Cyclic hemiacetal structures

Figure 9.5 Cyclic, hemiacetal structures of D-glucose. The reaction between an alcohol and aldehyde group within an aldohexose results in the formation of a hemiacetal. The only stable ring structures are five- or six-membered. Ketohexoses and pentoses also exist as ring structures due to similar internal reactions.

Below this GA concentration, the relative content of each monomeric structure was found to be virtually constant regardless of the GA concentration, and also pH (pH2- 8), in contrast to the temperature which has marked effect on it On the other hand, above this concentration, considerable amount of polymerized structures exist in GA solution, which are closely related with cyclic hemiacetal structure, that is, again not related with a, 0- unsaturated structure. The conversion velocity between the monomeric and polymerized structures is quite slow, compared to that between the monomeric structures. The chemistry of GA in concentrated aqueous solution will be discussed in detail elsewhere. 

He favored the currently accepted, cyclic hemiacetal structure for the glucosides, but formulated them with the furanoid ring for glucose proposed in 1883 by B. Tollens. W. N. Haworth later established that the glucose residue in such glucosides is present in the form of a pyranoid ring. 

Drawing Cyclic Monosaccharides Cyclic hemiacetal structures may seem complicated at first glance, but they can be drawn and recognized by following the process illustrated in Figure 23-6. 

FIGURE 2.17 Cyclic hemiacetal structures THF = tetrahydrofuran THP = tetrahydropyran. 

FIGURE 2.18 The cyclic hemiacetal structures of D-glucose and D-fmctose. 

The acyl azide (142 R= CON3) has been photolysed in the presence of several aromatic compounds, e.g. benzene, anisole, or aniline. The nitrenes that were produced were in the triplet state, and yielded 3-substituted dihydronaphtho[2,l-Z)]pyrans (142 R=CONHPh, C0NHCeH40Me, or CONHNHPh respectively). Coumarins behaved similarly. Spectral examination has shown that the equilibrium between the butanedione and the cyclic hemiacetal structure (143 R = H, Me, or MeO) lies entirely on the latter s... 

Remember, sugars in solution typically exist as equilibrium mixtures ol open chain plus cyclic hemiacetal structures. For glucose, this equilibrium mixture contains 63.6% /3-pyrano.se, 36.4% o-pyrano.sc, together with just traces of open chain and furanose structures. I hc interconversion of (3 and a anomers is culled nuilamialion. 

These differences show up in the different chemical reactions of D-glucose. Finally, C-1 in the cyclic, hemiacetal structure is a stereogenic center. It has four different groups attached to it (H, OH, OC-5, and C-2) and can therefore exist in two configurations, R or S. Let us consider this last feature in greater detail. 

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