Calculated chemical shifts as a fine tool of conformational analysis: an unambiguous solution for haouamine alkaloids

In this study, for the first time, conformational analysis by calculated chemical shifts (CCS) deals with a real conformational problem of a large biomolecule. This new methodology is applied to haouamine A, which is much more stereodynamically puzzled than the small models used to validate previous CCS-based conformational studies. Thorough NMR experiments by Zubia et al. on this exotic polyfunctional paracyclophane alkaloid could not determine which experimentally detected interconversion of this compound occurs in solution, rotation or N-inversion. The present study uses CCS to locate the lowest energy conformers and thus to identify the observed stereodynamic process. Molecular mechanics calculations were used to explore the conformational space of this polycyclic system, and then the geometry of located conformers was refined by ab initio calculations at the B3LYP/6-31G(d,p) level; an implicit model for acetone solution was employed. Calculated relative energies are considered too inaccurate to identify the lowest energy (i.e., those detectable by NMR) conformers. Instead, rational regression analysis of CCS for carbon atoms using B3LYP/6-31+G(d)//GIAO-based calculations singled out two conformers from a large set of alternative low energy structures, although solvation shell was not explicitly included in the model. For only these two conformers, the differences in CCS (Delta delta) for selected pairs of carbons are very similar to the experimental Delta delta values. Thus, the conformers monitored by NMR have now been identified; their piperideine ring is of (1)Sf and Sf 1 (sofa-shaped) geometry. This azacycle appears to be flexible despite the presence of the ethylenebiphenylene bridge in haouamines. Interconversion between the conformers probably occurs via a concerted process of inversion of the piperideine ring, N-inversion coupled with rotation around the C-N bond, and rotation around two C-C bonds in the ethylenebiphenylene bridge. This CCS method of conformational analysis is sufficiently simple and reliable that if chemical shifts for a pair of the same carbons are sufficiently different in routine (13)C NMR spectra of stereoisomers (ca. > or = 2 ppm), the "resolving power" of the CCS technique may rival that of NMR techniques.