Ergothioneine, discovered in 1909 by Tanret in ergot of rye, is an amino acid of natural origin, with antioxidant properties, corresponding to the following formula: O HN NH NMea Y s Its presence has been demonstrated not only in a large number of fungi and mycobacteria, but also in plants, animals and humans. It is at the level of their roots that plants absorb ergothioneine biosynthesised by the fungi attached thereto. It is only through their diet that higher organisms, and humans in particular, ingest this compound. Several syntheses of this molecule have thus been proposed in the literature, but only two of them make it possible to end up with L-ergothioneine (natural enantiomer). The international application WO 95/00 494 proposes a 5-step synthesis of L-ergothioneine by reaction of the methyl ester of Na,No(-dimethyl-histidine (itself obtained from L-histidine in 2 steps) with phenyl chlorothioformate, then reaction with ethyl chloroformate, formation of the quaternary ammonium and finally de-protection of the sulphur and the methyl ester. Thus, such a synthesis strategy necessitates the protection of sulphur, which cannot be present in free form, in order to enable the methylation of the dimethylamine group to lead to the betaine function. In addition, the phenyl chlorothioformate must be prepared from thiophosgene (CSCI2) , a toxic reagent 2 and available with difficultly in large quantity for use at the industrial scale. The patent application US 2009/093 642 also describes a 9-step synthesis of L-ergothioneine from histidine by opening of the imidazole cycle and reaction with a thiocyanate such as potassium thiocyanate, to give 2-thiohistidine (according to the method described by Heath, H. et al. , 1951, J.Chem.Soc, 2215), then protection of the sulphur by a tertiobutyl group, formation of quaternary ammonium and de-protection of the sulphur. Apart from the use of large volumes of hydrochloric acid, KSCN, used in acid medium, is a highly toxic reagent. These two methods have several common points. Apart from their high number of steps, they cumulate the drawbacks of using not only very toxic reagents but also considerable quantities of organic solvents and concentrated hydrochloric acid, which lead to risks in environmental terms. In terms of the synthesis strategy, these two methods have in common introducing sulphur, from histidine or one of the N-dimethylated derivatives thereof, before generating the betaine group, which has the drawback of making the synthesis unwieldy through additional steps of protection and de-protection. Thus, there exists a real need to develop a novel method for synthesizing ergothioneine and derivatives thereof that is applicable at the industrial level, in other words which does not have difficulties of purification, which does not use products or solvents dangerous and toxic for humans and the environment, and which makes it possible to obtain, at the industrial scale, the product with a good yield and a low cost. 3 In aiming to develop an environmentally friendly method, at one and the same time using low toxicity reagents, minimising the number of steps and carrying out reactions in aqueous medium, the Applicant has decided to opt for a "reversed" synthesis strategy, which consists in introducing the sulphur into an intermediate already possessing the betaine group. This approach, completely original, may be considered bio-mimetic in so far as the biosynthesis by enzymatic route of L-ergothioneine in fungi proceeds in the same way (Askari, A. and Melville, D.B., 1962, J. Biol. Chem., 237, 1615-1618). The object of the present patent application is thus a method for synthesizing a derivative of following formula (I) : O ^ (I), or a physiologically acceptable salt thereof, a tautomer, a stereoisomer or a mixture of stereoisomers in all proportions, in particular a mixture of enantiomers, and especially a racemic mixture thereof, for which: - Ri and R2 represent, independently of each other, a hydrogen atom or a (C1-C4) alkyl group such as methyl, at least one of the Ri and R2 groups representing a hydrogen atom, and advantageously each representing a hydrogen atom, and - R3, R4 and R5 represent, independently of each other, a (C1-C4) alkyl group such as methyl, 4 comprising the following successive steps: (i) cleavage reaction of a compound of following formula (II) : o S NHRg O (II), or a physiologically acceptable salt thereof, a tautomer, a stereoisomer or a mixture of stereoisomers in all proportions, in particular a mixture of enantiomers, and especially a racemic mixture thereof, for which: Xi^'^^ . RiN. /^N N^ .NR2 - ^ ^~^ ^ represents ' ^-^ or — ^, - Ri/ R2A R3A R4 arid R5 are as defined above, - Re represents a hydrogen atom or a (C1-C4) alkyl or -CO-( (C1-C4) alkyl) group, and in particular a hydrogen atom or a -COCH3 group, and more particularly a hydrogen atom, and - R7 represents a hydrogen atom or a (C1-C4) alkyl group, and in particular a hydrogen atom, in the presence of a thiol, preferably soluble in the reaction solvent which could be especially water, at a temperature above or egual to 60°C, to give a compound of formula (I), and (ii) separation of the compound of formula (I) obtained at the preceding step (i) from the reaction medium. "Tautomer" is taken to mean, according to the present invention, a constitutional isomer of the compound obtained 5 by prototropy, in other words by migration of a hydrogen atom and change of location of a double bond. The different tautomers of a compound are generally interconvertible and present in equilibrium, in solution, in proportions that can vary according to the solvent used, the temperature or even the pH. Within the framework of compounds of the invention, the 2-thioimidazole cycle may be present in the following different tautomer forms: ^^^ /W*^ /W^ HN 7 ^ N 7 ^ HN / ^ S HS HS In the present invention, "physiologically acceptable" is taken to designate what is generally safe, non toxic and neither biologically nor otherwise undesirable and which is acceptable for pharmaceutical, cosmetic or food (human or animal) use, in particular food. "Physiologically acceptable salts" of a compound is taken to designate salts that are physiologically acceptable, as defined above, and which have the desired activity (pharmacological, cosmetic or food) of the parent compound. Such salts comprise: (1) hydrates and solvates, (2) acid addition salts formed with inorganic acids such as hydrochloric acid, hydrobromic acid, sulphuric acid, nitric acid, phosphoric acid and similar; or formed with organic acids such as acetic acid, benzenesulphonic acid, benzoic acid, camphresulphonic acid, citric acid, ethanesulphonic acid, fumaric acid, glucoheptonic acid, gluconic acid, glutamic acid, glycolic acid, hydroxynaphthoic acid, 2-hydroxyethanesulphonic acid, 6 lactic acid, maleic acid, malic acid, mandelic acid, methanesulphonic acid, muconic acid, 2-naphthalenesulphonic acid, propionic acid, salicylic acid, succinic acid, dibenzoyl-L-tartaric acid, tartaric acid, p-toluenesulphonic acid, triinethylacetic acid, trifluoroacetic acid and similar, or (3) the salts formed when an acid proton present in the parent compound is either replaced by a metal ion, for example an ion of alkaline metal, an ion of alkaline-earth metal or an aluminium ion; or coordinated with an organic or inorganic base. Acceptable organic bases comprise diethanolamine, ethanolamine, N-methylglucamine, triethanolamine, tromethamine and similar. Acceptable inorganic bases comprise aluminium hydroxide, calcium hydroxide, potassium hydroxide, sodium carbonate and sodium hydroxide. "Stereoisomers" is taken to mean, according to the present invention, diastereoisomers and enantiomers. They are thus optical isomers. Stereoisomers that are not mirror images of each other are designated as "diastereoisomers", and stereoisomers that are mirror images of each other, but not superimposable, are designated as "enantiomers". A mixture containing egual quantities of two individual enantiomer forms of opposite chirality is designated as "racemic mixture". " (C1-C4) alkyl" group is taken to mean, according to the present invention, a saturated, linear or branched hydrocarbon chain comprising 1 to 4 atoms of carbon. It could be methyl, ethyl, propyl, isopropyl, butyl, isobutyl, sec-butyl, or tert-butyl groups. In particular, it could be the methyl group. 7 "Thiol" is taken to mean, according to the present invention, any reagent containing a SH group in its molecular structure. It will be more particularly a compound of formula R-SH with R representing a saturated, linear or branched,Ci to Cs hydrocarbon chain, especially C2 to Ce, substituted by one or more polar substituents. "Saturated hydrocarbon chain" is taken to mean, according to the present invention, a saturated, linear or branched hydrocarbon chain comprising advantageously 1 to 8 atoms of carbon. It could be more particularly a saturated, linear chain, such as a methyl, ethyl, propyl, butyl, pentyl or instead hexyl group. Polar substituents are taken to mean, according to the present invention, hydrophilic groups such as OH, SH, NH2 and COOH groups. "Cleavage reaction" is taken to mean, according to the present invention, that the compound engaged in this reaction is split into two parts during this reaction, to make it possible in the present case to form the thiocarbonyl function of the compound of formula (I). The compound of formula (I) could in particular be a compound of following formula (la): O ^ (la), or a physiologically acceptable salt, a tautomer, a stereoisomer or a mixture of stereoisomers in all proportions, in particular a mixture of enantiomers, and especially a racemic mixture thereof, for which Ri, R2, R3, R4 and R5 are as defined previously. 8 The compound of formula (I) will represent especially ergothioneine, and in particular L-ergothioneine. Step (i) : This cleavage reaction, carried out in the presence of a thiol, makes it possible to obtain the compound of formula (I) as well as pyruvic acid (CH3C (0)-CO2H) or one of the derivatives thereof, especially an ester (CH3C(0)-CO2R7) or a derivative obtained by reaction with the thiol, such as a thiocetalic derivative (two molecules of thiol can react with the ketone function of the pyruvic acid). Furthermore, the thiol should preferably be soluble in the reaction solvent which could be especially water, which has the additional advantage of being more ecological. The thiol used in this step (i) could be more particularly a thiol corresponding to the formula R-SH, with R representing an alkyl chain, linear or branched, and preferably linear, comprising from 1 to 8, especially 2 to 6, in particular 2 to 4, atoms of carbon, substituted by one or more groups chosen from OH, SH, NH2 and COOH. The presence of hydrophilic groups (OH, SH, NH2 and COOH) could make it possible especially to render the thiol more soluble in water, when water is used as solvent. The thiol could more particularly be chosen from cysteine, dithiothreitol, 2-mercaptoethanol, 2-mercaptopropionic acid, 3-mercaptopropionic acid and thioglycolic acid, and preferably will be 3-mercaptopropionic acid. It could also be mercaptoacetic acid and mercaptohexanoic acid. Advantageously, at least 2 molar equivalents of thiol will be used compared to the compound (II), in other words 9 at least 2 moles of thiol are used for one mole of compound (II) used. In particular, at least 5 molar equivalents of thiol, and especially 5 to 10 molar equivalents of thiol compared to the compound (II) could be used. The reaction mixture is heated to a temperature above 60°C because below this temperature the reaction kinetics would be too slow. The reaction could be carried out at a temperature ranqing between 60 and 120°C, advantageously between 80 and 100°C, more particularly after addition of the thiol. The reaction could be carried out especially in acid medium. Step (ii): The final product obtained (compound of formula (I)) could be separated from the reaction medium by techniques well known to those skilled in the art and applicable at the industrial scale, in particular by evaporation, if appropriate partial, of solvents, followed preferably by a recrystallisation to purify the product. Since the compounds of formula (I) are soluble in water, one or more prior extractions with an orqanic solvent, such as for example ethyl acetate or tert-butyl-methylic ether, could make it possible to eliminate the orqanic by-products formed durinq the reaction, such as pyruvic acid or derivatives thereof, as well as the excess of thiol. The product obtained could be purified if necessary by techniques well known to those skilled in the art, for example by recrystallisation, if appropriate after desalinisation of the aqueous phase containinq it, by techniques well known to those skilled in the art (for 10 example by electrodialysis, by addition of an adequate resin, or by reverse osmosis). Before or after this step (ii) , a salt of the formed compound could be prepared, if it is so desired, especially by addition of a physiologically acceptable acid or base as defined previously. The compound of formula (II) could be prepared from an acid addition salt, with the exclusion of the salt of hydriodic acid (HI) , of a compound of betaine type of following formula (III): O ^ 3 4 5 (jjj)^ or a tautomer, a stereoisomer or a mixture of stereoisomers in all proportions, in particular a mixture of enantiomers, and especially a racemic mixture thereof, for which ^^^c^ 2 ^ j^^^ j^^ 3j^(-j j^^ ^^-g ^s defined previously, by reaction successively with dibromine, then with a cysteine derivative of following formula (IV): NHRg ° (IV), or a stereoisomer or a mixture of stereoisomers in all proportions, in particular a mixture of enantiomers, and especially a racemic mixture thereof, in which Re and R7 are as defined previously. "Acid addition salt of the compound of betaine type of formula (III)" is taken to mean, according to the present invention, a salt of the compound of betaine type of 11 formula (III) obtained by addition of an acid, with the exclusion of hydriodic acid HI. The acid could in particular be hydrochloric acid or sulphuric acid. In this reaction, dibromine could be used at the rate of 1 to 1.5 molar equivalents compared to the compound of betaine type of formula (III) . Preferably, the dibromine is added cold (very rapid addition preferably), at a temperature below 10 °C, preferably below 5°C. The addition of dibromine could thus be carried out at a temperature ranging between -10°C and 10°C, advantageously ranging between -5°C and 5°C. The cysteine derivative could in particular be N-acetylcysteine or cysteine (especially in the D, L or racemic form), and in particular cysteine and especially L-cysteine. The cysteine derivative will be advantageously used in excess, in particular at the rate of 2 to 10, advantageously 3 to 7 molar equivalents of cysteine derivative compared to the compound of betaine type of formula (III), in other words that 2 to 10, advantageously 3 to 7 moles of cysteine derivative are used for one mole of compound (III) used. This reaction could be carried out in a solvent such as water. The yield of this step could be greater than or equal to 45%, or even greater than or equal to 60%. Preferably, the compound of formula (II) will not be isolated from the reaction medium but will be engaged directly in the following step (i) . Thus, the preparation of the compound (I) from the compound (III) may be carried out in a single reactor, without isolation of the intermediate compound (II) ("one-pot"). 12 The method for preparing a compound of formula (I) according to the invention could thus comprise the following successive steps: (al) reaction of an acid addition salt, with the exclusion of the salt of hydriodic acid, of a compound of betaine type of formula (III) as defined above, or a tautomer, a stereoisomer or a mixture of stereoisomers in all proportions, in particular a mixture of enantiomers, and especially a racemic mixture thereof, with dibromine, then with a cysteine derivative of formula (IV) as defined above or a stereoisomer or a mixture of stereoisomers in all proportions, in particular a mixture of enantiomers, and especially a racemic mixture thereof, and in particular with cysteine and especially L-cysteine, to give a compound of formula (II) as defined above, (bl) cleavage reaction of the compound of formula (II) obtained at the preceding step (al) in the presence of a thiol as defined previously, preferably soluble in the reaction solvent which could be especially water, at a temperature greater than or equal to 60°C, to give a compound of formula (I), and (cl) separation of the compound of formula (I) obtained at the preceding step (bl) from the reaction medium. The steps (bl) and (cl) correspond respectively to the preceding steps (i) and (ii) . The step (al) , for its part, corresponds to the step of preparation of the compound of betaine type of formula (II) described previously. Advantageously, the steps (al) and (bl) will be carried out in a same solvent, such as water, preferably, in a same reactor, in other words without isolation of the intermediate products (compound of formula (II) in particular). 13 Under these conditions, the reaction medium can contain a cysteine derivative used preferably in excess at step (al) . Before separating the compound of formula (I) from the reaction medium (step (cl) ) , it could thus be necessary to eliminate the excess of cysteine derivative in order to facilitate the isolation and the purification of the compound of formula (I) . Especially, in the case of a cysteine derivative for which R7 = H or (C1-C4) alkyl and in particular in the case of cysteine, benzaldehyde may then be added for example, which will then form with the cysteine derivative in excess a derivative of 2-phenylthiazolidine-4-carboxylic acid, a compound which precipitates in a solvent such as water. In this way, the cysteine derivative in excess could be recycled. The overall yield of preparation of the compound of formula (I) from the compound of betaine type of formula (III) could be greater than or equal to 40%. According to a particular embodiment of the invention, the compound of formula (I) is a compound of formula (la) and the method of preparation thereof comprises the following successive steps: (a2) reaction of an acid addition salt, with the exclusion of the salt of hydriodic acid, of a compound of betaine type of following formula (Ilia): O '\^X, NR3RX ,^^^^^_ or a tautomer, a stereoisomer or a mixture of stereoisomers in all proportions, in particular a mixture of enantiomers, and especially a racemic mixture thereof, 14 X <; ->X for which 1 ^<>^ 2^ R^^ R^ and R5 are as defined previously, successively with dibromine, then with a cysteine derivative of formula (IV) as defined above or a stereoisomer or a mixture of stereoisomers in all proportions, in particular a mixture of enantiomers, and especially a racemic mixture thereof, and in particular with cysteine and especially L-cysteine, to give a compound of betaine type of following formula (Ila): O S NHRg ^ (Ila), or a physiologically acceptable salt, a tautomer, a stereoisomer or a mixture of stereoisomers in all proportions, in particular a mixture of enantiomers, and especially a racemic mixture thereof, X -- '"X for which i'"~<>^ 2^ R^^ J^^^ R^^ R^ and R7 are as defined previously, (b2) cleavage reaction of the compound of betaine type of formula (Ila) obtained at the preceding step (a2) in the presence of a thiol as defined previously, preferably soluble in the reaction solvent which could be especially water, and in particular with cysteine, dithiothreitol, 2- mercaptoethanol, 2-mercaptopropionic acid, 3- mercaptopropionic acid or thioglycolic acid, and preferably with 3-mercaptopropionic acid, at a temperature above or equal to 60°C, to give a compound of formula (la), and 15 (c2) separation of the compound of formula (la) obtained at the preceding step (b2) from the reaction medium. The steps (a2), (b2) and (c2) correspond respectively to the preceding steps (al), (bl) and (cl). The compounds of formula (Ila) represent particular forms of the compound of formula (II) . Similarly, the compounds of betaine type of formula (Ilia) represent particular forms of the compound of betaine type of formula (III) . Another object of the present invention is a compound of following formula (II): o S NHRg O (II), or a physiologically acceptable salt, a tautomer, a stereoisomer or a mixture of stereoisomers in all proportions, in particular a mixture of enantiomers, and especially a racemic mixture thereof, for which: - ^ ^^^ ^ represents ^ ^-^ or ^-^ ^, and - Ri, R2, Rsf R4f Rsf Re and R7 are as defined previously, with the exclusion of the compound for which i^-<>^ 2 represents ^-^ or ^-^ , R3, R4 and R5 each represent a methyl group and Re and R7 each represent a hydrogen atom. 16 The excluded compound is described in: Ishikawa et al. J. Biol. Chem. 1974, 249(14), 4420. In particular, it will not be a compound of formula (II) for which 1 \-^ '^ represents ^-^ or ^-^ , and R3, R4 and R5 each represent a methyl group. It could be especially a compound of formula (Ila) as defined previously. In particular, this compound could be 2-{2- [ (2-ammonio-2-carboxyethyl) thio] -llf-imidazol-4-yl}-l-carboxy-N, N, W-trimethylethanaminium dihydrochloride (Herc-Cys, 2HC1). Another object of the present invention is a method for preparing a compound of following formula (II): O S NHRg O (II), or a physiologically acceptable salt, a tautomer, a stereoisomer or a mixture of stereoisomers in all proportions, in particular a mixture of enantiomers, and especially a racemic mixture thereof, for which: - ^ ^-^ ^ represents ^ — or ^^^ ^, and - Ri, ^2, R3/ R4, R5, Re and R7 are as defined previously, by reaction of an acid addition salt, with the exclusion of the salt of hydriodic acid, of a compound of betaine type of formula (III) as defined previously 17 or a tautomer, a stereoisomer or a mixture of stereoisomers in all proportions, in particular a mixture of enantiomers, and especially a racemic mixture thereof, for which ^^^ 2^ j^^, R4 and R5 are as defined in claim 1, successively with dibromine, then with a cysteine derivative of formula (IV) as defined in claim 7 or a stereoisomer or a mixture of stereoisomers in all proportions, in particular a mixture of enantiomers, and especially a racemic mixture thereof, and in particular with cysteine and especially L-cysteine to give a compound of betaine type of following formula (Ila): O S NHRg ° (Ila), or a physiologically acceptable salt, a tautomer, a stereoisomer or a mixture of stereoisomers in all proportions, in particular a mixture of enantiomers, and especially a racemic mixture thereof, for which 1^X;>"^, R3^ R^, R^, Rg and R7 are as defined in claim 1, (b2) cleavage reaction of the compound of betaine type of formula (Ila) obtained at the preceding step (a2) in the presence of a thiol, at a temperature above or equal to 60°C, to give a compound of formula (la), and (c2) separation of the compound of formula (la) obtained at the preceding step (b2) from the reaction medium. 9. Method according to any of claims 7 and 8, characterised in that the cysteine derivative is used in 31 excess, at the rate of 2 to 10, advantageously 3 to 7 molar equivalents of cysteine derivative compared to the compound of betaine type of formula (III) . 10. Method according to any of claims 7 to 9, characterised in that dibromine is used at the rate of 1 to 1.5 molar equivalents compared to the compound of betaine type of formula (III). 11. Method according to any of claims 7 to 10, characterised in that the preparation of the compound (I) from the compound (III) is carried out in a single reactor, without isolation of the intermediate compound (II). 12. Compound of betaine type of following formula (II): O S NHRg O (II), or a physiologically acceptable salt, a tautomer, a stereoisomer or a mixture of stereoisomers in all proportions, in particular a mixture of enantiomers, and especially a racemic mixture thereof, for which: - ^ ^^^ ^ represents ^ ^-^ or ^^ ^, and - Ri, R2, R3, R4, R5, Re and R7 are as defined in claim 1, with the exclusion of the compound for which ^"'<^ 2 HN. ^N N^/NH represents ^-^ or — , R3, R4 and R5 each 32 represent a methyl group and Rg and R7 each represent a hydrogen atom. 13. Method for preparing a compound of betaine type of following formula (II): O S NHRg o (II), or a physiologically acceptable salt, a tautomer, a stereoisomer or a mixture of stereoisomers in all proportions, in particular a mixture of enantiomers, and especially a racemic mixture thereof, for which: - ^ ^-^ ■^ represents ^ ^-^ or ^~^ ■^ , and - Ri, R2, R3/ R4, R5, Re and R7 are as defined in claim 1, by reaction of an acid addition salt, with the exclusion of the salt of hydriodic acid, of a compound of betaine type of formula (III) as defined in claim 7, with dibromine, then with a cysteine derivative of formula (IV) as defined in claim 7, and in particular with cysteine and especially L-cysteine. 14. Method according to claim 13, characterised in that dibromine is used at the rate of 1 to 1.5 molar equivalents compared to the compound of betaine type of formula (III). 15. Method according to any of claims 13 and 14, characterised in that the cysteine derivative is used in 33 excess, at the rate of 2 to 10, advantageously 3 to 7 molar equivalents of cysteine derivative compared to the compound of betaine type of formula (III).