Preparation of alkylthiobenzimidazoles

The invention concerns a new process for the preparation of biologically active 2-[(alkoxycarbonyl)amino]-5-(alkylthio)-1H-benzimidazoles of the formula (V) (V) wherein R and R1 independently stand for alkyl having 1 to 3 carbon atoms. According to the invention compounds of the formula (V) are prepared by reacting carbamic acid alkyl esters of the formula (I) (I) wherein R is as defined above, with (a) chlorosulfonic acid or (b) oleum to yield a sulfonic acid of the formula (II) (II) wherein R is as defined above, and subsequently with a chlorinating agent, reducing a sulfonic acid chloride of the formula (III) (III) wherein R is as defined above, obtained by process variant (a) or (b) and reacting a benzimidazole-thiol of the formula (IV) (IV) obtained, wherein R is as defined above, with an alkyl halide of the formula (VI) R1-Hal (VI) wherein R1 is as defined above and Hal represents a halogen atom.