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									Chapter 22: Carbohydrates.
hydrates of carbon: general formula Cn(H2O)n
Plants: photosynthesis
                         hn
    6 CO2 + 6 H2O              C6H12O6 + 6 O2
Polymers: large molecules made up of repeating smaller units
   (monomer)
Biopolymers:                           Monomer units:
   carbohydrates                       monosaccharides
   peptides and proteins               amino acids
   nucleic acids                       nucleotides
22.2: Nomenclature and Structure of Carbohydrates.
I. Number of carbohydrate units
    monosaccharides: one carbohydrate unit- simple
carbohydrates
    disaccharides: two carbohydrate units - complex carbohydrates
    trisaccharides: three carbohydrate units                258
II. Position of carbonyl group
       at C1, carbonyl is an aldehyde: aldose (reducing sugar)
       at any other carbon, carbonyl is a ketone: ketose
           (non-reducing sugar)




                                                           259
III. Number of carbons
       three carbons: triose   six carbons: hexose
       four carbons: tetrose   seven carbons: heptose
       five carbons: pentose   etc.




IV. Ring size of the Cyclic form
       6-membered ring – pyranose
       5-membered ring – furanose




                                                        260
V. Stereochemistry - the D/L convention; before the R/S
   convention, stereochemistry was related to (+)-glyceraldehyde
Fischer projections. Representation of a three-dimensional
molecule as a flat structure. Tetrahedral carbon represented by
two crossed lines:
   horizontal line is coming                    vertical line is going back
   out of the plane of the                      behind the plane of the
   page (toward you)                            paper (away from you)
                         substituent   carbon

(R)-(+)-glyceraldehyde




(S)-(-)-glyceraldehyde



                                                                              261
          D-glyceraldehyde                    L-glyceraldehyde
          R-(+)-glyceraldhyde                 S-(-)-glyceraldhyde
          (+)-rotation = dextrorotatory = d   (-)-rotation = levorotatory = l

The hydroxyl group of R-(+)-glyceraldhyde was assigned with the
OH group of the chiral carbon (C2) pointing to the right in the
Fischer projection.

Carbohydrates are designated as D- or L- according to the
stereochemistry of the highest numbered chiral carbon of the
Fischer projection. If the hydroxyl group of the highest numbered
chiral carbon is pointing to the right, the sugar is designated as
D (Dextro: Latin for on the right side). If the hydroxyl group is
pointing to the left, the sugar is designated as L (Levo: Latin for
on the left side). Most naturally occurring carbohydrates are of
the D-configuration. Other than glyceraldehyde, there is no
relationship between D/L, R/S, and the sign of the optical rotation
                                                                  262
(+/- or d/l).
For carbohydrates, the convention is to arrange the Fischer
projection with the carbonyl group at the top for aldoses and
closest to the top for ketoses. The carbons are numbered from
top to bottom.




                                                           263
Manipulation of Fischer Projections
1. Fischer projections can be rotate by 180° (in the plane of the
   page) only!




               180°                             180°




                         Valid                            Valid
                        Fischer                          Fischer
                       projection                       projection


                                                              264
a 90° rotation inverts the stereochemistry and is illegal!




                                           This is not the correct convention
                  90°                      for Fischer projections

                                          Should be projecting toward you
                                         Should be projecting away you


                                         This is the correct convention
                                         for Fischer projections and is
                                         the enantiomer



                                                                          265
2. If one group of a Fischer projection is held steady, the other
    three groups can be rotated clockwise or counterclockwise.




                         120°            120°
           hold             hold
          steady           steady             hold
                                             steady



                         120°            120°

                        hold             hold
                                                         hold    266
                       steady           steady
                                                        steady
Assigning R and S Configuration to Fischer Projections
1. Assign priorities to the four substitutents according to the
   Cahn-Ingold-Prelog rules
2. Perform the two allowed manipulations of the Fischer projection
   to place the lowest priority group at the top or bottom.
3. If the priority of the other groups 123 is clockwise then
   assign the carbon as R, if priority of the other groups 123
   is counterclockwise then assign the center as S.




                                                             267
Fischer projections with more than one chiral center:


                                                  (2S, 3R)




                                                             268
The cyclic form (hemi-acetal or hemi-ketal) of carbohydrates
Recall acetal/ketal formation from Chapter . . .




In the case of carbohydrates, cyclization to the hemi-acetal
creates a new chiral center.
                                               -D-Glucopyranose (64%)
                                                (-anomer: C1-OH and
                                                    CH2OH are cis)


                                               -D-Glucopyranose (36%)
                                                (-anomer: C1-OH and
                                                   CH2OH are trans)
                                                                  269
The hemiacetal or hemiketal carbon of the cyclic form of
carbohydrates is the anomeric carbon. Carbohydrate isomers that
differ only in the stereochemistry of the anomeric carbon are
called anomers. (Table 22.2, p. 1135)
Converting Fischer projections to Haworth projections




                                                          270
271
Aldotrioses (C3) - Glyceraldehyde is the simplest carbohydrate:
2,3-dihydroxypropanal




Aldotetroses – (C4)2,3,4-trihydroxybutanal




                                                             272
Aldopentoses (C5) -three chiral carbons, eight stereoisomers




Aldohexoses (C6) - four chiral carbons, sixteen stereoisomers




                                                               273
22.3: Formation of Carbohydrates
22.3a: Lengthening Chains in Carbohydrates. Kiliani-Fischer
Synthesis- one-carbon chain extension of monosaccharides




                                                        274
22.3a: Shortening Chains in Carbohydrates.
Ruff Degradation:




Wohl Degradation:




                                             275
22.4. Reactions of Carbohydrates.
22.4a: Mutarotation. The - and -anomers are in equilibrium,
and interconvert through the open form. The pure anomers can
be isolated by crystallization. When the pure anomers are
dissolved in water they undergo mutarotation, the process by
which they return to an equilibrium mixture of the anomer.


                                          64%




                                          36%

22.4b: Epimerization in Base. Recall stereochemical scrambling
in Chapter 19.3.


                                                          276
22.4c: Reduction. C1 of aldoses are reduced with sodium
borohydride to the 1° alcohol (alditols).



                                                          277
22.4d: Oxidation. C1 of aldoses can be selectively oxidized to the
carboxylic acid (aldonic acids) with Br2 or Ag(I) (Tollen’s test).




Reducing sugars: carbohydrates that can be oxidized to aldonic
acids.




                                          cellobiose and maltose
                                            are reducing sugar
                                                                   278
   lactose is a reducing sugar




sucrose is not a reducing sugar




  Is fructose a reducing sugar?




                          279
Oxidation to aldaric acids:




22.4e: Osazone Formation. Treatment of carbohydrates with
phenylhydrazine (C6H5-NHNH2) leads to phenylhydrazone
formation with the carbonyl. Treatment with 3 equiv. of phenyl-
hydrazine leads to osazone formation.




                                      Mechanism: Fig: 22.33
                                                              280
22.4f: Ether and Ester Formation.




                                    281
Glycosides. Mixed acetals and ketals of the cyclic form of
carbohydrates
                                                      Chapter 16.9




                                                              282
Note that only the anomeric hydroxyl group is replaced by ROH
                                                    Mechanism:
                                                    Fig. 22.39 and
                                                    Problem 22.21




When ROH is another carbohydrate unit, disaccharides
(complex carbohydrates) are formed.



                                                            283
22.4g: Modern Carbohydrate Chemistry (please read).
22.5: The Fischer Determination of the Structure of D-Glucose
(and the 15 Other Aldohexoses).

                         1) KCN                 CHO                        CO2H
                         2) H2, Pd
                                            H     OH               H         OH
                         3) H2O                             HNO3
                                            H     OH               H         OH
                                                CH2OH                      CO2H
                                        D-(-)-erythrose

            CHO
        H     OH         Killiani-Fischer
            CH2OH            synthesis

  D-(+)-glyceraldehyde


                                                CHO                         CO2H
                                        HO         H               HO         H
                                                            HNO3
                                            H     OH                   H      OH
                         1) KCN
                                                CH2OH                       CO2H
                         2) H2, Pd
                         3) H2O
                                            D-(-)-threose


                                                                                   284
285
286
enantiomers   287
22.6: Special Topic: An Introduction to Di- and
Polysaccharides.
monosaccharide + monosaccharide = disaccharide + H2O
  Cn(H2O)n     +   Cn(H2O)n      Cn(H2O)n + Cn(H2O)n - H2O




                                                     288
289
Polysaccharides. Cellulose: glucose polymer made up of 1,4’-
-glycoside linkages



Amylose: glucose polymer made up of 1,4’--glycoside linkages




Amylopectin and glycogens are branched polysaccharides related
to amylose and cellulose




                                                         290
The 8 essential carbohydrates




                                291
Glycoproteins: Glycosides of proteins




                                        292

								
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