The heterocyclic hydroxylamine- O -sulfonates constitute a novel family of formal O -substituted hydroxyguanidines and hydroxyamidines that serve as functional precursors to a variety of fused heterocyclic ring systems incorporating N-N, N-O, N-S, or N-N + moiety. They are readily accessible from the reaction of 2-chloroazoles, 2-chloroazines, and 2-chlorodiazines with hydroxylamine- O -sulfonic acid. They have a rich chemistry exemplified by tandem reactions, such as nucleophilic addition-electrophilic amination, nucleophilic addition-electrophilic 5- endo-trig cyclization or fluorogenic Mannich-electrophilic amination reaction. The heterocyclic hydroxylamine- O -sulfonates have significant potential for use in synthesis of anticancer, antiviral, and antimicrobial agents. The newly discovered fluorogenic reaction and fluorescent dyes (Safirinium-P and Safirinium-Q) have found applications in fluorescent detection and labeling.