Oligosaccharides side chain cleavage

Side chain cleavage is of interest as a tool for locating links in oligosaccharides (equation 18). Tri-ethylsilane, catalyzed by BF3 alone or by a 5 1 mixture of TMS-OTf and BF3, has proven to be useful for this purpose with both pyranosides and furanosides. In these reactions the hydride atom is delivered axially at C-1. ... 

Two repeated exposures of resin 38 to the catalyst (9% mol) for 18 h in dichloromethane at room temperature afforded the expected allyl lactoside in an encouraging isolated yield of 81% from resin 35 (90% per step). Traces of dimerized compounds resulting from cross-metathesis were detected as the only side products. Extension of the oligosaccharide chain was subsequently performed first by deacetylation (excess NaOMe in 4/1 CH2Cl2/MeOH at r.t.) and glycosylation with known lactosyl donor 40 in conditions similar to those mentioned above. Cleavage was performed twice as described above, but with a reduced reaction time of 6 h in this case tetrasaccharide 42 was isolated in 51% yield from 35 (84% per step). No dimerized products were detected. 

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