CHAPTER 25 CARBOHYDRATES SOLUTIONS TO TEXT PROBLEMS 25.1 (b) Redraw the Fischer projection so as to show the orientation of the groups in three dimensions. HOCH-CHo is equivalent to HOCH, -C--CHO OH Reorient the three-dimensional representation, putting the aldehyde group at the top and the primary alcohol at the bottom. CHO turn90° HOCH,C--CHO CHOH What results is not equivalent to a proper Fischer projection, because the horizontal bonds are n they should be"forward. The opposite is true for the vertical bonds. To make the drawing correspond to a proper Fischer projection, we need to rotate it 180 around a vertical axis CHO CHO HO-C-H is equivalent to HO H,OH CHOH rotate180° Now, having the molecule arranged properly, we see that it is L-glyceraldehyde 701 Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
701 CHAPTER 25 CARBOHYDRATES SOLUTIONS TO TEXT PROBLEMS 25.1 (b) Redraw the Fischer projection so as to show the orientation of the groups in three dimensions. Reorient the three-dimensional representation, putting the aldehyde group at the top and the primary alcohol at the bottom. What results is not equivalent to a proper Fischer projection, because the horizontal bonds are directed “back” when they should be “forward.” The opposite is true for the vertical bonds. To make the drawing correspond to a proper Fischer projection, we need to rotate it 180° around a vertical axis. Now, having the molecule arranged properly, we see that it is L-glyceraldehyde. is equivalent to CHO CH2OH HO H CHO CH2OH HO H C CHO CH2OH H OH C rotate 180 H OH HOCH2 C CHO CHO CH2OH H OH C turn 90 is equivalent to H OH HOCH2 CHO OH HOCH2 C H CHO Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
702 CARBOHYDRATES (c) Again proceed by converting the Fischer projection into a three-dimensional representation CHO CHO HOCH,H HOCH, C-H Look at the drawing from a perspective that permits you to see the carbon chain oriented ver tically with the aldehyde at the top and the Ch,oh at the bottom. Both groups should point away from you. When examined from this perspective, the hydrogen is to the left and the hydroxyl to the right with both pointing toward you. CHO CHO HOCH, -C-H is equivalent to H-C-OH CHOH The molecule is D-glyceraldehyde 25. 2 Begin by drawing a perspective view of the molecular model shown in the problem. To view the compound as a Fischer projection, redraw it in an eclipsed conformation H OH H OH CH HOCH HOCH CHEO HO H taggered conformation Same molecule in eclipsed The eclipsed conformation shown, when oriented so that the aldehyde carbon is at the top, vertical bonds back, and horizontal bonds pointing outward from their stereogenic centers, is readily trans- formed into the Fischer projection of L-erythrose CHO HOH H is equivalent to HOC H, CHEO HO--H CHOH CHOH 25.3 L-Arabinose is the mirror image of D-arabinose, the structure of which is given in text Fig ure 25.2. The configuration at each stereogenic center of D-arabinose must be reversed to trans form it into L-arabinose CHO H H-OH HO-H oh Ho-h CHOH CHOH D-(-)-Arabinose L-(+)-Arabinose Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
702 CARBOHYDRATES (c) Again proceed by converting the Fischer projection into a three-dimensional representation. Look at the drawing from a perspective that permits you to see the carbon chain oriented vertically with the aldehyde at the top and the CH2OH at the bottom. Both groups should point away from you. When examined from this perspective, the hydrogen is to the left and the hydroxyl to the right with both pointing toward you. The molecule is D-glyceraldehyde. 25.2 Begin by drawing a perspective view of the molecular model shown in the problem. To view the compound as a Fischer projection, redraw it in an eclipsed conformation. The eclipsed conformation shown, when oriented so that the aldehyde carbon is at the top, vertical bonds back, and horizontal bonds pointing outward from their stereogenic centers, is readily transformed into the Fischer projection of L-erythrose. 25.3 L-Arabinose is the mirror image of D-arabinose, the structure of which is given in text Figure 25.2. The configuration at each stereogenic center of D-arabinose must be reversed to transform it into L-arabinose. HO H H OH CHO H OH CH2OH d-()-Arabinose H HO HO H OH CHO H CH2OH l-()-Arabinose is equivalent to or CHO CH2OH HO C H HO C H L-Erythrose HO H HO H CHO CH2OH OH OH HOCH2 H H CH O OH H HOCH2 H HO CH O Staggered conformation Same molecule in eclipsed conformation OH OH HOCH2 H H CH O is equivalent to CHO CH2OH H OH C CHO OH HOCH2 C H is equivalent to CHO OH HOCH2 H CHO OH HOCH2 C H Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
CARBOHYDRATES 703 25.4 The configuration at C-5 is opposite to that of D(+)-glyceraldehyde. This particular carbohydrate therefore belongs to the L series. Comparing it with the Fischer projection formulas of the eight D-aldohexoses reveals it to be in the mirror image of D-(+)-talose; it is L-( 25.5 (b) The Fischer projection formula of D-arabinose may be found in text Figure 25.2. The Fischer projection and the eclipsed conformation corresponding to it are CHO HO→H HOCH, OH about H H HO H HO H CHOH Conformation suitable for of D-arabinose Cyclic hemiacetal formation between the carbonyl group and the C-4 hydroxyl yields the a-and B-furanose forms of D-arabinose. HOCH O H HO HO H HO H a-D- Arabinofuranose (c) The mirror image of D-arabinose [from part(b)] is L-arabinose CHO Ho-H OH HO CHOH H H OH HO-+H HOH∠C H OH Ho-+H CH,OH CHOH H OH L-Arabinose The C-4 atom of the eclipsed conformation of L-arabinose must be rotated 120 in a clock wise sense so as to bring its hydroxyl group into the proper orientation for furanose ring formation H H oH H o rotate about OH C-3-C-4 bond OCH Original eclipsed conformation Conformation suitable for uranose ring formation Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
25.4 The configuration at C-5 is opposite to that of D-()-glyceraldehyde. This particular carbohydrate therefore belongs to the L series. Comparing it with the Fischer projection formulas of the eight D-aldohexoses reveals it to be in the mirror image of D-()-talose; it is L-()-talose 25.5 (b) The Fischer projection formula of D-arabinose may be found in text Figure 25.2. The Fischer projection and the eclipsed conformation corresponding to it are Cyclic hemiacetal formation between the carbonyl group and the C-4 hydroxyl yields the - and -furanose forms of D-arabinose. (c) The mirror image of D-arabinose [from part (b)] is L-arabinose. The C-4 atom of the eclipsed conformation of L-arabinose must be rotated 120° in a clockwise sense so as to bring its hydroxyl group into the proper orientation for furanose ring formation. Original eclipsed conformation of l-arabinose HO H 4 OH H H OH 3 2 1 5 CH2OH Conformation suitable for furanose ring formation H HOCH2 OH H H OH OH rotate about C-3 C-4 bond C O H C O H CHO CH2OH HO H OH H OH H d-Arabinose CHO CH2OH H HO H HO H OH l-Arabinose Eclipsed conformation of l-arabinose HO H 4 HO H H OH 3 2 1 5 CH2OH C O H -d-Arabinofuranose H HOCH2 OH H H HO HO H O -d-Arabinofuranose H HOCH2 H OH H HO HO H O d-Arabinose H OH H OH CHO CH2OH HO H rotate about C-3 C-4 bond Eclipsed conformation of d-arabinose H HO 4 H HO HO H 3 2 1 5 CH2OH Conformation suitable for furanose ring formation HOCH2 H 4 H HO HO H C O H 3 2 1 5 OH C O H CARBOHYDRATES 703 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
704 CARBOHYDRATES Cyclization gives the a-and B-furanose forms of L-arabinose OH H OH H HOCH HOCH ar-L-Arabinofuranose B-L-Arabinofuranose In the L series the anomeric hydroxyl is up in the a isomer and down in the B isomer. (d) The Fischer projection formula for D-threose is given in the text Figure 25. 2. Reorientation of that projection into a form that illustrates its potential for cyclization is shown 10-tH is equivalent to 4 H CHOH Cyclization yields the two stereoisomeric furanose forms H H 0 H HO H HO B-D-Threofuranose a-D-Threofuranose 25.6 (b) The Fischer projection and Haworth formula for D-mannose are CHO HO-H HOCH HO-H OH OH 1o HO CS HOHO OH CHOH D-Mannose mannopyranose The Haworth formula is more realistically drawn as the following chair conformation HOCH HO- OH H Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
Cyclization gives the - and -furanose forms of L-arabinose. In the L series the anomeric hydroxyl is up in the isomer and down in the isomer. (d) The Fischer projection formula for D-threose is given in the text Figure 25.2. Reorientation of that projection into a form that illustrates its potential for cyclization is shown. Cyclization yields the two stereoisomeric furanose forms. 25.6 (b) The Fischer projection and Haworth formula for D-mannose are The Haworth formula is more realistically drawn as the following chair conformation: HO HO H H OH H H H OH HOCH2 O -d-Mannopyranose CHO CH2OH HO H OH H OH H HO H d-Mannose H HO HO OH H H HO H CH2OH -d-Mannopyranose (Haworth formula) H HO OH H HO H HOCH2 H HO H O C O H H HO OH H 4 3 2 O H 1 -d-Threofuranose OH H H HO HO H O -d-Threofuranose H OH H HO HO H O C O H H HO OH H 4 3 2 O H 1 d-Threose CH2OH HO H OH H C H 1 O 2 3 4 is equivalent to C O H -l-Arabinofuranose H HOCH2 OH H OH H H OH O -l-Arabinofuranose H HOCH2 H OH OH H H OH O 704 CARBOHYDRATES Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website
CARBOHYDRATES 705 Mannose differs from glucose in configuration at C-2. All hydroxyl groups are equatorial in B-D-glucopyranose; the hydroxyl at C-2 is axial in B-D-mannopyranose (c) The conformational depiction of B-L-mannopyranose begins in the same way as that of B-D-mannopyranose. L-Mannose is the mirror image of D-mannose. CHO CHO CH,OH H 10-H H HH HO-H CHOH D-Mannose L-Mannose Eclipsed conformation To rewrite the eclipsed conformation of L-mannose in a way that permits hemiacetal forma tion between the carbonyl group and the C-5 hydroxyl, C-5 is rotated 120 in the clockwise HO CH,OH H te about CHOH CHOH H C-S bond Translating the Haworth formula into a proper conformational depiction requires that a choice be made between the two chair conformations shown H HO CH,OHH H H H HH OH OH HO CHOH OH H OH Haworth formula of Less stable chair conformation More stable chair confor B-L-mannopyranose ChOH is axial CH,OH is equatoria (d) The Fischer projection formula for L-ribose is the mirror image of that for D-ribose CHO CHO CH2OH H OHHO→H HO HO HOHO H-OH L-Ribose Eclipsed conformation of L-ribose is Haworth formula of Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE Website
Mannose differs from glucose in configuration at C-2. All hydroxyl groups are equatorial in -D-glucopyranose; the hydroxyl at C-2 is axial in -D-mannopyranose. (c) The conformational depiction of -L-mannopyranose begins in the same way as that of -D-mannopyranose. L-Mannose is the mirror image of D-mannose. To rewrite the eclipsed conformation of L-mannose in a way that permits hemiacetal formation between the carbonyl group and the C-5 hydroxyl, C-5 is rotated 120° in the clockwise sense. Translating the Haworth formula into a proper conformational depiction requires that a choice be made between the two chair conformations shown. (d) The Fischer projection formula for L-ribose is the mirror image of that for D-ribose. CHO CH2OH H H H OH OH OH d-Ribose CHO CH2OH HO HO HO H H H l-Ribose Eclipsed conformation of l-ribose is oriented properly for ring closure. HO H HO H HO H CH2OH Haworth formula of -l-ribopyranose HO H HO H HO H H O OH C O H HO H H CH2OH HO H OH H H O OH Haworth formula of -l-mannopyranose Less stable chair conformation; CH2OH is axial OH H H CH2OH H H HO H OH OH O More stable chair conformation; CH2OH is equatorial OH OH OH H H H H HO HOCH2 O H HO H HO H HO H H OH CH2OH C H O O 1 3 2 4 5 5 6 HO H H CH2OH HO H H OH OH C H 1 3 2 4 -l-Mannopyranose (remember, the anomeric hydroxyl is down in the l series) HO H H CH2OH HO H H OH H OH O rotate about C-4 C-5 bond CHO CH2OH H HO H HO H OH H OH l-Mannose CHO CH2OH HO H OH H OH H HO H d-Mannose Eclipsed conformation of l-mannose HO H H H HO HO H OH CH2OH C O H CARBOHYDRATES 705 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website