Solid acids ester formation

Other Gas Reactions. Several other reactive gases or vapours were examined but found to be unsatisfactory. No ester formation ( 1745 cm"1) was found when oxidatized films were exposed to acetic acid or formic acid vapour. Alcohol/carboxylic acid reactions in the solid state have often been suggested as the source of ester products, but not substantiated (4,5). Gaseous ammonia reacted with carboxylic acid groups to give absorptions at 1550 cm"1 [-C(=0)-0 ] and 1300 cm"1 (NHi +). However, these absorptions were very broad and the method inferior to acid measurement by SF. Although N20 did not react with oxidized polyolefins, the reaction of N02 with oxi-... 

A comparison of the synthesis of Z-Phe-Leu-NH2 in ten different solvents revealed that the highest overall yields could be expected in solvents where the substrate solubility is minimized. The highest yields in terms of solid product were found in solvents where both substrate and product solubility are minimized [45]. These simple rules may not hold when special factors apply, such as the formation of solid solvates. This may account for a few apparent exceptions, such as the product precipitation in dichloromethane of both a peptide and a sugar fatty acid ester [45, 63]. 

Less reactive than acyl halides, but still suitable for difficult couplings, are symmetric or mixed anhydrides (e.g. with pivalic or 2,6-dichlorobenzoic acid) and HOAt-derived active esters. HOBt esters smoothly acylate primary or secondary aliphatic amines, including amino acid esters or amides, without concomitant esterification of alcohols or phenols [34], HOBt esters are the most commonly used type of activated esters in automated solid-phase peptide synthesis. For reasons not yet fully understood, acylations with HOBt esters or halophenyl esters can be effectively catalyzed by HOBt and HOAt [3], and mixtures of BOP (in situ formation of HOBt esters) and HOBt are among the most efficient coupling agents for solid-phase peptide synthesis [2]. In acylations with activated amino acid derivatives, the addition of HOBt or HOAt also retards racemization [4,12,35]. 

Diazines other than diketopiperazines can also be prepared on insoluble supports (Table 15.31 see also Figure 3.13 [382]). Most strategies are based on intramolecular nucleophilic substitutions or acylations. Several examples of the solid-phase preparation of quinoxalinones have been reported. In most cases, the compounds have been prepared from support-bound 2-fluoronitrobenzenes according to the strategies outlined in Figure 15.18. Alternatively, a-amino acid esters bound to polystyrene as IV-benzyl derivatives can be N-arylated with 2-fluoronitrobenzene. Reduction of the resulting 2-nitroaniline leads to the formation of quinoxalinones [383]. 1,4-Diazines have been chemically modified by N- or C-alkylation on insoluble supports (Entries 9 and 10, Table 15.31). 

Amino acid esters can be monodentate (1) or bidentate (2), and examples of both types of complex have been isolated and their solid state IR spectra studied.39 41 Many investigations have shown that formation of the monodentate ester species has similar effects to protonation of the a-amino group. Thus the pKa values of MNH2CH(R)C02H and NH3CH2C02H (carboxyl ionization) are usually quite similar.42... 

Addition of halogen fluorosulfates XOS02F (X=C1, Br, I) to fluoroolefins is considered an electrophilic reaction [8]. However, the question of whether this process is concerted or the reaction proceeds via an independent carbocationic intermediate (Eqs. 2 and 3) is still open. Formation of carboxylic acid esters as byproducts in the reaction of HFP with C10S02F, which was carried out in trifluoroacetic or heptafluorobutyric acids as solvents, could not be a solid proof of conjugate addition, since formation of esters may be a result of addition of C10C(0)Rf to olefin. These materials are known to be formed in the reaction of ClOS02F with fluorinated carboxylic acids, even at low temperature [99]. 

Volatile fatty acids are by-products in the formation of long-chain fatty acids, which are required for cell membrane phospholipid biosynthesis. The biosynthesis of volatile fatty acids is generally controlled by the same factors that control the formation of ethyl fatty acid esters, that is, oxygen, ergosterol and various insoluble solids (grape solids, clarification solids, yeast hulls) tends to suppress production whereas sugar concentration and clarification are stimulatory (Bardi et al. 1999 Delfini et al. 1992, 1993 Edwards et al. 1990 Houtman et al. 1980). 

Effect of Acyl Donors. TTie synthesis of glucose fatty acid esters was investigated with continuous by-product removal in a stirred-tank membrane reactor by azeotropic distillation using EMK containing 20% hexane as reaction solvent and different fatty acids as acyl donors. From previous studies on the lipase-catalyzed synthesis of glucose esters in a solid-phase system (17,19,22,23), it was already known that the fatty acid chainlength had a considerable influence on product formation. This was due... 

The acetic acid in the homogeneous system neutralises the amount of sodium hydroxide added, so that the phenolphthalein is decolorised. A solid residue of sodium acetate is obtained when the solution is evaporated to dryness. The ester formation can be promoted by the addition of small amounts of concentrated sulfuric acid (or P4O10) as these bind the water which is formed (eqn. 1)... 

Villalgordo et al. [22, 23] as well as Gayo and Suto [25] developed a strategy to cleave pyrimidines from the solid support. After oxidation of the thioether-linkage 17, aromatic substitution of the sulfonyl unit was performed with different N-nucleophiles as amines and azides to give free amino- or azido-pyrimidines 19 (Scheme 16.5). To demonstrate the stability of the linker, the resin-bound derivatives were subjected to different reactions such as saponification, ester reduction, acid chloride formation or Mitsunobu alkylation. A similar approach was presented later on by Hwang and Gong in the SPOS of 2-aminobenzoxazoles [26]. 

The synthesis of 2,4-dihydroxybenzophenone (compound 4 Eq. 5) was investigated over various solid catalysts by reacting a 1 1 molar mixture of resorcinol with benzoic acid in chlorobenzene as solvent, water resulting from ester formation being removed [3,22]. 

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