A convenient synthetic method for (±)-malyngolide (1), an antibiotic from the marine bluegreen alga Lyngbya majuscula GOMONT, is described. The compound (±)-1 was synthesized from 2-methylglutaric anhydride (starting material) in four steps, involving the TBHP-based osmium-catalyzed procedure for vicinal dihydroxylation of the methylene unit in unsaturated carboxylic acid as the key step. The optical resolution of (±)-l was accomplished by HPLC, in which diastereomeric (+)-2-(4-chlorophenyl)isovaleric acid [(+)-CPIA] esters of (±)-l derived from (+)-CPIA-C1 could be easily separated, and subsequent hydrolysis of each ester gave the enantiomeri-cally pure (−)-l and (+)-l, respectively. A synthetic method for α-methylene malyngolide, which is to be expected for biological activity, is also described.