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Carbohydrate a cetals and ket als

Carbohydrate a cetals and ket als.

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Carbohydrate a cetals and ket als

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  1. Carbohydrate acetals and ketals • Two types of structurally different derivatives of carbohydrates. The acetal (or ketal) group of the first type of carbohydrate acetals (ketals) originates from a carbonyl group of any carbonyl compound and two hydroxyl groups of carbohydrate (e. g., 2,3-O-isopropylidene-D-glyceraldehyde (I) from acetone and D-glyceraldehyde). The second type originates from a carbonyl group of a sugar and two hydroxyl groups of any alcohol (e. g., D-glyceraldehyde dimethyl acetal (II) from D-glyceraldehyde and methanol). .

  2. Carbohydrate acetals and ketals Also glycosides are sugar acetals (derived from aldoses) or ketals (derived from ketoses), e. g., methylα-D-glucopyranoside (III), but also internal glycosides, e. g., 1,6-anhydro--D-glucopyranose (IV) (red colour shows the acetal functional group).

  3. Carbohydrate acetals and ketals • Acetone, benzaldehyde, acetaldehyde and formaldehyde aremost often employed carbonyl compounds for preparation of ketals and acetals of the first type. • According to these starting carbonyl compounds, they are called as isopropylideneketals (1,2;5,6-di-O-isopropylidene--D-glucofuranose, 1,2-O-isopropylidene--D-glucofuranose, 1,2;3,4-di-O-isopropylidene--D-galactopyranose), andbenzylidene (4,6-O-benzylidene-D-glucopyranose), ethylideneand methylene acetals of carbohydrates.

  4. 1,2:5,6-di-O-isopropylidene--D-glucofuranose (I) (obsolete name,diacetone glucose) – crystalline compound, m. p. 110 °C, []D -180° (water), soluble in water and many organic solvents. The acid hydrolysis rate of its 5,6-O-isopropylidene group is 40-times higher than that in position 1,2. This is employed for preparation of another important derivative, 1,2-O-isopropylidene--D-glucofuranose (II). Ketal Iis employed as starting compound in many syntheses. Thus, e. g., the intermediates, obtained either after oxidation of its free hydroxyl group to 3-oxo derivative, or after its O-substitution, are employed for preparation of aminosaccharides, deoxysaccharides or branched-chain saccharides.

  5. 1,2;3,4-di-O-isopropylidene--D-galactopyranose • Is employed in synthesis of saccharides and their derivatives, e. g.,D-fucose (6-deoxy-D-galactose)or D-galacturonic acid.

  6. Carbohydrate acetals and ketals are stable in basic and neutral solutions. In acid solutions they decompose to the starting sugar and carbonyl compound. Their hydrolysis rate is highest for benzylidene acetals and decreases in the order isopropylidene ketals, ethylidene acetals and methylene acetals. From carbohydrate benzylidene acetals, the saccharide can be regenerated also by hydrogenolysis on paladium, similarly as from benzyl ethers. D-fucose (6-deoxy-D-galactose)

  7. 4,6-O-benzylidene-D-glucopyranose (I) 40 % of 4,6-O-benzylidene-D-glucopyranose (I) can be isolated by reacting D-glucosewith 1 mol of benzaldehyde.An excess of benzaldehydegives rise to 1,2:4,6-di-O-benzylidene-α-D-glucopyranose(II).

  8. Nowadays,more modern, transacetalization (transketalization) reagents are employed for preparation of carbohydrate acetals and ketals; acetone dimethylketalinstead of acetone andbenzaldehyde dimethylacetalinstead of benzaldehyde.

  9. Conformational analysis of the carbohydrate ketals and acetals Ketones (R1-CO-R2),reacting with hydroxyl groups of carbohydrates, preferentially provide the termodynamicallymore favourablefive-membered cyclic ketals of the 1,3-dioxolane type. The characteristic examples are O-isopropylidene ketals (R1 = R2 = Me). The reason is thatboth the bulky substituents R1, R2are placed inequivalent, degeneratedquasi-equatorial (orquasi-axial) positions. Aldehydes (R-CH=O),reacting with hydroxyl groups of carbohydrates, preferentially provide the termodynamicallymore favourablesix-membered cyclic ketals of the 1,3-dioxane type. The characteristic examples are O-benzylidene acetals (R = Ph). In this case, the bulky substituent Ris placed in equatorialposition andhydrogen atom in the axial position.

  10. Isopropylideneketals of common aldohexoses

  11. In case of employing more modern transketalization reagent,acetone dimethylketalinstead of acetone, D-mannose does not afford 1,2;5,6-di-O-isopropylidene--D-mannofuranose, but its glycoside, methyl 1,2;5,6-di-O-isopropylidene--D-mannofuranoside.

  12. Acetals and ketals of alditols D-glucitol (sorbitol) 2,4-O-benzylidene-D-glucitol 1,3:2,4:5,6-tri-O-benzylidene-D-glucitol 1,3:2,4-di-O-benzylidene-D-glucitol 1,2:5,6-di-O-izopropylidén-D-manitol alebo 1,2:3,4:5,6-tri-O-izopropylidén-D-manitol (rôzne podmienky) D-manitol

  13. Acetals and ketals of alditols D-glucitol (sorbitol) 1,3:2,4:5,6-tri-O-benzylidene-D-glucitol 1,2:5,6-di-O-isopropylidene-D-mannitol or 1,2:3,4:5,6-tri-O-isopropylidene-D-mannitol Acid (differentcondition) (rôzne podmienky) D-mannitol

  14. Synthetic employment of carbohydrate acetals and ketals 2,4-O-benzylidene-D-glucitol L-xylose D-allose 1,2;5,6-di-O-isopropylidene--D-glucofuranose 1,2;5,6-di-O-isopropylidene--D-allofuranose Free sugars can be practically released from all sugar acetals or ketals by hydrolysis with a 3 N strong acid at room temperature within 48 hours.

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