Esterification, during chromatography

Initially, the oxidation conditions chosen for 154 to 157 were the modified ruthenium tetraoxide conditions of Sharpless and co-workers.44 The crude oxidation products were converted to methyl esters 106,160, 161, and 162 and their C-2 epimers 163 to 166 using either diazomethane or trimethylsilyldiazomethane.77 The epimer ratios were determined from integration of the H NMR spectra of the crude esterification products to ensure that accurate ratios were obtained without losing minor isomers during chromatography. The results obtained are summarized in Scheme 60 and Table 15. 

Problems may also be caused by oxidation (see Section III,E), complex formation with metal ions (see Section IV,G), and by esterification. Furthermore, amino acids with low solubilities (cystine and tyrosine) may precipitate at the starting point and only be dissolved slowly during chromatography. 

The reasons for derivatizing carbamates, apart from their thermolability during the gas chromatographic analysis, include increased detector sensitivity (particularly with electron capture detection), increased volatility, better chromatography separation, apphcability to multiresidue and confirmatory analysis, and enhanced compound stability. There are two general approaches to the analysis of Af-methylcarbamates by derivatization, namely derivatization of the intact pesticides and derivatization of a hydrolysis product (one of which will always be the volatile methylamine). The reactions typically used to obtain derivatives of both intact and hydrolysis products of A-methylcarbamates are methylation, silylation, halogenation, acylation, and esterification. 

Polysaccharide fraction B, isolated from ammonium oxalate extraction, was a pectin of low degree of esterification. This material could not be fractionated satisfactorily by ion-exchange chromatography since only a small proportion was eluted from DEAE-Sephadex with neutral buffers. A more characteristic pectin sample, which was possibly lost during CWM preparation, was... 

The subsequent diastereoselective reduction of artemisinic acid to dihydroarte-misinic acid works on a rhodium catalyst in excellent yield and with high selectivity. There follows an esterification, since the esters show better selectivity during the ensuing oxidation steps. The latter can be performed without purification of the intermediate in a quasi one-pot reaction. Artemisinin is obtained after chromatography in 41 % yield. 

The oleic acid esterification is particularly interesting from the perspective of biodiesel production. Oleic acid is present in different type of vegetable oils, as well as also in domestic and industrial, therefore, it is required to study esterification of this fatty acid mainly when ethyl or methyl alcohol are employed during the proeess. Oleic acid as well as its alkyl esters are easily deteetable by gas chromatography analysis (Figure 4). 

Esterification of carboxylic acids followed by hydrocarbon extraction and gas chromatography has been widely used (e.g., Willey et al. 1975 Carothers and Kharaka 1978 Kharaka et al. 1985). The main advantage of this method is the ability to detect and quantify some of the longer-chain acids that are not detectable by other methods. The disadvantages are that (1) many of the esters of these carboxylic acids are more hydrophilic than hydrophobic, and thus are never extracted into the analate and (2) the short-chain difunctional acid esters are apt to either precipitate as insoluble Ca or Mg salts during evaporative concentration of the extract, or totally decarboxylate at the injection port, and thus are either not analyzed or are analyzed as CO2, respectively (Kawamura et al. 1986 Alltech Associates 1985). This method is also expensive and time-consuming. 

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