Process for producing sulfonic acid esters

A method for producing a sulfonate ester compound, which comprises reacting (a) a compound having a sulfo group (-SO3H); wherein the compound having a sulfo group is a compound represented by the general [4] formula: R4-SO2-OH [4] and (b) a compound having a group represented by the general formula [5]: R5- O-R1 [5] in the presence of an organic base that is capable of forming a salt with said sulfo group, wherein the organic base is a secondary amine, a tertiary amine or a quaternary ammonium salt, wherein R4 represents a group alkyl optionally substituted alkyl group containing heteroatoms optionally substituted, an alkenyl group optionally substituted, an alkynyl group optionally substituted, an optionally substituted aryl or heterocyclic group optionally substituted; R1 represents a sulfonyl group represented by the general formula [2]:-R2 -SO2 [2] wherein R2 represents a halogen atom, a haloalkyl group, an alkoxy group, an optionally substituted alkyl group or an aryl group optionally substituted; or an acyl group represented by the general [3] formula: wherein R3 represents an optionally substituted alkyl group or an aryl group optionally substituted; and R5 represents an alkyl group optionally substituted; alkyl group containing heteroatoms optionally substituted, an alkenyl group optionally substituted, an alkynyl group optionally substituted, an optionally substituted aryl group or a heterocyclic group optionally substituted.