Stereochemistry of 1,2-oxaphospholanes. Part X. Synthesis, X-ray structure and NMR spectroscopy of (2S^*,3R^*,5R^*)-2-methoxy-3,5-dimethyl-2-oxo-1,2-oxaphospholan-3-ol

Abstract

The title compound (4a) was synthesized from a phosphite compound obtained from 4-hydroxy-2-pentanone and dimethylphosphorochloridite, after addition of an equivalent of water. Thecis-configuration of the P=O and C3-OH groups was established based on³¹P NMR chemical shifts. In chloroform, as well as in benzene solution, the most favorable conformations of the title compound are ⁵₄ T,⁴ E, and ⁵₄ T. The crystal and molecular structure of4a has been determined by single-crystal X-ray diffraction data, and refined toR=0.056. The five-membered ring has an envelope⁴ E (C₅) conformation, with the asymmetry parameter ΔC ₅(4)=3.5°. Both the C5-CH₃ and the C3-CH₃ groups are equatorial. In the solid state, the molecules form dimers with hydrogen bonding between the hydroxyl groups and the phosphoryl oxygen atom of an adjacent molecule. A quantitative comparison of the structure of 4a with the structure of the related 2-mcthoxy-3,5,5-trimethyl-2-oxo-1,2-oxaphosphospholan-3-ol is given.