Aldohexoses anomers

The following structure is that of an l aldohexose in its pyranose form. Identify it, and tell whether it is an a or 0 anomer. 

The pyranoid monosaccharides provide a wide range of asymmetric molecules for study by the c.d. spectroscopist. However, these compounds are not without their difficulties. In aqueous solution, these compounds exist in a complex equilibrium involving the two possible chair conformers of the pyranoses, the furanoses, a and p anomers, and the acyclic form, as well as septanoses for aldohexoses and higher sugars. 

It follows that, when we dissolve a sugar such as glucose or ribose in water, we create a mixture of various equilibrating structures. The relative proportions of pyranose and furanose forms, and of their respective anomers for the eight aldohexoses, are shown in Table 12.1. In each case, the proportion of non-cyclic form is very small (<1%). 

The aldohexoses have not been extensively studied by X-ray crystal-lographers. Crystals of a- and jS-D-galactose were examined by Sheldrick,21 who gave directions for crystallizing both anomers. In one preparation, the crystals of a-D-galactose were small plates large prismatic crystals of jS-D-galactose were also obtained. 

The reaction is performed in a homogeneous liquid phase by use of less than 2 mol% of heteropolyacids with respect to acetylated aldohexose, and 70-90% yields and 60-98% mol% / -anomer selectivity are obtained. 

Convert each aldohexose to the indicated anomer using a Haworth projection, a. Draw the a anomer of b. Draw the a anomer of c. Draw the p anomer of ... 

Draw both pyranose anomers of each aldohexose using a three-dimensional representation using a chair pyranose. Label each anomer as a or p. 

The most stable conformation of the pyranose ring of most D-aldohexoses places the largest group, CH2OH, in the equatorial position. An exception to this is the aldohexose D-idose. Draw the two possible chair conformations of either the a or p anomer of D-idose. Explain why the more stable conformation has the CH2OH group in the axial position. 

How many different aldohexose stereoisomers are possible (excluding anomers) ... 

Furthermore, the —OH or —OCH3 group on C-1 is on the right in an a-D-anomer and on the left in a -D-anomer, as shown for Fig. 34.11 for aldohexoses. (Notice that on the right means down in the cyclic structure.)... 

The rate constants, kj, for the opening of the hemiacetal ring were calculated by the authors from equation (9) for equilibrium mixtures of the a and p anomers of some aldopentoses and aldohexoses. Assuming that the thickness (8, in cm) of the reaction layer equals the mean distance between two aldose molecules in solution, k values (k/ = kh S) can be calculated, and, from known values of k , k, and D, values of the equilibrium constant K) can also be found by using equation 12). The results are given in Table I. 

Sugars, especially those with five or six carbon atoms, normally exist as cyclic molecules rather than as the open-chain forms we have shown so far. The cycUzation takes place as a result of interaction between the functional groups on distant carbons, such as G-1 and G-5, to form a cyclic hemiacetal (in aldohexoses). Another possibility (Figure 16.5) is interaction between G-2 and G-5 to form a cyclic hemiketal (in ketohexoses). In either case, the carbonyl carbon becomes a new chiral center called the anomeric carbon. The cyclic sugar can take either of two different forms, designated a and P, and are called anomers of each other. 

The aldohexose D-galactose exists predominantly in cyclic forms. Given the structure below, draw the Haworth structure for the anomer. Label the new compound as a or p. 

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