Chemistry of conjugated diene-magnesium reagents: A novel approach to cyclic and functionalized molecules

Highly reactive magnesium reacts with a wide variety of substituted 1,3-dienes to give the corresponding (2-butene-1,4-diyl)magnesium reagents. Reactions of symmetrical (2-butene-1,4-diyl)magnesium with organic dihalides generates three-, four-, five-, and six-membered carbocycles. In some cases, uncyclized products can be obtained. Stepwise reactions of (2,3-dimethyl-2-butene-1,4-diyl)magnesium with two different electrophiles afford formal 1,2-additions, yielding polyfunctionalized ketones with the generation of a quaternary center. Substituted five- and six-membered cyclic ketones can also be synthesized by this approach. Treatment of unsymmetrical (2-butene-1,4-diyl)magnesium complexes with chlorotriorganosilanes followed by cyclohexanone results in additions across a terminal double bond in high regioselectivity with the introduction of two functional groups. Silicon-containing heterocycles or spiro compounds can be readily prepared by using these dienemagnesium reagents. Formation of magnesium complexes of 1,2-dimethylenecycloalkanes has also been accomplished by using highly reactive magnesium. Treatment of these dienemagnesium reagents with 1,n-dihaloalkanes or bromoalkylnitriles generates a wide variety of spiro olefins and spiroenones in one synthetic operation. Dispirocycles can also be synthesized by using the dienemagnesium reagents. This approach represents a general method for preparing spirocarbocycles with different combinations of ring sizes. Remarkably, fused bicyclic systems containing a substituted five-membered ring can be readily prepared by the reactions of the magnesium complexes of 1,2-dimethylenecycloalkanes with carboxylic esters. The reaction proceeds via a magnesium salt of a spiroenol containing a cyclopropane ring. Quenching the intermediate at an appropriate temperature creates an alternative pathway for the reaction, leading to a regioselective synthesis of $\beta$,$\gamma$-unsaturated ketones. Application of this approach to (2-methyl-3-phenyl-2-butene-1,4-diyl)magnesium provides a convenient method for synthesizing substituted 3-cyclopentenols. The recent development of the 1,3-diene-magnesium chemistry includes a one-step synthesis of spiro $\gamma$-lactones. In this method, conjugated diene-magnesium reagents are reacted with ketones, followed by CO$\sb2.$ Spiro $\gamma$-lactones are generated upon workup. The overall process efficiently assemble three individual species into a complex molecule