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25.13 Glycosides Glycosides Glycosides have a substituent other than OH at the anomeric carbon. Usually the atom connected to the anomeric carbon is oxygen. Glycosides O-Glycosides are mixed acetals. O-Glycosides are mixed acetals CH O O OH H CH2OH hemiacetal O-Glycosides are mixed acetals CH O O OH hemiacetal H CH2OH ROH O OR H acetal Preparation of Glycosides Glycosides of simple alcohols (such as methanol) are prepared by adding an acid catalyst (usually gaseous HCl) to a solution of a carbohydrate in the appropriate alcohol. Only the anomeric OH group is replaced. An equilibrium is established between the a and b-glycosides (thermodynamic control). The more stable stereoisomer predominates. Preparation of Glycosides CH O H OH HO H H OH H OH CH2OH D-Glucose HOCH2 HO HO CH3OH O OCH3 OH HCl + HOCH2 HO HO O OH OCH3 Preparation of Glycosides HOCH2 Methyl b-D-glucopyranoside HO HO O OCH3 OH + Methyl a-D-glucopyranoside (major product) HOCH2 HO HO O OH OCH3 Mechanism of Glycoside Formation HOCH2 HO HO •• • O• OH OH HCl HOCH2 HO HO •• O •• carbocation is stabilized by lone-pair donation from + oxygen of the ring H OH Mechanism of Glycoside Formation HOCH2 •• • O• HO HO OH HOCH2 HO HO CH3 HOCH2 O HO O •• + HO + H OH + O H3C •• H CH3 •• •• O •• O •• H + OH H Mechanism of Glycoside Formation HOCH2 HO HO •• • O• OH HOCH2 HO HO CH3 HOCH2 O HO O •• + HO + H + –H OH + O H3C •• H HOCH2 •• O •• OH HO HO •• OCH3 + •• •• O •• OH •• OCH 3 ••