SOLUTION STRUCTURE AND CONFORMATIONAL EQUILIBRIA OF A SYMMETRICAL CALIX[6]ARENE - COMPLETE SEQUENTIAL AND CYCLOSTEREOSPECIFIC ASSIGNMENT OF THE LOW-TEMPERATURE NMR-SPECTRA OF A CYCLOASYMMETRIC MOLECULE

被引:50
作者
MOLINS, MA
NIETO, PM
SANCHEZ, C
PRADOS, P
DEMENDOZA, J
PONS, M
机构
[1] UNIV BARCELONA,FAC QUIM,DEPT QUIM ORGAN,E-08028 BARCELONA,SPAIN
[2] UNIV AUTONOMA,DEPT QUIM,E-28049 MADRID,SPAIN
关键词
D O I
10.1021/jo00051a046
中图分类号
O62 [有机化学];
学科分类号
070303 ; 081704 ;
摘要
A calix[6]arene containing tert-butyl and chlorine substituents in alternate rings, with a 3-fold sequential symmetry, could be frozen in a completely asymmetrical conformation at 183 K in CD2C6 The H-1-NMR could be completely assigned by low-temperature ROESY, DQF-COSY, HMQC, and HMBC experiments a sample in which the phenolic protons had been partially exchanged by deuterons to reduce both spin diffusion and conformational exchange processes. The three-dimensional structure obtained using restrained molecular dynamics is a winged cone made asymmetric by the clockwise or anticlockwise sense of a cyclic array of hydrogen bonds. Three different types of exchange processes, leading to a statistically symmetric conformation at room temperature, could be identified in the NOESY and ROESY experiments. Hexa-tert-butylcalix[6]arene, with a potential 6-fold symmetry, seems to have a very similar conformation at low temperature with a C2 axis as the only symmetry element.
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收藏
页码:6924 / 6931
页数:8
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