Cis- isomers

CgHgO, PhCH = CHCOiH. Colourless crystals. Decarboxylales on prolonged heating. Oxidized by nitric acid to benzoic acid. Ordinary cinnamic acid is the trans-isomer, m.p. 135-136 C on irradiation with u.v. light it can be isomerized to the less stable cis-isomer, m.p. 42" C. 

Assuming that the cis isomer is the reactant, the bans isomer product is expected to be accompanied by others arising from secondary reactions of the biradical, as observed experimentally [58]. 

The Cyclooctene Isomerization. A reaction that attracted some attention in recent years is the cis-trans isomerization of cyclooctene [84]. The cis isomer is much less strained than the trans, but the latter is readily fonned upon direct photolysis and also upon photosensitization. In this case, two enantiomeric trans isomers are formed. The appropriate loop is a variation of that shown in Figure 14, as shown in Figure 34. This is a phase inverting i -type loop, that... 

The complete descriptor of this isomer is again obtained by multiplication of the descriptors of the two subunits. In this case, we obtain a value of (-r 1), the opposite of the value for the cis isomer, as desired. 

The object of this part of the project is to determine the energy ("enthalpy) levels in each the three con formers and so to determine the composition of the equilibrium conformational mixture. That having been done for the cis isomer, the procedure is repeated for the trans isomer. 

Revision Problem 1 Leaf alcohol (TM 392) is widespread in plants and has the characteristic smell of green leaves and grass. The cis isomer alone has tlfis smell and is used in perfumery. How would you make it ... 

All of the above isomerization recipes can, in theory, produce almost 100% yields of isosafrole. But about 20-30% of that isosa-frole is in a screwed up configuration called cis. This cis Isomer does not react the same way as trans and the drug that will be made from it will not be recognized in the same way as trans in the brain cells of users. Tsk. 

The chemist can try to separate the two isomers by careful fractional distillation but it will be next to impossible to do because both the cis and the trans have practically the same boiling point. There are a few things that the chemist can do or hope for to get rid of that cis isomer. The cis configuration is less stable than the trans and may switch over to the trans configuration with a little help. The chemist can gently heat the isosafrole oil to about 150°C for an hour or so. She can also try the same heating except have the oil mixed with some acetic acid. Also, the isosafrole can be fractionally distilled to ultra purity and then be allowed to simply sit for a couple of days. Trans isosafrole may spontaneously crystalize out while the cis form remains as an oil. They can then be separated by filtration. 

When all is said and done, the chemist may just wish to leave the damn cis isomer in with the trans. It s not really going to hurt anything and if it goes unreacted in some of the conversion steps it will be lost in the process long before X is made. It might even correct itself during some of the conversion steps. 

Both cis- and irans- 1,2-dibromoethylenes react with metal acetylides, but with different reactivity. For the competitive reaction of cis- and trans- fl-dibromoethylenes with the zinc acetylide 314 shows that the irans isomer is more reactive than the cis isomer[228,229]. It was also found by a competitive reaction with the zinc acetylide 317 that the irans vinyl monobromide 315 is more reactive than the cis isomer 316[230]. 

A key intermediate, 163, which possesses all but one chiral center of (+ )-brefeldin, has been prepared by the enantiocontrolled cycloaddition of the chiral fi,/3-unsaturated ester 162 to 154[107], Synthesis of phyllocladane skeleton 165 has been carried out by the Pd-catalyzed cycloaddition of the unsaturated diester 164 and cobalt-catalyzed cycloaddition of alkynes as key reactions[108]. Intramolecular cycloaddition to the vinylsulfone in 166 proceeds smoothly to give a mixture of the trans and cis isomers in a ratio of 2.4 1[109], Diastereocontrolled cycloaddition of the hindered vinylsulfone 167 affords a single stereoisomeric adduct, 168, which is used for the synthesis of the spirocarbocyclic ring of ginkgolide[l 10],... 

In the reaction of 2-aminoselenazoIe with ethyl propiolate, in addition to compound type 7 (50% yield), compounds 8 and 9 were also isolated. Compound 7 is probably formed through the cis isomer of 8 (Scheme 55). 

FIGURE 3 20 The enthalpy difference between as- and trans 1 2 dimethylcyclopropane can be determined from their heats of combustion Van der Waals strain between methyl groups on the same side of the ring make the cis isomer less stable than the trans... 

As m the 1 4 dimethylcyclohexanes the 6 kJ/mol (1 5 kcal/mol) energy difference between the more stable (trans) and the less stable (cis) stereoisomer is attributed to the strain associated with the presence of an axial methyl group m the cis isomer... 

By assuming that the heat of combustion of the cis isomer was larger than the trans structural assignments were made many years ago for the stereoisomenc 2 3 and 4 methylcyclohexanols This assumption is valid for two of the stereoisomenc pairs but is incorrect for the other For which pair of stereoisomers is the assumption incorrect Why" ... 

The pair of isomers designated as and trans 2 butene have the same constitution both have an unbranched carbon chain with a double bond connecting C 2 and C 3 They differ from each other however m that the cis isomer has both of its methyl groups on the same side of the double bond but the methyl groups m the trans isomer are on oppo site sides of the double bond Recall from Section 3 11 that isomers that have the same constitution but differ m the arrangement of their atoms m space are classified as stereoisomers as 2 Butene and trans 2 butene are stereoisomers and the terms as and trans specify the configuration of the double bond... 

The difference m stability between stereoisomeric alkenes is even more pronounced with larger alkyl groups on the double bond A particularly striking example compares as and trans 22 5 5 tetramethyl 3 hexene m which the heat of combustion of the cis stereoisomer is 44 kJ/mol (10 5 kcal/mol) higher than that of the trans The cis isomer IS destabilized by the large van der Waals strain between the bulky tert butyl groups on the same side of the double bond... 

Although both stereoisomers yield 4 tert butylcyclohexene as the only alkene they do so at quite different rates The cis isomer reacts over 500 times faster than the trans The difference in reaction rate results from different degrees of rr bond develop ment in the E2 transition state Since rr overlap of p orbitals requires their axes to be parallel rr bond formation is best achieved when the four atoms of the H—C—C—X unit he in the same plane at the transition state The two conformations that permit this are termed syn coplanar and anti coplanar... 

The reaction is reversible and its stereochemical requirements are so pronounced that neither the cis isomer of fumaric acid (maleic acid) nor the R enantiomer of malic acid can serve as a substrate for the fumarase catalyzed hydration-dehydration equilibrium... 

Disubstituted cyclohexanes can exist as cis-trans isomers as well as axiaEequatorial conformers. Two isomers are predicted for 1,4-dimethylcyclohexane (see Fig. 1.9). For the trans isomer the diequatorial conformer is the energetically favorable form. Only one cis isomer is observed, since the two conformers of the cis compound are identical. Interconversion takes place between the conformational (equatoriaEaxial) isomers but not configurational (cis-trans) isomers. 

FIGURE 1.9 Two isomers of 1,4-dimethylcyclohexane. (a) Trans isomer (b) cis isomer. 

When the asymmetric carbon atoms in a chiral compound are part of a ring, the isomerism is more complex than in acyclic compounds. A cyclic compound which has two different asymmetric carbons with different sets of substituent groups attached has a total of 2 = 4 optical isomers an enantiometric pair of cis isomers and an enantiometric pair of trans isomers. However, when the two asymmetric centers have the same set of substituent groups attached, the cis isomer is a meso compound and only the trans isomer is chiral. (See Fig. 1.15.)... 

Figure 1.3 shows several repeat units of cis-l,4-polyisoprene and trans-1,4-polyisoprene. Natural rubber is the cis isomer of 1,4-polyisoprene, and gutta-percha is the trans isomer. 

Figure 1.3 1,4-polyisoprene with R=CH3 (a) cis isomer natural rubber (b) trans... 

Examples of prolate near-symmetric rotors are the s-trans and s-cis isomers of crotonic acid, shown in Figure 5.8, the a axis straddling a chain of the heavier atoms in both species. The rotational term values for both isomers are given approximately by Equation (5.37) but, because A and B are different for each of them, their rotational transitions are not quite coincident. Figure 5.9 shows a part of a low-resolution microwave spectmm of crotonic acid in which the weaker series of lines is due to the less abundant s-cis isomer and the stronger series is due to the more abundant s-trans isomer. 

Butenediol. 2-Butene-l,4-diol [110-64-5] is the only commercially available olefinic diol with primary hydroxyl groups. The commercial product consists almost entirely of the cis isomer. 

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