Cyclic polyenes rearrangement

The best way to understand how orbital symmetry affects pericyclic reactions is to look at some examples. Let s look first at a group of polyene rearrangements called electrocyclic reactions. An electrocyclic reaction is a pericyclic process that involves the cycli/ation of a conjugated polyene. One 7r bond is broken, the other 7t bonds change position, a new cr bond is formed, and a cyclic compound results. For example, a conjugated triene can be converted into a cyclohexa-diene, and a conjugated diene can be converted into a cyclobutene. 

Isomerization of Polyene Compounds. Intramolecular rearrangement of Fe(CO)3(polyene) compounds are somewhat similar to the fluxional behaviour described above. Though one is concerned with acyclic rather than cyclic polyenes, and with 7r-bonding rather than a-bonding changes, the shift isomerizations (19) (20) (21) could be considered analogues... 

Thorough biochemical analysis of carotenoid biosynthesis, classical genetics, and more recently molecular genetics resulted in the elucidation of the main routes for the synthesis of acyclic and cyclic carotenoids at a molecular level (Sandmann 2001). Little is known, however, about the biosynthesis of carotenoids containing additional modifications of the end groups, the polyene chain, the methyl groups, or molecular rearrangements that contribute to the tremendous structural diversity of carotenoids. At present, hundreds of individual carotenoids have been characterized (Britton et al. 1998), and novel carotenoids continue to be isolated. All carotenoids are derived from the isoprenoid or terpenoid pathway. 

Larger polyenyl cations could be formed by prolonation of polyenes, and are characterized by intense colors due to extensive resonance effects and a shift of tt-jt electron transfers in the visible area. H-NMR spectra were able to characterize the polyenic character of such ions. However, no X-ray data, - C-NMR spectra, and vibrational. spectra have been reported owing to rapid rearrangements of these cations into cyclic isomers. 

The thermal cyclization of cM-l,3,5-hexatriene to yield 1,3-cyclohexadiene is an example of an electrocychc reaction. C-1 and C-6 of the original polyene are linked by a O bond in the cyclic product. This reaction is known as the Cope rearrangement. 

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