Six-membered hemiacetal

Both six-membered pyranose and five-membered furanose structures are encountered, a particular ring size usually being characteristic for any one sugar. Thus, although glucose has the potential to form both six-membered and five-membered rings, an aqueous solution consists almost completely of the six-membered hemiacetal form five-membered rings... 

The reactivity of the 2,5-anhydrides of aldoses is determined by two essential structural features that do not exist in the sugars, namely, the presence of an oxolane ring and of a carbonyl group (most frequently, free) a to the ring-oxygen atom. These two characteristics make the 2,5-anhydroaldoses closer to tetrahydro-2-furaldehyde than to the aldoses, where only in exceptional cases is the carbonyl group not masked by the formation of an intramolecular, five- or six-membered, hemiacetal ring. 

So far, no systematic investigation of the length of the side chain in VII/116 (Scheme VII/23) has been made. There have been, however, enquieries into lactonization reactions, in which the side chain contained two [94], three [4] [83] [95] [96], and four [93] carbon atoms. In these cases, the intermediate hemiacetal ring is a five-, six-, or seven-membered ring. The yields of the lactones, generated via five- and six-membered hemiacetals, are usually high (90 % and more). If a lactonization reaction involves a seven-membered hemiacetal, the yields are distinctly lower. 

Some terminal alkenes are oxidized to aldehydes depending on their structure. As described before, acrylonitrile and acrylate are oxidized to acetals of aldehydes in alcohols or ethylene glycol.Selective oxidation of terminal carbons in 4-hydroxy-1-alkenes (18) gave the five-membered hemiacetals (19), which can be converted to y-butyrolactones by PCC oxidation (Scheme 4). Formation of a tricyclic six-membered hemiacetal (62%) from a 5-hydroxy-1-alkene system was used for the synthesis of rosa-ramicin. Formation of aldehydes as a major product from terminal alkenes using (MeCN)2Pd(Cl)(N02) and CuCU in r-butyl alcohol under selected conditions was reported. The vinyl group in the -lactam was oxidized mainly to the aldehyde as shown below (equation 12). ... 

Hemiacetals are formed from hydroxy aldehydes " or ketones which are usually first in a protected form and are deblocked prior to cyclization. Ozonolysis of cyclohexene derivative 5 and reductive workup, followed by hydrolysis of the acid chloride formed in situ and borane reduction of the carboxylic acid, leads to the six-membered hemiacetal 6. a precursor for fluo-rinated sugars that are potentially versatile as molecular probes in the elucidation of biochemical processes. ... 

Fructose-1,6-diphosphate (FDP) aldolase catalyzes the reversible aldol addition of DHAP and D-glyceraldehyde-3-phosphate (G3P) to form D-fructose-1,6-diphosphate (FDP), for which eq 10 M in favor of FDP formation (Scheme 13.9). RAMA accepts a wide range of aldehyde acceptor substrates with DHAP as the donor to stereospecifically generate 3S,4S vicinal diols (Scheme 13.8). The diastereoselectivity exhibited by FDP aldolase depends on the reaction conditions. Racemic mixtures of non-natural aldehyde acceptors can be partially resolved only under conditions of kinetic control. When six-membered hemiacetals can be formed, racemic mixtures of aldehydes can be resolved under conditions of thermodynamic control (Scheme 13.10). 

The six-membered (hemiacetal) ring formed is called the pyranose ring. 

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. 

A six-membered hemiacetal ring is shown by the infix -pyran-, and a five-membered hemiacetal ring is shown by the infix -furan-. The terms furanose and pyranose are used because monosaccharide five- and six-membered rings correspond to the heterocyclic compounds pyran and furan ... 

As a rule of thumb, oxidation of the (S)- or pro-(S ) hydroxyl group occurs selectively with HLADH (Scheme 2.143). In the case of 1,4- and 1,5-diols, the intermediate y- and 8-hydroxyaldehydes spontaneously cyclize to form the more stable five- and six-membered hemiacetals (lactols). The latter are further oxidized in a subsequent step by HLADH to form y- or 5-lactones following the same (S)-or pro-(5) specificity [1035]. Both steps - desymmetrization of the prochiral or meso-diol and kinetic resolution of the intermediate lactol - are often highly selective. By using this technique, enantiopure lactones were derived from... 

The monosaccharide D-glucose, whose chemistry is representative of all aldoses containing four or more carbon atoms, exists predominantly in the two pyranosc forms 4 and 5. These are six-membered hemiacetals formed by the reversible cyclization of the acyclic polyhydroxy aldehyde 3 (Eq. 23.1). In the cyclic forms 4 and 5, the ring carbon that is derived from the carbonyl group is referred to as the anomeric carbon atom. The specific rotation, [a] (Sec. 7.5), of a-D-(+)-glucose (4) is +112 whereas that of the -anomer 5 is +19°. When crystals of either pure 4 or pure 5 are dissolved in water, the [a]p changes to an equilibrium value of +52.7°. This process is termed mutarotation. At equilibrium in water, the a- and p-forms are present in the ratio of 36 64 only about 0.03% of D-glucose is in the acyclic form 3. 

When a carbohydrate forms a six-membered hemiacetal ring, it is called a pyranose, and when a carbohydrate forms a five-membered hemiacefal ring, it is called a furanose. 

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