The reaction may be carried out by treatment of the compound of formula (13) with a base, for example an inorganic base such as a hydroxide, e.g. sodium hydroxide in a solvent such as an alcohol, e.g. ethanol, at an elevated temperature e.g. the reflux temperature, followed by acidification of the reaction mixture to a pH of around pH4 to around pH6 using an acid such as an inorganic acid, e.g. hy