Cleavage reagent, nucleophilic

The Si—S bond has some similarities with the Si—O bond. However, its resistance to cleavage by nucleophilic reagents is considerably smaller. In contrast to the C—S bond the Si-S linkage very readily undergoes solvolytic cleavage in water and alcohols. Silthianes are hydrolyzed even by atmospheric moisture, leading to the corresponding silanols. 

Substituted hydrazones of i6a,i7a-epoxypregnan-20-ones undergo epoxide cleavage and nucleophilic substitution at C(i ), providing a very convenient route to i6a,i7a-dihydroxy-pregnan-20-ones [21]. Ethoxycarbonylhydrazine appears to be the most successful reagent, and the reaction probably follows the route ... 

Substitutions. If heterolytic, these can be classified as nucleophilic or electrophilic depending on which reactant is designated as the substrate and which as the attacking reagent (very often Y must first be formed by a previous bond cleavage), a. Nucleophilic substitution (Chapters 10, 13). 

Depending on the chemical structure of the anchor and chemistry of its attachment to the resin, the product can be cleaved at the end of the synthesis either with acid, base or nucleophilic cleavage reagents, hydrogenolysis, enzymatic, palladium-catalyzed or photochemical, oxidative and reductive cleavage methods. In addition, the safety-catch anchors can be chemically modified at the end of the synthesis to provide a structure which is subsequently cleavable. 

The haloacetophenone type of polymer reacted with N-protected amino acids and peptides — as already known from esterifications with the monomer reagent in conventional syntheses [74] — under milder conditions but in better loading yield compared to chloromethylated polystyrene, resulting in enhanced reactivity of the peptide phenacyl ester bond on polymer towards nucleophilic cleavage reagents [74—76]. (For further details on peptide cleavage, see Sect. 3.5.)... 

Substitution Reactions on Side Chains. Because the benzyl carbon is the most reactive site on the propanoid side chain, many substitution reactions occur at this position. Typically, substitution reactions occur by attack of a nucleophilic reagent on a benzyl carbon present in the form of a carbonium ion or a methine group in a quinonemethide stmeture. In a reversal of the ether cleavage reactions described, benzyl alcohols and ethers may be transformed to alkyl or aryl ethers by acid-catalyzed etherifications or transetherifications with alcohol or phenol. The conversion of a benzyl alcohol or ether to a sulfonic acid group is among the most important side chain modification reactions because it is essential to the solubilization of lignin in the sulfite pulping process (17). 

Isothiazoles and 2,1-benzisothiazoles are stable to most nucleophilic reagents, but quaternized compounds are dequaternized and/or suffer cleavage of the N—S bond. 

Chiral oxazolines developed by Albert I. Meyers and coworkers have been employed as activating groups and/or chiral auxiliaries in nucleophilic addition and substitution reactions that lead to the asymmetric construction of carbon-carbon bonds. For example, metalation of chiral oxazoline 1 followed by alkylation and hydrolysis affords enantioenriched carboxylic acid 2. Enantioenriched dihydronaphthalenes are produced via addition of alkyllithium reagents to 1-naphthyloxazoline 3 followed by alkylation of the resulting anion with an alkyl halide to give 4, which is subjected to reductive cleavage of the oxazoline moiety to yield aldehyde 5. Chiral oxazolines have also found numerous applications as ligands in asymmetric catalysis these applications have been recently reviewed, and are not discussed in this chapter. ... 

No direct nucleophilic substitution of the hydrogen atoms in the isoxazole nucleus a or y to the nitrogen is as yet known. Thus, the Chichibabin reaction fails in the isoxazole series because of the cleavage of the heterocyclic nucleus under these conditions. It is the lability of the isoxazole ring toward nucleophilic reagents that makes the chemical behavior of isoxazole fundamentally different from that of pyridine. 

Methods of synthesis for carboxylic acids include (1) oxidation of alkyl-benzenes, (2) oxidative cleavage of alkenes, (3) oxidation of primary alcohols or aldehydes, (4) hydrolysis of nitriles, and (5) reaction of Grignard reagents with CO2 (carboxylation). General reactions of carboxylic acids include (1) loss of the acidic proton, (2) nucleophilic acyl substitution at the carbonyl group, (3) substitution on the a carbon, and (4) reduction. 

The preparative value of this compound lies in the surprising fact that bis(l,3-diphenylimidazolidinylidenc-2) behaves in many reactions ie.g., with aromatic aldehydes,2,7 and with carbon acids 2 7-fJ) as if it dissociated to form a nucleophilic carbene. The hydrolytic cleavage of these derived imidazolidine derivatives makes possible the preparation of formyl compounds, so that the amino olefin can be considered as a potential carbonyla-tion reagent. In many reactions it is not necessary to isolate... 

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