We report the discovery and validation of a series of benzoisothiazolones as potent inhibitors of phosphomannose isomerase (PMI), an enzyme that converts mannose-6- phosphate (Man-6-P) into fructose-6-phosphate (Fru-6-P) and, more importantly, competes with phosphomannomutase 2 (PMM2) for Man-6-P, diverting this substrate from critical protein glycosylation events. In congenital disorder of glycosylation type Ia, PMM2 activity ...