Process for preparation of bisphosphonic acid compounds

A process for preparation of bisphosphonic acid, a compound of formula 1 or a salt thereof,

• comprising reacting a carboxylic acid compound of formula 2 or a salt thereof
• wherein,
• A is a straight chain alkyl, a branched alkyl or a cyclic alkyl chain with up to 10 carbon atoms, which can optionally contain hetero atoms in between and,
• B is alkyl, aralkyl, aromatic or heteroaromatic group, which can be optionally substituted;
• or
• wherein, R1 and R2 may be selected from hydrogen or straight chain, branched or cyclic lower alkyl,
• with phosphorous acid and a phosphorous chloride selected from PCl3, PCl5 and POCl3, in sulfolane.