Synthesis of Fluorescent Alanines by a Rhodium-Catalysed Conjugate Addition of Arylboronic Acids to Dehydroalanine Derivatives

Several beta-arylalanine derivatives containing fluorescent groups were prepared in good yields using a rhodium-catalysed conjugate addition of arylboronic acids to N,N-diprotected and N-monoprotected dehydroalanines. The best conditions for these reactions required the use of an excess of arylboronic acid (4 equiv.), [Rh(COD)(2)]BF4 as catalyst, and CsF as base in dioxane/H2O (10:1) at 110 degrees C. These conditions were also applied to several dipeptides with dehydroalanine residues. The photophysical properties of some of the beta-arylalanines were studied in three solvents with different polarities. Due to the absence of the alpha,beta-double bond, the absorption and fluorescence emission of the new compounds are dominated by the photophysical properties of the polycyclic aromatic fluorophores (naphthalene, phenanthrene, and pyrene). Considering the relatively high fluorescence quantum yield of these compounds, some of them may be useful as fluorescent markers for peptides and proteins.