Expanding the Scope of Chemoselective Reductive Cyclization: Synthesis of Unsubstituted 9H‐Thioxanthenes and 10‐Methyl 9,10‐Dihydroacridines and Beyond

Unsubstituted 9 H ‐thioxanthenes are obtained from 2‐(arylthio)benzaldehydes via In(OTf) 3 ‐catalyzed chemoselective reductive cyclization in diisopropyl ether at elevated temperature. The reaction possesses broad substrate scope, good functional group compatibility and proceeds via chemoselective reduction of thioxanthylium ion in the presence of a highly reactive arylaldehyde. Similarly, unsubstituted 10‐methyl‐9,10‐dihydroacridines are also obtained via a Bronsted acid catalyzed chemoselective reductive cyclization of 2‐(methylphenylamino)benzaldehydes in isopropanol (solvent). The reductive cyclization method can also be utilized for the synthesis of 9‐trifluormethyl containing (thio)xanthenes. Mono‐deuterated 9 H ‐thioxanthene is obtained in moderate yield in deuterated solvent (THF‐d8).