Is y a single value?
I'm not entirely sure I know what you're asking for, but maybe something like this can work?
y = randi(100,1,1);
x = randi(50,5,1);
c = 1;
xc = [];
while length(x)>1
z = y-sum(xc)-x; % Calculate z values for all x
xc(c) = x(z==max(z)); % Record x which results in max z ( I know you want max R^2 but you didn't give a relationship for z and R^2)
x = x(x~=xc(c)); % Remove recorded x value from x
c = c + 1;
end
xc(c) = x; % Record last value of x
