Fischer conformation

Figure 1.2.3 Energy diagram for the conformers of butane. The most prominent stretched and cyclic (Fischer) conformers are given below.

Possible conformations of the calix[4]arene systems. (Figure adapted from Fischer S, P D J Groothenuis, roenen, PV P van Hoorn, F C j M van Geggel, D N Reinhoudt and M Karplus 1995. Pathways for mational Interconversion of Calix[4]arenes. Journal of the American Chemical Society 117 1611-1620.)... 

FIGURE 7 9 Repre sentations of (2/ 3R) dihy droxybutanoic acid (a) The staggered conformation is the most stable but is not properly arranged to show stereochemistry as a Fischer projection (b) Rotation about the C 2-C 3 bond gives the eclipsed conforma tion and projection of the eclipsed conformation onto the page gives (c) a correct Fischer projection... 

Construct a molecular model corresponding to the Fischer projection of rneso 2 3 dibro mobutane Convert this molecular model to a staggered conformation in which the bromines are anti to one another Are the methyl groups anti or gauche to one another in this staggered con formation" ... 

The aldotetroses are the four stereoisomers of 2 3 4 trihydroxybutanal Fischer pro jections are constructed by orienting the molecule m an eclipsed conformation with the aldehyde group at the top The four carbon atoms define the mam chain of the Fischer projection and are arranged vertically Horizontal bonds are directed outward vertical bonds back... 

Which aldotetrose is the structure shown Is it D erythrose D threose L erythrose or L threose (Be careful The conformation given is not the same as that used to generate a Fischer projection)... 

Notice that the eclipsed conformation of d ribose derived directly from the Fischer pro jection does not have its C 4 hydroxyl group properly oriented for furanose ring forma tion We must redraw it m a conformation that permits the five membered cyclic hemi acetal to form This is accomplished by rotation about the C(3)—C(4) bond taking care that the configuration at C 4 is not changed... 

Fischer projection formulas can be used to represent molecules with several stereogenic centers and are commonly used for caibohydrates. For other types of structures, a more common practice is to draw the molecule in an extended conformation witii the main chain horizontal. In this arrangement, each tetrahedral caibon has two additional substituents, one facing out and one in. The orientation is specified widi solid wedged bonds for substituents facing out and with dashed bonds for substituents that point in. 

Since the main chain in this representation is in an entirely staggered conformation, whereas in the Fischer projection formulas the conformation represented is completefy eclipsed, an anti relationsh between two adjacent substituents in an extended structure corresponds to being on the same side in a Fischer projection formula (erythro) whereas a syn relationship corresponds to being on opposite sides in die Fischer projection (three). 

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. 

Scheme 9. The threo 2,3-diol problem. The erythro/threo notation is based on Fischer projection formulas. For example, if threo 2,3-diol 28, shown here in a staggered zigzag conformation, was depicted in an eclipsed Fischer projection, the adjacent hydroxyls attached to carbons 2 and 3 would reside on opposite sides of the carbon chain. An alternative, perhaps less ambiguous, descriptor is the syn/anti notation.63...

Note. The Fischer projection is not intended to be a true representation of conformation. 2-Carb-4. Configurational symbols and prefixes 2-Carb-4.1. Use of d and l... 

The molecule is twisting and bending around all of the time, and the conformation with the bracelet-shaped skeleton is just one of the possible conformations. The molecule probably spends very little of its time like this (it is a relatively high energy conformation), but this is the conformation that we will use to draw our Fischer projection. 

Schamagl C, Raupp-Kossmann R, Fischer SF (1999) Molecular basis for pH sensitivity and proton transfer in green fluorescent protein protonation and conformational substates from electrostatic calculations. Biophys J 77 1839-1857... 

Fig. 3.—Formulas Representing KDO (1). [a, Fischer projection-formula (acyclic form) b, Haworth formula (ketopyranose) c, conformational drawing (ketopyranose).]...

Also, in 1890 Fischer had proven that the reduction of fructose with sodium amalgam yields a mixture of mannitol and sorbitol and pointed out that this conformed with the van t HofF-Le Bel theory (19). It seems, therefore, that the idea of asymmetric induction was clearly in a state of incubation prior to his publication of the relative configurations of the sugars in 1891. 

The carbene-bound alkyl groups are acidic pX [(CO)5Cr=C(OMe)Me in H2O] 12.3 and can be easily deprotonated and alkylated [45,211,212] or acylated [213] (Figure 2.16). Stereoselective aldol-type additions can be realized with the aid of Fischer-type alkylcarbene complexes [214-216]. In these reactions the metallic fragment can either play the role of a bulky carbonyl group or stabilize a given conformation of the substrate by chelate formation [216,217]. 

It should be appreciated that a Fischer projection involving more than one chiral centre actually depicts an eclipsed conformer, which is naturally a high-energy... 

Fischer projection is equivalent to viewing the eclipsed conformer from the top... 

However, as we shall see shortly, Fischer-projection-derived eclipsed conformers are particularly useful in deducing the stereochemistry in cyclic forms of sugars (see Box 3.16). 

The original concept offered to explain why enzymes exhibit such a high degree of substrate specificity. The active site of an enzyme was viewed as a topological template for a particular reactant hence, there is an enzyme-substrate complementarity . See Induced-Fit Model Ligand-Induced Conformational Change E. Fischer (1894) Ber. 27, 2985. 

The use of rotated Fischer projections has been retained in the present edition in order to provide a link with, and an explanation of, the bulk of existing published polymer literature, although the present common practice [4] is to depict main-chain bonds in planar, extended zigzag (all-trans) conformations, together with a stereochemical representation of side-groups at tetrahedrally-bonded atoms. 

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