Cyclobutadiene cobalt compounds formed by -cycloaddition of alkynes

1 77 -(Cyclobutadiene) cobalt compounds formed by [2+2]-cycloaddition of alkynes 

While trying to synthesize the ABCD taxane framework by using a [2- -2- -2]-cycloaddition reaction mediated by 

the intramolecular variants of the [2-f-2]-cycloaddition of two alkynes have been shown to occur, cyclobutene adduct 245 stabilized by a CoCp unit (Equation (42)). 

A somewhat related constrained cyclization occurred with an aryl unit 1,4-substituted by acyclic alkynes. Depending upon the nature of the alkyne substituents, 1,2-246 or 1,3-tethered products 247 were formed regio-selectively (Equation (43)).  

Polyether macrocycles containing a 77 -cyclobutadiene cobalt unit 248 and 249 were obtained in a related way 

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