Structure, 3-144 Tollens’ reagent

D-Mannose reacts with acetone to give a diisopropylidene derivative (Problem 25.61) that is still reducing toward Tollens reagent. Propose a likely structure tor this derivative. 

Thymine, electrostatic potential map of, 1104 structure of, 1101 Thyroxine, biosynthesis of, 551 structure of. 1020 TIme-of-flight (TOP) mass spectrometry, 417-418 Titration curve, alanine, 1023 TMS, see Tetramethylsilane see Trimethylsilyl ether Tollens reagent, 701 Tollens test, 992... 

An unknown carbohydrate of formula C12H22O11 reacts with Tollens reagent to form a silver mirror. An a-glycosidase has no effect on the carbohydrate, but a /3-galactosidasc hydrolyzes it to D-galactose and D-mannose. When the carbohydrate is methylated (using methyl iodide and silver oxide) and then hydrolyzed with dilute HC1, the products are 2,3,4,6-tetra-O-methylgalactose and 2,3,4-tri-(9-methylmannose. Propose a structure for this unknown carbohydrate. 

A test for reducing sugars, employing the same silver-ammonia complex used as a test for aldehydes. A positive test gives a silver precipitate, often in the form of a silver mirror. Tollens reagent is basic, and it promotes enediol rearrangements that interconvert ketoses and aldoses. Therefore, both aldoses and ketoses give positive Tollens tests if they are in their hemiacetal forms, in equilibrium with open-chain carbonyl structures, (p. 1118)... 

In the second place, oxidation by Fehling s or Tollens reagent cannot be used for the preparation of aldonic acids (monocarboxylic acids) from aldoses. Both Fehling s and Tollens reagents are alkaline reagents, and the treatment of sugars with alkali can cause extensive isomerization and even decomposition of the chain. Alkali exerts this effect, in part at least, by establishing an equilibrium between the monosaccharide and an enediol structure. 

Which of the following componnds will react with Tollens reagent (Ag, NH3, and H2O) For those that do react, draw a structural formula for the organic product. 

A number of oxidizing agents are used to identify functional groups of carbohydrates, in elucidating their structures, and for syntheses. The most important are (1) Benedicts or Tollens reagents, (2) bromine water, (3) nitric acid, and (4) periodic acid. Each of these reagents produces a different and usually specific effect when it is allowed to react with a monosaccharide. We shall now examine what these effects are. 

The Tollens reagent indicates the presence of an aldose or ketose when the surface of the reaction flask is coated with metaUic silver (looking like a mirror), while the other two tests indicate the presence of an aldose or ketose with the formation of a reddish precipitate (CU2O). These three reactions are used only as tests to obtain structural information about unknown carbohydrates but are not efficient as preparative methods when sufficient quantities of the aldonic acid are desired. A carbohydrate that tests positively for any of these three tests is said to be a reducing sugar because the carbohydrate can reduce the oxidizing agent. 

Aldehydes are reductants. They are oxidized, for example, by the complex ion tetraiodomercurate-(n) (Nessler s reagent). The mercury of the complex is reduced to the metallic state. Aldehydes also reduce the ammoniacal silver nitrate (Tollens reagent) with the formation of metallic silver. Concerning this point, let s us recall that aldoses are also reductants. For example, they reduce cupric copper to cuprous copper (Fehling s reagent). Ketoses also exhibit this property. In this case, this is due to the presence of the group enediol in their structure. 

Given the structure of a sugar, write equations for its reaction with each of the following reagents acetic anhydride, bromine water, nitric acid, sodium borohydride, and Tollens or Fehling s reagent. 

The chemical properties of monosaccharides are further complicated by the fact that they can exhibit tautomerism in aqueous basic solutions (Figure 1.15). This means that after a short time a basic aqueous solution of a monosaccharide will also contain a mixture of monosaccharides that will exhibit their characteristic chemical properties. For example, a solution of fructose will produce a silver mirror when treated with an ammoniacal solution of silver nitrate (Tol-len s reagent). This is because under basic conditions fructose undergoes tautomerism to glucose, whose structure contains an aldehyde group, which reduces Tollen s reagent to metallic silver. 

The typical aldehyde reactions of D-(-H)-glucose—osazone formation, and perhaps reduction of Tollens and Fehling s reagents—are presumably due to a small amount of open-chain compound, which is replenished as fast as it is consumed. The concentration of this open-chain structure is, however, too low (less than 0.5%) for certain easily reversible aldehyde reactions like bisulfite addition and the Schiff test. 

Problem 35.4 When (4 )-maltose is subjected to two successive one-carbon do-gradations, there is obtained a disaccharide that reduces Tollens and Fehling s reagents but does not form an osazone. What products would be expected from the acidic hydrolysis of this disaccharide What would these facts indicate about the structure of (+)-maItosc ... 

Problem 14.13. What structural feature in carbohydrates such as glucose and fructose is responsible for their positive tests with Tollens and Benedict s reagents ... 

---