The pathways for conversion of 2-carbenabicyclo[3.2.1]octa-3,B-diene (1), 3-carbenatetracyclo-[3.3.0.0.(2,8)0(4,6)]octane (2), and bicyclo[3.2.1]octa 2,3,6-triene (3) to common product endo-6-ethynylbicyclo-[3.1.0]hex-2-ene (4) were evaluated using density functional theory at the B3LYP/6-311 +G(3df,2p)//B3LYP/6-31G(d,p) and B3LYP/6-31 g(d,p)//B3LYP/6-31 G(d,p) levels. A manifold of equilibrating C8H8 species (1 2 reversible arrow 3) is found not to be important. Rearrangement of bivalent 1 to product 4 proceeds via a (pi)2(S)+(sigma)2(S)+(pi)2(S) carbene-to-carbene rearrangement process to generate endo-6-(2-carbenavinyl)bicyclo[3.1.0]hex-2-ene. Allene 3 and carbenatetracyclooctane 2 proceed directly to product.