Abstract
In-situ-generated N-aryl nitrile imines derived from trifluoroacetonitrile efficiently react with polycyclic 1,4-quinones, yielding fused pyrazole derivatives as the exclusive products. The reactions proceed via the initially formed [3 + 2]-cycloadducts, which undergo spontaneous aerial oxidation to give aromatized heterocyclic products. Only for 2,3,5,6-tetramethyl-1,4-benzoquinone, the expected [3 + 2]-cycloadduct exhibited fair stability and could be isolated in moderate yield (53%). The presented method offers a straightforward access to hitherto little known trifluoromethylated polycyclic pyrazoles. All products were isolated as pale colored solids with medium-intensity absorption maxima in the range of 310–340 nm for naphthoquinone-derived products and low-intensity bands in the visible region (≈400 nm) for the anthraquinone series.
Highlights
IntroductionSelective functionalization of 1,4-quinones is a challenging task in current organic synthesis, and diverse transformations are known to create new C–C bonds and/or to extend the (poly)cyclic system
The 1,4-quinone scaffold belongs to the most important structural motifs present in naturally occurring compounds as well as synthetic drugs and other functionalized organic molecules of great practical importance, e.g., in materials chemistry (Figure 1) [1,2,3,4].Selective functionalization of 1,4-quinones is a challenging task in current organic synthesis, and diverse transformations are known to create new C–C bonds and/or to extend thecyclic system
In-situ-generated N-aryl nitrile imines derived from trifluoroacetonitrile efficiently react with polycyclic 1,4-quinones, yielding fused pyrazole derivatives as the exclusive products
Summary
Selective functionalization of 1,4-quinones is a challenging task in current organic synthesis, and diverse transformations are known to create new C–C bonds and/or to extend the (poly)cyclic system. In this context, cycloadditions are of special importance, and Diels–Alder reactions have successful-. In the already reported reactions of propargylic 1,3-dipoles, such as nitrile oxides or nitrile ylides, with 1,4-quinones, competitive reaction courses involving either ethylenic C=C or carbonyl C=O bonds were observed. The slightly less polar benzonitrile benzylide underwent [3 + 2]-cycloaddition to the C=C bond exclusively [16]. [3 + 2]-cycloadditions of nitrile imines to the C=O group of 1,4quinones have not yet been reported
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