1.4- Bisallenes, cycloaddition

Hydrocarbon 136 subsequently cyclizes to a bismethylencyclobutene again, 137, in which the triple bonds of the substituent are so close that they can engage in a [2+ 2] cycloaddition, leading to the doubly annelated hydrocarbon 138. For less highly unsaturated bisallenes, a similar behavior is observed and they allowed the determination of the stereochemistry of the first ring-closure step [51]. 

With bisallene 11 having a three-carbon chain between the allenic parts, the intramolecular [2 + 2]-cycloaddition occurred at the inner carbon-carbon double bonds to afford 6,7-dimethylenebicyclo[3.2.0]heptane 12 [11]. 

Vinylallenes and bisallenes participate in the Diels-Alder-type cycloaddition as the diene component, providing a powerful tool for the construction of complex ring systems. They also undergo thermal electrocydic ring closure to form methylenecy-clobutene derivatives. 

A [2.2]paracydophane was prepared by dimerization of p-quinodimethane 232, which was obtained by a [4+ 2]-cycloaddition reaction of bisallene with DMAD [184], This sequence represents one of the most general approaches to functionalized para-cyclophanes. 

The CuBr2/amine-promoted 2 + 2-cycloaddition reaction of 1,4-bisallenes (17) produced bicyclo[4.2.0]octadiene derivatives (18) in a one-pot procedure (Scheme 7) Phosphoramidite ligands (19) activate the Au(I)-catalysed 2 + 2-cycloaddition reaction of A-allenylsulfonamides with styrenes, at -70 C, to form vinyl cyclobutane derivatives in high yields and enantioselectivity. A new Pt(II) catalyst with a hollow-shaped tri- ethynylphosphine activates the intermolecular 2 + 2-cycloaddition reaction of allenyl silyl ether with vinyl ethers to yield methylenecyclobutanes in good yield. 

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