Process for the preparation of (S)-N-tertbutyl-1,2,3,4- tetrahydroisoquinoline-3-carboxamide

Abstract: It is here described a new process for the synthesis of (S)-N-tertbutyl-1,2,3,4-tetrahydroisoquinoline-3-carboxamide wherein the N-carboxy anhydride of formula is treated with tert-butylamine in an organic inert solvent, preferably toluene, at a temperature of between -80 and -30°C.

Description: The present invention relates to a new process for the preparation of (S)-N-tert-butyl-1,2,3,4-tetrahydroisoquinoline-3-carboxamide, whose formula of structure is here below reported, and which is a key intermediate for the preparation of compounds having elevated pharmacological activity, which can be used in particular in the treatment and in the prevention of the infections caused by HIV. In the greatest part of said antiviral drugs, the (S)-N-tert-butyl-1,2,3,4-tetrahydroisoquinoline-3-carboxamide is not used directly as such but, contrarily, is previously hydrogenated to form the N-tert-butyl-decahydro-(4aS,8aS)-isoquinolin-3(S)-carboxamide, whose formula of structure is here below reported, and which, by suitable substitutions at the isoquinoline nitrogen which will be obvious to any person skilled in the art, is in turn converted into the pharmacologically active derivative. In US 5,196,438, herein incorporated as a reference, pharmacologically active compounds are disclosed whose formula of structure is here below reported and in which the decahydroisoquinoline residue derived from the N-tert-butyl-decahydro-(4aS,8aS)-isoquinolin-3(S)-carboxamide is immediately identifiable; among these, the derivative of most interest, and the structural formula of which is given below is known by the commercial name Saquinavir. Another anti-viral drug of substantial importance, which also contains the decahydroisoquinoline residue present in Saquinavir, is Nelfinavir, the structural formula of which is likewise given below Both Nelfinavir and Saquinavir are normally used in the form of the corresponding watersoluble salts and, in particular, in the form of the mesylated salts. In US 5,587,481 it is described a process for the manufacture of the (S)-N-tert-butyl-1,2,3,4-tetrahydroisoquinoline-3-carboxamide wherein the (3S)-1,2,3,4-tetrahydroisoquino-line-3-carboxylic acid is reacted with phosgene in ethyl acetate to produce the corresponding N-carboxyanhydride (NCA), here-below reported. The NCA is then directly reacted with tert-butylamine and subsequently hydrogenated to give the N-tert-butyl-decahydro-(4aS,8aS)-isoquinolin-3(S)-carboxamide. The process disclosed in US 5,587,481 is however characterized by drawbacks which cannot be neglected as low yields and, in particular, the fact of using phosgene, which is a toxic and highly dangerous gas. EP-A-751128 describes a similar synthesis process in which the conversion of the (3S)-1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid into the corresponding NCA can be effected also with triphosgene. The advantages resulting from this process are clear because the use of triphosgene which, unlike phosgene, is not a toxic gas, permits complete safety of implementation with a consequent saving in operating costs and plant costs; triphosgene is also a solid compound which enables it to be used more accurately and therefore without the typical secondary reactions which may occur with the use of a gaseous reagent in excess. Finally, WO-A-00/00494 describes an implemented process for the synthesis of the (S)-N-tert-butyl-1,2,3,4-tetrahydroisoquinoline-3-carboxamide characterized in that the (3S)-1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid is reacted with triphosgene in dioxane making it possible to obtain particularly high global yields also on an industrial scale. Both US 5,587,481, EP-A-751128 and WO-A-00/00494 present the common feature that the reaction between NCA and tert-butylamine is carried out at a starting temperature higher than -20°C. It has now been surprisingly found and it is the object of the present invention, that by carrying out the reaction between tert-butylamine and the NCA at a starting temperature lower than -50°C, the (S)-N-tert-butyl-1,2,3,4-tetrahydroisoquinoline-3-carboxamide is produced with a yield and a purity definitely higher than those obtainable by carrying out the reaction according to the previously described methods. In particular, as it will be appreciated by the following examples, the reaction carried out according to the present invention permits the obtainment of the (S)-N-tert-butyl-1,2,3,4-tetrahydroisoquinoline-3-carboxamide with yields of 15 to 20% higher than those obtainable by operating at a starting temperature higher than -20°C. Furthermore, the reaction between tert-butylamine and the NCA carried out according to the previously described methods produces a dimer by-product, whose formula is here-below reported, which is on the contrary produced in substantially reduced amounts if operating according to the method of the present invention. The reaction between NCA and tert-butylamine is carried out in an inert organic solvent, preferably toluene, THF, dioxane, methylene chloride, using from 1 to 10 moles of tert-butylamine, preferably from 3 to 5 moles, for each mole of NCA. The NCA is generally suspended in 5 to 10 liters of solvent for 1 kg of NCA. The so-obtained suspension is then normally cooled to a temperature lower than -50°C, preferably at a temperature comprised between -80 and -50°C, even more preferably at -75 to -65°C. The tert-butylamine is then added maintaining the temperature below -50°C, preferably below - 55°C; the temperature is then raised until room temperature and maintained as such until reaction completion. The method for the manufacture of the NCA is not limiting for the purposes of the present invention and can be carried out according to the several methods reported in literature; the preferred method is however that disclosed in WO-A-00/00494 and wherein the (3S)-1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid is reacted with triphosgene in dioxane at a starting concentration of 0.5 to 1.5 mol/l with 0.3 to 1.2 equivalents of triphosgene at temperatures of between +20 and +85°C. In the preferred embodiment of the invention, the NCA thus obtained is not isolated from the reactor; on the contrary, the solvent of the previous reaction is distilled off and the raw NCA is then directly suspended in the reaction solvent, preferably in toluene; normally 3000 to 5000 liters of solvent, preferably 3500 to 4500, are used for 500 kg of (3S)-1,2,3,4-tetrahydro-isoquinoline-3-carboxylic acid. The reaction mixture is then cooled to about -70°C and the tert-butylamine is then added dropwise into the solution for a time period of 20 to 40 minutes, maintaining the temperature below -55°C; the temperature is then raised to about - 20°C in 20 to 40 minutes and it is then raised to +15 to +20°C still in 20 to 40 minutes. The temperature is then maintained as such for about 2 hours and water is added afterwards in order to quench the reaction. The end product is then isolated in accordance with conventional techniques; preferably it is precipitated from acidic water by addition of a base, generally soda. The(S)-N-tert-butyl-1,2,3,4-tetrahydroisoquinoline-3-carboxyamide can then be hydrogenated to N-tert-butyl-decahydro-(4aS,8aS)-isoquinoline-3(S)-carboxamide in accordance with conventional techniques as those described in US 5,256,783, US 5,587,481, EP-A-751128 and WO-A-00/00494, herein incorporated as a reference; the reaction is normally carried out in an organic polar solvent, preferably an alcohol, even more preferably isopropanol, in the presence of a reduction catalyst in heterogeneous phase, preferably rhodium supported on alumina. Those and other aspects of the invention will become clear from the following examples which are to be regarded purely as non-limiting illustrations of the invention. EXAMPLE 1 (Reference) In the following table there are reported the results of experiments carried out in toluene on 5 g of isolated NCA (40 ml, K.F. = 0,02%, which means water content calculated with the Karl Fischer method) with 3 equivalents of tert-butylamine. The reactions have been carried out maintaining the temperatures indicated in column B for the whole time indicated in column D; only the temperature of test 8 was maintained at -40°C for the first hour and then raised to room temperature for the 3 following hours. The yields in solution were determined via HPLC. A B C D E F test temp. (°C) t-BuNH 2 addition time (min) total reaction time (hours) average yield (%) dimer (area%) 1 - 7°C 5 3 94,5 2,46 - 2,47 2 - 7°C 60 3 94,5 2,26 - 2,21 3 - 7°C 5 4 89,2 2,29 - 2,42 4 - 7°C 60 4 91,0 2,31 - 2,33 5 - 7°C 5 6 92,7 2,46 - 2,47 6 - 7°C 60 6 94,0 2,45 - 2,46 7 + 13°C 5 3 92,7 2,82 - 3,59 8 -40°C 1 hour 5 4 99,2 0,78 - 0,79 As it can be appreciated from the data listed in the table, by carrying out the reaction between tert-butylamine and NCA at a starting temperature lower than -20°C, it is possible to obtain substantially higher yields and a presence of dimer by-product considerably lower than by operating at a starting temperature higher than -20°C. EXAMPLE 2 A triphosgene solution (83 g, 0,28 mol) in dioxane (200 ml) has been added dropwise in 2 hours to a suspension of (3S)-1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid (100 g, 0,564 mol) in dioxane (300 ml) under reflux. The mixture has then been maintained at reflux (100°C) under stirring for 30 minutes until complete dissolution. The solution has then been cooled to +75°C to +85°C in 10 minutes and distilled off under vacuum in 2 hours and 30 minutes, maintaining the temperature at +40°C, until a dense but stirrable paste was obtained. The residue was then taken up twice with 100 ml of toluene and distilled off to give a stirrable paste. This was taken up again with 500 ml of toluene and cooled to -50°C; the t-butylamine (180 ml, 1,709 moles) was then added dropwise in 30 minutes under stirring and in an inert atmosphere. The temperature was then let to increase until +15°C in 1 hour, the temperature was maintained as such for 2 hours and 700 ml of water were then added. The organic phase was then extracted with water (800 ml) and HCI 30% at pH=1 and the product was thus precipitated for the aqueous layer by addition of NaOH 30%, filtered, washed with water and dried under vacuum. 112.4 grams of product were obtained. The results of this test are here-below listed titer 99,11 % purity HPLC (A%) 99,32 % dimer (HPLC, A%) 0,36 % enantiomeric purity (A%) 98,9 % K.F. 0,21 % NaCI 0,24 % yield 85,03 % COMPARATIVE EXAMPLES The reaction of example 2 was repeated by changing the starting temperature and the time of t-butylamine dropwise addition only; the results are reported in the table. test starting reaction temperature dropwise addition time (minutes) yield (%) dimer (area %) titer (%) purity (%) 2 -40 to -50°C 20 85 0.36 99.1 99.3 2a +10 to +15°C 90 71 3.8 98.6 96 2b -5 to -10°C 120 73 3.1 98.1 96.6 2c -5 to -10°C 120 73 1.3 99.8 98.6 EXAMPLE 3 A solution of triphosgene (400 kg) dissolved in dioxane (1000 I) is added dropwise, with agitation, over a period of 2 hours to a mixture of (3S)-1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid (500 kg) in dioxane (1500 I) heated to reflux. After one hour at that temperature, the whole is cooled to +40°C and distilled to leave a residue. The residue is taken up twice in toluene (300 l) and distilled off to leave a residue which is again taken up in toluene (3500 I). The so-obtained suspension is then cooled to -70°C and a solution of tert-butylamine (900 I) is added in about 30 minutes maintaining the temperature below -55°C. The temperature is then raised to about -20°C in 30 minutes and is raised to +15 to +20°C in 30 further minutes; it is then maintained as such for 2 other hours. The reaction is then quenched by adding demineralized water (3500 I); the organic phase is then extracted with water (3000 I) and HCI 30% (345 l), diluted with 2400 liters of water, treated with carbon, filtered and made basic with soda 30% until pH 11. The solid thus obtained is then filtered, washed with water and dried to give 524.4 kg of (S)-N-tert-butyl-1,2,3,4-tetrahydroisoquinoline-3-carboxyamide (yield 80%; dimer 1 to 2%) as white crystalline powder; purity > 97% (HPLC), titer > 98%. The so-obtained (S)-N-tert-butyl-1,2,3,4-tetrahydroisoquinoline-3-carboxyamide is then dissolved in about 1800 liters of isopropanol and subsequently hydrogenated at 45 bar on Rh/Al 2 O 3 (5%, 8 kg) for 3-4 hours at a temperature of +70°C; the catalyst is then filtered and washed with isopropanol. The solvent is evaporated and the raw material is taken up with heptane; the solvent is evaporated and the raw material is taken up again with heptane (120 I) and cooled to +0 to +5°C for 1 hour. The final product is then obtained after filtration and washing with heptane as a white crystalline powder (map. 113 to 115°C), with a yield of 88%.

Claims: A process for the production of (S)-N-tert-butyl-1,2,3,4-tetrahydroisoquinoline-3-carboxamide comprising the reaction between tert-butylamine and the N-carboxyanhydride of formula in an organic inert solvent, characterized in that said reaction is carried out at a starting temperature lower than -50°C. A process according to claim 1 characterized in that said starting temperature is between -80 and -50°C. A process according to claim 2 characterized in that said starting temperature is between -75 and -65°C. A process according to claim 1 characterized in that said organic inert solvent is selected from toluene, THF, dioxane and CH 2 Cl 2 . A process according to claim 4 characterized in that said organic inert solvent is toluene. A process according to claim 1 characterized in that said N-carboxyanhydride is treated with 1 to 10 tert-butylamine equivalents. A process according to claim 6 characterized in that said N-carboxyanhydride is treated with 3 to 5 tert-butylamine equivalents. A process according to claim 1 characterized in that said reaction is carried out in 5 to 10 liters of organic inert solvent for 1 kg of N-carboxyanhydride. A process according to claim 1 wherein: (a) a solution of said N-carboxyanhydride in toluene is cooled to -75 to -65°C, (b) tert-butylamine is added to the solution dropwise for a time period of 20 to 40 minutes maintaining the temperature below -55°C, (c) the temperature is then raised to about -20°C in 20 to 40 minutes and (d) it is then raised to +15 to +20 °C in 20 to 40 minutes and (e) maintained as such until reaction completion. A process according to anyone of the previous claims wherein said N-carboxyanhydride is obtained from the reaction between (3S)-1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid and triphosgene in an inert organic solvent. A process according to claim 10 characterized in that said inert organic solvent is dioxane. A process according to claim 10 characterized in that said reaction between (3S)-1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid and triphosgene is carried out at a temperature comprised between +20 and +105°C, preferably under reflux. A process according to claim 10 characterized in that (3S)-1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid is used at a starting concentration of 0.5-1.5 mol/l. A process according to claim 10 characterized in that (3S)-1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid is treated with 0.3 to 1.2 triphosgene equivalents. A process according to claim 10 characterized in that said N-carboxyanhydride is suspended in 3000 to 5000 liters of solvent, preferably in 3500 to 4500, for 500 kg (3S)-1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid. A process according to anyone of the previous claims wherein the so-obtained (S)-N-tert-butyl-1,2,3,4-tetrahydroisoquinoline-3-carboxyamide is converted into N-tertbutyldecahydro-(4aS,8aS)-isoquinolin-3(S)-carboxamide by catalytic hydrogenation. A process according to claim 16 characterized in that said catalytic hydrogenation is carried out in isopropanol in the presence of rhodium supported on alumina. A process for the manufacture of anti-viral drugs and of the corresponding saline derivatives characterized by comprising a process according to anyone of the previous claims. A process according to claim 18, characterized in that the anti-viral drugs are the following two compounds A process according to claim 19, characterized in that the salt derivatives of said antiviral drugs are mesylated salts.

Inventors: BANFI ALDO, BELLANI PIETRO, BANFI, ALDO, BELLANI, PIETRO
Applicants: CLARIANT LSM ITALIA SPA, CLARIANT LIFE SCIENCE MOLECULES (ITALIA) S.P.A.
Application year: 2000
Main classification: C07D217
IPC codes: C07K5, C07D491, C07D401, C07D498, A61P31, C07D217, A61K31, C07B61