Acylureas carboxylic acid esters

Carboxylic acid amides and urethans or carboxylic acid esters from acylureas... 

The esterification of support-bound carboxylic acids has not been investigated as thoroughly as the esterification of support-bound alcohols. Resin-bound activated acid derivatives that are well suited to the preparation of esters include O-acylisoureas (formed from acids and carbodiimides), acyl halides [23,226-228], and mixed anhydrides (Table 13.15). A-Acylurea formation does not compete with esterifications as efficiently as it does with the formation of amides from support-bound acids. Esters can also be prepared from carboxylic acids on insoluble supports by acid-catalyzed esterification [152,229]. Alternatively, support-bound carboxylic acids can be esteri-fied by O-alkylation, either with primary or secondary aliphatic alcohols under Mitsu-nobu conditions or with reactive alkyl halides or sulfonates (Table 13.15). 

Carboxylic acids react under Pfitzner-Moffatt conditions, resulting in the formation of methylthiomethyl esters and /V-acylureas.41 Nevertheless, although the authors are not aware of any report involving the selective oxidation of alcohols in the presence of a carboxylic acid, such outcome would be likely with carboxylic acids with little nucleophilicity, as standard Pfitzner-Moffatt oxidations are performed in the presence of trifluoroacetate that is known for not to interfere. 

In general, hydrazides may be prepared by many of the methods analogous to those used in the preparation of amides. For example, hydrazine salts of carboxylic acids and reactions of hydrazine with esters, acyl halides, acyl anhydrides, and amides may be used to produce hydrazides. A reaction analogous to the Hofmann degradation is the formation of hydrazides from ureides (acylureas) [54] (Eq. 4). 

Ester and Thioester Formation. These reactions occur through the same O-acylurea or anhydride active intermediate as in the amide coupling reactions, and the discussion of associated problems applies here as well. In general, alkyl and (particularly) aryl thiols can be efficiently coupled to carboxylic acids using DCC. Reactions of primary and secondary alcohols proceed reliably, but require the presence of an acylation catalyst. This is usually 4-Dimethylaminopyridine (DMAP), " (see also 1,3-Dicyclohexylcarbodiimide—4-Dimethylaminopyridine), but others have been used including 4-pyrrolidinopyridine and pyridine (.solvent) with catalytic p-Toluenesulfonic Acid The acylation of more hindered alcohols often re.sults in reduced yields however, even f-butanol can be acylated, providing a useful route to t-butyl esters. Various other carbodiimide derivatives have also been used in the preparation of esters. As with amides, which are not limited to intermolecular reactions, a wide variety of lactones can also be synthesized. ... 

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