Process for the production of the form I of the anhydrous terazosin monohydrochloride

A process for the production of form I of anhydrous 1-(4-amino-6,7-dimethoxy-2-quinazolinyl)-4-(tetrahydro-2-furoyl)piperazine monohydrochloride which comprises the steps of:

• (a) heating at a temperature between 50.degree. C. and the boiling temperature a suspension of 1-(4-amino-6,7-dimethoxy-2-quinazolinyl)-4-(tetrahydro-2-furoyl)piperazine in a mixture made of methanol and a solvent which is a member selected from the group consisting of C.sub.2 to C.sub.6 straight or branched alcohols, esters of the C.sub.1 to C.sub.8 aliphatic carboxylic acids with straight or branched C.sub.1 to C.sub.8 alcohols, C.sub.3 to C.sub.8 aliphatic ketones, C.sub.4 to C.sub.8 straight, branched or cyclic aliphatic ethers, aliphatic amides and aliphatic nitriles;
• (b) adding to said suspension under strong stirring in a period of time between 15 minutes and 2 hours from 0.95 to 1.00 equivalents of hydrochloric acid dissolved in methanol, while keeping the temperature between 50.degree. C. and the boiling temperature;
• (c) continuously heating at the boiling temperature for a period of time between 10 minutes and 3 hours;
• (d) cooling the suspension to 20.degree. C. under a nitrogen atmosphere;
• (e) adding an amount of 0.02 to 0.10 equivalents of hydrochloric acid with respect to said suspension of said 1-(4-amino-6,7-dimethoxy-2-quinazolinyl)-4-(tetrahydro-2-furoyl)piperazine , said hydrochloric acid being dissolved in methanol while keeping the suspension under strong stirring for a period of time between 10 and 30 minutes, filtering said suspension, washing the solid with the solvent and drying it in an oven under vacuum at a temperature between 70.degree. C. and 75.degree. C. for a period of time between 12 and 24 hours.