Euphornin， isolated from Euphorbia helioscopia L.(Toudai Gusa in Japanese)， isatoxic diterpene having antitumor activity. Euphornin is a highly oxygenatedditerpene containing two double bond included in a 12-membered ring. Oxidativecleavage of both double bonds afforded two segments， the C1-C5・C12-Cl5 segmentand the C6-C11 segment， which would be key intermediates for total synthesis ofeuphornin. The C1-C5・C12-C15 segment is a five membered ring compound includ-ing four asymmetric carbons on the ring and two on the side chain. And the C1-C5・C12-C15 segment equivalent to these of the nature diterpene has been synthes-ized from commercially available L-malic acid in our synthetic studies. In thepresent paper， the degradation reaction of euphornin has been carried out to affordthe C1-C5・C12-C15 segment which is expected to be coupled with the C6-C11segment giving rise to euphornin.