Radical-Mediated Cyclization of Thiobenzamides into Benzothiazoles

Significance

A new and general method for the intramolecular cyclization of thiobenzamides to benzothiazoles using cerium ammonium nitrate (CAN) or the hypervalent iodine reagent, phenyliodine(III) bistrifluoroacetate (PIFA), is reported. The reaction is suggested to proceed, without provision of evidence, via aryl radical cation intermediates. Usually PIFA afforded higher yields of products than CAN, except for R^¹ = OMe and R^² = 4,5-diOMe, in which cases no product was isolated with PIFA, whereas CAN afforded 32% yield. No rationalization for this behavior was offered. The reaction also failed for R^¹ = H and R^² = NO₂ substituted starting materials leading to the recovery of the resulting benzamide.