Fused eight-membered ring systems

All polycyclic pigments, with the exception of triphenylmethyl derivatives, comprise anellated aromatic and/or heteroaromatic moieties. In commercial pigments, these may range from systems such as diketopyrrolo-pyrrol derivatives, which feature two five-membered heteroaromatic fused rings (DPP pigments) to such eight-membered ring systems as flavanthrone or pyranthrone. The phthalo-cyanine skeleton with its polycylic metal complex is somewhat unique in this respect. 

In connection with the behavior of the eight-membered ring system, it is interesting to mention that the uncatalyzed thermolysis of the open-chain tetraene ether 230 in toluene at 150 °C (11 h) gives rise to a mixture of four intramolecular Diels-Alder products 231-234 in 80% total yield (equation 74)88. The thermolysis of dimethyl homologue 235 (toluene, 150 °C, 11 h, 81%) affords the c/s-fused cyclohexene derivative 236 and... 

Scheme 4). This cascade RCM applied to diene-ynamides leads to fused az-abicyclic products with good yields [11], and carbohydrates can be used to synthesize polyoxygenated bicyclic systems that contain seven- and eight-membered rings [12]. 

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