Trans-compound

Comments The diene A is symmetrical so it doesn t matter which double bond is attacked by the carbene. On the other hand, it may be difficult to stop carbene addition to the second double bond. The only control over the stereochemistry will be that the trans compound we want is more stable. Japanese chemists have recently synthesised optically active trans chrysanthemic acid by this route (Tetrahedron Letters. 1977, 2599). 

Hydrogenation of a-terpineol gives dihydroterpineol [498-81-7] a mixture of cis- and trans-compounds (120). Dihydroterpinyl acetate [80-25-1] produced from the mixture is also a useful fragrance compound. 

N- Aminoaziridines have been converted to alkenes by reaction with a variety of oxidizing agents (70JA1784). Usually, the deamination reaction is stereospecific. The oxidation of l-amino-2,3-diphenylaziridines with manganese dioxide, however, was not stereospecific. The trans compound gives entirely frans-stilbene, whereas the cfs-aziridine forms a mixture of 85% trans- and 15% c -aikene. cw-Stilbene is not isomerized to trans under the reaction conditions, and the results are explained in terms of an azamine intermediate which can isomerize through a tautomeric equilibrium. 

A classic example of neighboring-group participation involves the solvolysis of compounds in which an acetoxy substituent is present next to a carbon that is undergoing nucleophilic substitution. For example, the rates of solvolysis of the cis and trans isomers of 2-acetoxycyclohexyl p-toluenesulfonate differ by a factor of about 670, the trans compound being the more reactive one ... 

Stoddart and his coworkers have reported syntheses of the trans-syn-trans and the trans-anti-trans isomers of dicyclohexano-18-crown-6 The synthesis of these two compounds from trans-l,2-cyclohexanediol was accomplished in two stages. First, the diols were temporarily linked on one side by formation of the formal, and this was treated with diethylene glycol ditosylate and sodium hydride to form the hemi-crown formal. Removal of the formal protecting group, followed by a second cychzation completed the synthesis. The synthesis of the trans-anti-trans compound is illustrated below m Eq (3 12) and the structures of the five possible stereoisomers are shown as structures 1—5. 

Compare geometries of the cis and trans cycloalkenes. Are the double bonds incorporated into the trans compounds significantly more distorted than those incorporated into the analogous cis cycloalkenes Consider carbon-carbon bond lengths and the twisting and/ or puckering of the double bond. Are any distortions greater in trans-cycloheptene than in trans-cyclooctene ... 

Tetrahydro derivatives are formed when either quinoxaline or 6-chloroquinoxaline is reduced with lithium aluminum hydride in ethereal solution. Similar reduction of 2,3-dimethylquinoxaline gives the meso-(cts)-1,2,3,4-tetrahydro derivative. This is shown to be a stereospecific reduction since lithium aluminum hydride does not isomerize the dl-(trans)-compound. Low temperature, platinum catalyzed, hydrogenation of 2,3-dimethylquinoxaline in benzene also gives meso (cis) -l,2,3,4-tetrahydro-2,3-dimethylquinoxaline. ... 

Finally, it should be mentioned that the allylation of 2,4-disubstituted pyrrolidine derivatives proceeds with reasonable diastereosclectivity for the trans-compound 2,4-rra .v-l, but with low selectivity for the m-isomer 2,4-m-l131. 

Evidently, the structure of the transition state is such that cis as well as trans compounds can be formed with, more or less, a preference for the cis compounds. The reason for the latter may be that coordination of cis isomers is sterically favored. In agreement with this are the observations that cis compounds are more reactive than the corresponding trans compounds (49, 106). 

The related chromium compound [1140], trans-[Cr(pn)2Br2]Br H20, undergoes rapid dehydration at 395 419 K by a first-order process for which E = 96 kJ mole-1 and this is accompanied by some 10% isomerization to the cis compound. At higher temperatures, 433-473 K, the residual anhydrous trans compound isomerizes in the solid state this is also a first-order process and E = 180 kJ mole-1. 

Higher, the cis compound will have 12 different orien tations and the trans compound will have only 3 different orientations. 

The different ratios of 52/53 produced by cycloadditions performed at atmospheric and high pressure, and the forma tion of the unusual trans adducts 53, have been explained by the facts that (i) Diels-Alder reactions under atmospheric pressure are thermodynamically controlled, and (ii) the anti-endo adducts 52 are converted into the short-lived syn-endo adducts 54 which tautomerize (via a dienol or its aluminum complexes) to 53. The formation of trans compounds 53 by induced post-cycloaddition isomerization makes the method more flexible and therefore more useful in organic synthesis. 

UV irradiation of piperine, the most important hot substance of pepper, does not lead to the incorporation of atmsopheiic oxygen [8]. The all-trans compound is converted to the cis-trans isomer, this can be seen in the chromatogram above the all-rrans piperine (Fig. 16). 

Similar processes occur with azo compounds [10]. trans-Dimethylaminoazobenzene (butter yellow) yields some of the cis isomer on irradiation with long-wavelength UV light and this possesses a lower hRf than the trans compound on rechromatographing with the same mobile phase using the two-dimensional SRS technique (Fig. 17). IR and MS measurements were used to confirm that no oxygen had been incorporated into the molecule. 

We may now proceed to an analysis of the significance of the d values. If the transition state were close to the product, then the P values of the disubstituted ethylenes should be small, because in the product, the substituent is bonded to an sp hybridized carbon atom. Thus, it is incapable of resonance interaction. As the observed values of p for the trans-disubstituted dienophiles are very large, we conclude that the transition state is closer to reactants than to products. The cis-disubstituted dienophiles show a much smaller value of P than do the trans compounds. It therefore seems likely that the transition state for the CIS compounds will be closer to product than is the transition state for the trans compounds. The values of p for the reaction of the tnws-disubstituted dienophiles with 9,10-dimethylanthracene, while large, are much smaller than... 

Fig. 15. The overlap of (ligand) p orbital with a vacant orbital upon rearrangement of the trans compound to a trigonal bipyramid.

Geometrical cis-trans isomers — The UV-Vis spectra of most cis isomers are similar to those of the corresponding dll-trans isomer. However, a few consistent differences can be found in the spectra of cis isomers as compared to those from the corresponding a -trans compound a hypsochromic shift (2 to 6 nm for mono-dY through 34 nm for tetra-c ), a decrease in absorbance, a reduction of the spectral fine structure, and tlie appearance of a new absorption band known as a cis peak. For example, in a study in which the structures were confirmed by NMR, the isomers... 

The preferred stereoelectronic arrangement is perpendicular alignment of the acetate with respect to the double bond. For example, the cis and trans isomers of 1-vinyl-2-methylcyclohexyl acetate show divergent stereochemical results. Only the exocyclic E-isomer is formed from the cis compound, whereas the trans compound gives a 1 1 mixture of the E- and Z-isomers. This is the result of a strongly preferred conformation for the cis isomer, as opposed to a mixture of conformations for the trans isomer.32... 

Figure 11.1 The hydroboration-oxidation of 1-methylcyclopentene. The first reaction is a syn addition of borane. (In this illustration we have shown the boron and hydrogen both entering from the bottom side of 1-methylcyclopentene. The reaction also takes place from the top side at an equal rate to produce the enantiomer.) In the second reaction the boron atom is replaced by a hydroxyl group with retention of configuration. The product is a trans compound (trans-2-methyl-cyclopentanol), and the overall result is the syn addition of -H and -OH.

Formation of /1-lactams by the reaction of an acid chloride, a SchifFbase and a tertiary amine (Eq. 62) appears to involve multiple pathways, some of which are very fast at higher temperatures. When conducted in open vessels in unmodified micro-wave ovens, high level irradiation leads to preferential formation of the trans -lactams (55%) whereas, at low power, the cis isomer was obtained as the only product (84%). The failure of the cis isomer to isomerize to the trans compounds is an example of induced selectivity. 

The trans compound cannot be formed by a concerted reaction from the 33t-7t state because the predominant level perturbation is the almost degenerate interaction of K(jr) and 0(jr), that would give cis geometry of product. Since the trans adduct must be formed via biradical intermediate (the only other possibility), it has usually been convenient to suppose that the cis adduct is also formed from the same biradical intermediate. Another choice of mechanism is that cis compound is formed via a concerted reaction, and the trans compound arises from a biradical pathway. In this case, the spin prohibition could be outweighed by two factors, the favorable geometry and the stabilizing first-order perturbation. 

The trans compound melts at approximately 90 °C, and continued heating leads to isomerization to the cis structure. Geometrical isomerizations can also lead to a change in structure of the complex. For example, a change from square planar to tetrahedral structure has been observed for the complex [Ni(P(C2H5)(C6H5)2)2Br2]. 

The hydrolysis products of cis- and trans-DDP are known to undergo condensation reactions with formation of di- and oligo-p-OH species [11,15]. However, for geometric reasons less robust OH bridged species are expected for trans compounds compared to the cyclic species formed by the cis isomer [11]. 

The fact that the trans compound contains no deuterium, while the cis compound contains it, shows that the reaction proceeds by trans mechanism. 

According to this theory the labile normal diazotates are regarded as the syn- (= cis-) compounds, the stable wo-diazotates as the anti-(= trans-) compounds. 

Alkyl-substituted olefins are also epoxidized by CPO. The oxidation of functionalized terminal olefins by CPO catalysis with fBuOOH as oxygen donor affords the corresponding epoxides in good ee values (entries 20, 22, 24-26). While cz5-substituted olefins were epoxidized by CPO with H2O2 in good to excellent enantiomeric excesses (entries 32-35), the trans compounds were poorly converted. The enantioselective CPO-catalyzed epoxidation has been intensively investigated, although examples of diastereoselective oxidations (entries 43-45) are rare. 

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