Reaktionen Mit Trimethylsilylhalogeniden an Derivaten Der Digitoxose

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Abstract
Treatment of methyl 3,4-di-O-acyl-2,6-dideoxy-α-D-ribo-hexo-pyranoside 1 or 2 with trimethylsilyl halide leads to the formation of a complex mixture of α-D-ribo-hexopyranosyl halides 3 or 5 together with the educts 1 or 2 as well as their β-anomers 8 or 9. The bromides 3 and 5, suitable for glycosidations, are preferably obtained by reaction of the digitoxose acetate derivatives 6 and 7, respectively, which in turn are prepared from 1 and 2 by mild acetolysis. Further reaction of the halides 3 to 5 with trimethylsilyl halides gives rise to a quantitative formation of the 2,3,6-trideoxy-4-0-acyl-3-halo-α-D -arabino-hexopyranosyl halides 10 to 12. In another reaction sequence starting with the olivose triacetate 20 the formation of 10 via the halide 13 is demonstrated. Structural evidence for the halides 10 to 12 is given by 1H NMR data as well as by analyses of their glycosides 14 to 19. The results support a mechanistic interpretation for the formation of 10 to 12 via a 3,4-acetoxonium ion as the key intermediate obtained from 3 by an SNfi and from 13 and SN2i step. Final conversion into the terminal halodeoxy compounds 10 to 12 proceeds by and SN2 reaction with the halide ion.