Hydroxyl groups Subject

The phosphonium and carbenium salts are efficient reagents for activating and coupling A-alkoxycarbonylamino acids as well as peptide acids. However, the requirement for tertiary amine to effect the reaction has several implications. The base renders hydroxyl groups subject to acylation. Hence, the side chains of serine and threonine and any hydroxymethyl groups of a resin that have not been derivatized... 

This reaction has been used m an imaginative way to determine the ring size of glycosides Once all the free hydroxyl groups of a glycoside have been methylated the glycoside is subjected to acid catalyzed hydrolysis Only the anomeric methoxy group IS hydrolyzed under these conditions—another example of the ease of carbocation for matron at the anomeric position... 

Chemical Properties. Lignin is subject to oxidation, reduction, discoloration, hydrolysis, and other chemical and enzymatic reactions. Many ate briefly described elsewhere (51). Key to these reactions is the ability of the phenolic hydroxyl groups of lignin to participate in the formation of reactive intermediates, eg, phenoxy radical (4), quinonemethide (5), and phenoxy anion (6) ... 

The effect substitution on the phenolic ring has on activity has been the subject of several studies (11—13). Hindering the phenolic hydroxyl group with at least one bulky alkyl group ia the ortho position appears necessary for high antioxidant activity. Neatly all commercial antioxidants are hindered ia this manner. Steric hindrance decreases the ability of a phenoxyl radical to abstract a hydrogen atom from the substrate and thus produces an alkyl radical (14) capable of initiating oxidation (eq. 18). 

Hydroxyl Group. The OH group of cyanohydrins is subject to displacement with other electronegative groups. Cyanohydrins react with ammonia to yield amino nitriles. This is a step in the Strecker synthesis of amino acids. A one-step synthesis of a-amino acids involves treatment of cyanohydrins with ammonia and ammonium carbonate under pressure. Thus acetone cyanohydrin, when heated at 160°C with ammonia and ammonium carbonate for 6 h, gives a-aminoisobutyric acid [62-57-7] in 86% yield (7). Primary and secondary amines can also be used to displace the hydroxyl group to obtain A/-substituted and Ai,A/-disubstituted a-amino nitriles. The Strecker synthesis can also be appHed to aromatic ketones. Similarly, hydrazine reacts with two molecules of cyanohydrin to give the disubstituted hydrazine. 

In the endoplasmic reticulum of eukaryotic cells, the oxidation of the terminal carbon of a normal fatty acid—a process termed ch-oxidation—can lead to the synthesis of small amounts of dicarboxylic acids (Figure 24.27). Cytochrome P-450, a monooxygenase enzyme that requires NADPH as a coenzyme and uses O, as a substrate, places a hydroxyl group at the terminal carbon. Subsequent oxidation to a carboxyl group produces a dicarboxylic acid. Either end can form an ester linkage to CoA and be subjected to /3-oxidation, producing a... 

The reactions of olefins with peracids to form epoxides allows for the selective oxidation of carbon-carbon double bonds in the presence of other functional groups which may be subject to oxidation (for example, hydroxyl groups). The epoxides that result are easily cleaved by strong acids to diols or half-esters of diols and are therefore useful intermediates in the synthesis of polyfunctional compounds. 

To clarify the relationship between intermediate 11 and its predecessor, intermediate 12, it is instructive to recognize the correspondence between the C-ll hydroxyl group and the C-6 carboxyl group in 11. Thus, even though the structural relationship between intermediates 11 and 12 may not appear to be close, subjection of... 

Subjection of intermediate 16 to the action of 3 n aqueous HC1 in THF results in the formation of monocyclic lactol 14. In the presence of aqueous acid, the internal acetal grouping in intermediate 16 is hydrolyzed and lactol 14 is produced after the liberated secondary hydroxyl group attacks the terminal aldehyde carbonyl positioned five atoms away (see intermediate 15). Protection of the free aldehyde function in 14 with 1,1-dimethylhydrazine proceeds smoothly under dehydrating conditions and affords intermediate 13 in an overall yield of 72 %. 

Protein phosphorylation-dephosphorylation is a highly versatile and selective process. Not all proteins are subject to phosphorylation, and of the many hydroxyl groups on a protein s surface, only one or a small subset are targeted. While the most common enzyme function affected is the protein s catalytic efficiency, phosphorylation can also alter the affinity for substrates, location within the cell, or responsiveness to regulation by allosteric ligands. Phosphorylation can increase an enzyme s catalytic efficiency, converting it to its active form in one protein, while phosphorylation of another converts it into an intrinsically inefficient, or inactive, form (Table 9—1). 

L-dihydroxy-succinic acid (L(dexiro)-tartaric acid, CXIII). This result establishes the position of the double bond between C4 and C5 and demonstrates that C4 carries only one hydrogen atom while C5 has attached to it the enolic hydroxyl group. Treatment of the enol CXI with ethereal diazomethane gives 5-methyl-A4-D-glucosaccharo-3,6-lactone methyl ester (CXIY) which upon further methylation with silver oxide and methyl iodide yields 2,5-dimethyl-A4-D-glucosaccharo-3,6-lactone methyl ester (CXV). When the latter is subjected to ozonolysis there is formed oxalic acid and 3-methyl-L-threuronic acid (CXVI). Oxidation of this aldehydic acid (CXYI) with bromine gives rise to a monomethyl derivative (CXVII) of L-ilireo-dihydroxy-succinic acid. 

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