Anomeric mixtures

Reaction of 3-aminopropanol with an a and [3 mixture of 5-bromo-5-deoxy-2-xylofuranose (25) and 5-(9-tosyl-D-lyxofuranose in MeCN afforded anomeric mixtures of trihydroxypyrido[2,l-Zi]l,3]oxazines 20, 21 and 22-24, respectively (99T6759, 99T14251). 

MeCN, r.t., 24 h) gave, respectively, a mixture of fluorinated derivatives 287 (67%) and 290 (6%), and 290 (54%) and 287 (11%). In each case, the minor products were formed by C-5 epimerization of the starting compounds prior to the displacement, followed by fluorination. Reduction (NaBHJ of 287 and 290, followed by hydrolysis, gave an anomeric mixture of 5-deoxy-5-fluoro-L-idofuranose (from 287) and 5-deoxy-5-fluoro-D-glucofuranose... 

On treatment with sulfur tetrafluoride in HF (—78°, overnight) and subsequent glycosylation with methanol, an anomeric mixture of racemic... 

Condensation of 1,3-di-0-acetyl-5-0-benzyl-2-deoxy-2-fluoro-D-arabinofuranose (880) or 3-0-acetyl-5-0-benzoyl-2-deoxy-2-fluoro-D-ar-abinofuranosyl bromide (881) with 2,6-dichloropurine (fusion procedure for the former) gave, respectively, an anomeric mixture of 9-glycosylpurine... 

The a anomers of 2-acetamido-2-deoxy-D-glucose and 2-acetamido-2-deoxy-D-galactose have the same nir c.d. bands in 1 1 methanol-water at 0° as the anomeric mixtures have in aqueous solution. This indicates that the anomeric configuration has little influence on the nir c.d. band. 

However, workers do not agree as to the shape of the c.d. spectrum for these sugars at shorter wavelengths, as Fig. 15 demonstrates. The correct spectrum still remains an open question, but the intense c.d. band expected at 190 nm for the amide mr c.d. bands are of opposite sign for the two anomers and nearly cancel in the equilibrium mixture. Thus, differences in the anomeric mixtures could explain differences in the c.d. spectra. The amide irir c.d. band is obvious for the anomeric mixture from 2-acetamido-... 

Hultiple products are frequently observed for the separation of TMS-sugar derivatives. At equilibrium reducing sugars can exist in more than one isomeric form known as anomers. Formation of their THS derivatives followed by gas chromatography will result in multiple peaks corresponding in composition to the equilibriue anomeric mixture [436]. 

Chemical shift (p.p.m.) of model compounds in H20 relative to internal 1,4-dioxane (67.86 p.p.m.). Chemical shifts for these compounds are given at pH 5.5-7.5. Estimated precision for the chemical shifts is 0.05 p.p.m.h See Refs. 82 and 83. See Ref. 20.d These assignments may have to be interchanged. See Ref. 21 numbers in the brackets below the given chemical shift values refer to those published in Ref. 86. f See Ref. 19. The chemical shift for the -anomeric carbon atom was found to be 100.6 p.p.m. and was determined from an anomeric mixture of this compound. The existence of the a-Man — Ser unit was confirmed by the l]CH value (169 Hz) obtained for this compound. See Ref. 84. 

These model reactions were of great value in the extension of the C-glycosylation reaction with malonic esters to five-membered ring-systems. Treatment of 2,3 5,6-di-O-isopropylidene-a-D-manno-furanosyl bromide122,123 (150) with diethyl sodiomalonate led to an anomeric mixture of C-glycosyl compounds that could be separated by column chromatography, with the a (151) and /B (152) anomers in... 

Ohrui and Fox128 reported that treatment of 2,3-O-isopropylidene-5-O-trityl-jS-D-ribofuranosyl chloride (171) with diethyl sodiomalonate in 1,2-dimethoxyethane gave an anomeric mixture of C-glycosyl compounds (172) which, when allowed to equilibrate in ethanol containing sodium ethoxide, gave the )3 anomer (173) as the preponderant product. Similar observations were made with the C-... 

C-Glycosylation in the D-ribofuranose series can be effected under very mild conditions in the presence of stannic chloride as the catalyst, but the product may consist of anomeric mixtures. Thus, 2-j3-D-ribofuranosylnaphthalene (202), 2,4,6-trimethoxy-1-/3-D-... 

The 1,2-cyclic sulfites are readily obtained from anomeric mixtures of 1,2-diols by reaction with sulfinyl diimidazolide as mixtures of exo- and endo-isomers at sulfur (Scheme 4.34) [312-314],... 

Scheme 4.36 Formation of an anomeric mixture of glucosyl triflates.

HSBtaz, TsCI, Bu4NCI, C6H6/50% aq NaOH 95%, cep, 1 3 [439] (SPyr)2, Bu3P, CH2CI2 Anomeric mixture [440]... 

Bielawska and Michalska [511] have introduced S-(2-deoxyglycosyl)phosphor-othioates as glycosyl donors. Activation of these compounds for glycosidation with silver salts (AgF, AgC104 or AgOTf) resulted in the formation of 2 -deoxydisacchar-ides as anomeric mixtures. Thiem and coworkers reported [507] an efficient activation method for S-(2-deoxyglycosyl)phosphorothioate donors 102 by using N-iodo-succinimide (NIS) or iodonium bis(2,4,6-trimethylpyridine) perchlorate (IDCP) (Scheme 4.94). 

Treatment of a 1,2-dehydro derivative (1 mmol) with an equimolar amount of 0,0-diethyl S-hydrogen phosphorodithioate (1 mmol) in benzene (3 ml) for 24 h quantitatively yielded a crude anomeric mixture of the title compounds after evaporation in... 

Deoxy-a-D-ribosyl-l-phosphate 20, a key substrate in the preparation of 2 -deoxynucleosides, was stereoselectively prepared by crystallization-induced asymmetric transformation in the presence of an excess of ortho-phosphoric acid and tri( -butyl)amine under strictly anhydrous conditions (Scheme 2).7 Initial Sn2 displacement of Cl in ot-glycosyl chloride 16 by phosphoric acid resulted in a 1 1 a/p anomeric mixture of 17 and 18 due to the rapid anomerisation of the a-chloride in polar solvents. Under acidic conditions, in the presence of an excess of H3P04, an equilibration between the a and p anomers gradually changed in favour of the thermodynamically more stable a-counterpart. By selective crystallization of the mono tri( -butyl)ammonium salt of the a-phosphate from the mixture, the equilibrium could be shifted towards the desired a-D-ribosyl phosphate 18 (oc/p = 98.5 1.5), which was isolated as bis-cyclohexylammonium salt 19 and deprotected to furnish compound 20. 

Although addition of activated phosphoramidite to hemiacetals of manno-pyranoses under thermodynamic control has been reported to deliver exclusively a-phosphates in some cases,43 anomeric mixtures with preponderance of a-anomer have been reported in other examples.10,44 Since formation of phosphorotetrazolidite is a rate-limiting step of the process, initial activation of phosphoramidite followed by addition of nucleophilic hemiacetal should accelerate condensation and favour the formation of the thermodynamic a-product. Indeed, reaction of hemiacetal 101 with dibenzyl phosphorotetrazolidite assured exclusive a-selectivity of the resulting glycosyl phosphate 102.43 The accumulation in the reaction mixture of mildly acidic 1H-tetrazole, which is liberated upon reaction of tetrazolidite with hydroxylic component, could also favour predominant formation of the a-phosphate (Scheme 18, A). Conventional hydrogenolysis afforded the a-mannosyl phosphate 103. 

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