a known compound and may be prepared for example from 5 fluorouracil as described by g j durr in synthetic procedures in nucleic acid chemistry w w zorbach amp r s tipson eds page 92 or by r duschinsky in u s pat no 3 040 026 2 4 bis alkylthio 5 fluoropyrimidines are prepared by contacting 2 4 dichloro 5 fluoropyrimidine with an alkyl mercaptan in the presence of a base the reaction conditions employed are similar to those used for related exchange reactions of 2 and 4 chloropyrimidines see for example the pyrimidines by d j brown from the series the chemistry of heterocyclic compounds edited by weissberger and taylor the reaction is usually conducted in nonaqueous media using at least stoichiometric ratios of reactants thus for each equivalent of 2 4 dichloro 5 fluoropyrimidine at least 2 0 equivalents of alkyl mercaptan are employed while a stoichiometric excess of alkyl mercaptan might not be detrimental to the reaction such an excess is not generally recommended unless adequate provisions are made for the recovery and recycle or for the destruction of so odorous a reagent since it is desirable to convert the alkyl mercaptan completely into the corresponding thiolate anion for each equivalent of.