Aldohexoses

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... 

Like aldopentoses aldohexoses such as d glucose are capable of forming two fura... 

Other aldohexoses behave similarly m adopting chair conformations that permit the CH2OH substituent to occupy an equatorial orientation Normally the CH2OH group is the bulkiest most conformationally demanding substituent m the pyranose form of a hexose... 

Of the eight stereoisomeric aldopentoses Figure 25 2 shows the Fischer projections of the d enantiomers (d ribose d arabinose d xylose and D lyxose) Likewise Figure 25 2 gives the Fischer projections of the eight D aldohexoses... 

The aldohexoses are allose altrose glucose mannose gulose idose galactose and talose The mnemonic All altruists gladly make gum m gallon tanks is helpful m writing the correct Fischer projection for each one... 

From among the carbohydrates shown in Figure 25 2 choose the D aldohexoses that yield... 

Note The dipping solution, which can also be used as a spray solution, can be employed with silica gel, kieselguhr, cellulose, RP, NH2 and CN phases. Sugars (exceptions include, for example, fructose, melezitose and raffinose) yield brilliantly colored zones on an almost colorless background when the spray solution is employed. Aldohexoses appear brown, aldopentoses bright red and hexuronic acids orange in color [3]. The detection limit differs for different substances it ranges from 10 ng (pyridoxal) over 100 ng (cinnamaldehyde) up to 2 pg (citral). 

Heating the sugar with strong acid yields furfural derivatives. Aldohexoses can eliminate water and formaldehyde under these conditions yielding furfural. This adehyde reacts with amines according to I to yield colored Schiff s bases. Ketohex-oses condense with diphenylamine in acid medium with simultaneous oxidation according to II to yield the condensation product shown. 

Heating the sugars with strong add yields furfural derivatives. Under these conditions aldohexoses can eliminate formaldehyde and water to yield furfural. This aldehyde reacts with amines to yield colored Schiff s bases. 

Louis Fieser of Harvard University suggested the following procedure for remembering the names and structures of the eight o aldohexoses ... 

Which of the other six d aldohexoses yield optically active aldaric acids on oxidation, and which yield optically inactive (meso) aldaric acids (See Problem 25.19.)... 

Conversion of the aldehyde into a nitrile is accomplished by treatment of an aldose with hydroxvlamine to give an oxime (Section 19.8), followed by dehydration of the oxJme with acetic anhydride. The Wohl degradation does not give particularly high yields of chain-shortened aldoses, but the reaction is general for all aldopentoses and aldohexoses. For example, D-galactose is converted by Wohl degradation into n-lyxose. 

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