Bài giảng Organic Chemistry - Chapter 24: Carbohydrates

Carbohydrate = Cn(H2O)nCarbohydrates occur in nature in nucleic acids, fats, cellulose, fibers, starch, “table sugar,” antibiotics, and other biological molecules.“Hydrated carbon”A pentahydroxyaldehydeImportant recognition molecules on the cell surface:Chapter 24: CarbohydratesNamingThe simplest carbohydrates are the sugars or saccharides. They constitute polyhydroxy-aldehydes (aldoses) or -ketones (ketoses); they form oligomers by ether bridges (hence di-, tri-, tetrasaccharide, etc.).The simplest sugars, both C3(H2O)3: 2,3-Dihydroxypropanal(Glyceraldehyde)An aldotriose1,3-DihydroxyacetoneA ketotrioseChain length: Triose, tetrose, pentose, etc.ChiralFischer projections: Review chapter 5Some important monosaccharides:Dextrose, blood sugar, grape sugarSweetest natural sugar; fruitsRibonucleic acids4 Stereo-centers3 Stereo-centers**********Fischer Projection: A flat stencil CH3CH2CH3HBrEyes in the plane of the boardDepending on your starting dashed-wedged line structure, several Fischer projections are possible for the same molecule. BrHCCH3CH2CH3HBrRules reminder:180 º turn or double exchange leaves stereochemistry intactMost sugars are chiral and occur enantiomerically pure. Simplest case, one stereocenter:In almost all natural sugars, the stereocenter furthest away from carbonyl (drawn at the top) has the same absolute configuration as D-glyceraldehyde: “D-sugars”D and L are an older nomenclature (predates the knowledge of the absolute configuration of glyceraldehyde). The dextrorotatory enantiomer was called D, the other L. Later, D was found to be R, L therefore S.Commonly: Natural UnnaturalRules for arranging the Fischer stencil: Carbonyl on top, places bottom C*OH on the right in the D sugars. D-Threose L-ThreoseD-Erythrose L-ErythroseThe Family of D-AldosesrareunnaturalrareunnaturalunnaturalrareL-form more commonThe Family of D-KetosesFive-membered ringSix-membered ringTwo diastereomers: AnomersCyclic Hemiacetal Formation by GlucoseTwo diastereomers: AnomersOther ways of drawing cyclic structures:Not a carbon atomBest are conformational pictures: Mutarotation: Change in observed optical rotation when a sugar molecule equilibrates with its anomer.Anomeric carbonAnomeric carbonOH down: α-Anomer; crystallizesOH up: β-Anomer; more stable because all-equatorial Reactions of SugarsOxidationa. CHO  COOH (aldose  aldonic acid): Br2, H2OMechanism of bromine oxidation:b. Oxidation of both ends of aldoses → aldaric acidNote selectivity of nitric acid: Picks on primary OH function (after oxidizing the formyl group): Less hindered.Mechanism:Nitrous acidFor some sugars, this oxidation may give meso (achiral) aldaric acid. Can be used for proof of stereochemistry. D-(+)-Allose: chiral, dextrorotatoryAllaric acid: meso, achiral, no specific rotation Mirror planeSymmetry becomes obvious also in NMR, e.g. 13C NMR: 6 peaks3 peaksc. Oxidative cleavage: HIO4This reagent causes the rupture of vicinal diols to dialdehydes. How?(like a cyclic acetal)Periodic acidCompare:How does this work for sugars? Leads to complete degradation of the carbon chain.Note: Each carbon retains the same number of attachedhydrogen atoms as were present in the original sugar.Gives length of sugar and whether it is an aldose or ketoseNote: Each carbon retains the same number of attachedhydrogen atoms as were present in the original sugar.Another way to think about this is as a “dihydroxylative” cleavage of each chain C-C bond, e.g. fructose:OHOHOHOHOHOHOHOHOHOHAdd OHs to each side of the bond brokenKetose2. Reduction to alditolsSorbitol (“sugar alcohol”) is used as artificial sweetener in diet foods: 2.6 cal/g per versus 4 cal/g for normal sugar. Sorbitol also occurs naturally in many stone fruits.Note: Just as in the oxidation to aldaric acids, reduction may symmetrize the sugar.Ketoses and isomeric aldoses give the same alditols3. Esters and Ethers: ProtectionAcetalHemiacetalAcetal function can be deprotected selectivelyProtection of anomeric carbon  no mutarotation, no aldehyde oxidation (i.e. does not behave as reducing sugar), no reduction.Mild, does not touch normal etherAlternative exploitation of the special reactivity of the (hemi)acetal function: Turn it into an acetal, called glycoside for sugars.Protection as cyclic acetalsRecall:β-D-Altroseβ-D-Altrose bisacetonide-CH2OH often not engaged: Flexibility makes entropy of acetal formation worse4. Kiliani-Fischer Extension (Modified) Lindlar typeHeinrich Kiliani 1855 - 1945Emil Fischer 1852-1919Nobel prize 1902 Example:D-LyxoseD-TaloseD-Galactose**New stereocenter5. Ruff DegradationNote: Both diastereomers (R or S at the top stereocenter) degrade to the same lower sugar.Fe3+  Fe2+Fe3+  Fe2+Otto Ruff1871-1939Aldonic acidLower aldoseExample:D-LyxoseD-TaloseD-Galactose**StereocenterlostHigher SaccharidesSucrose: Disaccharide derived from glucose and fructoseEther bridge between respective anomeric centers: nonreducing“Table sugar”150 lb/ person/ year worldwideCellulose: Glucose polymer with β–acetal linksMolecular weight 500,000 (~3000 units of glucose; 1 unit = 178 molecular weight. Used in cell wall material: Rigid structure due to multiple hydrogen bonds. Wood is largely cellulose and lignin.Paper and cotton are nearly pure cellulose.