Method for preparing a salt of isocyclosporin a

A method for preparing a salt of isocyclosporin A by transesterification of cyclosporin A into a salt of isocyclosporin A, the method comprising the steps of:

• (a) dissolving the cyclosporin A in anhydrous methanol and adding trifluoroacetic acid, thereby obtaining a solution of dissolved cyclosporin A;
• (b) heating the solution obtained in step (a) by microwave radiation;
• (c) removing a liquid comprising the anhydrous methanol and the trifluoroacetic acid, thereby obtaining a residue;
• and
• (d) recovering the salt of isocyclosporin A with the trifluoroacetic acid from the residue.