1,3,7-Octatrien

Perform the same calculation for eyelization of 2,4,6-octatriene. Which isomer of dimethylcvclohexadiene is formed ... 

Electrocyclic reactions of 1,3,5-trienes lead to 1,3-cyclohexadienes. These ring closures also exhibit a high degree of stereospecificity. The ring closure is normally the favored reaction in this case, because the cyclic compound, which has six a bonds and two IT bonds, is thermodynamically more stable than the triene, which has five a and three ir bonds. The stereospecificity is illustrated with octatrienes 3 and 4. ,Z, -2,4,6-Octatriene (3) cyclizes only to cw-5,6-dimethyl-l,3-cyclohexadiene, whereas the , Z,Z-2,4,6-octa-triene (4) leads exclusively to the trans cyclohexadiene isomer. A point of particular importance regarding the stereochemistry of this reaction is that the groups at the termini of the triene system rotate in the opposite sense during the cyclization process. This mode... 

The most striking feature of electrocyclic reactions is their stereochemistry. For example, (2 ,4Z,6 )-2,4,6-octatriene yields only c/s-5,6-dimethyl-l,3-cyclo-hexadiene when heated, and (2 ,4Z,6Z)-2,4,6-octatriene yields only trnns-5,6-dimethyl-l,3-cyclohexadiene. Remarkably, however, the stereochemical results change completely when the reactions are carried out under what are called photochemical, rather than thermal, conditions. Irradiation, or photolysis,... 

Active Figure 30.5 Thermal cyciizaltons of 2,4,6-octatrienes occur by disrotatory ring closures. Sign in atwww.thomsonedu.com... 

Problem 30.2 Draw the products you would expect from conrotatory and disrotatory cyclizations of (2Z,4Z.6/)-2,4,6-octatriene. Which of the two paths would you expect the thermal reaction to follow ... 

Problem 30.4 What product would you expect to obtain from the photochemical cvclization of (2 ,4Z,6 )-2,4,6-octatriene Of (2 ,4Z,6Z)-2,4,6-octatriene ... 

We have already seen (p. 341) that the cyclisation of trans,cis,trans 2,4,6-octatriene (6) proceeds thermally to yield cis 1,2-dimethyl-... 

Consider now the 1,1,6,6-tetramethylated derivative of (Z)-l,3,5-hexatriene (83), a species more properly named (Z)-2,6-dimethyl-2,4,6-octatriene and occasionally and trivially called ds-allo-ocimene . To estimate its enthalpy of formation, let us use simple olefin additivity along with ... 

Very powerful tools for the study of dienes and, to some extent, polyenes (in particular annular polyenes) are both H and 13 C NMR spectroscopies, which will be discussed in a separate section. As previously mentioned 1,3-butadiene is more stable in the s-trans conformation and in the H NMR spectrum both butadiene (1) and 2,3,6,7-tetramethyl-2,4,6-octatriene (3) display the vinyl proton at a low chemical shift value. In these simple examples the S value can be predicted theoretically. The 111 NMR spectrum of a C25-branched isoprenoid was examined as part of the structural determination for biomarkers and is shown in Figure l6. The other spectral and structure assignments are described later in this review. 

Starting from 2,4,6-octatriene and pivaldehyde, the conjugated homoallylic alcohol 8 is obtained as the sole product. Cycloheptatriene-derived complexes react with aldehydes and C02 to afford mixtures of the isomeric 1,3- and 1,4-cycloheptadienyl carbinols or acids, respectively. Interestingly, analogous reactions with methyl chloroformate or dimethyl carbamoyl chloride produce the conjugated dienyl ester 9 or amide 10 as unique products [19,20]. 

The molecular orbitals derived from overlap of six p atomic orbitals as found in trienes are shown in Figure 8.7. Since there are six electrons to accommodate, as two paired electrons per orbital, the HOMO is n3. Thus, for 2,4,6-octatriene the disrotatory mode of reaction gives the trans isomer (Scheme 8.5). 

If a stepwise cyclization mechanism is assumed—for example, for /j-Cg—two octatriene intermediates may be formed, viz. 1,3,5-octatriene would lead to ethylbenzene, and 2,4,6-octatriene to o-xylene (Scheme II). The dehydrogenation of the latter would give octatetraene, which, in turn, gives styrene via vinylcyclohexadiene. Dehydrogenation and cyclization of octatriene were reported to compete over chromia and molybdena catalysts (67) with less hydrogen present (e.g., in a pulse system with in helium carrier gas), styrene formation is enhanced. 

EAE)- and (2Z,4 )-3,7-dimethyl-2,4,6-octatrienals] from acarid mite Histiogaster sp. A096 (Acari Acaridae). Bioscience, Biotechnology and Biochemistry 65 2749-2754. 

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