Abstract
Fluorinated fluorophores are valuable tools for studies of biological systems. However,
amine-reactive single-isomer derivatives of these compounds are often very expensive.
To provide an inexpensive alternative, we developed a practical synthesis of 4-carboxy-Pennsylvania
Green methyl ester. Derivatives of this hydrophobic fluorinated fluorophore, a hybrid
of the dyes Oregon Green and Tokyo Green, are often cell-permeable, enabling labeling
of intracellular targets and components. Moreover, the low pK
a of Pennsylvania Green (4.8) confers bright fluorescence in acidic cellular compartments,
such as endosomes, enhancing its utility in chemical biology investigations. To improve
access to the key intermediate 2,7-difluoro-3,6-dihydroxyxanthen-9-one, we subjected
bis(2,4,5-trifluorophenyl)methanone to iterative nucleophilic aromatic substitution
by hydroxide on scales in excess of 40 grams. The key intermediate was used to prepare
over 15 grams of pure 4-carboxy-Pennsylvania green methyl ester in 28% overall yield
without the use of chromatography. This compound can be converted into the amine-reactive
N-hydroxysuccinimidyl ester in essentially quantitative yield for the synthesis of
a wide variety of fluorescent molecular probes.
Key words
fluorophore - bioorganic chemistry - chemical biology - molecular probes - fluorine
- conjugation
