ating for example at the reflux temperature, a diacid of formula (V) where R is --CO.sub.2 H with an acyl chloride such as acetyl chloride.The homochiral acids of formula (IIa) may be prepared according to another feature of the invention by oxidation of an oxazolidinone of formula (VIII) ##STR12## (where Ph is a phenyl group) using an oxidizing agent such as peroxide, e.g. hydrogenperoxide in a s