Alditols optical activity

Problem 25.17 I Reduction of D-glucose leads to an optically active alditol (D-glucitol), whereas reduc-I tion of D-galactose leads to an optically inactive alditol. Explain. 

A D-aldohexose A is formed from an aldopentose B by the Kiliani-Fischer synthesis. Reduction of A with NaBH4 forms an optically inactive alditol. Oxidation of B forms an optically active aldaric acid. What are the structures of A and B ... 

It was known that aldoses could be reduced to alditols, and could be oxidized to the monocarboxylic aldonic acids and to the dicarboxylic aldaric acids. A theory of stereoisomerism and optical activity had been proposed (1874) by van t Hoff and Le Bel. Methods for separating stereoisomers were known and optical activity could be measured. The concepts of racemic modifications, meso compounds, and epimers were well established. 

Aldose E is optically active, but treatment with sodium borohydride converts it to an optically inactive alditol. Ruff degradation of E gives F, whose alditol is optically inactive. Ruff degradation of F gives optically active D-glyceraldehyde. Give the structures and names of E and F and their optically inactive alditols. 

A, B, and C are three aldohexoses. Compounds A and B yield the same optically active alditol when they are reduced with hydrogen and a catalyst A and B yield different phenylosazones when treated with phenylhydrazine B and C give the same phenylosazone but different alditols. Assuming that all are D sugars, give names and structures for A, B, and C. 

NaBH4 reduces D-glucose to D-glucitol. Do you expect the alditol formed under these conditions to be optically active or optically inactive Explain. 

There are four D-aldopentoses (Table 17.1). If each is reduced with NaBH4, which yield optically active alditols Which yield optically inactive alditols (See Example 17.6)... 

Account for the observation that the reduction of D-glucose with NaBH4 gives an optically active alditol, whereas the reduction of D-galactose with NaBH4 gives an optically inactive alditol. (See Example 17.6)... 

D-Arabinose and o-lyxose yield optically active alditols, while D-ribose and o-xylose yield optically inactive alditols. 

Another method for the establishment of the configuration of an alditol obtained synthetically is especially valuable when both of the 2-epimers are optically active. It consists of the synthesis of the same polyol from two different aldoses. Thus, from the fact that D-mannose and L-galactose by the cyanohydrin synthesis yield four heptoses, which on reduction give only three different heptitols, one of which (perseitol) is produced from both D-mannose and L-galactose, the configuration of the four heptoses and three heptitols could be deduced. (For a discussion of these methods and detailed references, see Hudson (64).)... 

Compound A is a D-aldopentose that is converted into an optically active alditol upon treatment with sodium borohydride. Draw two possible structures for compound A. 

X undergoes a Wohl degradation to produce an aldopentose, which is converted into an optically active alditol when treated with sodium borohydride. From the information presented, there are only two possible structures for compound X. Identify the two possibilities, and then propose a chemical test that would allow you to distinguish between the two possibilities and thereby determine the structure of compound X. 

Compound A is a D-aldopentose. When treated with sodium borohydride, compound A is converted into an alditol that exhibits three signals in its C NMR spectrum. Compound A undergoes a Kiliani-Fischer synthesis to produce two aldo-hexoses, compounds B and C. Upon treatment with nitric acid, compound B yields compound D, while compound C yields compound E. Both D and E are optically active aldaric acids. 

When subjected to a Ruff degradation, a D-aldopentose, A, is converted to an aldotetrose, B. When reduced with sodium borohydride, the aldotetrose B forms an optically active alditol. The NMR spectrum of this alditol displays only two signals. The alditol obtained by direct reduction of A with sodium borohydride is not optically active. When A is used as the starting material for a Kiliani-Fischer synthesis, two diastereomeric aldohexoses, C and D, are produced. On treatment with sodium borohydride, C leads to an alditol E, and D leads to F. The NMR spectrum of E consists of three signals that of F consists of six. Propose structures for A-F. 

An unknown 8-D-aldohexose has only one axial substituent. A Wohl degradation forms a compound which, when treated with sodium borohydride, forms an optically active alditol. This information allows you to arrive at two possible structures for the )S-D-aldohexose. What experiment can you carry out to distinguish between the two possibilities ... 

Two D-aldohexoses (A and A") give optically inactive alditols on reduction. A" is formed from B" by Kiliani-Fischer synthesis. Since B" affords an optically active aldaric acid on oxidation, B" is B and A" is A. The alternate possibility (A ) is formed from an aldopentose B that gives an optically inactive aldaric acid on oxidation. 

Only two D-aldopentoses (A and A") are reduced to optically active alditols CHO... 

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