Aldohexoses configurations

The task of relating carbohydrate configurations to names requires either a world class memory or an easily recalled mnemonic A mnemonic that serves us well here was pop ularized by the husband-wife team of Lours F Fieser and Mary Fieser of Harvard Uni versity m their 1956 textbook Organic Chemistry As with many mnemonics it s not clear who actually invented it and references to this particular one appeared m the chem ical education literature before publication of the Fiesers text The mnemonic has two features (1) a system for setting down all the stereoisomeric d aldohexoses m a logical order and (2) a way to assign the correct name to each one... 

Note. For simple aldoses up to aldohexoses, and ketoses up to hept-2-uloses, the anomeric reference atom and the configurational atom are the same. 

Van t Hoff had predicted, in 1874, that there should exist 16 normal straight-chain optically isomeric aldohexoses (8,13). It was the awesome challenge to substantiate this prediction that Fischer accepted in the mid-1880s, and that was met by his classical contribution in 1891 entitled, On the Configuration of Glucose and Related Compounds (4). 

The most important natural monosaccharide, D-glucose, is an aliphatic aldehyde with six C atoms, five of which carry a hydroxyl group (1). Since C atoms 2 to 5 represent chiral centers (see p. 8), there are 15 further isomeric aldohexoses in addition to D-glucose, although only a few of these are important in nature (see p.38). Most natural monosaccharides have the same configuration at C-5 as D-glyceraldehyde-they belong to the D series. 

Epimerization. In weakly alkaline solutions, glucose is in equilibrium with the ketohexose D-fructose and the aldohexose D-mannose, via an enediol intermediate (not shown). The only difference between glucose and mannose is the configuration at C-2. Pairs of sugars of this type are referred to as epi-mers, and their interconversion is called epimerization. 

Problem 22.11 The C -epimeric aldohexoses glucose and mannose give the same osazone as fructose. Write equations and explain the configurational significance. M... 

Problem 22.20 Two Ruff degradations of an aldohexose give an aldotetrose that is oxidized by HNO, to /n o-tartaric acid. What can be the family configuration of the aldohexose ... 

An aldopentose (A) of the D-configuration on oxidation with concentrated nitric acid gives a 2,3,4-trihydroxypentanedioic acid (a trihydroxyglutaric acid) (B) which is optically inactive. (A) on addition of HCN, hydrolysis, lactonization, and reduction gives two stereoiso-meric aldohexoses (C) and (D). (D) on oxidation affords a 2,3,4,5-tetrahydroxy-hexanedioic acid (a saccharic acid) (E) which is optically inactive. Give structures of compounds (A)-(E). 

Pentoses and hexoaes could be distinguished readily with phenylhydra-zine. The compound hitherto called isodulcitol was identified as a sugar by means of phenylhydrazine, and was designated as rhamnose from that time on. Glucose and fructose (which H. Kiliani had discerned to be an aldohexose and a ketohexose, respectively) and mannose were found to give one and the same phenylosazone the three sugars, therefore, have the same configuration at carbon atoms 3, 4, and 5. 

Fig. 5. Haworth formula indicating structural and configurational features essential for efficient binding of aldohexoses with glucose-6-phosphatase. From Lygre and Nordlie (126). Copyright (1965), Elsevier Publishing Co. Reproduced by permission.

The configuration at C-5 is opposite to that of D-(+)-glyceraldehyde. This particular carbohydrate therefore belongs to the l series. Comparing it with the Fischer projection formulas of the eight D-aldohexoses reveals it to be in the mirror image of D-(+)-talose it is l-(—)-talose... 

Chain extension of aldoses takes place at the aldehyde end of the chain. The aldehyde function of an aldopentose becomes C-2 of an aldohexose, which normally results in two carbohydrates diastereomeric at C-2. Thus, (-l-)-glucose and (-r)-mannose have the same configuration at C-3, C-4, and C-5 they have opposite configurations at C-2. The configuration at C-2, C-3, and C-4 of (-)-arabinose is the same as that at C-3, C-4, and C-5 of (+)-glucose and (-r)-mannose. 

Therefore we now know the configurations of C-3 and C-5 of (+(-glucose and (+(-mannose and that these two aldohexoses have opposite configurations at C-2, but the same (yet to be determined) configuration at CM. 

Application of the Kiliani-Fischer synthesis to the aldopentose D-arahinose produces aldohexoses D-glucose and D-mannose. Because the Kiliani-Fischer synthesis incorporates the stereocenters of D-arabinose without changes, D-glucose and D-mannose must have the same configurations at carbons 3,4, and 5 as does D-arabinose at carbons 2, 3, and 4, respectively. Furthermore, D-glucose and D-mannose must differ only in their configuration at carbon 2. 

Aldohexose 14, written with its primary alcohol group at the top of the chain and its aldehyde group at the bottom, also produces diacid 13 on oxidation with nitric acid. A rotation of 180° in the plane of the page (recall that such a rotation of a Fischer projection does not change the configuration of the compound) puts 14 in the more common form with the aldehyde group at the top of the chain. When drawn this way, it is apparent that 14 is an L-aldohexose and is a diastereomer of 7. 

Digitalin (A) is related to which D-aldohexose Provide a name for A, including the configuration at the anomeric carbon. Predict the products of the reaction of A with (a) CH3OH, H+ catalyst (b) CH3I, Ag20. 

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