Continuous flow synthesis method for preparing isoniazid

Abstract

A multi-step continuous flow synthesis method for preparing isoniazid (isonicotinic acid hydrazide), which includes reacting 4-cyanopyridine with NaOH within a specific molar ratio and temperature range to produce the intermediate isonicotinamide, and reacting this intermediate with hydrazine hydrate within a specific molar ratio and temperature range without separating it, to prepare isoniazid (isonicotinic acid hydrazide) with a yield greater than about 90%.

Description

Technical Field

[0001] The present invention relates to a multi-step continuous flow synthesis method for preparing isoniazid, and more particularly, but not limited to, a continuous flow synthesis method for preparing isoniazid with a yield exceeding 90%. Background of the Invention

[0003] Numerous methods and synthetic routes for preparing isoniazid have been described in the prior art.

[0004] However, the existing synthetic methods for producing these compounds are all based on processes of the standard stirred batch reactor type, in which a large amount of organic solvents are used. In addition, these processes generally separate process intermediates at each step of the process, thereby further increasing the use of solvents. Therefore, the cost of the active pharmaceutical ingredient produced in this way is relatively high, mainly due to the dependence on organic solvents and the inherent low process efficiency due to the separation of intermediates.

[0005] Microreactor technology (MRT), recently referred to as "flow chemistry", is an emerging technology that enables researchers and developers to rapidly screen reactions using continuous flow, thereby determining reaction conditions suitable for use at the production level. In addition, in addition to using conventional reaction methods, the inherent safety associated with the use of small reactor volumes allows users to use reaction conditions that were previously considered too dangerous to use in a production environment; for example, extreme reaction conditions or the use / production of "hazardous" compounds. Therefore, the types of reactions available to chemists have increased. To date, there has been no process for producing isoniazid based on flow chemistry methods.

[0006] Therefore, there is a need for an improved method for preparing isoniazid. In particular, there is a need for a fully continuous multi-step flow synthesis method for preparing isoniazid that preferably produces a yield of isoniazid exceeding 90%. Summary of the Invention

[0008] According to a first aspect of the present invention, there is provided a multi-step continuous flow synthesis method for preparing isonicotinohydrazide of formula 1, which is suitable for producing a yield greater than about 90%,

[0009]

[0010] comprising the following steps:

[0011] a) reacting 4-cyanopyridine of formula 12

[0012]

[0013] with NaOH at a temperature of about 90 °C to about 105 °C in a molar ratio of 4-cyanopyridine:NaOH of about 1:0.1 to about 1:0.7 to produce the intermediate isonicotinamide of formula 13

[0014]

[0015] b) React the intermediate isonicotinamide of formula 13 with hydrazine hydrate at a temperature of about 100 °C to about 120 °C with a molar ratio of 4-cyanopyridine:hydrazine hydrate of about 1:1.75 to about 1:2.50.

[0016] Wherein the residence time of the reaction in step a) is about 7 to about 32 minutes, and the residence time of the reaction in step b) is about 10 to about 25 minutes.

[0017] In one embodiment, in step a), 4-cyanopyridine is dissolved in a mixture of water and an alcohol, including methanol.

[0018] In a preferred embodiment, the alcohol-to-water ratio of the water and alcohol mixture is about 7:3.

[0019] In one embodiment, in step a), the molar ratio of 4-cyanopyridine to NaOH is about 1:0.15 to about 1:0.6, or about 1:0.15 to about 1:0.4.

[0020] In a preferred embodiment, in step a), the molar ratio of 4-cyanopyridine to NaOH is about 1:0.2.

[0021] In one embodiment, in step a), the temperature is about 90 °C to about 100 °C, or about 95 °C to about 100 °C.

[0022] In a preferred embodiment, in step a), the temperature is about 95 °C.

[0023] In a preferred embodiment, in step a), the residence time is about 10 to 15 minutes.

[0024] In a particularly preferred embodiment, in step a), the residence time is about 10 minutes.

[0025] In one embodiment, in step b), the molar ratio of 4-cyanopyridine to hydrazine hydrate is about 1:1.75 to about 1:2.25.

[0026] In a preferred embodiment, in step b), the molar ratio of 4-cyanopyridine to hydrazine hydrate is about 1:2.0.

[0027] In another embodiment, in step b), the temperature is about 100 °C to about 115 °C, or about 105 °C to about 115 °C.

[0028] In a preferred embodiment, in step b), the temperature is about 105 °C.

[0029] In a preferred embodiment, in step b), the residence time is from about 10 to about 20 minutes.

[0030] In a particularly preferred embodiment, in step b), the residence time is about 10 minutes.

[0031] In a preferred embodiment, the overall yield of the multi-step continuous flow synthesis method for preparing isoniazid from 4-cyanopyridine is greater than about 92%, greater than about 94% or greater than about 95%.

[0032] In a particularly preferred embodiment, the overall yield of the multi-step continuous flow synthesis method for preparing isoniazid from 4-cyanopyridine is about 96%. Brief Description of the Drawings

[0034] The present invention will now be described in more detail with reference to the following non-limiting embodiments and drawings, in which:

[0035] Figure 1 A schematic diagram of the general synthesis method of the present invention is shown;

[0036] Figure 2 A schematic diagram of the apparatus for studying the preparation of isoniazid 1 as a multi-step continuous process is shown;

[0037] Figure 3 A continuous flow apparatus for the synthesis of isonicotinamide by hydrolysis of 4-cyanopyridine is shown;

[0038] Figure 4 The effect of the relative molar percentage of sodium hydroxide on the conversion to 13 and the formation of by-product 15 is shown;

[0039] Figure 5 The effect of temperature on the conversion of 13 and the formation of by-product 15 is shown;

[0040] Figure 6 The effect of residence time on the conversion of 12 to 13 is shown; and

[0041] Figure 7 The effect of the total residence time on the conversion to 1 is shown, where the residence time of the reaction for the conversion to 13 is set to 10 minutes.

[0042] Detailed Description of Preferred Embodiments

[0043] The present invention will be described more fully hereinafter with reference to the drawings, in which some non-limiting embodiments of the present invention are shown.

[0044] The present invention as described hereinafter should not be construed as limited to the specific embodiments disclosed, and minor modifications and other embodiments are included within the scope of the present invention.

[0045] Although specific terms are used herein, they are used in a general and descriptive sense only and not for purposes of limitation.

[0046] As used herein, in this specification and the appended claims, the singular forms "a", "an", and "the" include plural forms unless the context clearly dictates otherwise.

[0047] The terms and phrases used herein are for descriptive purposes and should not be regarded as limiting. The terms "comprising", "containing", "having", "including" and their variants are intended to cover the items listed thereafter, their equivalents, and additional items.

[0048] When used in this specification, the terms "total continuous flow synthesis method", "multi-step continuous flow synthesis method", and "total continuous multi-step flow synthesis method" are used interchangeably and should be understood to represent a flow synthesis method, i.e., a method using microreactor technology, including multiple steps, wherein the final product of the method is obtained without separating any intermediates.

[0049] The present invention provides a method for preparing isoniazid by a total continuous flow synthesis method. Figure 1 A schematic diagram showing the general synthesis method and synthesis steps of an embodiment of the present invention for preparing isoniazid is shown.

[0050] Figure 1 The shown continuous flow synthesis method of isoniazid is a two-step reaction using microreactor technology. Generally, the first reaction to convert to isoniazid is the hydrolysis of 4-cyanopyridine 12 in a first microreactor or the first part of a suitable microreactor device to produce isonicotinamide 13. Isonicotinamide 13 reacts with hydrazine hydrate in a second microreactor or the second part of a suitable microreactor device, which is in continuous fluid communication with the first microreactor or the first part of the microreactor.

[0051] An example of a microreactor device for implementing the method of the present invention is Figure 2 shown. As Figure 2 can be seen, a continuous flow reaction device for implementing the present invention can be constructed by using two syringe pumps, three syringes, and various reactor plates. The system may include one-way flow check valves and back pressure regulators. The temperature of the reactor plate can be controlled, for example, by using an oil bath or in any other manner known in the art. Those of ordinary skill in the art will understand that various details of the experimental setup described above can be modified to arrive at further embodiments, but these embodiments will remain within the scope of the present invention.

[0052] The compatibility of reagents and solvents is particularly important in a flow system to ensure that products and intermediates remain in solution. Incorrect solvent, reagent ratios, reagent concentrations, reaction temperatures, and other reaction conditions can cause reagent precipitation. Precipitation not only affects reaction efficiency but also leads to system blockages, which in turn cause an increase in system pressure. System blockages will result in costly system shutdowns and overall inefficiencies in the reaction.

[0053] The individual synthetic steps of the method according to the invention will now be described in more detail with reference to the following non-limiting experimental examples and analytical data.

[0054] Experimental parameters

[0055] All chemicals used were purchased from Merck, Sigma Aldrich, and Industrial Analytica. They were used as received without further purification.

[0056] The reaction progress was monitored by thin-layer chromatography (TLC). TLC was performed on a 250 μm thick E. Merck silica gel plate (60F-254) with hexane and ethyl acetate (80:20 v / v) as the mobile phase. Detection of the spots was done at 254 nm using a Camag UV detector cabinet. The purity of the compounds was determined by a single spot on the TLC plate.

[0057] Recorded using a Bruker spectrometer (Bruker Ultrashield TM 400plus) 1 H and 13 C nuclear magnetic resonance (NMR) spectra, which were used at 400 MHz for the proton spectrum and 100 MHz for the carbon spectrum. The spectra were calibrated using the residual 1 H chemical shift in DMSO-d6 (2.62 ppm) (used as an internal standard). The chemical shift values for all spectra are given in parts per million (ppm).

[0058] 4-Cyanopyridine 12, isonicotinamide 13, isoniazid 1, and isonicotinic acid 15 were determined by analytical high-performance liquid chromatography (HPLC). HPLC analysis was performed using an Agilent 1220 Infinity LC instrument equipped with a diode array detector (DAD), equipped with an Agilent Eclipse Plus C18 reversed-phase chromatographic column. HPLC-grade methanol (organic mobile phase) from Merck was used. To obtain a buffer solution (mobile aqueous phase) with a pH of 7.0, 29.1 mL of 0.1 M sodium hydroxide and 50 mL of 0.1 M potassium dihydrogen phosphate (analytical grade) were added to 1000 mL of deionized water to prepare the solution. The buffer solution was degassed by a method combining ultrasonic and vacuum filtration. A 0.45 μM Whatmann filter paper was used during the degassing process. The organic mobile phase was also degassed before use. Chromatographic separation was achieved using an isocratic solvent system. The HPLC parameter settings are shown in the following table.

[0059]

[0060] FTIR characteristic peaks for identifying functional groups in the sample were recorded on a Bruker Alpha spectrophotometer with ATR fitting. Sample analysis was recorded in the range of 4000 - 400 cm -1 and the peaks were reported in wave numbers (cm -1 ). Both solid and liquid samples were analyzed without any modification. OPUS manager software was used for quantification.

[0061] GC was performed on an Agilent 7820A instrument using an HP-5 chromatographic column (30 m × 320 μm × 0.5 μm). The samples were detected using a flame ionization detector. Split-split less injection was used. The following table provides the GC parameter settings.

[0062] To confirm the purity of the synthesized compound, the melting point was determined using a Stuart SMP10 digital instrument. Before analysis, the analyte sample had to be dried overnight in a vacuum desiccator. A small amount of powdered analyte sample was loaded into a glass capillary sealed at one end to a depth of approximately 4 mm. The Stuart device was set at a platform temperature of 60 °C.

[0063]

[0064] Synthesis Step 1: Preparation of Isonicotinamide 13

[0065]

[0066] Scheme 1: Preparation of Compound 13

[0067] Scheme 1 shows the first synthetic step of the method, in which 4-cyanopyridine 12 is converted to isonicotinamide 13 in the presence of an aqueous sodium hydroxide solution. The first step of the method was fully developed before considering the introduction of hydrazine hydrate in the second step of the reaction.

[0068] These reactions were carried out using Figure 3 the microreactor device shown. The microreactor device consists of a Chemyx fusion200 modular syringe pump, a 10 mL SGE glass syringe, and borosilicate glass plate reactors LTF-V (volume: 1.7 mL, channel size: 1 mm, geometry: 115×60×6 mm) and LTF-VS (volume: 1.1 mL, channel size: 1 mm, geometry: 15×60×6 mm). The SGE glass syringe is connected to a microreactor plate by a PTFE tube with an inner diameter of 0.5 mm. A three-way t-connector (Omnifit labware, aperture: inner diameter 8.0 mm, outer diameter 0.5 - 4 mm) placed in front of the microreactor is used to mix the reagents pumped out from the two syringes. In order to allow the reactant solution to flow unidirectionally, check valves (CV outlet: 3302, 1 / 4 - 28M to 1 / 4 - 28F and CV inlet: 3301, 1 / 4 - 28M to 1 / 4 - 28F, 15 psi) are connected in series to each of the two reaction streams, as Figure 3 shown.

[0069] The microreactor was immersed in a temperature-controlled oil bath, and the reactants were delivered to the plate by a Chemyx fusion200 modular syringe pump equipped with two syringes. A 10 psi back pressure regulator (BPR) was installed in series between the reactor outlet and the collection bottle. Preliminary experiments summarized in Table 1 below show that the microreactor gives comparable performance.

[0070] Table 1: Summary of the conditions and results of preliminary tests on LTF-V and LTF-VS reactor plates.

[0071] Reactor

[12] (M) [NaOH] T(℃) <![CDATA[R t (minutes)]]> Conversion rate of 12 (%) Selectivity (%) LTF-V 0.5 0.05 90 15 85 93 LTF-VS 0.5 0.05 90 15 81 89

[0072] Reaction solvent

[0073] The compatibility of reagents and solvents is particularly important in a flow system to ensure that products and intermediates remain in solution. Incorrect solvents and other reaction conditions can lead to reagent precipitation and pressure build-up. The two solvent systems studied were water and a mixture of water and methanol. Although methanol was used in these experiments, those skilled in the art will recognize that similar lower alcohols, such as ethanol, are equally suitable as reaction solvents. Briefly, the effect of reaction temperature was also considered.

[0074] In the first preliminary experiment, an aqueous solution of 4-cyanopyridine (0.5 M) and an aqueous solution of sodium hydroxide (0.05 M) were fed into the microreactor using two syringes. In the second preliminary experiment, a solution of 4-cyanopyridine (0.5 M) in a 7:3 methanol / water mixture and sodium hydroxide (0.05 M) were fed into the microreactor using two syringes. The reactions were carried out at temperatures of 40 °C and 90 °C respectively, with a residence time of 15 minutes. The samples were collected and analyzed using off-line HPLC and GC.

[0075] It was observed that using only water resulted in solid precipitation in the reactor channels, thereby reducing the conversion to the desired intermediate isonicotinamide 13. Formation of precipitates in a continuous flow system is highly undesirable as it leads to blockage of connectors and ultimately an increase in pressure within the system. Therefore, it is important to select a suitable solvent for the reaction to keep the products and intermediates in solution.

[0076] The presence of the water-miscible solvent methanol helped to dissolve 4-cyanopyridine, and the resulting reaction products and by-products, and no precipitation was observed within the reaction channels and connectors. Table 2 below summarizes the results of these experiments, which showed that the use of an alcohol (such as methanol) and elevated temperature promoted the nitrile hydrolysis reaction.

[0077] Table 2: Summary of the conditions and results of the water and MeOH:water mixture experiments, where

[12] is the concentration of 4-cyanopyridine.

[0078] Operation

[12] (M) [NaOH] Solvent system T(℃) <![CDATA[R t (minutes)]]> Conversion rate (%) 1 0.5 0.05 Water 40 15 35 2 0.5 0.05 Water 90 15 62 3 0.5 0.05 MeOH / Water (7:3) 40 15 48 4 0.5 0.05 MeOH / Water (7:3) 90 15 83

[0079] Effect of reagent molar equivalent on the conversion rate of isonicotinamide 13

[0080] The molar equivalent of base to nitrile in the reaction is crucial for achieving reaction control, especially in terms of reaction selectivity. The following procedure was used to evaluate the effect of base concentration on the preparation of isonicotinamide 13.

[0081] A standard solution (0.50 M) of 4-cyanopyridine 12 in a methanol / water (7:3) solution was added from the first syringe (A), and a solution of sodium hydroxide in deionized water (0.04 M) was added from the second syringe (B). To maintain a constant reaction temperature (90 °C), the microreactor was immersed in a temperature-controlled oil bath. After a residence time of 15 minutes, the reaction products were collected in vials to obtain an indication of the reaction time before further study. To study the effect of the relative stoichiometry of the reagents (4-cyanopyridine 12 and sodium hydroxide) on the conversion of the product 13, the experiment was repeated using different sodium hydroxide solutions with a concentration range of 0 to 0.75 M. The products of each flow reaction were collected in vials and analyzed using off-line HPLC. Table 3 below shows a summary of the reaction parameters used and the results obtained (see Figure 4 ).

[0082] Table 3: Reaction parameters for studying the effect of molar equivalent on the conversion of 12 to isonicotinamide 13, where

[12] is the concentration of 4-cyanopyridine.

[0083]

[0084] Analysis of the reaction samples provided initial information on whether the reaction was complete and possible by-products. During the analysis of the samples in this optimization experiment, three prominent peaks were observed on the HPLC chromatogram. The first peak corresponded to the nitrile substrate 12, another peak corresponded to the amide product 13, and another peak corresponded to the by-product isonicotinic acid 15. Thus, the substrate conversion was measured directly, and the reaction selectivity was calculated based on this ratio. The conversion of the amide product 13 was used to determine the optimal reaction conditions.

[0085] Figure 6 A comparison between the conversion to the desired isonicotinamide 13 and the conversion to the by-product compound isonicotinic acid 15 is shown.

[0086] From Figure 4 It can be seen that in the absence of any base, the conversion was 5%. Since the nitrile group is not particularly reactive, an alkaline catalyst needed to be added to increase the reactivity of 4-cyanopyridine 12. However, the amount required for this reaction to achieve the desired yield and selectivity in a continuous flow system was unknown per se. Clearly, as the molar ratio of compound 12: sodium hydroxide increased, the conversion of compound 12 increased. When at least 0.134 M (0.2 molar equivalent) of sodium hydroxide was used, a maximum conversion of 100% was obtained.

[0087] However, even when the results showed a 100% conversion of compound 12, increasing the concentration of sodium hydroxide had an adverse effect on the selectivity of amide 13. Preferably, the molar ratio of 4-cyanopyridine 12 to sodium hydroxide is in the range of about 1:0.1 to about 1:0.7, including any specific ratio falling within this range, such as about 1:0.15, about 1:0.2, about 1:0.25, about 1:0.3, about 1:0.35, about 1:0.4, about 1:0.45, about 1:0.5, about 1:0.55, about 1:0.6, about 1:0.65, and about 0.7. Preferably, the molar ratio of 4-cyanopyridine 12 to sodium hydroxide is in the range of about 1:0.15 to about 1:0.6 or about 1:0.15 to about 1:0.4.

[0088] Effect of residence time on the conversion rate of isonicotinamide 13

[0089] A solution of 4-cyanopyridine 12 (0.50 M) in methanol / water (7:3) was added from the first syringe (A), and a solution of sodium hydroxide in deionized water (0.10 M, 0.2 molar equivalents) was pumped from the second syringe (B). The reaction products were collected in vials at different residence times. The reaction was carried out at a constant temperature of 90 °C. The effect of residence time on the conversion to isonicotinamide 13 was studied at eight different flow rates, with residence times ranging from 1.5 - 32 minutes.

[0090] Table 4: Reaction parameters for studying the effect of residence time on the conversion of 4-cyanopyridine 12.

[0091]

[0092] From the results in Table 4 above and Figure 6 the graphs therein, it can be seen that the conversion of 12 increases with increasing residence time and reaches an ideal conversion at a residence time of approximately 7 minutes. Preferably, the residence time for the conversion of 12 to 13 is from about 7 minutes to about 32 minutes, including any duration within this range. For example, the residence time can be about 7 minutes, about 8 minutes, about 9 minutes, about 10 minutes, about 11 minutes, about 12 minutes, about 13 minutes, about 14 minutes, about 15 minutes, about 16 minutes, about 17 minutes, about 18 minutes, about 19 minutes, about 20 minutes, about 21 minutes, about 22 minutes, about 23 minutes, about 24 minutes, about 25 minutes, about 26 minutes, about 27 minutes, about 28 minutes, about 29 minutes, about 30 minutes, about 31 minutes, and about 32 minutes. Preferably, the residence time for the conversion of 12 is from about 7 minutes to about 20 minutes, from about 7 to about 15 minutes, or from about 10 to about 15 minutes.

[0093] Effect of reaction temperature on the conversion rate of isonicotinamide 13

[0094] The following experimental procedure was used to study the effect of reaction temperature on the preparation of 13. A standard solution of 4-cyanopyridine 12 (0.50 M) in MeOH / H 2 O (7:3) solution was placed in the first syringe (A), and a solution of sodium hydroxide (0.10 M) in deionized water was placed in the second syringe (B). The effect of temperature on product selectivity and conversion was studied at temperatures between 80 °C and 110 °C. The temperature of the solution in the microreactor was controlled by a thermostatic bath. The products of each flow reaction were collected in vials and immediately analyzed using off-line HPLC. To ensure that the results obtained were due to the reaction occurring within the microchannel, the collected samples were immediately cooled by immersing the sealed vials containing the samples in a beaker of water at room temperature. Table 5 below summarizes the different parameters used in the experiment.

[0095] Table 5: Influence of temperature on the conversion of 12 to 13.

[0096]

[0097]

[0098] As the temperature increases, the conversion of 12 gradually increases and reaches 100% conversion after 95 °C. From Figure 5 it can be seen that the results indicate that the conversion of 13 is affected by the reaction temperature. As the temperature gradually increases from 80 - 95 °C, the conversion of the amide product also increases to a certain level (about 95%), and then starts to decline as more and more by-product 15 is formed. Therefore, without wishing to be bound by any particular theory, it is believed that as the temperature continues to increase, the reaction favors the formation of acid and thus does not increase the conversion of the amide. Preferably, the reaction temperature for the conversion of compound 12 to compound 13 is from about 90 to about 105 °C, more preferably from about 90 to about 100 °C.

[0099] Based on the results obtained in this study, using a temperature of 95 °C, the conversion of isonicotinamide is optimal. This temperature achieves a 99% conversion of 12 while forming fewer by-products (4%), resulting in a maximum selectivity of 96% for isonicotinamide 13. All subsequent reactions were carried out under the following optimal conditions: residence time 10 minutes, temperature 95 °C.

[0100] Effect of the concentration of 4-cyanopyridine 12 on the conversion rate of isonicotinamide 13

[0101] The influence of the concentration of compound 12 was studied in the range of 0.08 M - 1 M. The concentration of 12 was changed while keeping the relative molar equivalents between 12 and NaOH constant. The following experimental procedure was used to study the influence of concentration.

[0102] Using two SGE glass syringes and PTFE tubing (inner diameter 0.5 mm), 4-cyanopyridine (0.08 M in MeOH / H 2 O 7:3) and sodium hydroxide (0.016 M) were fed into the LTF-V microreactor. After a 10-minute residence time, samples were collected and analyzed. The reaction was repeated with different concentrations of 4-cyanopyridine 12 and sodium hydroxide as shown in Table 6 below. The molar equivalent between compound 12 and sodium hydroxide was kept constant at 1:0.2. Total conversion and selectivity were determined using off-line HPLC.

[0103] Table 6: Reaction parameters used to study the influence of the concentration of compound 12 on the conversion of 13.

[0104]

[0105]

[0106] As can be seen from the results shown in Table 6 above, the conversion of 4-cyanopyridine 12 is affected by the concentration of reagent 12. Generally, an increase in the concentration of 4-cyanopyridine results in an increase in the amount of product formed. However, surprisingly, for higher concentrations, the same trend is not followed. Increasing the concentration of 12 to 1 M results in a significant decrease in the conversion rate (5%).

[0107] Synthesis Step 2: Preparation of Isonicotinohydrazide (Isoniazid) 1

[0108]

[0109] Scheme 2: Preparation of Compound 1

[0110] Scheme 2 shows the reaction steps for converting the intermediate isonicotinamide 13 into isonicotinohydrazide (isoniazid) 1. As Figure 2 shown, the preparation of product 1 was studied through a two-step synthetic route, in which the second synthetic step, i.e., the reaction of the intermediate compound 13 with hydrazine shown above, was carried out continuously with in-situ prepared compound 13.

[0111] The continuous flow reaction device consists of two Chemyx fusion 200 modular syringe pumps, three 10 mL SGE glass syringes (the first two syringes are controlled by one pump and one syringe is controlled by the other pump), and different LittleThings Factory (LTF) glass reactor plates (equipped with PTFE tubes with an ID of 0.5 mm). These reactor plates include: 1 LTF-V residence plate (reactor volume: 1.7 mL, channel size: 1 mm, geometric size: 115×60×6 mm), 1 LTF-MS micromixer (reactor volume: 0.2 ml, channel size: 1 mm, geometric size: 115×60×6 mm), and two LTF-VS residence plates (reactor volume: 1.1 mL, channel size: 1 mm, geometric size: 115×60×6 mm). The system also includes one-way flow check valves (CV outlet: 3302, 1 / 4 - 28M to 1 / 4 - 28F and CV inlet: 3301, 1 / 4 - 28M to 1 / 4 - 28F) and a 35 psi back pressure regulator. These reactor plates are arranged in two temperature oil baths, where the first oil bath contains two plates: a single LTF-V residence plate (for synthesizing amide 13) and a single LTF-MS mixing plate, which contains two input channels (one from the continuous flow of the LTF-V residence plate and the second as the hydrazine hydrate inlet into the flowing stream). The second oil bath contains two LTF-VS residence plates (for the synthesis of isonicotinohydrazide 1). Hydrazine hydrate is introduced into the flow reaction using the LTF-MS micromixer. The second step of the reaction requires thorough mixing, so the LTF-VS plate is used. Two residence plates are used in the second step just to increase the residence time.

[0112] Effect of residence time on the conversion rate of isoniazid 1

[0113] Use Figure 2 the experimental setup shown, add 4-cyanopyridine 12 standard solution (0.67 M, in MeOH / H 2 O 7:3) from the first syringe (A), add sodium hydroxide solution (0.134 M, 0.2 eq) from the second syringe (B), and add hydrazine hydrate solution (1.34 M, 2.0 eq) from the third syringe (C). Synthesize the in-situ formed intermediate 13 using a residence time of 10 minutes and a bath temperature of 95 °C.

[0114] To study the effect of residence time (in the second part of the microreactor setup) on the conversion of product 1, a series of flow rates were studied after adding hydrazine hydrate. Hydrazine hydrate was pumped into the system at flow rates ranging from 0.05 - 0.3 mL / min. This resulted in a residence time for the hydrazine reaction of 2 - 25.2 minutes in this particular reactor setup. The temperature of the hydrazine reaction was kept constant at 110 °C. The product was collected in vials and analyzed using HPLC. Table 7 below shows the reaction parameters used in the optimization experiments.

[0115] Table 7: Reaction parameters used to study the effect of residence time of the hydrazine reaction on the conversion of 1.

[0116]

[0117]

[0118] Generally, as the residence time increases, the conversion of isoniazid 1 gradually increases. The residence time (R t ) shown in Table 7 above is the combined residence time of the reaction to convert to intermediate 13 and the subsequent hydrazine reaction to convert to product 1.

[0119] Therefore, as can be seen from Table 7 and Figure 7 the corresponding graph, an effective residence time of 11 minutes (i.e., 21 minutes total time minus the 10 minutes used for the first reaction to convert to 13) provides a conversion of 95.5%, and at higher residence times of 19.1 and 25.2 minutes, the conversion increases slightly. It was noted that there was complete selectivity from amide intermediate 13 to product 1. Therefore, the residence time of the hydrazine reaction is in the range of about 10 to about 25 minutes, including all durations within this range, such as 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, and 25 minutes. Preferably, the residence time of the hydrazine reaction is in the range of about 10 to about 20 minutes.

[0120] Effect of hydrazine hydrate concentration on the conversion rate of isoniazid 1

[0121] Hydrazine hydrate at different concentrations in the range of 0.67 M (1 equivalent) to 1.675 M (2.5 equivalents) relative to the starting compound 12 was studied. The residence time for the hydrazine reaction was 11 minutes (total residence time was 21 minutes), while the temperatures of the first and second reactions were maintained at 95 °C and 110 °C, respectively. 4-Cyanopyridine 12 (0.67 M), sodium hydroxide (0.134 M), and hydrazine hydrate solutions at different concentrations were used. The reaction conditions are summarized in Table 8 below. Samples were collected in vials and the conversion and selectivity of 1 were analyzed using HPLC.

[0122] Table 8: Reaction parameters for studying the effect of hydrazine hydrate concentration on the conversion of product 1, where R t is the total residence time.

[0123]

[0124]

[0125] The results shown in Table 8 above indicate that the hydrazine hydrate concentration has an effect on the conversion of intermediate 13 to isoniazid 1. The amount of hydrazine hydrate used was between 1 and 2.5 times relative to the starting material 4-cyanopyridine 12. The results show that it is very advantageous to use an excess (2.0 equivalents) of hydrazine hydrate based on the starting material. Preferably, the molar ratio of 4-cyanopyridine to hydrazine hydrate is in the range of about 1:1.50 to about 1:2.50, more preferably in the range of about 1:1.75 to about 1:2.25, including all sub-ranges contained within these ranges.

[0126] Effect of temperature on the conversion rate of isoniazid 1

[0127] 4-Cyanopyridine (0.67 M) was added to syringe A, sodium hydroxide (0.134 M) was added from syringe B, and hydrazine hydrate (1.34 M) was added from syringe C to a continuous flow system. The temperature of the first oil bath (synthesis of intermediate 13) was maintained at 95 °C, while the temperature of the second oil bath varied between 90 °C and 120 °C. To ensure that the results obtained were due to the reactions occurring within the microchannels, the collected samples were immediately cooled and then analyzed using HPLC. The reaction parameters are summarized in Table 9 below.

[0128] Table 9: Reaction parameters for studying the effect of the temperature of the hydrazine reaction on the conversion of 1.

[0129]

[0130] The effect of the temperature in the hydrazine reaction on the conversion of isoniazid 1 is shown by the results in the table above. The reaction temperature appears to have a significant effect on the conversion of 1. As the temperature increases, the amount of product 1 formed also increases non-linearly.

[0131] The foregoing description of some exemplary embodiments of the invention is for the purpose of illustrating how to make and implement the invention. Those of ordinary skill in the art will know that various details can be modified to obtain further embodiments, but many of these embodiments will still remain within the scope of the invention.

[0132] The non-metric units used in this specification and the drawings can be converted to the metric system by means of the following conversion factors:

[0133] 1 psi = 6,895 x 10 3 Pa.

Claims

1. A multi-step continuous microreactor flow synthesis method for preparing isoniazid of formula 1, which produces a yield greater than 90%, comprising the following steps: a) In a first microreactor or the first part of a microreactor device, 4-cyanopyridine of formula 12 is reacted with NaOH in a mixture of water and methanol at a temperature of 95 °C to 105 °C in a molar ratio of 4-cyanopyridine:NaOH of 1:0.2 to 1:0.4 to produce the intermediate isonicotinamide of formula 13 b) In a second microreactor or the second part of a microreactor device, which is in fluid communication with the first microreactor or the first part of the microreactor, the intermediate isonicotinamide of formula 13 is reacted with hydrazine hydrate at a temperature of 100 °C to 120 °C in a molar ratio of 4-cyanopyridine:hydrazine hydrate of 1:1.75 to 1:2.50, wherein the residence time of the reaction in step a) is 10 to 32 minutes, and the residence time of the reaction in step b) is 10 to 25 minutes.

2. The method according to claim 1, wherein the methanol:water ratio of the water and methanol mixture is 7:

3.

3. The method according to claim 1, wherein in step a), the molar ratio of 4-cyanopyridine to NaOH is 1:0.

2.

4. The method according to claim 1, wherein in step a), the temperature is 95 °C to 100 °C.

5. The method according to claim 1, wherein, in step a), the temperature is 95 °C.

6. The method according to claim 1, wherein in step a), the residence time is 10 to 15 minutes.

7. The method according to claim 1, wherein in step a), the residence time is 10 minutes.

8. The method according to claim 1, wherein in step b), the molar ratio of 4-cyanopyridine:hydrazine hydrate is 1:1.75 to 1:2.

25.

9. The method according to claim 1, wherein in step b), the molar ratio of 4-cyanopyridine:hydrazine hydrate is 1:2.

0.

10. The method according to claim 1, wherein in step b), the temperature is 100 °C to 115 °C, or 105 °C to 115 °C.

11. The method according to claim 1, wherein, in step b), the temperature is 105 °C.

12. The method according to claim 1, wherein in step b), the residence time is 10 to 20 minutes.

13. The method according to claim 1, wherein in step b), the residence time is 10 minutes.

14. The method according to claim 1, wherein the overall yield of the multi-step continuous flow synthesis method for preparing isoniazid from 4-cyanopyridine is greater than 92%, greater than 94% or greater than 95%.

15. The method according to claim 14, wherein the overall yield of the multi-step continuous flow synthesis method for preparing isoniazid from 4-cyanopyridine is 96%.

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