Synthesis and properties of dimethyltetradehydrothia-[19]-, -[21]-, -[23]-, and -[25]-annulene. Diatropic and paratropic analogues of thiophene

Abstract

The title thia-annulenes (9), (11), (14), and (16) have been synthesized by Wittig reactions of (2E,4Z)-5-methylhepta-2,4-dien-6-ynal (6) and its vinylogous aldehydes (7), (12) with bis-[(triphenylphosphonio)methyl] sulphide dibromide (5), followed by intramolecular oxidative coupling of the resulting acyclic sulphides. The properties of the thia-annulenes are discussed on the basis of ¹H NMR and electronic spectra and compared with those of their lower homologues.