Hemiacetals nomenclature

The (non-detachable) prefix carba- signifies replacement of a heteroatom by carbon in general natural product nomenclature [26], and may be applied to replacement of the hemiacetal ring oxygen in carbohydrates if there is a desire to stress homomorphic relationships. If the original heteroatom is unnumbered, the new carbon atom is assigned the locant of the non-anomeric adjacent skeletal atom, with suffix a . 

Haworth representation, cyclic monosaccharides, 61-63 Hemiacetal groups disaccharides with, 149-150 without, 148-149 nomenclature, 122-123 oligosaccharides with, 153-154 without, 151-153... 

The cyclic hemiacetal and hemiketal forms of monosaccharides are capable of reacting with an alcohol to form acetals and ketals (see Section 7.2). The acetal or ketal product is termed a glycoside, and the non-carbohydrate portion is referred to as an aglycone. In the nomenclature of glycosides we replace the suffix -ose in the sugar with -oside. Simple glycosides may be synthesized by treating an alcoholic solution of the monosaccharide with an acidic catalyst, but the reaction mixture usually then contains a mixture of products. This is an accepted problem with many carbohydrate reactions it is often difficult to carry out selective transformations because of their multifunctional nature. 

Draw the structural formulae of both pyranose forms which would result from ring closure (hemiacetal formation) ofD-idose. What is the relationship between these structures and the corresponding pyranose forms derived from L-idose Check your answer by determining the configuration of the chirality centres according to the RIS nomenclature. 

Pyranose and Furanose Names Cyclic structures of monosaccharides are named according to their five- or six-membered rings. A six-membered cyclic hemiacetal is called a pyranose, derived from the name of the six-membered cyclic ether pyran. A five-membered cyclic hemiacetal is called a furanose, derived from the name of the five-membered cyclic ether ran. The ring is still numbered as it is in the sugar, not beginning with the heteroatom as it would be in the heterocyclic nomenclature. These structural names are incorporated into the systematic names of sugars. 

The labels acetal and hemiacetal are part of a nomenclature that is routinely used in talking of natural products. The functional groups that are represented by these labels, a diether to the same... 

Not all carbohydrates exist in equilibrium with six-membered hemiacetal rings in several instances the ring is five membered. (Even glucose exists, to a small extent, in equilibrium with five-membered hemiacetal rings.) Because of this variation, a system of nomenclature has been introduced to allow designation of the ring size. 

Anomeric carbon (Section 22.2C) The hemiacetal or acetal carbon in the cyclic form of a carbohydrate. The anomeric carbon can have either the a or /3 stereochemical configuration (using carbohydrate nomenclature), resulting in diastereomeric forms of the carbohydrate called anomers (a-anomers and /3-anomers). Anomers differ on/y in the stereochemistry at the anomeric carbon. 

In the nomenclature of oligosaccharides, trivial names are still used for a number of substances (e.g. maltose, cellobiose or trehalose. Figure 4.5). In the case of reducing disaccharides, systematic nomenclature takes as its basis the name of the monosaccharide with a free hemiacetal hydroxyl group, which is preceded by the name of the substituted monosaccharide and the respective anomeric configuration is indicated (a- or P-). Disaccharide maltose (Figure 4.5), formed by condensation of... 

This nomenclature has recently been accepted by both the American Chemical Society and the Chemical Society. Glucosiduronic acid should replace the term glu-curonide which has commonly been used to denote the hemiacetal of the free acid. Glucosiduronate now replaces "glucuronidate as the name of the carboxylate anion (or ester) of the hemiacetal (111). 

These rules were empirical and based on observations of limited samples of compounds. Subsequent to Hudson s rule, it was found that there were exceptions to the rule, and it was proposed that a better system of nomenclature would be based on the absolute configurations in which the hemiacetal hydroxyl that is on the same side (cis) as the hydroxyl group reacting with the aldehyde or ketone is a-, and the hemiacetal hydroxyl that is on the opposite side (trans) is P-. 

In reality, the cyclic hemiacetal forms of sugars and their derivatives may assume a wealth of different ring conformations. For purposes of nomenclature, however, linear Fischer Projections and Haworth Representations with their quasi conformation-averaging planar-cyclic habit are best suited to illustrate the manifold stereochemicd relationships significant for carbohydrates. 

Formerly it was conventional to use the terms hemiacetal and acetal for compounds formed by reaction between aldehydes and alcohols. Similar reactions between ketones and alcohols gave rise to compounds called hemiketals and ketals. Current nomenclature uses hemiacetal and acetal for compounds derived from either an aldehyde or a ketone, but reserves hemiketal and ketal for those derived from ketones. In other words, the ketals are a subclass of the acetals and the... 

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