Patent Description:
Tirbanibulin, N-benzyl-<NUM>-(<NUM>-(<NUM>-(<NUM>-morpholinoethoxy)phenyl)pyridin-<NUM>-yl) acetamide of formula I
<CHM>
is a product used in the treatment of the actinic keratosis, a chronic recurrent skin condition caused by lengthy exposure to solar radiation.

<CIT> reports the first published synthesis for tirbanibulin. In said document the compound is identified as Compound <NUM> - KX2-<NUM>, and is prepared on a small scale according to the reaction reported in Scheme <NUM>, wherein Fibrecat™ is polymer bound di(acetate)dicyclohexylphenylphosphine palladium(II).

The same synthesis is reported in <CIT> and <CIT>.

<CIT>, <CIT>, <CIT>, <CIT> and <CIT> disclose the synthesis of tirbanibulin on a small scale (up to <NUM>; Example <NUM>), on an intermediate scale (at least <NUM> of tirbanibulin dihydrochloride; Example <NUM>) and on a large scale (at least <NUM> of tirbanibulin dihydrochloride; Example <NUM>). The small-scale synthesis is the same as described in Scheme <NUM>, whereas the larger-scale synthesis follows the steps shown in Scheme <NUM> (which corresponds, including the numbering of the intermediates, to the scheme reported in Example <NUM> of <CIT>).

<CIT> discloses a crystalline form of tirbanibulin. The free base is obtained by the procedure described in <CIT>, which belongs to the patent family of the above-mentioned <CIT>.

Finally, the supporting information to the publication <NPL>, wherein tirbanibulin is identified as "compound <NUM> (KX2-<NUM>)", describes the synthesis reported in Scheme <NUM>, which corresponds, including the numbering of the intermediates, to the scheme reported in the above-mentioned publication.

In all the known syntheses, the key reaction appears to be construction of the <NUM>-pyridylphenyl system with a Suzuki reaction between a <NUM>-bromopyridine derivative and a phenylboronic ester substituted at the <NUM> position by the <NUM>-(<NUM>-morpholinoethoxy) group (Schemes <NUM> and <NUM>), or between <NUM>-fluoro-<NUM>-pyridylboronic acid and <NUM>-(<NUM>-(<NUM>-morpholino)ethoxy)bromobenzene, the latter being followed by further multi-step processing to introduce the N-benzylacetamide residue.

As is clear from schemes <NUM>-<NUM>, the known syntheses require multiple steps for the construction of the intermediates to undergo the Suzuki reaction and/or for the introduction of the N-benzylacetamide group.

A more efficient method of preparing tirbanibulin with high yields and purity therefore needs to be developed.

The object of the invention is a process for the preparation of tirbanibulin, which comprises the following steps:.

The process according to the invention is illustrated in Scheme <NUM>.

A molar ratio of ester II to <NUM>-hydroxyphenylboronic acid III ranging between <NUM> and <NUM>, preferably <NUM>, will preferably be used in step a).

The catalyst is preferably selected from Pd(OAc)<NUM>, Pd(PPh<NUM>)<NUM> and palladium on carbon (Pd/C), and is preferably used in a molar ratio ranging between <NUM> and <NUM> to compound II.

More preferably the catalyst is Pd(PPh<NUM>)<NUM>, used in a molar ratio of <NUM> to compound II.

The inorganic base is selected from Na<NUM>CO<NUM>, K<NUM>CO<NUM> and Cs<NUM>CO<NUM>, preferably Na<NUM>CO<NUM>, and is used in a molar ratio ranging between <NUM> and <NUM>, preferably <NUM>, to compound II.

The alkali metal fluoride is preferably KF, and it is used in a molar ratio ranging between <NUM> and <NUM>, preferably <NUM>, to compound II.

The reaction is conducted in a mixture of acetone/water <NUM>/<NUM> or dioxane/water <NUM>/<NUM>, operating at a temperature ranging between <NUM> and the boiling point of the mixture of solvents.

A II/dioxane/water <NUM>/<NUM>/<NUM> w/v/v ratio and a II/acetone/water <NUM>/<NUM>/<NUM> w/v/v ratio are preferably used.

In specific embodiments of the invention, step a) is conducted in the presence of Pd(OAc)<NUM>, Pd(PPh<NUM>)<NUM> or Pd/C in the presence of KF in a mixture of dioxane/water under reflux.

In another specific embodiment, step a) is conducted in the presence of Pd(OAc)<NUM> and Na<NUM>CO<NUM> in a mixture of dioxane/water under reflux.

In a preferred embodiment, step a) is conducted in the presence of Pd(PPh<NUM>)<NUM> and potassium fluoride in a <NUM>/<NUM> mixture of dioxane/water.

The use of Pd/C as catalyst, which can be recovered at the end of the reaction, is advantageous in terms of the environmental sustainability of the process.

After conventional isolation and purifications, for example by dilution/trituration in a solvent belonging to the family of alkyl ethers, preferably in diisopropyl ether, compound IV is obtained with yields of around <NUM>% and a purity suitable for use in the next step (<NUM>%).

Step b) is typically conducted at a molar ratio of compound V to compound IV ranging between <NUM> and <NUM>, preferably <NUM>.

The compounds of formula V are known and commercially available. In a preferred embodiment, a compound of formula V is N-(<NUM>-chloroethyl)morpholine hydrochloride.

The reaction is typically conducted in the presence of <NUM>-<NUM> molar equivalents to compound IV of an inorganic base selected from K<NUM>CO<NUM> and Cs<NUM>CO<NUM>, preferably in the presence of <NUM> molar equivalents of Cs<NUM>CO<NUM>, and optionally in the presence of a phase-transfer catalyst.

The reaction is conducted in an alcohol-based solvent selected from methanol, ethanol and isopropanol, operating at the reflux temperature, or in dimethylformamide at a temperature not exceeding <NUM>. The reaction is preferably conducted in ethanol under reflux, optionally in the presence of tetrabutyl ammonium bromide.

After conventional processing, compound VI is isolated with a yield of <NUM>-<NUM>% and with suitable purity (about <NUM>%) for use in the next step.

If the reaction is conducted in an alcohol-based solvent other than the alcohol residue present in ester IV, the resulting compound VI consists of a mixture of esters derived from partial transesterification, which can be used "as is" in the next step.

Typically, step c) is conducted using <NUM> to <NUM>, preferably <NUM>, molar equivalents of benzylamine to compound VI, in, toluene. The reaction is conducted at the reflux temperature of the solvent, in the presence of <NUM>-<NUM> molar equivalents to the compound of formula VI, preferably <NUM> molar equivalents, of <NUM>-pyridone. The use of said catalysts allows the reaction to be conducted at lower temperatures but with reaction rates and yields comparable to those of reactions conducted in the absence thereof, using a solvent in normal industrial use such as toluene. Moreover, the crude reaction products of reactions conducted in the presence of said catalysts contain fewer by-products and/or impurities.

The crude reaction product can be purified according to the procedures described in the prior art, such as those described in <CIT>, to provide tirbanibulin base.

Tirbanibulin base can optionally be converted to tirbanibulin dihydrochloride according to the methods described in the prior art, for example in <CIT>.

Compound II is a known compound, obtainable by known methods. It is preferably obtained by esterification of <NUM>-(<NUM>-bromopyridin-<NUM>-yl)acetic acid of formula VII, which is commercially available,
<CHM>
with a (C<NUM>-C<NUM>)- alkyl alcohol, preferably methanol, in the presence of acid catalysis, preferably with sulphuric acid.

A further object of the present invention is therefore a process for the preparation of tirbanibulin wherein a compound of formula II is obtained by esterification of <NUM>-(<NUM>-bromopyridin-<NUM>-yl)acetic acid VII, with a C<NUM>-C<NUM> alkyl alcohol, preferably methanol, in the presence of acid catalysis.

The process of the invention is characterised by steps that take place in easily controlled temperature ranges, times and reaction conditions, requiring simple purifications, and supplying the corresponding reaction products with high yields and purity.

The invention is further illustrated by the following examples.

The <NUM>H and <NUM>C NMR spectra were recorded with a Bruker <NUM> instrument.

The HPLC analyses were performed with a Perkin Elmer instrument, using the method described in Example <NUM>.

The mass spectra were recorded with a Waters Micromass ZQ quadrupole instrument.

The reactions are conducted in a nitrogen atmosphere. <NUM> (<NUM> mmol) of <NUM>-(<NUM>-bromopyridin-<NUM>-yl)acetic acid VII is suspended in <NUM> of methanol in a <NUM>-necked <NUM>-litre flask. The suspension is heated to about <NUM> to obtain solubilisation, and <NUM> (d <NUM>; <NUM> mmol) of <NUM>% sulphuric acid is then cautiously added. The temperature of the reaction mixture rises spontaneously to <NUM>. The mixture is heated under reflux (<NUM>-<NUM>) for about three hours, to obtain a complete reaction. The reaction volume is reduced to about a third of the initial volume, and <NUM> of dichloromethane and about <NUM> of a <NUM>% aqueous solution of potassium bicarbonate are added until a pH of between <NUM> and <NUM> is reached. The phases are separated and the aqueous phase re-extracted with <NUM> x <NUM> of dichloromethane. The combined organic phases are washed with <NUM> x <NUM> of a <NUM>% aqueous solution of potassium bicarbonate, and then with <NUM> x <NUM> of water. The mixture is dried over sodium sulphate and filtered, and the solvent distilled, to obtain <NUM> of methyl ester as oil (<NUM> mmol; yield <NUM>%).

Following the same procedure, but using ethanol instead of methanol, ethyl <NUM>-bromopyridin-<NUM>-acetate (compound II, R = CH<NUM>CH<NUM>) is prepared.

The reaction is conducted in a nitrogen atmosphere, using solvents thoroughly degassed by bubbling nitrogen through them. <NUM> (<NUM> mmol) of methyl <NUM>-(<NUM>-bromopyridin-<NUM>-yl)acetate, <NUM> of <NUM>,<NUM>-dioxane (degassed) and <NUM> of purified water (degassed) are loaded into a <NUM>-necked <NUM> flask. <NUM> (<NUM> mmol) of <NUM>-hydroxyphenylboronic acid III, <NUM> (<NUM> mmol) of KF and <NUM> (<NUM> mmol) of Pd(PPh<NUM>)<NUM> are added to the resulting solution. The mixture is left under stirring under reflux for about <NUM> hours; the reaction is biphasic. A slight excess of <NUM>-hydroxyphenylboronic acid (<NUM>, <NUM> mmol) is added. A second excess of <NUM>-hydroxyphenylboronic acid (<NUM>, <NUM> mmol) is added after <NUM> hour, and the reaction is left to continue for a further <NUM> hours. The mixture is cooled to room temperature, the phases are separated, and the aqueous phase is re-extracted with <NUM> x <NUM> of dichloromethane + <NUM>% dioxane. <NUM> of dichloromethane and <NUM> of water are added to the combined organic phases. The phases are separated, and the organic phase dried over sodium sulphate. The organic phase is filtered and distilled at low pressure to obtain a dark yellow solid, which is broken up in <NUM> of diisopropyl ether. The solid is filtered, washed with <NUM> x <NUM> of diisopropyl ether, and dried under vacuum at <NUM>°±<NUM>.

The reaction is conducted in a nitrogen atmosphere. <NUM> (<NUM> mmol) of methyl <NUM>-[<NUM>-(<NUM>-hydroxyphenyl)-<NUM>-pyridyl]acetate in <NUM> of ethanol is loaded into a <NUM>-necked <NUM>-litre flask. <NUM> (<NUM> mmol) of <NUM>-(<NUM>-chloroethyl)morpholine hydrochloride, <NUM> (<NUM> mmol) of caesium carbonate and <NUM> (<NUM> mmol) of tetrabutylammonium bromide are added. The mixture is heated to reflux. After <NUM> hours under reflux, the reaction is completed and the formation of two products due to partial transesterification is observed in HPLC (method indicated in Example <NUM>). The caesium carbonate is filtered. The solvent is distilled at low pressure, and the residue (dark orange oil) is taken up with <NUM> of ethyl acetate. The organic phase is washed with <NUM> x <NUM> of water. After phase separation, the organic phase is decoloured with <NUM> of carbon, heating it in an inert atmosphere at <NUM>-<NUM> for <NUM>-<NUM> minutes. The carbon is filtered off, the filtrate is dried over sodium sulphate, the sodium sulphate is filtered, and the solvent is distilled at low pressure. The residue is taken up with heptane (<NUM>) and the solvent is distilled at low pressure, to obtain a yellow oil deemed sufficiently pure to be used "as is" in the next step.

The conversion is deemed to be <NUM>%, as the weight of the oil exceeds the theoretical value (<NUM>) due to the presence of residual solvent.

HPLC analysis conducted by the method described in Example <NUM> shows <NUM>% purity, considered sufficient for the next step.

The reaction is conducted in a nitrogen atmosphere. The product obtained according to Example <NUM>, deemed to contain <NUM> (<NUM> mmol) of methyl-<NUM>-(<NUM>-(<NUM>-(<NUM>-morpholinoethoxy)phenyl)pyridin-<NUM>-yl)acetate, dissolved in <NUM> of toluene, is loaded into a <NUM>-necked <NUM> flask. <NUM> (<NUM> mmol) of benzylamine and <NUM> (<NUM> mmol) of <NUM>-pyridone are added. The mixture is heated under reflux for <NUM>-<NUM> hours. The solvent is distilled at low pressure until a solid residue is obtained, which is taken up by dissolving it with <NUM> of dichloromethane. The organic phase is repeatedly washed with water (<NUM> x <NUM>). The phases are separated and the organic phase is decoloured by adding <NUM> of carbon and leaving it under stirring for <NUM>-<NUM> minutes at room temperature. The carbon is filtered, and the filtrate is dried over sodium sulphate and filtered. The dichloromethane is distilled to obtain a solid residue, which is taken up with <NUM> of toluene. The mixture is left under stirring to break it up at room temperature for <NUM>-<NUM> minutes, and <NUM> of a toluene:heptane <NUM>:<NUM> solution is then added. The mixture is left under stirring at room temperature for about <NUM>-<NUM> minutes; <NUM> of heptane is then added, and the resulting mixture is left under stirring at room temperature for about <NUM>-<NUM> minutes. The broken-up solid is filtered and washed, first with toluene:heptane <NUM>:<NUM> (<NUM>), next with toluene:heptane <NUM>:<NUM> (<NUM>), and finally with heptane (<NUM>), as described in <CIT>. After stove-drying under vacuum at <NUM>-<NUM>, <NUM> of tirbanibulin I is obtained in the form of a straw-coloured solid.

Claim 1:
A process for the preparation of tirbanibulin of formula I
<CHM>
comprising the following steps:
a) Suzuki reaction between a compound of formula II
<CHM>
wherein R is a (C<NUM>-C<NUM>)-alkyl, and <NUM>-hydroxyphenylboronic acid of formula III
<CHM>
in the presence of a palladium-based catalyst and in the presence of an inorganic base or of an alkali metal fluoride, to give a compound of formula IV
<CHM>
wherein R is as defined above in a mixture of acetone/water <NUM>/<NUM> or dioxane/water <NUM>/<NUM>, operating at a temperature ranging between <NUM> and the boiling point of the mixture of solvents;
b) reaction of a compound of formula IV with a compound of formula V
<CHM>
wherein X is selected from chlorine, bromine, iodine, (C<NUM>-C<NUM>)-alkyl sulphonate and aryl sulphonate, preferably chlorine, to give a compound of formula VI
<CHM>
wherein R is as defined above;
c) reaction of a compound of formula VI with benzylamine in toluene in the presence of <NUM>-pyridone a to give tirbanibulin of formula I.