Cisltrans isomerism

The 1,2-diazetidine and 1,3-diazetidine isomers have about the same PAs. In the case of 1,2-diazetidine, there is a 20 kj mol-1 difference between the as- and /ram-isomers in their PA. This should not be surprising, as this molecule also has an NH-NH moiety. For 1,3-diazetidine, the NH groups are on opposite corners of the ring, so the effects on the PA due to cisltrans-isomerism should be minimized (Table 13). 

Very recently, the azobenzene norbornene derivative XI (Scheme 2) was successfully prepared and polymerized with initiator 5 and 9, and it showed a nematic mesophase. The polymer was photo-switchable due to photo cisltrans-isomerization of the N=N double bond [65]. This is the first step towards ROMP-derived materials that may be used for optical data storage. 

Like morphiceptin and PL017, cisltrans isomerization occurs around the Tyr-Pro amide bond in endomorphin-1, with similar populations (75%trans, 25%cis) found for the two conformations (819). Based on structural comparison of the cis and trans conformations of endomorphin-1 to other /x- and 8-selective opioid peptides, Podlogar et al. proposed that the trans conformation was the bioactive form (819). Recently, Schiller and coworkers reported endomorphin-2 and morphiceptin analogs containing a pseudoproline derivative in... 

Cytokinin specificity is determined by the structure of its side chain, where even a small modification can significantly affects the biological activity. Among enzymatic modifications that have been already proved in vivo are ranked 0-glycosylation and 0-acetylation of zeatin, cisltrans isomerization of its hydroxyl group, double bond reduction to dihydrozeatin and hydroxylation of A4-isopen tenyl moiety of isopentenyladenine type to zeatin-types of cytokinins. 

Aryl derivatives of 101 (R=Ar) are planar, and the bond lengths indicate a fully delocalized 10 re-electron aromatic system. By contrast, the (dimethyl-amino) derivative 102 adopts a folded structure. Dithiatetrazocines with exo-cyclic N(Me)Bu groups exhibit both cisltrans and ring inversion isomerism.257... 

Under analogous conditions, carbonylation of chloral affords cis or trans 2,5-bis(trichloromethyl)-l,3-dioxolan-4-ones. The stereochemical outcome of the reactions can be controlled by the concentration of the employed sulfuric acid (90-99%) and the reaction time. The cis isomer is predominantly formed under more acidic conditions after 10 min (cis/trans 95/5 48% yield), whereas complete isomerization to the trans isomer (cisltrans 0/100 65% yield) takes place at lower acidity (90%) and prolonged reaction time (7 h) [63]. 

Throughout this chapter, the EIZ nomenclature will be employed to denote geometric isomerism about the double bonds of the polyene chain, while the cisltrans ( dt ) nomenclature will be used only to denote conformational isomerism about the formal single bonds. 

Solvent was found to affect the cisjtrans ratio of the products. Thus, cis-1,2-di-p-toluenesulphonylethylene gave 90 and 80% retention with thiophenoxide ion in ether and methanol, respectively, while azide ion gave 90% retention in aqueous acetonitrile and lower degrees of retention in dimethyl sulphoxide and in aqueous methanol (Meek and Fowler, 1968). Since isomerization was found to take place in the last case, the significance of the other results is not clear, although it may be expected that in systems which are prone to racemization, a solvent effect on the cisltrans ratio will be observed. 

Thus they treated the cisltrans mixture obtained by isomerization with iron pentacarbonyl and after decomposition of the complex with ferric chloride obtained the desired all-trans-tetraene ester (4) in 51% yield. The remaining steps in the synthesis involved lithium aluminum hydride reduction (80% yield) and Mn02 oxidation (52% yield). 

Under similar conditions, by using tributylstannyl chloride as an electrophile in place of isopropanol, allyl acetates are converted to allylstannanes in good yield (Scheme As compared with protonation, tributylstannylation is much more regio- and stereoselective. The stannylation of geranyl and neryl acetates proceeds withont loss of the stereochemical integrity and selectively provides (E)- and (Z)-isomers, respectively, in good yield. On the other hand, the stannylation of cyclohexenyl derivatives accompanies the stereochemical isomerization mixtures of cis- and trani -cyclohexenylstannanes in the same ratio (cisltrans = ca. 1 2) are obtained from the mixtures of cw-rich and trans-rich cyclohexenyl acetates (Scheme 14). ... 

Isomerization between o j-2-butene and 1-butene preferentially takes place over isomerization between cis and Irons and between Irons and Tbutene (cisltrans=20 from 1-butene and l/trons=10 from cw-2-butene). High selectivity to os-2-butene and 1-butene in the hydrogenation of butadiene has also been observed. All of these reactions proceed via a zr-allyl intermediate tmd these facts indicate that the surface of CdO is basic (see Section 4.1). However, no direct measurement of the basicity has been reported. 

Configurational isomers also constitute different chemical individuals and cannot be converted into others without rupture of chemical bonds. This chemical stmctural property, which includes cisltrans [Z/E) isomerism and different tactici-ties, is also readily accessible from the IR and Raman spectra of polymers. Figure 8 depicts the Raman spectra of three poly (hutadiene)s with different compositions of the l,4-ct5, l,4-tran5, and 1,2-configtrrational isomers. The selectivity of... 

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