Synthese von bromosubstituierten Butenoliden II

Synthesis of Bromosubstituted Butenolides II . Methyl 4,4′‐dibromosenecioate ( 2 ) was prepared by double N‐bromosuccinimide bromination of methyl senecioate ( 1 ) and converted to methyl 4,4′‐diiodo‐senecioate ( 3 ) with sodium iodide and to 3‐bromomethyl‐2‐buten‐4‐olide ( 4 ) with aqueous hydrobromic acid. A mixture of methyl ( Z )‐ and ( E )‐4‐bromosenecioate ( 8 and 9 ) yielded 3‐methyl‐2‐butenolide ( 5 ) with aqueous hydrobromic acid and a mixture of ( Z )‐and ( E )‐4‐methoxy‐senecioic acid ( 10 and 11 ) with methanolic potassium hydroxide. N‐Bromosuccinimide treatment of the butenolide 5 afforded 4‐bromo‐3‐methyl‐2‐buten‐4‐olide ( 6 ) and 4,4‐dibromo‐3‐methyl‐2‐buten‐4‐olide ( 7 ).