Demethylation of 5,n-di-tert-butyl-8,n-dimethoxy[2.n]metacyclophane-1-ynes with BBr3 to afford novel [n]benzofuranophanes

Abstract Novel [ n ]benzofuranophanes ( n  = 8 & 10) 2a – b have been prepared by successive intramolecular cyclization from 5,19-di- tert -butyl-8,22-dimethoxy[ n ]metacyclophane-1-yne ( syn - 1a – b ) by treatment with BBr 3 in CH 2 Cl 2 at room temperature for 8h. [2. n ]Benzofuranophanes 2a – b were also obtained by treatment of 1,2-di- endo -bromo-5,19-di- tert -butyl-8,22-dimethoxy[ n ]metacyclophane ( meso - 3a – b ) with BBr 3 in CH 2 Cl 2 by using the same reaction conditions. 1 H NMR spectra of 2a – b reveals the formation of intramolecular hydrogen bonding between hydroxyl proton with the oxygen of the furan moiety and X-ray analysis shows that the lengths between H (OH) and O (furan) are 1.981 and 1.823 A, respectively. The conformation of [8]benzofuranophane 2a in solution is rigid with restricted rotation around the diaryl linkage rather than [10]benzofuranophane 2b because of weak intramolecular hydrogen bonding and the short length of the cross-linking chain.