A green approach to produce the indolyl derivatives from four natural quinones (perezone, isoperezone, menadione, and plumbagin) was performed; in this regard, a comparative study was accomplished among the typical mantle heating and three nonconventional activating modes of reaction (microwave, near-infrared, and high speed ball milling or tribochemical), under solventless conditions and using bentonitic clay as a catalyst. In addition, the tribochemical production of isoperezone from perezone is also commented on. It is also worth noting that the cytotoxicity of the synthesized indolylquinones in human breast cancer cell was tested by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay, with the 3-indolylisoperezone being the most active. The structural attribution of the target molecules was performed by typical spectroscopic procedures; moreover, the experimental and computed1H and13C NMR chemical shifts data, with previous acquisition of the corresponding minimum energetic structures, were in good agreement.

The structure of pyridine-2,6-dicarboxylic acid, C(7)H(5)NO(4), has been determined at 0.71 A resolution. The molecule is located on a site with mirror symmetry. A one-dimensional supramolecular structure is stabilized in the solid state through a strong symmetric double hydrogen bond, with H* * *O distances of 1.86 (3) A and O-H* * *O angles of 167 (3) and 171 (5) degrees. This arrangement is similar but not identical to that reported for the isoelectronic isophthalic acid (benzene-1,3-dicarboxylic acid).

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