Ch_322b_22.09

Ch_322b_22.09 - Glycosides: Full Acetals Hemiacetals and...

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Glycosides: Full Acetals Hemiacetals and Full Acetals Two equivalents of an alcohol reversibly add to an aldehyde in the presence of an acid catalyst to form an hemiacetal and then a full acetal : RCH O = + R'OH H + RCH OH OR' hemiacetal H + , R'OH RCH OR' OR' + H 2 O acetal When D-(+)-glucose is dissolved in methanol in the presence of a small amount of acid, a full acetal is formed with the addition of one equivalent of methanol to the hemiacetal. O HOCH 2 HO HO OH either ! or " -anomer OH H + , CH 3 OH O HOCH 2 HO HO OH O HOCH 2 HO HO OH H H OCH 3 OCH 3 + methyl- ! -D-glucopyranoside methyl- -D-glucopyranoside mp 165 o C, [ ! ] = +158 o D 25 mp 107 o C, [ ! ] = -33 o D 25
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Glycosides The full acetals (glycosides) are stable to bases but hydrolyze under aqueous acidic conditions back to the hemiacetals and alcohols. Glycosides do not show mutarotation because there is no equilibrium between the two anomers. O
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This note was uploaded on 10/20/2009 for the course CHEM 322BL at USC.

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Ch_322b_22.09 - Glycosides: Full Acetals Hemiacetals and...

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