Application of Residual Dipolar Couplings and Selective Quantitative NOE to Establish the Structures of Tetranortriterpenoids from Xylocarpus rumphii

Nine triterpenoid derivatives were isolated from the heartwood of Xylocarpus rumphii and were identified as xylorumphiins E (1), C (2), L (3), and M–R (4–9). Compounds 4–9 have a hemiacetal group in the triterpenoid side chain, making them impossible to purify. Purification was achieved after acetylation and subsequent separation of the epimeric mixtures of acetates; however differentiaition of the R and S epimers was not possible using standard NMR techniques. In one case, the relative configuration of a remotely located stereocenter with respect to the stereocenters in the main skeleton was unambiguously determined using residual dipolar couplings. Dipolar couplings were collected from the sample oriented in compressed poly­(methyl methacrylate) gels swollen in CDCl₃. In another case, the relative configuration was determined using 1D selective quantitative NOE experiments. Xylorumphiin K (10), xyloccensin E, taraxer-14-en-3β-ol, (22S)-hydroxytirucalla-7,24-diene-3,23-dione, and 25-hydroxy-(20S,24S)-epoxydammaran-3-one were isolated from the bark of the same plant. Compounds 3–10 are new compounds. Compounds 1–6 and xyloccensin E were tested at one concentration, 1 × 10^–5 M, in the NCI59 cell one-dose screen but did not show significant activity.