Complexation in situ of 1-methylpiperidine, 1,2-dimethylpyrrolidin, and 1,2-dimethylpiperidine with rhodium(II) tetracarboxylates: Nuclear magnetic resonance spectroscopy, chiral recognition, and density functional theory studies

Complexation in situ of 1-methylpiperidine, racemic 1,2-dimethylpyrrolidin, and racemic 1,2-dimethylpiperidine with rhodium(II) tetracarboxylates in chloroform was studied by ¹ H and ¹³ C nuclear magnetic resonance (NMR) spectroscopy and density functional theory (DFT) methods. As substrates, three dirhodium(II) compounds were applied, tetraacetate, tetrakistrifluoroacetate, and a derivative of optically pure Mosher's acid. Due to conformational flexibility, free and complexed ligands can adopt potentially various conformations. The NMR titration experiments revealed the subsequent formation of 1:1 and 1:2 complexes, depending on the molar ratio of substrate to ligand. Conformations of free and complexed ligands were examined by the comparison of experimental and DFT gauge-independent atomic orbital (GIAO) calculated chemical shifts and by the analysis of the internal energy of the compounds. For some ligand and substrate combinations, a mixture of complexes differing in ligand conformations was formed. Complexes of Mosher's acid derivative of rhodium(II) with racemic 1,2-dimethylpyrrolidin and 1,2-dimethylpiperidine exhibited NMR chiral recognition phenomenon, manifested by splitting of signals in ¹³ C NMR and ¹ H,¹³ C HSQC spectra.

Plakilactones G and H from a marine sponge. Stereochemical determination of highly flexible systems by quantitative NMR-derived interproton distances combined with quantum mechanical calculations of (13)C chemical shifts

In this paper the stereostructural investigation of two new oxygenated polyketides, plakilactones G and H, isolated from the marine sponge Plakinastrella mamillaris collected at Fiji Islands, is reported. The stereostructural studies began on plakilactone H by applying an integrated approach of the NOE-based protocol and quantum mechanical calculations of (13)C chemical shifts. In particular, plakilactone H was used as a template to extend the application of NMR-derived interproton distances to a highly flexible molecular system with simultaneous assignment of four non-contiguous stereocenters. Chemical derivatization and quantum mechanical calculations of (13)C on plakilactone G along with a plausible biogenetic interconversion between plakilactone G and plakilactone H allowed us to determine the absolute configuration in this two new oxygenated polyketides.