Hemiacetal hydroxyl groups

Hemiacetal hydroxyl groups of carbohydrate molecules also may be coupled to amine-containing molecules to form N-glycosidic linkages, such as those in nucleic acids and oligonucleotides. 

The hemiacetal hydroxyl group of the aldopyranosuloses 325 was usually protected in the form of the methyl glycoside by treatment with methyl orthoformate in the presence of a Lewis-acid catalyst.208 Higher yields of the corresponding methyl glycosides could be obtained by treatment of a glycopyranosulose with methyl iodide in the presence of silver oxide.211 One-step transformation of the dihydro-furan derivatives 324 into the methyl glycosides (326, 327) by treatment with methanol and formic or trifluoroacetic acid has been reported.212... 

Supports activated with divinyl sulfone, like those activated with bisoxirane, can be coupled to underivatized carbohydrates.21 The supports activated with divinyl sulfone couple to the hemiacetal hydroxyl group of the carbohydrate by the vinyl group. Accordingly affinity adsorbents prepared by this method will contain glycosyl units. The structure of the reaction product of L-fucose with agarose activated with divinyl sulfone is shown in 5. 

An unsubstituted hexose, such as D-glucopyranose (1), contains four alcoholic hydroxyl groups and one hemiacetal hydroxyl group. Such a polyfunctional1 monomer can polymerise to give a multibranched polymer, such as (2). A trisubstituted hexose, such as 2,3,6-tri-0-(JV-phenyl-carbamoyl)-D-glucopyranose (3),1W contains only one alcoholic hydroxyl... 

It is reasonable to assume that reducing sugars which exist in the pyranose or furanose form must revert to their open-chain aldehydo or keto forms before any significant C—H ionization occurs. This reversion is brought about by ionization of the acidic hemiacetal hydroxyl group, followed by ring opening. Subsequent carbanion formation at the carbon... 

The hemiacetal hydroxyl group is more reactive than the other secondary ones. Thus, trityl 5-0-tritylpentopyranosides are obtained from pentoses when an excess of trityl chloride [290] or elevated temperature [291] is used. For D-lyxose, the ditrityl derivative is formed considerably even at room temperature with only one molar equiv. of the reagent. Under such conditions, the other three pentoses give the 5-trityl ethers in ca. 60% yields [291]. Further re-investigation of the monotritylation of D-xylose at 50 °C revealed the 5-, 4-, 3-, and 1- substitution in the ratio [292] of 100 25 0 36. 

For aldohexoses, OH-1, OH-2, and OH-6 seem to be particularly reactive [399,400]. Nevertheless, the initial product of the treatment of D-a/tro-3-heptulose (coriose) with chlorotrimethylsilane in pyridine is claimed to have a free hemiacetal hydroxyl group [401]. 1,6-Anhydro-P-D-glucopyranose yielded 87% of the 2,4-disilyl ether [402],... 

About two-thirds of glucose in solution exists as this stereoisomer, but hemiacetal formation and cleavage is rapid, and this is in equilibrium with a further one-third that carries the hemiacetal hydroxyl group axial (<1% is in the open-chain form). 

Protonation of the hemiacetal hydroxyl group is followed by loss of water. Attack by the second hydroxyl group of ethylene glycol forms the cyclic acetal ring. 

Glycosyl trifluoroacetimidates have recently been introduced as glycosyl donors [321]. The compounds are prepared by the reaction of the hemiacetal hydroxyl group with N-substituted trifluoroacetimidoyl halides 138, providing anomeric mixtures of the imidates 139 (Scheme 4.24). 

The 1,6-anhydride ring can be produced by an intramolecular, nucleophilic displacement of any reactive substituent on C-6 (such as the p-tolylsulfonyloxy, iodo, or chloro group), with a hemiacetal hydroxyl group or its anion.93-95 Levoglucosan (6) also appears as a minor product in the reaction mixture after alkaline hydrolysis of /3-D-glucopyranosc 6-phosphate.96... 

Compound (9) showed, in the infrared, no absorption corresponding to a free aldehyde group. On acetylation, it gave a crystalline monoacetate (acetylated on the hemiacetal hydroxyl group) and the diacetate. The monoacetate yielded a crystalline methylsulfonyl ester. 

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