Described is a process for the preparation of a compound of the formula wherein R is unsubstituted or substituted C1–C12alkyl, C2–C4alkenyl, C2–C4alkynyl, C3–C6cycloalkyl, aryl or heteroaryl, or —SR1; R1 is unsubstituted or substituted C1–C12alkyl, C2–C4alkenyl, C2–C4alkenyl, C2–C4alkynyl, cycloalkyl, aryl or heteroaryl; and X is a leaving group; which comprises a) reacting a compound of the formula wherein R is as defined for formula (I), with a water removing reagent.