Fischer formulas

A more eflicient and general synthetic procedure is the Masamune reaction of aldehydes with boron enolates of chiral a-silyloxy ketones. A double asymmetric induction generates two new chiral centres with enantioselectivities > 99%. It is again explained by a chair-like six-centre transition state. The repulsive interactions of the bulky cyclohexyl group with the vinylic hydrogen and the boron ligands dictate the approach of the enolate to the aldehyde (S. Masamune, 1981 A). The fi-hydroxy-x-methyl ketones obtained are pure threo products (threo = threose- or threonine-like Fischer formula also termed syn" = planar zig-zag chain with substituents on one side), and the reaction has successfully been applied to macrolide syntheses (S. Masamune, 1981 B). Optically pure threo (= syn") 8-hydroxy-a-methyl carboxylic acids are obtained by desilylation and periodate oxidation (S. Masamune, 1981 A). Chiral 0-((S)-trans-2,5-dimethyl-l-borolanyl) ketene thioketals giving pure erythro (= anti ) diastereomers have also been developed by S. Masamune (1986). 

In the same way that a Fischer formula is a projection of the eclipsed conformation onto the page, the line drawn through its center is a projection of the plane of symmetry that is present in the eclipsed conformation of meso-2,3-butanediol. 

First law of thermodynamics, 24 645-648 First limiting amino acid, 2 601 First-order irreversible chemical kinetics, 25 286-287, 292-293 First-principle approach, in particle size measurement, 13 153 First sale doctrine, 7 793 Fischer, Emil, 16 768 Fischer carbene reaction, 24 35-36 Fischer esterification, 10 499 Fischer formula, 4 697 Fischer-Indole synthesis, 9 288 Fischer lock and key hypothesis, 24 38 Fischer-Tropsch (FT) synthesis, 6 791, 827 12 431... 

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Problem 22.30 (a) Draw the chair conformations for a- and /3-D-(-t-)-glucopyranose. The bulky CHjOH is usually assigned an equatorial position. In flat Fischer formulas. OH s on the right are tram to CHjOH, and those on the left are as. (b) Why are most naturally occurring glycopyranosides /3-anomers -4... 

After the introduction of the Fischer formulas came the use of the Haworth representation, which was an attempt to give a more accurate spatial view of the molecule. Because the Haworth formula does not account for the actual bond angles, the modem con-... 

The first new result from the application of the lactone rule was the assignment of a complete configurational formula to natural rhamnose. The configuration of carbon atom 5 was indicated by the sign of rotation of the rhamnotetronic lactone, and therefore the Fischer formula could now be made complete it became... 

The configurations of monosaccharides are described by several types of formulas. As an example, the following formulas are shown for a-D-glucopyranose structural, Haworth, modified Fischer, and Fischer. In Fischer formulas the -H or -OH groups are above or below the plane formed by the hemiacetal bonds and the carbon chain (-H bonds are shown shorter). 

Haworth structures more closely depict proper bond angles and lengths than do Fischer representations. To convert from the traditional Fischer formula of a D-pen-tose or D-hexose to a Haworth formula, the following steps should be followed ... 

Let s now examine our problem of comparing a hashed-wedged line formula with a Fischer projection. The actual problem is how to interconvert hashed-wedged line and Fischer formulas. The key to this is recognizing that Fischer projections are pictures of a molecule in an eclipsed conformation. Very important. So the first step in our comparison is to get the hashed-wedged line formula into an eclipsed conformation. Any 60° rotation will do, such as rotation of the left-hand methyl group up out of the page, toward you ... 

Comment It must be emphasised that, since Haworth representations are not projection formulae as is the Fischer formula, they cannot be rotated in the plane of the paper. A Haworth-t3rpe formula can be rotated without change about a central vertical axis. It is always necessary to keep the same groups above and below the plane of the ring. 

Representation of the cyclic forms could be simplified by using the special shorthand notation called the Haworth formula (in the honor of Sir Norman Haworth). Fischer formulas can be easily transformed into Haworth formulas by the rules given in the following scheme. 

Make 3D models of the molecules in Figure 5.12 to help you visualize them. Notice that these structmes are derived from an eclipsed conformation of the molecule, viewed from above so that horizontal groups project toward the viewer. The actual molecule is an equiUbrium mhrture of several staggered conformations, one of which is shown below. Fischer formulas are used to represent the correct configurations, but not necessarily the lowest energy conformations of a molecule. 

The aldohexose D-glucose in the open-chain Fischer formula (I), the Fischer-Tollens hemiacetal ring formula (II), and the Haworth formula (III). The numbering system is shown. 

In the preceding discussion, the structure and configurations of the two isomeric glucoses have been developed. The structure and configuration may be represented by the cyclic form of the Fischer formula as in (I) for a-glucose and as in (II) for methyl a-glucoside. 

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