Functional groups alkene

The term Diels-Alder reaction in a general sense refers to the reaction between a diene and a dienophile. Retro Diels-Alder reaction is a process that, under certain conditions, produces diene and olefin or a compound containing a C=C bond. The application of flash vacuum pyrolysis to effect the retro Diels-Alder reaction, as shown in Schemes 5-46 and 5-47, has become the standard procedure since the introduction of the method by Stork et al.74 in the 1970s. Therefore, alkenes that are difficult to access by conventional methods may be obtained via retro Diels-Alder reactions.75 In particular, this reaction allows the preparation of thermodynamically less stable compounds such as 4,5-dialkyl cyclopenta-2-en-one. In this case, the alkene functional group can be regarded as being protected by cyclopentadiene (as shown in 154 or 157), which, after subsequent reaction, can easily be removed through quick pyrolysis. 

For alkenes, several carbon oxidation levels are again possible. Furthermore, both carbon atoms must be considered as part of the same alkene functional group. While the total oxidation level can go from —4 for ethylene (as the sum of the oxidation level of both carbon atoms in the functional group) to 0 for a tetrasub-stituted alkene, we again recognize that all are of the same functional class. 

Consideration of the oxidation level reveals diat while one carbon is reduced (the one to which hydrogen adds), die other is oxidized (die one to which the oxygen adds). There is no net change in oxidation level of the alkene functional group. Likewise die reverse processes of these addition reactions, namely, elimination of HX from alkyl halides and dehydration of alcohols to give alkenes, are not redox processes. Additions of water to alkynes is analogous. In this case, however, the product is a ketone, the oxidation level of the ketone is seen to be the same as the alkyne, and so no net change in oxidation level has occurred. 

Cis/Trans Isomerism There is normally free rotation around the carbon-carbon single bonds in alkanes. The alkene functional group has two carbon-carbon bonds. The introduction of the second bond freezes rotation around the... 

The functional group in alkenes is ( > C = C< ). The alkene functional group (R,C = CR,) is planar in shape with bond angles of 120. The two carbon atoms involved in the double bond are both sp2 hybridised. Each carbon has three sp2 hybridised orbitals which are used for a bonds while the p orbital is used for a n bond. Thus, the double bond is made up of one a bond and one n bond ... 

To deduce the location of the double bond within the lipid backbone, the mixture (500 ng) was subjected to consecutive bisthiomethylation of the alkene85 and O-methyloxime formation (Equation 3). GC—MS study of the fragmentation of these derivatives (e.g., see 31, derived from 24) allowed simultaneous determination of the cleavage site (between C24 and C25) and of which portion contained the original ketone (i.e., the odd versus even mass fragments of 17 3 and 426 for 31). All of the monounsaturated lipid ketones had the alkene in the same downstream location in other words, they varied in the number of methylene units between the ketone and alkene functional groups but were constant in their -octyl terminal alkyl moiety. The four most major components (24, 25, 27, and 28) were prepared by chemical synthesis and used to confirm their identity in the natural pheromone and their pheromonal activity both alone and in admixtures. 

Isomerization of drugs typically involves racemization or epimerization of optical isomers or cis-trans isomerization of compounds containing an alkene functional group. Examples of epimerization include pilocarpine (55), rolitetracycHne (56), and ergotamine (57). Drugs susceptible to racemization include benzodiazepines, penicillins, and cephalosporins. The rearrangement of amphotericin B is an example of cis-trans isomerization (58). 

The study of the electrophilic addition of activated electrophiles across the alkene functional group by Zefirov and coworkers237 has led to the synthesis of a wide range of sulphamate esters. The general approach involves the insertion of S03 into a weak electrophile which generates an activated electrophile that adds readily across carbon-carbon double bonds of olefins (equation 65). 

If both directions lead to the same number for the alkene functional group suffix and the same low number(s) for one or more of the substituents, then those substituents are ignored and the direction is chosen that gives the lowest number to one of the remaining substituents. 

Because sodium borohydride cannot reduce carbon-carbon double bonds, a carbonyl group in a compound that also has an alkene functional group can be selectively reduced, as long as the double bonds are not conjugated (Section 18.13). Acid is not used in the second step of the reaction, in order to avoid addition to the double bond. 

In general, intermolecular [2 -I- 2] photocycloaddition of simple alkenes does not show satisfactory regioselective control. An approach to improve the reaction selectivity is use of a tether to constrain the reactive alkene functional groups proximate to each other and therefore allow the photocycloaddition to proceed. Tethers which have been employed include sugar alcohols such as d-maimitol and L-erythritol [101], diazacrown ethers [102], cyclophane [103], and silanes [104]. In all cases, the tether preorganized alkenes yield head-to-head adducts. The silane-tethered method can bind different alkene groups as well as alkynes, as shown in Scheme 27 [105]. Photoreactions of polyacetylenes and alkenes have also been investigated [106]. 

The two sides of alkenes are due to the nature of the four-electron double bond. In the simplest picotre, we assign two of its electrons to a basic, garden-variety CT bond between the atoms. The other two electrons are then placed in two parallel p orbitals, overlapping sideways to fomi the ir bond. This tr-type overlap prevents the carbons at each end of the double bond from rotating with respect to one another. Ethene, therefore, is a perfectly flat molecule, and, in general, the carbons of the alkene functional group and all the atoms attached to them will lie in a plane, with the n electrons above and below. 

You see here logical extensions of carbonyl and alkene chemistry. The carbon-carbon double bond in an a, (3-unsaturated carbonyl compound generally shows the same addition reactions typical of simple alkenes. However, the highly polar nature of the carbonyl strongly affects the reactivity of the alkene functional group. This... 

WHY DO THESE TOPICS MATTER ] At the end of the chapter, we will show how simple changes in the placement of alkene functional groups can lead to distinct properties, from the strength of the rubber in our tires to our ability to see. 

Enolates provide many ways to functionalize the a-carbon of a cartronyl compound. Most importantly, enolates provide ways to form new catbon-carbon bonds, me of these synthetic connections are shown here. Previously studied reactions of carbonyl, alcohol, and alkene functional groups (e.g., reduction, oxidation, addition, substitution) lead to or from some of these pathways. 

The reactions of lipids represent many reactions that we have studied in previous chapters, especially reactions of carboxylic acids, alkenes, and alcohols. Ester hydrolysis (e.g., saponification) liberates fatty acids and glycerol from triacylg-lycerols. The carboxylic acid group of a fatty acid can be reduced, converted to an activated acyl derivative such as an acyl chloride, or converted to an ester or amide. Alkene functional groups... 

Three very dissimilar substances—vitamin A from sources including dark green leafy vegetables, cholesterol from animals, and rubber from certain trees—have something in common. Their molecules all have carbon-carbon double bonds—the alkene functional group. 

If one of the functional groups is an alkene, suffix endings are used for both functional groups and the alkene functional group is stated first, with its e ending omitted to avoid two successive vowels (Section 7.2). 

---