The Effect of Fusion of Angular Strained Rings on Benzene: Crystal Structures of 1,2‐Dihydrocyclobuta[a]cyclopropa[c]‐, 1,2,3,4‐Tetrahydrodicyclobuta[a,c]‐, 1,2,3,4‐Tetrahydrodicyclobuta[a,c]cyclopropa[e]‐, and 1,2,3,4,5,6‐Hexahydrotricyclobuta[a,c,e]benzene

Noticeable deformations of the benzene framework caused by annelation with strained rings are observed in the crystallographic studies on 1–4. These are the most strained benzene derivatives yet known. The angles at the sp2 carbon atoms in 1 and 3 indicated by an arrowhead are the largest known: 176.9 and 174.9°, respectively. Molecules 2 and 4 display a slight bond length alternation in the arene moiety (0.010–0.023 Å).