Chiral transformations of D-ribose to 2(S)-amino-2-[2,5-dihydro-5(R)-methylfuran-2(R)-yl]ethanoic acid, a diastereomer of the antibiotic (+)-furanomycin

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Abstract
The chain-extended 2-(2,3-O-isopropylidene-β-D-ribofuranosyl)-1,3-diphenylimidazolidine derivative (3a) was prepared by a known sequence from D-ribose. Benzylation of the primary alcohol function of 3a, deprotection of the masked aldehyde, chain extension using a modified Strecker-type synthesis, and acetylation of the resulting epimeric 2-hydroxyl groups gave the appropriately blocked seven-carbon 3,6-anhydro compounds (5a, b). The primary alcohol function was deprotected and then removed by mesylation, iodide replacement, and hydrogenolysis. The 2-hydroxyl group was deprotected, mesylated, and replaced by azide. Methanolysis of the amide and isopropylidene groups gave the α-azido ester diols (13a, b). Treatment of 13a with thiocarbonyldiimidazole followed by the Corey–Winter reaction with trimethylphosphite effected concomitant reduction of the azide function and reductive elimination of the cyclic thionocarbonate group. Saponification of the ester intermediate gave the target α-amino acid, 2(S)-amino-2-[2,5-dihydro-5(R)-methylfuran-2(R)-yl]ethanoic acid (1). Comparison of the properties of 1 and (+)-furanomycin confirmed the necessity of revision of stereochemistry for the antibiotic.

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