methyl (4E,8E)-2-(2,3-dihydroxy-2-methyl-butanoyl)oxy-3-(3-hydroxy-2-m ethyl-butanoyl)oxy-8-methyl-13-methylidene-12-oxo-11-oxabicyclo[8.3.0] trideca-4,8-diene-4-carboxylate