Mechanistic studies in the chemistry of urea. Part 8. Reactions of urea, 1-methylurea, and 1,3-dimethylurea with some acyloins and butane-2,3-dione (diacetyl) in acid solution

Urea and 1-methylurea react with both aromatic and aliphatic acyloins to form 4-imidazolin-2-ones but, under the same reaction conditions, 1,3-dimethylurea does not react. However, 1,3-dimethylurea does react with diacetyl to form 4,4′-methylenebis-(1,3,5-trimethyl-4-imidazolin-2-one)(4a). The carbon atom lost in the formation of (4a) is eliminated as formaldehyde. We propose a reaction mechanism which involves formation of an oxetan ring. With 1-methylurea and urea the main products of reaction with diacetyl are the bicyclic compounds (6a–c). However, it is possible that reactions analogous to that leading to the formation of (4a) also occur and that the carbonium ion (5c) is the coloured species formed in the well known colorimetric procedure for the determination of urea concentrations in biological liquids.