Some benzo- and naphtopyrans by novel cyclisations
It has recently been shown that the naphthalene dimethyl ethers E-2-Hydroxymethyl-1,4-dimethoxy-3-pent-1-enylnaphthalene and E-2-Hydroxyethyl-1, 4-dimethoxy-3-prop-1-enylnaphthalene cyclise to afford naphtho[2,3-c]pyrans when treated with two molar equivalents of cerium(IV) ammonium nitrate. It has also been shown that these naphthalenes cyclise readily and in high yield to give naphthopyrans when treated with potassium-t-butoxide in dimethylformamide under anaerobic conditions. In order to determine which structural features present in these compounds are required for each mode of cyclisation, a series of ortho-alkenylbenzyl and -naphthyl alcohols, unsubstituted or carrying methoxy or ethyl substituents at appropriate positions on the aromatic ring have been synthesized. The investigations which were carried out revealed that as far as the cerium(IV) ammonium nitrate reactions were concerned, a methoxy group ortho to the alkenyl side chain appeared to be imperative for the success of the reaction. A mechanism for the reaction is proposed. Although no conclusive general mechanism could be inferred from the results obtained from the base-promoted cyclisations, the explanation for this unusual reaction may involve steric effects, since it was those alcohols in which both the hydroxyalkyl and alkenyl side chains were flanked by bulky groups (forcing the reacting centres into close proximity) which cyclised in high yield.
Networked Digital Library of Theses and Dissertations