Oxazol-5(4$H$)-ones I, compounds derived by cyclization of $N$-acyl $\ce{\upalpha\!\mbox{-}amino}$ carboxylic acids, $\ce{RC(=O)NHCR2C(=O)OH}$, through formal loss of the elements of water. 4-Hydrocarbylideneazlactones II are often referred to as 'unsaturated azlactones'.