A CONVENIENT SYNTHESIS OF PHOSPHONIC ANHYDRIDES - TRIMERS [RPO2](3) (R = TERT-BUTYL, 2-METHYLPHENYL, 2,4,6-TRIMETHYLPHENYL) - THEIR STRUCTURES AND REACTION-PRODUCTS

By reaction of RP(O)Cl-2 with RP(O)(OSiMe3)(2), phosphonic anhydrides (RPO2)(3) (R = tBu, 2-methylphenyl, 2,4,6-trimethylphenyl) Za-e are co nveniently obtained. In contrast to 1b and 1c, compound 1a is remarkab ly stable against protolysis. Intermediates of hydrolysis of 1a, namel y tris(tert-butyl)triphosphonic acid (2) and bis(tert-butyl)diphosphon ic acid (3), can also be isolated in good yield. The structures of 1-3 were determined mainly by NMR spectroscopy (H-1, C-13, P-31). Assumin g an energetic preference for the chair conformations in solution, and considering the steric requirements of the bulky substituents R, conf igurations Ia (point group C-s, two R in equatorial positions) for 1a and b, and IIa (point group C-3v, all R equatorial) for 1c are suggest ed. - Reaction of 1a with N-benzyloxycarbonylglycine (4) in methanol a ffords strong evidence that in the first step of peptide synthesis wit h (RPO2)(3), a mixed anhydride of triphosphonic acid and the N-protect ed amino acid is formed. - The crystal structure of 1a (monoclinic, sp ace group P2(1)/n) widely corresponds to the suggested configuration I a, but reveals an envelope conformation for the six-membered ring with a P3O2 plane in the crystal. In the crystal structure of the octahydr ate of the disodium salt of 2 (monoclinic, space group P2(1)/c), it ca n be seen that the polar end groups of the anions [tBu(3)P(3)O(7)](2-) together with the water molecules and the Na+ cations form hydrogen-b onded double-layers, strictly separated from each other by the non-pol ar tert-butyl groups of the anions.