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M utarot cia
Mutarotácia

je všeobecne acidobázicky katalyzovaná vzájomná premena anomérno-tautomérnych foriem redukujúceho sacharidu cez jeho acyklickú formu prebiehajúca v čerstvom roztoku sacharidu až do ustálenia rovnováhy. Táto premena sa navonok prejavuje ako časová zmena optickej otáčavosti []D z počiatočnej do rovnovážnej hodnoty a schématicky sa označuje vodorovnou šípkou (napr. pre -anomér D-glukózy []D +111º  +53º (voda)).


Mutarot cia zahr uje anomeriz ciu a tautomeriz ciu
Mutarotácia zahrňuje anomerizáciu a tautomerizáciu

α-D-glukopyranóza

β-D-glukopyranóza

Zdroj: Monosaccharides. Their Chemistry and Their Roles in Natural Products, P.M. Collins, R.J. Ferrier, Wiley,Chichester, 1995.


Mutarotácia zahrňuje anomerizáciu a tautomerizáciu

α-D-glukopyranóza38 %

β-D-glukopyranóza62 %

A

aaaaldehydo-aaaD-glukóza- hydrátglukaaaa+aaaaaaaaa aldehydo-aaaD-glukóza 0,02 %

α-D-glukofuranóza0,05 %

β-D-glukofuranóza0,14 %


Zdroj: Monosaccharides. Their Chemistry and Their Roles in Natural Products, P.M. Collins, R.J. Ferrier, Wiley,Chichester, 1995.


Prejav mutarotácie v 500 MHz 1H NMR spektre α-D-glukopyranózy: a) ihneď po rozpustení v D2O; b) po dosiahnutí rovnováhy vD2O. Zdroj: Monosaccharides. Their Chemistry and Their Roles in Natural Products, P.M. Collins, R.J. Ferrier, Wiley,Chichester, 1995.


20 MHz 13C NMR spektrum mutarotačnej rovnováhy α- a β-D-glukopyranózy vo vode. Zdroj: Monosaccharides. Their Chemistry and Their Roles in Natural Products, P.M. Collins, R.J. Ferrier, Wiley,Chichester, 1995.


Lobry de Bruyn – Alberda van Ekenstein reaction (1900)

aldose

epialdose

1,2-enediol

2-ketose


Lobry de Bruyn-Alberda van Ekenstein reaction

a base catalyzed and non-equilibrium mutual isomerization of an epimeric pair of aldoses and the corresponding 2-ketose, proceeding through an 1,2-enediol intermediate. In a preparative scale, this reaction is being used for synthesis of 2-ketoses from aldoses; the yields of the corresponding 2-epimeric aldoses (epialdoses) are low. Under a prolonged treatment of the catalytic base, the reaction can proceed further along the entire sugar chain and result in a complex mixture of ketoses and aldoses. Moist pyridine is usually the reaction medium instead of often used aqueous solutions of alkaline earth hydroxides, which catalyze subsequent reactions as well.


Epimerization – in general, a reaction causing a mutual interconversion of epimeric sugars.

The most expedient method for preparation of epimeric aldoses is the Bílik reaction.

In a smaller extent, the epimerization of aldoses and ketoses occurs under the conditions of the Lobry de Bruyn-Alberda van Ekenstein reaction as well.

Also aldonic acids undergo a base-catalysed epimerization.


Bílik reaction – the intramolecular reaction of aldoses, proceeding in aqueous solutions in the presence of a catalytic amount of molybdic acid at 70-90 oC within 2-8 hours and providing an equilibrium mixture of the epimeric pair of aldoses. The compositions of the equilibrium mixtures for the epimeric pairs of D-aldoses are:

D-threose :D-erythrose 4 : 3

D-arabinose : D-ribose 2 : 1

D-xylose : D-lyxose 7 : 5

D-glucose : D-mannose 3 : 1

D-galactose : D-talose 4 : 1

D-allose : D-altrose 3 : 2

etc.

The equilibrium ratios are the same also for the epimeric pairs of L-aldoses.

The reaction has enormous significance for the production of several aldoses, e. g., D-mannose, D-ribose, L-ribose, D-lyxose and D-talose.


A general scheme of the Bílik reaction

aldose

epialdose

The reaction has been discovered and in 1971-1972 published by the Slovak chemist Vojtech Bílik (1929-1994).


Stereoselective hydroxylation of glycals

D-galactal

D-talose 95 %

D-galactose 5 %

The discovery od the Bílik reaction was preceded by the discovery of another reaction of sugars catalyzed by molybdate ions, the stereoselective hydroxylation of glycals, described by V. Bílik in 1969-1970.


General conditions of the c 2 epimerization of aldoses catalyzed by molybdate ions
General conditions of the C-2 epimerization of aldoses catalyzed by molybdate ions

  • 10-20 % aqueous solution of an aldose (0,06-1,1 mol/L)

    in 0,1 % aqueous molybdic acid (0,006 mol/L)

  • pH 2,9, 80-95 ºC

90 ºC

90 ºC

D-glucose D-glucose + D-mannoseD-mannose

2 h

10 h

73 % 27 %


C 2 epimerization of d glucose and d mannose catalyzed by molybdic acid
C-2 Epimerization of D-glucose and D-mannose catalyzed by molybdate ionscatalyzed by molybdic acid

H2MoO4

H2O

D-glucose D-mannose

73 % 27 %


The equilibrium ratios of the epimeric aldoses in the catalyzed by molybdate ionsBílik reaction


The (partly incorrect) mechanism of the catalyzed by molybdate ions Bílik reaction proposed in 1975


The B catalyzed by molybdate ionsílik reaction is the intramolecular rearrangement of aldoses, in which the C-2C-3 bond breaks under the simultaneous formation of the C-1C-3 bond. This has been proved by the following transformations:

Source: Hayes ML, Pennings NJ, Serianni AS, Barker R (1982) J Am Chem Soc 104:6764


The mechanism of the catalyzed by molybdate ionsBílik reaction


The application of the catalyzed by molybdate ionsBílik reaction for the mutual transformation of 2-ketoses and 2-C-hydroxymethyl-aldoses (1996)

2-ketose

2-C-hydroxymethyl-aldose

D-hamamelose

2-C-hydroxymethyl-D-ribóza

2-C-hydroxymethyl-D-ribo-hexose

D-fructose

D-arabino-hex-2-ulose


Petrus L, Petrusova M, Hricoviniova Z, catalyzed by molybdate ions

Topics in Current Chemistry 215: 15-41 2001


Schematic representation of the intramolecular rearrangement of the aldose chain, which occurs in the Bílik reaction as the consequence of the tetradentate binding of the aldose in the binuclear molybdate complex represented by the couple of the fused tetragonal bipyramides


The structure of the molybdate complex of erythritol of the aldose chain, which occurs in the


The structure of the molybdate complex of D-lyxose of the aldose chain, which occurs in the


The other known systems catalyzing of the aldose chain, which occurs in the the C-1-C-2 transposition

C-2 “epimerization” of aldoses

D-[1-13C]glucoseD-[2-13C]mannose

- Ni(II) – N-alkylated ethylenediamine1- Co(II) – N-alkylated ethylenediamine2- Ca(OH)2, partially Sr(OH)2ref. 3- La(OAc)3 in aqueous NaOHref. 4


The intramolecular rearrangement, characteristic for the of the aldose chain, which occurs in the Bílik reaction, also occurs in Nature. The non-mevalonate metabolic synthesis of isoprenoides, which includes the NADPH-dependant rearrangement andreduction of xylulose-5-phosphate to 2-C-methyl-D-erythritol-4-phosphate, is catalyzed by the Mn2+-metalloenzyme 1-deoxy-D-xylulose-5-phosphate-reductoisomerase (DXR). This enzyme, in addition to the rearrangement, is also reducing the intermediate C-2 branched chain aldose, the thermodynamic equilibrium of which with the unbranched-chain ketose is strongly shifted in favour of this ketose

J. Biol. Chem. 282, 2007, 19905-19916.

The Bílik reaction

Topics Curr. Chem. 215, 2001, 15-41


The catalysis of the transposition rearrangement of reducing sugars has been observed only with the complexes of elements (metals) possessing the d-orbitals.

These d-orbitalsare apparently essential for the occurrence of this rearrangement, which probably is dependent on the delocalization of the carbonyl group of the reducing sugar to the carbon atom neighboring with this carbonyl group, namely through the interaction of the d-orbitals of thecatalytic element with the p-orbitals of the participating carbon and oxygen atoms of the reducing sugar..


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