Racemates formation

A coverage dependence of 2D conglomerate or racemate formation was first observed for NN on Au(lll). In that case, the transition from homochiral chain structures to heterochiral 2D domains is strongly influenced by the hcp-fcc substrate reconstruction sites of the substrate [83]. 

In a structure-based drug-discovery programme in the field of influenza virus neuraminidase inhibitors, researchers at BioCryst Pharmaceuticals discovered aminocarbafuranose sialyl mimetic 89 as a very potent candidate against influenza A and B viruses [17]. To arrive at this highly substituted compound in a non-racemic format, the viable route of Scheme 14 was executed. 

In planning the synthesis of a series of carbapyranoses in their chiral non racemic format, Zanardi and co-workers [7d] utilized the seven-carbon lactone 149, in turn prepared by elaboration of butenolide 12 (vide supra, Scheme 3). 

The same CALB preparation was appUed in many dynamic kinetic resolutions combining two types of catalysts with each other. In the presence of homogeneous transition metal catalysts that catalyze the racemization and heterogeneous acids or bases or immobilized transition metals Novozym 435 was not deactivated [1, 26-28]. This is all the more remarkable since the reactions catalyzed by these catalysts include redox reactions at elevated temperatures (>60°C). When Novozym 435 was applied for the enantioselective synthesis of cyanohydrin acetates (10) from aliphatic aldehydes (7), good results were achieved (Scheme 2.2) for this dynamic kinetic resolution (DKR) [29]. Here NaCN is used as the base for the dynamic racemic formation and degradation of the cyanohydrins (6 and 8). 

Metal compounds possessing Lewis acid character are often used in the Michael addition reaction, and the methodology is reasonably applied to the asymmetric reaction in the presence of chiral ligands. The mechanism could involve either purely Lewis acidic activation of the Michael acceptor or generation of new orga-nometallic species by the transmetalation or C-H activation, although they were not clear in many cases. The system of Sn(OTf)2 and chiral (S)-diamine developed by Mukaiyama promoted the asymmetric addition of trimethylsilyl enethi-olate 68 to P-arylvinyl ketones (Scheme 13) [70, 71]. The diamine-coordinated tin enolate was considered to be involved, and slow addition of 68 was essential to inhibit the racemate formation process. 

Weissbuch, I., Berfeld, M., Bouwman, W., Kjaer, K., Als-Nielsen, J., Lahav, M., Leiserowitz, L. Separation of enantiomers and racemate formation in two-dimensional crystals at the water surface from racemic - amino acid amphiphiles design and structure, J. Amer. Chem. Soc. 119 (1997) 933-942. 

Acid-catalyzed reaction of lb with acetone and simultaneous azeotropic removal of water provides diethyl (27 ,3i )-2,3-0-isopropylidenetartrate 3 in 82—83% yield after distillative purification [3]. However, it has been reported that upon application of this method in petroleum ether as solvent, significant racemate formation was observed [4]. The preferred method for the preparation of 3 involves a transketalization reaction of lb with 2,2-dimeth-oxypropane under acidic catalysis. The yield is nearly quantitative with no loss of optical integrity [5]. 

Histidine is one of the most problematic building blocks in peptide synthesis. Imidazole ring of His is easily A-acylated. Undesired side reactions such as racemization, formation of cyclic imidazolides and A-guanylations of the imidazole ring have been observed. These problems may be avoided by reversible masking of the imidazole function such as benzyl (Bzl), 2,4-nitropheyl (Dnp), benzyloxymethyl (Bom), tert-butoxycarbonyl (Boc), triphenyhnethyl (Trt), diphenylmethyl (Dpm), pyridyldi-phenyhnethyl (Pdpm), 4-toluenesulfonyl (Tos), allyl (Al) and allyloxymethyl (Alom). 

E. Vedejs, R. W. Chapman, S. Lin, M. Muller, D. R. Powell, Crystallization-induced asymmetric transformation vs quasi-racemate formation in tetrava-lent boron complexes, /o r a/ of the American Chemical Society 2000, 122, 3047. 

The correlations given in the Atlas are in general the most recent and rehable available. Where a chemical correlation between substances has been carried out, this is given preference over correlation by chiroptical methods, asymmetric synthesis, quasi-racemate formation, etc., and these in turn are given precedence over any older work involving comparison of monochromatic rotations. The latter is included only in a few cases where no more recent information is available and where the correlation is of considerable interest. 

Furthermore, he showed that these considerations apply to catalytic asymmetric syntheses and decompositions alike, though in the former the stability of the optically active state (measured by the ratio of the time required for racemate formation to the time required for attainment of maximum optical activity) will be considerably greater than in the latter. 

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