Zinc complexes conformation

The zinc nitrate complexes of pyridyl functionalized 12[ane]N3 ligands l-(2-pyridylmethyl) -1,5,9-triazacyclododecane and l-(2-pyridyl-2 -ethyl)-l,5,9-triazacyclododecane were formed. 13C NMR studies were used to determine conformation in solution suggesting the former gave a trigonal bipyramidal isomer in solution with a water bound and the latter gave a 2 1 mixture of tetrahedral and asymmetric trigonal bipyramidal isomers. The crystal structure of the zinc complex of l-(2-pyridyl-2 -ethyl)-1,5,9-triazacyclododecane reveals a tetrahedral geometry in the solid state.679... 

An alternative access to L-amino acids was found by using chloroform as solvent in the asymmetric Strecker synthesis with galactosyl imines [24], This interesting reverse of asymmetric induction compared to the reactions shown in Scheme 5 can be explained by considering the zinc complex A as the crucial reactive species. Due to the exo anomeric effect, which is a delocalization of the 7r-electrons into the a of the ring C-O-bond, the imines adopt the conformation represented in Scheme 8. 

The conformation is proved by a significant NOE between the aldimine proton and the anomeric proton [17,24]. In polar solvents, free cyanide attacks the complex A, preferably from the unshielded Si-side. In unpolar solvents like chloroform, cyanide is not set free from the silyl derivative. The activation of the cyanide proceeds by an interaction between the exo chloride of the zinc complex and the silyl group. Thus, the cyanide is directed to the Re-side of the glycosyl imine (see Scheme 8). This nucleophilic attack produces L-aminonitriles with moderate or good stereoselectivity (S R 3-9 1) and high yields. 

This is in agreement with the data obtained by CD which showed that patellamide C underwent only slight conformational change when forming zinc complexes. The final energy of the structure was 11.8 kJ/mol and the backbone RMSD is 0.24 0.09 A with no nOe violations. An overlay of the zinc complex structure and the uncomplexed structure is shown in Figure 13 to demonstrate their similarity. 

Coordination of the oxazolidinone 9 with the zinc complex activated the electrophi-licity of the alkene moiety toward addition of the nucleophilic radicals, but the stereodetermining step was the subsequent addition-fragmentation reaction of the intermediate radical with an allyltin reagent. A transition state XVIII similar to FV was proposed for the bis(oxazoline)-Mg complex-catalyzed Diels-Alder reaction reported by Corey [13], As the conformation of the bound a-amidyl radical formed by reaction with tert-butyl radical is s-cis [29a], the back face of the prostereogenic radical in XVni is shielded by one of the phenyl substituents on the oxazoline rings. So, the addition reaction occurred from the front face to the radical intermediate XVIII to give the (/ ) product from the (R,R) ligand 12. 

Retro-inversion of hydroxamate enkephalinase inhibitors [89] leads to a 5-fold difference in potency between the (/ ) and (5) isomer compared with the 100-fold difference obtained with retrothiorphan. This difference between the thiol and hydroxamate inhibitors may be explained in terms of the geometrical parameters existing in their inhibitor-zinc complexes since in the thermolysin-thiol inhibitor complex, the zinc is tetra-coordinated [87], whereas in the thermolysin-hydroxamic acid inhibitor complex it is penta-coordinated [88]. The conformational space accessible to the thiol function is less than that accessible to the hydroxamate function, as there is only one degree of freedom in retrothiorphan (a rotation around the alpha... 

Ishizaki and Hoshino prepared optically active secondary alkynyl alcohols (up to 95% e.e.) by the catalytic asymmetric addition of alkyl zinc reagents to both aromatic and aliphatic aldehydes. The chiral ligands studied were based on the pyridine scaffold. Of the three aryl substitutions studied, the a-napthyl derivative was found to be superior (Scheme 21.10). Mechanistically, it was proposed that (S)-l would react with dialkynyl zinc alkoxide A and ethyl zinc alkoxide B. Coordination of additional di-alkynyl zinc and alkynylethyl zinc with these alkoxides (A, B) would give C and D, respectively (Scheme 21.11). More bulky alkoxide (C) would have severe steric interactions with the alkynyl group and pyridine moiety, which might cause undesired conformational changes of the l-zinc complexes. Consequently, the enatioselectivity would be decreased. 

Figure 12 4-f-Butyloxacalix[3]arene (left) cone and partial cone conformers and (right) a zinc complex model of an adamantyl... 

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