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The conformations of calix [4] arenes can be deduced from the 13C NMR chemical shift of the methylene groups connecting each pair of aromaticrings. Inspection of 24 cases revealed that when the phenol rings beside each methylene are in a syn orientation (ie, in cone conformations), the methylene signals appear around 31, whereas they appear around 37 when both phenol rings are anti oriented (ie, in 1, 3-alternate conformations). Steric effects are believed to be the main cause of such large differences.

The conformations of calix [4] arenes in solution have long been studied since the pioneering work of Gutsche. 1 Among the number of methods that can be used, 2 those based on the NMR methylene or aromatic signals and their multiplicities have been shown to be the most useful to distinguish the four main conformations of these cyclic tetramers of phenols. 3 456For calix [4] arenes with the same …