group to a hydroxyl group, which would yield an alditol. These catalysts that limit hydrogenation to one step are called poisoned catalysts; Lindlar palladium is another example. The reactions below illustrate this improved method for the conversion of L-threose to L-lyxose and L-xylose. Uses and limitations Both enantiomers of glyceraldehyde are commercially available, so one can access any stereoisomer of any chain-length aldose by an appropriate number of repeated applications of the Kiliani–Fischer synthesis. The triose D-glyceraldehyde (1) leads to the tetroses D-erythrose (2a) and D-threose (2b). Those lead to the pentoses D-ribose (3a) and D-arabinose (3b), and D-xylose (3c) and D-lyxose (3d), respectively. The next iteration leads to the hexoses D-allose (4a) and D-altrose (4b), D-glucose (4c) and D-mannose (4d), D-gulose (4e) and D-idose (4f), and D-galactose (4g) and D-talose (4h). The D-heptoses and beyond are available by continuing the sequence, and enantiomeric L series is available by starting the sequence with L-glyceraldehyde. In practice, the Kiliani–Fischer synthesis is usually used for production of sugars that are difficult or impossible to obtain from natural sources. While it does provide access to every possible stereoisomer of any desired aldose, the process is limited in by its low yield and use of toxic reagents. In addition, the process requires having a supply of the previous sugar in the series, which may itself require substantial synthetic work if it is not readily available. For example, if successive iterations of the Kiliani–Fischer synthesis are used, the overall yield drops approximately exponentially for each additional iteration. The process only provides direct access to aldoses, whereas some sugars of interest may instead be ketoses. Some ketoses may be accessible from similar aldoses by isomerization via an enediol intermediate; for example, on standing in aqueous base, glucose, fructose, and mannose will slowly interconvert since they share an enediol form. (See Lobry de Bruyn–van Ekenstein transformation). Some unusual sugars are also accessible via aldol addition. See also Homologation reactions Wohl degradation Ruff degradation References Carey, Francis A. (2006). Organic Chemistry, Sixth Edition, New York, NY: McGraw-Hill. . Addition reactions Organic redox reactions Name reactions Homologation reactions