You seem to be equivalencing 'structure field' and 'workspace variable' names - they are two different things. If you are trying to load a .mat file variable into a different named variable from your lookup table, I think you would have to first use something like matfile to map the .mat file then use the eval function to create the workspace new variable:
% Given: oldVar is a string or char array containing the original var name
% newVar is a string or char array containing the desired var name
% fileName is a string or char array containing the .mat filename
matMap = matfile(fileName);
eval([newVar '= matMap.' oldVar ';']);
If trying to rename a structure's fields maybe something like:
% Given: oldField is a string or char array containing the original field name
% newField is a string or char array containing the desired field name
% varName is the structure name
% fileName is a string or char array containing the .mat filename
matMap = matfile(fileName);
varName.(newField) = matMap.varName.(oldField); % loop/repeat this for all varName fields
