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Home , Pyranose

Chapter 24: [Sections: 24.1–24.10]

Carbohydrates definition • naturally occuring compounds derived from carbon, oxygen and hydrogen • the net molecular formula comes from each carbon having an equivalent of water, hence, hydrates of carbon

= C6H12O6 = 6 carbons and 6 water molecules (6 x H2O) • commonly referred to as "" or "saccharides" (driving from saccharum, which is latin for )

simple carbohydrates complex carbohydrates

• carbohydrates that CANNOT be • carbohydrates that CAN be hydrolyzed hydrolyzed to simpler carbohydrates to simpler carbohydrates + H3O

"" "" "" ""

Monosaccharides: polyhydroxy aldehydes and ketones polyhydroxy aldehydes polyhydroxyketones

CH2OH group CH2OH at top CHO group O H ketone carbonyl at top group at 2nd O OH bearing carbon chiral carbons OH bearing H OH H OH chiral carbons CH2OH CH2OH CH2OH group CH2OH group at bottom at bottom

3 carbons in length = 4 carbons in length =

CH2OH CHO O HO H HO H H OH H OH HO H H OH CH2OH

CH2OH • "ose" ending implies a • most common in nature are aldopentose, aldohexose, ketopentose, ketohexose • each of these are examples of monosaccharides The simplest : D– and L-

O H O H O H

H * OH H OH HO * H

CH2OH CH2OH CH2OH

Fischer projection • D-gylceraldehyde (the R enantiomer) rotates a plane of polarized light in the dextroratory (+) direction in a polarimeter while L-glyceraldehyde is levoratory (–) • remember: we cannot predict whether any given enantiomer is (+) or (–) without either: i) conducting an experiment with a polarimeter, or ii) knowing the direction that light is rotated by the other enantiomer • D-glyceraldehyde is naturally–occurring, while L-glceraldehyde is not! • All naturally-occurring monosacharrides derive from D-glyceraldehyde

Kiliani-Fischer Synthesis CN CHO

O D- H H2 , Pd/BaSO4 H OH HCN H OH H2O H * OH NaCN CH2OH CH2OH CH2OH CN CHO D-glyceraldehyde (aldotriose) D- H2 , Pd/BaSO4 H OH H2O H OH CH OH O 2 CH2OH

CHO H NH O

R R C N R R

H H

CH2OH L-threose ?

• Kiliani-Fischer synthesis produces two new monosaccharides with one additional carbon atom • two stereoisomeric products result. Type of stereoisomers = • monosaccharide stereoisomers of this type are also called "epimers" because they differ ONLY in stereochemistry of a single stereogenic carbon atom • both stereoisomers called "D" NOT because they are (+)-rotating[they are actually both levoratory] but because they derive from D-glyceraldehyde and have the D-stereochemistry at the bottom-most stereogenic carbon • both are naturally occurring. L-sugars are NOT naturally–occurring! • ALL naturally occurring sugars, both aldoses and , are D-sugars Glucose • Glucose is a naturally-occurring aldohexose. Given this information, draw as much of its structure as possible:

• given this limited information, how many possible stereoisomers could be drawn for glucose? Problems: 1, 2 A more complete perspective and summary:

• naturally-occurring monosaccharides, whether aldoses or ketoses, share in common the stereochemistry of the CHOH group at the stereogenic carbon nearest the bottom. • D-glucose and D- are particularly important sugars as far as human energy supplies are concerned

Problems: 1–4 Acetals and hemi-acetals O + MeO OMe CH3OH, H

R H R H acetal

O CH3OH

R H

O CH OH • in the absence of an acid catalyst, the 3 HO OMe reaction of an aldehyde with an alcohol cannot proceed past the hemi-acetal stage R H R H • this is an equilibrium process that in aldehyde (99%) hemi-acetal (1%) ordinary cases heavily favors the starting aldehyde

O O O HO H HO H pyran

• however, if a molecule contains BOTH the alcohol group for the reaction AND the aldehyde group, formation of the hemi-acetal is strongly favored when a stable 5- or 6-membered ring can be formed

Pyranose rings

CHO CH2OH CH2OH H OH O H H O HO H H H OH H + OH H H OH OH OH H OH H OH H OH

CH2OH

• upon closure to form the 6-membered pyranose ring, two different stereoisomers (called ) are formed, based on the stereochemistry of the OH group at the hemi-acetal linkage (or anomeric carbon) • when pyranoses are drawn in this conventional manner, the α- has the OH group pointing down (trans to the CH2OH group) and the β-anomer has the OH group pointing up (cis to the CH2OH group) • the stereochemistries of the other OH groups are defined by the stereochemistries in the monosaccharide predicting the structure of, and naming, pyranose rings:

CHO H H OH CH OH H H OH H 2 H HO H H OH HOH2C CHO OH H O H OH OH OH OH H OH H OH H OH

CH2OH glucose H CH OH CH2OH 2 O H O OH H O H H OH H OH H H OH H OH CH2OH H OH OH H OH H H β-anomer H OH H O Name: OH H CH2OH H OH CH2OH O O H H H H H H OH H OH H OH O OH OH H OH H OH α-anomer Name:

Shortcut for drawing pyranose rings of D-sugars CHO H OH HO H H OH H OH

CH2OH

predict the structures, and corresponding names, of the pyranose rings of D-:

CHO HO H HO H HO H H OH

CH2OH

Problems: 5 • the two anomers of glucose can be separated and purified • the β-anomer has [α] = +18.7°, the α-anomer has [α] = +112.2° • no matter which isomer is started with, however, upon sitting in aqueous solution the final optical rotation from the polarimeter = +52.6° which is somewhere in between the readings of either pure compound • this results from the fact that each ring is in equilibrium with the "open form" and hence in equilibrium with the other anomer as well • the result is a mixture of the two anomers with a net resulting optical rotation CHO CH2OH CH2OH H OH pure β H O H pure α H O OH HO H H H OH H +18.7° OH H H OH +112.2° H OH OH OH H OH H OH H OH CH2OH β-D-glucopyranose "open form" glucose α-D-glucopyranose 63% < 0.01% 37% • the interconversion of one anomer with the other in this way is known as "mutarotation"

Relative stability of the two glucopyranose anomers CH OH 2 CH2OH O H H O H O OH O H H OH H OH H OH OH OH H H OH H OH

Glycoside formation

O + CH3OH O H HO H H+ OH hemi-acetal CH2OH CH2OH H O OH H H O OCH3 OH H H OH H OH H methyl β-D-glucopyranoside OH H CHO H OH H OH H OH CH3OH CH OH, H+ HO H + 3 H OH H+ x H OH CH2OH CH2OH CH2OH H O H H H O H OH H H D–glucose OH H methyl α-D-glucopyranoside OH OH OH OCH3 H OH H OH • formation of an acetal via a "glycoside" linkage •when formed from a sugar molecule it is called a "pyranoside" since it comes from glucopyranose • in the absence of an acid catalyst, the anomeric carbon of pyranosides is locked in place • pyranosides will not undergo mutarotation under neutral conditions Problems: 6

Reducing sugars: Distinguishing between pyranose and pyranoside molecules.

CHO CO2H H OH + H OH HOH2C Ag in NH OH O 4 HO HO H HO H OH + Ag° H OH HO OH Tollen's reagent H OH H OH H OH CH OH pyranose 2 CH2OH

+ HOH2C Ag in NH4OH HO O OCH3 HO OH Tollen's reagent pyranoside

Disaccharides

HOH C HOH C 2 O 2 O HO HO HOH2C HO O HO OH HO OH OH OH + HO OH H3O CH2OH α-glucopyranose O O

HO OH OH

• maltose is a disaccharide and a complex carbohydrate (can be hydrolyzed) • the two monosaccharides are connected via a "glycoside" linkage • in maltose, the α-pyranoside linkage is retained between the two sugars and is fixed • the hemi-acetal linkage, however, is still able to mutatorate • is a made up of thousands of repeating glucose units • all of the units are connected by the α- glycoside link • amylose is a major consituent of in food that humans rely upon for food/energy • more complex branched polymers () are formed by the body as a way of storing glucose for energy

HOH2C HOH2C HOH2C O O O HO HO HO HOH2C O OH O HO OH HO OH + HO OH H3O OH HO OH OH β-glucopyranose • cellobiose is also a disaccharide and a complex carbohydrate (can be hydrolyzed) • in cellobiose the β-gylycoside linkage is retained between the two sugars and is fixed • the hemi-acetal linkage is still able to mutatorate • is a polysaccharide made of ~7,000 units on average of glucose • all of the units are connected by the β- glycoside link

• humans have enzymes specific for hydrolysis of the α-glycoside link and can therefore hydrolyze starches like amylose to release the glucose for energy • humans lack the enzymes necessary to hydrolyze the β-glycoside link and cannot, therefore, liberate the glucose from like cellobiose • cows also do not naturally produce enzymes necessary to hydrolyze the β-glycosidee link but their stomachs contain microorganisms that DO produce the required enzymes so that cows are able to digest cellulose (e.g., grasses) Some other interesting polysaccharides... • hydrolyzed in the body by an enzyme called lactase • lactase production begins to decline for children past age 2 and continues • ~75% of adults have intolerance

D- D-Glucose

type of glycoside link?

D-Fructose 2008 Georgia sugar refinery explosion type of glycoside link to glucose? D-Glucose where is the glycoside link for fructose?

31% glucose 38% fructose 17% water 7% maltose 1%

nectar: 90% water with a mixture of compounds comprised of 55% sucrose, 24% glucose, 21% fructose plus aroma chemicals and proteins

• derivatized • similar structure to cellulose • NH bonds allow for hydrogen bonding which increases the strength of strands • main constituent of insect exoskeletons Chapter 24 Essential Concepts

1. Know the molecular formula for all carbohydrates and be able to differentiate complex from simple carbohydrates 2. Know the basic structure of all aldoses and all ketoses, understand how to name the structures, and how to determine if they are (D) naturally occurring or (L) non-naturally occurring. 3. Understand the sequence involved in the Kiliani-Fischer synthesis. Be able to predict products. 4. Know why sugars adopt ring structures, be able to form pyranose rings from a given D- aldohexose sugar, and be able to assign proper names for the anomers. 5. Understand the process of mutarotation and why one anomer might be favored over another. 6. Be able to draw simple alkyl (typically methyl) pyranosides and assign names. 7. Know Tollen’s reagent, why it is used, and what it determines about a sugar’s structure. 8. Understand how complex carbohydrates are built up from simple carbohydrates. You should be able to pick out glycoside (acetal) versus hemi-acetal linkages. 9. Be familiar with some of the common complex carbohydrates discussed in class.