Synthesis of Nonracemic Dimethyl α-(Hydroxyfarnesyl)phosphonates via Oxidation of Dimethyl Farnesylphosphonate with (Camphorsulfonyl)oxaziridines

Abstract

Several strategies for synthesis of nonracemic dimethyl α-(hydroxyfarnesyl)phosphonate and the parent phosphonic acid have been explored. Separation of diastereomeric derivatives prepared by esterification of racemic α-hydroxy phosphonate with ( S )-(+)- O -methylmandelic acid was possible, and these diastereomers could be assigned absolute stereochemistry on the basis of literature precedent. However, hydrolysis to the α-hydroxy phosphonic acid was accompanied by extensive isomerization. Addition of a nonracemic phosphonamidite to farnesal also gave nonracemic material, but again hydrolysis was problematic. Oxidation of dimethyl farnesylphosphonate anion with nonracemic (camphorsulfonyl)oxaziridines was shown to be regio- and stereoselective for formation of the α-hydroxy phosphonate. Enantiomeric excess of ∼70% ee was established by conversion of the oxidation products to their ( S )-(+)- O -methylmandelate derivatives. Although hydrolysis of these methyl esters was accompanied by extensive racemization, both enantiomers of α-(hydroxyfarnesyl)phosphonic acid were obtained in low ee by this strategy.