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107. 4-Cyano-6,7-dimethoxy-carbostyrils With Solvent and pH Independent High Fluorescence Quantum Yields and Emission Maxima.
A. B. Avhale, H. Prokocpová, J. Šefcovicová, W. Steinschifter, A. E. Täubl, G. Uray and W. Stadlbauer, Eur. J. Org. Chem. 2008, 563-571.

Highly fluorescent and stable 6,7-dimethoxy-2-oxoquinoline-4-carbonitriles 11 were synthesized starting from appropriate 4-hydroxyquinolones 3 via reactive 4-chloroquinolones 8 using toluenesulfinates as catalysts. In contrast to the well described 4-trifluoromethyl substituted analogues 18, N-substituted derivatives 11 fluoresce in water, polar and unpolar solvents in a narrow 430 - 440 nm window with almost constant quantum yield of 0.5. Equal excitation is possible in the broad double maximum between 385 and 410 nm yielding identical data between pH 1 and 11. These properties could lead to a broadly usable fluorescence standard. N-Alkylation with bromoacetate yields ester 13 in good yields. Reactive succinimidoyl (OSu) ester 15 was prepared via saponification to acid 14. With amino acids or peptides, linking to labeled derivatives 17 was achieved under mild conditions in slightly basic aqueous media

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