Their number is further increased by spatial inversion of the dicarboxylic acids formed on oxidation, followed by reduction; for example: d- and /-glucose yield d-and l-gulose; and also by Lobry de Bruyn and Van Ekenstein's discovery that hexoses are transformed into mixtures of their isomers when treated with alkalis, alkaline earths, lead oxide, &c.
L-Xylose was discovered by Koch in 1886; its enantiomorph is prepared from d-gulose by Wohl's method.
Other forms are: d- and l-gulose, prepared from the lactones of the corresponding gulonic acids, which are obtained from d- and /-glucose by oxidation and inversion; d- and l-idose, obtained by inverting with pyridine d- and l-gulonic acids, and reducing the resulting idionic acids; d- and l-galactose, the first being obtained by hydrolysing milk sugar with dilute sulphuric acid, and the second by fermenting inactive galactose (from the reduction of the lactone of d, l-galactonic acid) with yeast; and d- and l-talose obtained by inverting the galactonic acids by pyridine into d- and l-talonic acids and reduction.
Since both d-glucose and d-gulose yield the same active (d) saccharic acid on oxidation, the configuration of this and the corresponding /-acid must be sought from among those numbered 5-10 in the above table.
Arabinose being convertible into /-glucose and xylose into l-gulose, the alternative formulae to be considered are CH 2 (OH) - - - +COH CH 2 (OH) + + - COH.