extracting diagonals from 3d matrices

I try to extract diagonals from 7*7*217 matrices into 1*7*217. How can i make it? I tried below code but it did't work. I would appreciate if you help me.

for k=1:217
D(:,:,k)=diag(diag(L(:,:,k)));
end

Answer 1

[Matt J]

   Lr=reshape(L,1,49,[]);
   D=Lr(1,1:8:end,:);