Double-bonded molecules, isomerism

Isomerism.—From an examination of the above formulas it will be seen that the positions occupied by the added hydrogen atoms in the original cymene molecule or, what is the same thing, the positions occupied by the double bonds, makes isomerism possible both in the tetra-hydro cymenes or menthenes with one double bond and the dihydro cymenes or mentha-di-enes with two double bonds. In the former case six isomers are possible while in the latter there are fourteen. This will be clear if we give the skeleton formulas for the six possible menthenes. 

CIS double bond to isomerize to trans, and the conformational changes in the protein molecule that result trigger a cascade of reactions that u Itim ately leads to a nerve signal being sent to the brain. 

Fig. 3A-C. Some examples of photoisomerizable molecules A trans-cis Isomerization around the azobenzene double bond. B Isomerization between spiropyran and merocyanine states. C The reversible electrocyclic ring closure of diarylethenes...

Cleave C-C, C-N, C-O bonds to produce double bonds, or add groups to double bonds Intramolecular isomerization Linking of molecules to form C-O, C-N, C-C, C-S bonds... 

CHO, and the double bond between carbons 11 and 12 of the side chain is in the less stable c/s configuration. When rhodopsin absorbs light energy, the less stable 11-c/s double bond is converted to the more stable 11-frans double bond. This isomerization changes the shape of the rhodopsin molecule, which in turn causes the neurons of the optic nerve to fire and produce a visual image. 

Nevertheless, such reactions catalyzed by zeolites have been discussed in the review of 2001 (1) isomerization (double-bond shift, isomerization of tricyclic molecules, like synthesis of adamantane, isomerization of terpenes, diverse rearrangements, conversion of aldehydes into ketones), (2) electrophilic substitution in arenes (alkylation of aromatics, including the synthesis of linear alkylbenzenes, alkylation and acylation of phenols, heteroarenes and amines, aromatics nitration and halogenation), (3) cyclization, including the formation of heterocycles, Diels-Alder reaction, (4) nucleophilic substitution and addition,... 

The compounds of this article, ie, ftve-membered heterocycles containing two adjacent nitrogen atoms, can best be discussed according to the number of double bonds present. Pyrazoles contain two double bonds within the nucleus, imparting an aromatic character to these molecules. They are stable compounds and can display the isomeric forms, (1) and (2), when properly substituted. Pyrazoles are scarce ia nature when compared to the imidazoles (3), which are widespread and have a central role ia many biological processes. 

Because the integrity of the dihydrothiazine ring and its C-4 carboxyl substituent is crucial to useful antimicrobial activity, reactions involving this part of the cephalosporin molecule are usually undesirable. The possibilities for sulfur oxidation or alkylation, substitution at C-2 which is adjacent to both sulfur and a double bond, double bond isomerization and addition reactions, and the influence of a free carboxylic acid must all be considered in designing reactions to selectively modify other cephalosporin functionalities. 

The stereoisomers of olefin saturation are often those derived by cis addition of hydrogen to the least hindered side of the molecule (99). But there are many exceptions and complications (97), among which is the difficulty of determining which side of the molecule is the least hindered. Double-bond isomerization frequently occurs, and the hydrogenation product is the resultant of a number of competing reactions. Experimentally, stereochemistry has been found to vary, sometimes to a marked degree, with olefin purity, reaction parameters, solvent, and catalyst 30,100). Generalizing, it is expedient, when unwanted products arise as a result of prior isomerization, to avoid those catalysts and conditions that are known to favor isomerization. 

Hydrogenation of aromatics under mild conditions gives mainly the all-cis isomer as if hydrogen addition takes place from only one side of the molecule (23,24). Reductions under more vigorous conditions may give other isomers by isomerization of the initially formed all-cis product. Under mild conditions, other isomers are accounted for by desorption and readsorption in a new orientation of intermediate olefins, as well as by double-bond migration in the... 

Conjugation is crucial not only for the colors we see in organic molecules but also for the light-sensitive molecules on which our visual system is based. The key substance for vision is dietary /3-carotene, which is converted to vitamin A by enzymes in the liver, oxidized to an aldehyde called 11-frans-retinal, and then isomerized by a change in geometry of the C11-C12 double bond to produce 11-cis-retinal. 

These pentahydrides have attracted attention as catalysts for hydrogenation of the double bond in alkenes. IrH5(PPr3)2 catalyses vinylic H-D exchange between terminal alkenes and benzene, the isomerization of a,f3-ynones, isomerization of unsaturated alcohols and dehydrogenation of molecules such as secondary alcohols [176],... 

Isopentenyl diphosphate is isomerized by a shift of the double bond to form dimethylallyl diphosphate, then condensed with another molecule of isopentenyl diphosphate to form the ten-carbon intermediate ger-anyl diphosphate (Figure 26-2). A further condensation with isopentenyl diphosphate forms farnesyl diphosphate. Two molecules of farnesyl diphosphate condense at the diphosphate end to form squalene. Initially, inorganic pyrophosphate is eliminated, forming presqualene diphosphate, which is then reduced by NADPH with elimination of a further inorganic pyrophosphate molecule. 

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