Synthesis and X-Ray-Structure Analysis of Azelastine Azelastine hydrochloride, 4-(p-Chlorobenzyl)-2-(hexahydro-)-methyl-1H-azepin-4-y1)-1(2H)-phthalazinone-hydrochloride ( 2 ) is a new orally effective, highly potent and long-acting antiallergic agent. Starting with phthalic acid anhydride two alternative syntheses of Azelastine hydrochloride are described. X-ray crystallographic analysis of Azelastin-monohydrate shows two independent molecules A and B with remarkable conformational differences. The structural differences and conclusions with regard to structure-activity-relationships will be discussed.