Abstract
The addition of various dialkyl phosphite derivatives to ( S)- N, N-dibenzylphenylglycinal 6 led to the preponderance of anti over syn diastereoisomers: from 75:25 when NEt 3 or Ti(O iPr) 4 were used to 51:49 for Li or Mg salts. In the NEt 3-catalysed reaction, partially racemised phosphonates were formed, while enantiomeric products were obtained after addition of lithium O, O-dimethylphosphonate to ( S)- 6 at −70°C. Because the syn-isomers were found to be resistant to O-benzoylation, mixtures of diastereoisomers were easily separated after esterification of the anti products. Hydrogenation of the syn-phosphonates in the presence of Boc 2O gave enantiomeric dialkyl N-Boc-2-amino-1-hydroxy-2-phenylethylphosphonates—phosphonate analogues of the docetaxel C-13 side chain.
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