Revised Structure of Antidesmone, an Unusual Alkaloid from Tropical Antidesma Plants (Euphorbiaceae)

Structure Validation of Natural Products by Quantum-Mechanical GIAO Calculations of 13C NMR Chemical Shifts GIAO=gauge including atomic orbitals.

“…These concerned the alkaloid antidesmone (15), [9] a Cichorium sesquiterpene lactone (12) [7] and a sesquiterpene from Ligularia sagitta (20), [10] represented in Scheme 2. In the original papers [7,9,10] it is stressed that the small amount of sample and the structural complexity of natural products can cause the lack of key diagnostic NMR correlations in the two-dimensional spectra necessary for a safe assignment. In these cases, a validation method for structure elucidation of natural compounds could be a powerful supporting tool to avoid incorrect structural interpretations of experimental NMR data.…”

“…For this purpose, we calculated 13 C NMR chemical shift values by the method described here, at both the HF and the B3LYP level, for the right and wrong structures for the compounds listed above (see Scheme 2). [7, 9, 10, 12±15] Antidesmone: In a recent paper, [9] the structure of antidesmone, an alkaloid from Antidesma membranaceum and Antidesma venosum, was revised to become structure 15 (Scheme 2), rather than the isoquinoline derivative 16, as assumed previously. [12] To explore the potential of the validation method proposed, structures 15 and 16, for which the octyl fragment at C-6 was substituted in our calculation by the shorter propyl group, were optimized at the HF and the B3LYP level and 13 C NMR chemical shifts were determined.…”

“…[12] To explore the potential of the validation method proposed, structures 15 and 16, for which the octyl fragment at C-6 was substituted in our calculation by the shorter propyl group, were optimized at the HF and the B3LYP level and 13 C NMR chemical shifts were determined. Figure 3 shows the calculated and corresponding experimental 13 C NMR chemical shift [9] The values of intercept, slope, and r of the least-squares linear fits are reported in Table 2. The r value, relative to structure 15, is higher than that observed for structure 16.…”

“…The NMR spectrum interpretation method is tested on three cases in which misinterpretation of experimental data led to erroneous structure determinations. [7,9,10] Interestingly, it was observed that electron correlation contributions were more important for species showing 13 C NMR signals in the low-field region of the spectrum. This observation can be used as a rule of thumb for using post-HF methods for the calculation of 13 C NMR chemical shifts.…”

Toxicity Testing of Two Medicinal Plants, Bridelia micrantha and Antidesma venosum

“…Various alkaloids and specifically the glycine-derived acetogenic quinoline alkaloid, antidesmone, have been detected in A. venosum [18,19].…”