Fischer projections drawing mirror images

Drawing Monosaccharide Structures The Fischer Projection. From the stereochemical point of view, monosaccharides are considered to be derived from the two trioses, D- and L-glyceraldehyde (see Fig. III-l). These two parent compounds differ only in the steric arrangement of the atoms about the central asymmetric carbon they are mirror images (enantiomorphs, optical antipodes) of one another. 

The method used to draw stereoisomers of this type is to first draw one isomer in the Fischer projection. Then we draw its mirror image and see if they are super-imposable, if they are not, they are stereoisomers, specifically enantiomers (see structures I and II of Fig. 2). Then we switch the first left hand group with the first right hand group and compare it with the original structure to see if they are superimposable. If they are not superimposable, they are stereoisomers (compare structure I with structure III in Fig. 2). We continue this process with all the chiral centers until we reach the other end of the molecule. 

Similar drawings are provided for diastereomer 45 (45A and 45B), and another line drawing is provided for 46 (46A and 46B), which is the mirror image of 45. Careful inspection of 45 and 46 reveals something different from previous stereoisomers Structure 45 is superimposable on 46, which means that these two structures are the same (they represent one molecule, 45 = 46). Make a model of both 45A and 46A. Pick up 45A, rotate it by 180°, and lay it on top of 46A. They are a perfect fit all atoms match up. Note thcU the atoms will match only in an eclipsed rotamer. Examine Fischer projection 45B to see that both bromine atoms are on the same side of the molecule, as are both hydrogens. The up and the down groups are both methyl. 

It is possible to represent mirror-image isomerism in the plane of the paper through a perspective drawing (Fig. 1). Clearer, however, is E. Fischer s projection formula now widely used in print. 

Fischer projections can also be drawn for compounds that have two or more chiral carbons. For example, in the mirror images of erythrose, both of the carbon atoms at the intersections are chiral. To draw the mirror image for L-erythrose, we need to reverse the positions of all the —H and the —OH groups on the horizontal lines. For compounds with two or more chiral carbons, the designation as a d or l stereoisomer is determined by the position of the —OH group attached to the chiral carbon farthest from the carbonyl group. 

Determine if each Fischer projection is chiral or achiral. If chiral, identify it as the D or L stereoisomer and draw the mirror image. 

Draw the Fischer projection for the mirror image of the ribulose in Sample Problem 13.4. 

To draw the mirror image, all the —OH groups on the chiral carbon atoms are drawn on the opposite side. L-Ribulose has the following Fischer projection ... 

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