Fused Four-membered Carbocyclic Systems

An X-ray structure determination for the stable benzocyclobutadiene (49) indicates, as expected, that the canonical form shown best describes the structure. Sophisticated calculations have been carried out for a wide range of benzo-fused cyclobutadiene derivatives. The agreement between calculated and experimental transition energies and moments for the few known compounds is impressive enough to suggest that predictions for the (as yet) unknown compounds are reliable. Because of the discrepancy between the calculated spectra and the data reported for the tentatively claimed 1,2-diphenylphenan-thro[l]cyclobutene (50), the alternative structure (51) has been proposed.  

Some n.m.r. spectra for benzo- and dibenzo-cyclobutadiene dications indicate that these species are fully delocalised six- and ten-electron aromatic systems respectively. Flash-pyrolytic extrusion of nitrogen from benzocinnoline analogues has been employed in the generation of the fused cyclobutadiene derivatives (52), (53), and (54). 1,2-Dimethylbenzocyclobutadiene gives the 

Full details of the formation of 3,4-cyclobuta[l,2]cyclohepten-6-one (58) derivatives from the dichlorocarbene ring-expansion reaction of 1- and 2-methoxybiphenylenes have appeared. Spectral data suggest that there is considerable bond fixation in the seven-membered ring, to minimise cyclo-butadienoid character in the four-membered ring. Similar considerations apply to the fused tropolones (59) and (60), which exist in the tautomeric forms shown and undergo methylation only at the points indicated. 

A new synthesis of 9,10-diphenylbicyclo[6.2.0]decapentaenes (61) has been reported. Preliminary spectral data suggest a nearly planar structure. The 

Fujino, S. Ebine, and J. Tsunetsugu, Tetrahedron Letters, 1978, 143. 

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Other Four Heteroatoms 2 2 Five or More Heteroatoms Tricyclic Systems Central Carbocyclic Ring with Fused Five-membered Rings Tricyclic Systems Central Carbocyclic Ring with Fused Five- and Six-membered Rings Tricyclic Systems Central Carbocyclic Ring with Fused Six-membered Rings... 

Voliime 6 reviews the literature published during 1976. The format of the earlier volumes is retained, with individual chapters on three-membered rings four-membered rings five- and six-membered rings and related fused systems medium-and large-ring compounds and bridged carbocycles. 

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