Isomerization of , cis/trans

TABLE 7 Photochemical Isomerizations of cis-trans Isomeric Dianthrylethylenes 38 and 39 in Cyclohexane. (Excitation wavelength 366 nm)... 

The reaction of T with the geometrical isomers of DMC lead mainly to the T-substituted isomer with configuration similar to the reactant however, there is a 5% trans contamination in the case of T 4- cis-DMC and an 8 % cis contamination in the case of T -f- trans-DMC. In other hot T for H substitution, it was found " that there is retention of the configuration. Here, however, substitution with inversion or late isomerization of the T-substituted parent molecule cannot be distinguished. The activation energy for the thermal isomerization of cis - trans and trans - cis is about 2 kcal mol lower than that of isomerization of pentenes. 

In this study, the occurrence and the growth temperature-dependent alteration of trans-unsaturated fatty acid in Vibrio ABE-1 are described. It is suggested that the isomerization of cis/trans unsaturation of fatty acids may result in the growth temperature-dependent shift in the phase transition temperature of the membrane phospholipids. 

Schoenlein R W, Peteanu L A, Wang Q, Mathles R A and Shank C V 1993 Femtosecond dynamics of cis-trans Isomerization In a visual pigment analog Isorhodopsin J. Phys. Chem. 97 12 087-92... 

Furylfuramide. a[(5-Nitro-2-furaiiyl)-2-methylene]-2-furaiiacetamide, withdrawn from the market in Japan in 1974 because of mutagenicity, is prepared by condensation of 5-nitro-2-furancarboxaldehyde with 2-furanacetic acid followed by chlorination and amination (30). The isomerization of cis to trans form of fudyfuramide has been shown to occur in the presence of a variety of biological reducing agents (31). 

A -Bridgehead compounds have been obtained from the photochemical cyclization of cfi-1-styrylimidazoles. For example, irradiation of the imidazole (532) in methanol in the presence of I2 resulted in cyclization at the 2-position of the imidazole ring with the formation of an imidazo[2,l-a]isoquinoline (533) (76JCS(Pl)75). Isomerization of the trans-to the c/i-styrylimidazole was followed by photodehydrocyclization, trans- 1-Styrylbenzimidazole (534) was isomerized under Pyrex-filtered light in the presence of one molar equivalent of I2. The resulting cis isomer on irradiation through quartz gave the benzimidazo[2,l-a]isoquinoline (535) in 53% yield. 

Related to stereoregularity is the possibility of cis, trans isomerism. The molecule of natural rubber is a c/s-1,4-polyisoprene whilst that of gutta percha is the trans isomer. 

Fig. 13.12. Energy diagram illustrating differential in energy deficit for photosensitized isomerization of cis and trans isomers.

Although lupinine is thus a comparatively simple alkaloid its detailed chemistry has been difficult to unravel owing (a) to the presence in its molecule of two asymmetric carbon atoms as asterisked in (XI), and (6) the possibility of cis-trans isomerism in certain of its proximate (ieriva-tives. Winterfeld and Holschneider have pointed out that a further complexity arises from the presence in natural Z-lupinine of a structural isomeride, aZZolupinine for which formula (XII) is suggested. They also quote Kreig s observation that by the action of sodium on a benzene solution of Z-lupinine (m.p. 68-9° [ajo — 23-52°), the latter is converted... 

Cis-trans isomerization is an important step in the ehemistry of vision, for example, the light-initiated, enzyme-eatalyzed isomerization of cis to trans-XQtmul. 

Pyrethrolone and cinerolone make up the keto alcohol moiety of the pyrethrins. Both of these keto alcohols have one asymmetric carbon at the 4-position and a double bond in the side chain which is capable of cis-trans isomerism in the 2-position. It is possible, therefore, to have four stereoisomers for each keto alcohol. Katsuda et al. (22) show that only the ( + ) form occurs in the natural esters. Elliott (8) has shown recently, by a new procedure developed to obtain pure ( + ) pyrethrolone, that the hitherto unidentified prye-throlone C is in reality pyrethrolone contaminated with thermally isomerized material. (+) Pyrethrolone forms a crystalline monohydrate from which the pure alcohol is obtained. The natural configurations of the keto alcohols in the esters are insecticidally more active, as is the case with the acid moiety. 

Another case to which we can apply the principle of microscopic reversibility is the isomerization of cis- and rrans-(H20)4CrClJ and the loss of Cl- from each. This system has been studied both by careful chromatographic separation of the components20 in quenched solutions and by simultaneous multiwavelength spec-trophotometric determinations.21 The scheme is as follows, where the subscript c indicates cis, t trans, and m the monochloro complex ... 

This ring structure requires the existence of cis-trans isomerism, as shown in structures (U) and (12) ... 

Fig. 16 Detection of cis/trans isomerization of piperine by the SRS technique after UV irradiation (A) originai chromatogram, (B) schematic representation.

No cases of cis/trans isomerism have yet been reported among organo-cobalt(III) complexes, even among compounds not covered by this review. Complexes such as [R2Co(bipy)2] all have the cis configuration steric hindrance would be greatly increased by the coplanarity of the two bipyridyl ligands in the trans isomer 123). 

On the other hand, in order to preserve the cold properties of the fuel (Cloud Point, Pour Point and low-temperature filterability), it is mandatory not to increase the melting point, that in turn depends on both the saturated compound (stearic acid, C18 0) content and the extent of cis/trans and positional isomerization as the difference in melting point between the cis and trans isomer is at least 15°C according to double bond position as shown in Table 1. 

More recently, a new mode of cis-trans isomerization of a disilene has been suggested for the extremely hindered disilene 27. As will be detailed in Section VIII. B, 27 undergoes thermal dissociation into the corresponding silylenes. Monitoring the thermolysis of (Z)-27 at 50°C by H and 29Si NMR reveals a competitive formation of the isomerized ( >27 and benzosilacyclobutene 37, which is most likely formed by intramolecular insertion of silylene 36 into the C—H bond of the o-bis(trimethylsilyl)-methyl group (Scheme 3).22,59 This suggests the possible occurrence of cis-trans isomerization via a dissociation-association mechanism. 

Thus isomerization of the trans to cis ketone is stereospecific while isomerization of cis to trans results in about 28% racemization. Postulating different pathways for trans to cis and cis to trans isomerizations would appear at first hand to violate the principle of microscopic reversibility. However, what we are dealing with here are the excited state intermediates as reactants and these are not necessarily in equilibrium for the two isomers. For example, taking the analogy... 

The spontaneous isomerization of all-trans- carotenoids at room temperature is a slow process, and its rate depends on the solvent and the pigment structure. For example, the initial solutions of P-carotene in a mixture of tetrahydrofuran (THF), methanol, and acetonitrile containing ca. 95% of all -trans- and 5% of 9-cis- plus 13-co-isomers was transformed to 90% all-fra ns- p -ca rot ene and 9% of 9-cis- plus 13-cA-carotene after 24h of spontaneous isomerization at 25°C (Pesek et al. 

In addition, the results indicated that the efficiency of cis —> trans increased as the initial cis double bond configuration is shifted from the center of the polyenic chain, consistent with the 7j, triplet excited state potential curve that has a very shallow minimum at the 15-cis position compared to the deep minima at the all-trans position. The results strongly suggest that isomerization takes place via the 7j state of the carotenoid even in the case of direct photoexcitation, with their photosensitized process because of the very low intersystem crossing quantum yield, isc ([Pg.246]

Wache, Y., A. Bosser-DeRatuld et al. (2003). Effect of cis/trans isomerism of beta-carotene on the ratios of volatile compounds produced during oxidative degradation. J. Agric. Food Chem. 51(7) 1984—1987. 

Triafulvenes of type 224 capable of cis-trans rotational isomerism in most cases occur as equilibrium mixtures of structures 224A and 224B their configura-... 

You CS, Parker RS, Goodman KJ, Swanson JE and Corso TN. 1996. Evidence of cis-trans isomerization of 9-cis- 3-carotene during absorption in humans. Am J Clin Nutr 64 177-183. 

One of the possible mechanisms of cis-trans isomerization of olefins is excitation to the triplet or diradical state.98-96 The two paths, one by way of singlet and triplet states and the other solely by way of singlet states, are diagrammed in Fig. 1. The two lines with minima at 0° and... 

The protocol developed by Jacobsen and Katsuki for the salen-Mn catalyzed asymmetric epoxidation of unfunctionalized alkenes continues to dominate the field. The mechanism of the oxygen transfer has not yet been fully elucidated, although recent molecular orbital calculations based on density functional theory suggest a radical intermediate (2), whose stability and lifetime dictate the degree of cis/trans isomerization during the epoxidation <00AG(E)589>. 

Thermal isomerization of certain cis-l,3,4-trisubstituted azetidin-2-ones 76 provided the trans isomer in good yield . Bases caused the isomerization of cis-3-substituted-4-formylazetidin-2-ones and of sulfonic acid derivatives of 3-aminoazetidin-2-ones during the formation of a Schiff base <00T3985>. 4-Acyloxy-iV-o-azidobenzoyl-P-lactams underwent ring expansion to produce l,3-oxazin-6-ones 77 . 

---