Mycolactones and Tetrafibricin 3764 3.5. UDB Scope and Limitation: Altromycin B and 2′-Substituted Taxanes 3764 4. Quantum Mechanical Calculation of NMR Properties in the Configurational Assignment of Organic Molecules 3767 4.1. Quantum Chemical Calculation of NMR Parameters 3768 4.2. QM-NMR in Structural and Conformational Analysis 4.3. Relative Configuration Assignments by Combined MM/QM Approaches 4.4. Conformational and Configurational Analysis via 13 C NMR GIAO Chemical Shifts Prediction on MM Geometries 4.5. Stereochemical Analysis of the 3R-and 3β-Hydroxy Metabolites of Tibolone through NMR and Quantum Chemical Investigations 4.6. Determination of the Relative Configuration of Flexible Organic Compounds through Boltzmann Weighted GIAO 13 C NMR Chemical Shift Calculations 4.7. Quantum Mechanical Calculations of NMR J-Coupling Values: Toward the Automatic Determination of Relative Configuration in Organic Compounds 4.7.1. Quantum Mechanical Calculations of NMR J-Coupling Values: Relative Configuration Assignment of the C23−C33 Reidispongiolide Fragment 4.7.2. New Combined NMR−Quantum Mechanical Strategy in the Determination of the Relative Configuration of Steroids: Application to Stemmosides C and D 5. Conclusion and Future Perspectives 6. Addendum 7. Notes and References

The influence of the calculation method in mimicking experimental (13)C NMR chemical shifts of 15 low-polarity natural products singularly containing 10-20 carbon atoms was investigated by employing different quantum chemistry approaches and basis sets, both in the preliminary geometry optimizations and in the following single-point (13)C GIAO calculations of the NMR chemical shifts. The geometries of the involved species were optimized at the PM3, HF, B3LYP and mPW1PW91 levels whereas the (13)C NMR parameters were determined at the HF, B3LYP and mPW1PW91 levels. Different combinations of basis sets were also tested. The consistency and efficiency of the considered combinations of geometry optimizations and GIAO (13)C NMR calculations were thoroughly checked by the analysis of statistical parameters concerning computed and experimental (13)C NMR chemical shift values.

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Two new metabolites, named halipeptins A and B, have been isolated from the marine sponge Haliclona sp. Their structures were determined by extensive use of one- and two-dimensional NMR experiments, mass spectrometry, and UV and IR spectroscopy. Halipeptin A is a novel 17-membered cyclic depsipeptide, consisting of five residues including two alanines (with L stereochemistry) and three new residues that appear to be previously undescribed from natural sources: 1,2-oxazetidine-4-methyl-4-carboxylic acid, 3-hydroxy-2,2,4-trimethyl-7-methoxydecanoic acid (HTMMD), and N-methyl-delta-hydroxyisoleucine. The HTMMD residue is substituted with 3-hydroxy-2,2,4-trimethyl-7-hydroxydecanoic acid in halipeptin B. Halipeptin A was found to possess very potent anti-inflammatory activity in vivo, causing about 60% inhibition of edema in mice at the dose of 300 microg/kg (i.p.).