Polymer acylating agents

Treatment of tertiary benzylamines with acylating agents can lead to debenzylation. If the benzyl group is linked to an insoluble polymer, acylation and debenzylation will lead to the release of an acylated amine into solution (Entry 6, Table 3.9). These cleavage reactions generally yield products that are contaminated with acylating agent and so require further purification. 

It is possible to achieve selective esterification at OH-6 in good yield even with acetyl chloride by employing low temperature in the presence of a controlled amount of acylating agent.13,93 Some tin derivatives (such as dibutyltin oxide) also show significant selectivity towards position 6 (Scheme 7).94,95 A polymer-bound stannyl reagent was also applied for the selective C-6 acetylation of... 

The vapor-phase acylation of pyrrole was carried out over zeolite catalysts such as HZSM-5(19.7), HZSM-5(30), HZSM-5(280), HY, and cation-promoted modified zeolites like CeHZSM-5(30), LaHZSM-5(30), and CeHY, in a fixed-bed reactor at atmospheric pressure using acetic anhydride as an acylating agent <1998CAL95>. The catalytic activity of the zeolite catalysts was dependent on the reaction temperature and the type of cation polymer used in the modification of the zeolite surface. The acylation was found to be more active on Br0nsted acidic sites available over zeolite systems. The yield of l-(l//-pyrrol-2-yl)-l-ethanone 376 with respect to the conversion of pyrrole on HZSM-5(280) at 250 °C was 75.5% (Equation 87). 

In a related and more recent study a variety of solid-supported acylating agents were synthesized and used for microwave-mediated transformation of amines, alcohols, phenols, and thiophenols [123]. In a microwave-mediated procedure, Mer-rifield resin was first modified by attaching 1,4-butanediol to introduce a spacer unit. Bromination and subsequent reaction with commercially available 6-methyl-2-thiouracil then treatment with corresponding acyl chloride afforded the desired polymer-bound pyrimidines (Scheme 16.81). The acylating ability of this supported reagent has been proven by reaction with benzylamine. 

Scheme 16.82. A polymer-supported imide-based acylating agent.

Finally, while it is not obvious how both the amine-component and the carboxyl component could be applied in polymeric form, this dilemma was solved (Cohen et al. 1981) by the introduction of a mediator. A solution of imidazole is circulated through separate beds of the insoluble reactive carboxyl-component and the polymer-bound amine-component. The acyl-imidazoles generated in the active ester bed are excellent acylating agents which transfer... 

The mechanism of DCC coupling involves conversion of the carboxyl group of the acid to a good acylating agent (steps 1 and 2) followed by acylation of the free amino group of the polymer-bound amino acid (steps 3 and 4). 

A lipase-catalyzed acylation of comb-like methacrylate polymers was induced through OH groups in the side chains using the activated esters, vinyl acetate, phenyl acetate, 4-fluorophenyl acetate, and phenyl stearate, as acylating agents. The OH groups in the side chains of the methacrylate/styrene copolymer were... 

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