Catalytic hydrogenation standard procedure

Nitro sugars have been broadly used as precursors of the corresponding amino sugars, most of them naturally occurring and biologically relevant compounds. The standard procedure involves a catalytic hydrogenation and several catalysts have been used for this purpose. 

The Na-Z- or 7V"-Boc-protected lipo-amino acids and their esters are obtained by standard protocols.113X In contrast, for the synthesis of the A -Fmoc derivatives intermediate bis-silylation and the use of Fmoc-Cl as acylating agent are recommended. 129 The standard procedure with Fmoc-OSu leads to low yields (25%) mainly due to low solubility of the amino acids. 133 Alternatively, the improved solubility of the 4-toluenesulfonate salt of the benzyl esters is exploited for reaction with Fmoc-OSu. The benzyl ester is then removed by catalytic hydrogenation, although the Fmoc group is known to be reduced under these conditions unless particular precautions are taken. 133 156-159 ... 

The standard procedure is illustrated by the preparation of cinnamic acid and furylacrylic acid (Expt 6.138). The cinnamic acid obtained is the more stable ( )-isomer. It may be readily reduced to the saturated acid (3-phenylpropanoicacid) and two procedures are described. Catalytic hydrogenation is a convenient method, but the conjugated double bond may also be reduced with, for example, sodium amalgam in the presence of alkali. 

Obvious shortcomings of the Reichstein procedure are the modest yield (50— 60% from L-sorbose is often mentioned) and the acetone protection-deprotection sequence. Overall, 1 ton of sodium sulfate is produced per ton of ASA (acetone is recycled). It should be noted, on the other hand, that the catalytic hydrogenation and microbial oxidation steps are in all respects highly efficient, even by today s standards [153] and impeccably green [154]. 

Trichloroacetimidates, CCl,C(NH)OR, have been prepared under mild conditions by the reactions of alcohols with trichloroacetonitrile under basic conditions promoted by catalytic amounts of tetra-n-butylammonium hydrogen sulphate [72]. The procedure is far superior to the standard methods which normally require anhydrous reaction conditions. 

Experiments. A standard method of pretreatment was developed to ensure constant catalyst activity. The procedure consisted of three steps. First, air was passed over the catalyst for 22 hours at a rate of 0.02 SCFM (standard cubic feet/minute) to regenerate the catalyst to a constant oxidized state. Hydrogen was then used to reduce the catalyst. The hydrogen pretreatment was studied, and an optimum period or 2 hours at a rate of 0.115 SCFM was determined. The experimental run then followed. This procedure was cyclic and repeated without interruptions. Standard runs were conducted at the beginning and at the end of a series of runs to ensure constant catalytic activity. 

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