Short answer: don't used these variable names: https://www.mathworks.com/matlabcentral/answers/304528-tutorial-why-variables-should-not-be-named-dynamically-eval, instead use a 2D array or a cell array.
The next best thing to do it to use 'a1', 'a2', ..., 'a3' as filednames of the struct. For example
myStruct.a1=[1 2 3 4];
myStruct.a2=[10 11 15 11];
myStruct.b2=[4 5 6 7];
myStruct.c2=[8 9 10 11];
x={'a1' 'a2' 'b2' 'c2'}; % you need a cell array here. The code line in your question will not work
Access them like this
>> myStruct.(x{1}) % same as myStruct.a1
ans =
1 2 3 4
>> myStruct.(x{3})
ans =
4 5 6 7
As a last resort. If you are insistent on using these variable names, then use eval(): https://www.mathworks.com/help/matlab/ref/eval.html or evalc(): https://www.mathworks.com/help/releases/R2020a/matlab/ref/evalc.html
a1=[1 2 3 4];
a2=[10 11 15 11];
b2=[4 5 6 7];
c2=[8 9 10 11];
x={'a1' 'a2' 'b2' 'c2'}; % you need a cell array here. The code line in your question will not work
Access them using eval
>> eval(x{1})
a1 =
1 2 3 4
>> eval(x{4})
c2 =
8 9 10 11
