# How many stereoisomers of a heptulose are possible? How many are D and how many are L sugars? How many names will be needed for all the isomers?

Jun 6, 2015

There are 46 heptulose isomers, so you need 46 names. There are 22 D isomers, 22 L isomers, and 2 meso compounds.

A heptulose is a ketoheptose, that is, a carbohydrate with seven carbon atoms and a ketone group.

The ketone group can be at $\text{C-2}$, $\text{C-3}$, or $\text{C-4}$.

Each positional isomer has 4 chiral centres, so there are 3 × 2^4 = 48 possible stereoisomers.

$\text{C-2}$ Heptuloses

The Fischer projections of the D isomers are:

And there are also eight corresponding L isomers.

$\text{C-3}$ Heptuloses

The Fischer projections of the D isomers are:

And there are also eight corresponding L isomers.

$\text{C-4}$ Heptuloses

The Fischer projections of the D isomers are:

Two of these are meso compounds, so there are six D isomers and six L isomers.

That makes a total of 16 + 16 + 12 + 2 = 46 isomers.

Of these 46 isomeric heptuloses, only two are commonly found in nature.

Sedoheptulose plays an important role in the formation of hexoses from smaller fragments.

Mannoheptulose is found in avocados. It inhibits the breakdown of glucose and the secretion of insulin.