mula (I) which comprises reacting the carboxylic acid (II) or an acid addition salt thereof with propionic anhydride and the reaction of theresultant product with methanol.The reaction of the compound of formula (II) with propionic anhydride is conveniently carried out in an aprotic solvent such as a hydrocarbon e.g. toluene or an ester e.g. ethyl acetate or isopropyl acetate and preferably in the