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 Website25.4 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 clock￾wise 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