Hemiacetals special

The reaction starts of with a protonation - use the catalyst. Resist the urge to protonate the 4-hydroxyl, but go for the one at position 1 that has the added functionality of the hemiacetal linkage. It is going to be the more reactive one. Protonation is followed by loss of water as leaving group. The intermediate oxonium cation shown is actually a resonance form of the simpler carbocation now you can see the role of the adjacent oxygen. The reaction is completed by attack of the nucleophile, the 4-hydroxyl of another molecule. This is not special, but is merely another version of the hemiacetal synthesis done in part (a). 

Dialdehyde starch,176-178 block reactive starch179 and cationic aldehyde starch180 181 comprise a special class of starches that can react with cellulose through the formation of covalent hemiacetal or acetal bonds. The zwitterion group is another functional group that has been utilized in conjunction with cationic and acetal groups. 

Hemiacetal formation is reversible, and hemiacetals are stabilized by the same special structural features as those of hydrates. However, hemiacetals can also gain stability by being cyclic—when the carbonyl group and the attacking hydroxyl group are part of the same molecule. The reaction is now an intramolecular (within the same molecule) addition, as opposed to the intermolecular (between two molecules) ones we have considered so far. 

To answer this problem, one must know the structures of the molecules in question and a couple of definitions. By definition, epimers are a pair of molecules that differ from each other only in their configuration at a single asymmetric center. Anomers are special epimers that differ only in their configuration at a carbonyl carbon hence, they are usually acetals or hemiacetals. An aldose-ketose pair is obvious. Inspection of Fischer representations of the molecular pairs leads to the conclusion that (a), (c), and (e) are aldose-ketose pairs (b) and (f) are epimers and (e) are anomers. 

Heterocycies can be divided into two subgroups nonaromatic and aromatic. We have already encountered a few nonaromatic heterocycies—ethylene oxide and other cyclic ethers (Chapter 8), cyclic hemiacetals such as glucose (Chapter 9), cyclic esters called lactones (Chapter 10), and cyclic amines such as morphine (Chapter 11). In general, these nonaromatic heterocycies behave a great deal like their acyclic analogs and do not require special discussion. 

As a special case, the formation of hemiacetals 2 (lactolization) during the hydroformylation of hydroxy-functionalized olefins, such as allyl or homoallyl alcohols, has to be mentioned (1, Y= O, Scheme 5.70). With these substrates, the reaction occurs in an intramolecular manner. In the presence of an external alcohol, the cyclic hemiacetal can further react to give a nonsymmetric cyclic acetal 3. Hemiacetals can be subjected to hydrogenation to afford diols 4. Under reducing conditions and in the presence of amines, amino alcohols 5 are formed both are valuable building blocks in fine chemistry. Alternatively, oxidation gives lactones 6 [5]. By dehydration of hemiacetals, cychc vinyl ethers 7 are formed. The same transformation with allylamines (Y=NR) gives cyclic hemiaminals, A/ ,0-acetals, lactames, or vinyl amines. 

Entry 6 reminds us that the furanose and pyranose forms of carbohydrates are specialized examples of hemiacetals and capable of interconversion. Entries 7 and 8 connect enolization of the open-chain form to interconversion of C-2 epimers. Entry 9 illustrates another reaction involving the enol of the open-chain form. Here, the enediol intermediate connects aldose and ketose isomers. In entry 10, an enzyme-catalyzed reverse aldol reaction cleaves a 6-carbon chain to two 3-carbon fragments. 

The product of this reaction is called an acetal. When we form an acetal from a ketone, there is one intermediate that gets a special name, because it is the only intermediate that does not have a charge. It is called a hemiacetal, and you can think of it as half-way toward making an acetal ... 

A special acetal or ketal that results when a hemiacetal or hemiketal reacts with a noncarbohydrate alcohol, and water is released. 

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