Transformations, Fischer projection

Acyclic Alditols. - A simple method has been described for transforming Fischer projections into zig-zag forms. Dipole-dipole interactions of D-sorbitol in water and methanol solution have been studied, and evidence for solute-solvent interactions presented. The preparation of 1,4-di-O-benzyl-L-threitol from L-tartaric acid in 4 steps has been reported. A study of optical rotations of (S)-propane-l,2-diol, D-t/ reo-butane-2,3-diol as well as some alditols and some deoxyalditols in aqueous and non-aqueous solution has been reported D-glucitol, for example, is dextrorotaiy in DMSO or HMPT but levorotary in water or pyridine. The dependence of rotation on the solvent is considered in detail. The mass spectra of some per-O-benzoylalditols have been described. ... 

The eclipsed conformation shown, when oriented so that the aldehyde carbon is at the top, vertical bonds back, and horizontal bonds pointing outward from their stereogenic centers, is readily transformed into the Fischer projection of L-erythrose. 

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. 

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. 

As an example, formula V will be transformed to a Fischer projection. Inspection... 

Example Transformation of a Horizontal Fischer Projection of D-Glucose into a Haworth Perspective Formula. 

PROBLEM 22.11 Transform the Fischer projection of D-mannose (Rg. 22.8) into the chair structure of P-D-mannopyranose using the method described in Rgures 22.17 and 22.19. Confirm your answer by using the fact that D-mannose is the C(2) epimer of D-glucose. 

Draw the hashed-wedged line structures corresponding to Fischer projections A and B, above. Is it possible to transform A into B by means of a rotation about a single bond If so, identify the bond and the degree of rotation required. Use models if necessary. 

Draw the Fischer projection of L-(-)-glucose and illustrate its transformation into the corresponding six-membered cyclic hemiacetal. 

---