Acetic anhydride cyclodehydration agent

It was shown previously that saturated 5(4//)-oxazolones or 2-oxazolm-5-ones with only one substituent at C-4 can be considered as the tautomeric form of saturated 5(2//)-oxazolones or 3-oxazolin-5-ones. These compounds can also be considered as amino acid derivatives and, indeed, cyclization procedures are the most commonly used to prepare these compounds. The cyclization reaction employs a variety of cyclodehydrating agents and the general method is shown in Scheme 7.23, with an A-acyl-a-amino acid being the most typical starting material used. In this way, 5(4//)-oxazolones derived from most natural amino acids 99 (R3 = H) have been obtained by heating the corresponding A-acyl derivatives in the presence of acetic anhydride. 

The first procedure to prepare unsaturated 5(4//)-oxazolones was the Erlenmeyer synthesis" " that was described more than one hundred years ago and is still used extensively with some variations in the experimental conditions. In general, the reaction employs an acylamino acid, for example, A-acetyl- or A-benzoylglycine are the most common, and a carbonyl compound, usually an aldehyde, in the presence of a cyclodehydrating agent such as acetic anhydride (Scheme 7.114). Hundreds of unsaturated oxazolones 363 have been obtained via this procedure and these compounds are valuable intermediates for the synthesis of many interesting organic compounds. 

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