Abstract:
The present invention is directed to an improved stereoselective tandem substitution and reduction of an imine precursor to the cis form of 3-substituted-4-(phenylanilo)piperidine compounds or their substituted derivatives.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a stereoselective tandem substitution and reduction reaction of 4-(cinalino) piperidine derivatives. The resulting 3,4-cis compounds are useful intermediates leading to intravenous analgesics. 
     U.S. Pat. No. 4,584,303 discloses a method of preparing compounds of the following formula: ##STR1## wherein R 1  is a lower alkyl, lower cyclic alkyl, lower alkyl lower cyclic alkyl, lower alkenyl, lower alkyl phenyl; R 2  is phenyl or aromatic heterocycle ring either of which may be substituted; and R 3  may be a variety of groups including alkyl phenyl. With respect to the R 1  and NHR 2  substitutents, it is desirable to obtain the more pharmacologically active cis isomer. 
     The method taught in U.S. Pat. No. 3,584,303 for preparing compounds of formula (I) requires a multi-step synthetic route involving the preparation of cis isomers at an advanced stage of the synthetic route. Furthermore, a cis:trans ratio of not greater than 70:30 can be achieved. 
     J. Burke, Jr. et al., J. Med. Chem., Vol. 29, No. 6, pp 1087-1093 (1986), discloses a method to prepare piperidine derivatives, which method suffers the same disadvantages of the above-mentioned prior art method. Furthermore, the multi-step method of Burke et al. requires expensive starting materials and protection of the piperidine nitrogen. 
     The deprotonation of aliphatic imines followed by alkylation is generally known as, for example, shown by Collum et al., J. Am. Chem. Soc., Vol. 106, pp 4865-4869 (1984). The reduction of an imine is in itself also conventional. 
     SUMMARY OF THE INVENTION 
     The invention is directed to a method of preparing a compound of the following formula: ##STR2## wherein R 1  is lower alkyl, lower cylic alkyl, lower alkyl lower cyclic alkyl, lower alkenyl, or lower alkyl phenyl; R 2  is a phenyl or aromatic heterocycle ring either of which may be substituted with one or more groups selected from the group consisting of lower alkoxy, lower alkyl, halogen, or combinations thereof; and R 3  can be a wide variety of groups, for example, selected from phenyl lower alkyl, thiazolyl lower alkyl, which can be substituted in the 4-position with a lower alkyl group, (4,5-dihydro-5-oxo-1H-tetrazol-1-yl) lower alkyl, which can be substituted in the 4-position with a lower alkyl, and substituted phenyl lower alkyl in which the substituents on the phenyl ring are selected from halogen, lower alkoxy, lower alkyl or combinations thereof. 
     According to the method of the present invention, an aryl imine precursor is capable of being stereoselectively substituted in a single reactor without requiring intermediate separations or purifications. Readily available starting materials are utilized and no special protection of the piperidine nitrogen is necessary. Very high selectivity of the cis isomer (typically 96%) can be achieved, resulting in an easier purification at an early stage of the overall synthetic route leading to the final product. Production of the nonrecyclable trans isomer is minimized. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Applicants have found an improved method for the efficient and economical preparation of compounds according to formula (I) below: ##STR3## wherein R 1  is lower alkyl, lower cyclic alkyl, lower alkyl lower cyclic alkyl, lower alkenyl, or lower alkyl phenyl; R 2  is a phenyl or aromatic heterocycle ring either of which may be substituted with one or more groups selected from the group consisting of lower alkoxy, lower alkyl, halogen, or combinations thereof; and R 3  is selected from a wide variety of groups, including phenyl lower alkyl, thiazolyl lower alkyl, which can be substituted in the 4-position with a lower alkyl group, (4,5-dihydro-5-oxo-1H-tetrazol-1-yl) lower alkyl, which can be substituted in the 4-position with a lower alkyl, and substituted phenyl lower alkyl in which the substituents on the phenyl ring are selected from halogen, lower alkoxy, lower alkyl or combinations thereof. 
     By the term lower alkyl, lower alkoxy, lower cyclic alkyl, or lower alkenyl are meant branched or unbranched chain groups with seven or less carbon atoms and preferably 5 or less carbons. By aromatic heterocycle ring is meant a fully unsaturated ring having 5 or 6 members including 0-3 nitrogen atoms and 0-2 oxygen or sulfur atoms. The term halogen is meant to include fluoro, chloro, bromo, or iodo substituents. 
     Examples of suitable R 1  groups include methyl, ethyl, cyclopropyl, cyclopropylmethyl, allyl and benzyl. 
     Examples of suitable R 2  groups include phenyl, 2-fluorophenyl, 2-methylphenyl, 2-methoxyphenyl, pyrazolyl, or thienyl. 
     Examples of suitable R 3  groups include phenyl ethyl, 2-(4-ethyl-4-4,5-dihydro-5-oxo-1H-tetrazol-1-yl)ethyl, and 2-(4-methyl-thiazoyl)ethyl. 
     The route to compounds of the formula (I) proceeds as follows: ##STR4## wherein R 1 , R 2  and R 3  are as defined above and R 1  X is a reagent capable of adding the R 1  group to the 3 position of the piperidine ring, for example, a lower alkyl iodide alkylating agent. 
     It is noted that the present invention does not involve the N-substituted-3-substituted-4-piperidone intermediate compound conventionally employed, for example, in the prior art synthetic route of U.S. Pat. No. 4,584,303. This latter compound is difficult to make. Instead, Applicants&#39; above described reaction scheme begins with the intermediate compound N-substituted-4-piperidone, which is commercially available from Aldrich Chemical Co. (Milwaukee, Wisc.) or Chemical Dynamics Corp. (So. Plainfield, N.J.) or else easily made by a Mannich condensation, as will be readily appreciated by those skilled in the art. The latter piperidone compound is then reacted with a substituted or unsubstituted phenyl amine or aromatic heterocyclic amine to form the imine precursor (III) above. This reaction is suitably carried out in the presence of p-toluene sulfonic acid and refluxed while collecting water. The imine precursor (III) is then stirred at a low temperature with a non-nucleophilic base such as freshly prepared lithium diisopropyl amide in order to deprotonate the imine. Other suitable non-nucleophilic bases include LDC (lithium dicyclohexylamide), LIC (lithium isopropylcyclohexylamide), LTP (tetramethylpiperamide) and LHS [lithium bis(trimethylsilyl)]amide. 
     By removal of the alpha (to the schiff base) proton, the following anion is generated: ##STR5## 
     Addition of a substituting agent R 1  X such as the alkylating agent alkyl iodide results in the anion IIIa attaching the electrophile R 1  X to form the following alkylated imine intermediate. ##STR6## 
     The most preferred substituting agents are iodides, bromides and chlorides. Tosylates and mesylates and other substituting agents known to those skilled in the art are also suitable. 
     Finally, without the necessity of isolating (IIIb), a very selective reduction to the desired valuable intermediate (IV) above is carried out. This reduction is a steric approach control reaction wherein the least crowded side of the intermediate (IIIb) is preferably attached by a relatively highly hindered reducing agent such as the hindered borohydride reducing agents. 
     The most preferred reducing agent is &#34;Super-Hydride&#34; because of its superior combination of cis selectivity and speed. Other preferred hindered borohydride reducing agents include L-selectride (lithium tri-sec-butylborohydride, 1.0 molar in THF) and Nβ-Enantride (lithium hydride, a 9-ββN-nopolbenzyl ether adduct, 0.5 molar in THF). A cis:trans selectivity of greater than 90 percent and typically 96 percent is thereby obtainable. Other suitable reducing agents include Red-Al (sodium bis(2-methoxy-ethoxy)aluminum hydride, 3.4 molar in toluene, and LiAlH 4 . Typically, a selectivity of only about 60-70 percent cis:trans is obtained with the latter reducing agents. All of the above named reducing agents are commercially available from Aldrich Chemical Co. (Milwaukee, Wisc.). 
     A major advantage of the above described synthetic route is that the reactions occur in a single vessel by the sequential introduction of the necessary reactants. Consequently, successive isolations and purifications are obviated, thereby greatly simplifying the overall route to a variety of therapeutic agents, notably of the fentanyl type, which are used in humans for inducing anesthesia or analgesia, for example A-3331. The remaining steps in the overall synthesis to such compounds are disclosed in detail in U.S. Pat. No. 4,584,303 to Huang et al., which hereby is incorporated by reference. Other compounds which may be prepared by a synthetic route within the scope of the present invention include β-hydroxyl-3-methylfentanyl and cis-3-methyl fantanyl (known in the chemical and pharmaceutical literature as F-7302 and R 26,800, respectively) which have demonstrated potent analgesic activity. 
    
    
     EXAMPLE 1 
     This example illustrates the preparation of 1-benzyl-3-(cis-methyl)-4-[N-(20fluorophenyl)amino]piperidine. A solution of 1-benzyl-4-piperidone, 15.20 gms (80.31 mmol), 2-fluoroaniline, 9.02 gms (81.17 mmol) and p-toluenesulfonic acid monohydrate, 0.48 gms in 200 ml of toluene was refluxed overnight under argon collecting water in a Dean-Stark trap. After 18 hours the theoretical amount of water had separated and the trap was drained several times to distill off 100 mls of toluene. The reaction was cooled under argon and diluted with 40 mls anhydrous THF. The deep orange solution of crude imine 1 was used &#34;as is&#34; for next step. ##STR7## 
     LDA was prepared under an atmosphere of argon by adding freshly distilled diisopropyl amine, 15 ml (107 mmol) dropwise to a cold (0° C.) solution of 55 mls of 1.6M n-butyllithium (88 mmol) in hexane in 60 mls of anhydrous THF. After 15 minutes at 0° C. the LDA solution was cooled to -78° C. in an isopropanol-dry ice slush bath. The solution of imine 1 (80.31 mmol) was added dropwise with stirring. The reaction was stirred at -78° C. for 1 hour followed by rapid addition of methyl iodide, 12.9 gms (90.88 mmol). The reaction was stirred at -78° C. for 15 minutes followed by warming to room temperature. After 10 minutes at -78° C. a precipitate formed which redissolved as the reaction warmed up. After stirring at room temperature for 2 hours the reaction was cooled to 0° C. in an ice bath and 120 mls 1.0M Lithium triethlyborohydride in THF was added dropwise via syringe. The reaction was stirred overnight (16 hours) warming to room temperature. The reaction was then cooled back down to 0° C. and 50 mls of water was added slowly dropwise (exotherm and vigorous gas evolution). The quenched reaction mixture was concentrated to a pasty oil which was dissolved in 200 mls toluene and washed with 100 mls of water. The toluene layer was separated, dried over anhydrous sodium sulfate and concentrated to give a crude mixture of cis and trans diamine. Analytical LC showed the cis/trans diamine ratio to be 92.2:7.8. This crude oil was flash chromatographed on Silica 60, 230-400 mesh (400 gms) eluting with 1:10 EtOAc/Hex with 0.1% ammonium hydroxide added to give 9.74 gms pure cis diamine (40.6%) shown below and 1.57 gms cis and trans diammine mixture. Total isolated yield of cis and trans diammine was 47.2%. ##STR8## 
     M.P.: 55°-56° C. 
     NMR: 7.40(s,5H), 7.20-6.35(m,4H), 3.90(br s,1H), 3.40 (br s,2H), 3.00-1.30(complex,8H), 0.95(d,3H) 
     EXAMPLE 2 
     In an analogous manner to the synthesis of Example 1, except using allyl bromide as the substituting agent, the compound 1-benzyl-3-(cis-allyl)-4-[N-(2-fluoro-phenyl)amino]piperidine was prepared having the following formula: ##STR9## oil at room temperature 
     NMR: 7.30 (m,5H), 6.95(m,2H), 6.60(m,2H), 5.65(m,1H), 4.45(m,2H), 3.95(br 2,1H), 3.60(m,1H), 3.50 (ABq,2H), 2.60-1.65(complex,9H) 
     EXAMPLES 3-5 
     In an analogous manner to the synthesis of Example 1, except using 1-iodopropane, benzyl bromide, and (bromomethyl)-cyclopropane, respectively as the substituting agent, the following compounds were prepared: ##STR10## 1-benzyl-3-[cis-(n-propyl)]-4-[N-(2-fluorophenyl)amino]-N piperidine oil at R.T. 
     NMR: 7.40(s,5H), 7.40-6.50(m,4H), 3.95(br s, 1H), 3.60(m,1H), 3.50(ABq,2H), 2.60-0.30(complex,14H) ##STR11## 1-benzyl-3-(cis-benzyl)-4-[N-(2-fluorophenylamino)]piperidine oil at R.T. 
     NMR: 7.40-6.40(complex,14H), 4.05(m,1H), 3.50(br s,2H). 3.85-1.45(complex,10H). ##STR12## 1-benzyl-3-[cis-(cyclopropyl)methyl]-4-[N-(20fluorophenylamino)]piperidine oil at R.T. 
     NMR: 7.50-6.50(m,9H), 4.35(m,1H), 3.40(br s,2H) 3.35-0.80(complex,10H), 0.60-0.05(m,5H) 
     It will be understood that the embodiments described herein are merely exemplary and that a person skilled in the art may make variations and modifications without departing from the spirit and scope of the invention. All such modifications and variations are intended to be included within the scope of the invention as defined in the appended claims.