Rearrangement reactions in the fluorination of 3-deoxy-3- C-methyl-3-nitro-hexopyranosides (and hexo-1-thiopyranosides) of the d- and l-series by the DAST reagent

Abstract

Fluorination of diverse 3-deoxy-3- C-methyl-3-nitro-hexopyranosides and hexo-1-thiopyranosides of the d- and l-series by the DAST reagent was studied in order to establish, on this 3-branched-chain sugar domain, the influence of the stereochemical relationship of the substituents at positions 1 and 2, as well as the protection of the HO-4, on the kinds of rearrangement reaction promoted by this fluorinating agent. Three classes of pyranosidic substrate were employed: ( a) 1,2- trans configured, irrespective of whether HO-4 is protected or not; ( b) 4- O-protected 1,2- cis configured; and ( c) 4- O-unprotected 1,2- cis configured. They were prepared starting from simple glycosides through routes involving a Baer reaction and subsequent transformations by known methodology. All the substrates of class a essayed underwent, on treatment with DAST at room or higher temperatures, a rearrangement involving a 1,2-shift to give both the anomeric 2-inverted pyranosyl fluorides (or, for the 1-thioglycosides, only the α fluoride). Substrates of class b led, through a mechanism similar to that proposed for transformations of related substrates, to ring-contracted 2,5-anhydro-1-fluoro-1- O-methyl- (or 1-deoxy-1-phenylthio-)hexitol derivatives (sometimes as 1-epimers), which are precursors of 2,5-anhydro- aldehydo-sugars. Substrates of class c led to 4,5-anhydro-1-fluoro-1- O-methylalditol derivatives in all cases, together with a ring-contracted 5-fluoro-hexofuranoside in only one case; a rationalisation for their formation is proposed.