Macrocyclic cyclo[n]malonates -: Synthetic aspects and observation of columnar arrangements by X-ray crystallography

A variety of achiral and chiral macrocyclic oligomalonates were synthesised in a one-step procedure through condensation of malonyl dichloride with alpha,omega-diols. We have investigated the applicability of this method by varying the length and type of the spacers in the diol. Product distribution analysis revealed that the preferential formation of monomeric, dimeric, or trimeric macrocyclic malonates can be controlled by choosing diols with specific spacers connecting the hydroxy groups. Of special interest are the macrocyclic bismalonates, as they show pronounced crystallisability and arrange into columnar motifs in the solid state. They feature distinctive dihedral angles: all ester moieties adopt anti conformations whereas the planes of the carboxy moieties of each malonate residue arrange in an approximately orthogonal fashion. The latter geometry is enforced by the macrocyclic structures, as revealed by a conformational search in the Cambridge Structural Database. The X-ray diffraction data show that C=O center dot center dot center dot H-C, and C-O center dot center dot center dot H-C hydrogen bonds stabilise the columnar arrangement of the dimeric rings with formation of tubular assemblies. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)