Conformational mobility of the bridged calix[6]arenes with allyl and ethyl groups at the lower rim

The conformational mobility of the bridged calix[6]arenes with allyl and ethyl groups at the lower rim was investigated. Allylation of the tetrahydroxy compounds afforded two isomers: 5a (cone) and 5b (1,2,3-altemate), which were separated by silica-gel chromatography. These isomers underwent slow isomerization at room temperature with a half-life time of about 2 weeks. On the other hand, the ethyl derivatives 6-9 were found to exist as mixtures of conformational isomers in solution; they were observed independently on the NMR time-scale but were inseparable on the laboratory time-scale. The ratios of two isomers of 6-9 depended on the functionality on the bridging unit, the effects of which were investigated by molecular mechanics calculations. In the crystalline state, the ethyl derivative 6 with a bromo functionality was found to take the 1,2,3-altemate conformation, while compound 7 with an ethynyl group adopted the cone conformation.