One-Pot Synthesis of Metalated Pyridines from Two Acetylenes, a Nitrile, and a Titanium(II) Alkoxide

Four-component coupling process involving two acetylenes, a nitrile, and a divalent titanium alkoxide reagent, Ti(O-i-Pr)4/2i-PrMgCl, directly yielded titanated pyridines in a highly selective manner. The reaction can be classified into four categories: (i) a combination of an internal acetylene, a terminal acetylene, sulfonylnitrile, and the titanium reagent to yield α-titanated pyridines, (ii) a combination of an internal acetylene, a (sulfonylamino)acetylene, a nitrile, and the titanium reagent to yield alternative α-titanated pyridines, (iii) a combination of an internal acetylene, a (sulfonylamino)acetylene, a nitrile, and the titanium reagent to yield titanated aminopyridines, and (iv) a combination of an acetylenic amide, a terminal acetylene, a nitrile, and the titanium reagent to yield pyridineamides with their side chain titanated. Some of these reactions enabled virtually completely regioselective coupling of two different, unsymmetrical acetylenes and a nitrile to form a single pyridine. Synthetic applications of these reactions have been illustrated in the preparation of optically active pyridines and medicinally useful compounds.