Synthesis of the penta‐glutamyl derivative of N‐[4‐[N‐[3‐(2,4‐diamino‐1,6‐dihydro‐6‐oxo‐5‐pyrimidinyl)‐propyl]amino]benzoyl]‐L‐glutamic acid (5‐DACTHF). An acyclic analogue of tetrahydrofolic acid