88607238-2721/21/037:38 AM Page242ac113ac11:aEDL 242 Part I Structure and Catalysis OH HO- o-R =R- R- -C-oR+H,o H Ho—R Aldehyde Alcohol Hemiacetal Acetal FIGURE 7-5 Formation of hemiacetals and hemiketals An aldehyde or ketone can react with an alcohol in a 1:1 ratio to yield a hemiacetal or hemiketal, respectively, creating a new chiral center at the carbonyl carbon. r- -C=0+HO- eRCOR+Ho Substitution of a second alcohol molecule produces HO-R nother sugar molecule, the bond produced is a glycosidic bond (p 245 group along the chain. The formation of these ring struc- istry of ring forms of monosaccharides. However, the tures is the result of a general reaction between alco- six-membered pyranose ring is not planar, as Haworth hols and aldehydes or ketones to form derivatives called perspectives suggest, but tends to assume either of two hemiacetals or hemiketals (Fig. 7-5), which contain"chair" conformations(Fig. 7-8). Recall from Chapter 1 an additional asymmetric carbon atom and thus can ex (p. 19) that two conformations of a molecule are in- ist in two stereoisomeric forms. For example, D-glucose terconvertible without the breakage of covalent bonds exists in solution as an intramolecular hemiacetal in which the free hydroxyl group at C-5 has reacted with the aldehydic C-l, rendering the latter carbon asyn metric and producing two stereoisomers, designated a and B(Fig. 7-6). These six-membered ring compounds are called pyranoses because they resemble the six- membered ring compound pyran(Fig. 7-0. The sys- tematic names for the two ring forms of D-glucose ar a-D-glucopyranose and B-D-glucopyranose H-C-OH Aldohexoses also exist in cyclic forms having five- membered rings, which, because they resemble the five- CH2OH membered ring compound furan, are called furanoses However, the six-membered aldopyranose ring is much more stable than the aldofuranose ring and predomi- CH2OH ates in aldohexose solutions. Only aldoses having five -OH or more carbon atoms can form pyranose rings Isomeric forms of monosaccharides that differ only in their configuration about the hemiacetal or heike- tal carbon atom are called anomers. The hemiacetal (or carbonyl) carbon atom is called the anomeric carbon. The a and B anomers of D-glucose interconvert in aque ous solution by a process called mutarotation. Thus a solution of a-D-glucose and a solution of B-D-glucose eventually form identical equilibrium mixtures having 6CH2OH 6CH2OH identical optical properties. This mixture consists of about one-third a-D-glucose, two-thirds B-D-glucose and very small amounts of the linear and five-membered ring(glucofuranose) forms OH H Ketohexoses also occur in a and B anomeric forms In these compounds the hydroxyl group at C-5(or C-6) reacts with the keto group at C-2, forming a furanose Cor pyranose)ring containing a hemiketal linkage (Fig. FIGURE 7-6 Formation of the two cyclic forms of D-glucose. Read 7-5). D-Fructose readily forms the furanose ring (Fig. tion between the aldehyde group at C-1 and the hydroxyl group at 7-7); the more common anomer of this sugar in com- C-5 forms a hemiacetal linkage, producing either of two stereoiso- bined forms or in derivatives is B-D-fructofuranose mers, the a and B anomers, which differ only in the stereochemistry Haworth perspective formulas like those in Fig- around the hemiacetal carbon. The interconversion of a and B anomers ure 7-7 are commonly used to show the stereochem- is called mutarotation242 Part I Structure and Catalysis H C -D-Glucopyranose C H OH H 1 5 C 6CH2OH 4 C OH CH2OH 6 C 5 HO H OH C H 3 H C 4 HO C3 OH H H 2 OH C 1 5 6CH2OH 4 C O OH HO OH C H H C3 H C H H 2 OH OH C 1 5 6CH2OH 4 C O HO OH C H H C3 H C H H 2 OH OH D-Glucose -D-Glucopyranose H C O O 1C H 2 FIGURE 7–6 Formation of the two cyclic forms of D-glucose. Reac￾tion between the aldehyde group at C-1 and the hydroxyl group at C-5 forms a hemiacetal linkage, producing either of two stereoiso￾mers, the
and anomers, which differ only in the stereochemistry around the hemiacetal carbon. The interconversion of
and anomers is called mutarotation. FIGURE 7–5 Formation of hemiacetals and hemiketals. An aldehyde or ketone can react with an alcohol in a 1:1 ratio to yield a hemiacetal or hemiketal, respectively, creating a new chiral center at the carbonyl carbon. Substitution of a second alcohol molecule produces an acetal or ketal. When the second alcohol is part of another sugar molecule, the bond produced is a glycosidic bond (p. 245). istry of ring forms of monosaccharides. However, the six-membered pyranose ring is not planar, as Haworth perspectives suggest, but tends to assume either of two “chair” conformations (Fig. 7–8). Recall from Chapter 1 (p. 19) that two conformations of a molecule are in￾terconvertible without the breakage of covalent bonds, R3 O  HO C Ketal R1 C OR4 R 1 H Aldehyde Hemiketal R2 HO C H OH R 1 OR3 Hemiacetal OR3 R2 R 1 C O R2 Alcohol   H2O C  H2O OH R1 OR2 Ketone Alcohol C H Acetal OR3 OR2 HO R4 R2 R1 HO R4 HO R3 HO R3 group along the chain. The formation of these ring struc￾tures is the result of a general reaction between alco￾hols and aldehydes or ketones to form derivatives called hemiacetals or hemiketals (Fig. 7–5), which contain an additional asymmetric carbon atom and thus can ex￾ist in two stereoisomeric forms. For example, D-glucose exists in solution as an intramolecular hemiacetal in which the free hydroxyl group at C-5 has reacted with the aldehydic C-1, rendering the latter carbon asym￾metric and producing two stereoisomers, designated
and (Fig. 7–6). These six-membered ring compounds are called pyranoses because they resemble the six￾membered ring compound pyran (Fig. 7–7). The sys￾tematic names for the two ring forms of D-glucose are
-D-glucopyranose and -D-glucopyranose. Aldohexoses also exist in cyclic forms having five￾membered rings, which, because they resemble the five￾membered ring compound furan, are called furanoses. However, the six-membered aldopyranose ring is much more stable than the aldofuranose ring and predomi￾nates in aldohexose solutions. Only aldoses having five or more carbon atoms can form pyranose rings. Isomeric forms of monosaccharides that differ only in their configuration about the hemiacetal or hemike￾tal carbon atom are called anomers. The hemiacetal (or carbonyl) carbon atom is called the anomeric carbon. The
and anomers of D-glucose interconvert in aque￾ous solution by a process called mutarotation. Thus, a solution of
-D-glucose and a solution of -D-glucose eventually form identical equilibrium mixtures having identical optical properties. This mixture consists of about one-third
-D-glucose, two-thirds -D-glucose, and very small amounts of the linear and five-membered ring (glucofuranose) forms. Ketohexoses also occur in
and anomeric forms. In these compounds the hydroxyl group at C-5 (or C-6) reacts with the keto group at C-2, forming a furanose (or pyranose) ring containing a hemiketal linkage (Fig. 7–5). D-Fructose readily forms the furanose ring (Fig. 7–7); the more common anomer of this sugar in com￾bined forms or in derivatives is -D-fructofuranose. Haworth perspective formulas like those in Fig￾ure 7–7 are commonly used to show the stereochem- 8885d_c07_238-272 11/21/03 7:38 AM Page 242 Mac113 mac113:122_EDL: