“Process for the preparation of nor-ursodeoxycholic acid” ************************** DESCRIPTION The present invention is directed to a process for the synthesis of nor-ursodeoxycholic acid of formula (I): Nor-ursodeoxycholic acid (nor-UDCA) is a derivative of cholic acid, 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 into the bile and transported into the lumen of the small intestine where they act as emulsifiers promoting the digestion and absorption of fats, as well as endocrine molecules capable of controlling different signaling routes. Bile acids (BA) are not only digestive surfactants, but also important cell signaling molecules, which stimulate various signaling pathways to regulate some important biological processes. The bile acid-activated nuclear receptor, the farnesoid X receptor (FXR), plays a fundamental role in the regulation of homeostasis of bile acids, lipids and glucose, as well as in the regulation of inflammatory responses, barrier function and prevention of bacterial translocation in the intestinal tract. As expected, FXR is involved in the pathophysiology of a wide 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 is thus important for the discovery of new possible therapies. Steroid derivatives having one less carbon atom on the side chain, such as BAR502 and nor-UDCA were considered particularly interesting. Nor-ursodeoxycholic acid is a bile acid, derivative of ursodeoxycholic acid (UDCA), in which there is one less carbon atom on the side chain. Based on its specific pharmacological properties, nor-UDCA is a promising drug for some cholestatic disorders of the liver and bile duct. Recently, nor-UDCA was successfully clinically tested in patients with primary sclerosing cholangitis (PSC) as the first application in patients. Furthermore, hepatic enrichment of nor-UDCA facilitates direct therapeutic effects on both parenchymal and non-parenchymal liver cells, thus counteracting cholestasis, steatosis, hepatic inflammation and fibrosis, inhibiting hepatocellular proliferation and promoting autophagy. This could open up its therapeutic use to other non-cholestatic and metabolic liver diseases. Hence the intention to develop a new process for the synthesis of nor-ursodeoxycholic acid that uses simple and repeatable reactions on an industrial scale. Various syntheses of nor-ursodeoxycholic acid are known in the literature. The patent IT1424122 B1 describes a method for the synthesis of nor-UDCA starting from ursodeoxycholic acid (UDCA), as shown in scheme 1: Scheme 1 However, the described synthesis requires the use of dangerous and difficult to control reagents. Furthermore, it leads to the formation of multiple impurities with consequent purification difficulties and reduction of the global yield. The patent application CN 114276401 A describes a method for the synthesis of nor- ursodeoxycholic acid through a series of redox reactions starting from (3α,5β,7α,23R)-3,7,23-trihydroxycholan-24-oic acid, as shown in scheme 2: In this case the synthesis is complex due to the used reagents and requires the use of expensive enzymes and coenzymes. We have now found an innovative process for the synthesis of nor-ursodeoxycholic acid which uses mild reagents and allows the desired product to be obtained in a few steps only. The object of the present invention is therefore a process for the synthesis of nor- ursodeoxycholic acid of formula (I) comprising the following steps: a) Oxidation of the primary alcohol of formula (II) in the presence of an oxidizing agent, a radical initiator and a phase transfer system, to give the corresponding aldehyde of formula (III) b) Conversion of the aldehyde of formula (III) thus obtained into the corresponding carboxylic acid of formula (IV) c) Reduction of the carbonyl group in said compound of formula (IV) in the presence of a reducing reagent to give the desired nor-ursodeoxycholic acid of formula (I). Step a) of the process object of the present invention consists of an oxidation reaction in the presence of an oxidizing agent selected from sodium hypochlorite, sodium chlorite, hydrogen peroxide, potassium permanganate, preferably sodium hypochlorite. According to the present invention, said oxidation reaction occurs in the presence of a radical initiator, preferably TEMPO, a phase transfer system in a mixture of water and an apolar aprotic solvent. Any phase transfer system known in the art can be used in step a) of the present invention. Preferably, said phase transfer system is a mixture of sodium bromide and tetrabutylammonium bromide. More preferably the molar ratio between sodium bromide and tetrabutylammonium bromide is 1:2. In step a) of the process of the present invention, the apolar aprotic solvent used is preferably selected from dichloromethane, dimethylacetamide, dimethylformamide, tetrahydrofuran, methyl-tetrahydrofuran, toluene and mixtures thereof with water. A mixture of dichloromethane with water is preferably used. According to an embodiment of the present invention, the primary alcohol of formula (II) is dissolved in a mixture of water and an apolar aprotic solvent. This primary alcohol of formula (II) can be synthesized according to one of the methodologies described in the art, and in particular according to the process described by the Applicant of the present invention in the patent application IT 102022000008861. Subsequently, the phase transfer system is added to the reaction mixture. According to an embodiment of the present invention, in said phase transfer system, sodium bromide is present in a molar amount comprised between 0.1 equivalents and 1 equivalent, preferably about 0.55 equivalents, compared to the molar amount of the alcohol of formula (II). According to a further embodiment of the present invention, in said phase transfer system, the tetrabutylammonium bromide is present in a molar amount comprised between 0.2 equivalents and 2 equivalents, preferably of about 1.1 equivalents, with respect to the molar amount of the alcohol of formula (II). As it is well known to those skilled in the art, this phase transfer system can be added as such or can be dissolved in a suitable solvent, for example in water. Preferably, such phase transfer system is added as an aqueous solution. Once the reaction mixture has cooled, the radical initiator and the oxidizing agent are added sequentially. Preferably, the radical initiator is added in a molar amount comprised between 0.1 equivalents and 0.5 equivalents, more preferably about 0.3 equivalents, with respect to the molar amount of the alcohol of formula (II). According to a preferred embodiment of the present invention, the oxidizing agent is added to the reaction mixture as an aqueous solution in a concentration comprised between 1 and 10 equivalents, more preferably about 6 equivalents. Once the reaction is completed, the aldehyde of formula (III) thus obtained is isolated using techniques well known to those skilled in the art and subsequently subjected to a variant of the Pinnick reaction in step b) of the process of the present invention. As it is well known to those skilled in the art, this reaction involves reacting an aldehyde dissolved in a polar protic solvent with sodium chlorite in the presence of sodium dihydrogen phosphate to obtain the corresponding carboxylic acid. Examples of polar protic solvents that can be used in step b) are methanol, ethanol, isopropanol, tert-butanol and mixtures thereof, more preferably it is tert-butanol. Preferably, according to the present invention, hydrogen peroxide is added to the reaction mixture in a molar amount comprised between 10 and 100 equivalents, more preferably about 62 equivalents, compared to the molar amount of the aldehyde of formula (III). As it will be clear to those skilled in the art, steps a) and b) can be carried out as described above, i.e. by isolating the aldehyde of formula (III) formed at the end of step a). Alternatively, steps a) and b) can be carried out "one pot", i.e. without isolating the intermediate compound. In this case, the oxidizing system, i.e. sodium chlorite and hydrogen peroxide, is added directly to the reaction mixture containing the aldehyde of formula (III), without isolating or purifying the aldehyde present in said mixture. Preferably, steps a) and b) are carried out "one pot". According to the present invention, the carbonyl group present in position 7 of the carboxylic acid of formula (IV) is reduced in step c) of the process, with methods described in the literature such as for example in Tetrahedron Letters, 1983, 24, 2487- 2490. According to one embodiment of the present invention, the carboxylic acid of formula (IV) is dissolved in a polar protic solvent. Examples of polar protic solvents that can be used in step c) are methanol, ethanol, isopropanol, nor-butanol, tert-butanol and mixtures thereof, more preferably it is tert- butanol. A reducing agent, such as for example metallic potassium or metallic sodium, is subsequently added to the reaction mixture thus formed. Thanks to the process of the present invention, and in particular to specific intermediates such as the compounds of formula (II), (III), (IV), it was possible to synthesize nor-ursodeoxycholic acid of formula (I) in a few steps only. As described above, the process of the present invention uses mild reagents, is repeatable on an industrial scale and provides a product with a purity suitable for use in the pharmaceutical sector. A further object of the present invention is therefore the use of the compounds of formula (II) and (IV) in the synthesis of the nor-UDCA acid of formula (I). Although 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 should not be seen as limiting the scope of the invention. EXAMPLE 1. Synthesis of (3R)-3-((3R,10S,13R,17R)-3-hydroxy-10,13-dimethyl- 7-oxohexadechydro-1H-cyclopenta[α]phenanthren-17-yl)butanoi c acid (IV). In a reaction flask, under nitrogen, (3R,10S,13R,17R)-3-hydroxy-17-((R)-4- hydroxybutan-2-yl)-10,13-dimethylhexadechydro-7H-cyclopenta[ α] phenanthren-7- one (1.0 g, 1 eq), a mixture (6:1) of dichloromethane (40.0 ml, 40 vol) and water (7.0 ml, 7 vol), a 1,0 M aqueous solution of sodium bromide (1.5 ml, 1.5 vol), a 1.0 M aqueous solution of tetrabutylammonium bromide (TBAB, 3 ml, 3 vol) were loaded. The reaction was brought to approximately 5-10°C and TEMPO (0.13 g, 0.3 eq), a saturated solution of sodium bicarbonate (7.5 ml, 3.5 eq), a 1.8 M aqueous solution of sodium hypochlorite (9.3 ml, 6 eq) were added. The reaction was heated to 20°C and maintained under these conditions for approximately 2 hours. At the end of the reaction, hydrochloric acid (3.0 ml, 1.1 eq) was added to the reaction mixture and kept under stirring for about 5 minutes. Subsequently, tert-butanol (28.0 ml, 28 vol), a 9.8 M aqueous solution of hydrogen peroxide (17.5 ml, 62 eq), a 1.0 M aqueous solution of sodium chlorite (33.0 ml, 12 eq), a 1.0 M aqueous solution of sodium dihydrogen phosphate (6.0 ml, 7.5 eq) were added and the mixture was maintained at a temperature of 20-25 °C for approximately two hours. Once the reaction was complete, the organic phase was washed with water (3 x 40.0 ml), 1.0M sodium hydroxide (1 x 40.0 ml), 2.0 N hydrochloric acid (1 x 20.0 ml) and again with water (3 x 40.0 ml). The organic phase was then dried over magnesium sulphate and reduced to a residue by vacuum distillation to give (3R)-3- ((3R,10S,13R,17R)-3-hydroxy-10,13-dimethyl-7-oxohexadechydro - 1H- cyclopenta[α]phenanthren-17-yl) butanoic acid. The product was purified by chromatographic column (eluent hexane:1-butanol 9:1) (70%). ¹ H-NMR (DMSO, 400 MHz): δ 11.8 (1H, s), 3.37 (3H, m), 2.90 (1H, m), 2.49 (1H, t), 2.08 (1H, m), 1.96 (1H, m), 1.80 (4H, m), 1.44 (6H, m), 1.15 (11H, m), 0.90 (4H, m), 0.63 (3H, s). ¹³ C-NMR (DMSO, 400 MHz): δ 211.9, δ 174.7, δ 69.6, δ 59.0, δ 55.4, δ 49.3, δ 49.1, δ 45.9, δ 45.5, δ 42.7, δ 42.6, δ 39.3, δ 37.9, δ 35.2, δ 34.3, δ 32.8, δ 30.3, δ 28.5, δ 24.9, δ 23.3, δ 21.7, δ 19.3, δ 12.3 δ. EXAMPLE 2. Synthesis of nor-ursodeoxycholic acid (I). The desired product (I) was obtained starting from the compound (3R)-3- ((3R,10S,13R,17R)-3-hydroxy-10,13-dimethyl-7-oxohexadechydro -1H-cyclopenta[α] phenanthren-17-yl) butanoic acid (IV) following the method described in Tetrahedron Letters, 1983, 24, 2487-2490 (entry 8) with a yield of 94%. ¹³ C-NMR (CD3OD, 400 MHz): δ 177.3, δ 72.1, δ 71.9, δ 57.5, δ 56.6, δ 44.8, δ 44.5, δ 44.0, δ 42.5, δ 41.4, δ 40.7, δ 38.6, δ 38.0, δ 36.1, δ 35.2, δ 34.9, δ 31.0, δ 29.7, δ 27.9, δ 23.9, δ 22.4, δ 20.2, δ 12.7.