Conversion of meso to dl form and vice versa in bisphosphonates:: Synthesis and structures of new meso, dl and (meso plus dl) bisphosphonates derived from cyclic phosphites and dialkyl acetylenedicarboxylates

The bisphosphonates [(OCH2CR2CH2O)P(O)CHC(O)OR'](2) [R = R' = Me (5), R = Me, R':= Et (6) R = Et, R' = Me (7), R = R' = Et (8)], [CH2(6-t-Bu-4-MeC6H4O)(2)P(O)CHC(O)OR](2) [R = Me (9), Et (10)] and [(OCH2CMe2-CH2O)P(S)CHC(O)OR]2 [R = Me (11), Et (12)] have been prepared by the reaction of the corresponding cyclic phosphites with dimethyl acetylenedicarboxylate or diethyl acetylenedicarboxylate in the presence of a base [Et3N or n-BuLi]. The structures of dl-, meso- and (dl + meso) forms [6a, 6b and 6ab] of 6 as well as the meso form of 8 are determined from X-ray crystallography. By means of combined X-ray structural and P-31 NMR spectroscopic investigations, it is shown that the meso form can be converted to the dl form and vice versa. A possible rationale invoking the intermediacy of an enolic form is proposed for this observation.