Intramolecular hydrogen bonding (P=O--H) stabilizes the chair conformation of six-membered ring phosphates

reaction: see text] Six-membered cyclic phosphates (2-phenoxy-2-oxo-1,3,2-dioxaphosphorinanes) bearing an internal protected or unprotected hydroxyl group were designed, synthesized, and studied by NMR and computational methods. Selective opening of O-isopropylidene-protected 1,2-diols at the primary site was achieved with either triethylsilane or trimethylallylsilane in the presence of BF3.OEt2. Applied to 5,6-O-isopropylidenepentofuranosides, this reaction gave rise to the formation of the corresponding 1,3-diol precursors for the six-membered ring phosphates containing an O-isopropyl or O-1,1-dimethyl-3-butenyl functional group at C-6. The O-1,1-dimethyl-3-butenyl protecting group was efficiently removed after the phosphorylation with BF3.OEt2, and the six-membered cyclic phosphates containing free hydroxyl groups were obtained. A cyclic phosphate with a free hydroxyl group oriented cis to the phosphoryl group shows a vicinal coupling constant 3J(HP) that is in accordance with the chair conformation. This is due to the formation of a seven-membered intramolecular hydrogen-bonded ring structure that stabilizes the chair conformation. Thus, the strong tendency of the phenoxy group to be in an axial position is diminished by the internal hydrogen bonding interaction. Computational studies provided strong support for the experimental observation.