Conjugated 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. 

Dienes and polyenes show a pronounced molecular ion in the mass spectra and hence the molecular weight of polyenes can be determined by positive ion mass spectra. The easy removal of a 7r-electron from a diene is usually the reason for the distinct M +. The mass spectral investigation of conjugated polyenes is somewhat similar to that of aromatic structures, due to the high stability of the rearranged ions formed after the... 

The transformations of compounds which are precursors for vitamin A and carotenoids have a special position among the rearrangements of the conjugated polyenes. Numerous isomerizations such as cw-fraws-isomerization, the dehydration of polyunsaturated acetylenic carbinols etc. were utilized to prepare the various carotenoides (e.g. /1-carotene, lycopene, cryptoxanthin, zeaxanthin) (for reviews, see References 146 and 147). However, one of these rearrangements turned out to be a considerable hindrance for the synthesis of target products. 

Sondheimer, F., Ben Efraim, D. A., Wolosovsky, R., Unsaturated Macrocyclic Compounds. XVII. The Prototropic Rearrangement of Linear 1,5 Enynes to Conjugated Polyenes. The Synthesis of a Series of Vinylogs of Butadiene, J. Am. Chem. Soc. 1961, 83, 1675 1681. 

IV. REARRANGEMENTS OF NON-CONJUGATED DIENES AND POLYENES A. Dienone-Phenol Rearrangements... 

Large annulenes tend to undergo conformational distortion, cis-trans isomerizations, and sig-matropic rearrangements (p. 40 and p. 100). Methylene-bridged conjugated (4n + 2(-rt cyclo-polyenes were synthesized with the expectation that these almost planar annulenes should represent stable HOckel arenes (E. Vogel, 1970, 1975). 

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