<P> More recently, an improved reduction method has been developed that produces somewhat higher yields of the larger sugars . Instead of conversion of the cyanohydrin to a lactone, the cyanohydrin is reduced with hydrogen, using palladium on barium sulfate as the catalyst and water as the solvent, to form an imine . Due to the presence of water, the imine quickly hydrolyzes to form an aldehyde, thus the final sugars are produced in just two steps rather than three . The separation of the isomers is then performed at the stage of the sugar products themselves rather than at the lactone intermediates . The special catalyst is needed to avoid further reduction of the aldehyde 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 . </P> <P> 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 . </P> <P> 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 . </P> <P> 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 . </P>

Name the reagent used in fischer killani synthesis