Pyrenylamino acids : synthesis, photophysical and electrochemical studies

Abstract

Several beta-pyrenyldehydroamino acids and a pyrenylalanine derivative were synthesized in good to high yields from dehydroamino acids using several types of reactions. A beta-[(pyren-1-yl)methylamino]alanine was prepared treating the methyl ester of N,N-(di-tert-butoxycarbonyl)dehydroalanine with 1-pyrenemethylamine hydrochloride in the presence of an excess of potassium carbonate. The methyl esters of N-(tert-butoxycarbonyl)-beta-(1,2,4-triazol-1-yl) dehydroalanine and dehydroaminobutyric acid were reacted with 1-pyrenemethylamine hydrochloride in the presence of triethylamine to give the E-isomers of the beta-aminomethylpyrene dehydroalanine and dehydroaminobutyric acid derivatives in 84% and 45% yield, respectively. Beta-(Pyren-1-yl) dehydrophenylalanine and dehydroaminobutyric acid derivatives were obtained from the pure stereoisomers of the corresponding beta-bromodehydroamino acids and 1-pyrene boronic acid using Suzuki cross-couplings. This reaction was also applied successfully to beta-bromodehydrodipeptides. The electrochemical behaviour of some of the compounds prepared was studied by cyclic voltammetry. The peak potentials for oxidation and reduction of pyrenylalanine were similar to those referred for pyrene. However, it was found that, when the pyrene ring was conjugated with the dehydroamino acid moiety, the compounds were more difficult to oxidize and easier to reduce than pyrene. The fluorescence properties of four of the pyrenylamino acids synthesized were evaluated in cyclohexane and alcohols. The of methyl ester of N,N-bis(tert-butoxycarbonyl)-beta-[(pyren-1-yl)methylamino] alanine presented high fluorescence quantum yield in cyclohexane and ethanol (0.45 and 0.35, respectively).