Carbohydrate Reactivity in Hydrogen Fluoride. XX. Di-L-erythro-2-pentulose Dianhydrides

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Abstract
Protonic activation of L-erythro-2-pentulose (L- ribulose ) with pyridinium poly(hydrogen fluoride) afforded di-L-erythro-2-pentulofuranose dianhydrides in almost quantitative yield. The outcome of the reaction was found to depend strongly on reaction conditions and appears to be governed by stereoelectronic factors, mainly the anomeric effect, which maintains axially disposed the O5 substituents in the dioxan ring in a chair conformation or in a seven- membered twist-chair arrangement. Thus, the dispirodioxan α,β-1,2':2,1'-dianhydride structure was found to be kinetically favoured at a low HF-to-pyridine ratio, and it readily isomerized into its di-β-1,2?:2,3' isomer under slightly more acidic conditions. Under thermodynamic reaction conditions, the previously unknown di-β-L 1,2':2,4'-dianhydride bridged structure was the major entity present in the reaction mixture, besides its 3'-O-α-L-erythro-2-pentulofuranosyl derivative. The differences observed in the protonic reactivity of this pentulose as compared to hexuloses can be rationalized in terms of a higher flexibility of the C5 unsubstituted furanose ring.

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