DIPOLAR 1-1 ADDUCTS FROM THE REACTION OF 3-AMINO-2H-AZIRINES WITH 1,3,4-OXADIAZOL-2(3H)-ONES AND 1,3,4-THIADIAZOL-2(3H)-ONES

Dipolar 1:1 Adducts from the Reaction of 3‐Amino‐2H‐azirines with 1,3,4‐Oxadiazol‐ and 1,3,4‐Thiadiazol‐2(3H)‐ones 3‐Amino‐2H‐azirines 1 react with 5‐(trifluoromethyl)‐1,3,4‐oxadiazol‐2(3H)‐one (2) as well as with different 5‐substituted 1,3,4‐thiadiazol‐2(3H)‐ones (5a–e) in 2‐propanol at room temperature to give dipolar 1:1 adducts of type 3 and 6, respectively, in reasonable‐to‐good yields (Schemes 3 and 6, Tab. 1 and 2). The structure of two of these dipolar adducts, 6a and 6e, which are formally donor‐acceptor‐stabilized azomethin imines, have been elucidated by X‐ray crystallography (Figs. 1‐4). In the reaction of 2 and sterically crowded 3‐amino‐2H‐azirines 1c–e with a 2‐propyl and 2‐propenyl substituent, respectively, at C(2), a 4,5‐dihydro‐1,2,4‐triazin‐3(2H)‐one of type 4 is formed as minor product (Scheme 3 and Table 1). Independent syntheses of these products proved the structure of 4. Several reaction mechanisms for the formation of compounds 3 and 4 are discussed, the most likely ones are described in Scheme 4: reaction of 2 as an NH‐acidic compound leads, via a bicyclic zwitterion of type A, to 3 as well as to 4. In the latter reaction, a ring‐expanded intermediate B is most probable. Copyright © 1990 Verlag GmbH & Co. KGaA, Weinheim