Synthesis of α- and β-C-Aryl Δ2-Glycopyranosides from p-tert-Butylphenyl Δ2-Glycopyranosides via Grignard Reagents

Treatment of p-tert-butylphenyl 4,6-di-O-benzyl-2,3-dideoxy-alpha-D-erythro-hex-2-enopyranoside (1aalpha) or the 4,6-di-O-(tert-butyldimethylsilyl) analogue (1balpha) with various functionalized arylmagnesium bromides in the presence of a catalytic amount of PdCl(2)(dppf) at 25 degrees C in THF afforded the corresponding unsaturated C-arylglycosides 2-14 having the alpha-configuration in quite good yields. Benzyl-, allyl-, and vinylmagnesium bromides gave also the corresponding unsaturated alpha-C-glycosides 15-18, although in lower yields. When the same reaction was performed in the presence of NiCl(2)(dppe) as the catalyst at -40 degrees C, only the formation of the corresponding unsaturated C-arylglycosides having the beta-configuration was observed. As expected, reaction of phenylmagnesium bromide with p-tert-butylphenyl 4,6-di-O-benzyl-2,3-dideoxy-beta-D-erythro-hex-2-enopyranoside (1abeta) in the presence of NiCl(2)(dppe) gave only the unsaturated beta-C-phenylglycoside 2abeta, while palladium-catalyzed reaction led to the preponderant formation of C-phenylglycoside 2aalpha. Reaction of PhMgBr with p-tert-butylphenyl 4-O-benzyl-2,3,6-trideoxy-alpha-L-erythro-hex-2-enopyranoside (20) afforded stereospecifically the unsaturated alpha- and beta-C-phenylglycoside 25 in the presence of PdCl(2)(dppf) and NiCl(2)(dppe), respectively.