the outset step d comprises esterifying a compound of formula ia with an alcohol of formula roh to produce compound of formula ib esterifying with an alcohol of formula roh to produce compound of formula ib is typically performed in an alcohol such as methanol ethanol n propanol i propanol n butanol i butanol 2 butanol or t butanol preferably methanol in the presence of a mineral acid or a substance generating a mineral acid in situ e g socl 2 the reaction temperature depends on the boiling temperature of the alcohol used and as a rule lies in the range of 60 u00b0 c to 150 u00b0 c the reaction time is about 1 to 24 hours preferably 1 hour the following examples shall illustrate the invention in more detail without limiting it 11 05 g 2 4 lutidine 0 1 mol were dissolved in 50 ml thf the solution was cooled and stirred maintaining the temperature at between u2212 70 to u2212 50 u00b0 c 81 25 ml nbuli 0 13 mol was added over 0 25 hour the dark red solution was stirred for a further 0 5 hour then 15 14 ml diethylamine 0 145 mol was added in.