Efficient synthesis and crystal structure of 2‐propyl‐5‐(substituted)phenyl‐1,4‐dioxo‐1,2,3,4,5,6,7,8‐octahydro‐[1,4,2]diazaphosphorino[1,2‐a] [1,3,2]benzodiazaphosphorine 3‐oxides