Synthesis of D-cycloserine

Comment and reaction mechanism 

It is an extremely simple synthesis, conceptually speaking.  As starting reagent we have one of the 20 natural aminoacids, D-serine. The first reaction is a transesterificazione (Fisher transesterification, acid-catalyzed) with methanol. As result of the reaction we are protecting at the same time the carboxylic and the amino function. It's easy then to chlorinate the hydroxyl function with phosphorous pentachloride. In the end we close the 5-membered cycle with hydroxylamine, where the selectivity of the reaction is well explained by the hard-soft theory.