CARBOHYDRATES 711 The pyranose form arises by closure to a six-membered cyclic hemiacetal, with the C-5 hydroxyl group undergoing nucleophilic addition to the carbonyl. In the B-pyranose form of The preferred conformation of B-D-xylopyranose is a chair with all the hydroxyl groups OH is better represented OH Haworth formula of Chair conformation of L-Xylose is the mirror image of D-xylose CHO CHO H OH HO-H CHOH HO-H H HO CHO OH HO-H CHOH CHOH D-Xylose L-Xylose ation To construct the furanose form of L-xylose, the hydroxyl at C-4 needs to be brought into the proper orientation to form a five-membered ring CH,OH rotate about 々 H HO CHO C3=c4bm H HO HOCH HOCH The a anomeric hydroxyl group is up in the L series (e) Methyl a-L-xylofuranoside is the methyl glycoside corresponding to the structure just drawn OCH3 HOCHA HO H (f) Aldonic acids are derived from aldoses by oxidation of the terminal aldehyde to a carboxylic acid H OH H HO HO H-OH H-OH CH,OH CHOH D-Xylose Xylonic acid Back Forward Main Menu TOC Study Guide Toc Student OLC MHHE WebsiteThe pyranose form arises by closure to a six-membered cyclic hemiacetal, with the C-5 hydroxyl group undergoing nucleophilic addition to the carbonyl. In the -pyranose form of D-xylose the anomeric hydroxyl group is up. The preferred conformation of -D-xylopyranose is a chair with all the hydroxyl groups equatorial. (d) L-Xylose is the mirror image of D-xylose. To construct the furanose form of L-xylose, the hydroxyl at C-4 needs to be brought into the proper orientation to form a five-membered ring. The anomeric hydroxyl group is up in the L series. (e) Methyl -L-xylofuranoside is the methyl glycoside corresponding to the structure just drawn. ( f ) Aldonic acids are derived from aldoses by oxidation of the terminal aldehyde to a carboxylic acid. d-Xylose H OH HO H CH2OH H OH d-Xylonic acid H OH HO H CH2OH H OH C H O C HO O OCH3 H H HOCH2 H HO HO H O HO H HO HO H H CH2OH CHO 1 3 2 4 5 H HOCH2 HO HO H H O H C O H rotate about C-3 C-4 bond OH H H HOCH2 H HO HO H O HO H H OH CHO H OH CH2OH H HO HO H OH H CH2OH CHO d-Xylose l-Xylose Eclipsed conformation of l-xylose HO H HO H H HO CH2OH CHO is better represented as H HO OH H OH H H OH O Haworth formula of -d-xylopyranose Chair conformation of -d-xylopyranose HO HO HO OH O CARBOHYDRATES 711 Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website