Several mono-and bi-cyclic polyhydroxylated alkaloids, isolated from plants', are specific and potent glycosidase inhibitors3, including those involved in the processing of glycoproteins4. Convenient intermediates for the synthesis of these compounds are 5-azido-5-deoxy sugar derivatives.
Attempts to synthesise 5-azido-5-deoxy derivatives of D sugars by treatment of 6-O-acetyl-3-O-benzoyl-5-chloro-5-deoxy-1,2-O-isopropylidene-~-~-ido-5
or -talo-furanose' with potassium azide in N,N-dimethylformamide at -95" resulted in considerable decomposition. As bromide is a better leaving group, an attempt was made to use the corresponding 5-bromide. However, treatment of 3-0-benzoyl-6-O-tert-butyldiphenylsilyl-l,2-O-isopropylidene-5-O-mesyl-a-D-allofuranose
(2), obtained by mesylation of 1, with potassium bromide in N,N-dimethylformamide at -90" for 7 days gave two C-5-epimeric bromides (3 and 4) in the ratio 1.6: 1.
The 13C-n.m.r. spectra of 3 and 4, which were nearly identical, clearly showed a downfield shift of the C-5 resonance by -18 p.p.m. compared to that of the 5-hydroxy derivative' (S 53.7 for3 and 53.2 for 4). The 'H-n.m.r. spectra of3 and 4 agreed well with the proposed structure. The J4,5 values of 4.2 and -0 Hz for 4 and 3, respectively, are consistent with the ~-ah and L-t& configuration@, respectively.
It is likely that the formation of the epimers 3 and 4 is due to the combined effects of the drastic reaction conditions and steric hindrance by the tert-butyldiphenylsilyl group. Bromides epimeric at C-5 were also obtained in the ratio -1.4:1 when the corresponding 6-benzoate was treated as above for 2; the bromides could not be separated, but their r3C-n.m.r. spectra contained signals for C-5 at 6 49.9 and 49.6. The bromide 4 was synthesised conveniently by the reaction of 3,5-0-benzylidene-6-O-tertbutyldiphenylsilyl-1,2-0-isopropylidene-a-D-glucofuranose with N-bromosuccinimide, followed by Zemplen debenzoylation, an oxidation-reduction sequence, and benzoylation'. *5-Azido-5-deoxy+talofuranose Derivatives, Part. 3. For Part 2, see ref. 1.