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Palladium-catalyzed synthesis of coumarin - …
Given the difficulties with existing syntheses, we sought an alternative route wherein the C(aryl)–N bonds of rhodamines are formed at a late stage.() We envisoned using the Buchwald–Hartwig cross-coupling of nitrogen nucleophiles with fluorescein ditriflates (Scheme , Route B). Pd-catalyzed cross-coupling has emerged as a powerful method for C–N bond formation,() yet these amination reactions have seen little use in the preparation of xanthene dyes. Lippard and co-workers reported a single example of coupling pyrrolidine to a dibromofluoran.() Peng and Yang prepared a rhodol library via cross-coupling of a fluorescein monotriflate with various amines.() We recently used this strategy to construct a precursor for a photoactivatable “caged” rhodamine.() These examples inspired us to further explore the C–N cross-coupling (Route B) as a general strategy for the direct conversion of fluorescein ditriflates to not only N-alkyl rhodamines but also N-aryl (5, HNR3R4 = aniline) and N-acyl derivatives (HNR3R4 = amide, carbamate, etc.). Most importantly, isomerically pure 5- and 6-substituted fluorescein dyes are readily synthesized on a multigram scale,() making this approach a convenient and divergent synthetic route to regioisomerically pure rhodamine dyes.
In summary, we prepared -Fe2O3@HAp-Ag NPs as a novel, efficient, and magnetically recyclable catalyst and characterized the material by SEM, TEM, FT-IR, XRD, and VSM spectroscopy. In this study, we have developed an efficient, green, and simple procedure for the synthesis coumarin derivatives by the Pechmann condensation reaction under solvent-and halogen-free conditions at 80 ℃. The significant advantages of the present method include the non-chromatographic purification of the products, a simple experimental procedure, short reaction times and green content of the procedure, high yields of products, and ease of recovery of the reusable catalyst from the reaction mixture using an external magnet.
Palladium-catalyzed synthesis of benzofurans and coumarins by ..
Despite the utility and flexibility of rhodamines, the established synthetic approach to this dye scaffold is archaic and difficult. Rhodamines are typically prepared through the acid-catalyzed condensation of an aminophenol (1) with a phthalic anhydride (2) to yield 3 (Scheme , Route A). Unfortunately, only a limited number of 3-aminophenols are compatible with this century-old route. Use of phthalic anhydrides bearing a substituent (R2 in 3) for bioconjugation yields products as intractable mixtures of 5- and 6-substituted regioisomers. Consequently, commercially available functionalized rhodamines are expensive and often sold as regioisomeric mixtures.() Furthermore, derivatization of the already elusive rhodamines into fluorogenic derivatives via N-acylation is often inefficient, due primarily to the low nucleophilicity of the rhodamine nitrogens.,
The -Fe2O3@HAp-Ag NPs were characterized by FT-IR, TEM, SEM, XRD, and VSM techniques, and applied as a novel, magnetically recyclable, and heterogeneous nanocatalyst for the synthesis of coumarin derivatives by the one-pot condensation of various phenols with ethyl acetoacetate under solvent-and halogen-free conditions ().
Scheme for synthesis of coumarin 3 catalyzed by Pd ..
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