Conformations, chelate-like

The stereochemical course of the reaction may be rationalized by assuming a six-mem-bered chelate which presumably exists in a chair-like conformation. Molecular models support the assumption that the nucleophile preferentially attacks from the bottom face of the complex due to the steric hindrance of the axial A-methyl group. 

G=G bond approximately perpendicular to the coordination plane, with the triethylsilylmethyl group adopting an axial or pseudo-axial position about the hexenyl chelate. Gomplex 41 presumably adopts a boat-like conformation to avoid unfavorable 1,3-diaxial interaction between the triethylsilylmethyl group and one of the methoxycarbonyl groups. 

Evidence very recently presented by O Farrell ei ah [3J indicates that the preferred conformation (XIII) of 2 hydroxy-5,6,7,8-tetra-hydro- 1-acetonaphthone is that which is unfavourable for chelation. However, this is not necessarily inconsistent with our views. Certainly XIII is in equilibrium with that conformation (XIV) which is favourable for chelation. Our argument merely postulates that chelation in a conformation like XIV is favoured by the bulk of the methylene group in the alicyclic ring. 

Probably the best means of protein attachment is through highly specific affinity interactions (e.g., streptavidin-biotin or His-tag-nickel-chelates) [111]. Proteins fused with a high-affinity tag, at their amino or carboxyl terminus, are finked to the surface of the chip via this tag, and hence all the attached proteins should orient uniformly away from the surface. Using this method, immobilized proteins are more likely to remain in their native conformation, while the analytes have easier access to the active sites of the proteins. 

The BF3-catalyzed reaction of a-aminoaldehydes with 10 is valuable for highly stereoselective synthesis of 2,3,5-trisubstituted pyrrolidines with all -cis configurations (Equation (50)).197 The stereochemical outcome like chelation-controlled stereochemistry might result from the inherent conformational arrangement of the aldehyde-BF3 complex. />-Quinoneimines, o-quinones, and a-alkoxyhydroperoxides undergo similar types of [3 + 2]-cycloadditions with allylsilanes to afford dihydro indoles, dihydrobenzofurans, and 1,2-dioxolanes, respectively.164,175,198... 

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