How to draw a Haworth projection from an acyclic aldohexose

Example: Convert D-mannose into a Haworth projection.

CHO HO H HO H H OH H OH

CH2OH D-mannose

Step [1]: · Draw a hexagon and place the oxygen atom in the upper right corner.

O O in upper right corner

Step [2]: · Place the anomeric carbon on the first carbon clockwise from the oxygen. · For an anomer, the OH is drawn down. · For a anomer, the OH is drawn up.

This C becomes the anomeric C.

1CHO O H O OH HO H 1 1 HO H OH H H OH anomer anomer H OH anomeric carbon - CH2OH first C clockwise from O

· Always keep in mind that the anomeric carbon comes from the carbonyl carbon in the acyclic form.

Step [3]: · Add the substituents of the three chiral carbons closest to the C=O. · The substituents on the right side of the Fischer projection are drawn down. · The substituents on the left are drawn up.

CHO HO 2 H add H O H O H 3 C2 - C4 H HO H 4 OH OH 4 OH OH H 4 OH HO OH HO OH 3 2 3 2 H H H OH H H anomer anomer CH2OH Haworth convention - 2

Step [4]: · For D sugars the CH2OH group is drawn up. For L sugars the CH2OH group is drawn down.

CHO CH2OH CH2OH HO H H O H H O H H HO H H OH OH OH OH H OH HO OH HO OH H H H OH H H anomer anomer CH2OH

This OH on the right side CH OH is drawn up. makes this a D sugar. 2

· The structures are now complete. This stepwise procedure can be used for any aldohexose. Notice that we have added substituents in a clockwise fashion around the ring.

Sample problem – Going in the opposite direction. Convert the following Haworth projection to the acyclic form of the aldohexose.

CH2OH OH O H H OH H H OH H OH

Begin by vertically drawing the six carbon atoms of the acyclic aldohexose. To draw the substituents on this skeleton, start at the pyranose O atom, and work in a counterclockwise fashion around the ring.

[1] Draw the carbon skeleton, placing the CHO on the top and the CH2OH on the bottom.

CHO Proceed in a counterclockwise fashion Begin here. around the ring. CH2OH OH O H H OH H H OH H OH CH2OH

[2] Decide if the sugar is D or L. [3] Add the three other chiral centers.

• The CH2OH drawn "up" means a D sugar. • "Up" groups go on the left. • D sugars have the OH on the bottom chiral • "Down" groups go on the right. ANSWER: center on the right. CHO CHO CHO CH OH CH2OH 2 O OH H OH OH O H OH H H H HO HO H OH H OH H OH H OH H HO HO H H OH H OH OH OH H OH CH OH CH2OH 2 CH2OH

Work upwards in placing OH groups. Both anomers give the same acyclic hydroxy aldehyde.

ã Janice G. Smith, 2003