Hydroxyl group reactivity

Chemical intuition, based on numerous literature precedents21 26 demonstrating highly varying hydroxyl group reactivities, would have predicted that the major products in the above fucosylation reactions would be those formed by glycosylation... 

F. W. Lichtenthaler, P. Pokinskyj, and S. Immel, Sucrose as a renewable organic raw material New, selective entry reactions via computer simulation of its solution conformations and its hydroxyl groups reactivities, Zuckerind., 121 (1996) 174-190. 

Deposited from solution in TV-methyhnorpholine-TV-oxide, original and crosslinked with a hydroxyl group reactive cross-linker. 

While the chemical structure of a polysaccharide is complex, most are composed of components with similar characteristics, which allow simultaneous manipulation of the entire polymer. Almost all polysaccharide modifications involve either cleaving an oxygen-carbon linkage between monomers or a reaction at a hydroxyl group. Reactivity at hydroxyls is reported either as the degree of substitution (DS), which is the average number of substitutions per monomer, or molar substitution (MS), which is the average number of moles of substituent per monomer. 

The choice of conditions for alkylation (or any protection method) is dictated by the existing functionality within the carbohydrate. Where there are alternatives within a category, the specific choice is usually made on the basis of previous experience or if the method offers a desired level of selectivity for protection of a particular site. For example, the alkylation conditions within category (a) above [with the exception of (vi)] and that under (c) lead to more or less equivalent levels of selectivity (summarised in section 2.2, Hydroxyl group reactivity) and will not be discussed further. However alkylations and acylations proceeding via 0-stannyl intermediates (b) and via copper chelates [(a)(vi)] follow set patterns that may differ from these general trends. These alternatives are discussed within sections 23.1, Benzyl ethers and 2.3.2, Allyl ethers. 

The magnitude of any effect is influenced by its position on the pyranoside. While the substituents affect the reactivity in a predictable manner, the magnitude of this effect depends oti the position of the group in most cases (similarly observed by Ley). This trend is most easUy observed in the tribenzylated thiogalactoside bearing one free hydroxyl group. Reactivity increases, as the hydroxyl group is available at the C6 (2.3) < C2 (3.1) < C3 (5.1) < C4 (11.8)... 

We shall describe a specific synthetic example for each protective group given above. Regiosdective proteaion is generally only possible if there are hydroxyl groups of different sterical hindrance (prim < sec < tert equatorial < axial). Acetylation has usually been effected with acetic anhydride. The acetylation of less reactive hydroxyl groups is catalyzed by DMAP (see p.l44f.). Acetates are stable toward oxidation with chromium trioxide in pyridine and have been used, for example, for protection of steroids (H.J.E. Loewenthal, 1959), carbohydrates (M.L. Wolfrom, 1963 J.M. Williams, 1967), and nucleosides (A.M. Micbelson, 1963). The most common deacetylation procedures are ammonolysis with NH in CH OH and methanolysis with KjCO, or sodium methoxide. 

The reaction of an alcohol with a hydrogen halide is a substitution A halogen usually chlorine or bromine replaces a hydroxyl group as a substituent on carbon Calling the reaction a substitution tells us the relationship between the organic reactant and its prod uct but does not reveal the mechanism In developing a mechanistic picture for a par ticular reaction we combine some basic principles of chemical reactivity with experi mental observations to deduce the most likely sequence of steps... 

Both parts of the Lapworth mechanism enol formation and enol halogenation are new to us Let s examine them m reverse order We can understand enol halogenation by analogy to halogen addition to alkenes An enol is a very reactive kind of alkene Its carbon-carbon double bond bears an electron releasing hydroxyl group which makes it electron rich and activates it toward attack by electrophiles... 

The use of hydroxyethyl (also hydroxypropyl) methacrylate as a monomer permits the introduction of reactive hydroxyl groups into the copolymers. This offers the possibility for subsequent cross-linking with an HO-reactive difunctional agent (diisocyanate, diepoxide, or melamine-formaldehyde resin). Hydroxyl groups promote adhesion to polar substrates. 

Rea.ctlons, Propargyl alcohol has three reactive sites—a primary hydroxyl group, a triple bond, and an acetylenic hydrogen—making it an extremely versatile chemical intermediate. 

Methylbutynol. 2-Methyl-3-butyn-2-ol [115-19-5] prepared by ethynylation of acetone, is the simplest of the tertiary ethynols, and serves as a prototype to illustrate their versatile reactions. There are three reactive sites, ie, hydroxyl group, triple bond, and acetylenic hydrogen. Although the triple bonds and acetylenic hydrogens behave similarly in methylbutynol and in propargyl alcohol, the reactivity of the hydroxyl groups is very different. 

Neopentyl glycol, or 2,2-dimethyl-1,3-propanediol [126-30-7] (1) is a white crystalline soHd at room temperature, soluble ia water, alcohols, ethers, ketones, and toluene but relatively iasoluble ia alkanes (1). Two primary hydroxyl groups are provided by the 1,3-diol stmcture, making this glycol highly reactive as a chemical intermediate. The gem-A methy configuration is responsible for the exceptional hydrolytic, thermal, and uv stabiUty of neopentyl glycol derivatives. 

Following this work, the y -12F-diol was used for the direct reaction with hexamethylene-1,6-diisocyanate in the presence of dibutyltin dilaurate to produce a cross-linked elastomer or a reactive prepolymer which was terminated with either isocyanate or hydroxyl groups, depending on which reactant was in excess (142,143). 

Dibasic Acid Esters. Dibasic acid esters (diesters) are prepared by the reaction of a dibasic acid with an alcohol that contains one reactive hydroxyl group (see Esters, organic). The backbone of the stmcture is formed by the acid. The alcohol radicals are joined to the ends of the acid. The physical properties of the final product can be varied by using different alcohols or acids. Compounds that are typically used are adipic, azelaic, and sebacic acids and 2-ethyIhexyl, 3,5,5-trimethyIhexyl, isodecyl, and tridecyl alcohols. 

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) ... 

Liquid crystal polyesters are made by a different route. Because they are phenoHc esters, they cannot be made by direct ester exchange between a diphenol and a lower dialkyl ester due to unfavorable reactivities. The usual method is the so-called reverse ester exchange or acidolysis reaction (96) where the phenoHc hydroxyl groups are acylated with a lower aHphatic acid anhydride, eg, acetic or propionic anhydride, and the acetate or propionate ester is heated with an aromatic dicarboxyHc acid, sometimes in the presence of a catalyst. The phenoHc polyester forms readily as the volatile lower acid distills from the reaction mixture. Many Hquid crystal polymers are derived formally from hydroxyacids (97,98) and thein acetates readily undergo self-condensation in the melt, stoichiometric balance being automatically obtained. 

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