Irans-Cyclooctene

The method is not suitable for trisubstituted alkenes or for a highly strained alkene such as Irans-cyclooctene. 

Vic-diols can thus be easily converted to alkenes through their reaction with /V,A"-thiocarbonyldiinidazole. The reported synthesis of irans-cyclooctene is illustrative of the method [219]. It should be noted that continuous elimination of /ram-cyclooctene by a stream of argon was necessary to avoid isomerization to the cis isomer. The conversion of cis-cyclooctene to fram-cyclooctene through a trithiocarbonate is described in the same paper. 

Normally, a trans alkene is more stable than its cis isomer. irans-Cyclooctene, how-1 ever, is less stable than m-cyclooctene by 38.5 kJ/mol. Explain. 

Inoue, Y., Takamuku, S., and Sakurai, H., Direct cis-trans photoisomerization of cyclooctene. A convenient method for preparing Irans-cyclooctene, Synthesis, 111, 1977. 

Even a simple strained molecule can show conformational enantiomerism. trans-Cyclooctene is the smallest stable trans-cycloalkene, and it is strained. If trans-cyclooctene existed as a planar ring, even for an instant, it could not be chiral. Make a molecular model of trans-cyclooctene, however, and you will see that it cannot exist as a planar ring. Its ring is folded into the three-dimensional structure pictured in Figure 5-18. The mirror image of this structure is different, and trans-cyclooctene is a chiral molecule. In fact, the enantiomers of iran.v-cyclooclcnc have been separated and characterized, and they are optically active. 

Die Stereochemie dicser Reaktion ist nicht eindoutig geklart. Wahrend flexible Systeme wie Cyclo-hexen oder Cycloocten zu cis- und trana-Addukten fiihren, ergeben starre Molekiile wie Bicyclof2.2.11 hepten nur irans-Addukte4. 

The ring-opening polymerization of a simple cyclic olefin such as cyclooctene yields two structures of maximum order, which are distinguished by the configuration (cis or irans) of their main-chain olefins. In contrast, polymers made from bicyclic olefins such as norbornene are inherently more complicated and have four structures of maximum order (Scheme 24). In addition to cis- and trans-olefins, the polymers can also be isotactic or syndiotactic. The stereochemistry of these polymers becomes even more complicated when the monomer is asymmetric, since head—head, head—tail, and tail—tail regioisomers are possible. Nevertheless, single-site metathesis catalysts have been developed that can control polymer stereochemistry to an impressive degree by both chain-end and site-control mechanisms. ° ° ... 

The Corey-Winter reaction proceeds with complete stereospecificity by a syn elimination pathway, allowing the stereospecific synthesis of alkenes. Thus, anti-1,2-diphenylethane-1,2-diol was converted into cis -stilbene (2.45), whereas the corresponding syn-diol gave irans -stilbene. The strained fi-cyclooctene was prepared from the Z-isomer using this procedure (2.46). An alternative stereospecific route to alkenes proceeds from the diol with ethyl orthoformate or WA(-dimethylformanude dimethyl acetal, or by conversion of the diol to a 2-phenyl-1,3-dioxolane and treatment with an organolithium reagent to promote proton abstraction at C-2 and... 

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