Thus Wohl prepared l-threose from l-xylose and l-erythrose from l-arabinose, and Ruff obtained d- and l-erythrose from d- and l-arabonic acids, the oxidation products of d- and l-arabinoses.
Eight stereoisomeric pentaldoses are possible, and six are known: d- and l-arabinose, d- and l-xylose, /-ribose, and d-lyxose.
L-Xylose was discovered by Koch in 1886; its enantiomorph is prepared from d-gulose by Wohl's method.
/-Ribose and d-lyxose are prepared by inversion from l-arabinose and l-xylose; the latter has also been obtained from d-galactose.
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.
When xylose is combined with hydrocyanic acid and the cyanide is hydrolysed, together with l-gulonic acid, a second isomeric acid, l-idonic acid, is produced, which on reduction yields the hexaldose l-idose.