An Instructive Update of Fischer’s Strategy for Assigning Configurations of Chiral Carbons in Linear Aldohexoses

Abstract

The double salt with Na+ and Rb+ of (+)-tartaric acid made it more seeable by X rays diffraction. Three successive chain shortening of linear aldohexoses produces either D- or L-glyceraldehyde. Those giving D-glyceraldehyde, produces D-erythrose or D-threose by two successive chain shortening, and in this way the configuration of C-4 relative to C-5 is disclosed. Analysis of products of Malaprade oxidation cleavage of 1,2-5,6-di-O-isopropylidene hexitols indicates the configuration of C-2 relative to C-5. A similar result for C3 and C4 as a whole is obtained by isopropylidenation to 1,2-3,4-5,6-tri-O-isopropylidene hexitols, regioselective removing of terminal isopropylidene residues, oxidation cleavage and reducing; C3 and C4 are recovered as isomers of 2,3-O-isopropylidene tetritol, and the configuration of C-4 is cleared by two independent paths. Chain shortening followed by ends equalization, and optical activity measuring, is a versatile method for discerning between symmetrical, irrechi (configurational) and constitutional monosaccharides. Correlation of the results of the above essays unequivocally discloses the structure of all lineary isomeric hexitols and aldohexoses.