Biologically Relevant Phosphoranes: Synthesis and Structural Characterization of Glucofuranose-Derived Phosphoranes with Penta- and Hexacoordination at Phosphorus

Carbohydrate-based phosphoranes were synthesized by reacting the appropriate diphenol with phosphorus trichloride
followed by the addition of chloralose to form 1 and by the addition of isopropylidene-d-glucofuranose to form 2
and 3. Phosphorane 4 was obtained by reacting 1,2-O-isopropylidene-α-d-glucofuranosyl-3,5,6-phosphite (13) with
a diphenol. For the synthesis of 5−9, the appropriate phosphite was reacted with isopropylidene−glucofuranose.
X-ray analyses of 1−9 were carried out successfully. Hexacoordinated structures resulted via oxygen donor action
at phosphorus in the cases of phosphoranes 1−3 and via sulfur donor action for phosphoranes 4−6. Trigonal
bipyramidal structures formed for 7−9 with the carbohydrate components occupying axial−equatorial sites. The
eight-membered ring of the diphenol moiety with weak or no donor groups in 7−9 occupied diequatorial sites of the
trigonal bipyramid. Solution NMR data are in agreement with the assigned solid-state structures. Isomerism between
penta- and hexacoordination is present in solution for 7. The isomerism observed for 7 and our previous study
showing a rapid exchange process that reorients the carbohydrate component of the trigonal bipyramidal phosphorane
suggest that these biophosphoranes may serve as models for active sites of phosphoryl-transfer enzymes. At an
active site, this type of pseudorotational behavior provides a mechanism that could bring another active site residue
into play and account for a means by which some phosphoryl-transfer enzymes express promiscuous behavior.