The reaction of 3-(dimethylamino)-2H-azirines 2 with 1, 3-benzoxazole-2 (3H))-thione (3, which can be considered as NH-acidic heterocycle (pKa ca. 7.3), in MeCN at room temperature, leads to 3-(2-hydroxyphenyl)-2-thiohydantoins 6 and thiourea derivatives of type 7 (Scheme 2). A reaction mechanism for the formation of the products via the crucial zwitterionic intermediate A'is suggested. This intermediate was trapped by methylation ...