Worked Example 21.1 Assigning R or S Configuration to a Fischer Projection Going from first-to second-to third-highest priority requires a counterclockwise turn,corresponding to S stereochemistry 4 4 H H H 2H02C NH21= 2 HO2C-C-NH2 H3C--C--NH2 CH3 CH3 HO2C 3 →3 S configuration
▪ Going from first- to second- to third-highest priority requires a counterclockwise turn, corresponding to S stereochemistry Worked Example 21.1 Assigning R or S Configuration to a Fischer Projection
21-3 D,L Sugars Glyceraldehyde Simplest aldose One chirality center Two enantiomeric (mirror-image)forms Only dextrorotatory enantiomer (-)-glyceraldehyde occurs naturally (+)-Glyceraldehyde has the R configuration (R)-(+)-glyceraldehyde is also referred to as D- glyderaldehyde (D for dextrorotatory) (S)-(-)-glyceraldehyde in also known as L-glyceraldehyde (L for levorotatory)
Glyceraldehyde ▪ Simplest aldose ▪ One chirality center ▪ Two enantiomeric (mirror-image) forms ▪ Only dextrorotatory enantiomer (–)-glyceraldehyde occurs naturally ▪ (+)-Glyceraldehyde has the R configuration ▪ (R)-(+)-glyceraldehyde is also referred to as Dglyderaldehyde (D for dextrorotatory) ▪ (S)-(–)-glyceraldehyde in also known as L-glyceraldehyde (L for levorotatory) 21-3 D,L Sugars
D,L Sugars Virtually all naturally occurring monosaccharides have the same R stereochemical configuration as D-glyceraldehyde at the chirality center farthest from the carbonyl group In Fischer projections most naturally occurring sugars have the hydroxyl group at the bottom chirality center pointing to the right Such compounds known as D sugars c=0 H H H CH2OH C=0 H OH H OH H OH HO H CH2OH H OH HO H H OH D-Glyceraldehyde H OH H OH [(R)-(+)-glyceraldehyde] H OH CH2OH H OH CH2OH CH2OH D-Ribose D-Glucose D-Fructose
Virtually all naturally occurring monosaccharides have the same R stereochemical configuration as D-glyceraldehyde at the chirality center farthest from the carbonyl group ▪ In Fischer projections most naturally occurring sugars have the hydroxyl group at the bottom chirality center pointing to the right ▪ Such compounds known as D sugars D,L Sugars
D,L Sugars L sugars have an S stereochemical configuration at the chirality center farthest from the carbonyl group -OH group pointing to the left in Fischer projections An L sugar is the mirror image(enantiomer)of the corresponding D sugar D and L sugars can be either dextrorotatory or levorotatory D and L designations only specify the stereochemical configuration at the one chirality center farthest away from the carbonyl group c=0 H H HO- 一H HO H H -OH CH2OH OH HO -H L-Glyceraldehyde HO- -H H OH [(S)-(-)-glyceraldehyde] HO- 一H H 一OH CH2OH CH2OH L-Glucose D-Glucose (not naturally occurring)
L sugars have an S stereochemical configuration at the chirality center farthest from the carbonyl group ▪ –OH group pointing to the left in Fischer projections ▪ An L sugar is the mirror image (enantiomer) of the corresponding D sugar D and L sugars can be either dextrorotatory or levorotatory ▪ D and L designations only specify the stereochemical configuration at the one chirality center farthest away from the carbonyl group D,L Sugars
21-4 Configurations of the Aldoses Aldotetroses are four-carbon sugars with two chirality centers and an aldehyde carbonyl group 22=4 possible stereoisomeric aldotetroses Two D,L pairs or enantiomers named erythrose and threose Aldopentoses are five-carbon sugars with three chirality centers and an aldehyde carbonyl group 23 =8 possible stereoisomeric aldopentoses Four D,L pairs of enantiomers named ribose,arabinose, xylose,and lyxose All but lyxose occur widely D-Ribose is an important constituent in RNA L-Arabinose is found in plants -D-Xylose is found in both plants and animals
Aldotetroses are four-carbon sugars with two chirality centers and an aldehyde carbonyl group ▪ 2 2 = 4 possible stereoisomeric aldotetroses ▪ Two D,L pairs or enantiomers named erythrose and threose Aldopentoses are five-carbon sugars with three chirality centers and an aldehyde carbonyl group ▪ 2 3 = 8 possible stereoisomeric aldopentoses ▪ Four D,L pairs of enantiomers named ribose, arabinose, xylose, and lyxose ▪ All but lyxose occur widely ▪ D-Ribose is an important constituent in RNA ▪ L-Arabinose is found in plants ▪ D-Xylose is found in both plants and animals 21-4 Configurations of the Aldoses