Amine alkylation represents a common method to get to imine precursors. After protonation, a cyclic [3,3]-sigmatropic rearrangement occurs producing an imine. Subsequent hydrolysis of the imine leads to release of the aldehyde product, allysine. NBS electrophilically brominates the amine, which is followed by decarboxylation and release of an imine. Ammonium ion is added forming an imine and releasing ammonia. Hydrolysis of the imine derives "N"-carbamoylputrescine followed with hydrolysis of the urea to form putrescine. It resembles a pyrrole more than a simple imine. PLP forms an imine with the amino acid derivative. The methylamine-derived ammonia is then released, restoring the original imine. The resulting chloramine undergoes dehydrohalogenation to afford the cyclic imine. The final acid is generated by hydrolysis of the imine. The imine nitrogen is basic and exhibits pi-acceptor properties. Aldehydes can also form using a hydrogen donor followed by "in-situ" hydrolysis of an imine. The process is catalysed by metal oxides and is assumed to proceed via the imine. A rearrangement in carbohydrate chemistry involving an imine is the Amadori rearrangement.