Development of optically active fluorescent Edman-type reagents

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Abstract
The optically active fluorescent Edman-type reagents 4-(3-isothiocyanatopyrrolidin-1-yl)-7-nitro-2,1,3-benzoxadiazole [(R)-(–)- and (S)-(+)-NBD-PyNCS] and 4-(3-isothiocyanatopyrrolidin-1-yl)-7-(N,N-dimethylaminosulfonyl)-2,1,3-benzoxadiazole [(R)-(–)- and (S)-(+)-DBD-PyNCS] were synthesized and characterized by their physico-chemical properties. High optical purities of all four enantiomers were identified by high-performance liquid chromatography (HPLC) with a chiral stationary phase (CSP) column. Good stability in both solid and liquid forms is one of the excellent properties of these novel Edman-type reagents. The applicability of these reagents to the resolution of mirror-image enantiomers of amines, including amino acids and some drugs (β-adrenergic blocking agents), by reversed-phase HPLC was demonstrated. The proposed reagents read quantitatively with primary and secondary amino functional groups under mild conditions (55 °C for 10 min) in the presence of triethylamine (TEA) to produce the corresponding fluorescent thiourea derivatives. The maximum excitation/emission wavelengths of the resulting diastereomers in acetonitrile–water (1 + 1) are around 490/530 nm for NBD-PyNCS derivatives and around 460/550 nm for DBD-PyNCS derivatives. The fluorescence wavelengths are independent of the amino compounds. The diastereomers that were derived from the amines were efficiently resolved on an achiral ODS column with water–acetonitrile containing 0.05% trifluoroacetic acid (TFA). The Rs values were in the range 0.83–4.12 for the diastereomers obtained with NBD-PyNCS and 1.35–6.06 for those with DBD-PyNCS. The diastereomer corresponding to the R configuration of the amines elute faster than those of the S configuration with the (R)-(–)-enantiomer of the Edman-type reagents used as the derivatization reagent. As expected, the opposite elution order is obtained with the use of the (S)-(+)-enantiomers.

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