ANCHIMERIC PARTICIPATION OF A METHOXY GROUP IN A REACTION OF A METATHIOPHOSPHATE

Thermolysis of derivatives of the 2,3-oxaphosphabicyclo[2.2.2]octene r ing system with phosphorus in the phosphonothionoate state accomplishe s the extrusion of O-alkyl metathiophosphates, ROP(S)O, as highly reac tive intermediates. With alcohol present, the intermediate is trapped as an O,O-dialkyl phosphorothionoate, (RO)(R'O)P(S)OH. With R = (RS)-s ec-butyl in the metathiophosphate released, the phosphorothionoate is formed as a 1:1 mixture of diastereoisomers, but when R = (RS)-1-metho xy-2-propyl in the metathiophosphate, a 4:1 isomer mixture is formed i n CHCl3, and 2:1 in toluene. This can be explained by anchimeric parti cipation of the methoxy group, giving diastereoisomeric 1,3,2-dioxapho spholane intermediates in amounts that are unequal due to small stabil ity differences that arise from steric effects. The structure of the b icyclic precursor was established by X-ray diffraction analysis and le d to the prediction that the major isomer would have the R(c),S*(p) c onfiguration. This was proved by an independent synthesis of the same 1:1 isomer mixture from thionation of (RO)PH(O)(OR'), separation by ch romatography, and X-ray diffraction analysis of the dicyclohexylamine salt of one of the isomers. This isomer had the R(c),R*(p) configurat ion and was identical to the minor isomer (with the more upfield P-31 NMR signal) obtained from the metathiophosphate.