Stereochemistry hemiacetal formation

Aldoses exist almost exclusively as their cyclic hemiacetals very little of the open chain form is present at equilibrium To understand their structures and chemical reac tions we need to be able to translate Fischer projections of carbohydrates into their cyclic hemiacetal forms Consider first cyclic hemiacetal formation m d erythrose To visualize furanose nng formation more clearly redraw the Fischer projection m a form more suited to cyclization being careful to maintain the stereochemistry at each chirality center... 

Generating Haworth formulas to show stereochemistry m furanose forms of higher aldoses is slightly more complicated and requires an additional operation Furanose forms of D ribose are frequently encountered building blocks m biologically important organic molecules They result from hemiacetal formation between the aldehyde group and the C 4 hydroxyl... 

Fig. 9.6. Stereochemistry of the hemiacetal formation from D-glucose in an aqueous solution at 20 °C (in the formulas, the carbon skeleton of 5-valerolactol is black—which emphasizes the relationship to the reaction from Figure 9.5— whereas the extra substituents as well as the bonds leading to them are red).

We have not previously considered stereochemistry. Hemiacetal (and acetal) formation is thermodynamic control as aU the reactions are reversible. The dimer and trimer crystallize frc-liquid so the stereochemistry may be governed by the formation of the most stable p . compound or by the fact that the crystallization of the least soluble diastereoisomer removes the equilibrium and so more is formed. We can see some reasons why the diastereoisomer i might be the most stable. The as ring junction between the two five-membered rings is much m stable than the trans, the two acetyl groups may prefer to be trans to each other, and there ma. > H bond in the crystal. We cannot be sure of these reasons but they are explored more in Chap . ... 

FIGURE 7-6 Formation of the two cyclic forms of D-glucose. Reaction between the aldehyde group at C-l and the hydroxyl group at C-5 forms a hemiacetal linkage, producing either of two stereoisomers, the a and fi anomers, which differ only in the stereochemistry around the hemiacetal carbon. The interconversion of a and fi anomers is called mutarotation. 

The stereochemistry of the alkene product in Wittig reactions is thought to be influenced by the reversibility of formation of the isomeric threo and erythro oxaphosphetanes (or betaines) which undergo stereospecific loss of triphenyl-phosphine oxide to give the trans (E) and cis (Z) alkenes, respectively (Scheme 4). Factors that enhance the reversibility of this initial step favour the threo intermediate and hence the (E) alkene. Stabilized phosphoranes give a predominance of the (E) alkene while non-stabilized phosphoranes give the (Z) alkene. In general, stabilized phosphoranes react readily with aldehydes (see Protocol 4) while non-stabilized phosphoranes will react with aldehydes, hemiacetals (see Protocol 5) and ketones.2,3... 

Notes and discussion. This procedure is used for the stereoselective formation of a, 2-trans configured fluoride. The use of the orthoester precursor allows differentiation of the 0-2, which is a strategic manipulation that is often useful in oligosaccharide synthesis. The product also has participatory acetyl protection of 0-2, which may be used to direct the stereochemistry of its glycosylation reaction. The direct treatment with DAST has the advantage that only a single anomer is formed, compared to the anomeric mixture which results from the two step process of hydrolysis to the hemiacetal followed by DAST treatment. 

The chemistry of the cellulosic fibers is similar to that of the simple sugars, but more complex, as the stereochemistry of the alcohol and hemiacetal structures is affected by degree of crystallinity, e.g., the -OFI groups on the carbon atoms 2, 3, and 6 have different reactivities. Reaction with mineral acids leads to cleavage of the glycosidic bond and formation of a reducing end (RO-CFI-OFI). 

First, it is difficult to control the stereochemistry of the glyco-sidic bond (a or /3) because the formation of an acetal (from a hemiacetal) proceeds through an Sfjl mechanism a mixture of a- and /3-glycosidic bonds results. In addition, it is difficult to form only the desired 1,4-linkage, because any one of the hydroxyl groups located on carbons 2, 3, 4, and 6 could be used to form the bond. 

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