Mechanochemical reaction of ninhydrin with aromatics, enols and amines: Synthesis, crystal structure and supramolecular self-assembly of cyclic and acyclic adducts

Abstract

Ninhydrin and related indanone scaffolds are employed as effective synthon in organic and bioorganic chemistry. In the present work, the reactivity of ninhydrin is exploited to synthesize various cyclic and acyclic compounds using mortar and pestle grinding method. The reaction of ninhydrin with various naphthols, phenols, acetylacetone/ethylacetoacetate under grinding condition led to fused/spirocyclic products. On the other hand, 2-methoxynaphthalene, 4-hydroxycoumarin, 1,3-indanedione, indole reacted with ninhydrin to afford simple C-2 arylated acyclic adducts. Single crystal X-ray diffraction study revealed that, ninhydrin-o-cresol adduct, which preferentially remains in the cyclic hemiketal form 6, self-assembled to adopt zipper architecture. However, ninhydrin-2-methoxynaphthalene adduct 7 was found to display corrugated sheet like arrangement. Scanning electron microscopy (SEM) study reveals that cyclic adducts 6, 8, 10 exhibit fibrillar morphology, whereas acyclic adducts 7, 12, 15 disclose sheet morphology.