18315-89-4

A critical step in the synthesis of the rare sugars, l-lyxose and l-ribose, from the corresponding d-sugars is the oxidation to the lactone. Instead of conventional oxidizing agents like bromine or pyridinium dichromate, it was found that a heterogeneous catalyst, Pd-Bi/C, could be used for the direct oxidation with molecular oxygen. The composition of the catalyst was optimized and the best results were obtained with 5:1 atomic ratio of Pd:Bi. The overall yields of the five-step procedure to l-ribose and l-lyxose were 47% and 50%, respectively. The synthetic procedure is advantageous from the viewpoint of overall yield, reduced number of steps, and mild reaction conditions. Furthermore, the heterogeneous oxidation catalyst can be easily separated from the reaction mixture and reused with no loss of activity.

An improved process for the preparation of potassium ribonate and of ribonolactone, which is an interesting intermediate for the preparation of riboflavin (vitamin B2), starting from potassium arabonate. An aqueous potassium arabonate solution is epimerized by adding a water-soluble non-ionic organic solvent, after which the greater part of the non-epimerized potassium arabonate can be separated off in a crystalline form. The mother liquor, which essentially contains potassium ribonate in addition to a small amount of unconverted potassium arabonate, is greatly concentrated and cooled, whereupon pure potassium ribonate crystallizes out; the latter can be lactonized in the conventional manner. Alternatively, the mixture of potassium arabonate and potassium ribonate, contained in the concentrated mother liquor, can be lactonized and the resulting lactone mixture, containing at least 70% by weight of ribonolactone, can be separated by fractional crystallization using dioxane or ethylene glycol monomethyl ether.