Esterification solid phase

During the first decade when solid-phase synthesis was executed using Fmoc/tBu chemistry, the first Fmoc-amino acid was anchored to the support by reaction of the symmetrical anhydride with the hydroxymethylphenyl group of the linker or support. Because this is an esterification reaction that does not occur readily, 4-dimethylaminopyridine was employed as catalyst. The basic catalyst caused up to 6% enantiomerization of the activated residue (see Section 4.19). Diminution of the amount of catalyst to one-tenth of an equivalent (Figure 5.21, A) reduced the isomerization substantially but did not suppress it completely. As a consequence, the products synthesized during that decade were usually contaminated with a small amount of the epimer. In addition, the basic catalyst was responsible for a second side reaction namely, the premature removal of Fmoc protector, which led to loading of some dimer of the first residue. Nothing could be done about the situation,... 

Chromatographic fixed-bed reactors consists of a single chromatographic column containing a solid phase on which adsorption and reaction take place. Normally a pulse of reactant is injected into the reactor and, while traveling through the reactor, simultaneous conversion and separation take place (Fig. 3). Since an extensive overview of the models and applications of this type of reactor was presented by Sardin et al. [ 132], only a few recent results will be discussed here. Most of the practical applications have been based on gas-liquid systems, which are not applicable for the enzyme reactions, but a few reactions were also reported in the liquid phase. One of these studies, performed by Mazzotti and co-workers [ 141 ], analyzed the esterification of acetic acid into ethyl acetate according to the reaction ... 

Transesterification is the main reaction of PET polycondensation in both the melt phase and the solid state. It is the dominant reaction in the second and subsequent stages of PET production, but also occurs to a significant extent during esterification. As mentioned above, polycondensation is an equilibrium reaction and the reverse reaction is glycolysis. The temperature-dependent equilibrium constant of transesterification has already been discussed in Section 2.1. The polycondensation process in the melt phase involves a gas phase and a homogeneous liquid phase, while the SSP process involves a gas phase and two solid phases. The respective phase equilibria, which have to be considered for process modelling, will be discussed below in Section 3.1. 

Besides direct solid phase fixation, the functional OH-group of 49 may be used for further transformations. Esterification with acetyl chloride at the OH-linker (Scheme 27) leads to 50, providing a heteroscorpionate ligand with the protected OH-linker. An intramolecular hydrogen bridge is found in the X-ray structure [d(01-N21) = 2.478 (3) A] between the carboxylic acid and the pyrazole nitrogen (Fig. 32b). 

Following extraction/cleanup, quinoxaline-2-carboxylic acid can be detected by electron capture, or mass spectrometric techniques, after gas chromatographic separation on capillary or conventional columns. A prerequisite of quin-oxaline-2-carboxylic acid analysis by gas chromatography is the derivatization of the molecule by means of esterification. Esterification has been accomplished with methanol (419, 420, 422), ethanol (421), or propanol (423) under sulfuric acid catalysis. Further purification of the alkyl ester derivative with solid-phase extraction on a silica gel column (422), thin-layer chromatography on silica gel plate (420), or liquid chromatography on Hypersil-ODS, 3 m, column (423), has been reported. 

Tertiary aliphatic alcohol linkers have only occasionally been used in solid-phase organic synthesis [73], This might be because of the vigorous conditions required for their acylation. Esterification of resin-bound linker 4 with /V-Fmoc-prolinc [72,74] could not be achieved with the symmetric anhydride in the presence of DMAP (20 h), but required the use of /V-Fmoc-prolyl chloride (10-40% pyridine in DCM, 25 °C, 10-20 h [72]). A further problem with these linkers is that they can undergo elimination, a side reaction that cannot occur with benzyl or trityl linkers. Hence, for most applications in which a nucleophile-resistant linker for carboxylic acids is needed, 2-chlorotri-tyl- or 4-acyltrityl esters will probably be a better choice than ferf-alkyl esters. 

Support-bound, enantiomerically pure alcohols can be converted into phosphonates by Mitsunobu esterification, which results in complete inversion at the stereo-genic center. This strategy has been used to prepare peptidyl phosphonates on solid phase. These are interesting transition-state analogs with potential utility as peptidase inhibitors (Figure 11.3 [12,13]) or tyrosine phosphatase inhibitors [14]. Serine or threonine derivatives can be converted into phosphonates by direct phosphonylation with an activated monoalkyl phosphonate [15] or by treatment with phosphonamidites RP(OR)NR2 in the presence of tetrazole followed by oxidation [16]. 

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]. 

The goal of most studies of esterification reactions on insoluble supports has been the attachment of N-protected amino acids to polystyrene-derived alcohols. In recent years, however, the scope of esterifications has been expanded to other types of alcohol and acid. In the following section, the most important strategies for the preparation of esters on solid phase are discussed. These have been organized according to type of functionality that is initially linked to the support. 

Support-bound primary or secondary aliphatic alcohols can be acylated under conditions similar to those used in solution, provided that these conditions are compatible with the chosen linker. For instance, acids can be activated with a carbodiimide either as symmetric anhydrides or as O-acylisoureas, which quickly react with alcohols in the presence of a catalyst, such as DMAP or another base, to yield esters (Table 13.12). Further acid derivatives suitable for esterification reactions on solid phase include acyl halides and imidazolides. HOBt esters react only slowly with alcohols, but enable the selective acylation of primary alcohols in the presence of secondary alcohols (Entry 5, Table 13.12). 

Standard solid-phase peptide synthesis requires the first (C-terminal) amino acid to be esterified with a polymeric alcohol. Partial racemization can occur during the esterification of N-protected amino acids with Wang resin or hydroxymethyl polystyrene [200,201]. /V-Fmoc amino acids are particularly problematic because the bases required to catalyze the acylation of alcohols can also lead to deprotection. A comparative study of various esterification methods for the attachment of Fmoc amino acids to Wang resin [202] showed that the highest loadings with minimal racemization can be achieved under Mitsunobu conditions or by activation with 2,6-dichloroben-zoyl chloride (Experimental Procedure 13.5). iV-Fmoc amino acid fluorides in the presence of DMAP also proved suitable for the racemization-free esterification of Wang resin (Entry 1, Table 13.13). The most extensive racemization was observed when DMF or THF was used as solvent, whereas little or no racemization occurred in toluene or DCM [203]. 

Solid-phase preparation of trans 3-alkyl (3-lactams was reported by Mata et al. recently (Scheme 14) [103]. The synthetic sequence involved the start from Fmoc-glycine tethered to Wang resin, followed by addition of a controlled excess of 43 (4 equiv) and triethylamine (8 equiv) to the resin bound imine 42 in refluxing toluene. Cleavage form resin surface and esterification afforded the 3-alkyl (3-lactams, 45, as a single product with excellent trans-selectivity. 

Water concentrations in liquid and solid phases were measured by the Karl Fisher method using the Karl Fisher Titrator (Mettler DL 18). Butanol and butyl butyrate were determined by gas chromatography using a 6-ft, 5% DEGS on a Chromosorb WHP, 80/10 mesh column (Hewlett Packard, Palo Alto, CA) and hexanol as internal standard. Acid consumption was monitored by volumetric titration of samples diluted in ethanol employing 0.02 M KOH alcoholic solution and phenolphthalein as pH indicator. Esterification was expressed as molar percent of consumed reactant, according to Eq. 1 ... 

For immobilized lipase preparations, a more complex mechanism is expected to occur since esterification efficiency is also highly dependent on the hydration state of the enzyme preparation, which can be greatly modified by the nature of the substrate and the support (1,4)- In the case of butyl butyrate synthesis, analysis of substrate polarity measured as partition coefficient (Table 1) showed a higher value for butanol than for butyric acid, favoring butanol migration to the solid phase (immobilized lipase). Thus, there should be more alcohol than acid at the active site of the immobilized lipase, requiring an excess of acid in the reaction medium to provide equimolar amounts of reactants and satisfactory yields (7). 

After completion of each esterification reaction, the medium was centrifuged, and the solid phase (immobilized enzyme) was recovered according to the method described by Langone and Sant Anna (11). Then, the experiments were performed reusing the enzyme recovered after eachbatch reaction assay. 

Following this, a reaction of type A + C is considered. Many esterification and ester hydrolysis reaction, can be brought to this form. The analysis reveals that, for the latter reaction, total conversion is always possible provided that the feed concentration is sufficiently low. In contrast to this, limitations will arise for high feed concentrations similar to the previous case - that is, if the reactant has highest or lowest affinity to the solid phase. It is worth noting that these results are consistent with experimental observations [28]. 

For a higher accuracy of the isotherm, the activities in the liquid phase instead of the concentrations can be used. If a thermodynamic model for the computation of the solid-phase activities exists, the equilibrium concentration q and qi can also be calculated from the condition of equal activity, as was done for example for the esterification of acetic acid on Amberlyst 15 using the Flory-Huggins activity model for the solid phase and a UNI FAC model for the liquid phase [17]. 

Y. Miura, Y. Shinohara, J.-i. Furukawa, N. Nagahori, and S.-I. Nishimura, Rapid and simple solid-phase esterification of sialic acid residues for quantitative glycomics by mass spectrometry, Chem. Eur. J., 13 (2007) 4797 1804. 

In view of the rapid reaction of carbodiimides with water they are often used in dehydration reactions. Major examples are the intra- and intermolecular esterification reactions of carboxylic acids, and the formation of peptides from carboxylic acids and protected amino acids. Especially, dicyclohexylcarbodiimide (DCC) or diisopropylcar-bodiimide (DIPCD) are often used in carbodiimide mediated reactions because the corresponding urea byproducts are insoluble in most organic solvents and water, and therefore are readily removed by filtration. Also, water soluble carbodiimides, such as N-ethyl-N -(3-dimethylamino)propylcarbodiimide (EDC) or its hydrochloride (EDCCl, sometimes referred to as EDAC) are often used in these reactions. EDC reacts with carboxyl groups at pH of 4.0-6.0, but loses its reactivity at lower pH. Sometimes solid phase reactions are conducted using carbodiimide terminated linear or crosslinked polymers. 

Figure 5 Chemoenzymatic approaches for the production of novel bioactive compounds. In this example, the enzymatic buildup of the linear precursor of daptomycin by its NRPSs (DptA, DptBC, and DptD) is substituted by solid-phase synthesis (a). By using the 4 Ppan transferase Sfp and the CoA-thioester of the linear peptide, the opo-enzyme PCP-TE and be modified, and after trans-esterification cyclized by the TE domain (b). Because the resulting ho/o-enzyme cannot be modified again, this is a single turnover reaction. Another strategy uses thiophenole-esters of the linear peptides to be cyclized (c). When these compounds are used, no PCP domain is necessary. The TE domain is readily acylated, and regiospecific and stereospecific cyclization toward daptomycin or, depending on the linear peptide provided, toward variants thereof occurs. Because the enzyme is not altered in any way after product release, this setup results in a multiple turnover.

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