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Synlett 2007(1): 0164-0166  
DOI: 10.1055/s-2006-958410
LETTER
© Georg Thieme Verlag Stuttgart · New York

Efficient Procedure for Reductive Opening of Sugar 4,6-O-Benzylidene Acetals in a Microfluidic System

Katsunori Tanaka, Koichi Fukase*
Department of Chemistry, Graduate School of Science, Osaka University, Machikaneyama 1-1, Toyonaka, Osaka 560-0043, Japan
Fax: +81(6)68505419; e-Mail: koichi@chem.sci.osaka-u.ac.jp;
Further Information

Publication History

Received 30 September 2006
Publication Date:
20 December 2006 (online)

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Abstract

An efficient procedure for the reductive opening of 4,6-O-benzylidene acetals was established under microfluidic conditions. 4,6-O-Benzylidene acetals of the glucose, glucosamine, and galactose derivatives were selectively converted into the corresponding 4- or 6-O-benzyl derivatives in nearly quantitative yields.

Key words

microfluidic system - reductive opening - 4,6-O-benzylidene acetal - oligosaccharides

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A new micromixing device, ‘Comet X-01’ (Techno Applications Co., Ltd., 34-16-204, Hon, Denenchofu, Oota, Tokyo 1450072, Japan), was used. It exhibited much superior mixing efficiency to the simple T-shape mixer and similar efficiency to the IMM micromixer for acid-catalyzed reaction, such as dehydration (data not reported). This new device is well suited for a large-scale synthesis and for establishing a micro chemical plant. The detailed structure and mixing system will be reported elsewhere.

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The products ratio of 4- and 6-O-benzyl derivatives was same as those obtained by batch reaction.

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Microfluidic Reaction; Typical Procedure: A solution of BF3·OEt2 (3 mL, 23.7 mmol, 1.0 M) in CH2Cl2 (23.7 mL) was injected in advance to the micromixer by using a syringe pump at a flow-rate of 0.5 mL/min. Subsequently, a solution of benzylidene acetal 1 (1 g, 2.69 mmol, 0.1 M) and Et3SiH (4.3 mL, 26.9 mmol, 1.0 M) dissolved in CH2Cl2 (26.9 mL) was also injected to the micromixer by another syringe pump at the flow rate of 0.5 mL/min and mixed at 0 °C. After the reaction mixture was allowed to flow at r.t. for an additional 45 s through a Teflon reactor tube (Φ = 1.0 mm, l = 0.9 m), the mixture was quenched by pouring it to a sat. NaHCO3 solution at 0 °C. It took about 7 min to consume 1 g of the substrate 1 under above conditions. The mixture was extracted with EtOAc, washed with brine, dried over Na2SO4, filtered, and concentrated in vacuo to give the crude product. The residue was purified by preparative TLC on silica gel (50% EtOAc in hexane) to afford 5 (935 mg, 93%). The conditions established herein can be readily applied to the scale-up synthesis simply by preparing the stock solutions of substrate and reagents and pumping them continuously into the micromixer.