Question

Draw the following monosaccharides, using chair conformations for the pyranoses and Haworth projections for the furanoses.

$$\begin{gathered} \mathbf{\left(}\mathbf{a}\mathbf{\right)}\mathbf{}\mathbf{\alpha }\mathbf{‐}\mathbf{D}\mathbf{‐}\mathbf{mannopyranose}\left(C2\mathrm{epimer}\mathrm{of}\mathrm{glucose}\right) \\ \mathbf{\left(}\mathbf{b}\mathbf{\right)}\mathbf{}\mathbf{\beta }\mathbf{‐}\mathbf{D}\mathbf{‐}\mathbf{galactopyranose}\mathbf{(}\mathbf{C}\mathbf{4}\mathbf{}\mathbf{epimer}\mathbf{}\mathbf{of}\mathbf{}\mathbf{glucose}\mathbf{)} \\ \mathbf{\left(}\mathbf{c}\mathbf{\right)}\mathbf{}\mathbf{\beta }\mathbf{‐}\mathbf{D}\mathbf{‐}\mathbf{allopyranose}\mathbf{}\mathbf{(}\mathbf{C}\mathbf{3}\mathbf{}\mathbf{epimer}\mathbf{}\mathbf{of}\mathbf{}\mathbf{glucose}\mathbf{)} \\ \mathbf{\left(}\mathbf{d}\mathbf{\right)}\mathbf{}\mathbf{\alpha }\mathbf{‐}\mathbf{D}\mathbf{‐}\mathbf{arabinofuranose} \\ \mathbf{\left(}\mathbf{e}\mathbf{\right)}\mathbf{}\mathbf{\beta }\mathbf{‐}\mathbf{D}\mathbf{‐}\mathbf{ribofuranose}\mathbf{}\mathbf{(}\mathbf{C}\mathbf{2}\mathbf{}\mathbf{epimer}\mathbf{}\mathbf{of}\mathbf{}\mathbf{arabinose}\mathbf{)} \end{gathered}$$

Short answer

(a)

$\mathrm{\alpha }‐\mathrm{D}‐\mathrm{mannopyranose}$

(b)

$\mathrm{\beta }‐\mathrm{D}‐\mathrm{galactopyranose}$

(c)

$\mathrm{\beta }‐\mathrm{D}‐\mathrm{allopyranose}$

(d)

$\mathrm{\alpha }‐\mathrm{D}‐\mathrm{arabinofurinose}$

(e)

$\mathrm{\beta }‐\mathrm{D}‐\mathrm{ribofuranose}$

Step-by-step solution

Pyranose, furanose, Epimers and Anomers

Pyranose is a six-membered cyclic hemiacetal while a furanose is a five-membered cyclic hemiacetal.

Epimers are sugars with a common structure, such as the arrangement of H and OH around carbon atoms, except for the carbon present at position or carbon number 2. Glucose and mannose, for example, are epimers of each other.

Anomers are the diastereomers that result during cyclization. Only the arrangement surrounding the first carbon C1, referred known as the anomeric carbon, differs (hemiacetal carbon atom).

Identifying α‐and β‐  anomers of pyranoses and furanoses

For $\alpha \mathit{‐}$anomer, hydroxy group (-OH) on the anomeric carbon is down (axial position) while for $\beta \mathit{‐}$anomer, hydroxy group (-OH) on the anomeric carbon is up (equatorial position). Again, for $\alpha \mathit{‐}$anomer, the anomeric hydroxy(-OH) group is placed trans to the terminal $-{\mathrm{CH}}_2\mathrm{OH}$group while for $\beta \mathit{‐}$ anomer, the anomeric hydroxy (-OH) group is placed cis to the terminal${\mathrm{CH}}_2\mathrm{OH}$ group.

Structure of monosaccharides

(a) In chair conformation of $\mathrm{\alpha }‐\mathrm{D}‐\mathrm{mannopyranose}$, the anomeric carbon is C1 . The hydroxy group (-OH) on the anomeric carbon is down (axial position).

$\alpha ‐\mathrm{D}‐\mathrm{mannopyranose}$

In chair conformation of $\beta ‐\mathrm{D}‐\mathrm{galactopyranose}$, the anomeric carbon is C1. The hydroxy group (-OH) on the anomeric carbon is up (equatorial position).

$\beta ‐\mathrm{D}‐\mathrm{galactopyranose}$

(c) In chair conformation of $\beta ‐\mathrm{D}‐\mathrm{alllopyranose}$, the anomeric carbon is C1 . The hydroxy group (-OH) on the anomeric carbon is up (equatorial position).

$\beta ‐\mathrm{D}‐\mathrm{allopyranase}$

(d) In Haworth projection of $\alpha ‐\mathrm{D}‐\mathrm{arabinofuranose}$, the anomeric carbon is C1. The hydroxy group (-OH) on the anomeric carbon is down which is trans to the terminal ${\mathrm{CH}}_2\mathrm{OH}$group

$\alpha ‐\mathrm{D}‐\mathrm{arabinofuranose}$

(e) In Haworth projection of , the anomeric carbon is . The hydroxy group on the anomeric carbon is up which is cis to the terminal $‐{\mathrm{CH}}_2\mathrm{OH}$group.

$\beta ‐\mathrm{D}‐\mathrm{ribofuranose}$