Macrocyclic-yliden malonates 8-10 have been prepared by Knoevenagel condensation of differently sized crown malonates 5-7 with several aldehydes. The spectroscopic investigations of derivatives 8-10a in several solvents (acetone, acetonitrile, end dichloromethane) in the presence of sodium, lithium, barium, and magnesium perchlorate, evidenced, in some cases, the formation of different complexes, whose structure depends on the cation and on the dimension of the crown-ether unit. The Mg(II) complexation of 9-10 involves both carbonyl oxygen atoms, and this specific interaction has a strong catalytic effect on the Diels-Alder reaction between 9-10b with cyclopentadiene. (C) 1998 Elsevier Science Ltd. All rights reserved.