An improved process for the preparation of pyrimidine derivatives is provided comprising reacting a Wittig reagent of the general formula wherein R is an alkyl of from 1 to 10 carbon atoms, aryl or arylalkyl, R1 is a substituted or unsubstituted hydrocarbon group, R2 and R3 are the same or different and are hydrogen or a substituted or unsubstituted hydrocarbon group; Z is sulfur, oxygen, sulfonyl, or imino which may be substituted by formyl, acetyl, propionyl, butyryl, isobutyryl, valeryl, isovaleryl, amino substituted by sulfonyl or alkylsulfonyl, and sulfonyl substituted by alkyl, amino or alkylamino and X is a halogen; with an aldehyde of the general formula wherein R4 is hydrogen, a lower alkyl or a cation capable of forming a non-toxic pharmaceutically acceptable salt and each R5 are the same or different and are hydrogen or a hydrolyzable protecting group, or each R5, together with the oxygen atom to which each is bonded, form a hydrolyzable cyclic protecting group, or each R5 is bonded to the same substituent which is bonded to each oxygen atom to form a hydrolyzable protecting group; in the presence of a base.