Tropone, aromatic character

The early unsuccessful search for aromatic character in 2//-azepin-2-one (14) and 47/-azepin-4-one (15), the 6n-monoaza analogs of tropones, has been surveyed.46... 

Another seven-membered ring that shows some aromatic character is tropone (44). This molecule would have an aromatic sextet if the two C=0 electrons stayed away from the ring and resided near the electronegative oxygen atom. In fact, tropones are stable compounds, and tropolones (45) are found in nature. However, analyses of dipole moments, NMR spectra, and X-ray diffraction measurements show that tropones and tropolones display appreciable bond alternations. ... 

In the series of heterocyclic fused analogs, Cook et al. (68T4501, 68TL3771) compared the properties of cyclohepta[c]furan-6-one (307a) with those of benzo[d]tropone and tropone by means of HMO calculations, spectra, and chemical reactivity (carbonyl derivatization). Whereas the two fused systems are comparable, their aromatic character is still less than that of tropone. 

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