Dimethylenecycloalkane-Magnesium Reagents

Equation 42 Reaction of hard electrophile with (2,3-dimethyl-2-butene-l,4 liyl) magnesium. 

The overall result is that one can effect a net 2,1-addition or a 1,2-addition by choosing appropriate electrophiles. Complex, highly functionalized molecules have been prepared using this approach and are shown in Table 44 [81]. 

6 Carbocyclization of (1,4-Diphenyl-2-butene-l, 4-diyl) magnesium with Organic Dihalides 

It should be noted that the (l,4-diphenyl-2-butene-l,4-diyl)barium complex has been prepared in our laboratories and exhibits higher reactivity than its magnesium-diene counterpart. 

Based upon the bis-nucleophilicity of 1,3-diene-magnesium intermediates, reactions of these intermediates with bis-electrophiles can lead to spiro or fused bicyclic molecules, depending upon the regioselectivity of the cyclization. It has been shown that treatment of magnesium complexes of l,2-bis(methylene) cyclohexane with l, -dibromoalkanes results in overall 1,2-cyclizations of the original dienes, affording spirocarbocycles in good to excellent yields [115]. 

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In comparison with previous methods, the use of 1,2-dimethylenecycloalkane-magnesium reagents provides a simple approach for spiroannulation as well as access to a wide variety of spirocyclic systems. Furthermore, spirocarbocycles prepared by this method contain functional groups, such as the exocyclic double bond or a keto group in one of the rings which may be used for further elaboration of these molecules. 

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