Chiral 1,2-diamines, conformations

The asymmetric Horner-Wadsworth-Emmons (HWE) reaction of l,3-dioxan-5-ones with phosphonate 184 and a chiral diamine was reported. With the /i r/-butyl-substituted l,3-dioxan-5-one, the product possesses a chiral axis. It was obtained in good yield and with 80% ee (Scheme 53) <2002TL281>. The HWE reaction with similar heterocyclic substrates was used to provide conformationally restricted arachidonic acid derivatives <1999TA139>. 

Helix-sense-selective polymerization of methyl, benzyl, and f-butyl methacrylates was attempted by using the complexes of chiral crown ethers, 91 and 92, and that of a chiral diamine 93 with n-BuLi however, these esters seem to be too small to form and maintain helical conformation [138,139], The complexes of BuLi with 58a and 59 failed in producing an optically active, helical polymer in the polymerization of methyl and benzyl methacrylates [104b]. 

A zinc(n) meso-meso linked porphyrin oligomer 57 exists in a nonhelical conformation in solution, but may adopt a dynamic helical conformation upon complexation with an achiral urea 58 through complementary hydrogen bonding interactions [115]. In the presence of the chiral diamine (S)-59, the 57-58 complex forms a predominantly one-handed helical conformation, thus showing a characteristic ICD in the absorption region of the porphyrin chromophore. This system may be used to sense the chirality of chiral diamines. 

Chiral auxiliaries derived from (S)-proline appeared to be particularly attractive since they possess conformationally rigid pyrrolidine ring(s), a prerequisite to the above specified criteria. In this chapter highly stereoselective asymmetric reactions, employing the chiral diamines 1 and chiral diamino alcohols 2 and 3 (Fig. 1),... 

An asymmetric approach to differentially substituted cw-1,2-diamino cyclohexanes also utilized the Curtius rearrangement. The chiral diamine products are components of biologically active small molecules and useful as conformationally restricted peptide-like scaffolds. y Amino acid 54 was prepared in enantiomerically pure form using asymmetric reductive amination and converted to phthaloyl-protected y -amino acyl azide 56. Curtius rearrangement, followed by addition of benzyl alcohol and further heating provided chiral c/5-1,2-diamine 57 with orthogonal A-protection. 

Metal salen complexes can adopt non-planar conformations as a result of the conformations of the ethane-1,2-diyl bridge. The conformations may have Cs or C2 symmetry, but the mixtures are racemic. Replacement of the ethylenediamine linker by chiral 1,2-diamines leads to chiral distortions and a C2 chiral symmetry of the complex due to the half-chair conformation of the 5-membered ring of the chelate. Depending on substitution at the axial positions of the salen complex, the symmetry may be reduced to Q, but as we have seen before in diphosphine complexes of rhodium (Chapter 4) and bisindenyl complexes of Group 4 metals (Chapter 10) substitution at either side leads to the same chiral complex. Figure 14.10 sketches the view from above the complex and a front view. 

A-Boc-A -isopropylimidazolidine (175) is deprotonated by the (—)-sparteine method and alkylated to form imidazolidines 176 (equation 41). The yields remain below 50%, since only a part of 175 exists in the shown conformation, which is required for the directed deprotonation. At low temperature, the interconversion between the s-trans- and i-c -conformation is too slow. Ring cleavage furnishes synthetically useful chiral 1,2-diamines 177. 

Among several chiral cyclic and acyclic diamines, (R,R)-cyclohexane-l,2-diamine-derived salen ligand (which can adopt the gauche conformation) was most effective in providing high enantioselectivity [38]. Further, the introduction of substituents at the 3,4, 5 and 6 positions on the aromatic ring of catalyst 39c was not advantageous, and resulted in low enantioselectivity [32,37,39]. The metal ions from first-row transition metals - particularly copper(II) and cobalt(II) - that could form square-planar complexes, produced catalytically active complexes for the asymmetric alkylation of amino ester enolates [38]. 

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