New Process for the Synthesis of Rifaximin and a New Pseudo-Crystalline Form of Rifaximin Obtained Thereby

A process for the preparation of rifaximin comprising the following steps:

• a) Reacting under stirring a molar equivalent of rifamycin O with an excess of 2-amino-4-methylpyridine in a solvent comprising water, ethyl alcohol and acetone, at a temperature comprised between 15° C.-40° C. and for a period of time comprised between 20-35 hours;
• b) Precipitating the solid by cooling the solution;
• c) Filtering and washing the precipitated solid;
• d) Suspending the precipitated solid in a mixture of ethyl alcohol and water comprising at least 95% ethyl alcohol, under heating at a temperature comprised between 35° C.-45° C. for 2.5-3.5 hours under stirring;
• e) Cooling the suspension to a temperature of 10° C. in 60 minutes and keeping the solution at 10° C. for at least 30 additional minutes;
• f) Filtering and washing the resulting solid;
• g) Drying the solid.