Unexpected degradation of the bisphosphonate P-C-P bridge under mild conditions

Unexpected degradation of the P-C-P bridge from novel bisphosphonate derivative 1a and known etidronate trimethyl ester (1b) has been observed under mild reaction conditions. A proposed reaction mechanism for the unexpected degradation of 1a and 1b is also reported.

Typical examples of 1 H and 31 P NMR spectra of 1b decomposition experiment in triethyl amine S16 General experimental details: 1 H, 31 P and 13 C NMR spectra were recorded on a Bruker Avance 500 spectrometer operating at 500.1, 202.5 and 125.8 MHz, respectively. TMS or TSP (for D 2 O solutions) was used as an internal standard for 1 H and 13 C measurements, and 85% H 3 PO 4 was used as an external standard for 31 P measurements. The 3 J HH couplings are indicated by the letter " J" . The n J HP couplings were calculated from proton spectra and all J values are given in Hz. The n J CP couplings were calculated from proton decoupled carbon spectra with the coupling constants given in parenthesis as hertz. The purity of the compound 1a was determined from 1 H and 31 P NMR spectra and was ca. 96 %. Electronspray ionization mass spectra were acquired by an LCQ quadrupole ion trap mass spectrometer with an electronspray ionization source.
Procedure for the preparation of etidronate derivative 1a: Acetylated etidronic acid (5) (150 mg, 0.60 mmol), Na 2 CO 3 (388 mg, 3.66 mmol) and ethyl chloroformate (3 ml) was refluxed for overnight under the nitrogen atmosphere. Reaction mixture was evaporated to dryness, diethyl ether (8 ml) was added and reaction mixture was stirred for 15 minutes before the precipitate was removed by centrifugation. The remaining solution was evaporated to dryness and compound 1a was obtained as colorless oil in 55% yield. Typical example of 1a degradation experiment: Compound 1a (70.5 mg, 0.143 mmol) was dissolved in MeOH (2 ml), 40% NaOH (58 µl, 4 eq) was added and the reaction mixture was stirred for 0.5 h at room temperature. Reaction mixture was evaporated to dryness and dried in vacuo. Product (ca. 63 mg) was obtained as white powder and contained mixture of acetate and phosphites 2-4 as identified from 1 H and 31 P NMR spectra (see pages S12-13).
Typical example of 1b degradation experiment: Compound 1b (50 mg) was dissolved in H 2 O (1 ml), pH was adjusted to 11 with 6 M NaOH (this needed only 1 pasteur pipette drop) and the reaction mixture was stirred for 1 h at room temperature before evaporating it to dryness in vacuo.
Product was obtained as white powder and contained mixture of acetate and phosphite 4 as S3 identified from 1 H and 31 P NMR spectra (see pages S14-15; in 31 P NMR spectrum the remaining H-P coupling can be seen because of insufficient decoupling power).
Typical example of 1b degradation experiment in triethyl amine: Compound 1b (50 mg) was dissolved in H 2 O (1 ml), triethyl amine (122 µl, 5 eq) was added and the reaction mixture was stirred for 1 h at 60 ºC before evaporating it to dryness in vacuo. Product was obtained as slightly yellow solid and contained mixture of acetylphosphonate 7 and phosphite 3 as identified from 1 H and 31 P NMR spectra (see pages S16-17; in 31 P NMR spectrum the remaining H-P coupling can be seen for the compound 3 because of insufficient decoupling power).