Synthesis of L-nogalose, L-sibirosamine, and related branched-chain carbohydrates

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Description: In this study synthetic approaches to the naturally occurring branched-chain sugars sc L-nogalose, sc L-sibirosamine, sc L-vinelose and sc L-kansosamine were developed. The C-3 methyl branch common to all four structures was introduced by the addition of methylmagnesium iodide to a ketosugar in its furanose form; namely, methyl 2,5-di-O-benzyl-6-deoxy-$\alpha$- sc L-mannofuranosid-3-ulose. The keto-sugar was synthesized in 6 steps from commercially available 6-deoxy- sc L-mannose (sc L-rhamnose). Addition of the Grignard reagent occurred from the $\beta$-face with complete diastereoselectivity. The syntheses of all four desired compounds were developed from the resulting 3-C-methyl branched sugar via the synthesis of the key intermediate methyl 6-deoxy-3-C-methyl-$\alpha$- sc L-mannopyranoside (72). Nogalose was made from 72 by permethylation. The remaining three syntheses diverged as each target compound required further elaboration at C-4. Synthesis of sibirosamine and the kansosamine precursor were based on the ring opening of a 3,4-anhydrosugar (Methyl 3,4-anhydro-6-deoxy-3-C-methyl-$\alpha$- sc L-talopyranoside, 68) with a nitrogen nucleophile. The anhydrosugar 68 was reacted with sodium N-methylbenzenesulfonamide to make methyl-N-benzenesulfonyl-$\alpha$- sc L-sibirosaminide. The anhydrosugar 68 was reacted with sodium azide to make the kansosamine precursor methyl 4-amino-4,6-dideoxy-3-C-methyl-$\alpha$- sc L-mannopyranoside. Both nucleophilic ring opening reactions occurred with regioselectivity and diastereoselectivity. The vinelose precursor required inversion of configuration at C-4. This was done by oxidation of methyl 6-deoxy-2,3-O-isopropylidene-3-C-methyl-$\alpha$- sc L-mannopyranoside to a C-4 ulose and subsequent reduction of the carbonyl group. Hydrogen addition to the carbonyl occurred so as to give the desired configuration at C-4 exclusively. sc L-nogalose was synthesized in eleven steps, and methyl N-benzenesulfonyl-$\alpha$- sc L-sibirosaminide was made in fourteen steps, from 6-deoxy- sc L-mannose. Formation of any side products in the final routes to the targets was not detected. Confusion in the literature regarding the anomeric configuration of the enantiomer of 72 was resolved by chemically transforming 72 into compounds of known anomeric configuration ($\alpha$), performing N.O.E. difference spectroscopy and X-ray crystallography on 72.
Language: English
Format: Degree Work