Polyenes heteroatom containing

Fig. 4.1 illustrates the first few members of the series of neutral polyenes the equilibria between butadiene 4.1 and cyclobutene 4,2, between hexadiene 4,3 and cyclohexadiene 4.4, and between octatetraene 4.5 and cyclooctatriene 4,6. There are of course heteroatom-containing analogues, with nitrogen or oxygen in the chain of atoms, and the systems can be decked out with substituents and other rings. To appreciate what the fundamental reaction is, it is only necessary to tease out the components—the longer conjugated... 

The non-planar polyene nature of azepines renders them susceptible to a variety of intra-and inter-molecular pericyclic processes. The azepine-benzeneimine valence isomerization has been discussed in Section 5.16.2.4, and the ring contractions of azepines to benzenoid compounds in the presence of electrophiles is covered in Section 5.16.3.3. In this section the thermal and photochemical ring contractions of azepines to bicyclic systems, their dimerizations and their isomerizations via sigmatropic hydrogen shifts are discussed. Noteworthy is a recent comprehensive review which compares and contrasts the many and varied valence isomerizations, dimerizations and cycloadditions of heteroepins (conjugated seven-membered heterocycles) containing one, two and three heteroatoms (81H(15)1569). 

W. Wu, Y. Luo, L. Song, S. Shaik, Phys. Chem. Chem. Phys. 3, 5459 (2001). VBDFT(s) A Semiempirical Valence Bond Method Application to Linear Polyenes Containing Oxygen and Nitrogen Heteroatoms. 

Although these and several other [25,32,79-81] active retinoids have been described which contain heteroatoms substituted in rings or along the polyenic side chain, the structural variations possible with hetero-substituted retinoids have been scarcely explored. The potential for specificities of anti-cancer action and of favourable toxicity profiles necessitates further chemical and biological evaluation of this class of compounds. 

A Semiempirical Valence Bond Method Application to Linear Polyenes Containing Oxygen and Nitrogen Heteroatoms. 

The DTHDA reaction is a special case of the DTDA (diene-transmissive Diels-Alder) reaction in which one or more heteroatoms are contained within a cross-conjugated triene/polyene (heterodendralene) it-system, a dienophile it-system, or both. The DTHDA protocol is an efficient and attractive method for the stereoselective synthesis of a variety of heterocyclic systems. 

Wu W et al (2001) VBDFT(s) - a semi-empirical valence bond method application to linear polyenes containing oxygen and nitrogen heteroatoms. Phys Chem Chem Phys 3 5459-5465... 

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