Stereospecific total synthesis of chromomoric acid C-I and C-II methyl esters and delicate elucidation of their structures have been reported. A stereospecific five-step synthesis, starting from enone 5, with one-pot threecomponent coupling and retro Diels-Alder reaction as the key reactions provided a target molecule which was proved to be chromomoric acid C-I methyl ester (l), but its physical data were identical with that of the naturaI product originally assigned to be chromomoric acid C-IV methyl ester (4). Structural elucidation with a lanthanide shift reagent showed the original assignment should be revised to 1. Total synthesis and structural confirmation of chromomoric acid C-II methyl ester (2) was thereafter fulfilled. Chromomoric acid C I-IV methyl esters (l-4) are a group of optically active natural products isolated from Chromolaena morii and Chromolaena chasleae by Bohlmann et al. 23 This family of octadecanoids are metabolites of linolenic acid and bear structural resemblance to prostaglandin? which are biosynthetically derived from arachidonic acid and exhibit diverse pharmacological properties.' This distinct feature greatly arouses our interests in the possible biological activities of this group of new substances. However, they exist only in minute amounts and their stereochemistry has not yet been determined. Therefore, a total synthesis should be of high interest and significance. We detailed here the stereospecific total synthesis and the delicate structure elucidation of chromomoric acid C-I and C-II methyl esters (1 and 2 respectively).' Chromomoric acid C-I methyl ester (1) H Chromomoric acid C-III methyl ester (3) H Chromomoric acid C-II methyl ester (2) H Chromomoric acid C-IV methyl ester (4)