Chiral ammonium center

Homolytic fission of the chiral ammonium ylide (143) generates two diastereomeric radical pairs which evolve by two different routes (a) cage recombination leads to the two diastereomeric ketoamines (161) and (162), where the migrating center has retained its absolute stereochemistry (b) radical escape gives achiral free radicals that combine to yield racemic products (Scheme 35). 

The transmission of chirality in membranes and other types of aggregates formed by surfactants was widely studied. The superstructures of bilayers formed by gemini dicationic surfactants based on quaternary ammonium centers can be modulated through the employ of enantiomeric tartrate salts. The twisted ribbons (Fig. 4) are formed when an enantiomeric excess of one of the tartrates is present, with the period (helical pitch) shorten-... 

In 2006, Shibasaki and co-workers [91] developed phase-transfer-catalyzed asymmetric Michael addition of glycine Schiff base 237 and dienone 238 to afford dienone 240 in 84% isolated yield and 82% ee for the total synthesis of (+)-cylin-dricine C (242) and a formal synthesis of (—)-lepadiformine (244), using a chiral two center organocatalyst 239 (TaDiAS Tartrate-derived Di-Ammonium Salt) (Scheme 17.41). (+)-cylindricine C (242) was achieved three steps from 240 using... 

On the other hand the external asymmetric induction strongly depended on the chiral auxiliary. The careful analysis of the hypothetical zwitterionic intermediates c-a and c-fi indicated the formation of a stereogenic ammonium center. In terms of the well-known 1,3 chirality transfer of 3,3sigmatropic rearrangements, the... 

This peculiar chirality was generated by the directionality of the cone calix[6] arene wheel which upon threading with 12a differentiates the two benzylic units around the ammonium center (Scheme 30.3) [29]. It is here worth noting that while beautiful examples of dissymmetric supramolecules assembled from achiral molecules are known [31], to the best of our knowledge, this is the first example of a chiral supramolecule in which an atomic stereogenic center is generated supramolecularly. 

The Darzens condensation is an old methodology for the construction of a,p-epoxy carbonyl compounds 49 with control of two formed stereogenic centers. This reaction includes an aldol reaction (C-C bond formation), which normally requires stoichiometric amounts of base to achieve good yields. Only phase transfer catalysts, usually chiral ammonium halides have shown to be efQdent to perform this transformation in a catalytic and enantioselective form. 

These salts are obtained from 1,2 (or l,3)-diamines. The cyclization step is generally the condensation of the diamines on ethyl orthoformate in the presence of ammonium tetrafluoroborate (Scheme 19). By using enantiop-ure diamines, chiral salts 34 bearing stereogenic centers on the backbone, on the nitrogen substituents, or on both were prepared [1,2,4,5,26]. Several dicyclophane imidazolinium salts have been prepared by one-pot three-step procedure [27]. 

Amines with three different substituents are potentially chiral because of the pseudotetrahedral arrangement of the three groups and the lone-pair electrons. Under normal conditions, however, these enantiomers are not separable because of the rapid inversion at the nitrogen center. As soon as the lone-pair electrons are fixed by the formation of quaternary ammonium salts, tertiary amide N-oxide, or any other fixed bonding, the inversion is prohibited, and consequently the enantiomers of chiral nitrogen compounds can be separated. 

Enantioselective reagents for ammonium ions include, for example, a mixture containing a host chiral crown ether such as 196, possessing four (R) centers and symbolized as M, a host achiral crown ether of similar functionality, symbolized as R, and a salt of a guest chiral amine, symbolized as A, which is analyzed by fast atom bombardment MS (FAB-MS), and the relative peak intensity of the equilibrium complexes 7(MA)//(RA) is measured and correlated with the chirality of the guest molecule. Many host and guest molecules have been investigated405. 

The requisite hydroxylamine function for such cyclizations can also be generated from a precursor having a nitro group. This novel route has provided access to hitherto unknown l-hydroxy-6-allyl-, and -6,6-bisallyl-piperazine-2,5-diones (91UP1). The starting material is an W-nitroacetyl amino acid ester that can be either mono-or bis-allylated at the methylene adjacent to the nitro group. Reduction of the N02 to NHOH using zinc/ ammonium chloride, followed by cyclization, leads to the desired products (Scheme 76). Compound (215) is unique in that it possesses a chiral center at C-3 and a quaternary carbon at C-6 on a l-hydroxypiperazine-2,5-dione system. 

Chiral crown ethers based on IB-crown-6 I Fig. 4> can form inclusion complexes with ammonium ions and proionated primary amines. Immobilization of these chiral crown ethers on a chromatographic support provides a chiral stationary phase which can resolve most primary amino acids, amines and amino alcohols. However, the stereogenic center must be in fairly close proximity in the primary aininc lor successful chiral separalion. Significantly, ihe chiral crown ether phase is unique in that ii is one of the few liquid chromatographic chiral stationary phases that does not require the presence of an aromatic ring to achieve chiral separations. 

Study of the chemical evolution of chirality started in 1809 with the discovery of Haiiy [4], who postulated from crystal cleavage observations that a crystal and each of its constituent space-filling molecules are images of each other in overall shape. Later, in 1848, Pasteur reported the different destruction rates of the dextro- and levorotatory forms of ammonium tartrate by the mold Penicillium glaucum [5]. These observations could not be explained properly at that time, but in 1874 Le Bel [6] and van t Hoff [7] independently proposed that the four valences of the carbon atom are directed toward the vertices of an atom-centered tetrahedron. This finding allowed the development of the theory of the three-dimensional structure of molecules by which the phenomenon of chirality and Pasteur s discovery were explained scientifically. 

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