Hydrogenation, base catalyzed Subject

The primary aldehyde product is reduced to the desired butanol, or it is subjected to a base-catalyzed aldol condensation and then hydrogenated to give 2-ethylhexanol. The phthalic ester of the latter is used as a plasticiser in PVC. The first process was based on a Co2(CO)s catalyst, a precursor of HCo(CO)4. The pressure is high, ca. 200-300 bar, in order to maintain the catalyst s stability. In the 60s Shell developed a process using phosphine ligands which allowed the use of lower pressures. The catalyst is less active but it directly produces alcohols with a somewhat higher linearity. 

As already mentioned, the steric course of acid- or base-catalyzed conjugate additions is usually subject to thermodynamic control. An interesting example of how hydrogen bonding in the product(s) may affect the stereochemical outcome of the addition reaction is provided by ajmalicine (25). When the enoate 25 was treated with 5% aqueous sulfuric acid, the 17-hydroxy-derivative 26 was obtained as a single diastereomer in 40% yield20. The configuration of the... 

To ascertain the stereochemistry of the two isomers obtained from condensation of L-dopa with acetaldehyde, the cis acid 2 was converted to the ethyl ester hydrochloride and subjected to X-ray analysis. By effecting the cycliza-tion at pH 7 instead of in dilute mineral acid, the yield of the trans acid 3 was increased from 5 to 257q. Amino acids 1 and 2 are natural products, the former being found in the seed embryos of Mucuna mustisiana, and the latter in the velvet bean. It is worth noting that base-catalyzed epimerization of derivatives of 2 and 3, where all the active hydrogens were protected, led mostly to the 1,3-trans products. ... 

Kissel, Ramsden, and other researchers at Pfizer and Chirotech jointly published a novel chiral synthesis of pregabalin (2) in 2003 based on asymmetric hydrogenation (Burk et al., 2001, 2003). Their synthesis started with the condensation of isobutyralde-hyde with acrylonitrile under Baylis-Hillman conditions to give allylic alcohol 65. This alcohol was activated as the carbonate 66 and subjected to palladium-catalyzed car-bonylation conditions to give cyanoester 67. The ester 67 was hydrolyzed and converted to... 

You probably studied acid-base chemistry as part of your undergraduate studies. However, acids and bases play such a key role in medicine and physiology that we believe the subject merits another look. The central theme in acid-base chemistry is relatively simple. Acids donate hydrogen ions to bases. The uses of this reaction are myriad in variation and in application. By controlling the acid-base conditions, you can ensure that a medication stays in solution. If the acid content of blood changes by a tiny amount, the patient dies. The acidic and/or basic properties of amino acids located in the active site of an enzyme catalyze a staggering number of chemical transformations which are essential for life. 

At The American University, Isbell s major interest in research turned to the study of the oxidation of saccharides with hydrogen peroxide. In collaboration with Dr. Frush, he published some forty papers on the subject. A number of major discoveries were made, including that of a stepwise degradative peroxidation, which is catalyzed by base or by such metals as iron(II). It starts at the anomeric carbon of an aldose, either in the acyclic or the cyclic form, and affords the lower aldose and formic acid (see Fig. 8). Two mechanisms were recognized an ionic one prevalent in strong alkali, and a free-radical process catalyzed by Fe(II) (see Fig. 9). 

Figure 11.13 Reactions at a-carbon of a-amino acids catalyzed by pyridoxal enzymes All three substituents at C are subject to labilization in the three types of a-carbon reactions. The hydrogen is labilized in recemization reactions, the amino group is labUized in the transamination and the carboxyl group is labilized in decarboxylation. a-Amino acid condenses with pyridoxal phosphate to yield pyridoxylidene imino acid (an aldimine). The common intermediate, aldimine and distinct ketimines leading to the production of oxo-acid (in transamination), amino acid (in racemization) and amine (in decarboxylation) are shown. The catalytic acid (H-A-) and base (-B ) are symbolic both can be from the same residue such as Lys258 in aspartate aminotransferase.

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