O-Glycosyl Donors
3.4
637
clearly showed that the protective groups in the acceptors, regardless of pyranoses or furanoses and of primary alcohols or secondary alcohols, were the crucial factor for the outcome
of the stereochemistry in glycosylations with 409. This observed stereoselectivity was also
donor dependent, since glycosylation with 2-benzyl-3,5-dibenzoyl CB arabinofuranoside was
not as stereoselective [211].
5.4.1 Synthesis of an Octaarabinofuranoside
Based on Stereoselective β-D-Arabinofuranosylation
The authors applied this acceptor-dependent β-arabinofuranosylation method to the synthesis
of octaarabinofuranoside 417. Their retrosynthesis of compound 417 led to three components,
a linear methyl trisaccharide 416, a branched BCB trisaccharide 415, and to CB furanosyl
donor 409. Levulinyl protective groups were chosen in fragments 415 and 416 for selective
deprotection prior to furanosyl coupling ( > Scheme 66). Three arabinose building blocks were
used in the assembly.
Arabinofuranosyl donor 418 glycosylated acceptor 419, to yield after levulinyl-deprotection
and repetitive glycosylation with 419, the linear trisaccharide 420 ( > Scheme 67a). Coupling
of latent BCB donor 422, with active CB donor 421, led after deprotection of the levulinyl
groups to diol 423 ( > Scheme 67b). The crucial double β-arabinofuranosylation of diol 423
⊡ Scheme 66
Retrosynthesis of octaarabinose 417. A component of lipoarabinomannan in mycobacterial cell wall
3.4
637
clearly showed that the protective groups in the acceptors, regardless of pyranoses or furanoses and of primary alcohols or secondary alcohols, were the crucial factor for the outcome
of the stereochemistry in glycosylations with 409. This observed stereoselectivity was also
donor dependent, since glycosylation with 2-benzyl-3,5-dibenzoyl CB arabinofuranoside was
not as stereoselective [211].
5.4.1 Synthesis of an Octaarabinofuranoside
Based on Stereoselective β-D-Arabinofuranosylation
The authors applied this acceptor-dependent β-arabinofuranosylation method to the synthesis
of octaarabinofuranoside 417. Their retrosynthesis of compound 417 led to three components,
a linear methyl trisaccharide 416, a branched BCB trisaccharide 415, and to CB furanosyl
donor 409. Levulinyl protective groups were chosen in fragments 415 and 416 for selective
deprotection prior to furanosyl coupling ( > Scheme 66). Three arabinose building blocks were
used in the assembly.
Arabinofuranosyl donor 418 glycosylated acceptor 419, to yield after levulinyl-deprotection
and repetitive glycosylation with 419, the linear trisaccharide 420 ( > Scheme 67a). Coupling
of latent BCB donor 422, with active CB donor 421, led after deprotection of the levulinyl
groups to diol 423 ( > Scheme 67b). The crucial double β-arabinofuranosylation of diol 423
⊡ Scheme 66
Retrosynthesis of octaarabinose 417. A component of lipoarabinomannan in mycobacterial cell wall