The absolute configuration of the branched methyl group in ante -iso type side chain moiety on the long chain base of glucocerebroside, HLC-2-A, which was isolated from the sea cucumber Holothuria leucospilota was determined. Oxidation of the glucocerebroside with ozone afforded C₁₃-fragment including the ante -iso moiety. The optically active C₁₃-fragment was synthesized asymmetrically by using the Wittig reaction from chiral synton for comparison with the natural fragment.