Esters crown catalyzed formation

A particularly useful application of the crown mediated ester synthesis is in the preparation of phenacyl esters [21, 22]. The crown catalyzed reaction of potassium carboxylates with a,4-dibromoacetophenone yields derivatives which are usually crystalline and which can be readily detected by uv spectroscopy [23]. Moreover, the phenacyl ester is a useful protecting group which can be removed under mild conditions [24]. Phenacyl ester formation has also been catalyzed by tertiary amines [5]. Examples of the condensation according to equation 6.7 are given in Table 6.6. 

Potassium chromate in HMPT reacts with a variety of substrates in a crown catalyzed nucleophilic displacement. Both dicyclohexyl- and dibenzo-18-crown-6 catalyze the formation of chromate esters which decompose according to equation 11.5 to yield oxidized product [11]. 

The Merrifield solid phase peptide synthesis requires the formation of an esterified resin as the first step. The substitution of an N-protected amino acid for a chloride in chloromethylated polystyrene must be quantitative or products which are one amino acid too short will ultimately be isolated. 18-Crown-6 has been shown to catalyze quantitative ester formation between the potassium salts of boc-amino acids and chloromethylated resin in DMF solution. Dichloromethane was reactive under these conditions [26]. The results of these displacement reactions according to equation 6.10 are presented in Table 6.8. 

The reactions of halide and azide ions (see above), cyanide ion (see Chap. 7) and thiocyanate ions (see Sect. 13.7) have all been discussed. In the general context of halides and pseudohalides, it should be noted that nitrite ion has been successfully phase-transferred [31, 32]. The yields for the formation of nitroalkanes in either crown [31 ] or quaternary ion catalyzed [32] processes are fair to good for primary alkanes and poor for secondary systems. Bromocyclohexane, for example, afforded only traces of nitrocyclohexane when treated with nitrite ion, the by-products resulting presumably from oxygen attack (nitrite ester formation) or elimination. The conversion of n-octy bromide into the corresponding nitro-compound is formulated in equation 9.14 and several examples of the transformation are recorded in Table 9.6. 

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