Acid-catalysed reactions of activated benzofuranylmethanols: formation of calixbenzofurans

Regiochemistry of electrophilic substitution of the 3-substituted-4,6-dimethoxybenzofurans is largely controlled by the substituents at C3. The presence of a 3-aryl group results in the preferred formation of 2-substituted benzofuran derivatives whereas a bulky tert-butyl 3-substituent leads to a dominance of 7-substituted products. Acid-catalysed reactions of 2-hydroxymethylbenzofurans give mainly unsymmetrically linked calix[3]benzofurans. However, acid treatment of 7-hydroxymethylbenzofurans affords only the symmetrically linked calix[3]benzofurans in high yields. Benzofuranyl glyoxylamides, glyoxylates and ketones can be obtained in high yields and the acid-catalysed reactions of their corresponding alcohol reduction products generally give moderate to high yields of the symmetrically linked calix[3]benzofurans together with a minor amount of a calix[4]benzofuran.