706 CARBOHYDRATES Of the two chair conformations of B-L-ribose, the one with the greater number of equatorial AOH HO HO H Less stable chai More stable chair conformation of 25.7 The equation describing the equilibrium is HOCH2 O HO HOCH, OH OH HOCH OH CH=O OH a-D-Mannopyranose Open-chain form of D-mannose B-D-Mannopyranose Let a= percent a isomer; 100-A= percent B isomer. Then A(+29.39)+(100-A)(-17.0°)=100+14.2° 46.3A=3120 Percent a isomer =67%o Percent B isomer =(100-A)=33% 25.8 Review carbohydrate terminology by referring to text Table 25.1. A ketotetrose is a four-carbon ke- tose. Writing a Fischer projection for a four-carbon ketose reveals that only one stereogenic center is present, and thus there are only two ketotetroses. They are enantiomers of each other and are known as D- and L-erythrulose CHOH CHOH H-OH CHOH CHOH 25.9(b) Because L-fucose is 6-deoxy-L-galactose, first write the Fischer projection formula of D-galactose, and then transform it to its mirror image, L-galactose. Transform the C-6 CH,OH roup to CH3 to produce 6-deoxy-L-galactose CHO CHO DH HO HO H o-H H H H H HO-H CH,OH CH D-Galactose - Galactose 6-Deoxy-L-galactose (from Figure 25.2) fucose Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE WebsiteOf the two chair conformations of -L-ribose, the one with the greater number of equatorial substituents is more stable. 25.7 The equation describing the equilibrium is Let A  percent isomer; 100 A  percent isomer. Then A(29.3°)  (100 A)(17.0°)  100(14.2°) 46.3A  3120 Percent isomer  67% Percent isomer  (100 A)  33% 25.8 Review carbohydrate terminology by referring to text Table 25.1. A ketotetrose is a four-carbon ke￾tose. Writing a Fischer projection for a four-carbon ketose reveals that only one stereogenic center is present, and thus there are only two ketotetroses. They are enantiomers of each other and are known as D- and L-erythrulose. 25.9 (b) Because L-fucose is 6-deoxy-L-galactose, first write the Fischer projection formula of D-galactose, and then transform it to its mirror image, L-galactose. Transform the C-6 CH2OH group to CH3 to produce 6-deoxy-L-galactose. H HO HO H OH CHO H OH H CH2OH d-Galactose (from Figure 25.2) HO H H OH CHO HO H H OH CH2OH l-Galactose HO H H OH CHO HO H H OH CH3 6-Deoxy-l-galactose (l-fucose) H C O OH CH2OH CH2OH d-Erythrulose HO C O H CH2OH CH2OH l-Erythrulose OH HOCH2 HO HO OH O -D-Mannopyranose [] 20  29.3 D OH HOCH2 HO OH HO O -D-Mannopyranose [] 20 17.0 D Open-chain form of D-mannose HOCH2 HO HO CH O OH OH HO H HO H HO H H O OH Less stable chair conformation of -l-ribopyranose OH HO OH OH O More stable chair conformation of -l-ribopyranose OH HO OH O HO 706 CARBOHYDRATES Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website