Medium‐sized heterocycles: Synthesis of a 2,10‐Bis(alkoxycarbonyl)‐substituted 6‐Oxo‐12H‐dibenzo[d,g][1,3,2]dioxaphosphocin

The synthesis and characterization of a 2,10‐bis(alkoxycarbonyl)‐substituted 6‐oxo‐12H‐dibenzo[d,g][1,3,2]‐dioxaphosphocin, 6, is described. The lack of an observable 5JHP coupling of phosphorus to the downfield C‐12 methylene proton and the observation of a 2JHCH geminal coupling constant of 14 Hz suggest that the conformation of 6 in solution is not solely a boat‐chair conformation, but may involve a rapid equilibrium of the boat‐chair with either a boat or twist‐boat conformation.