Synthetic studies of naturally occurring branched-chain carbohydrates Syntheses of axenose and evermicose

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Description: New stereocontrolled syntheses of the branched-chain carbohydrates, axenose and evermicose, constituents of the antimicrobial axenomycins and everninomycins, respectively, have been developed using an unusual furanoside approach from 2-deoxy-D-ribose. In the course of this work, the beta-anomer of metyl-D-digitoxose was also prepared. A new, short enantioselective synthesis of amicetose, also a constituent of the axenomycins, is also described. A key step of the amicetose synthesis is an asymmetric hydroboration/oxidation. Enantiomeric purity of the isobutyl glycosides of amicetose was established by a chiral gas chromatographic method. Studies, herein, of organometallic additions to methyl 3-O-benzyl-2-deoxy-5-alpha, beta-D-erythro-pentodialdo-1,4-furanosides have refined the model for acyclic diastereocontrol in chain-extension reactions of dialdoses. These studies suggest that the simple chelation model often cited in the literature is not sufficient to predict the stereochemical outcome of additions to pentodialdofuranosides. Addition reactions to the beta-anomer are predicted by a Felkin-Ahn model; addition reactions to the alpha-anomer appear to be chelation controlled. Th methylcerium organometallic reagent was found to be particularly useful for the introduction of the methyl branch-point in branched-chain carbohydrate synthesis utilizing furanoside-3-uloses as substrates. Use of the methylcerium reagent overcomes beta-elimination and enolization side-reactions common to this type of transformation with other organometallic reagents.
Language: English
Format: Degree Work