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A Macrobicyclic Calix[6]arene Phosphocine: Conformational Studies Involving High‐Resolution NMR Analysis and Molecular Modeling
Author(s) -
RegnoufdeVains JeanBernard,
Cartier Alain,
Fenet Bernard,
PelletRostaing Stéphane
Publication year - 2005
Publication title -
helvetica chimica acta
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.74
H-Index - 82
eISSN - 1522-2675
pISSN - 0018-019X
DOI - 10.1002/hlca.200590082
Subject(s) - chemistry , steric effects , moiety , intramolecular force , crystallography , enantiomer , molecule , crystal structure , substructure , nmr spectra database , proton nmr , nuclear magnetic resonance spectroscopy , stereochemistry , organic chemistry , spectral line , astronomy , physics , structural engineering , engineering
The solution structure of the phosphorylated hexa( tert ‐butyl)calix[6]arene 1a was elucidated through a series of high‐resolution, one‐ and two‐dimensional 1 H‐ and 13 C‐NMR experiments, in combination with 31 P‐NMR measurements. The NMR results were cross‐checked and refined with partial structural information obtained from a poorly resolved X‐ray crystal‐structure analysis. The latter, that could not be upgraded due to the low quality of the crystals and due to strong thermal motions, confirmed that 1a was a racemate in which two adjacent phenol units are bridged by a mono‐ethyl phosphate moiety, generating a macrocyclic dibenzo[1,3,2]dioxaphosphocine substructure, while the four residual aromatic rings bear regular diethyl phosphate arms. One of these rings is inverted, generating the main asymmetry in the molecule, which gives rise to the racemic nature of the macrobicycle. The two enantiomers, however, do not interconvert on the NMR time scale in solution and resemble the solid‐state structure, as expected from the steric hindrance due to the pendant phosphoesters and the high intramolecular tension due to the presence of the phosphocine substructure. The most‐probable conformation of 1a in solution was determined, starting from the NMR and X‐ray data, by means of semi‐empirical PM3 calculations with the GEOMOS program, including an unprecedented large number of 232 atoms.