Five-membered ring hemiketal

Ketoses also exist in solution predominantly in cyclic forms. For example, D-fructose forms a five-membered-ring hemiketal because its C-5 OH group reacts with its ketone carbonyl group. If the OH group bonded to the new asymmetric center is trans to the primary alcohol group, then the compound is a-o-fmctofuranose if it is cis to the... 

In a similar manner, ketones can react with alcohols to form hemiketals. The analogous intramolecular reaction of a ketose sugar such as fructose yields a cyclic hemiketal (Figure 7.6). The five-membered ring thus formed is reminiscent of furan and is referred to as a furanose. The cyclic pyranose and fura-nose forms are the preferred structures for monosaccharides in aqueous solution. At equilibrium, the linear aldehyde or ketone structure is only a minor component of the mixture (generally much less than 1%). 

The C-2 keto group in the open-chain form of a ketohexose, such as fructose, can form an intramolecular hemiketal by reacting with either the C-6 hydroxyl group to form a six-membered cyclic hemiketal or the C-5 hydroxyl group to form a five-membered cyclic hemiketal (Figure 11.5). The five-membered ring is called a furanose because of its similarity to... 

Figure 11.5. Furanose Formation. The open-chain form of fructose cyclizes to a five-membered ring when the C-5 hydroxyl group attacks the C-2 ketone to form an intramolecular hemiketal. Two anomers are possible, but only the a anomer is shown.

Five- and six-membered ring hemiacetals and hemiketals are common configurations of many sugars. Ribose, which forms a part of the structure of RNA, closes to form a five-membered ring preferentially. 

Fructose is a ketose, or ketone sugar. Recall that the reaction between an alcohol and a ketone )delds a hemiketal. Thus the reaction between the C-2 keto group and the C-5 hydroxyl group in the fructose molecule produces an intramolecular hemiketal. Fructose forms a five-membered ring structure. 

Fructose is a ketohexose sugar. The cyclic structure formed is called a hemiketal, and the five membered ring that is formed is called furanose ring. The straight chain structure and the cyclic structures of fructose are shown in Figure 29-5. 

There are obviously many aldose monosaccharides and many ketose monosaccharides, but the discussion will focus only on the d diastereomers, as with the aldoses. The triose is l,3-dihydroxy-2-propanone (41 also called glycerone) and the tetrose is d-glycero-tetrulose (42). Two pentoses are named d-ribulose (43) and d-xylulose (44) and four hexoses are named d-psicose (45), d-fructose (46), d-sor-bose (47), and d-tagatose (48). All of these compounds are ketoses and are further classified according to the number of carbon atoms, as noted in Section 28.1. For example, 42 is a ketotetrose, 43 is a ketopentose, and 45 is a ketohexose. For 43-48, cyclization is possible to form a hemiketal. Just as aldehydes and alcohols react to form a hemiacetal, ketones and alcohols react to form a hemiketal (see Chapter 18, Section, 18.6). For 43, a five-membered ring (a furanose) is formed if the terminal CH2OH unit (in violet) reacts with the ketone carbonyl. The two anomers formed are 49 (a-d-ribulofuranose) and 50 (P-d-ribulofuranose). 

The cyclic hemiacetal or hemiketal forms of aldo- and ketohexoses and pentoses are the predominant forms of these sugars, rather than the open-chain structures we have discussed to this point. Cyclic hemiacetals and hemiketals of carbohydrates that contain five-membered rings are called furanoses. Cyclic hemiacetals and hemiketals that contain six-membered rings are called pyranoses. These names are based the cyclic rings of furan and pyran. 

Cyclic hemiacetals or hemiketals of sugars with a five-membered (tetrahydrofuran) ring are called furanoses, those with a six-membered (tetrahydropyran) ring pyran-oses. For sugars with other ring sizes see 2-Carb-5. 

The reactivity of carbohydrates is dominated by the reactivity of the aldehyde group and the hydroxyl on its next-neighbor (/ ) carbon. As illustrated by the middle row of Fig. 2.3, the aldehyde can be isomerized to the corresponding enol or be converted into its hydrate (or hemiketal) form upon reaction with water (or with an hydroxyl-group). These two reactions are responsible for the easy cycliza-tion of sugars in five- and six-membered rings (furanose and pyranose) and their isomerization between various enantiomeric forms and between aldehyde- and ketone-type sugars (aldose and ketose). 

Lactones have also been reacted with trimethyl(trifluoromethyl)silane under fluoride catalysis in tetrahydrofuran however, only the silylated hemiketals could be isolated cleanly in the case of five- and six-membered ring lactones (in 70 and 75% yield, respectively). In the case of the seven-membered ring lactone, a mixture of products resulted. 

D-Fructose, a ketohexose, can potentially form either a five-membered (furanose) or a six-membered (pyranose) ring involving formation of an internal hemiketal linkage between C2 (the anomeric carbon atom) and the C5 or C(, hydroxyl group, respectively. The hemiketal linkage introduces a new asymmetrical center at the C2 position. Thus, two anomeric forms of each of the fructo-furanose and fructopyranose ring structures are possible (Figure 9-10). In aqueous solution at equilibrium, fructose is present predominantly in the )3-fructopyranose form. 

---