Derivatives Solid esters

Of the common esters, methyl oxalate (solid, m.p. 54°) and ethyl oxalate (liquid) give amides almost immediately upon shaking with concentrated ammonia solution. The resulting oxamide, m.p. 417°, is valueless as a derivative. The esters may, however, be easily hydrolysed and identified as above. 

Solid esters are easily crystallisable materials. It is important to note that esters of alcohols must be recrystallised either from non-hydroxylic solvents (e.g. toluene) or from the alcohol from which the ester is derived. Thus methyl esters should be crystallised from methanol or methanol/toluene, but not from ethanol, n-butanol or other alcohols, in order to avoid alcohol exchange and contamination of the ester with a second ester. Useful solvents for crystallisation are the corresponding alcohols or aqueous alcohols, toluene, toluene/petroleum ether, and chloroform (ethanol-free)/toluene. Esters of carboxylic acid derived from phenols... 

Acid-labile linkers are the oldest and still the most commonly used linkers for carboxylic acids. Most are based on the acidolysis of benzylic C-O bonds. Benzyl esters cleavable under acidic conditions were the first type of linker to be investigated in detail. The reason for this was probably the initial choice of polystyrene as an insoluble support for solid-phase synthesis [13]. Polystyrene-derived benzyl esters were initially prepared by the treatment of partially chloromethylated polystyrene with salts of carboxylic acids (Figure 3.3). 

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]. 

Fats are solid esters of glycerol and (mostly) saturated acids. Oils are liquid esters that are derived primarily from unsaturated acids and glycerol. The acid portion of a fat usually contains an even number of carbon atoms, often 16 or 18. Some acids that occur frequently in fats and oils are... 

TABLE XVI. ORGANIC DERIVATIVES OF ESTERS b) Solids (Listed in order of increasing m.p.) ... 

PTC is a method of choice for the Wittig-Horner reaction of carbanions derived from esters of alkanephosphonic acids, activated with additional car-banion stabilizing groups, with aldehydes and ketones.. E-Alkenes are usually formed as the main products in good yields. Solid-liquid system KOH/benzene with 18-crown-6 as the catalyst is recommended for these processes however, K2C03/H20/dioxane or 50% NaOH aq was also used. Although the yields are sometimes low, because of the simplicity of the procedure, it can be favorably compared with other base/solvent systems used for this reaction (eqs. 72 and 73). 

Improvements in the analysis of benzoylecgonine from urine have been reported recently from three groups [14,23,24]. All techniques used El in conjunction with SIM, and all employed deuterated benzoylecgonine as an internal standard. Solid phase extraction in conjunction with one step esterification of benzoylecgonine into either its propyl or isopropyl esters resulted in LODs of at least 10 ng/ml [23]. An automated technique was developed utilizing SPE and trimethylsilyl derivatization that has the capability of analyzing over 100 samples in 2 hr. No LODs were reported [24]. Finally, SPME was utilized to extract the hexylchloroformate derivative (hexyl ester) of benzoylecgonine from urine (Fig. 3). The LOD was reported to be 30 ng/ml for this rapid, solvent-free extraction technique [14]. 

---