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Wood Chemistry PSE 406 Lecture 4 More Sugars: Disaccharides and Rings PSE 406 - Lecture 4 1 Hemiacetal Formation A hemiacetal is a compound having the C(OH)OR group that results from the reaction of an aldehyde with one mole of an alcohol. Sugar molecules contain an aldehyde group and lots of alcohol groups. PSE 406 - Lecture 4 2 Hemiacetal Formation in Sugars (1) CH2OH H C C H OH HO CH2OH O .. H H C C C H OH O H H C C H O .. OH C C H OH H C C H OH HO CH2OH HO H O OH C H CH2OH H H HO C C C H OH O H C C H H O OH C OH H C C H OH H Notes PSE 406 - Lecture 4 3 Hemiacetal Formation in Sugars (2) β-D-glucopyranose α-D-glucopyranose PSE 406 - Lecture 4 4 Important Monosaccharide Projections CHO H HO CH2OH O OH H H OH H O HO H H OH OH OH CH2OH D-glucose Fisher HO H H OH HO H OH HO OH α-D-glucopyranose Haworth α-D-glucopyranose Chair Configuration Notes PSE 406 - Lecture 4 5 Conformation: The Chair a a=axial e=equatorial e HO CH2OH O HO OH Chair OH -d-glucopyranose β-D-glucopyranose The most stable form of a sugar in a six membered ring is in the chair formation. The substituent groups on the molecule are either in an axial or equatorial position depending on what gives the most stable molecule. PSE 406 - Lecture 4 6 Conformation: Other Forms Skew Boat Boat We won’t be worrying about these forms. PSE 406 - Lecture 4 7 The Haworth Projection This is a very common way that wood chemists draw sugars in the ring form. In this projection, the thick bonds are perpendicular to the plane of the paper CH 2OH O OH OH HO OH Substituents are parallel to the plane of the paper and lie either above or below the plane of the molecule PSE 406 - Lecture 4 8 Formation of Anomers Ring closure creates new chiral center at C1; new pair C1 epimers termed anomers. For D sugars, the following is true: » alpha form: OH below the ring » beta form: OH above the ring For L sugars, the opposite is true. H OH H O HO H HO H OH H OH a-D-glucopyranose H OH H O HO OH HO H H -D-glucopyranose PSE 406 - Lecture 4 OH H 9 Nomenclature H O H H OH HO H CH2OH O OH H OH -D-xylopyranose H OH OH H OH H H OH H OH H H CH2OH O O H HO H H -D-xylofuranose H H H OH OH H H OH OH a-D-xylofuranose a-D-xylopyranose PSE 406 - Lecture 4 10 Mutarotation (1) CH2OH O H H OH HO CH2OH OH H H OH HO H H H OH H OH H O H H H C OH H CH2OH OH H H OH HO H H HO CH(OH)2 H H OH d-glucose (hydrate form) CH2OH OH O OH H H OH H OH a-d-glucofuranose OH O H OH d-glucose (open chain) CH2OH HO OH a-d-glucopyranose CH2OH OH H H OH HO H H H -d-glucopyranose Notes O H -d-glucofuranose PSE 406 - Lecture 4 11 Mutarotation (2) Note: the cyclic form must contain a hemi-form in order to have a migratory hydrogen and therefore be capable of ring-opening (that is, mutarotation). PSE 406 - Lecture 4 12 Fischer to Haworth In order to convert a Fischer projection of a sugar to a Haworth project, certain rules must be followed in order to maintain correct orientation of the molecule. Remember, the horizontal bonds in the Fischer projection (solid wedges) are out of the plane and the vertical wedges (broken wedges) are into the plane. CH3 H C OH CH3 H OH C2H5 C2H5 PSE 406 - Lecture 4 13 Fischer to Haworth CHO H OH H 1 OH H OH H OH H 2 OH H OH H OH H 3 OH HO 4 H HO H H OH CH2OH HO H H O HOH2C 5 H 6 CH2OH O 5 HO 4 1 3 OH 2 6 CH2OH O OH OH Modify the Fischer formula so that all of the ring atoms lie along a vertical line. When rotating substituents around a chiral carbon, to keep the correct configuration, after a 90 rotation, the opposite substituents must be switched. Proceed around the Haworth formula placing the groups on the left above the hexagon and those on the right below the plane. PSE 406 - Lecture 4 14 Linkages Between Sugars In the tree, monosaccharides are linked through enzymatic processes. Linkage proceeds through a dehydration process (loss of H2O) (acetal formation). The bond between the glucoses units in cellobiose is a glycosidic bond. Once linked, the glucose unit on the left is no longer a hemiacetal, it is now an acetal. OH CH2OH O O OH OH HO OH O OH CH2OH Cellobiose 4-O-(-D-Glucopyranosyl)-D-Glucopyranose Cellobiose is disaccharide produced from the partial hydrolysis of cellulose. It is not naturally found in wood. The squiggly bond on C1 above means it can be either a or . PSE 406 - Lecture 4 15 Linkages Between Sugars The glycosidic linkage between the glucose units is between the C1 of one glucose to C4 of the other glucose. So this bond is a (14) linkage. The glucose molecules in the drawing on the right are opposite to make drawing the (14) linkage easy. OH CH2OH O O OH OH HO OH O CH2OH OH Cellobiose 4-O-(-D-Glucopyranosyl)-D-Glucopyranose C6 CH 2OH C5 O C4 HO PSE 406 - Lecture 4 C1 OH C3 C2 OH OH 16 More Disaccharides Neither of these sugars are found in wood. Sucrose occurs naturally in sugarcane and in sugar beets. Maltose is formed through the degradation of starch. CH2OH CH2OH O O HO OH O HO CH2OH OH OH Sucrose a-D-Glucopyranosyl--D-Fructofuranoside CH2OH CH2OH O O OH OH OH O HO OH OH Maltose 4-O-(a-D-Glucopyranosyl)-D-Glucopyranose PSE 406 - Lecture 4 17