Stereochemistry of sugars

At the turn of the twentieth century, Emil Fischer was studying the stereochemistry of sugars (Chapter 23), which contain as many as seven asymmetric carbon atoms. To draw these structures in perspective would have been difficult, and to pick out minor stereochemical differences in the drawings would have been nearly impossible. Fischer developed a symbolic way of drawing asymmetric carbon atoms, allowing them to be drawn rapidly. The Fischer projection also facilitates comparison of stereoisomers, holding them in their most symmetric conformation and emphasizing any differences in stereochemistry. 

The stereochemistry of sugars used to be represented by Fischer projections. The carbon backbone was laid out in a vertical line and twisted in such a way that all the substituents pointed towards the viewer. 

SCHEME 6.35 Oxygen-based nucleophiles can be used to invert the stereochemistry of sugar hydroxyl groups. 

Checking on your understanding of the stereochemistry of sugars and their selective protection. 

The depictions of glucopyranose and fructofuranose shown in Figures 11.4 and 11.5 are Haworth projections. In such projections, the carbon atoms in the ring are not explicitly shown. The approximate plane of the ring is perpendicular to the plane of the paper, with the heavy line on the ring projecting toward the reader. Like Fischer projections, Haworth projections allow easy depiction of the stereochemistry of sugars. 

The concept itself of asymmetric synthesis, stoichiometric or catalytic, took a long time appear. One important step was the investigations of Fischer in 1894-1899 on the structure and stereochemistry of sugars [3,4,5]. He observed the formation of diastereomers on addition of HCN to the aldehyde function of... 

The stereochemistry of sugars used to be represented by Fischer projections. The carbon backbone was laid out in a vertical line and twisted in such a way that all the substituents pointed towards the viewer. Fischer projections are so unlike real molecules that you should never use them. However, you may see them in older books, and you should have an idea about how to interpret them. Just remember that all the branches down the side of the central trunk are effectively bold wedges (coming towards the viewer), whiie the central trunk lies in the plane of the paper. By mentally twisting the backbone into a reaiistic zig-zag shape you shouid end up with a reasonabie representation of the sugar molecule. 

A major advance came with detailed insight into the stereochemistry of sugars (Fischer, 1891). This crucial description clarified their chemical relationships, but would suggest that, in Brown s experiments, D-glucose (3) should be reduced to D-glucitol (7), not to o-mannitol (8). The former... 

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