A-Acylimidazoles

Acyclic dienes, 433 Acylation, Friedel-Crafts, 329 A-Acylimidazoles, 78 Acyloxyphosphonium salts, 79 Adamantane, 354... 

A-Acylimidazoles are easily reduced to the corresponding aldehydes with LiAlH4 in THF or ether as solvent.[1] Thus, aliphatic, conjugated aliphatic, aromatic, conjugated aromatic, and heteroaromatic aldehydes can all be obtained in this way in moderate to high yields. 

Irradiation of 1-acyl-1,2,4-triazoles, contrary to A-acylimidazoles affords no photo-Fries products, but instead forms aldehydes and other products via the corresponding acyl radicals.t9]... 

The mechanism of CDI-mediated acylation of amines is well understood. The first step involves a partial protonation of the basic imidazole-nitrogen, protonated A-acetylimidazole has a p a of 3.6,f l leading to an activated species which is then attacked by the carboxylate. The resulting mixed anhydride extrudes carbon dioxide giving rise to A-acylimidazole which on treatment with an amine compound leads to the desired anoide (Scheme 1). An important advantage of this method over the carbodiimide method is that the byproducts carbon dioxide and imidazole are readily and quantitatively separated from the reaction product by simple washing procedures. 

Acylation of imidazole produces A-acylimidazoles via loss of proton from the initially-formed A-3-acyl-imidazolium salt. A-Acyl-imidazoles are even more easily hydrolysed than A-acyl-pyrroles moist air is sufficient. The ready susceptibility to nucleophilic attack at carbonyl carbon has been capitalised upon commercially available l,l -carbonyldiimidazole (CDI), prepared from imidazole and phosgene, can be used as a safe, phosgene synthon, and also in the activation of acids for formation of amides and esters via the A-acyl-imidazole. ... 

To our surprise, compound 58 refused to undergo Af-acylation, decomposing more or less rapidly during all such attempts. Exposure to mild acylating agents, such as A-acylimidazoles or 4-nitrophenyl esters in the presence of HOBt, induced slow degradation and produced none of the desired products. Complete destruction of the substrate ensued upon... 

The major classes of carbonyl compounds include aldehydes, ketones, carboxamides, esters, carboxylic acids and anhydrides, and carbonyl halides (acyl halides). These groups differ in the identity of the substituent X on the carbonyl group. At this point we concentrate on these examples, but a number of other carbonyl derivatives have important roles in synthetic and/or biological reactions. These other compounds include acyl cyanides, acyl azides, A-acylimidazoles, 0-aryl esters, and thioesters. The carbonyl compounds are arranged below in the order of the increasing reactivity toward nucleophilic addition. 

RCOOH RCH2OH. On conversion into the A-acylimidazole, a carboxylic acid becomes reducible to the primary alcohol by NaBH in the presence of water. ... 

In another application, A -acylimidazoles react with lithium aluminium hydride at... 

By the same token, acylation of imidazole produces A>acylimidazoles via loss of proton from the initially formed A -3-acylimidazolium salt. A device which has been employed frequently for the synthesis of 1-acylimidazoles is to use two mol equivalents of the heterocycle for one of the acylating agent, the second mole of imidazole serving to deprotonate the first-formed A-acylimidazolium salt. 

Acyl halides, activated esters, carboxylic anhydrides and A(-acylimidazoles react widi alkylidenepbos-phoranes to yield acylated... 

Protection of an alcohol function by esterification sometimes offers advantages over acetal-protecting groups such as the tetrahydropyranyl ethers. Generally, acetals are stable in base and labile in acid, while esters are more stable in acid than acetals and are readily hydrolyzed in base. Esters are especially useful in oxidations, but are not suitable in organometallic reactions. Acetates and benzoates are the most common ester-protecting groups they can be conveniently prepared by reaction of unhindered alcohols with acetic anhydride or benzoyl chloride, respectively, in the presence of pyridine or other tertiary amines. The use of A-acylimidazoles (imidazolides) allows the reaction to be carried out in the absence... 

Oxazoline was shown to be an efficient directing group for arylation of arene derivatives with Ru(II) catalyst [(Eq. 5)] [61], A -Acylimidazole was also shown to be a directing group promoting ortho monoarylation of arene. Ort/io-alkenylation of 2-phenyl oxazoline with alkenylbromide was performed using the same mthe-nium(II) catalyst [61]. 

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