Fructose cyclic hemiketal forms

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%). 

Monosaccharide structures may be depicted in open-chain forms showing their carbonyl character, or in cyclic hemiacetal or hemiketal forms. Alongside the Fischer projections of glucose, ribose, and fructose shown earlier, we included an alternative... 

As already mentioned, <5-hydroxyfeto s may be present in the open-chain form or as an isomeric cyclic hemiketal, depending on their substitution pattern. For example, a polyhy-droxylated 5-hydroxyketone, D-fructose, is present exclusively in the form of hemiketals (Figure 9.7). Responsible for this equilibrium position is the fact that the carbonyl group involved contains an electron-withdrawing group in both a-positions, which favors the addition of nucleophiles (see Section 9.1.1). 

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... 

Ketohexoses such as o-fructose also exist as cyclic compounds. Addition of the OH at C-5 to the carbonyl group forms the 5-membered cyclic hemiketal (Fig. 18-4). The 5-membered ring is often referred to as a furanose ring. The a- and /3-notations are the same as for o-glucose. The OH groups at C-2 in a-o-fructose and /S-D-fructose are irons and cis, respectively, to the CH2OH at C-5. 

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. 

Cyclic hemiacetals and hemiketals are much more stable than the open-chain compounds discussed earlier. In Chapter 7, we ll see that sugars such as glucose and fructose exist predominantly in the form of cyclic hemiacetals and hemiketals. 

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... 

---