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This example refers to reaction b of the process of the invention.
20 kg of the intermediate of formula (III) prepared as described in example 1 are suspended in 50 l of Dowtherm A under nitrogen flow. In an inert atmosphere, it is heated to approximately 200° C. for approximately 5 hours. Upon completion of the reaction (TLC) a clear red-brown solution is obtained, without the formation of black pitch. The reaction mixture is cooled slowly to 25° C. (a partial precipitation is observed). 100 l (5 volumes) of cyclohexane are added and it is cooled to between 0 and 5° C. for one hour. It is filtered by washing with cyclohexane and dried at reduced pressure and T=45° C. for at least 12 hours. 16.8 kg of yellow solid are obtained which is refluxed in 80 l of toluene in the presence of decolouring carbon. The suspension is filtered, washing it with hot toluene. Part of the solvent is distilled at reduced pressure until the beginning of crystallisation. It is cooled at room temperature and then to between 0 and 5° C. for at least one hour.
The filtered solid is washed with cold toluene and dried at reduced pressure at T=45° C. for at least 12 hours. 15.3 kg of intermediate (IV) are obtained in the form of an almost white solid of quality suitable for continuation of the synthesis.
1H-NMR and mass spectroscopic analyses are performed on part of the product thus obtained, purified by chromatography for analytical purposes (silica gel, 7 parts in volume of heptane—3 parts in volume of ethyl acetate), obtaining the following results: