Achiral groups, defined

Bottom Pictorial representation of (a) the achiral, S8 symmetric homodimer 16-16 and of (b) the two enantiomeric forms of a calixarene capsule heterodimer. The head-to-tail directionality of the urea groups (arrow) defines the chirality. 

Hydrogen atom transfer implies the transfer of hydrogen atoms from the chain carrier, which is the stereo-determining step in enantioselective hydrogen atom transfer reactions. These reactions are often employed as a functional group interconversion step in the synthesis of many natural products wherein an alkyl iodide or alkyl bromide is converted into an alkane, which, in simple terms, is defined as reduction [ 19,20 ]. Most of these reactions can be classified as diastereoselective in that the selectivity arises from the substrate. Enantioselective H-atom transfer reactions can be performed in two distinct ways (1) by H-atom transfer from an achiral reductant to a radical complexed to a chiral source or alternatively (2) by H-atom transfer from a chiral reductant to a radical. 

In retrospect, it seems unfortunate that in 1971 Morrison and Mosher8 generalized the definition, while keeping the term, an asymmetric synthesis is a reaction in which an achiral unit in an ensemble of substrate molecules is converted by a reactant into a chiral unit in such a manner that the stereoisomeric products arc produced in unequal amounts ( Footnote The substrate molecule must have either enantiotopic or diastereotopic groups or faces) . Obviously the phrase "an achiral unit in an ensemble of substrate molecules is too inexact and requires a great deal of additional explanation, which was partially given by the footnote (note that molecule, i.e., singular, was used ). Currently, the Morrison-Mosher term appears to be equivalent to stereoselective reaction. Unfortunately, this term was only defined in the modem sense by Izumi in 1971, i.e., in the same year the Morrison-Mosher definition was published. 

The phase-transfer-catalyzed asymmetric alkylation of 1 has usually been performed with achiral alkyl halides, and hence the stereochemistry of the reaction with chiral electrophiles has scarcely been addressed. Nevertheless, several groups have tackled this problem. Zhu and coworkers examined the alkylation of 1 with stereo-chemically defined (5S)-N-benzyloxycarbonyl-5-iodomethyl oxazolidine using 4d to prepare (2S,4R)-4-hydroxyornithine for the total synthesis of Biphenomycin. Unexpectedly, however, product 7 with a 2 R absolute configuration was formed as a major isomer, and the diastereomeric ratio was not affected by switching the catalyst to pseudoenantiomeric 2d and even to achiral tetrabutylammonium bromide (TBAB), indicating that the asymmetric induction was dictated by the substrate (Scheme 2.3) [21]. 

In all the examples of exodendrally functionalized enantioselective den-drimer catalysts, the active sites in the periphery of the support were well-defined immobilized molecular catalysts. An alternative is provided by the possibility of attaching chiral multi-functional molecules to the end groups of dendrimers which, due to their high local concentrations, may interact more or less strongly with an achiral reagent and thus induce enantioselectivity in a transformation of a prochiral substrate. Asymmetric induction thus occurs by way of a chiral functionalized microenvironment for a given reaction. 

The chirality of the compounds discussed so far, especially of those in Section 2 is caused entirely by the chirality of their substituents. Due to their nonplanar shape, calixarenes offer various additional possibilities to produce chiral derivatives, the chirality of which is not based on a chiral group or subunit but on the absence of a symmetry plane, an inversion center, or an alternating axis in the molecule as a whole. This means, that opening of the macrocyclic structure would lead to an achiral linear molecule. Such molecules may therefore be called inherently chiral which should not be confused with the term intrinsically chiral,110 A graph whose chirality is independent of its embedding in the three-dimensional space is intrinsically chiral, while the inherent chirality defined above is due to the three-dimensional structure. 

The compounds in this report usually contain a chirotopic stereogenic carbon ring atom, and were prepared as racemic mixtures. Hypothetically, if the BC conformation prevails, then one can imagine two enantiomers in solution (reference, 5)-BC 9 and (retro-inverso,R)-BC 9-bar. Since this stereochemistry is complicated, it will be helpful if we refer to the descriptor for only one enantiomer. Therefore, in an arbitrary but consistent manner in this report, we will always define the reference ring chirality and label tropicity to be that of the (S)-enantiomer. For example, suppose a racemic mixture of (reference,S)-BC 9 and (retro-inverso,R)-BC 9-bar affords crystals belonging to an achiral space group so that both enantiomers in the racemic compound are present in the crystal lattice. Let us further suppose that dissolution of these crystals will give the same solution-state conformation. We will write that the solid-state (reference,S)-BC 9... 

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