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Abstract
Racemic ethyl hydrogen (1-hydroxy-2-methylsulfanyl-1-phenylethyl)phosphonate was resolved with (R)-1-phenylethylamine. The (R)-configuration of the (-)-enantiomer was determined by chemical correlation. Esterification of the (-)-enantiomer with a substituted diazomethane derived from 3-hydroxy-1,3,5(10)-estratrien-17-one delivered two epimeric phosphonates separated by HPLC. Methylation with methyl fluorosulfate at the sulfur atom and treatment with a strong base induced an α-hydroxyphosphonate-phosphate rearrangement with formation of dimethyl sulphide and two enantiomerically pure enol phosphates. Their oily nature interfered with a single crystal X-ray structure analysis to determine the stereochemistry at the phosphorus atom.
Highlights
When phosphoric acid derivatives (±)-1 are treated with strong bases in stoichiometric amounts such as alkyl lithiums or lithium amides at low temperatures, they are deprotonated to give short-lived organolithiums (±)-2 containing dipole-stabilised[1] carbanions (Scheme 1)
We prepared a racemic ethyl hydrogen α-hydroxyphosphonate, resolved it with (R)-1-phenylethylamine and esterified it with a diazomethane derived from 3-hydroxy1,3,5(10)-estratrien-17-one
Each epimer obtained by HPLC separation was methylated at the methylsulfanyl substitutent and treated with base to induce α-hydroxyphosphonate-phos
Summary
When phosphoric acid derivatives (±)-1 are treated with strong bases in stoichiometric amounts such as alkyl lithiums or lithium amides at low temperatures, they are deprotonated to give short-lived organolithiums (±)-2 containing dipole-stabilised[1] carbanions (Scheme 1). The eluate was concentrated under reduced pressure and dried (0.5 mbar/RT) to give phosphonic acid (−)-10 (0.710 g, 94%) as colourless gum, which crystallised; mp 61–63 °C (i-Pr2O/few drops of CH2Cl2); [α]1D8–16.9 The free phosphonic acid obtained by general procedure A from (R)-1-phenylethylammonium salt hemihydrate (R)-11 × (−)-10 × 0.5H2O (0.80 g, 1.97 mmol) was dissolved in a mixture of ethanol (12 mL) and water (8 mL) and neutralised with KOH (10%, phenolphthalein).
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