A synthesis of ethyl-, butyl-, hexyl- and dodecyl-substituted fimbrolides from alkyl-substituted levulinic acid derivatives through bromination and acid promoted lactonisation is described. The underlying reactions have been investigated using levulinic acid as a model, and the effects of varying the bromination conditions and changing acid concentration on product distribution are discussed. Dibromination proceeds best in CHCl 3 and proceeds in EtOH-free CHCl 3 without the complication of ester formation. Cyclisation occurs with concomitant oxidation in 98-100% H 2 SO 4 but gives highest yields of fimbrolides in 100% H 2 SO 4 . The formation of related beckerelide substances is also described.