2 Kiliani also showed that arabinose, C 6 11 12 0 61 a sugar found in cherry gum, was an aldopentose, and thus indicated an extension of the idea of a " sugar."
Also Kiliani found that the lactone derived from the cyanhydrin of natural arabinose (laevo) was identical with the previous lactone except that its rotation was equal and opposite.
Certain of these relations are here summarized (the starting substance is in italics): l-Glucose f- 1-arabinose --- l-mannose - l-mannoheptose; glucononose fa-gluco-octose F - a-glucoheptose f- d-glucose - 0-glucoheptose - > /-gluco-octose; d-mannose--> d-mannoheptose--> manno-octose--> mannononose; d-glucose --> d-arabinose - i d-erythrose.
B-arabinose -+ l-erythrose.
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.
Scheibler discovered l-arabinose in 1869, and regarded it as a glucose; in 1887 Kiliani proved it to be a pentose.
D-Arabinose is obtained from d-glucose 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.
To settle this point it is necessary to consider the configuration of the isomeric pentoses - arabinose and xylosefrom which they may be prepared.
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.
As a matter of fact, only arabinose gives an active product on oxidation; it is therefore to be supposed that arabinose is the - - - compound, and consequently CH 2 (OH) - - - + COH = /-glucose CH 2 (OH) + - - - COH = l-gulose.
Sulphuric acid converts it into l-arabinose; and nitric acid oxidizes it to oxalic acid (without the intermediate formation of mucic acid as in the case of gum arable).