Crystal and Molecular Structure of 5-Ethoxytrimethylenephosphoric Acid

5-Ethoxytrimethylenephosphoric acid crystallizes in the monoclinic space group P2₁/c with a = 5.416 (1)Å, b - 18.687 (4)Å, c = 16.677 (3)Å, β = 92.50 (1)°, and Z = 8. The structure was solved by direct methods and refined to an R value of 0.040 for 2276 unique reflections. The cyclic phosphate rings of the two molecules in the asymmetric unit are both in chair conformations, and both ethoxy groups are oriented axially. A comparison of the structure of this monocyclic phosphodiester with the cyclic phosphodiesters found in glycoside cyclic phosphates, including 3',5'-cyclic nucleotides, reveals that the axial orientation of the ethoxy group observed in the present study and the equatorial orientation of the endocyclic glycoside oxygen atoms in the other cyclic esters is the most striking structural difference. This difference is important in the explanation for the unusual thermochemical behavior of the glycoside cyclic phosphates.