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CONCEPT: INTRODUCTION TO CARBOHYDRATE MONOSACCHARIDES Sugars or saccharides are also referred to as carbo-hydrates, implying that carbon has been combined with Monosaccharides are the most basic units of sugars All unmodified monosaccharides have the same general formula:, where n 3 Monosaccharides can be represented as straight chains OR rings One oxygen attached to each and every carbon atom, and 1 IHD regardless of form Monosaccharides begin as either or Aldehyde sugar = Ketone sugar = General Features: Monosaccharides can be described by both generic names and specific names. Generic naming involves: Carbonyl type (i.e. aldo ) Carbon Chain Length (Pre-IUPAC) with ose ending (i.e. triose, -tetrose, pentose, -hexose, etc) The total number of stereoisomers possible is described by Epimers are stereoisomers of monosaccharaides differing at only one chiral carbon EXAMPLE: Epimers of aldohexose Page 2
PRACTICE: Provide the generic name for the following monosaccharide. PRACTICE: How many possible stereoisomers AND epimers exist for the following aldopentose? Draw all of the possible epimers. Page 3
PRACTICE: Identify and label all of the possible stereoisomers of aldotetrose as enantiomers, diastereomers or epimers of each other. Page 4
CONCEPT: MONOSACCHARIDES D and L-ISOMERISM All monosaccharides come in dextrorotary (D) and levorotary (L) forms. These are enantiomers of each other. Monosaccharide stereochemistry is determined by the penultimate carbon or chiral carbon NOTE: This carbon (C-5) will be used as a later in this chapter D = - configuration (-OH pointing RIGHT) L = - configuration (-OH pointing LEFT) PRACTICE: Provide the generic name, including stereochemistry, for the following monosaccharides: Page 5
CONCEPT: MONOSACCHARIDES DRAWING FISCHER PROJECTIONS In 1891, Emil Fischer devised a representation known as Fischer Projections specifically to depict carbohydrates. The most atom is always represented on It is necessary to learn how to convert Bondline to Fischer and Fischer to Bondline Bondline to Fischer: Similar to our lessons in Orgo 1, we can use the caterpillar method to convert Bondline to Fischer Alternatively we can just swap the stereochemistry of all downward facing alcohols Fischer to Bondline: Reverse caterpillar is the most reliable method. Two possible answers Page 6
PRACTICE: Convert the following monosaccharide into its Fischer representation. Is it a D or L-isomer? PRACTICE: Convert the following monosaccharide into a bondline representation. Page 7
CONCEPT: MONOSACCHARIDES COMMON STRUCTURES Pentoses and hexoses are highly prevalent in biological systems. You probably only need to know a few of these. Biological systems tend to recognize D-monosaccharides more than L- monosaccharides The other important monosaccharides can all be related back to these basic structures. Page 8
CONCEPT: MONOSACCHARIDES FORMING CYCLIC HEMIACETALS By definition, monosaccharides contain at least one carbonyl group and multiple alcohols. The nucleophilic addition of 1 eq. alcohol produces hemiacetals. A second equivalent produces acetals Recall that the only stable hemiacetals are cyclic (5 and 6-membered rings) Thus many monosaccharides can undergo reversible intramolecular, ring-forming hemiacetal mechanism EXAMPLE: D-Glucose undergoes nucleophilic addition to form a cyclic, 6-membered hemiacetal. Page 9
PRACTICE: Provide the mechanism for the cyclic hemiacetal formation of the following hydroxycarbonyl. Page 10
CONCEPT: MONOSACCHARIDES CYCLIZATION In aqueous solutions, monosaccharides are most stable in a cyclic form. Furanose = -carbon cyclic sugar Pyranose = -carbon cyclic sugar Nucleophilic addition of the penultimate alcohol to the electrophilic carbonyl carbon leads to cyclization The carbonyl carbon is, so it can be attacked from either the top or bottom When monosaccharides cyclize, two different C-1 epimers are possible. These are known as anomers - The α-anomer = anomeric oxygen is with the stereodescriptor (C-5) carbon - The β-anomer = anomeric oxygen is with the stereodescriptor (C-5) carbon The OH s on the in the straight chain point in the ring The OH s on the in the straight chain point in the ring - D-Sugars: Stereodescriptor (C-5) faces on the ring PRACTICE: Draw the β-anomer predicted through the cyclization of D-mannose. Page 11
CONCEPT: MONOSACCHARIDES HAWORTH PROJECTIONS Simplified drawings of sugars in their cyclic form, imagining that the ringed structure is planar. NOTE: The anomeric carbon is always drawn at the far in Haworth Projections This is true for both pyranose and furanose rings PRACTICE: Properly number the following Haworth Projections. Identify each anomer and predict anomeric equilibrium. PRACTICE: Draw the Haworth Projection that would be expected from cyclization of D-Fructose into α-d-fructofuranose. Page 12
CONCEPT: MONOSACCHARIDES MUTOROTATION Pyranose and furanose rings are constantly hydrolyzing back and forth between cyclic and straight-chain forms. Mutorotation: The process by which the anomeric position interconverts between α and β forms Anomers always differ in optical activity. e.g. α-d-glucopyranose = +112.2 ; β-d-glucopyranose = +18.7 They are NOT enantiomers of each other. Hence the unrelated activities In solution, any D-glucopyranose will always equilibrate to +52.5, indicating mutorotation Acid-Catalyzed Mechanism: Page 13