2,3,4,5-Tetra-O-benzyl-D-glucopyranose

To a stirred solution of 2,3,4,6-tetra-O-benzyl-D-glucopyranose (3 540 mg, 1 mmol) in CHC13 (5 mL) under argon was added 1-fluoro-A, Ai-diisopropyl-2-methylprop-1 -enamine (2 173 mg, 1.1 mol) at rt. The reaction was followed by TLC. Once the reaction was complete, the solvent was removed under reduced pressure and the residue subjected to flash chromatography (silica gel, petroleum ether/EtOAc 2 1) to give 4 yield 537 mg (99%) (a// ) 28 72. 

OL-Glycosyl esters.1 The anomeric lithium alkoxide of 2,3,4,5-tetra-O-benzyl-D-glucopyranose (2) reacts with 1 to form mainly an a-glycosyl ester (3, a/p = 16 1). This reaction is generally strongly a-selective, even with highly hindered... 

In a different approach, phase-transfer catalyzed glycosylation of simple alcohols using tetra-O-benzyl-D-glucopyranose and /7-toluenesulfonyl chloride in the presence of BnEt3NCl has been also reported [408]. 

Allevi, P, Anastasia, M, Ciuffreda, P, Fiecchi, A, Scala, A, The first direct method for C-glucopyranosyl derivatization of 2,3,4,6-tetra-O-benzyl-D-glucopyranose, J. Chem. Soc., Chem. Commun., 1245-1246, 1987. 

Related to the acetate activating group is the more labile trifluoroacetate group. In a reaction between methylphenol and l-trifluoroacetoxy-2,3,4,6-tetra-O-benzyl-D-glucopyranose, shown in Scheme 2.4.6, Allevi, et al.,53 observed the formation of a 50% yield of the p coupled product. This reaction actually proceeded through a one pot process in which the trifluoroacetate was formed followed by introduction of the nucleophile and addition of the Lewis acid. In spite of the relatively low yield, the ease of this chemistry and selectivity for the p anomer illustrates its usefulness. 

Similar acid mediated cyclizations have been reported on a variety of substrates. For example, as shown in Scheme 7.3.2, Casiraghi, etal.,14 treated 2,3,4,6-tetra-O-benzyl-D-glucopyranose with a pyrrole-derived Grignard reagent and a titanium based Lewis acid. As shown, the addition proceeded in 64% yield with the formation of a single diasteriomer. Subsequent exposure to an acidic DOWEX resin provided a 50% yield of the p-C-glycoside and an additional 21% isolated as the corresponding a anomer. 

Scheme 2. Synthesis of gabosine I from tetra-O-benzyl-D-glucopyranose.220...

The synthesis of (+)-nojirimycin (1) from tetra-O-benzyl-D-glucopyranose (11) as depicted in Scheme 10 °° was found to be unreliable. Compound 11 underwent anomeric oxidation with DMSO-AC2O to afford tetra-0-benzyl-D-glncono-l,5-lactone (52) in 84% yield, which was treated with liquid ammonia solntion in the presence of trace amounts of Amberlite IR-120 (H+) resin in dioxane to give the glnconolactam 53 (50%). Treatment of 53 with sodinm borohydride followed by removal of the benzyl protecting groups afforded... 

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