Abstract
The bisphosphonates [(OCH2 CR2 CH2 O)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-Me-C6 H4 O)2 P(O)CHC(O)OR]2 [R = Me (9 ), Et (10 )] and [(OCH2 CMe2 CH2 O)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 [Et3 N 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 31 P 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.
Key words
bisphosphonates - epimerization - diastereomers - X-ray structures -
31 P NMR
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