Fischer projection formulas group

Because the hydroxyl group at C-4 of ( (-arabinose is at the right in a Fischer projection formula (evidence of step 1), the hydroxyl at C-2 must be to the left in order for the aldaric acid to be optically active. 

Fischer projection formulas show three-dimensional structures in two dimensions. In such formulas, horizontal groups project toward the viewer, and vertical groups project away from the viewer. 

If you view the second formula from the top, you will see that it is just a three-dimensional representation of the Fischer projection. Horizontal groups at each stereogenic center come up toward you, and vertical groups recede away from you. The second formula represents an eclipsed conformation of D-threose. The third and fourth formulas represent sawhorse and Newman projections, respectively, of a staggered conformation of D-threose. 

In order to determine the conformation, the substitution pattern at carbon atoms C2 and C3 must first of all be established. Three different groups are attached to each. Hence, the groups of highest priority on each are taken as the reference groups (shown bold in the diagram below), thus this must be the antiperiplanar conformation. In order to transform the formula into a Fischer projection formula, the part of the formula which is closer to the observer is rotated until an eclipsed conformation is obtained in which the groups forming the main chain are oriented synperiplanar to each other. 

The first step is to convert the Fischer projection formulae into sawhorse projections. Note that for a base-induced elimination reaction of the E2 type, the tosyl group must be antiperiplanar to the proton or deutron being abstracted from the neighbouring carbon atom. As a consequence of this reaction mechanism, the configuration of the resulting double bond can be predicted. Because of the higher steric hindrance in the transition states leading to the Z isomers, in both cases the E isomer should be expected as the major product. 

The stereodescriptor erythro indicates that the chlorine atom and the hydroxy group lie on the same side of the main chain in a Fischer projection formula. Since it does not define an absolute configuration, two enantiomers must be considered. The Fischer projection formula, which represents an eclipsed conformation, is simplest first converted into a sawhorse projection (also in its eclipsed conformation) and then one side of the molecule rotated until both the reference groups, the chlorine atom and the hydroxy group, adopt an antiperiplanar arrangement. The required Newman projection formula can then be derived from these formulae. 

In order to be able to decide which transformations must be carried out at which groups in the starting molecule it is first of all best to convert the Fischer projection formula of the target molecule to a zigzag projection which corresponds to the orientation of the substituents in the starting material. Once this has been accomplished it is a relatively simple task to identify the necessary reaction steps. 

The formation of a hemiacetal ring gives rise to a new chiral center in the aldoses, namely, at the C-1 atom. This leads to two C-1 epimers, termed anomers. When the hydroxyl group at C-1 (glycosidic hydroxyl) in the Fischer projection formula is located on the same side as the hemiacetal ring, the anomer is termed the a form in the opposite case it is the j3 form (Fig. 2-5). 

D-Sugar (Section 27.2C) A sugar with the hydroxy group on the stereogenic center furthest from the carbonyl on the right side in the Fischer projection formula. 

Figure 6 Structures of the four stereoisomers of sphingosine. Sphingosine has two chiral carbon atoms (C-2 and C-3). The Fischer projection formula of each structure is also shown, with C-1 at the top, to illustrate the D/L and erythro/threo stereochemical nomenclature. C-3 has an erythro orthreo configuration as it relates to C-2, depending on whether the similar groups (amino and hydroxy) are on the same or opposite side of the Fischer projection. D versus L refers to the configuration at C-2 relative to the configuration of D-glyceraldehyde versus L-glyceraldehyde.

Hudson lactone rule. The value of the rotation of aldonic acid lactones is decisively affected by the configuration of that carbon atom whose hydroxyl group is engaged in the cyclization. If, in the normal Fischer projection formula, the lactone ring is written on the right, the lactone is dextrorotatory if it is written on the left, the lactone is levorotatory. 

In D-family sugars, the OH on the chiral carbon farthest from the carbonyl group is on the right side in a Fischer projection formula. So both (+)-glucose and (-)-fructose are D-sugars despite their rotation of plane-polarized light in opposite directions. 

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