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

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