Abstract
Herein we report a significant, valuable extension of a recently implemented pyrrole benzannulation methodology that, employing versatile nitrodienes from our lab as useful C4 building blocks, led to indole derivatives characterized by unusual patterns of substitution. The 6-nitro-7-arylindoles resulting from suitably derivatized, non-symmetric dienes are of foreseeable synthetic interest in search for new polyheterocyclic systems. As an example, pyrrolocarbazoles with a rarely reported ring fusion were synthesized with the classical Cadogan protocol. Furthermore, the proven easy reducibility of the nitro group to amine will surely open the way to further interesting elaborations.
Highlights
Due to the outstanding importance of indole derivatives in many applicative fields—as natural products, pharmaceuticals, fine chemicals and materials, as well as core structures in more complex heterocyclic systems—their synthesis [1,2,3] keeps on being a very active research subject
Some examples of benzannulating procedures in which a C4 building block is involved have been reported: an approach to be generally preferred when specific substituents are needed at the 4–7 positions, as they can be conveniently placed in the appropriate positions of the employed reagent
24 h necessary to observe, by TLC, the complete disappearance of 4a; the analysis of the chromatographed were necessary to observe, by TLC, the complete disappearance of 4a; the analysis of the final reaction mixture highlighted the formation of three different indole derivatives (6a–8a, Scheme 2), chromatographed final reaction mixture highlighted the formation of three different indole
Summary
Due to the outstanding importance of indole derivatives in many applicative fields—as natural products, pharmaceuticals, fine chemicals and materials, as well as core structures in more complex heterocyclic systems—their synthesis [1,2,3] keeps on being a very active research subject. Classic syntheses tend to build up the penta-atomic heteroring, exploiting the functionalities of properly derivatized benzene substrates [4], rather than assembling the hexa-atomic ring onto a pyrrolic starting material [4,5,6]. Some examples of benzannulating procedures in which a C4 building block is involved have been reported: an approach to be generally preferred when specific substituents are needed at the 4–7 positions, as they can be conveniently placed in the appropriate positions of the employed reagent. 4,5-dicyanopyridazine [8]; more frequently, ionic processes have been reported, where various sources of the C4 fragment were used, combined with the appropriate pyrrole derivative. It should be mentioned that the benzene ring of the final indole can be built up starting from pyrrolic intermediates that already contain all, or some, of the carbon atoms of the eventual new ring
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