Synthesis of Carfentanil

"For instance, carfentanil is approximately 4000 times as potent as heroin and has an extremely favorable therapeutic index [...]. Hence, an easy week's work for two chemists could provide 1 (one) kilogram of carfentanil which would be equivalent to four metric tons of pure heroin"

Donald A. Cooper (DEA) in "Future Synthetic Drugs of Abuse"

Very, very good idea, Mr. Cooper. Thanks, DEA!

Carfentanil synthesis [US Pat. 4,179,569]

To a mixture of 102 parts of phenethyl-piperidone-4, 47 parts of aniline and 370 parts of acetic acid is added dropwise a soln of 36 parts of KCN in 100 parts of water, at a temperature between 35-45°C (exotermic reaction). After the addition complete, the cooling-bath is removed and the whole is stirred for 20 hrs at room temperature. The reaction mixture is poured into 650 parts of ammonium hydroxide and 500 parts of crushed ice are added. The mixture is extracted with chloroform. The organic extract was dried over potassium carbonate, filtered and evaporated. The resudie (solid) was triturated in diisopropylether (DIPE). After keeping at RT, 4-anilino-4-cyano-1-(2-phenylethyl)-piperidine (I) is obtained; mp 120-121°C.

To 4500 parts of conc. H2SO4 are added portionwise 710 parts of (I), while keeping the T below 25 C. Upon completion, stirring is continued overnight at RT. The reaction mixture is poured onto a mixture of 10.000 parts of crushed ice and 3600 parts of ammonium hydroxide. The product was extracted with chloroform, extract dried, filtered and evaporated. The residue is stirred in 140 parts of DIPE, cooled, the product filtered off and dried, yielding 4-(phenylamino)-1-(2-phenylethyl)-4-piperidinecarboxamide (II), mp 182.5°C

A mixture of 105 parts of (II), 53.7 parts of KOH and 275 parts of ethylene glycol is stirred and refluxed for 20 hrs. After cooling the RM is poured onto 1000 parts of water and the whole is filtered. The filtrate is strongly acidified with HCl soln till the formed precipitate enters solution. Then the soln is strongly alkalized with a conc NaOH soln (exotermic rxn) and filtered hot.The sodium salt is allowed to crystallize from the filtrate. It is filtered off and recryst from water, yielding 4-(phenylamino)-1-(2-phenylethyl)-4-piperidinecarboxylic acid, sodium salt (III), mp >300°C (dec)

A solution of 62.3 parts of (III), in 340 parts of HMPA (hexamethylphosphoric triamide, can be subst. to DMSO) is stirred and heated to 100°C. After cooling to 10°C, there are added dropwise 28.4 parts of iodomethane, (slightly exotermic reaction). Upon completion, stirring is continued for 24 hrs at RT. Then the reaction mixture is poured onto water (800 parts) and product is extacted with toluene. The extract is washed with water, dried, filtered and evaporated. The oily residue solidifies on trituration in DIPA, yielding methyl 4-(phenylamino)- 1-(2-phenethyl)-4-piperidinecarboxylate (IV), mp 94.9 C.

A mixture of 33.8 parts of (IV), and 100 parts of propionic anhydride is stirred and refluxed for 6.5 hrs. The reaction mixture is allowed to cool overnight to RT while stirring and poured onto ice-water. The product is extracted with toluene, extract washed with water, dried, filtered and evaporated. The residue converted onto the citrate salt in IPA and diethyl ether. The oily salt is triturated in a mixture of IPA and ether, the solid product filtered off and recryst. twice first from IPA, then from acetone, dried in vacuo at 100 C, yielding carfentanil citrate, mp 151-154°C. For oxalate salt ED50 = 0.0006 mg/kg i/v, this mean that for 100-kg body you need only 0.06 mg.

FYI, for 3-methylfentanyl (ED50=0.00058 mg/kg i.v.) look up is synthesis in J. Med. Chem. vol 17, No. 10, p. 1047 (1974).

Carfentanil synthesis [US Pat. 5,106,983]

A solution of 60.5 g (0.929 mole) of potassium cyanide in 181 mL of water was added slowly to a stirred solution of 127 g (0.625 mole) of 1-(beta-phenethyl)-4-piperidone and 87.1 g (0.935 mole) of aniline in 635 mL of glacial acetic at 25°-30°C. After stirring for a total of 45 hours at room temperature, the reaction mixture was poured into a mixture of 900 g of ice and 1610 mL of concentrated ammonium hydroxide with stirring for two hours during which a brown solid precipitated out. The solid was filtered and washed with water. Recrystallization from isopropanol gave a total of 125 g (66% yield) of product, 1-(beta-phenethyl)- 4-((N-phenylamino)-4-piperidine carbonitrile, as tan crystals, m.p. 119°-120°C.

Formic acid (600 mL was added to 600 mL of acetic anhydride at a rate such that the temperature of the mixture did not exceed 42°C. To this solution was added 119 g (0.39 mole) of 1-(beta-phenethyl)- 4-(N-phenylamino)-4-piperidine carbonitrile at 5-10°C. with stirring. After standing at room temperature overnight, the reaction mixture was poured into ice-water and made alkaline to pH 7.4 with dilute sodium hydroxide solution during which a tan solid precipitated out. The solid was filtered and washed with water. Recrystallization from methanol gave 100.4 g (77.3% yield) of product 1-(beta-phenethyl)-4-(N-formyl- N-phenylamino)-piperidine carbonitrile, as white crystals, m.p. 136-138°C.

To a suspension of 100 g (0.30 mole) of 1-(beta-phenethyl)-4-(N-formyl-N-phenylamino)-piperidine carbonitrile m 1.0 L of anhydrous methanol was added slowly a solution of 670 g (18.4 mole) of hydrogen chloride in 2.0 L of anhydrous methanol at 3-10°C. The resulting yellow solution was refluxed two hours, and then the methanol was allowed to distill off until 1.7 L was collected over the next three hours. During this time a white solid precipitated out. The reaction mixture was cooled and filtered, yielding 79 g (64.2% yield) of product, methyl 1-(beta-phenethyl)-4-(N-phenylamino)-4-piperidineimidate dihydrochloride, as white solid, m.p. 196-198°C (dec).

A suspension of 79 g (0.19 mole) of methyl 1-(beta-phenethyl)-4-(N-phenylamino)-piperidineimidate dihydrochloride in 675 ml of water was made alkaline to pH 9 with dilute sodium hydroxide solution. The solid was filtered and washed with water. The solid was dried in a vacuum desiccator to give 61.5 g (98.8% yield) of product, 1-(beta-phenethyl)-4-(phenylamino)- piperidine carboxamide, as a white solid m.p. 181-183°

A solution of 28.4 g (0.088 mole) of 1-(beta-phenethyl)-4-(N-phenylamino)-4-piperidine carboxamide and 17 g of potassium hydroxide in 114 mL of ethylene glycol was allowed to reflux for 3 hours. The reaction mixture was poured into 228 mL of ice-water and made acidic to pH 6-6.5 with concentrated hydrochloric acid during which a tan solid precipitated out. The solid was filtered and washed with cold water. The solid was suspended in benzene and water was removed by azeotropic distillation. The mixture was cooled and filtered yielding 28 g (98.3% yield) of product, 1-(beta-phenethyl)-(N-phenylamino)-piperidinecarboxylic acid, as a white solid, m.p. 254-255°C (dec).

Concentrated sulfuric acid (12 mL) was added slowly to a stirred suspension of 25.6 g (0.079 mole) of 1-(beta-phenethyl)-4-(N-phenylamino)-4-piperidine carboxylic acid in 95 mL of anhydrous methanol. The resulting solution was refluxed for a total of 73 hours. The reaction mixture was cooled and poured into 1 L of icewater, upon which a gummy brown solid precipitated out. The mixture was made basic to pH 7.4 with dilute sodium hydroxide solution and extracted with methylene chloride. The extracts were dried over magnesium sulfate and evaporated to give 21 g of crude product which was recrystallized from N-hexane to give 19.0 g (71.1% yield) of desired product, methyl 1-(beta-phenethyl)-4-(N-phenylamino)-4-piperidine carboxylate, as white crystals, m.p. 92-93°C.

A mixture of 10.0 g (0.03 mole) of methyl 1-(beta-phenethyl)-(N-phenylamino)-piperidine carboxylate and 100 g (0.77 mole) of propionic anhydride was heated to reflux for 6 hours. Most of the propionic anhydride was then removed by distillation under reduced pressure. The residue was slurried in about 100 mL of ice-water and made alkaline to pH 8 with ammonium hydroxide. The mixture was extracted with chloroform and the extracts dried over magnesium sulfate and evaporated under reduced pressure. The residue, which weighed about 12 g, was dissolved in 50 mL of isopropanol and treated with a solution of 3.8 g (0.03 mole) of oxalic acid dissolved in 50 mL of isopropanol. The product was allowed to crystallize out overnight at room temperature. When filtered and dried, there was obtained 12.2g (85% yield) of carfentanil oxalate, or methyl 1-(beta-phenethyl)-(N- propionyl-N-phenylamino)-4-piperidine carboxylate oxalate, m.p. 183-185°C.