Hemiacetals structure

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.

Negative evidence for the importance of the free aldehyde group is found in the failure of all efForts - to condense amides with free monosaccharides, which have a hemiacetal structure. 

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. 

In any event a model valid for enzymic saccharide syntheses as far apart in behavior as those stemming from substrates with mixed acetal and hemiacetal structures might be expected to carry certain assumptions and implications for the entire discipline. This is the case with the type of reaction mentioned at the outset,... 

Two of the key ideas used by Fischer can be illustrated best with aldo-tetroses because they have only two chiral carbons and far fewer possible structures to consider. Writing the four possibilities as the aldehyde rather than hemiacetal structures, we have 4-7. Of these, 4 and 5 constitute a pair of enantiomers, as do 6 and 7. These pairs can be identified by careful oxidation of the terminal groups to give the corresponding tartaric (2,3-dihydroxy-... 

As we stated previously, glucose forms some, but not all, of the common carbonyl derivatives. The amount of free aldehyde present in solution is so small that it is not surprising that no hydrogen sulfite derivative forms. With amines, the product is not a Schiff s base but a glucosylamine of cyclic structure analogous to the hemiacetal structure of glucose, Equation 20-3. The Schiff s base is likely to be an intermediate that rapidly cyclizes to the glucosylamine ... 

Detailed studies of the periodate oxidation of dextran and inulin (4-6) have shown evidence for the occurrence of hemiacetal structures formed by reaction of aldehydes with hydroxyl groups of either the same unit (intra-residual) or a neighbouring unit (inter-residual). Inter-residual hemiacetal formation reduces the number of diol struc-... 

Vomifoline has been isolated63" from the leaves of Rauwolfia vomitoria collected from the Ivory Coast, and a re-examination of its chemical behaviour and its physical properties has shown that it is identical with peraksine (177) previously the alternative hemiacetal structure had been accepted as the structure of vomifoline. 

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. 

Arene oxide 80 has been reported as a stable metabolite of 5,5-diphenyl-hydantoin. Since no substituted benzene oxides have previously been isolated as metabolites and since an oxirane hemiacetal structure would be predicted to be highly unstable, the structural assignment in this report is highly questionable. 

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

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

The fact that, from periodate-oxidized methyl a-n-glucopyranoside, a mono-(formazan) could be prepared,constitutes evidence in favor of the latter s hemiacetal structure (or the hydrate form) as expressed in the preceding formula. 

Compound XXXII is an aldehyde-hemiacetal phenylhydrazone XXXIII is a hemiacetal-aWehydo phenylhydrazone and XXXIV is a so-called hemialdal structure. Of them, only XXXIII is suited to formation of a formazan. The successful formazan reaction furnishes proof of the predominance of this hemiacetal-oZde do phenylhydrazone structure. On these grounds, it is safe to assume that, to the oxidized polysaccharides themselves, this aldehyde-hemiacetal structure (or the hydrate form) is to be assigned. 

Formation of the hemiacetal structure of glucose. Both the a- and /J-anomers of glucose form. 

Similarly, methyl diformylacetate, a /(-tricarbonyl compound, is almost totally enolized towards the aldehyde and reacts smoothly with alkenes to give a hemiacetal structure 7 present in many of the iridoid glucosides113-116. 

In addition to these equilibria, in aqueous solution, dehydroascorbic acid rapidly forms a hemiacetal structure. 

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... 

Compounds showed weak activity in both assays, due to their hemiacetal structure. In general, all compounds showed potent antifeedant and insecticidal activity at concentrations of 16.0-63.7 pg/cm2. These compounds contained the following partial structures an a,P-unsaturated carbonyl in ring A, vicinal hydroxy groups at Cn and Ci2, a methyleneoxy bridge between Cg and C13 and a side chain atCis [111]. 

Compounds A and B contain three hydroxyl groups, two of which are phenolic, and Robertson and coworkers suggested that the two compounds have the hemiacetal structure (30), the two isomers differing only in their... 

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