A Simple Rule for the Conversion of Fischer Monosaccharide Projection Formulae into Haworth Representations J. M. Argilk Facultat de Blologia, Universitat de Barcelona, Diagonal 645, 08071 Barcelona, Spain Fischer projection formulae are widely used to represent compounds containing chiral centers. In these, the horizontal bonds are assumed to extend forward from the plane of the paper and thevertical bonds to extend to the rear. All the monosaccharides except dihydroxyacetone contain one or more asymmetric or chiral carhon atoms and can thus be represented with Fischer projection formulae. However, sugars often form intramolecular hemiacetals, in which there is an addition of an hydroxyl group to the carboxyl group (either from an aldehyde or from a ketone group). The formation of these bonds induces a cyclic configuration in the molecule and the appearance of a new asymmetric carbon thus originating the anomers alfa and beta of each D- or L-sugar. The anomeric carhon is the one which contains the reactive carbonyl. Hemiacetals with five-membered rings are called furanoses, and hemiacetals with six-membered rings are called pyranoses. In cases where either five- or six-membered rings are possible, the five-membered ring usually predominates, but there are notable exceptions such as glucose. The furanoses and pyranoses are more realistically represented by pentagons or hexagons by the Haworth convention. The lower edge of the heavy line is to be thought of as projecting out, and the other edge is to be thought of as projecting into the paper. Haworth structures are not easy for students to memorize and, although they are easy to draw and unambiguous in
depicting configurations, the lack of clear and easy-to-remember rules for their genesis from the Fischer projection formulae makes them difficult for students to work with. Our rule, which is quite simple and universal, consists of joining the anomeric carhon with the other carhon atom which participates in the hemiacetalic bond (normally C , - I , ~being the number of carbon atoms of the molecule) with either a ~ - s h a p e dline (for D-sugars) or ans-shaped line (for L-sugars. .. . (see structure below). All OH substituents or 011-vontaining suhstuuents tie.. -CH,OHJ that fall inside the "letter" l i c e downward in [he Hawmth structures: the ones that fall outside the "letter" are represented facing up in the Haworth models. The S-shaped line, in the case of Lsugars, has to have its second loop between the anomeric carbon and the carbon following suit in the numeration of the compound. The mnemonic title to remember is very easy: InsideLetter-Down, where the "S" from the S-shaped line (from "sinister") is contained and is equivalent to the L-form, contained in the word "Letter". "Down" explains both the direction of the substituent in the Haworth model and the Dshaped line corresponding to the D-form. Acknowledgment The author thanks M. Alemany and R. Gassol for their valuable comments.
CHzOH
I
C=O
HOCHZ-C-OH
Ho-L-H
I H-C-OH I H-C-OH I
CH20H
D-fructose CHZOH
I
C=O H-LoH
I I HO-C-H I
HO-C-H
HO-C-H
I
H-C-OH H-C
,
I
CHzOH a-D-fructose
HO-~;,
CHzOH
CHzOH
L-fructose
LY-L-~~UC~OS~
Volume 63 Number 11 November 1966
927
