5(4H)-Oxazolones. Part X. Acid and base effects on the translactonization reaction of 4-(2-oxa-alkylidene)-5(4H)-oxazolones: New synthesis of 5-alkylidene-3-benzoylamino-2(5H)-furanones

4-(2-Oxa-alkylidene)-5(4H)-oxazolones (azlactones) 1 can be transformed in acidic conditions (anhydrous HBr/CHCl3) into 5-alkylidene-3-benzoylamino-2(5H)-furanones 2 which have Z configuration at the exocyclic double bond. The same reaction conducted in acetic acid as solvent gives, besides the alkylidene-furanones 2, the furanyl acetates 5. The result of azlactone transformation in the presence of base (DBU) depends on the steric hindrances in the starting material. The less hindered oxazolones 1a,b give condensation products 8, whereas the more hindered azlactone 1c gives furanone 2c or, in presence of an alkylating agent, furanone 2d.