Pyranosyl fluoride

Introduction of F2 into 3,4,6-tri-O-acetyl-D-glucal (61) in CCI3F (Freon 11) at —78° in a manner used for the non-labeled compound (so-called cold synthesis) gave a 4 1 mixture of 3,4,6-tri-0-acetyl-2-deoxy-2-[ F]fluoro-a-D-gluco- (574) and ) -D-manno-pyranosyl fluorides (575),... 

No simple rationalization for the observed values of the 19F resonances could be made for the peracetylated 2-deoxy-2-fluoro-D-gIuco-and -manno-pyranosyl fluorides, apart from the fact that the anomeric fluorine substituent always gives the lower field-resonance of the two198 this should, however, be compared with the explanation described287 (see later in this Section) for the observed differences in 19F... 

It has also been observed281 that, for a number of fluorinated monosaccharides, geminal 19F- H coupling-constants for the pyranose series are larger when the fluorine substituent is axially rather than equatorially disposed. Exceptions to this generalization were subsequently found, as in the virtually identical value of/F 2, H-2 for peracetylated 2-deoxy-2-fluoro-a-D-gluco- and -manno-pyranosyl fluorides.71... 

The signs of geminal, F H coupling-constants (in contrast to those of geminal, iH-1H coupling-constants) are absolutely positive. They were determined for derivatives of glycopyranosyl fluorides, 75,292,293 2-deoxy-2-fluoro-D-gluco- and -manno-pyranosyl fluorides,71... 

C-Glycosides A variety of organoaluminum reagents react with furanosyi or pyranosyl fluorides to form C-glycosides in 68-93% yield. 

F-C) values in a number of glycosyl fluorides are given by Bock and Pedersen. (142) The absolute value of /(F—C) values in the cr-pyranosyl fluorides (fluorine atom axial [25] are generally 10 Hz larger than the values in the corresponding / -pyranosyl fluorides (fluorine atom equatorial [26]. For example, for R, = R2 = R3 = H, /(F-C)o is... 

V(F-C3) values in the / -pyranosyl fluorides are 4-10 Hz, while in the a-derivatives they are 0-6 Hz. In the latter the fluorine and C3 atoms have a gauche orientation, while in the former they are trans oriented. On the basis of the results obtained for other three-bond coupling constants in saturated systems, one would expect 3/(F-C) to depend on dihedral angle. However, keeping in mind the results discussed for V(C-C) and 3/(N-C), and the results discussed by Stothers, (1) it is unlikely that one will be able to describe this dependence by a simple Karplus equation. 

Precursor 18.28 of unit E is prepared from L-serine. The secondary amino function is protected by derivation to 9-fluorenylmethylcarbamate (R = C13H9CH2OCO) which can be cleaved in moderate alkaline conditions. Coupling of the pyranosyl fluoride 18.28 with 18.29 uses AgClO4 (2 eq) and SnClj (2 eo) promotors ct/fi = 4.5 1). In this way the AE fragment is... 

Several pyranosyl halides have been studied as acetylated or benzoylated derivatives. In both the fluorides and chlorides, the equatorial carbon-halogen bonds are shorter than the axial bonds, with the data for fluorides illustrated in Fig. 14. The C - F bond-lengths in both derivatives of -D-xylo-pyranosyl fluoride are significantly shorter than the 143.2 pm found for the non-anomeric C-F bond in l,3,4-tri-C -acetyl-2-deoxy-2-fluoro-D-xylo-pyranose. There is also a variation of 4.4 pm in the C-1 - 0-5 bond-lengths. 

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