1,5-Anhydro-3-O-benzyl-2,6-dideoxy-4-O-(3,4-di-O-benzyl-2,6-dideoxy- β-d-arabino-hexopyranosyl)-d-arabino-hex-1-enitol (17), which corresponds to the BC fragment of various orthosomycins, was prepared from phenyl 2,3-di-O-benzyl-6-deoxy-4-O-(3,4-di-O-benzyl-2,6-dideoxy-β-d-arabin o-hexopyranosyl)-1-thio-β-d-glucopyranoside (16) by reductive lithiation. The synthesis of 16 involved a stereoselective coupling of phenyl 2,3-di-O-benzyl-6-deoxy-1-thio-β-d-glucopyranoside (9) and 1,2-di-O-acetyl-3,4-di-O-benzyl-6-deoxy-β-d-glucopyranose (14) followed by deoxygenation at C-2′. Glycosylation of methyl 2-O-benzyl-6-deoxy-4-O-methyl-β-d-galactopyranoside (25) with 3,4,6-tri-O-acetyl-2-deoxy-2-phthalimido-β-d-glucopyranosyl trichloroacetimidate, followed by deamination at C-2′, led stereospecifically to methyl 2-O-benzyl-6-deoxy-4-O-methyl-3-O-(3,4,6-tri-O-acetyl-2-deoxy-β-d-a rabino-hexopyranosyl)-β-d-galactopyranoside (26). The 2-deoxy unit of 26 was then modified by consecutive axial introduction of a C-Me group at position 3′, protection of HO-3′, and deoxygenation at C-6′, in order to obtain methyl 3-O-(3-O-benzoyl-2,6-dideoxy-3-C-methyl-β-d-arabino-hexopyranosyl)- 2-O-benzyl-6-deoxy-4-O-methyl-β-d- galactopyranoside (39), which corresponds to the DE fragment of orthosomycins. A glycosyloxyselenation-oxidation-elimination sequence was performed on 39 and either 1,5-anhydro-3,4-di-O-benzyl-2,6-dideoxy-d-arabino-hex-1-enitol (40) or 1,5-anhydro-3-O-benzyl-2,6-dideoxy-4-O-(3,4-di-O-benzyl-2,6-dideoxy- β-d-arabino-hexopyranosyl)-d-arabino- hex-1-enitol (17) to give the CDE tri- and BCDE tetrasaccharide fragments, respectively. Each fragment contained the spiro-ortholactone junction with an (R) configuration at the anomeric carbon atom of the C-unit. © 1990.
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