Aluminum chloride (2.7 g) is added to tetrahydrofuran (60 mL) at 25-30° C. The solution is cooled to 0-10° C. and lithium aluminum hydride (1.08 g) is added. The mixture is stirred for 15-30 minutes. A solution of trans-5-chloro-2,3,3a,12b-tetra hydro-2-methyl-1H-dibenz[2,3:6,7]oxepino[4,5-c]pyrrol-1-one (6.0 g) in tetrahydrofuran (120 mL) is added drop-wise at 0-10° C. and the mixture is maintained for 1 hour. Sodium hydroxide solution (0.6N, 100 mL) is added at 0-10° C. The mixture is extracted with ethyl acetate (75 mL) and layers are separated, then the aqueous layer is washed with ethyl acetate (25 mL). The combined organic layer is washed with saturated brine solution (2×100 mL). The organic layer is dried over anhydrous sodium sulfate (10 g) and the solvent from the organic layer is evaporated at 45° C. The obtained residue is dissolved in 1-propanol (20 mL) at 25-30° C. This solution is added to a solution of maleic acid (3.2 g) in 1-propanol (10 mL) at 25-30° C. and stirred for 10 minutes, then seeded with asenapine maleate monoclinic form (120 mg). The mixture is stirred at 25-35° C. for 24 hours and the obtained solid is collected by filtration, washed with 1-propanol (5 mL), and dried under reduced pressure at 65° C., to afford 4.9 g of the title compound. Purity by HPLC: 99.59%; PXRD pattern is shown as FIG. 1; endotherm by DSC: 147° C.; weight loss by TGA: 0.04%; moisture content by Karl Fischer (KF): 0.12%; residual solvents: 1-propanol: 150.3 ppm; ethyl acetate: not detected; tetrahydrofuran: not detected; particle size distribution: d(10)=5.671 μm, d(50)=24.077 μm, d(90)=58.716 μm.
