The crystal structure of methyl (1 S ,2 S )-2-methyl-1-((3 R ,5a S ,9a S )-6,6,9a-trimethyl-1,4-dioxo-1,3,4,5,5a,6,7,8,9,9a-decahydronaphtho[1,2-c] furan-3-yl)cyclopropane-1-carboxylate, C ₂₁ H ₂₈ O ₅