Identification of guayule triterpenes by two‐dimensional and multipulse NMR techniques

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AbstractFive triterpene natural products have been isolated from the resin of the guayule bush (Parthenium argentatum) and four of these identified using two‐dimensional (2D) and multipulse NMR techniques. Of the five structures, two have been reported previously. Two of the structures contain an uncommon seven‐membered ring incorporating an ether linkage. The 2D carbon–carbon chemical shift correlation (2D INADEQUATE) and DEPT multiplicity experiments are used as the primary techniques for determining the molecular skeleton. Supporting evidence is provided by the carbon–proton and proton–proton (COSY) chemical shift correlation experiments, and by preparation of simple derivatives. The merits and problems of the various techniques are discussed. We reverse the previous assignments of C‐27 and C‐29 of cycloartanone.