Anomers of Glucose anomerie carbon H H CHOH CH2OH CHOH HO HO HO H H HO -H HO HO OH OH OH OH H OH H H H a-D-glucopyranose open-chain form B-D-glucopyranose Copyright2010 Pearson Prentice Hall,Inc. The hydroxyl group on the anomeric (hemiacetal) carbon is down (axial)in the a anomer and up (equatorial)in the B anomer. The B anomer of glucose has all its substituents in equatorial positions. The hemiacetal carbon is called the anomeric carbon, easily identified as the only carbon atom bonded to two oxygens. Chapter 23 16
Chapter 23 16 Anomers of Glucose ▪ The hydroxyl group on the anomeric (hemiacetal) carbon is down (axial) in the α anomer and up (equatorial) in the β anomer. ▪ The β anomer of glucose has all its substituents in equatorial positions. ▪ The hemiacetal carbon is called the anomeric carbon, easily identified as the only carbon atom bonded to two oxygens
Anomers of Fructose anomeric carbon cis =B HOCH2O. CH2OH HOCHOH HOCH2O H HO入2 5KH HOC2 CH2OH OH H HO H43 OH 3 CH2OH OH H OH H trans=c☒ a-D-fructofuranose B-D-fructofuranose Copyright2010 Pearson Prentice Hall.Inc The a anomer of fructose has the anomeric -OH group down,trans to the terminal -CH2OH group. The B anomer has the anomeric-OH group up,cis to the terminal -CH2OH. Chapter 23 17
Chapter 23 17 Anomers of Fructose ▪ The a anomer of fructose has the anomeric —OH group down, trans to the terminal —CH2OH group. ▪ The b anomer has the anomeric —OH group up, cis to the terminal —CH2OH
Mutarotation CHOH H CH OH HO CHOH ● HO HO H 0 H HO OH H OH C~H a anomer open-chain form 阝anome crystallize helow 9c qulibru中solutio y llizc above9W H CHOH CH,OH HO大 HO, HO- 40 -H -0H OH OH H OH H Copyright2010 Pearson Prentice Hall,Inc. ■ An aqueous solution of D-glucose contains an equilibrium mixture of a-D-glucopyranose,B-D- glycopyranose,and the intermediate open-chain form. Crystallization below 98C gives the a anomer,and crystallization above 98C gives the B anomer. Chapter 23 18
Chapter 23 18 Mutarotation ▪ An aqueous solution of D-glucose contains an equilibrium mixture of α-D-glucopyranose, β-Dglycopyranose, and the intermediate open-chain form. ▪ Crystallization below 98°C gives the α anomer, and crystallization above 98°C gives the β anomer
Base-Catalyzed Epimerization of Glucose H、 =0 H H HO HOH C-OH C一OH HO-C✉H HO- 一H HO -H HO -H H-Q-H HO- 一H+ -OH ←→ H -OH H -OH H -OH H 一OH H -OH H OH H -OH H -OH CH2OH CHOH CH2OH CH2OH D-glucose enolate D-mannose Copyright2010 Pearson Prentice Hall,Inc. ■ Under basic conditions,stereochemistry is lost at the carbon atom next to the carbonyl group. The enolate intermediate is not chiral,so reprotonation can produce either stereoisomer. Because a mixture of epimers results,this stereochemical change is called epimerization. Chapter 23 19
Chapter 23 19 Base-Catalyzed Epimerization of Glucose ▪ Under basic conditions, stereochemistry is lost at the carbon atom next to the carbonyl group. ▪ The enolate intermediate is not chiral, so reprotonation can produce either stereoisomer. ▪ Because a mixture of epimers results, this stereochemical change is called epimerization
Enediol Rearrangement Step I:Remove Step 2: Step 3: Step 4:Reprotonate on CI the a proton Reprotonate Deprotonate on O the O on C2 H H OH OH +OH ·TOH OH +-OH H-C-OH OH -OH =0 HO- H HO H HO H →H0 H HO H H -OH OH H OH H OH H OH H-OH -OH H- -OH H OH H -OH CH2OH CH2OH CH2OH CH,OH CHOH D-glucose enolate enediol enolate D-fructose In base,the position of the carbonyl can shift. Chemists use acidic or neutral solutions of sugars to prevent this rearrangement. Chapter 23 20
Chapter 23 20 Enediol Rearrangement ▪ In base, the position of the carbonyl can shift. ▪ Chemists use acidic or neutral solutions of sugars to prevent this rearrangement