Comparison with tetraenes

The interest in azacyclooctatetraenes is mainly due to their structural similarity to cycloocta-tetraene.1 3 Most synthetic efforts have concentrated on a comparison of the unusual chemical and physical properties of cyclooctatetraene with its aza analogs. Besides studies concerning the aromaticity, the primary focus of much work has concerned the stability of the eight-membered ring and its valence isomerization. 

The reader will recall equations 9 and 10 are generally ca 5 and 42 kJ moT1 endothermic. However, we generally lack enthalpies of formation for the compounds with the same affixed X as the above with which to compare reactions 9,10 and 16. Possibly, though very unlikely, the reaction of two conjugated dienes to form a linearly conjugated tetraene will show considerable additional stabilization. We note one comparison of related 4 and 3 chromophore species. Consider the de-acetylenation reaction... 

In addition to the CD exciton chirality method, we have recently reported that the theoretical calculation of the CD spectra by the jt-electron SCF-CI-dipole velocity MO method8-l4 has become an important tool for determination of the absolute configuration of a variety of twisted and conjugated n-electron systems. In fact, we have recently determined the absolute stereochemistry of (8aS)-(+)-l,8a-dihydro-3,8-dimethylazulene 10, a labile biosynthetic intermediate for 1,4-dimethylazulene 11 isolated from a liverwort, by application of the MO method to the theoretical calculation of the CD spectra of the twisted tetraene system (8a/ )-12.15 In that case, we have also succeeded in the experimental verification of the absolute configuration theoretically determined, by comparison of the CD spectra of the natural product with those of synthetic chiral model compounds (8a5)-(+)-... 

Particular care is required in naming retinoids possessing additional substituents in the olefmic side chain. Comparison of the stereochemical designation of ethyl (all- )-retinoate (24) with the ester (29), which is substituted in the 10-position by fluorine, shows clearly that because of established rules (lUPAC, 1979) on the priority of substituents at double bonds, the stereochemical prefix has to be changed from E to Z, although the stereochemical configuration of the tetraene side chain has remained unchanged. 

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