Synthesis of fluorescent alanines by a rhodium-catalysed conjugate addition of arylboronic acids to dehydroalanine derivatives

Several β-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)]BF as catalyst, and CsF as base in dioxane/HO (10:1) at 110 °C. These conditions were also applied to several dipeptides with dehydroalanine residues. The photophysical properties of some of the β-arylalanines were studied in three solvents with different polarities. Due to the absence of the α,β-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. Fluorescent β-arylalanine derivatives were prepared in good yields using a rhodium-catalysed conjugate addition of arylboronic acids to N-protected dehydroalanines. The photophysical properties of some of the β-arylalanines were studied in solvents of different polarities. Considering the high fluorescence quantum yield of these compounds, some of them may be useful as fluorescent markers.