D:A-Friedo-24-noroleana-1,3,5(10),6-tetraen-29-oicacid, 2-[[(4x,20a)-3,4-epoxy-29-methoxy-2,29-dioxo-D:A-friedo-24-noroleana-1(10),5,7-trien-3-yl]oxy]-3-hydroxy-,methyl ester, (20a)-