Carbon-13 NMR chemical shifts. A single rule to determine the conformation of calix[4]arenes

The conformations of calix [4] arenes can be deduced from the 13 C NMR chemical shift of the methylene groups connecting each pair of aromatic rings. Inspection of 24 cases revealed that when the phenol rings beside each methylene are in a syn orientation (i.e., in cone conformations), the methylene signals appear aroung δ31, whereas they appear around δ37 when both phenol rings are anti oriented (i.e., in 1,3-alternate conformations). Steric effects are believed to be the main cause of such large differences