215F07-notes-11-9-07-carbs2

215F07-notes-11-9-07-carbs2 - Chem 215 F07 Notes Dr Masato...

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Chem 215 F07 Notes – Dr. Masato Koreeda - Page 1 of 8. Date: November 9, 2007 Chapters 14.8, 23-1,2, 5, and 7: Carbohydrates (continued) II. Glycosides – A general term used to describe organic molecules covalently bound to carbohydrate molecules (usually through anomeric bonds). (1) Formation of glycosides O HO HO HO OH OCH 3 H H H HO OH H O HO O HO HO HO OH HO 1 2 4 3 5 6 1 anomeric carbon anomeric carbon ! -anomer " -anomer C1-epimers; anomers; diastereomers CH 3 OH, 0.7 % HCl, 10 °C (short time) HO H CH 2 OH H OCH 3 H H HO OH H O HO H CH 2 OH 2 3 4 5 6 1 + ! -anomer " -anomer methyl " - D -glucofuranoside methy ! - D -glucofuranoside anomeric carbon anomeric carbon 1 O HO HO HO OCH 3 HO O HO HO HO OCH 3 HO 1 2 4 3 5 6 anomeric carbon anomeric carbon ! -anomer " -anomer 1 CH 3 OH, 4 % HCl, rt methyl " - D -glucopyranoside methyl ! - D -glucopyranoside Kinetic conditions (for this reaction)! Thermodynamic conditions! MAJOR PRODUCT MINOR PRODUCT ~66% ~33% + In general, (5-membered) furanosides are formed preferentially under the kinetic conditions, whereas (6-membered) pyranosides are formed under the thermodynamic conditions, i.e., more stable. Five membered systems have a number of eclipsing interactions, thus less stable.
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Chem 215 F07 Notes – Dr. Masato Koreeda - Page 2 of 8. Date: November 9, 2007 (2) Mechanism for the formation of anomeric glycosides O HO HO HO O HO O HO HO HO OH HO 1 2 4 3 5 6 anomeric carbon ! -anomer 1 When protonation occurs on the anometic OH. O HO HO HO O HO 1 H H O HO HO HO HO 1 lone pair-assisted ionization. H O CH 3 or When protonation occurs on the ether oxygen atom. H H O HO HO HO O HO H H H H O CH 3 or lone pair-assisted ionization. 1 O HO HO HO HO H H OH O CH 3 H stereochem. mixture H O CH 3 1 O HO HO HO HO H H O O CH 3 H H O CH 3 H 1 O HO HO HO HO H H O O CH 3 H H lone pair-assisted ionization. 1 O HO HO HO HO H H O CH 3 1 O HO HO HO HO H H O CH 3 O HO HO HO OCH 3 HO 1 O HO HO HO OCH 3 HO O HO HO HO OCH 3 HO O HO HO HO OCH 3 HO H H H H H H H O CH 3 H O CH 3 ! -anomer " -anomer rotation along the C 1 -C 2 bond 2 Comments : • The α -anomeric hemiacetal undergoes similar processes to produce a mixture of anomeric glycosides. • Protonation on the lone pairs of the oxygen atoms other than the anomeric (i.e., C 1 -O) and ether ring oxygen ones does not lead to the ready elimination of the protonated hydroxyl groups due to the lack of the lone pair-assisted ionization.
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Chem 215 F07 Notes – Dr. Masato Koreeda - Page 3 of 8. Date: November 9, 2007
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