Xue Hou, Weicheng Yuan, Dong-mei Fang
Mar 1, 2013
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Journal
Journal of mass spectrometry : JMS
Abstract
Oxindole scaffolds are versatile and useful building blocks, and commonly present in a wide variety of structurally complex and biologically active compounds, which were proved to have potent antimicrobial, antitumor and oviposition-stimulant biological activities. For the significance of those interesting molecules, numerous methods were made to construct these molecules. Among them, 3-substituted oxindoles were widely used as donors in many asymmetric strategies, such as Michael additions, Mannich reaction and D-A reaction. Recently, 3-isothiocyanato oxindoles were designed by Yuan’s group, which were applied to react with ketone with high reactivities. [10,11] Matsunaga’ group also reported that 3-isothiocyanato oxindoles exhibited high reactivities with imine. The high reactivity might indicate special character of 3-isothiocyanato oxindoles. Structural characterization is the key to further development of their functions. Herein, 3-isothiocyanato oxindoles were studied with electrospray ionization mass spectrometry (ESI-MS) in our laboratory. ESI is a soft ionization mode. It will not destroy the molecules, even the thermally unstable intermediates. The regular ions generated by electrospray quadrupole time-of-flight MS (ESI-QTOF-MS) in positive-ion mode are protonated ions, other ionic adducts including [M+Na] and [M+K]. For few compounds, odd-electron ions might be observed as M or M during ESI. In our study, a series of unexpected radical ions, [M-H+Na]•, were detected. HPLC grade acetonitrile and CD3OD were purchased from Fisher Scientific (Pittsburgh, PA) and Cambridge Isotope Laboratories (Andover, MA), respectively. MS experiments were performed on a Bruker micrOTOF-Q mass spectrometer (Bremen, Germany). The mass resolution was 10 000 FWHM at 922 u. Argon gas was used as the collision gas and high-purity nitrogen gas as the nebulizer and dry gas. The ESI source conditions were as follows: capillary V, 4500 V, end plate voltage, 4000 V; capillary exit voltage, 120 V and dry gas temperature, 180 C. The 3-isothiocyanato oxindoles were synthesized in Yuan’s laboratory. The samples with the concentration of 10 ng/ml introduction rate was 180 ml/h. Mass data were processed using Bruker Compass DataAnalysis 4.0. Theoretical calculations were performed on the bond dissociation energy (BDE) of C–H bond homolithic cleavage for the tested substrates by the hybrid density