Carbon-centred

The use of a stereogenic carbon centre allowed an efficient asymmetric induction in the benzannulation reaction towards axial-chiral intermediates in the synthesis of configurationally stable ring-C-functionalised derivatives of al-locolchicinoids [51]. The benzannulation of carbene complex 52 with 1-pen-tyne followed by oxidative demetalation afforded a single diastereomer 53 (Scheme 33). 

However, deuteration at carbons 9, 10, 12 and 13 did change the spectrum to one consisting of only a nitrogen triplet, indicating that the radical trapped had a deuterium attached to the carbon centre. This would imply that the carbon-centred radical lies at one of these four positions. 

In 1997, Chelucci et al. developed new chiral 2-(l-p-tolylsulfinyl)alk-ylpyridines and assessed these ligands in the enantioselective addition of ZnEt2 to benzaldehyde." This produced in all cases catalysts in good yields but with low enantioselectivities (< 19% ee). An examination of the results collected in Scheme 3.33 indicates that the enantioselectivity and the configuration of the resulting carbinol were independent of both the substituent and the configuration at the carbon centre. Therefore, this suggests that the stereochemical... 

The Heck reaction has proven to be an extremely useful method for the formation of C-C bond at a vinyl carbon centre. There are numerous reported examples of enantioselctive Pd catalyzed C-C bond forming reactions.10"13 Surprisingly, reports of Heck transformations using amino acid based phosphine, phosphinite ligands are rare. Recently Gilbertson reported a proline derived phosphine-oxozoline ligand in a catalytic asymmetric Heck reaction.5 In this paper we present some novel amino acids derived ligands as part of a catalytic system for use in asymmetric Heck reactions. 

We referred earlier to the significance of reactions at the alkylidyne carbon atoms of the dimetal species. Our studies in this area are in a preliminary stage, but Schemes 1 and 2 summarise some chemistry at the bridged carbon centres for the compounds (1 ) and (3,)(12). It will be noted that protonation of the neutral bridged al ylidyne compounds yields cationic alkylidene species in which one C—C bond of the tolyl group is n2 co-ordinated to tungsten, a feature revealed by both n.m.r. and X-ray diffraction studies. 

Amino acid formation in the Urey-Miller experiment and almost certainly in the prebiotic environment is via the Stecker synthesis shown in Figure 8.3. This reaction mechanism shows that the amino acids were not formed in the discharge itself but by reactions in the condensed water reservoir. Both HCN and HCO are formed from the bond-breaking reactions of N2 and H2O in a plasma, which then react with NH3 in solution. The C=0 group in formaldehyde or other aldehydes is replaced by to form NH and this undergoes a reaction with HCN to form the cyano amino compound that hydrates to the acid. The Strecker synthesis does not provide stereo-control over the carbon centre and must result in racemic mixtures of amino acids. There is no room for homochirality in this pathway. 

From these data, and the similarity of the data for the other radicals contained in Table 1, it therefore appears that, unlike carbon-centred radicals, the tricoordinate trialkyl radicals of Group IV elements have the tetrahedral structure 1. 

The number of spatial isomers increases with increasing number of asymmetric carbon centres with the addition of each new asymmetric centre, the number of isomers doubles. The trihydroxy glutaric acid molecule has three identical asymmetric carbon atoms. 

Compounds with one asymmetric carbon atom would be active because of a tetrahedral structure. When there are two or more such asymmetric carbon centres, we will have to take into consideration the concepts of the plane of symmetry. 

As the temperature is raised above 77 K, the anion radical IV disappears at approximately 190 K and at room temperature only the carbon centred radicals V - VII are observed. 

The nucleophilic carbon centres of lithium enolates may also be pyrami-dalized, though there is little experimental evidence of this (Seebach et al., 1985, 1991). Silyl enol ethers are generally planar, but two (67 R = Ph and OMe] derived from an imidazolidinone used in amino acid synthesis show... 

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