Question

Emil Fischer synthesized L-gulose, an unusual aldohexose that reduces to give D-glucitol. Suggest a structure for this L sugar, and show how L-gulose gives the same alditol as D-glucose. (Hint: D-Glucitol has -CH₂OHgroups at both ends. Either of these primary alcohol groups might have come from reduction of an aldehyde.)

Short answer

Aldoses and ketoses can be reduced to their corresponding polyalcohols, known as sugar alcohols or alditols by using the reagents such as sodium borohydride (NaBH₄) or by catalytic hydrogenation using a nickel (Ni) catalyst.

Step-by-step solution

Alditols

Aldoses and ketoses can be reduced to their corresponding polyalcohols, known as sugar alcohols or alditols by using the reagents such as sodium borohydride (NaBH₄) or by catalytic hydrogenation using a nickel (Ni) catalyst.

Naming of alditols

Alditols are named by adding a suffix -itol to the root name of the sugar.

D- and L- series of sugars

For D - series of sugars, the OH group of the bottom asymmetric carbon atom is on the right side in the Fischer projection. Again, for L - series of sugars, the OH group of the bottom asymmetric carbon atom is on the left side in the Fischer projection. D - and L - refers to the absolute configuration around the asymmetric carbon atom while (+) and (-) refers to the direction of rotation of the plane polarised light.

Synthesis