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-alpha-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.