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

• Novel hydroxy[n]benzofuranophanes have been synthesized by intramolecular cyclization.
• Intramolecular hydrogen bonding between hydroxyl proton and the oxygen of furan ring was observed.
• Weak intramolecular hydrogen bonding causes the rigid structures of [n]benzofuranophanes.

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 BBr3 in CH2Cl2 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 BBr3 in CH2Cl2 by using the same reaction conditions. 1H 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 Å̊, 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.

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