Syntheses of Highly Fluorescent GFP-Chromophore Analogues

Eight B-containing compounds, i.e., 1a−h, were prepared as mimics of the green fluorescent
protein (GFP) fluorophore. The underlying concept was that synthetic GFP chromophore analogues are
not fluorescent primarily because of free rotation about an aryl−alkene bond (Figure b). This rotation is
not possible in the β-barrel of GFP; hence, the molecule is strongly fluorescent. In compounds 1a−h,
radiationless decay via this mechanism is prevented by complexation of the BF2 entity. The target materials
were prepared via two methods; most were obtained according to the novel route shown in Scheme b,
but compound 1f was made via the procedure described in Scheme . Both syntheses involved formation
of undesired compounds E-4a−h that formed simultaneously with the desired isomeric intermediates Z-4a−h. Both compounds form BF2 adducts, i.e., 1a−h and 5a−h, respectively. Methods used for spectroscopic
characterization and differentiation of compounds in the series 1 and 5 are discussed, and these are
supported by single-crystal X-ray diffraction analyses for compounds 1c, 5c, 1f, and 5f. Electronic spectra
of compounds 1a−h and 5a−h were studied in detail. Those in the 5 series were shown to be only weakly
fluorescent, but the 1 series were strongly fluorescent compounds (comparable to the boraindacene,
BODIPY, dyes). Compounds 1g and 1h are water soluble, and 1h has particularly significant potential as
a probe, since it also has a carboxylic acid group for attachment to biomolecules.