Abstract Double aldol reactions of the Co 2 (CO) 6 complex of acetylenedicarbaldehyde with several enol ethers have been carried out and compared with the double Nicholas reactions of the corresponding bis(diethyl acetal) complex. The selectivity was found to depend on (i) the nucleophile, (ii) the solvent, and (iii) the number of equivalents of the auxiliary BF 3 ·OEt 2 . While silyl enol ethers give mono‐ and 1,4‐bis‐dialkylation products only, trimethoxybenzene affords 1,1‐bis‐ and 1,1,4‐tris‐arylation products. In dichloromethane as solvent, the trimethylsilyl enol ether of acetophenone exhibits novel behaviour. In this solvent, and in the presence of an excess acid, the primary bis(aldol) product evolves to the Co 2 (CO) 6 complex of 1‐phenyl‐4‐(5‐phenyl‐2‐furyl)‐but‐3‐yn‐1‐one and to 3‐(5‐phenyl‐2‐furyl)‐6‐phenyl‐2 H ‐pyran‐2‐one. These products are rationalised in terms of a common double aldol−cyclodehydration process, followed by α‐slippage of the µ‐Co 2 (CO) 6 unit, and by a cyclocarbonylation−decomplexation process, respectively. Reaction of the trimethylsilyl enol ether of acetophenone with the homologous (dibenzoylacetylene)Co 2 (CO) 6 complex led to Z ‐3‐[(3,5‐diphenyl‐2‐furyl)methylene]‐2−3 H ‐ benzofuran‐1‐one, apparently by an aromatic C−H bond activation. The structures of the pyrone and benzofuranone molecules were established either by X‐ray crystallography and/or by 2D NMR spectroscopy. A general mechanistic scheme is proposed to account for the versatility of these reactions. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)