4‐Cyano‐6,7‐dimethoxycarbostyrils with Solvent‐ and pH‐Independent High Fluorescence Quantum Yields and Emission Maxima

Abstract

Abstract 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 by using toluenesulfinates as catalysts. In contrast to the well‐described 4‐trifluoromethyl‐substituted analogues 18 , N ‐substituted derivatives 11 fluoresce in water, polar, and apolar 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 by saponification to acid 14 . With amino acids or peptides, linking to labeled derivatives 17 was achieved under mild conditions in slightly basic aqueous media. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)