CHM040PAT-WO AL & Partners Srl - 1 - “Process for the preparation of cholic acid derivatives” ************************** DESCRIPTION The present invention relates to a process for the preparation of cholic acid derivatives, in particular of compounds of formula (V): The present invention also relates to compounds useful for the preparation of such derivatives. Cholic acid is a primary bile acid synthesized in the liver from cholesterol through multiple complementary enzymatic processes. Bile acids include a group of molecular species with similar chemical structures, which are secreted in the bile and transported into the lumen of the small intestine where they act as emulsifiers aiding fat digestion and absorption, as well as endocrine molecules capable of controlling different signaling pathways. Bile acids (BA) are not only digestive surfactants, but also important cell signaling molecules, which stimulate different signaling pathways to regulate some important biological processes. The nuclear bile acid-activated receptor, the farnesoid X receptor (FXR), plays a critical role in the regulation of bile acid, lipid, and glucose homeostasis, as well as in the regulation of inflammatory responses, barrier function, and the prevention of bacterial translocation in the intestinal tract. As expected, FXR is involved in the pathophysiology of a broad range of diseases of the gastrointestinal tract, including inflammatory bowel disease, colorectal cancer, and type 2 diabetes. The identification of new steroid molecules capable of binding and modulating the FXR receptor has proven to be of significant importance for the discovery of new possible therapies. Particularly interesting in this sense were steroid derivatives having one carbon atom less on the side chain, such as BAR502 and nor-UDCA. CHM040PAT-WO AL & Partners Srl - 2 - Most of the synthetic protocols involving BA therefore include side chain modification and for this purpose the carboxylic functional group placed in C-24 plays a crucial role. The dehomologation of carboxylic acids allows the adaptation of a carbon chain to the desired length and can be obtained with different methodologies. There are several general protocols known in the literature for the dehomologation of the carboxylic functional group to the corresponding alcohol, with specific reference to examples of steroid derivatives. M.F. Saraiva et al., Tetrahedron, 2009, 65, 3563–3572 describes the following process for the transformation of cholic acid derivatives into the corresponding bromides with one carbon atom less by the Barton-McCombie reaction using 2- mercaptopyridine N-oxide sodium salt and bromotrichloromethane. Hernández-Huerta E. et al; “A Straightforward One-Pot Two-Step Conversion of Bile Acids into Dehomologated Alcohols”, Steroids 2021, 176, 108917 describes a one- pot procedure which allows the synthesis of dehomologated alcohols from carboxylic acids with the use of hypervalent iodine, MCPBA and photochemical irradiation. The above examples are not industrially convenient, as they involve complex technologies and isolation and purification procedures. Another example of an approach reported in the literature, with reference to cholic acid derivatives, consists in the rearrangement of the carboxylic acids into the corresponding dehomologated amines. Pellicciari R. et al. J. Med. Chem.2004, 47, 4559-4569 describes a process in which the Schmidt reaction is used. The preparation of the corresponding acyl chloride is followed by the reaction with sodium azide to give the corresponding acyl azide. This, in turn, is rearranged into the corresponding isocyanate which is reacted with an CHM040PAT-WO AL & Partners Srl - 3 - alcohol to give the corresponding carbamate and subsequently hydrolyzed to the final dehomologated amine. Again, the multi-step procedure is complex as it requires the protection of the hydroxyl groups which are not compatible with the formation of the acyl chloride using thionyl chloride. Furthermore, this procedure leads to mixtures of products making it unsuitable for industrial scalability. There is therefore the need to find a new synthetic approach to dehomologate the side chain of derived cholic acids which employs simple and repeatable reactions on an industrial scale. The object of the present invention is therefore a process for the preparation of a compound of formula (V) which comprises: a) Reacting a compound of formula (I) CHM040PAT-WO AL & Partners Srl - 4 - with an organic azide of formula R-N3 in an aprotic solvent in the presence of a base wherein R is selected from diphenylphosphoryl, trimethylsilyl, trifluoromethanesulfonyl, preferably a diphenylphosphoryl group. to give the corresponding isocyanate of formula (II) in which R1 represents a -C=O group, or an OH group either in α or β configuration; R2 represents a hydrogen atom, or an OH group; b) Reacting the isocyanate of formula (II) thus obtained with an alcohol of formula R3- OH to give the carbamate of formula (III) in which R1 and R2 have the above meanings; R3 represents a linear or branched C1-10 alkyl, aryl or alkylaryl. c) Hydrolyzing the carbamate of formula (III) in the presence of an acid or a base to give the amine of formula (IV) CHM040PAT-WO AL & Partners Srl - 5 - in which R1 and R2 have the above meanings. d) Subjecting the compound of formula (IV) to a diazotization reaction in the presence of a suitable reagent to provide the corresponding desired alcohol of formula (V). The process of the present invention is a dehomologation process for the preparation of an alcohol derivative of a cholic acid starting from the corresponding carboxylic acid, in which said alcohol derivative contains one carbon atom less than the starting carboxylic acid. The term "dehomologation" according to the present invention means one or more reactions which allow the side chain of the starting cholic acid derivative to be shortened by a -CH2 group. Step a) of the process object of the present invention consists of a Curtius reaction which, as it is well known to the skilled person, is carried out by reacting a carboxylic acid of formula (I) dissolved in an aprotic solvent with an organic azide of formula R- N3, wherein R is selected from a diphenylphosphoryl, trimethylsilyl, trifluoromethanesulphonyl group, preferably it is a diphenylphosphoryl group. Examples of aprotic solvents which can be used in step a) are N-methylpyrrolidone, N,N-dimethylacetamide, N,N-dimethylformamide, tetrahydrofuran, methyltetrahydrofuran, toluene, methylene chloride and mixtures thereof, preferably tetrahydrofuran. To the reaction mixture containing the acid of formula (I) dissolved in the above aprotic solvent and the azide, a tertiary amine selected from triethylamine, tributylamine, diisopropylethylamine, N,N-dimethylaminopyridine is added. Preferably said tertiary amine is triethylamine. According to the present invention, said reaction is carried out at a temperature ranging from room temperature to the reflux temperature of the solvent for a time comprised between 2 to 6 hours, preferably about 4 hours. CHM040PAT-WO AL & Partners Srl - 6 - Once the reaction has been completed, the isocyanate of formula (II) is isolated by techniques well known to those skilled in the art, such as for example filtration, extraction, precipitation, crystallization. Subsequently, the isocyanate of formula (II) thus obtained is subjected in step b) of the present process to a reaction with an alcohol of formula R3-OH, wherein R3 is a linear or branched alkyl C1-C4 group, an aryl or an alkylaryl. Said alcohol is preferably selected from methanol, ethanol, isopropanol, tert-butanol, more preferably it is tert-butanol. As it will be clear to the person skilled in the art, steps a) and b) can be carried out as described above, i.e. by isolating the isocyanate of formula (II) formed at the end of step a). Alternatively, steps a) and b) can be performed "one pot", i.e. without isolating the intermediate compound. Preferably, steps a) and b) are carried out "one pot", i.e. with the use of R ₃ -OH directly as the reaction solvent. According to the present invention, the carbamate of formula (III) is hydrolyzed in step c) of the process in the presence of an acid preferably selected from hydrochloric acid, sulfuric acid, or in the presence of a base preferably selected from sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate, more preferably potassium hydroxide. Preferably, said acid or said base is used in a molar amount comprised between 0.8 equivalents and 5 equivalents, more preferably comprised between 1 equivalent and 2.5 equivalents, with respect to the molar amount of carbamate of formula (III). This hydrolysis reaction takes place in a polar protic solvent selected from methanol, ethanol, isopropanol, 1-methoxy-2-propanol, butanol, preferably 1-methoxy-2- propanol. Contrary to the previous synthetic methods described in the art, thanks to the process of the present invention, it was possible to isolate the amine of formula (IV) with a yield of 87% and a conversion of 95%. Once the amine of formula (IV) has been isolated by techniques well known to the skilled person, according to the present invention, said amine is subjected to a diazotization reaction in step d). This reaction is carried out in the presence of a suitable reagent capable of releasing a nitrosonium ion, such as for example sodium nitrite, in acetic acid alone or in a CHM040PAT-WO AL & Partners Srl - 7 - mixture with water, preferably in acetic acid alone, or an organic nitrite R-NO2, where R is a linear or branched alkyl group. Preferably, sodium nitrite is used in a molar amount comprised between 1 equivalent and 3 equivalents, more preferably of about 2 equivalents, with respect to the molar amount of amine of formula (IV). According to a particularly preferred embodiment, the process of the present invention allows to dehomologate the side chain of a cholic acid derivative, such as 7K-LCA (compound of formula (Ia)), for the preparation of a key intermediate of the BAR502, or the compound of formula (Va): or of the corresponding acetyl derivative of formula (Vb), in turn hydrolyzed to give the compound of formula (Va). Therefore, the present invention is preferably directed to a process for the preparation of a compound of formula which comprises: a) Reacting a compound of formula (Ia) CHM040PAT-WO AL & Partners Srl - 8 - with an organic azide of formula R-N3 in an aprotic solvent in the presence of a base wherein R is selected from diphenylphosphoryl, trimethylsilyl, trifluoromethanesulfonyl, to give the corresponding isocyanate of formula (IIa) b) Reacting the isocyanate of formula (IIa) thus obtained with R ₃ -OH to give the carbamate of formula (III) wherein R3 is selected from methyl and tert-butyl, c) Hydrolyzing the carbamate of formula (IIIa) or (IIIb) in the presence of a base to give the amine of formula In which R ₁ and R ₂ have the above meanings. CHM040PAT-WO AL & Partners Srl - 9 - d) Subjecting the compound of formula (IVa) to a diazotization reaction in the presence of sodium nitrite in acetic acid to give the corresponding desired alcohol of formula (Va). As can be seen, the compounds of formula (Ia), (IIa), (IIIa), (IIIb), (IVa) and (Va) correspond to the compounds of formula (I), (II), (III), (IV) and (V) respectively in which the meanings of R1, R2 and R3 are as follows: - R1 represents a group -C=O; - R2 represents a hydrogen atom; - R ₃ represents a methyl or tert-butyl group. The compounds of formula (IIa), (IIIa), (IIIb) and (IVa) are new and constitute a further object of the present invention. Furthermore, these compounds are useful intermediates for the synthesis of cholic acid derivatives, in particular of a key intermediate for the synthesis of BAR502, i.e. the compound of formula (Va). A further object is therefore the use of the compounds of formula (IIa), (IIIa), (IIIb), (IVa) as intermediates in the synthesis of cholic acid derivatives and preferably in the synthesis of the compound of formula (Va). While the invention has been described in its characteristic aspects, modifications and equivalents which are apparent to those skilled in the art are included in the following invention. The present invention will now be illustrated by means of some examples, which are not to be seen as limiting the scope of the invention. EXAMPLES EXAMPLE 1. Synthesis of methyl ((3R)-3-((3R,10S,13R,17R)-3-hydroxy-10,13- dimethyl-7-oxohexadecahydro-1H-cyclopenta[a]phenanthren-17-y l) butyl)carbamate. 7-keto lithocholic acid (20.0 g, 1 eq), tetrahydrofuran (100.0 mL, 5 vol) was charged to a reaction flask, the temperature was brought to about 25°C, and triethylamine (8.5 ml, 1.2 eq) and diphenylphosphorylazide (DPPA, 15.5 g, 1.1 eq) were added. The reaction was heated to reflux and kept under these conditions for about 2 hours. The temperature was brought to room temperature and the resulting solution was dropped for about 4 hours in a 30% solution of sodium methoxide in methanol (48.3 mL, 5 eq). The reaction mixture was kept under these conditions for about two hours and, when the reaction was finished, methyl tetrahydrofuran (200.0 ml, 10 vol) and water (100.0 CHM040PAT-WO AL & Partners Srl - 10 - ml, 5 vol) were added. The organic phase was washed with 1M sodium hydroxide (2 x 200.0 mL), dried with magnesium sulfate and reduced to a residue by distillation under reduced pressure to give 20.5 g of methyl ((3R)-3-((3R, 10S,13R, 17R)-3- hydroxy-10,13-dimethyl-7-oxohexadecahydro-1H-cyclopenta[a]ph enanthren-17- yl)butyl)carbamate (96%). ¹ H-NMR (d6-DMSO, 400MHz): δ 6.98 (t, 1H), δ 4.5 (s, 1H), δ 3.5 (s, 3H), δ 3.35 (m, 1H), δ 3.03 (m, 1H), δ 2.9 (m, 2H), δ 2.46 (t, 1H), δ 2.07 (m, 1H), δ 1.8 (d, 1H), δ 1.7 (m, 5H), δ 1.5 (m, 3H), δ 1.4 (m, 4H), δ 1.13 (m, 10H), δ 0.9 (d, 4H), δ 0.62 (s, 3H). ¹³ C-NMR (d6-DMSO, 400MHz): δ 211.6, δ 157.03, δ 69.6, δ 55.02, δ 51.5, δ 49.3, δ 49.05, δ 42.65, δ 39.1, δ 38.26, δ 37.85, δ 36.16, δ 35.2, δ 34.36, δ 33.48, δ 30.27, δ 28.41, δ 24.89, δ 23.23, δ 21.71, δ 21.32, δ 19.01, δ 12.28. EXAMPLE 2. Synthesis of (3R,10S,13R,17R)-17-((R)-4-aminobutan-2-yl)-3- hydroxy-10,13-dimethylhexadecahydro-7H-cyclopenta[a]phenanth rene-7-one. In a reaction flask, methyl-((3R)-3-((3R,10S,13R,17R)-3-hydroxy-10,13-dimethyl-7- oxohexadecahydro-1H-cyclopenta[a]phenanthrene- 17-yl)butyl)carbamate (10.0 g, 1 eq), 1-methoxy-2-propanol (50.0 ml, 5 vol) and potassium hydroxide (3.0 g, 2 eq) were charged and the temperature was brought to about 25°C. The resulting suspension was heated to reflux and kept under these conditions for about 16 hours. At the end of the reaction, the mixture was brought to room temperature and filtered on paper. The residual solid was washed with 1-methoxy-2-propanol (2 x 25.0 mL) and concentrated by distillation under reduced pressure to give 7.5 g of (3R,10S,13R,17R)-17-((R)-4-aminobutan-2-yl)-3-hydroxy-10,13- dimethylhexadecahydro-7H-cyclopenta[a]phenanthren-7-one (87%). ¹ H-NMR (d6-DMSO, 400MHz): δ 3.35 (m, 1H), δ 2.89 (m, 1H), δ 2.5 (s, 2H), δ 2.44 (m, 2H), δ 2.05 (m, 1H), δ 1.93 (d, 1H), δ 1.8 (m, 3H), δ 1.4 (m, 5H), δ 1.09 (s, 3H), δ 1.07 (m, 4H), δ 1.02 (d, 2H), δ 0.88 (d, 4H), δ 0.62(s, 3H). ¹³ C-NMR (d6-DMSO, 400MHz): δ 211.85, δ 129.2, δ 69.6, δ 55.36, δ 49.3, δ 49.05, δ 42.65, δ 37.9, δ 35.2, δ 34.36, δ 33.6, δ 30.3, δ 28.55, δ 24.89, δ 23.26, δ 21.69, δ 20.75, δ 19.3, δ 12.3 EXAMPLE 3. Synthesis of tert-butyl ((3R)-3-((3R,10S,13R,17R)-3-hydroxy-10,13- 1H-cyclopenta[a]phenanthrene-17- 7-keto lithocholic acid (10.0 g, 1 eq), tert-butanol (50.0 ml, 5 vol) and tetrahydrofuran (10.0 ml, 1 vol) were charged to a reaction flask to give a colorless solution. CHM040PAT-WO AL & Partners Srl - 11 - Triethylamine (4.3 ml, 1.2 eq) and diphenylphosphorylazide (DPPA, 7.75 g, 1.1 eq) were added to the solution and the mixture was heated to reflux and kept under these conditions for about 4 hours. At the end of the reaction, the temperature was brought to about 20°C and methyl tetrahydrofuran (100.0 ml, 10 vol) and a 2M sodium hydroxide solution (30.0 ml, 3 vol) were added. The organic phase was washed with 2M sodium hydroxide (2 x 50.0 mL) and brine (50.0 mL), dried with magnesium sulfate and reduced to a residue by distillation under reduced pressure to give 16.0 g of tert- butyl ((3R)-3-((3R,10S,13R,17R)-3-hydroxy-10,13-dimethyl-7-oxohexa decahydro- 1H-cyclopenta[a]phenanthren-17-yl)butyl)carbamate (100%). ¹ H-NMR (d6-DMSO, 400MHz): δ 5.7 (t, 1H), δ 4.5 (s, 1H), δ 3.5 (s, 3H), δ 3.03 (m, 1H), δ 2.9 (m, 2H), δ 2.44 (t, 1H), δ 2.07 (m, 1H), δ 1.9 (d, 1H), δ 1.7 (m, 3H), δ 1.5 (m, 3H), δ 1.4 (m, 7H), δ 1.13 (m, 8H), δ 0.9 (d, 4H), δ 0.62(s, 3H) ¹³ C-NMR (d6-DMSO, 400MHz): δ 211.6, δ 158.6, δ 139.6, δ 69.6, δ 55.4, δ 55.2, δ 49.3, δ 49.05, δ 45.85, δ 45.53, δ 42.65, δ 42.64, δ 39.1, δ 37.9, δ 37.21, δ 36.7, δ 35.2, δ 34.85, δ 34.36, δ 33.48, δ 30.89, δ 30.27, δ 28.7, δ 28.41, δ 24.89, δ 23.23, δ 21.71, δ 19.11, δ 12.37 EXAMPLE 4. Synthesis of (3R,10S,13R,17R)-17-((R)-4-aminobutan-2-yl)-3- hydroxy-10,13-dimethylhexadecahydro-7H-cyclopenta[a]phenanth rene-7- one. Tert-butyl ((3R)-3-((3R,10S,13R,17R)-3-hydroxy-10,13-dimethyl-7- oxohexadecahydro-1H-cyclopenta[a]phenanthren-17-yl)butyl)car bamate (14.0 g, 1eq), acetonitrile (70.0 ml, 5 vol) and methanol (70.0 ml, 5 vol) were charged into a reaction flask and the temperature was brought to about 25°C. A 37% hydrochloric acid solution (4.0 mL, 1.5 eq) was dropped over 10 minutes into the reaction. The reaction was maintained under these conditions for about 6 hours. When the reaction was complete, the solvent was removed and methyl tetrahydrofuran (210.0 mL, 15 vol) and tetrahydrofuran (70.0 mL, 5 vol) were added. The mixture was washed with hydrochloric acid (280.0 mL) and filtered through celite washing with methyl tetrahydrofuran (70.0 mL, 5 vol) and tetrahydrofuran (28.0 mL, 2 vol). The aqueous phase was basified at pH = 10 with sodium carbonate. Methyl tetrahydrofuran (140.0 mL, 15 vol) and tetrahydrofuran (52.0 mL, 5 vol) were added. The organic phase was then dried with magnesium sulfate, filtered and reduced to a residue by distillation under reduced pressure to give 9.3 g of (3R,10S,13R,17R)-17-((R)-4-aminobutan-2- yl)-3-hydroxy-10,13-dimethylhexadecahydro-7H-cyclopenta[a]ph enanthrene-7-one (80%). CHM040PAT-WO AL & Partners Srl - 12 - ¹ H-NMR (d6-DMSO, 400MHz): δ 3.35 (m, 1H), δ 2.89 (m, 1H), δ 2.5 (s, 2H), δ 2.44 (m, 2H), δ 2.05 (m, 1H), δ 1.93 (d, 1H), δ 1.8 (m, 3H), δ 1.4 (m, 5H), δ 1.09 (s, 3H), δ 1.07 (m, 4H), δ 1.02 (d, 2H), δ 0.88 (d, 4H), δ 0.62(s, 3H). ¹³ C-NMR (d6-DMSO, 400MHz): δ 211.85, δ 129.2, δ 69.6, δ 55.36, δ 49.3, δ 49.05, δ 42.65, δ 37.9, δ 35.2, δ 34.36, δ 33.6, δ 30.3, δ 28.55, δ 24.89, δ 23.26, δ 21.69, δ 20.75, δ 19.3, δ 12.3 EXAMPLE 5. Synthesis of (3R,10S,13R,17R)-3-hydroxy-17-((R)-4-hydroxybutan- 2-yl)-10,13-dimethylhexadecahydro-7H-cyclopenta[a]phenanthre n-7 -one. (3R,10S,13R,17R)-17-((R)-4-aminobutan-2-yl)-3-hydroxy-10,13- dimethylhexadecahydro-7H-cyclopenta[a]phenanthrene-7-one (1.0 g, 1 eq) and acetic acid (5.0 ml, 5 vol) were charged into a reaction flask, the reaction temperature was brought to about 25°C and sodium nitrite was added in small portions (0.38 g, 2 eq). The mixture was kept under stirring under these conditions for about 16 hours. At the end of the reaction, this mixture was brought to pH = 9 by adding sodium bicarbonate. The product was extracted with ethyl acetate (10.0 mL, 10 vol) and the organic phase was washed with an aqueous solution of sodium bicarbonate (55.0 mL, 5 vol) and brine (5.0 mL, 5 vol). The solvent was removed by vacuum distillation to give an oily mixture containing the acetylated derivative. The mixture was subsequently treated with a solution of 30% sodium hydroxide (5 eq) in methanol (10 vol) at reflux until complete conversion after 16 hours to give (3R,10S,13R,17R)-3- hydroxy-17 -((R)-4-hydroxybutan-2-yl)-10,13-dimethylhexadecahydro-7H- cyclopenta[a]phenanthren-7-one (80%) and an analytically pure sample was purified by column chromatography (eluent dichloromethane :methanol 98:2). ¹ H-NMR (d6-DMSO, 400MHz): δ 4.49 (m, 1H), δ 4.25 (m, 1H), δ 3.45 (s, 1H), δ 3.35 (s 4H), δ 2.9 (m, 1H), δ 2.44 (t, 1H), δ 2.1 (m, 1H), δ 1.9 (m, 1H), δ 1.8 (m, 2H), δ 1.7 (m, 3H), δ 1.14 (s, 3H), δ 1.08 (m, 5H), δ 0.9 (d, 4H), δ 0.62 (s, 3H). ¹³ C-NMR (d6-DMSO, 400MHz): δ 211.85, δ 69.6, δ 58.9, δ 55.4, δ 49.3, δ 49.05, δ 45.85, δ 45.5, δ 42.7, δ 42.65, δ 37.9, δ 35.2, δ 34.36, δ 30.3, δ 28.55, δ 24.89, δ 23.26, δ 21.69, δ 19.3, δ 12.3