A regioselective 1,3-dipolar cycloaddition for the synthesis of novel spiro-chromene thiadiazole derivatives

A variety of 4′-(4-R-phenyl)-4-(methylthio)-2′-phenyl- 2′H-spiro[chromene-2,5′-[1,2,3]thiadiazole] 5a-d and 5′-(4-R-phenyl)-4-(methylthio)-3′-phenyl-3′H-spiro[chromene-2, 2′-[1,3,4]thiadiazole] 6a-d were synthesized regioselectively through the reaction of 4-(methylthio)-2H-chromene-2-thione 2 with diarylnitrilimines under refluxing dry chloroform. Whilst the reaction of 4-(allylthio)-2H-chromene-2- thione 3 with diarylnitrilimines under similar reaction conditions afforded the corresponding 4-(allylthio)-5′-(4-R-phenyl)-3′-phenyl-3′H- spiro[chromene-2,2′-[1,3,4]thiadiazole] 7a-d and 5-(4-R-phenyl)-2′- methyl-3-phenyl-2′,3′-dihydro-3H-spiro[[1,3,4]thiadiazole-2, 4′-thieno[3,2-c]chromene] 9a-d as two unisolable diastereoisomeric forms. The structures of the obtained spiro cycloadduct thiadiazoles have been assigned by means of spectroscopic measurements.