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(A) Key Rotating Frame Overhauser Effect Spectroscopy (ROESY) correlations of β-methyl-γ-lactone moiety in compounds 1 and 2; (B) key ROESY correlations of β-methyl-γ-lactone moiety in compound 3; (C) key ROESY correlations of tetrahydroxanthone moiety in compounds 1–3.
