61. Reaction of 3-(Dimethylamino)-2H-azirines with 1,3-Benzoxazole-2(3H)-thione

Article abstract of DOI:10.1002/hlca.19900730308

The reaction of 3-(dimethylamino)-2H-azirines 2 with 1,3-benzoxazole-2(3H)-thione (5), which can be considered as NH-acidic heterocycle (pK_a 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 duringworkup 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 deg C, 2a and 5 react to yield a mixture of 6a, 2-(isopropylthio)-2,3-benzoxazole (12), and the thioamide 13 (Scheme 5).A mechanism for the surprising alkylation 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.