Haworth forms

All of this work was set aside in 1932, when Haworth formed most of the carbohydrate workers into a team (or syndicate ) to investigate the structure of Szent-Gyorgyi s hexuronic acid, which proved to be vita-... 

FIGURE 22.17 Four steps to transpose a Fischer projection into the line representation called a Haworth form. 

P anomer or a squiggly line to indicate both. This kind of planar representation is called a Haworth form, after its inventor, Walter N. Haworth (1883-1950). Figure 22.18 repeats the process, transposing D-ribose into the a and P anomers of D-ribofiiranose. 

FIGURE 22.18 Constructing the Haworth forms of the a and P anomers of D-ribofuranose. 

PROBLEM 22.9 Draw the Haworth form of (a) the a anomer of D-glucofuranose ... 

FIGURE 22.19 The Haworth form of a pyranose is much more accurately shown as a chair structure. 

WORKED PROBLEM 22.10 Transpose the Haworth form of a-D-glucopyranose (Fig. 22.17) into the chair structure. 

Haworth form (Section 22.2) The representation of sugars in which they are shown as planar rings with the ring perpendicular to the plane of the paper. 

Structural drawings of carbohydrates of this type are called Haworth formulas, after the British chemist Sir Walter Norman Haworth (St Andrew s University and the University of Birmingham) Early m his career Haworth contributed to the discovery that carbohydrates exist as cyclic hemiacetals rather than m open chain forms Later he col laborated on an efficient synthesis of vitamin C from carbohydrate precursors This was the first chemical synthesis of a vitamin and provided an inexpensive route to its prepa ration on a commercial scale Haworth was a corecipient of the Nobel Prize for chem istry m 1937... 

Generating Haworth formulas to show stereochemistry m furanose forms of higher aldoses is slightly more complicated and requires an additional operation Furanose forms of D ribose are frequently encountered building blocks m biologically important organic molecules They result from hemiacetal formation between the aldehyde group and the C 4 hydroxyl... 

Haworth formulas are satisfactory for representing configurational relationships in pyranose forms but are uninformative as to carbohydrate conformations X ray crystal lographic studies of a large number of carbohydrates reveal that the six membered pyra nose ring of D glucose adopts a chair conformation... 

Haworth formulas (Section 25 6) Planar representations of furanose and pyranose forms of carbohydrates... 

Chemical Synthesis. The first synthesis of ascorbic acid was reported ia 1933 by Reichsteia and co-workers (14,39—42) (Fig. 4). Similar, iadependent reports pubHshed by Haworth and co-workers followed shordy after this work (13,43—45). L-Xylose (16) was converted by way of its osazone (17) iato L-xylosone (18), which reacted with hydrogen cyanide forming L-xylonitfile (19). L-Xylonitfile cyclized under mild conditions to the cycloimine of L-ascorbic acid. Hydrolysis of the cycloimine yielded L-ascorbic acid. The yield for the conversion of L-xylosone to L-ascorbic acid was ca 40%. 

Write Haworth formulas corresponding to the furanose forms... 

In the 1920s, Haworth and his school proposed the terms furanose and pyranose for the two forms. Haworth also introduced the Haworth depiction for writing structural formulae, a convention that was soon widely followed. 

The orientation of the model described above results in a clockwise numbering of the ring atoms. Groups that appear to the right of the modified Fischer projection appear below the plane of the ring those on the left appear above. In the common Haworth representation of the pyranose form of D-aldohexoses, C-6 is above the plane. 

Note. In writing Haworth formulae, the H atoms hound to the carbon atoms of the ring are often omitted to avoid crowding of the lettering in the ring. For the sake of clarity, the form with H atoms included is preferred in this document. 

Figure 13-1. o-Glucose. A straight chain form. B a-D-glucose Haworth projection. C a-o-glucose chair form. 

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