Brønsted Acid Promoted Thermal-Ring-Rearrangement of Fluorenopyrans to 2-(1H-Inden-3-yl)-9H-fluoren-3-ols Bearing Two All-Carbon-Quaternary Centres

Abstract

4-Aryl-fluorenopyrans bearing quaternary centre at C2 and C6 positions underwent a thermal-ring-rearrangement with a catalytic amount of pTsOH to furnish 2-(1H-inden-3-yl)-9H-fluoren-3-ols with two all carbon-quaternary centres, in which one is retained and the other is created. Interestingly, these compounds exhibit atropisomerism, when they have at least one unsymmetrical quaternary centre. DFT studies of structure and energetics of these atropisomers were offered, and the calculated 1H-NMR chemical shifts of the hydroxyl (–OH) functional group in the atropisomers corroborate well with the experiment.