Syntheses and properties of methano-bridged dichlorodidehydro-[16]-, -[20]-and-[24]-annulenediones

Abstract

Bisethynylation of the vinylogous dialdehydes of cyclohepta-1,3,5-triene-1,6-dicarbaldehyde with ethynylmagnesium bromide followed by oxidation of the resulted diols afforded the diacetylenic diketones. Intramolecular oxidative coupling of these diketones under the Glaser conditions unexpectedly afforded three annulenediones with a 1,4-dichlorobutatriene moiety. The unusual structures were confirmed by X-ray crystallographic analysis of 10,13-dichloro-11,12-didehydro-1,6-methano[16]annulene-9,14-dione. These annulenediones exhibited strong diatropicity in D₂SO₄ due to the dicationic 14π-, 18π- and 22π-electron species and their diatropicities were shown to increase with increasing ring size.