of 2 and 4JZ1 and in terms of an increased conjugative interaction between the hydrocarbon framework and the CO groups (5 a15 b) in the case of 5.
Among the chemical reactions of 5, that which deserves special attention, aside from the protonation and reductive acetylation already mentioned, is its reduction with sodium borohydride in methanol/dichloromethane. The reaction product formed thereby-formally with saturation of the central C=C bond-is the lOb,IOc-dihydro compound, the no longer planar syn-I ,6:8,13-ethanediylidene[l4]annulene-7,14-dione 4 [red rhombs (from acetone/hexane); slow decomposition above 165°C; yield 20%]; when dissolved in concentrated sulfuric acid, 4 is re-oxidized by atmospheric oxygen to 5. If one assumes a relatively high contribution of the tropylium resonance structures to 5, then the initial step of the observed hydrogenation might be interpreted as a nucleophilic addition of a hydride ion at C-lob (or C-~OC).['~] Proof of the structure of 4 is based not only on the spectra (Table 1) and their analogy to those of 2, but also on an independent synthesis of this new [14]annulenequinone, which, together with the structure and electrochemical investigations on 4, will be reported upon in a separate communication." 6l