Ring Cleavage Reactions of Methyl α-D-Allopyranoside Derivatives with Phenylboron Dichloride and Triethylsilane

Masaru Kojima,Seiji Takeuchi,Yuusuke Ito,Yutaka Nakamura

doi:10.3390/molecules161210303

Masaru Kojima, Seiji Takeuchi + Show 2 more

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https://doi.org/10.3390/molecules161210303

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Abstract

In the course of our studies on the regioselective carbon-oxygen bond cleavage of the benzylidene acetal group of hexopyranosides with a reducing agent, we found that a combination of a Lewis acid and a reducing agent triggered a ring-opening reaction of the pyranose ring of methyl α-D-allopyranosides. The formation of an acyclic boronate ester by the attachment of a hydride ion at C-1 indicated that the unexpected endocyclic cleavage of the bond between the anomeric carbon atom and the pyranose ring oxygen atom proceeded via an oxacarbenium ion intermediate produced by the chelation between O5/O6 of the pyranoside and the Lewis acid, followed by nucleophile substitution with a hydride ion at C1.

Highlights

Lewis-acid-induced regioselective carbon-oxygen bond cleavage of the benzylidene acetal group of hexopyranosides with a reducing agent is an important reaction in carbohydrate chemistry for the syntheses of complex oligosaccharides and glycoconjugates
Benzylidene-α-D-allopyranoside 1 with PhBCl2/Et3SiH (Scheme 1, Eq 1). This unexpected side reaction is caused by the reductive cleavage of the fluorous benzylidene acetal group and subsequent endocyclic cleavage of the pyranosides
When methyl 2,3-di-O-benzyl-4,6-O-Fbenzylidene-α-Dglucopyranoside 4 and phenyl 2,3-di-O-benzyl-4,6-O-Fbenzylidene β-D-allopyranoside 6 were reacted under the same reaction conditions, this unexpected side reaction was not observed (Scheme 1, Eqs. 2,3)

Summary

Introduction

Lewis-acid-induced regioselective carbon-oxygen bond cleavage of the benzylidene acetal group of hexopyranosides with a reducing agent is an important reaction in carbohydrate chemistry for the syntheses of complex oligosaccharides and glycoconjugates. Efficient and expeditious syntheses of natural products [16], oligosaccharides [15], Molecules 2011, 16 and modified monosaccharides have been accomplished by utilizing regioselective ring-opening reduction of fluorous benzylidene acetal groups and solid-phase extraction with a fluorous reversephase silica gel column. In the course of our studies on the expeditious synthesis of these products using fluorous benzylidene acetal groups, we isolated an interesting side product, the acyclic compound 3, during the regioselective ring-opening reduction of methyl 2,3-di-O-benzyl-4,6-OF benzylidene-α-D-allopyranoside 1 with PhBCl2/Et3SiH (Scheme 1, Eq 1). This unexpected side reaction is caused by the reductive cleavage of the fluorous benzylidene acetal group and subsequent endocyclic cleavage of the pyranosides. When methyl 2,3-di-O-benzyl-4,6-O-Fbenzylidene-α-Dglucopyranoside 4 and phenyl 2,3-di-O-benzyl-4,6-O-Fbenzylidene β-D-allopyranoside 6 were reacted under the same reaction conditions, this unexpected side reaction was not observed (Scheme 1, Eqs. 2,3)

Methods

Results

Conclusion