2- Or 3-Aryl Substituted Imidazo[1,2-A]pyridines

1. Field of the Invention

The present invention relates to novel 2- or 3-aryl substituted imidazo[1,2-a]pyridines as described further below. The substituted imidazo[1,2-a]pyridines are useful as local anesthetic agents, calcium channel blocking agents and antisecretory agents. Local anesthetics are known to exhibit antiarrhythmic activity. Calcium channel blocking agents are known to lower blood pressure. These compounds may further be useful in ophthamology.

2. Description of the Prior Art

No examples of local anesthetics with a 2- or 3-aryl substituted imidazo[1,2-a]pyridine structure have been seen in the prior art.

Local anesthetics are drugs which reversibly block nerve conduction near their site of application or injection and thus produce temporary loss of feeling or sensation in a limited area of the body. Local anesthetics are used to prevent pain in surgical procedures, injury, and disease. Local anesthetics can act on any part of the nervous system and on every type of nerve fiber. Since ionic mechanisms of excitability are similar in nerve and muscle, it is not surprising that local anesthetics also have prominent actions on all types of muscular tissue.

Local anesthetics prevent both the generation and the conduction of a nerve impulse. The main site of action is the cell membrane, and there is seemingly little direct action of physiological importance on the axoplasm. The axoplasmic effects that do occur may be secondary to the membrane action.

Known local anesthetics block conduction by interfering with the fundamental process in the generation of a nerve action potential, namely, the large transient increase in the permeability of the membrane to sodium ions that is produced by a slight depolarization of the membrane.

One theory of how local anesthetics block nerve conduction is that they compete with calcium at some site that controls the permeability of the membrane. Local anesthetics also reduce the permeability of resting nerve to potassium as well as to sodium ions.

Adverse reactions to local anesthetics can be divided into two groups: systemic and local adverse reactions. Systemic adverse reactions are usually associated with high blood levels of the drug and usually result from overdosage, rapid systemic absorption, or inadvertent intravenous administration. The reactions usually involve the central nervous and cardiovascular systems. Local adverse reactions to known local anesthetic drugs are either cytotoxic or allergic.

The present invention is directed to 2- or 3-aryl substituted imidazo[1,2-a]pyridines of the formula ##STR1## where X may be hydrogen, halogen, hydroxy, alkoxy having 1-3 carbon atoms, benzyloxy, or C.sub.1 -C.sub.6 alkyl either singularly or in combination;

R may be H or Ar;

R.sub.1 may be H, CH.sub.3 or Ar;

Ar may be ##STR2## Y may be C.sub.1 -C.sub.3 alkyl or C.sub.1 -C.sub.3 alkoxy; p may be 1 or 2;

R.sub.2 may be a C.sub.1 -C.sub.6 alkyl;

n may be 0 or 1 when R is Ar; or

n may be 1 when R.sub.1 is Ar; and

m may be 2-6, with the proviso that both R and R.sub.1 cannot be Ar at the same time and at least one of R and R.sub.1 is Ar.

The compounds of formula I are useful as local anesthetic agents, calcium channel blocking agents and antisecretory agents. Local anesthetics are known to exhibit antiarrhythmic activity. The compounds of formula I may further be useful in ophthamology.

The invention in its broadest aspects relates to 2- or 3-aryl substituted imidazo[1,2-a]pyridine compounds which have local anesthetic activity in mammals. The 2- or 3-aryl substituted imidazo[1,2-a]pyridine compounds of the invention demonstrating local anesthetic activity, calcium channel blocking activity and antisecretory activity are shown above.

The preferred compounds of the present invention are those wherein X is methyl; Y is methyl, methoxy or dimethoxy; R.sub.2 is butyl and m is 3.

The 2-aryl substituted imidazo[1,2-a]pyridine compounds where n is 0 are prepared in accordance with Scheme I. ##STR3##

In Scheme I, a substituted p-hydroxyacetophenone or p-hydroxypropiophenone, R.sub.1 is H or CH.sub.3 respectively, is treated with a 1-bromo-.omega.-chloro alkane such as 1-bromo-2-chloroethane, 1-bromo-3-chloropropane, 1-bromo-4-chlorobutane or 1-bromo-5-chloropentane or 1-bromo-6-chlorohexane by refluxing in an alcoholic base for about 12 to 48 hours to produce a p-chloroalkoxyphenone 1 as a liquid. The alcohol is preferably methanol, and the base may be potassium hydroxide or sodium hydroxide.

The p-chloroalkoxyphenone 1 is then reacted with bromine in either an ether solvent or glacial acetic acid or carbon disulfide. Suitable ethers include tetrahydrofuran, diethyl ether, or dimethoxy ether. The reaction takes place at a temperature of about 10.degree. C. to 65.degree. C. for about 2 to 24 hours to produce .alpha.-bromo-ketone 2.

The .alpha.-bromoketone 2 is then subjected to a condensation reaction with 2-aminopyridine or a substituted 2-aminopyridine in an alcoholic solvent. The condensation is conducted at about 65.degree. C. to 86.degree. C. for about 2 to 24 hours to yield a chloroalkoxyphenol imidazopyridine 3. Suitable substituted 2-aminopyridines which may be utilized in the condensation reaction include 3-methyl-2-aminopyridine, 5-bromo-2-aminopyridine, 4-methyl-2-aminopyridine, 3-benzyloxy-2-aminopyridine, 3-hydroxy-2-aminopyridine, and 4,6-dimethyl-2-aminopyridine. The alcoholic solvent may be methanol, ethanol or isopropanol.

The chloroalkoxyphenyl imidazopyridine 3 is treated with an amine solvent, such as dimethylamine, diethylamine, dipropylamine, dibutylamine, dipentylamine or dihexylamine at about 100.degree. C. to 150.degree. C. for 4 to 64 hours to yield the 2-aryl substituted imidazo-[1,2-a]pyridine 4.

The 2-aryl substituted imidazo[1,2-a]pyridines where n is 1 are produced according to Scheme II. ##STR4##

The .alpha.-bromoketone 2 which may be prepared as described in Scheme I, is treated in a polar solvent with an aqueous solution of a base, such as sodium hydroxide, at about 20.degree. C. to 50.degree. C. to produce .alpha.-hydroxyketone 5. Suitable polar solvents include dimethylformamide, dimethylsulfoxide, hexamethyl phosphoramide and N-methyl pyrrolidone.

The .alpha.-hydroxyketone 5 is then oxidized with an oxidizing agent, such as pyridinium chlorochromate, chromium trioxidepyridine, dimethyl sulfoxide-oxalyl chloride or chromic acid-sulfuric acid, in an inert solvent to yield a diketone 6. Suitable inert solvents include methylene chloride, chloroform and acetone.

The diketone 6 is reacted with bromine in either an ether solvent glacial acetic acid or carbon disulfide at about 10.degree. C. to 65.degree. C. for about 2 to 24 hours to produce .alpha.-bromodiketone 7. Suitable ethers include tetrahydrofuran, diethyl ether or dimethoxyether.

The .alpha.-bromodiketone 7 is subjected to the condensation as previously described in Scheme I to produce compound 8 which is reacted with an amine solvent as described in Scheme I to yield the 2-aryl substituted imidazo[1,2-a]pyridines 9.

The 3-aryl substituted imidazo[1,2-a]pyridines where n is 1 are produced in accordance with Scheme III which follows. ##STR5##

In Scheme III, 2-aminopyridine or a substituted 2-aminopyridine, such as 3-methyl-2-aminopyridine, 5-bromo-2-aminopyridine, 4-methyl-2-aminopyridine, 3-benzyloxy-2-aminopyridine, 4,6-dimethyl-2-aminopyridine or 3-hydroxy-2-aminopyridine is reacted with dimethylformamide, dimethylacetal or triethyl orthoformate in an inert solvent at about 60.degree. C. to 120.degree. C. for about 4 to 12 hours to produce an amidine 10. Suitable inert solvents include benzene, toluene, xylenes or acetone.

The amidine 10 is then subjected to a condensation reaction with a substituted .alpha.-bromoketone 11 in an alcoholic solvent, such as methanol, ethanol or isopropanol at about 60.degree. C. to 85.degree. C. for about 2 to 24 hours to yield a 3-aryl substituted imidazopyridine 12 which is a solid. The .alpha.-bromoketone 11 can be produced by reacting p-hydroxyacetophenone with bromine in either an ether solvent or glacial acetic acid or carbon disulfide at a temperature range of about 10.degree. C. to 65.degree. C. for approximately 2 to 24 hours. Suitable ethers include tetrahydrofuran, diethyl ether or dimethoxy ether.

The 3-aryl substituted imidazopyridine 12 is alkylated with a chloroalkyl dialkylamine to yield the 3-aryl substituted imidazo-[1,2-a]pyridine 13. The reaction is conducted in an alcoholic base such as potassium hydroxide in methanol and in the presence of catalytic iodine at a temperature of about 60.degree. C. to 80.degree. C. for about 8 to 9 hours. The chloroalkoxy dialkylamine used in the reaction is prepared by treating a 1-bromo-.omega.-chloroalkane with a dialkylamine at about 100.degree. C. to 150.degree. C.

For topical administration of the compounds of formula I as a local anesthetic, the carrier may take a wide variety of forms depending on the form of preparation, such as creams, dressings, gels, lotions, ointments or liquids. The 2- or 3-aryl substituted imidazo[1,2-a]pyridine will be present in the pharmaceutical composition from about 1% by weight to about 10% by weight, depending on the particular form employed.

An injectable form of the 2- or 3-aryl substituted imidazo-[1,2-a]pyridine is usually administered intradermally, subcutaneously, or submucosally across the path of nerves supplying the area to be anethesized. The injection may also be given intramuscularly. The 2- or 3-aryl substituted imidazo-[1,2-a]pyridines will be present in an injectable pharmaceutical composition from about 0.1% by weight to 10% by weight. The injectable preparation may also contain isotonicity adjusting agents such as sodium chloride, pH adjusting agents such as hydrochloric acid and preservatives such as methylparaben. Injectable preparations may be in the form of solutions or suspensions.

The pharmaceutical compositions for local anesthetic use described above, containing a compound of the present invention as the active ingredient in intimate admixture with a pharmaceutical carrier, can be prepared according to conventional pharmaceutical compounding techniques.

Pharmaceutical compositions for calcium channel blocking and antisecretory uses, containing a compound of the present invention as the active ingredient in intimate admixture with a pharmaceutical carrier, can be prepared according to conventional pharmaceutical compounding techniques. The carrier may take a wide variety of forms depending on the form of preparation desired for administration, e.g., intravenous, oral or parenteral. In preparing the compositions in oral dosage form, any of the usual pharmaceutical media may be employed, such as, for example, water, glycols, oils, alcohols, flavoring agents, preservatives, coloring agents and the like in the case of oral liquid preparations (such as, for example, suspensions, elixirs and solutions); or carriers such as starches, sugars, diluents, granulating agents, lubricants, binders, disintegrating agents and the like in the case of oral solid preparations (such as, for example, powders, capsules and tablets). Because of their ease in administration, tablets and cpasules represent the most advantageous oral dosage unit form, in which case solid pharmaceutical carriers are obviously employed. If desired, tablets may be sugar-coated or enteric-coated by standard techniques. For parenterals, the carrier will usually comprise sterile water, though other ingredients, for example, to aid solubility or for preservative purposes, may be included. Injectable suspensions may also be prepared, in which case appropriate liquid carriers, suspending agents and the like may be employed. For these utilities, the pharmaceutical compositions will generally contain per dosage unit, e.g., tablet, capsule, powder, injection, teaspoonful and the like, from about 1.0 to about 100 mg/kg, and preferably from about 5 to about 25 mg/kg of the active ingredient.

The following examples describe the invention in greater particularity and are intended to be a way of illustrating but not limiting the invention.

To a mixture of p-hydroxyacetophenone (50.7 g, 0.37 mol) and 1-bromo-3-chloropropane (160 ml, 1.5 mol) in methanol (250 ml) was added portionwise potassium hydroxide (63 g, 1.12 mol). The mixture was stirred at reflux for 24 hours, cooled to room temperature, filtered through Celite and evaporated in vacuo. The residual semi-solid was diluted with diethyl ether (500 ml) and washed with H.sub.2 O (2.times.300 ml). The ether solution was dried over MgSO.sub.4, filtered and evaporated in vacuo to give p-chloroproxy acetophenone as a liquid in 68% yield (53.38 g). .sup.1 H NMR (CDCl.sub.3): .delta. 7.98-7.89 (d, J=8.9 Hz, 2H), 7.02-6.92 (d, J=8.9 Hz, 2H), 4.16 (t, J=5.9 Hz, 2H), 3.75 (t, J=6.4 Hz, 2H), 2.52 (s, 3H), 2.34-2.16 (m, 2H).

To a stirred solution of p-chloropropoxyacetophenone (53.3 g, 0.25 mol) in diethyl ether (250 ml) was slowly added bromine (13 ml, 0.25 mol) and allowed to stir at room temperature for 16 hours. The dark mixture was poured into an aqueous saturated sodium bicarbonate solution (300 ml) and the organic layer separated. The ether layer was washed with an aqueous saturated sodium bicarbonate solution (300 ml) and with water (300 ml) and dried over MgSO.sub.4. The solution was filtered and evaporated in vacuo to yield .alpha.-bromo-4-chloropropoxy acetophenone (64.4 g, 88% yield) as a dark oil. .sup.1 H NMR (CDCl.sub.3): .delta. 7.96 (d, J=8.9 Hz, 2H), 6.95 (d, J=8.9 Hz, 2H), 4.41 (s, 2H), 4.19 (t, 2H), 3.75 (t, 2H), 2.26 (m, 2H).

A mixture of .alpha.-bromo-4-chloropropoxy acetophenone (12.8 g, 44 mmol) and 2-aminopyridine (4.0 g, 44 mmol) in ethanol (80 ml) was stirred at reflux for 3 hours, cooled at room temperature and filtered to give 2-(4-chloropropoxyphenyl)imidazo[1,2-a]pyridine (5.2 g, 32% yield) as a white solid. .sup.1 H NMR (CD.sub.3 OD): .delta. 8.81 (d, J=6.7 Hz, 1H), 8.51 (s, 1H), 7.97-7.01 (m, 7H), 4.19 (t, J=5.9 Hz, 2H), 3.79 (t, J=6.3 Hz, 2H), 2.25 (m, 2H).

A suspension of 2-(4-chloropropoxyphenyl)imidazo[1,2-a]-pyridine (5.2 g, 14 mmol) in dibutylamine (30 ml) was stirred at reflux for 5 hours. The excess dibutylamine was removed by distillation and the resulting oil was flash chromatographed (silica gel, 9:1 CH.sub.2 Cl.sub.2 :acetone) to give the free base of the title compound (5.1 g, 93% yield) as an oil. The HCl salt was prepared by dropwise addition of concentrated hydrochloric acid to a solution of the free base in methanol, concentrated and recrystallized from methanol acetone to yield 2-(4-dibutylaminopropoxyphenyl)imidazo[1,2-a]pyridine as a white crystalline solid, mp 179.degree. C. to 183.degree. C. IR(KBr): 3400, 2620, 1650, 1620 cm.sup.-1. MS: 380 (MH.sup.+). .sup.1 H NMR (CD.sub.3 OD): .delta. 8.80 (d, J=8 Hz, 1H), 8.52 (s, 1H), 7.92-7.49 (m, 5H), 7.18 (d, J=8 Hz, 2H), 4.23 (t, J=4.6 Hz, 2H), 3.50-3.13 (m, 6H), 2.25 (m, 2H), 1.82-1.34 (m, 8H), 1.01 (m, 6H).

Theor. C.sub.24 H.sub.33 N.sub.3 O.3HCl: C, 58.95; H, 7.42; N, 8.59. Found: C, 59.25; H, 7.71; N, 9.03.

When in the above procedure 2-amino-6-picoline was used as the starting material, the hydrochloride salt of 2-(4-dibutylaminopropoxyphenyl)-5-metylimidazo[1,2-a]pyridine, mp 221.degree. C. to 223.degree. C., was obtained as the product.

When in the above procedure p-hydroxypropiophenone is employed as the starting material, 2-(4-dibutylaminopropoxyphenyl)-3-methylimidazo[1,2-a]pyridine is obtained as the resultant product.

The title compound was prepared as described in Example 1 using 4'-hydroxy-3'-methylacetophenone and 3-methyl-2-aminopyridine and was converted to the HCl salt, mp 63.degree. C. to 66.degree. C. IR(KBr): 3410, 2670, 1645 cm.sup.-1. MS: 408(MH.sup.+).

Theor. C.sub.26 H.sub.37 N.sub.3 O.3HCl.1/2H.sub.2 O: C, 59.37; H, 7.86; N, 7.99. Found: C, 59.22; H, 7.56; N, 8.04.

When in the above procedure dipropylamine is employed as the starting material, the corresponding hydrochloride salt of 2-(4-(3-dipropylaminopropoxy)-3-methylphenyl)-8-methylimidazo[1,2-a]-pyrid ine was obtained as the resultant product.

When in the above procedure 1-bromo-4-chlorobutane is used as the starting material, the hydrochloride salt of 2-(5-(4-dibutylaminobutoxy)-3-methylphenyl)-8-methylimidazo[1,2-a]pyridine is obtained.

The title compound was prepared as described in Example 2 using 4'-hydroxy-2'-methylacetophenone and converted to the HCl salt, mp 78.degree. C. to 81.degree. C. IR(KBr): 3419, 2640, 1615 cm.sup.-1. MS: 408(MH.sup.+).

Theor. C.sub.26 H.sub.37 N.sub.3 O.3HCl.H.sub.2 O: C, 58.37; H, 7.91; N, 7.85. Found: C, 58.43; H, 8.01; N, 7.85.

The title compound was prepared as described in Example 2 using 4'-hydroxy-3',5'-dimethoxyacetophenone and converted to the HCl salt, mp 178.degree. C. to 180.degree. C. IR(KBr): 3415, 2700, 1625 cm.sup.-1. MS: 454(MH.sup.+).

Theor. C.sub.27 H.sub.39 N.sub.3 O.sub.3.3HCl: C, 57.60; H, 7.52; N, 7.46. Found: C, 57.40; H, 7.27; N, 7.27.

The title compound was prepared as described in Example 2 using 4'-hydroxy-3'-methoxyacetophenone and converted to the HCl salt, mp 127.degree. C. to 130.degree. C. IR(KBr): 3420, 2700, 1625 cm.sup.-1. MS: 424(MH.sup.+).

Theor. C.sub.26 H.sub.37 N.sub.3 O.sub.2.3HCl: C, 58.59; H, 7.56; N, 7.88. Found: C, 58.77; H, 7.59; N, 7.56.

When in any of the above procedures 1-bromo-2-chloroethane, 1-bromo-4-chlorobutane or 1-bromo-5-chloropentane is used in place of 1-bromo-3-chloropropane, the corresponding 2-(4-(2-dibutylaminoethoxy)-3-methylphenyl)-substituted imidazo[1,2-a]pyridines, 2-(4-(4-dibutylaminobutoxy)-3-methylphenyl)-substituted imidazol[1,2-a]pyridines or 2-(4-(5-dibutylaminopentoxy)-3-methylphenyl)-substituted imidazo[1,2-a]pyridines are obtained.

When in any of the above procedures dimethylamine, diethylamine, dipropylamine, dipentylamine or dihexylamine is used in place of dibutylamine, the corresponding 2-(4-(3-dimethyl-, 2-(4-(3-diethyl-, 2-(4-(3-dipropyl-, 2-(4-(3-dipentyl- or 2-(4-(3-dihexylaminopropoxy)-3-methylphenyl)-substituted imidazo[1,2-a]pyridine derivatives are obtained.

The local anesthetic activity of the above compounds was tested as follows.

The test compounds were dissolved or suspended in a 0.5% aqueous methylcellulose solution containing 0.4% (v/v) of Tween 80, the polyoxyethylene derivative of a sorbitan ester. Doses of up to 100 mg/kg were administered orally by gavage tube to groups of three male albino overnight-fasted mice (18 to 24 g) which were observed intermittantly for one hour. The mice were gently restrained and 0.05 ml of a 1% (w/v) solution or suspension of the test compound was injected into the quadriceps femoris muscle of one hind leg. Five minutes later, the mice were individually placed on a wire mesh screen. The wire mesh screen was then inverted. Compounds that possess local anesthetic activity impaired the ability of the mice to grasp the inverted screen with the injected leg. The response to the test compounds was compared to a similarly treated vehicle control group of mice.