Stereocenter designating configuration

The designations D and L refer to the configuration of the stereocenter farthest from the aldehyde or ketone group on a monosaccharide chain. When the monosaccharide chain is drawn as a Fischer projection, a D-monosaccharide has the -OH on this carbon on the right an L monosaccharide has it on the left. 

The three-dimensional arrangement of groups about a stereocenter in a molecule is termed its configuration. One way to designate the configurations of cis-trans isomers is to use the terms cis and tram. However, these terms are ambiguous in many instances. For example, is the following isomer of 1-chloro-l-fluoro-l-propene the cis- or the trans-stereoisomer ... 

Cahn-lngold-Prelog sequence rules (Section 6.2) Rules that are used to assign priorities to groups attached to a stereocenter so that the configuration of the compound can be designated. 

The chemistry of spiroketals, especially l,7-dioxaspiro[5,5]undecanes, is well-studied and reviewed in the literature (15,16). Generally, the ratio of isomers of spiroketals may be controlled by several stabilizing factors such as stereoelectronic effects and 1,3-diaxial interaction of the substituents. Utilizing well-designed spiroketals, one can selectively prepare the most stable isomer by thermodynamic equilibration. Based on this idea, we planned to prepare a spiroketal represented by II which possesses several substituents with established (Cy and C5O and unestablished (Ca) configurations. The spiroketal center and its a-position in II can isomerize to the more stable forms by steric effects caused by the established stereocenters on the ring. The prepared most stable isomer... 

The relative configuration of adjacent substituents in a Fischer projection formula are designated erythro if they are on the same side and threo if they are on the opposite side. The stereochemistry of adjacent stereocenters can also be usefully represented... 

Ans. There is no relationship between the direction of optical rotation and the D- or L-configuration. D- or L-refers to the actual configuration at the stereocenter and can be determined only by x-ray crystallography. Only after a determination of the configuration by X-ray crystallography can we use the convention described in Problem 17.18 to designate one enantiomer as d- and the other as L-. Optical rotation is determined by using a polarimeter and its direction is unrelated to the d- or L-nomenclature. 

For a sugar having n stereocenters, there are 2" isomers, so that there are 2 =16 aldohexoses, 8 in the d series and 8 in the l series. The names of the D-aldoses and their stereochemical relationships are illustrated in Scheme 5. These names form the basis of the group-configurational designators alio, altro, gluco,... 

In order to designate the configuration of a stereocenter, the priority of groups must be read in a clockwise or... 

FIG. 3 The polyenoyltetramic acid antibiotic a-lipomycin (7) and its aglycon P-lipomycin (8). The (stereo)structures of these compounds are depicted as formulated before our syntheses were accomplished. How the information shown here was gained is detailed in Section 3.1. The asterisks designate stereocenters with previously unknown configurations. 

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