Ytterbium(III) triflate-catalysed preparation of calix[4]resorcinarenes: Lewis assisted Brønsted acidity

Calix[4]resorcinarenes 1–5 have been prepared in good yields by the ytterbium(III) triflate hydrate-catalysed condensation of equimolar amounts of aldehydes and resorcinol. The lanthanide salt was easily recovered and re-used following the reaction thus providing an atom economic synthesis of resorcinarenes. Evidence is presented that indicates that ytterbium(III) triflate can significantly increase the Brønsted acidity of organic acids.