The D,L nomenclature divides sugars into two groups. 293ae268eroew R苍 necessary imply (+)and 生毛 8 目 盖重盖 国 24-2 Conformations and Cyclic Forms of Sugars nolecular hemiacetals Fischerr depict all-ecipsed 地 tve-membered 出 eadsto theal 2 2 The D,L nomenclature divides sugars into two groups. For a pentahydroxyhexanal there are 16 stereoisomers divided into two groups: 8 stereoisomers labeled D, and their 8 enantiomers labeled L. Systematic nomenclature of sugar molecules leads to long complex names. As a result, the common names of most sugars are usually used, for example: erythrose and threose for the four aldotetroses. Note that the D (or R) label does not necessarily imply (+) and L (or S) does not necessarily imply (-). D-glyceraldehyde is detrorotatory. D-erythrose is levorotatory. Almost all naturally occurring sugars have the D absolute configuration. 24-2 Conformations and Cyclic Forms of Sugars Fischer projections depict all-eclipsed conformations. The all-eclipsed Fischer projection representation of a sugar molecule can be translated into an all-eclipsed dashed-wedged line structure: Rotation at C3 and C5 by 180 degrees leads to the all-staggered conformation. Sugars form intramolecular hemiacetals. Hexoses and pentoses exist in solution as an equilibrium mixture with their cyclic hemiacetal isomers, in which the hemiacetals strongly predominate. Six-membered rings are the preferred products, however, five membered rings are known. Three- and four-membered rings are too strained to form. Six-membered rings are based upon the six-membered cyclic ether, pyran, and are called pyranoses. Five-membered rings are based upon the five-membered cyclic ether, furan, and are called furanoses