Di-cis isomer

This bi-exponential behavior confirms the presence of reversible isomerization steps coupled with irreversible degradation steps and accounts for the role of the di-cis isomers as reaction intermediates, according to the general reaction scheme presented in Figure 12.1. The dependence of the rate constant of each elementary step on temperature allowed the calculation of the respective activation... 

By using continuous-flow 400 MHz HPLC- H NMR spectra obtained using a cyanopropyl-silica gel column with heptane as the eluent, and by examination of the spectra extracted from the pseudo-2-dimensional representation, it was possible to identify the all -trans molecule, as well as the 9-cis, 11 -cis, 13-cis and 11, 13 di-cis isomers. 

Hengartner U, Bernhard K and Meyer K (1992) Synthesis, isolation, and NMR-spectroscopic characterization of fourteen (Z)-isomers of lycopene and of some acetylenic didehydro-and tetrahydrolycopenes. Helv Chim Acta 75 1848-1865 Hu Y, Hashimoto H, Moine G, Hengartner U and Koyama Y (1997) Unique properties of the 11 -cis and 11,11 -di-cis isomers of )3-carotene as revealed by electronic absorption, resonance Raman and H and NMR spectroscopy and by HPLC analysis of their thermal isomerization. J Chem Soc Perkin Trans 2 2699-2710... 

In the 7,7 -diacetylenic series it has recently been shown that no all-trans isomer can be detected in the iodine catalyzed stereomutation mixtures examined by HPLC of alloxanthin (31) and 7,8,7, 8 -tetra-dehydroastaxanthin (34) diacetate, in which the 9,9 -di-cis isomers are dominant (76). However, in the absence of iodine aW-trans (31) and all-trans (34) diacetate are rather stable. In the monoacetylenic series exemplified by diatoxanthin (41) and the diacetate of 7,8-didehydroasta-xanthin (80), appreciable amounts of the aW-trans isomers are present in the iodine catalyzed stereomutation mixture 76,89). Again the preference for cw-configuration is probably caused by both steric and electronic factors. 

If the negligible contribution of di-cis isomers is ignored, the composition of a typical photoisomerate (incandescent lamp) in ethanol is 22% 13-cis, 29% 11-cis, 11% 9-cis, 0.7% 7-cis, and 37% all-trans (Bridges and Alvarez, 1982b). If the 11-cif-retinaldehyde from this reaction is collected fix>m the column effluent. 

Four cis isomers of P-carotene (13,15-di-di-, 15-cis-, l3-cis-, and 9-cis-) and three of a-carotene (15-di-, 13-di-, and 9-cis-) were formed during heating of their respective dll-trans carotene crystals at 50,100, and 150°C. Isomerization catalyzed by heat was considered as a reversible first-order degradation reaction — a trans-to-cis conversion two- to three-fold slower than the backward (cis-to-trans) reaction (Table 4.2.6). The 9-cis- and 13-di- were the major P-carotene isomers formed and the 13 -cis- formed at a two- to three-fold faster rate than O-cw-P-carotene. In this system, a-carotene showed lower stability than P-carotene (Table 4.2.6). The activation energy (EJ was not reported since practically no degradation was observed... 

Among thermal processes, canning caused the largest trans-to-cis isomerization of provitamin A carotenoids, increasing the total cis isomers by 39% for sweet potatoes, 33% for carrots, 19% for collards, 18% for tomatoes, and 10% for peaches 13-di-P-carotene was the isomer formed in highest amonnts. ... 

Canning at 121°C for 30 min was also responsible for the highest losses of carotenoids in carrot juice, reaching 60% for P- and a-carotene, whereas the lutein level decreased 50%, all accompanied by the formation of 13-c -p-carotene in the largest amount, followed by 13-cA-lutein and 15-cA-a-carotene. Canning (T x = 121°C, F = 5) of sweet com resulted in a decrease of lutein by 26% and zeaxanthin by 29%, accompanied by increased amounts of 13-cis- lutein, 13 -CM-lutein, and 13-c/i-zeaxanthin. ° The relative amounts of cis isomers of lutein, mainly the 13-cis, increased by 15% and of 13-di-zeaxanthin by 20% after com canning." ... 

Extensive carrot blanching (100°C for 60 min) cansed losses of 26 to 29% in total p-carotene content, along with increased IS-CM-P-carotene contents up to 10% after pasteurization 95°C, P = 3) and to 14% after sterilization 121°C, F = 5). ° However, unheated juices produced from carrots blanched at 80°C for 10 min were devoid of cis isomers, and further pasteurization or sterilization processes formed only IS-di -P-carotene, at 2 and 5%, respectively. ... 

Similar effects could be observed in oil-rich foods such as pahn fruits in which losses of dll-trans a- and al -trans p-carotene were found to be, respectively, ca. 23 and 44% after sterilization. In terms of relative composition after processing, all-trans a-carotene decreased from about 30 to 23% and aH-trans P-carotene from 65 to 27%, whereas the cis isomers increased as follows D-di -a-carotene from less than 1 to 10%, D-cd-P-carotene from 3 to 23%, and O-cd-P-carotene from less than 1 to 18%. ... 

In dark conditions, the spontaneous isomerization of carotenoids occurs in solution the rate is dependent on temperature, solvent, and carotenoid structure. In the case of P-carotene, 13-di-P-carotene was formed approximately three times faster than the 9-cis- isomer at room temperature and at 150°C. ... 

Only 6% of the iifitial total lycopene prepared as a thin film on the surface of each vial remained after 144 hr under fluorescent light (2000 to 3000 lux) at 25°C under N2. Lycopene degradation occurred as a first-order reaction at 2.93 x 10" /min, and the concentration of aU lycopene mono-c isomers already present in the sample, 5-cis-, 9-cis-, l3-cis- and 15-d5 -, showed an inconsistent change in this period. Nevertheless, formation of lycopene di-c isomers was observed after 32 hr of light exposure and when considering relative percentage, loss of 13% of all-trani-lycopene occurred while an increase of 11% for total cis isomers was found after 144 hr. ... 

Thermolysis of 16e,f in either solution or gas phase (150-350 °C) gave deuteriated ethylenes (i.e. 40e from 16e and 41f from 16f) with about 95% retention of stereochemis-try ". Similarly, pyrolysis of the stereoisomeric 2,3-diphenylthiirane oxides 16g,h proceeded smoothly to yield stilbenes and sulfur monoxide in more than 70% yield . The extrusion of SO from the trans-isomer proceeds almost stereospecifically, while that from the cis-isomer occurs with complete loss of stereochemistry. This indicates the intervention of a stepwise mechanism, and not a symmetry-allowed nonlinear chelatropic reaction . Based on the fact that all attempts to trap the intermediate with 1,3-dipolarophiles were in vain, whereas a 1 1 adduct was obtained in good yield (about 60%) with the carbon radical scavenger di-p-anisyl thioketone, a mechanistic scheme as depicted in equation 10 has been proposed . Although the radical intermediates are capable of internal rotation about the carbon-carbon bond, for the 2,3-diphenyl case (i.e. 16g,h), the rotation would be... 

The 14-membered macrocycle 1,4,8,11-tetraazacyclotetradecane (cyclam or [14]aneN4), unlike cyclen, is capable of encircling most transition metal ions and in the case of Co111 the trans configuration is much preferred by comparison with the folded cis isomer. Electrochemical reduction of A,v-[Co(cyclam)(OI I)2]+ in 3M NaOH leads to rapid isomerization to the trans form, and the relative stabilities of the trans and cis isomers of the di- and trivalent complexes were determined from a thermodynamic cycle.702 This preference for trans orientation of the non-macrocyclic donors has enabled the isolation and investigation of many Co complexes without the complications of isomerization. Some novel examples include /r[Pg.61]

Upon low conversion direct photolysis the cis isomer (10) gave only the cis isomer (12) and the trans isomer (11) gave only the trans isomer (13). The triplet sensitized reaction of (10) and (11) gave rise only to cis-trans isomerization. Thus the di-ir-methane photorearrangement from the triplet state cannot compete with triplet state deactivation via cis-trans isomerization (Zimmerman has termed this the free rotor effect). Several other examples of regio-specilicity and stereospecificity in di-w-methane photoreactions are as followsa8 a3) ... 

Englert, G. and Vecci, M. 1980. Trans/cis isomerization of astaxanthin diacetate/isolation by HPLC and identification by 1H-NMR spectroscopy of three mono-civ- and six di-civ-isomers. Helv. Chim. Acta 63 1711-1717. 

The degradation of a- and P-carotene crystals upon heating at 150°C fitted a reversible first-order model, trans- to cis- conversion occurred two- to threefold slower than that observed for the backward reaction in other words, the equilibrium toward the all-trans- isomer was favored (Chen et al. 1994). Four cis- isomers of P-carotene (13,15-di-m-, 15-m-, 13-civ-, and 9-cis-) and three isomers of a-carotene (15-d.v-, 13-d.v-, and 9-cis-) were formed during the heating of their respective all-trans- carotene crystals. The 13-d.v isomer of both carotenes was found in greater amounts (Chen et al. 1994). In this system, a-carotene degraded faster than p-carotene (Table 12.2). 

---