may be prepared from the corresponding compounds of the formula i wherein r 1 is benzyl hereinafter referred to as compounds of the formula i g as described below and illustrated in scheme 5 first a compound of the formula i g is reacted with a chloroformate of the formula str10 wherein r 9 is clchch 3 ch 2 ch 3 or ch 2 c 6 h 5 the preferred reactant is 1 chloroethyl chloroformate this reaction which affords the corresponding compound of the formula xiv is generally carried out in a reaction inert solvent such as methylene chloride chloroform dichloroethane thf or toluene preferably toluene at a temperature from about 60 u00b0 c to about 100 u00b0 c preferably from about 80 u00b0 c to about 85 u00b0 c heating the compound of formula xiv so formed in a c 1 c 4 alcohol preferably methanol or ethanol yields the corresponding compound of the formula xv the reaction temperature may range from about 80 u00b0 c to about the reflux temperature of the solvent and is preferably about the reflux temperature of the solvent the compound of the formula xv formed in the foregoing step is then alkylated and thus converted into the corresponding compound of formula i by reacting it with a compound of the formula r 1 l wherein l is a leaving group in the presence of a base examples of suitable leaving groups are chloro bromo iodo mesylate tosylate and triflate otf suitable bases include pyridine triethylamine dimethylaminopyridine and potassium carbonate triethylamine is preferred generally the.