The conformations of aromatic ethers as determined from their ultraviolet absorption spectra

Abstract

A number of heterocyclic analogues of choline phenyl ether have been synthesised in order to investigate the relationship between conformation and biological activity in choline aryl ethers. The intensity of the Ph–O π→π* transition has been used to determine the angle (θ) between the plane of the benzene ring and the plane containing the oxygen bonds. The calculated angles increase in the order 2,3-dihydrobenzofuran < 2,3-dihydro-7-methylbenzofuran < chroman < homochroman; but the value of θ obtained for the 2,3-dihydro-7-methyl benzofuran is anomalously high and is explained on the basis of steric inhibition to sp² hybridisation of oxygen. Two β-methylated derivatives of choline phenyl ether bromide have also been investigated and the time-averaged values for θ calculated. It is deduced that unlike choline phenyl ether bromide these molecules are unable to adopt a conformation in which θ= 0°.