Abstract
This thesis describes a new, eight-step synthesis of the [n] (2,5) pyridinophanes starting from the appropriate macrocyclic ketones. By this method [9] (2,5) and [7] (2,5)pyridinophane have been prepared. [7] (2,5)Pyridinophane represents the shortest para-bridged aromatic compound to be synthesised and the spectral properties of this highly strained system are discussed in detail. A simplified acyloin cyclisation procedure is described which employs a Vibro-stirrer for the generation of a fine sodium dispersion instead of a high-speed stirrer which is normally used. This apparatus has been used to prepare [9] paracyclophane and a C[10] and a C[16] macropyclic ketone. An attempted acyloin cyclisation of 2,5-di-(4-methoxycarbonylbutyl)-N-ethylpiperidine met with failure. The bridging central methylene groups in a paracyclophane experience a shielding effect due to the induced ring current in the aromatic cycle. By replacing the benzene with a pyridine ring in this system the methylene groups will still be shielded but the degree of shielding will be related to the magnitude of the ring current and hence, aromaticity, of the new aromatic ring. Using this approach a comparison of the nuclear magnetic resonance spectra of [9](2,5)pyridinophane and [9] paracyclophane has been made to determine the aromaticity of the pyridine ring. Of the two compounds, the [9] (2,5)pyridinophane showed greater shielding of a central methylene proton. This leads to the conclusion that either, pyridine is slightly more aromatic than benzene; or, if the aromaticity of pyridine is the same as that of benzene then the additional shielding effect may possibly be due to the influence of the nitrogen atom in the pyridine ring. A high field shift of a central methylene proton was observed on formation of the N-methyl quaternary salts of [9] and [7](2,5)-pyridinophane. Evidence is presented to show that this was due to changes in the conformation of the bridging group caused by steric interaction with the N-methyl group, A survey of spectroscopically and structurally interesting bridged compounds and a discussion of ring current effects on the chemical shifts of protons are also included. As a result of the synthetic work, several possible routes to other bridged heteroaromatics are suggested.