Ch_322b_22.10 - Enolization Tautomerization and...

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Unformatted text preview: Enolization, Tautomerization and Isomerization When D-glucose is dissolved in an aqueous solution of calcium hydroxide and allowed to react for a few days, several products are recovered including D-mannose (an aldohexose) and D-fructose (a ketohexose). Monosaccharides such as D-glucose react under basic conditions through enolate ions produced from the open-chain aldo- or ketoforms. O O CH CH - : OH OH HO H HO H H H OH OH H H OH OH CH2OH CH2OH enolate anion = H2O HO- HO HO H H O CH H H OH OH CH2OH = D-mannose = tautomerization and isomerization O O CH CH - : OH OH HO H HO H H H OH OH H H OH OH CH2OH CH2OH enolate anion = H2O HO- OH CH OH HO H H OH H OH CH2OH enediol (note: pKa ~ 9-10) HOH2O : H 2O HO - CH2OH =O HO H H OH H OH CH2OH D-fructose Cyclic Acetals: Acetonides Aldehydes and ketones react with 1,2-diols to form cyclic acetals: R O C R' + CH2-CH2 HO OH ethanediol H+ O R O R' + H2O In a monosaccharide, formation of a cyclic acetal can occur when the vicinal (1,2) hydroxyl groups are to each other. HO HOCH2 = cyclic acetal + HO H HO OH !-D-glucopyranose H O O CH3CCH3 acetone = H+ HO HO HOCH2 H O H O O CH3 The cyclic acetals formed with acetone are called acetonides. CH3 HO HO HOCH2 HO O H H H OH !-D-mannopyranose O + CH3CCH3 = H+ CH3 HO O H CH3 CH2OH O O H H OH Compounds with hydroxyl groups or hydroxyl and aldehyde/ketone functions on adjacent carbons undergo oxidative cleavage reactions with aqueous periodic acid (HIO 4). The reaction is thought to proceed by way of a cyclic periodate intermediate (similar to the cyclic intermediates in syn hydroxylation with permanganate or osmium tetroxide): C OH C OH + O +7 O=I-OO == (-H2O) OC O I+7 O CO O = = periodate ion +7 oxidation state = = = C=O C OH O +7 + O=I-O O - HO OC O I+7 O CO O : = two electron redox OC O I+7 O CO O = = C=O + C=O O I +5 =O : O HO C=O O O = I +5 + O C=O = = == Structure-Cleavage Relationships When there are three or more contiguous "active functions" ( -OH, C=O), multiple bond cleavages occur. H H C=O formaldehyde + O H-C-OH formic acid + H C=O formaldehyde H = H H H H OH OH OH + 2 HIO4 H glycerol (1,2,3-propanetriol) The products of the periodate reaction may be predicted by replacing each cleaved C-C bond with -OH groups on the carbons. Water is removed as needed to give carbonyl functions. examples H H H H H OH OH OH + 2 HIO4 H H-C OH OH + OH H-C OH OH + OH H-C OH H H H C=O (-H2O) + O H-C-OH (-H2O) + H C=O (-H2O) H = H OH + 2 HIO4 =O OH H H 1,3-dihydroxyacetone H H H-C OH OH + OH C=O OH + OH H-C OH H H H C=O (-H2O) + O=C=O (-H2O) + H C=O (-H2O) H Quantitative Analysis Because the periodic acid oxidative cleavage reaction usually is very efficient, it is possible to use the number of molar equivalents of HIO4 consumed as a tool in structure analysis. one equivalent O OHO CH3-C-CH-C-CH3 OH OCH3 HOCH2-CH-CHOCH3 two equivalents OH O fast hydrolysis HOCH -CH-CH 2 = HOCH2-CH2-CH2OH no cleavage Quiz 22.010 Show how the products of the periodic acid oxidative-cleavage reaction can distinguish the two general tetrose structures below. CHO CHOH CHOH CH2OH 3 eq HIO4 HCO2H + HCO2H + HCO2H + H2C=O CH2OH C=O CHOH CH2OH 3 eq HIO4 H2C=O + CO2 + HCO2H + H2C=O ...
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This note was uploaded on 10/20/2009 for the course CHEM 322BL at USC.

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