1,2-Bridged Calixarene Monocrowns and Biscrowns in the 1,2-Alternate Conformation†
journal contributionposted on 02.12.1998, 00:00 authored by George Ferguson, Alan J. Lough, Anna Notti, Sebastiano Pappalardo, Melchiorre F. Parisi, Ada Petringa
The condensation of 1,2-di[(2-pyridylmethyl)oxy]calixarenes 1 (R = tBu, H) with glycol ditosylates 2a−e in anhydrous toluene in the presence of tBuOK has led to a mixture of 1,2-alternate and cone 1,2-calixarene crown conformers 3 and 4, respectively. Similarly, the reaction of 1,2-bridged calixarene crown-4 5 with tri- to pentaethylene glycol ditosylates 2a−c has produced 1,2-alternate and cone calixarene biscrown conformers 6 and 7, respectively. The distribution of conformers depends on the para-substituent at the upper rim and the length and bulkiness of the ditosylates. The best yields of the 1,2-alternate conformer (up to 51%) are observed in the absence of tBu groups and with the longer polyether chain. The conformational features of 3 and 6 have been deduced by NMR spectroscopy and by X-ray crystallography. X-ray analysis of 25,26-27,28-biscrown-4-calixarene (6a) shows that there are two independent molecules in the asymmetric unit, both in the 1,2-alternate conformation.