Methanesulfonic acid (ca. 0.99 eq.) was added, with stirring to a suspension of Imatinib (10.647 g) in ethyl alcohol (173 mL) heated previously to 65° C. and the mixture was stirred for 5 min. The seeds of the α-crystal form (0.279 g) were added to the solution at approx. 44° C. followed by dropwise addition of isopropyl alcohol (65 mL) and ethyl acetate (238 mL). Stirring at room temperature (approx. 24-25° C.) was continued for 4 hours and then the mixture was left without stirring for 64 h. The product was filtered off and washed with 50 mL of ethyl acetate. The crystals were dried under reduced pressure at room temperature to afford 11.85 g (93.1%) of α-crystal form of Imatinib monomesylate.
