Condensation of ninhydrin with phenols: Regioselective formation of diverse organic scaffolds and crystal structure studies

Abstract

In the present work, a range of phenolic compounds have been reacted with ninhydrin in acid medium to afford different indanone-based molecules via regioselective C–C bond formation. Ninhydrin reacts with 2,7-Dihydroxynaphthalene and 4-cyanophenol in AcOH producing ortho selective adduct 3a and 3b respectively; however, unlike 3a remaining as cyclic hemiketal structure, 3b exists as ring-opened diketo form. On the other hand, ortho-substituted phenols like o-cresol, guaiacol, o-chloro/iodophenol condense with ninhydrin to accomplish para selective diarylated adducts 5a-d, whereas 3-methoxy phenol provides corresponding monoarylated adduct 5e as the major product. Different hydroxy benzoic acids deliver versatile scaffolds like diarylated indanone, indenofuran, benzofuran or spirolactone 7a-e depending upon the substitution pattern and acidity of the reaction medium. All the compounds are characterized by 1H and 13C NMR spectra. In the solid state, scissors-shaped molecule 5a has been found to form inclusion complex with disordered o-cresol molecule and function as building unit of supramolecular network. In the crystal structure of spirolactone 7d, anti-parallel motif of dipolar···dipolar (CO(δ−)···C(δ+) = O) interaction results ladder-like arrangement.