Lead acetate Erlenmeyer azlactone synthesis

The preparation of the first unsaturated azlactone was reported in 1883 by Plochl/40 who condensed benzaldehyde with hippuric acid in presence of acetic anhydride. This approach was later used by Erlenmeyer/41 who extended the procedure to include other aldehydes and also established the usefulness of azlactones as intermediates in the synthesis of DHAs. The method involves the condensation of an A-acylglydne 4 with aldehydes and ketones in the presence of acetic anhydride and anhydrous sodium acetate (Scheme 2)J41 t5l Other catalysts such as copper(II) acetate/46 lead acetate/47,48 potassium carbonate/49 or potassium hydrogen carbonate 50 have also been used. The reaction proceeds via formation of an azlactone 5, which then condenses with the appropriate aldehyde or ketone to give unsaturated azlactone 6. Reaction of 6 with a nucleophile such as OH, OR, or NHR leads to the corresponding A-acyl-DHA derivatives 7. Reaction with the sodium salt of an amino acid gives a DHA containing dipeptide acid. 51 ... 

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