(S)-4-(N,N-Dimethylaminosulfonyl)-7-(3-aminopyrrolidin-1-yl)-2,1,3-benzoxadiazole is used as for the preparation of optically active 7-(3-isothiocyanatopyrrolidin-1-yl)-2,1,3-benzoxadiazole derivatives as fluorescent labeling reagents for amine analysis.