Process for production of methanediphosphonic acid compound

A process for production of a methanediphosphonic acid compound represented by the following general formula (1):

• ##STR5## wherein R.sub.1 is a pharmaceutically allowable cation, hydrogen atom, or linear or branched alkyl group having 1 to 6 carbon atoms, and may be the same or different, R.sub.2 is an alkyl group having 1 to 20 carbon atoms or an aryl group having 3 to 20 carbon atoms, and R.sub.4 is a hydrogen atom, or linear or branched alkyl group having 1 to 6 carbon atoms;
• comprising:
• reacting a tetraalkyl methanediphosphonate represented by the following formula:
• ##STR6## wherein R.sub.1 'is a linear or branched alkyl group having 1 to 6 carbon atoms, and may be the same or different, R.sub.3 is a hydrogen, sodium, potassium, or lithium atom, and R.sub.4 is the same as above, with a dialkyldisulfide or diaryldisulfide represented by the following formula:
• R.sub.2 S--SR.sub.2
• wherein R.sub.2 is the same as above, in the presence of a base;
• removing alkylthiolate or arylthiolate as an insoluble material or an insoluble salt out of the reaction mixture;
• then adding aqueous solution to obtain the resulting diphosphonate ester;
• and
• recovering a methanediphosphonic acid compound product.