D-Xylopyranosyl fluoride

The Noyori procedure was applied to a total synthesis of baiyunoside, a sweet principle, using 2,3,4-tri-<9-benzyl-D-xylopyranosyl fluoride (18 see Table 1), and a synthesis of glycotriosyl ceramide. A model experiment for the synthesis, using 18, showed a solvent dependence for the a ratio of the products. In this case, the use of acetonitrile, oxolane, or ether gave the a anomer (1,2-a.v), and the use of toluene or hexane gave the P anomer (1,2-trans), preponderantly. 

Fig. 14.—The Valence Geometry Parameters in Two Forms of D-Xylopyranosyl Fluoride Derivatives (from Ref. 98).

If the symmetry of the system is broken, as in glycosyl derivatives, it is best to adopt the nonequivalent (p-type and n-type) representation of the lone pairs on oxygen. This is illustrated with a-D-xylopyranosyl fluoride in Fig. 2. With this representation it is important to note that the p-type lone pair is tilted at a dihedral angle of ca. 30% to the axial C—F bond. 

Tetra-O-acetyl-a-D-xylopyranose in hydrogen fluoride is converted rapidly into tri-0-acetyl-a,/3-D-xylopyranosyl fluoride, and this product subsequently undergoes further transformations that have not yet been clarified.In contrast, tetra-O-benzoyl-a- or /3-D-xylopy-ranose (175), when dissolved in hydrogen fluoride, at once shows the n.m.r. spectrum of the benzoxonium ion 176. If this solution is rapidly processed, tri-0-benzoyI-a,/3-D-xyIopyranosyI fluoride (177) is obtained. An ion-pair of the cation 176 and HF2 probably exists in the solution these unite, to form 177, as the solution is concentrated. 

Tri-O-acetyl- and -O-benzoyl-jS-D-xylopyranosyl fluoride i Ci and C4 conformations, respectively), D-glucose oxime (exists as the cyclic 3-gluco-... 

Di-0-benzoyl-2-0-metbyl-p-D-xylopyranosyl fluoride, X-16 l,2 3,5-Di-0-benzylidene-a-D-xylofuranose, X-81 2,4 3,5-Di-0-benzylidene-D-xylose diethyl dithioacetal, X-83... 

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