In the Search for New Anticancer Drugs. Part 17 . Linear and Cyclic Polyether Analogues of N,N:N′,N′ :N",N"‐Tri‐1,2‐ethanediylphosphoric Triamide and N,N:N′,N′:N",N"‐Tri‐1,2‐ethanediylphosphorothioic Triamide.