How can I synthesize 2‐amino‐3‐(chlorosulfonyl)benzoic acid from toluene or benzene?
Here is a proposed route starting from aniline.
First protect the -NH2 with t-BOC
Directed lithiation of t-BOC-aniline (sBuLi/Et2O, -75C) and quench with MeSSMe to install -SMe in the 2 position.
Directed lithiation under the same conditions quenching with Ethyl Chloroformate to install  -CO2Et in the 6 position.
Oxidise the -SMe group to sulfoxide (MCPBA, DCM)
Pummerer rearrangement of the sulfoxide (Ac2O, aq. base work up) gives the free thiol.
Oxidation of the thiol to the sulfonic acid, one example of possible conditions hydrogen peroxide with Rh catalysis
Form the sulfonyl chloride (conditions using trichlorotriazene here. Depending on your choice of conditions you can remove the -BOC group or preserve it. I think preserving it will give a flexible late stage intermediate that is reasonably easily handled. It is possible to directly form the sulfonyl chloride from the -SMe with chlorine in aq acetic acid conditions here but the conditions are harsh and risk removing the -BOC group