NaHSO<sub>4</sub>/SiO<sub>2</sub> catalyzed generation of <i>o</i>-quinone/ <i>o</i>-thioquinone methides: synthesis of arylxanthenes/ arylthioxanthenes <i>via</i> oxa-6π-electrocyclization
Muthupandi Karthick, Edwin Konikkara Abi, Someshwar Nagamalla, Savarimuthu Philip Anthony, Chinnasamy Ramaraj Ramanathan
Abstract
ortho-Quinone methides, very reactive transient intermediates, are utilized successfully in synthesizing complex organic molecules of natural and biological significance. Among several synthetic protocols, the acid catalyzed generation of ortho-quinone methides from suitably substituted phenols is a promising method for further exploitation in organic synthesis. Such an interesting reactive species is conveniently employed in the synthesis of conformationally restricted triarylmethane derivatives such as 12/9-arylxanthenes/arylthioxanthenes starting from symmetrical/unsymmetrical 2-(hydroxydiarylmethyl)phenol/thiophenol, respectively, using SiO2-NaHSO4. Conformationally restricted 12/9-arylxanthenes/arylthioxanthenes were obtained in 52 to 96% yields using this protocol, which is believed to involve the formation of o-quinone methides followed by electrocyclic ring closure and isomerization at elevated temperature. Photophysical studies of selected examples in acidic media showed turn-on fluorescence by hydride ion transfer mediated π-conjugated xanthylium salt formation and suggested the application potential in bio-imaging and fluorescent sensors.