Method for the preparation of solifenacin and intermediate thereof

A method for the preparation of (1S)-(3R)-1-azabicylco[2.2.2]oct-3-yl-3,4-dihydro-1-phenyl-2(1H)-isoquinolinecarboxylate (Solifenacin), of formula (I) or its pharmaceutically acceptable salts

• the method comprising:
• a. reacting (R)-quinuclidin-3-ol a compound of formula (II) with bis (aryl) carbonate of formula (IIIa), optionally in the presence of a base, to form (3R)-1-azabicyclo[2.2.2]oct-3-yl-4-nitrophenyl carbonate of formula (IVa)
• wherein Ar is phenyl, or a substituted phenyl selected from the group consisting of p-nitrophenyl, 4-(trifluoro methyl) phenyl, 4-cyano phenyl, ethyl benzoate, and benzoic acid,
• optionally, isolating the compound of formula (IVa);
• and
• b. treating the reaction mixture comprising the formula (IVa) obtained from step (a) or optionally, after isolating the compound of formula (IVa) with (1S)-1-phenyl-1, 2, 3, 4-tetrahydro isoquinoline of formula (V) to obtain the compound of Formula (I), and optionally converting the base into its pharmaceutically acceptable salts