Protection of Double Bonds

The introduction of double bonds into organic systems via the elimination of hydrogen halides is a widely applicable transformation [137] (see also Sect. 3.4.2.1). Dehalogenations have been used as a means for the purification of olefins, for the temporary protection of double bonds, and for generating a new double bond as part of a synthetic sequence[137]. 

This reaction has very wide application in organic synthesis, such as the formation of quinones," benzoguinones," heterocycles (e.g., oxazoles, imidazoles,pyridazine," pyrrole, furan" ) and olefins with stereospecificity as well as the protection of double bonds. 

Numerous routine synthetic applications have appeared which make use of the stereoselective ring-opening of cyclopropylcarbinyl derivatives to E-olefins. The allylcarbinyl-cyclopropylcarbinyl interconversion is frequently used for the protection of double bonds in steroids during synthetic procedures for the preparation of homosteroids and may be involved in the fluorination of A -3-tri-methylsiloxy-steroids. ... 

These oxazolines have cationic surface-active properties and are emulsifying agents of the water-in-oil type. They ate acid acceptors and, in some cases, corrosion inhibitors (see Corrosion). Reaction to oxazoline also is useful as a tool for determination of double-bond location in fatty acids (2), or for use as a protective group in synthesis (3). The oxazolines from AEPD and TRIS AMINO contain hydroxyl groups that can be esterified easily, giving waxes (qv) with saturated acids and drying oils (qv) with unsaturated acids. 

The retro Diels-Alder reaction usually requires high temperatures in order to surmount the high activation barrier of the cycloreversion. Moreover, the strategy of retro Diels-Alder reaction is used in organic synthesis to mask a diene fragment or to protect a double bond [47]. Some examples are illustrated in Scheme 1.11. 

Bromination followed by debromination is useful in the purification of alkenes and in protecting the double bond. 

An interesting method is to protect one double bond by addition of cyclopentadienyl dicarbonyl iron during hydrogenation and afterwards to regenerate the product (equation 17)57. 

The sulfone moiety was reductively removed and the TBS ether was cleaved chemoselectively in the presence of a TPS ether to afford a primary alcohol (Scheme 13). The alcohol was transformed into the corresponding bromide that served as alkylating agent for the deprotonated ethyl 2-(di-ethylphosphono)propionate. Bromination and phosphonate alkylation were performed in a one-pot procedure [33]. The TPS protecting group was removed and the alcohol was then oxidized to afford the aldehyde 68 [42]. An intramolecular HWE reaction under Masamune-Roush conditions provided a macrocycle as a mixture of double bond isomers [43]. The ElZ isomers were separated after the reduction of the a, -unsaturated ester to the allylic alcohol 84. Deprotection of the tertiary alcohol and protection of the prima-... 

The technique of methoxymercuration-demercuration was utilized to determine the position of double bonds in the side chains. Since this method is not successful with the free alkaloids (272), the secondary amino groups must be protected as the A -heptafluorobutyramide. These amides are treated with mercuric acetate and methanol followed by reduction with sodium borohydride to yield the methoxylated compounds (273). The mass spectra of these compounds show a fragment ion (274) at m/z 59 indicating terminal double bonds in every case (Scheme 22) 16,25,410,411). 

A second mechanism in the. aging of CTPB propellants also exists and proceeds concurrently with the reactions proposed above. It consists of an attack at the reactive points of unsaturation in the backbone polymer, which causes additional crosslinking and hence an increase in propellant modulus, particularly at the surface. The exposed surface of CTPB propellants changes, as indicated by an increase in hardness. Heavy metal ions are particularly harmful, and it was found that an increase from 10 to 80 p.p.m. of iron caused a significant increase in surface hardening by catalytic attack on the double bonds. Antioxidants in general provide sufficient protection for polymer storage. In CTPB propellants the antioxidant selected to protect the double bond is very important. Amine-type antioxidants have provided better surface stability than phenolic compounds. 

The presence of double-bonded fatty acids in proteins and their isomerization was found to help some bacteria to adapt to ambient temperature changes62. The alteration of C=C bonds in liposomes plays a role in protection against radiation-induced damage63. 

The dibromide itself is usually prepared from the same alkene and so the reaction is not particularly useful for the synthesis of alkenes. It is useful, however, in protection strategy. During a lengthy synthesis, it may be necessary to protect a double bond so that it does not undergo any undesired reactions. Bromine can be added to form the dibromide and removed later by denomination in order to restore the functional group. 

Indeed, starting from the densely hydroxylated compound 122, formation of stannane 123 and [2,3]-sigmatropic rearrangement under Still s condition easily gave cyclohexene 124, whose hydroboration-oxidation led to protected carbapyranose 125. On the other hand, silylation of the hydroxymethyl moiety and hydration of double bond in... 

Furthennore, the presence of electnxi-withdrawing substituents, such as an allylic hydroxy group, is deactivating, an effect intensified by esterification to a degree such that acylation of an allylic alcohol may be sufficient to protect the double bond during photooxidation at another site. ... 

Figure 33.7 depicts the influence of plasma pretreatment of CRS surface as well as the hydrophilicity of the plasma polymers on corrosion test results. The left half of the bar graph represents hydrophilic interface and some of top surface are also hydrophilic. The right half of the bar graph represents the water-insensitive interface and nonhydrophilic top surfaces except the plasma polymer of CH4, which was intentionally kept in air for 10 min before application of E-coat. The figure indicates two important factors, i.e., the removal of oxides from CRS/plasma polymer interface, and nonhydrophilic top surface of plasma coatings, for corrosion protection of CRS by plasma interface engineering, which involves application of cathodic E-coat. While the air exposure of plasma polymer of CH4 severely deteriorated the corrosion protection of E-coated sample, the same exposure of TMS surface showed no effect. This difference seems to reflect the reactivity of double bonds described in Chapter 7. 

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