Processes for preparing montelukast sodium

A process for preparing an ester of montelukast comprising the following steps:

• a) activating 2-(2-(3(S)-(3-(7-chloro-2-quinolinyl)-ethenyl)phenyl)-3-((α-hydroxy)propyl)phenyl-2-propanol with a benzylsulfonyl or mesyl group;
• and
• b) reacting the product of step (a) with 1-(mercaptomethyl)cyclopropaneacetic acid alkyl ester in a solvent and in the presence of a cosolvent and a base to obtain an ester of montelukast.