6.4. New Red-Shifted 4-Styryl Coumarins as Potential Fluorescent Labels for Biomolecules

Abstract

Cellular biology, medicine, pharmacy, environmental sciences and other important scientific areas require highly sensitive analytical techniques to track and detect nucleic acids, oligonucleotides, antibodies, amino acids, proteins, lipids, carbohydrates, and other biomolecules. Of all sensitive analytical techniques, fluorescent labeling presents numerous advantages as it allows the use of small sample quantities as well as the respective fluorescent labels [1,2]. The availability and the development of new fluorophores are now enabling previously impossible studies of cellular processes and the detection of specific components of complex biomolecular assemblies with selectivity and exquisite sensitivity, in vitro and in vivo, as well the analysis of their interactions [3]. In this context, due to Chem. Proc. 2022, 11, 1 63 of 127 the high cost of the available commercial fluorescent labels, coumarin derivatives can be a solution to develop low-cost new fluorophores with absorption and emission at long wavelengths, combined with large Stokes shifts. In this work, we developed an effective synthetic strategy to produce new red-shifted 4-styryl coumarins using 7-diethylamino-4- methylcoumarin (1) as a starting material. These could be used to produce fluorescent labels for biomolecules. The main synthetic strategy to obtain 4-styryl coumarins was based on the high acidity of the methyl protons present at position 4 in 2-(7-(diethylamino)-4-methyl- 2H-chromen-2-ylidene)malononitrile (3), that enable aldol condensation reactions. The mentioned dicyanomethylene-coumarinylmethyl derivative, with a higher bathochromic shift than 100 nm when compared with its precursor, was obtained by the incorporation of two cyano groups in position 2 after the thionation of the carbonyl group of lactone. With the objective to extend the delocalization of the -electron system, we have designed and synthesized new 4-styryl coumarin derivatives, with absorption and emission at long wavelengths, combined with large Stokes shifts, using a simple, low-cost and efficient synthetic strategy (Scheme 1). sensitive analytical techniques to track and detect nucleic acids, oligonucleotides, antibodies, proteins, lipids, carbohydrates, and other biomolecules. Of all sensitive analytical techniques, presents numerous advantages as it allows the use of small sample quantities as well as the respective labels [1,2]. The availability and the development of new fluorophores are now enabling previously studies of cellular processes and the detection of specific components of complex biomolecular selectivity and exquisite sensitivity, in vitro and in vivo, as well the analysis of their interactions context, due to the high cost of the available commercial fluorescent labels, coumarin derivatives to develop low-cost new fluorophores with absorption and emission at long wavelengths, combined Stokes shifts. In this work, we developed an effective synthetic strategy to produce new red-shifted coumarins using 7-diethylamino-4-methylcoumarin (1) as a starting material. These could be fluorescent labels for biomolecules. The main synthetic strategy to obtain 4-styryl coumarins high acidity of the methyl protons present at position 4 in 2-(7-(diethylamino)-4-methyl-2H-chromen-ylidene)malononitrile (3), that enable aldol condensation reactions. The mentioned dicyanomethylenecoumarinylmethyl derivative, with a higher bathochromic shift than 100 nm when compared with was obtained by the incorporation of two cyano groups in position 2 after the thionation of the the lactone. With the objective to extend the delocalization of the π-electron system, we have synthesized new 4-styryl coumarin derivatives, with absorption and emission at long wavelengths, large Stokes shifts, using a simple, low-cost and efficient synthetic strategy (Scheme 13). Scheme 13. Synthesis of the 4-styryl coumarin derivatives