Glycals oxidation reactions

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. 

Under carefully controlled conditions in which the uptake of gas was limited to 2 moles per mole, the products of direct hydroformyla-tion of glycal esters were found to be obtainable. Thus, from di-O-acetyl-D-xylal, 4,5-di-0-acetyl-2,6-anhydro-3-deoxy-D-lyxo- and -xylo-hexose (19 and 20) were isolated in a combined yield of 20% (by way of hydrazone derivatives). Alternatively, these compounds were much more readily prepared by application of the Pfitzner-Moffatt oxidation reaction to the corresponding hydroxymethyl analogs. With tri-O-acetyl-D-glucal, the addition reaction was more suitable for the preparation of the 2,6-anhydroaldose compounds, the yield33 being 70%. 

The haem peroxidases are a superfamily of enzymes which oxidise a broad range of structurally diverse substrates by using hydroperoxides as oxidants. For example, chloroperoxidase catalyses the regioselective and stereoselective haloge-nation of glycals, the enantioselective epoxidation of distributed alkenes and the stereoselective sulfoxidation of prochiral thioethers by racemic arylethyl hydroperoxides [62]. The latter reaction ends in (i )-sulfoxides, (S)-hydroperoxides and the corresponding (R)-alcohol, all In optically active forms. 

Sulfonamides can also be alkylated by support-bound electrophiles (Table 8.10). Polystyrene-bound allylic alcohols have been used to N-alkylate sulfonamides under the conditions of the Mitsunobu reaction. Oxidative iodosulfonylamidation of support-bound enol ethers (e.g. glycals Entry 3, Table 8.10) has been used to prepare /V-sulfonyl aminals. Jung and co-workers have reported an interesting variant of the Baylis-Hillman reaction, in which tosylamide and an aromatic aldehyde were condensed with polystyrene-bound acrylic acid to yield 2-(sulfonamidomethyl)acrylates (Entry 4, Table 8.10). 

Oxidation of glycol esters to ene-S-lactones. Reaction of glycal esters with m-chloroperbenzoic acid involves the expected epoxidation followed by cleavage of the oxide by the /n-chlorobenzoic acid formed. However, in the presence of BF3, the reaction produces a,/ -unsaturated 5-lactones, usually in excellent yield. This oxidation is also possible with pyridinium chlorochromate and BF3 as catalyst, but in lower yield.1... 

Transition-metal-catalysed reactions of glycals with carbon nucleophiles provide an important route to C-glycosides.24 These reactions may proceed by two different pathways depending on the oxidation state of the transition metal used and the nature of the carbon nucleophile (Scheme 3.5c). For example,25 transmetallation of Pd(OAc)2 with pyrimi-dinyl mercuric acetate results in the formation of a pyrimidinyl palladium complex, which adds to the double bond of glycal derivatives. The resulting intermediate can undergo different transformations depending on the reaction conditions. 

Methylenation of 58 with an excess of the Takai reagent and subsequent RCM with the second generation Grubbs catalyst 7 (35 mol%) gave an intermediate (bis)-C-glycal which was directly subjected to hydroboration by BH3 THF and subsequent oxidative work-up to afford the C-trisaccharide 60 in 49% yield over three steps (26). Although double RCM reactions are known (27), this was the first time it has been used in the context of C-saccharide synthesis. 

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). 

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