Synthesis and Unexpected Rearrangement of a Hydroxyphostone (1)

Abstract

During our investigations on the wide range of stereoselective alkylations of 6-membered ring phostones,¹ we uncovered a novel rearrangement of 3-(diphenylhydroxymethyl)-2-ethoxy-2-oxo-1,2-oxaphosphorinane (1). The cis/trans diastereomers of 1 were prepared by the reaction of benzophenone with the appropriate ylide of the parent phostone (3).² When trans- 1 was left unattended at rt in CH₂Cl₂ a new compound, 2, was isolated which showed a significant downfield ³¹P shift and a higher melting point. Upon heating to the melting point 2 decomposed to give 4, which suggests that 2 may be an intermediate in the conversion of 1 to the Wittig-like product 4. The IR, ¹H, ¹³C, ³¹P, and 2-D NMR spectral data along with independent synthesis confirmed the identity of 2. Subsequently, 2 was also produced in 70% yield when cis- 1 was treated with CH₂Cl₂/ether-HC1_aq at 50–60°C for 2 weeks, but this product was contaminated with 30% of the exocyclic alkene 5. No rearrangement was observed when 1 was treated with TsOH/EtOH or HPF₆; only 5 was produced. The stereochemistry and mechanisms of these transformations are presented.