Synthesis of N, N-Dialkylamino- nor-Dihydroxanthene-Hemicyanine Fused Near-Infrared Fluorophores and Their First Water-Soluble and/or Bioconjugatable Analogues.

The effective synthesis of extended conjugated N, N-dialkylamino- nor-dihydroxanthene-based fluorophores is described from diversely functionalized salicylic aldehydes. The access to these original fluorescent derivatives proceeded in two steps through a one-pot construction of the unusual nor-dihydroxanthene ( nor-DHX) scaffold followed by a diversification step providing a wide variety of nor-DHX-hemicyanine fused dyes emitting in the range of 730-790 nm. The versatility of our approach has enabled a further extension to the late-stage introduction of negatively/positively charged polar groups onto their terminal nitrogen heterocyclic subunit, thereby giving access to the first water-soluble and/or bioconjugatable members of this emerging class of NIR fluorophores. Our water-solubilizing method is easily implementable, and the nor-DHX-hemicyanine skeleton maintains satisfying fluorescence quantum yields (5-20 %) under physiological conditions. Finally, the bioconjugation ability of fluorescent derivatives bearing a free carboxylic acid was demonstrated through the covalent labeling of a model protein, namely, bovine serum albumin. [ABSTRACT FROM AUTHOR]