Glycals stereochemistry

Cheng, J C Y, Daves, G D Jr, C-glycosides from palladium-mediated reactions of pyranoid glycals. Stereochemistry of formation of intermediate organopalladium adducts and factors affecting their stability and decomposition, J. Org. Chem., 52, 3083-3090, 1987. 

W. R. Roush, D. P. Sebesta, and R. A. James, Stereoselective synthesis of 2-deoxy-p-glycosides from glycal precursors. 2. Stereochemistry of glycosidation reactions of 2-thiophenyl- and 2-selenophenyl-a-D-g/uco-pyranosyl donors, Tetrahedron, 53 (1997) 8837-8852. 

The mechanism of glycal addition was further examined by Horton and coworkers,86 who have used this method to form (7S,9S)-4-demethoxy-7-0-(2,6-dideoxy-2-iodo-a-L-mannopyranosyl)-adriamycinone87 and -daunomycinone,88 which are iodinated analogs of natural antitumor compounds. The nature of the solvent was found to be critical. In particular, they were able to generalize that in non-coordinating polar solvents, where no possibility for interaction with iodine exists—that is, no lone-pair interactions—the formation of the iodonium intermediate was irreversible, and the resultant stereochemistry reflects the electronic... 

The fluorination and stereochemistry of the reaction of pyranoid and furanoid glycals with acetyl hypofluorite are described41. In related work42 the synthesis of 2-deoxy-2-fluoro-D-galactapyranose by treatment of 2-deoxy-D-galactapyranose with acetyl hypofluorite is described and the 18F-labelled product was similarly prepared through the use of Ac018F. 

The cycloaddition of glycals (21a-d) and dibenzylazidocarboxylate gives compounds (22a-d) in yields greater than 70% (Scheme 5) <87JA285>. These [4 + 2] adducts are useful intermediates in the preparation of 2-amino-2-deoxy carbohydrates <89JA2295>. The cycloaddition is stereospecific and is controlled by the stereochemistry at the C-3 position stereochemistry has been assigned by nuclear Overhauser enhancement (NOE) studies. 

To avoid isolating the glycal, the reaction was quenched with an excess of borane in THF and, upon oxidative work-up under basic conditions, C-disaccharide 38 was obtained in 64% yield over two steps (Scheme 11) (77). The stereochemistry of the hydroboration step was verified on the corresponding acetate by examination of the Jy%T and J2 y coupling constants in the proton NMR spectra. 

Yields refer to chromatographically homogeneous material. bYields are for three steps methylenation, RCM (35-40 mol% of 7) and hydroboration-oxidative work-up. Stereochemistry at C-l and C-2 determined by acetylation and analysis of //-2 coupling constant in H NMR. dA fair amount of unreacted mono-1,6-ester was isolated from the reaction mixture. cIn this case, the RCM reaction was stopped early, the (bis)C-glycal was isolated and purified (48%, unoptimized) and then subjected to hydroboration (66%, unoptimized). 

Expanding upon the lithiation of 2-phenylsulfinyl glycals, Schmidt and Dietrich [33] effected coupling reactions with benzaldehyde (Scheme 7.59). As illustrated, this reaction resulted in an approximate yield of 86%. An important observation is the induction of stereochemistry at the newly formed center with a diastereomeric ratio of approximately 4 1. 

In other experiments, a range of nucleophilic agents were reacted with sulfonyl hex-2-enopy-ranoside 103, and corresponding glycals 104 and 105 were obtained stereoselectively (O Scheme 3S) [209]. In this case, C-3 stereochemistry was regulated by the nature of the incoming anion. 

The stereochemistry of 3-C-nitro glycals has been studied in some detail [210]. For example, when nitroanhydroglucitol 106 was subjected to reaction with triethylamine, it gave the elimination product 107, which was then rearranged to an equilibrated mixture of glycals 108 and 109 (O Scheme 36) [211,212]. Some researchers have tried to explain this equilibrium shift by arguing that the quasi-equatorial anomeric proton is made more acidic by the stereoelectronic effect [213]. 

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