Acid anhydrides from acylimidazoles

The requisite W-methoxy-W-methylamides may be pre >ared from acid chlorides by employing a slight excess of the commercially available W,G-dimetiiylhydroxylamine hydrochloride in the presence of pyridine. They have also been pre >ared from acylimidazoles and from mixed anhydrides of carboxylic acids. Once prepared, these systems X)ssess stability equivalent to that of most tertiary amides and, in... 

A soln. of 1-heptanoylimidazole prepared from heptanoic acid and N,N -thiongl-diimidazole in tetrahydrofuran allowed to react with the Mg-enolate of monoethyl malonate (s. Synth. Meth. 14, 888) ethyl heptanoylacetate. Y 74%.— The use of mild acylating agents, such as 1-acylimidazoles or carboxylic alkoxy-formic anhydrides, prevents diacylation, which occurs with acid chlorides. F. e. and modifications s. G. Bram and M. Vilkas, Bl. 1964, 945. 

Thioesters.—Mixed anhydrides prepared from 2,4,6-trichlorobenzyl chloride and a carboxylic acid react with various thiols in the presence of 4-dimethyl-aminopyridine to give thioesters in 78—86% isolated yield.Somewhat less impressive yields were obtained in a few of the cases studied when l-fluoro-2,4,6-trinitrobenzene was used to couple the acid and the thiol.Treatment of 1-acylimidazoles with thiols in the presence of a catalytic amount of Mg(OEt)2 furnishes thioesters in good yields. Diphenyl-2-oxo-3-oxazolinylphosphonate brings about effective reaction between acids and thiols. Thiolacetates are efficiently prepared using the reaction of an alcohol with triphenylphosphine and di-isopropyl azodicarboxylate in the presence of thiolacetic acid/ A synthesis of some amino-acid derivatives containing the thioester functional group is achieved by the reaction of a vinyloxyborane with a Schiffs base (Scheme 56). ... 

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