Correct structures of Diels–Alder adducts from the natural cyclolignan thuriferic acid and its 8-epimer

A detailed analysis of one- and two-dimensional 1H and 13C NMR data for the endo and the exo adducts, obtained by Diels–Alder reaction of thuriferic and epithuriferic acids with cyclopentadiene is described. The unequivocal spectral data assignment of the endo and exo structures was complemented with molecular modelling studies and confirmed through X-ray diffraction studies.

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