Conformational and configurational study of 2-phenyl-4-aryl-1,3,2-dioxaphosphonates derived from xylofuranose derivatives. The anomeric effect in the spontaneous isomerization of cyclic phosphonates

A series of 2-phenyl-4-aryl-1,3,2-dioxaphosphonates derived from xylofuranose derivatives were synthesized in two-steps from diacetone-d-glucose in order to carry out conformational and configurational studies. The absolute stereochemistry at the phosphorus atom was assigned by analyzing the 1H NMR chemical shifts for each diastereoisomeric pair. This empirical assignment criterion was corroborated by X-ray crystallographic studies. Additionally, in the case of two cyclic phosphonates, spontaneous conversion into two more stable diastereoisomeric phosphonates occurred either in column chromatography or in solution (approx. 300 h in solution CDCl3). This is attributed to the cleavage of the O3–C4 bond within the P-heterocyclic ring, which is induced by the anomeric effect (nO → π∗PO).