Bases weak solid-phase

Prepai ative isolation of nonvolatile and semivolatile organic compounds fractions (hydrophobic weak acids, hydrophobic weak bases, hydrophobic neutrals, humic and fulvic acids) from natural and drinking waters in optimal conditions was systematically investigated by solid-phase extraction method with porous polymer sorbents followed by isolation from general concentrate of antropogenic and/or toxic semivolatile compounds produced in chlorination and ozonation processes. 

The two most commonly used types of allyl alcohol linker are 4-hydroxycrotonic acid derivatives (Entry 1, Table 3.7) and (Z)- or ( )-2-butene-l, 4-diol derivatives (Entries 2 and 3, Table 3.7). The former are well suited for solid-phase peptide synthesis using Boc methodology, but give poor results when using the Fmoc technique, probably because of Michael addition of piperidine to the a, 3-unsaturated carbonyl compound [167]. Butene-l,4-diol derivatives, however, are tolerant to acids, bases, and weak nucleophiles, and are therefore suitable linkers for a broad range of solid-phase chemistry. 

Support-bound triacylmethanes (e.g. 2-acetyldimedone) readily react with primary aliphatic amines to yield enamines. These are stable towards weak acids and bases, and can be used as linkers for solid-phase peptide synthesis using either the Boc or Fmoc methodologies, as well as for the solid-phase synthesis of oligosaccharides [456]. Cleavage of these enamines can be achieved by treatment with primary amines or hydrazine (Entries 2 and 3, Table 3.23 see also Section 10.1.10.4). 

Silyl ethers of aliphatic alcohols are inert towards strong bases, oxidants (ozone [81], Dess-Martin periodinane [605], iodonium salts [610,611], sulfur trioxide-pyridine complex [398]), and weak acids (e.g., 1 mol/L HC02H in DCM [605]), but can be selectively cleaved by treatment with HF in pyridine or with TBAF (Table 3.32). Phenols can also be linked to insoluble supports as silyl ethers, but these are less stable than alkyl silyl ethers and can even be cleaved by treatment with acyl halides under basic reaction conditions [595], Silyl ether attachment has been successfully used for the solid-phase synthesis of oligosaccharides [600,601,612,613] and peptides [614]. 

Silyl ethers that have been used in solid-phase synthesis include TES, TIPS, TBS, and TBDPS ethers. Polystyrene-bound phenols can be converted into TIPS ethers by treatment with TIPS-OTf/imidazole in DMF for 5 min [105], These silyl ethers are stable towards bases and weak acids, but can be selectively removed by treatment with TBAF (Entries 6 and 8, Table 7.8) or pyridinium hydrofluoride (THF, 25 °C, 15 h [24,75,106]). 

Numerous researchers have employed thiols as weak bases in the thioalkylation reaction to ligate unprotected peptides with a haloacetyl group to form thioethers at pH 7 8.5[90 91 131-133 or thioesters at acidic to basic conditions. 108"110 Of these two reactions, thioether formation is often the choice because thioesters suffer from instability in aqueous basic conditions. Haloacetyl derivatives, either as carboxylic acids or active esters, can be attached to either the N-terminal or side-chain amines during the stepwise solid-phase synthesis of either the peptide or the core and are stable to either HF or TFA cleavage conditions. Capping an amino group with a chloroacetyl group is compatible with Fmoc chemistry when used at a terminal step. 

In adsorption chromatography the mobile phase is usually a liquid and the stationary phase is a finely-divided solid adsorbent (liquid-solid chromatography). Separation here depends on the selective adsorption of the components of a mixture on the surface of the solid. Separations based on gas-solid chromatographic processes are of limited application to organic mixtures. The use of ion-exchange resins as the solid phase constitutes a special example of liquid-solid chromatography in which electrostatic forces augment the relatively weak adsorption forces. 

The Fmoc group protection is common in solid-phase peptide synthesis. Fmoc is resistant to acidic conditions and easily deprotected by weak bases, particularly secondary amines. Deprotection occurs through base-catalyzed abstraction of the (3-proton of the protecting group with elimination leading to formation of dibenzofulvene (1.83) (Scheme 1.36). 

For reactions carried out in homogeneous solution or under solid-phase conditions the use of Fmoc amino acid chlorides is limited by the competition between their aminolysis and the formation of the less reactive oxazol-5(4//)-ones in the presence of tertiary amines, which are essential components of such reaction systems. To improve the results under these conditions a hindered base, e.g. 2,6-di-/er/-butylpyridine, can be used as a hydrogen chloride acceptor since conversion to oxazol-5(4//)-one is slow with such bases. Although shown to be advantageous in certain cases, Fmoc amino acid chlorides are used in homogeneous solution synthesis only in particular cases. They react efficiently in the presence of pyridine with weak nucleophiles such as imine 2P l (Scheme 2) where other activated species such as an active ester, anhydride, acyl fluoride, and acyl imidazolide fail. 

The purification procedure should remove potentially interfering compounds and, moreover, fractionate the entire spectrum of cytokinins into groups. It is usually not possible to analyze nucleotides and O-glucosides together with free bases, ribosides and iV-glucosides. Classical liquid-liquid partition steps [274] have been recently replaced by less labor-intensive, rapid and selective solid-liquid extraction. When applied in neutral aqueous solutions, cytokinin nucleotides are retained on weak anion-exchangers (DEA -Sephadex, DEAE-cellulose), while the other cytokinin forms are retained on reversed phase sorbents. Dobrev and Kaminek [275] used mixed-mode solid-phase extraction for step-wise... 

The gas-phase exothermicity is due to the enhanced resonance stabilization of aniline compared to benzene for the nonzwitterionic amino acids, as found in the gas phase. On the other hand, the aniline resonance stabilization is lost in the zwitterionic amino acids of the solid phase and thus the reaction is essentially thermoneutral. This is, of course, related to the weak basicity of aniline compared to related nonaromatic bases such as cyclohexylamine, as exhibited by the ca 50 kJ mol-1 exothermicity of the formal reaction 29. 

In the spectral area with low Si/Al ratio, besides a strong peak for the species of Al(OH), several broadened, weak peaks in the range 72 80ppm could be found in the 27A1-NMR spectra, which is a typical feature of 5-coordinated intermediates. This structure is unstable in solution, but very stable in the solid phase. The aluminate compounds containing 5-coordinated Al species can be found in nature. One example is andalusite. To interpret the polymerization reaction that occurred in aluminosilicate solution, 5-coordinated aluminate was assumed to be existed. Based on the assumption of the presence of 5-coordinated aluminate, Dent Glasser and colleagues proposed another mechanism (described below) for the polymerization of silicate and aluminate ions in the solution,[41] which was named the 5-coordinated intermediate mechanism or base-delayed mechanism. The reactions will follow the steps described in Equation (5.7) ... 

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