Isomerization of a double bond

The rearrangement proceeds from the Si-state of the 1,4-diene 1. The Ti-state would allow for different reactions like double bond isomerization. Rigid systems like cyclic dienes, where EfZ -isomerization of a double bond is hindered for steric reasons, can react through the Ti-state. When the rearrangement proceeds from the Si-state, it proves to be stereospecific at C-1 and C-5 no -isomerization is observed. Z-l,l-Diphenyl-3,3-dimethyl-l,4-hexadiene 5 rearranges to the Z-configured vinylcyclopropane 6. In this case the reaction also is regiospecific. Only the vinylcyclopropane 6 is formed, but not the alternative product 7. ... 

For many radicals, step 1 (C=C+Y- -C—C—Y) is reversible. In such cases, free radicals can cause cis — trans isomerization of a double bond by the pathway. ... 

There are several reagents that lead to isomerization of a double bond to form a new alkene. Allyl decyl ether (CH2=CHCH20CioH2i) was isomerized to 1-decyloxy-l-propene (CH3CH=CHOCioH2i) by treatment with NaHFe(CO)4. Many of these reactions were discussed in 12-2. 

Fig. 12. Relationship between configuration and the ease of the palladium-catalyzed isomerization of a double bond in the steroid nucleus.

One way to accomplish the isomerization of a double bond into conjugation with a carbonyl group, one of the reactions needed for the last part of the synthesis of progesterone from diosgenin, is to treat the compound with base as shown in the following equation. Show a mechanism for this reaction. 

The isomerization of a double bond that occurs through the delocalization of an allylic intermediate. (p. 679)... 

There are several categories of photochromic reactions electrocyclization of conjugated polyenes [3], E-Z isomerization of a double bond [4], radical forma-... 

Reversible transformations are by no means rare and often lead to confusion because they are not easy to detect using simple analytical means. The most prominent example is that of the cis A trans isomerization, of a double bond. The best-documented example of this transformation is EHMC as outlined in Figure 2. 

Hydrozirconation of vitamin D3 (39) gives a mixture of four diastereomers resulting not only from addition to either face of the reacting exocyclic double bond, but also isomerization of the nonreacting 5,6-double bond (equation 41), even though no isomerization of unreacted (39) was detected. No explanation was offered. Partial isomerization of a double bond during hydrozirconation of a triple bond has also been observed. ... 

Fig. 1.1 Schematic representation of general cis-trans isomerism of a double bond.

Pregnenolone is derived from cholesterol and is the precursor of all other steroid hormones. For example, pregnenolone is coverted to progesterone by oxidation of the hydroxyl group to a ketone and isomerization of a double bond (see here). Synthesis of other steroid hormones from progesterone is shown in Figure 19.24. 

The same catalyst has been used by this research group in synthesis of j -amino-aldehydes (146) by combining two catalytic reactions, i.e. a Ni(II) complex-catalyzed isomerization of a double bond and a chiral phosphoric acid (127)-catalyzed aza-Petasis-Ferrier rearrangement in a highly dia-stereo- and enantioselective manner (Scheme 39). ... 

Since then, numbers of catalytic systems were developed. Regarding the intramolecular cyclization, in 2004, Nishida and co-workers reported a new type of palladium-catalyzed cyclization, which proceeds via selective isomerization of a double bond in the enamine structure followed by 5-endo cyclization. Various fused indoles were produced in good yields (Scheme 2.110). Interestingly, this procedure was also applied for the synthesis of the azocinoindole skeleton. 

Some bacterial cells contain mono-unsaturated fatty acids with a rran -confuguration as acyl components of membrane phospholipids. We found the temperature-dependent alteration in levels of a rran -mono-unsaturated fatty acid, 16 l(9t), in a psychrophilic bacterium, Vibrio sp. strain ABE-1 [1]. Morita et al. [2] presented the evidence for the direct in vivo isomerization between 16 l(9c) and 16 l(9t) in cells of this bacterium using stable isotope tracers, namely, [2,2-%2]16 l(9c) and [2,2-%2]16 l(9t). This isomerization is quite unique because of configurational isomerization of a double bond without shifting of the double bond position. In this article, we decribe purification and characterization of cis-trans isomerase from a psychrotrophic bacterium. Pseudomonas sp. strain E-3 Pseudomonas E-3), another organism that contains 16 l(9t) [1], and the possible physiological roles of rran -mono-unsaturated fatty acid in this bacterium. 

Morita, N., Shibahara, A., Yamamoto, K., Shinkai, K., Kajimoto, G., and Okuyama, H. (1993) Evidence for cis-trans isomerization of a double bond in the fatty acids of the psychrophilic bacterium Vibrio sp. strain ABE-1. J. BacterioL, 175, 916-918. 

The intramolecular Diels-Alder cyclization of two trienecarboxylate diesters derived from ( )-hexa-3,5-dienoic acid occurs only after isomerization of a double-bond, to form the sorbate derivative, has occurred. A stereocontrolled route to racemic eremophilane and valencane sesquiterpenes via an intramolecular Diels-Alder reaction has been described.2 Full papers have appeared describing the intramolecular Diels-Alder reactions of 2-substituted butadienes, which lead to bridgehead alkenes (Scheme 76). ... 

Cleavage of allylic ethers can occur using acetic anhydride in the presence of iron(III) chloride (eq 45). The reaction takes place without isomerization of a double bond, but optically active ethers are cleaved with substantial racemization. ... 

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