2,99	165,0±14,0
Trimecaine	33,6±0,33	20,0±1,7	38,3±1,05	47,3±8,4	56,9±12,8
Lidocaine	23,1±0,9	14,2±0,8	30,8±0,8	65,0±18,4	90,8±18,4
Novocaine	25,0±1,0	10,0±1,2	29,1±1,5	35,2±7,1	42,3±13,6

Thus, based on the results of biological studies, it can be concluded that the fluorobenzoic esters of secondary alcohols cause deep and long-term anesthesia, the introduction of fluorine leads to local anesthetics with varying degrees of efficiency. Of greatest interest are the compounds under the code MAB-199, MAB-201, which have a pronounced effect on the duration of action and have an advantage over comparison drugs, and therefore can be recommended for further study.

Experimental chemical part:

The course of the reaction and the individuality of the compounds are controlled by TLC on aluminum oxide of the III degree of activity, with the appearance of spots by iodine vapor. IR spectra were recorded on a Nicolet 5700 spectrometer in KBr tablets and between KBr plates. The ¹³C NMR spectra of the studied compounds in CDCl₃ were recorded on a JNOL-ECA400 spectrometer from JEOL with an operating frequency of 400 MHz on hydrogen nuclei and 100 MHz on carbon nuclei. Internal standard - HMDS. The data of elemental analysis of all synthesized compounds are given in the respective tables in the discussion of the experiment.

1-Propyl-4- (p-fluorobenzoyloxy) piperidine hydrochloride (5):

1.5g (0.01 Mole) of 1-propyl-4-hydroxypiperidine is dissolved in a small amount of absolute dioxane, then a solution of 2.46ml (0.02mol) of p-fluorobenzoyl chloride in absolute dioxane was slowly added dropwise with stirring. The reaction mixture is kept for 24 hours at room temperature. The progress of the reaction was monitored by TLC. The precipitation is washed with diethyl ether, filtered off on a SCHOTTE funnel, recrystallized from isopropyl alcohol. 1.52g (50.3% of theory) of 1-propyl-4- (p-fluoro) benzoyloxypiperidine hydrochloride are obtained with a m.p. 147-149⁰С, R_f 0.84 (Al₂O₃, eluent - benzene: dioxane - 4: 1).

Found, %: C 59.71; H 7.15. C₁₅H₂₁ClFNO₂.

Calculated, %: C 59.70; H 7.01.

1-Propyl-4- (m-fluorobenzoyloxy) piperidine hydrochloride (6):

In a round-bottom, three-neck flask equipped with a mechanical stirrer, a reflux condenser and a dropping funnel was placed 2g (0.014mol) of 1-propyl-4-hydroxypiperidine dissolved in absolute dioxane and a solution of 3.4ml (0.028mol) m-fluorobenzoyl chloride in absolute dioxane was slowly added. The reaction mixture is heated with stirring for one hour at a temperature of ~ 60°C and left at room temperature for 24 hours. Dioxane is evaporated to dryness on a rotary evaporator. The residue is washed with diethyl ether, the crystalline product is filtered off and recrystallized from isopropanol.

Obtained 2.02g (47.2% of theory) of 1-propyl-4- (m-fluoro) benzoyloxypiperidine hydrochloride with m.p. 154-156°C, R_f 0.87 (Al₂O₃, eluent - benzene: dioxane - 4:1).

Found, %: C 60.01; H 7.09. C₁₅H₂₁ClFNO₂.

Calculated, %: C 59.70; H 7.01.

1-Propyl-4- (o-fluorobenzoyloxy) piperidine hydrochloride (7):

2g (0.014Mole) of 1-propyl-4-hydroxypiperidine is dissolved in a small amount of absolute dioxane, then a solution of 3.3ml (0.028mol) of o-fluorobenzoyl chloride in absolute dioxane was slowly added dropwise with stirring. The reaction mixture is heated at a temperature of 50- 60°C for 3-4 hours and incubated for 48 hours at room temperature. The progress of the reaction was monitored by TLC. The precipitation is washed with diethyl ether, filtered off on a SCHOTTE funnel, recrystallized from isopropyl alcohol. 1.71g (40.5% of theory) of 1-propyl-4- (o-fluoro) benzoyloxypiperidine hydrochloride are obtained with a m.p. 131-133⁰С, R_f 0.85 (Al₂O₃, eluent - benzene: dioxane - 4:1).

Found, %: C 59.83; H 7.31. C₁₅H₂₁ClFNO₂.

Calculated, %: C 59.70; H 7.01.

1-Benzyl-4- (p-fluorobenzoyloxy) piperidine hydrochloride (8):

1.15g (0.005Mole) of 1-benzyl-4-hydroxypiperidine is dissolved in a small amount of absolute dioxane, then a solution of 1.23ml (0.01mol) of p-fluorobenzoyl chloride in absolute dioxanewas slowly added dropwise with stirring. The reaction mixture is kept for 24 hours at room temperature. The progress of the reaction was monitored by TLC. The precipitation is washed with diethyl ether, filtered off on a SCHOTTE funnel, recrystallized from isopropyl alcohol. 3.13g (73.7% of theory) of 1-propyl-4- (p-fluoro) benzoyloxypiperidine hydrochloride are obtained with a m.p. 202-204⁰С, R_f 0.89 (Al₂O₃, eluent - benzene: dioxane - 4:1).

Found, %: C 65., 39; H 6.15. C₁₉H₂₁ClFNO₂.

Calculated, %: C 65.23; H 6.05.

CONCLUSION:

Studies have been carried out on guinea pigs using the Bulbring and Wade method and rats using the “tail flick” method. The infiltration method is based on the principle of the summation of threshold mechanical stimuli applied by a certain rhythm and makes it possible to judge the intensity of the anesthetic effect. Each concentration was tested on 6 animals. The average values of the research results for 30 minutes were taken as the anesthesia index. Compounds and reference drugs (novocaine, lidocaine, trimecaine) were compared by the time of onset of anesthesia, the duration of complete anesthesia and the total duration of the anesthetic effect of the drug.

1) Infiltration anesthetic activity was studied by Bulbring-Wade method on guinea-pigs weighing 200-250g. In the back area of each animal, after removing hair from it, at 4 points (at the corners of a square with a side of 3cm) were intracutaneously injected in a volume of 0.2ml isotonic solutions of the studied compounds and reference drugs. Local anesthetic activity was evaluated 6-8 times for each of the selected concentrations. Sensitivity at the injection site was determined by touching a blunted injection needle, in a series of 6 touches at intervals of 3-4, every 5 minutes, for 30 minutes. The depth of anesthesia expressed in “anesthesia indices” (average of 6 experiments, maximum index - 36), duration of complete anesthesia, and total duration of anesthetic effect were determined. The activity of the compounds was compared with reference preparations - with trimecaine, lidocaine and novocaine at appropriate concentrations. Compound and comparision drugs were tested in 0.25% solutions.

2) Model of conduction anesthesia. The modified “tail flick” method of rats. The method of "tail flick" was developed at the Department of Pharmacology of the St. Petersburg Medical University named after academician I.P. Pavlov. It allows to determine the rate of onset of anesthesia, its depth, the duration of full anesthesia and the total duration of the anesthetic effect of the drug. The activity of compounds and reference agents was studied in 1% solutions. The study was conducted on outbred white male rats, weighing 200-250grams.

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Received on 16.08.2019 Modified on 04.10.2019

Research J. Pharm. and Tech 2020; 13(5): 2261-2265.

DOI: 10.5958/0974-360X.2020.00407.2