The reaction of 3-(dimethylamino)-2H-azirines 2 with 1,3-benzoxazole-2(3H))-thione (5), 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 with MeI and hydrolysis to give 9 (Scheme 4). Under normal reaction conditions, A' undergoes a ring opening to B which is hydrolyzed during workup to yield 6 or rearranges to give the thiourea 7. A reasonable intermediate of the latter transformation is the isothiocyanate E (Scheme 3) which also could be trapped by morpholine. In i-PrOH at 55-65°, 2a and 5 react to yield a mixture of 6a, 2-(isopropylthio)-1,3- benzoxazole (12), and the thioamide 13 (Scheme 5). A mechanism for the surprising akylation of 5 via the intermediate 2-amino-2-alkoxyaziridine F is proposed. Again via an aziridine, e.g. H (Scheme 6), the formation of 13 can be explained.