The present invention provides a process for the production of a compound of formula (I) or a salt thereof, wherein R is hydrogen, a straight, branched or cyclic C1-C6 alkyl group or an aryl group which may optionally be substituted. This process comprises: (a) reacting a compound of formula (II), with a compound of formula (III), wherein R is as defined above and X is a leaving group, in the presence of N,N-di-isopropylethylamine.