Polyenes reactions

No. of electrons in the relevant M.O. of polyene Reaction type Examples Ground State Excited State... 

Ohmine, I., Morokuma, K., Photoisomerization of Polyenes Reaction Coordinate and Trajectory in Triplet Mechanism, J. Chem. Phys. 1981, 74, 564 569. 

Finally, a very recent synthesis of the halenaquinol-related natural product (-i-)-xestoquinone by Keay and co-workers [98] has provided confirmation of the suitabihty of the AHR for inclusion in Pd-mediated cascade polyene reactions [ 99]. The one-pot transformation of triflate 112 into the pentacyclic product 113... 

Cyclization of polyenes. Reaction of nerolidol (1) with this brominating agent (1) in CH2CI2 (20°, 3 hours) yields a- and /J-snyderol, (2) and (3), in low yield. These bromine-containing monocyclic sesquiterpenes have been isolated recently from a marine red algae species. ... 

I. Ohraine and K. Morokuma, Photoisomerization of polyenes reaction coordinate and trajectory in triplet mechanism, J. Chem. Phys. 74 564 (1981). 

Adopting the view that any theory of aromaticity is also a theory of pericyclic reactions [19], we are now in a position to discuss pericyclic reactions in terms of phase change. Two reaction types are distinguished those that preserve the phase of the total electi onic wave-function - these are phase preserving reactions (p-type), and those in which the phase is inverted - these are phase inverting reactions (i-type). The fomier have an aromatic transition state, and the latter an antiaromatic one. The results of [28] may be applied to these systems. In distinction with the cyclic polyenes, the two basis wave functions need not be equivalent. The wave function of the reactants R) and the products P), respectively, can be used. The electronic wave function of the transition state may be represented by a linear combination of the electronic wave functions of the reactant and the product. Of the two possible combinations, the in-phase one [Eq. (11)] is phase preserving (p-type), while the out-of-phase one [Eq. (12)], is i-type (phase inverting), compare Eqs. (6) and (7). Normalization constants are assumed in both equations ... 

This reaction illustrates one of the methods by which R. Kuhn synthesised long polyene chains terminated at each end by phenyl groups. 

Intramolecular condensation reactions to generate six-membered carbocycles are mentioned in section 1.12, the polyene cyclization in section 1.15. 

Open-chain 1,5-polyenes (e.g. squalene) and some oxygenated derivatives are the biochemical precursors of cyclic terpenoids (e.g. steroids, carotenoids). The enzymic cyclization of squalene 2,3-oxide, which has one chiral carbon atom, to produce lanosterol introduces seven chiral centres in one totally stereoselective reaction. As a result, organic chemists have tried to ascertain, whether squalene or related olefinic systems could be induced to undergo similar stereoselective cyclizations in the absence of enzymes (W.S. Johnson, 1968, 1976). 

The early Escherunoser-Stork results indicated, that stereoselective cyclizations may be achieved, if monocyclic olefins with 1,5-polyene side chains are used as substrates in acid treatment. This assumption has now been justified by many syntheses of polycyclic systems. A typical example synthesis is given with the last reaction. The cyclization of a trideca-3,7-dien-11-ynyl cyclopentenol leads in 70% yield to a 17-acetyl A-norsteroid with correct stereochemistry at all ring junctions. Ozonolysis of ring A and aldol condensation gave dl-progesterone (M.B. Gravestock, 1978 see p. 279f.). 

Among the appHcations of lower valent titanium, the McMurry reaction, which involves the reductive coupling of carbonyl compounds to produce alkenes, is the most weU known. An excellent review of lower valent titanium reactions is available (195). Titanium(II)-based technology is less well known. A titanium(II)-based complex has been used to mediate a stetio- and regio-specific reduction of isolated conjugated triple bonds to the corresponding polyenes (196). 

Cyclization of polyenes by intramolecular rr-attack on an oxirane is an important biochemical reaction (Section 5.05.5.2). 

Although the Hiickel method has now been supplanted by more complete treatments for theoretical analysis of organic reactions, the pictures of the n orbitals of both linear and cyclic conjugated polyene systems that it provides are correct as to symmetry and the relative energy of the orbitals. In many reactions where the n system is the primary site of reactivity, these orbitals correctly describe the behavior of the systems. For that reason, the reader should develop a familiarity with the qualitative description of the n orbitals of typical linear polyenes and conjugated cyclic hydrocarbons. These orbitals will be the basis for further discussion in Chapters 9 and 11. 

When we recall the symmetry patterns for linear polyenes that were discussed in Chapter 1 (see p. 33), we can fiorther generalize the predictions based on the symmetry of the polyene HOMO. Systems with 4 n electrons will undergo electrocyclic reactions by conrotatoiy motion, whereas systems with 4 4- 2 n electrons will react by the disrotatoiy mode. 

Correlation diagrams can be constructed in an analogous fashion for the disrotatory and conrotatory modes for interconversion of hexatriene and cyclohexadiene. They lead to the prediction that the disrotatory mode is an allowed process whereas the conrotatory reaction is forbidden. This is in agreement with the experimental results on this reaction. Other electrocyclizations can be analyzed by the same method. Substituted derivatives of polyenes obey the orbital symmetry rules, even in cases in which the substitution pattern does not correspond in symmetiy to the orbital system. It is the symmetry of the participating orbitals, not of the molecule as a whole, that is crucial to the analysis. 

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