Ring structure diene-conjugated compounds

Examples 5.3 and 5.4 are molecules with very similar formulas, but different structures. The double bonds are in different positions. The wavelength of the absorption maximum for Example 5.3 is 309 nm, which is 54 nm longer than in Example 5.4. The major difference between the two compounds is that the molecule in Example 5.3 has a two double bonds within one ring and a conjugated system of three double bonds, that is, the double bonds are each separated by one single bond. That results in the addition of 36 nm for the diene in a ring and 30 nm for the double bond extension. In Example... 

Let us bear in mind that for compounds of the 3-ionone series we can consider two possibilities of reaction through six-center transition states leading respectively to heterocyclic rings (lactone or oxide) and to isomers with exocyclic conjugated diene structures. The two mechanistic t3rpes are shown in Figure 1. 

The smallest conceivable conjugated monocyclic diene is 1,3-cyclobutadiene. Several complexes involving cyclobutadiene are known. The compound itself is unstable and has not been studied by structural methods. It will therefore not be included in the present discussion. 1,3-Cyclobutadiene has, however, been isolated in argon matrices, and it has been established that the molecule has symmetry. For the tetra-tert-butyl derivative an envelope conformation (twist angle 7°) was found by X-ray methods, however the distances in the ring were obviously too similar for an antiaromatic system [1.464(3) and 1.483(3) A]. A redetermination at even lower temperatures gave more reasonable results (1.441 and 1.527 A) and a further analysis of the anisotropic parameters revealed that some residual disorder is still responsible for some equilibration and distances of 1.34 and 1.60 A were assumed to be the correct ones. ... 

In addition to the acyclic compounds just discussed, it is common to see cyclic molecules with conjugated 7i-bonds. An important cyclic diene is cyclopen-tadiene (5). The numbers are omitted from the name because there is only one possible structure. This is not the case for cyclohexadiene derivatives because there is the conjugated 1,3-cyclohexadiene (6) and the nonconjugated 1,4-cy-clohexadiene, 7. With all dienes that have larger rings, there is one conjugated isomer but there may be several possible nonconjugated isomers. 

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