Asymmetry transfer

Pyrrolines and pyrroles can be readily prepared from the rearrangement of a-aminoallenes. Optically enriched a-aminoallene 137 is rearranged to pyrroline 138 by catalytic silver nitrate (Eq. 13.45) [53], The yield of the reaction is high and the cyclization occurs with high levels of asymmetry transfer. Annulated 3-pyrroline 140 is the product of rearrangement of allenyl pyrrolidine 139 (Eq. 13.46) [53]. 

Der vorliegende Artikel wird sich im wesentlichen auf die Behandlung der extemen und der katalytisch-asymmetrischen Synthese sowie der Asymmetrie-Transfer-Prozesse beschranken. Femer soil kurz auf das Problem der absoluten asymmetrischen Synthese" (ohne optisch aktive Hilfsstoffe) eingegangen werden. Enzymatisch kontroUierte asymmetrische Synthesen konnen in diesem Rahmen nur an wenigen Beispielen be-leuchtet werden. 

In der Folgezeit hat sich diese Vorstellung auch zur Interpretation ahn-licher Asymmetrie-Transfer-Prozesse bewahrt. So fand man einen voUig analogen sterischen Verlauf bei der Reduktion von Ketonen und 1-Deu-terio-aldehyden mit Mg-Salzen optisch aktiver Alkohole [272, 273, 275, 276, 303). Man braucht in Ilia nur das A1 durch Mg zu ersetzen, um das sterische Resultat korrekt zu beschreiben. Asymmetrische C-Atome, die nicht direkt an das 0-Atom von I gebunden sind, beeinflussen die Stereo-... 

Haloetherification is closely related to halolactonization although it is used less often. Its main application is the diastereoselective synthesis of synthetically useful substituted tetrahy-drofurans. The mechanism of the asymmetry transfer of the reaction has been investigated using a transition-structure model based on AM1 calculations167. The strong correlation between model and experimental results makes the proposed rationale extremely attractive. 

The choice of the IV-protec ting group (Al-Boc) proved to be critical for achieving a high enantiomeric excess of the cyclization reaction. In contrast to KHMDS, lithium amide bases (LHMDS or LiTMP) did not afford detectable quantities of the anticipated heterocycles. The mechanism of asymmetry transfer was proposed to rely on the formation of axially chiral nonracemic enolate (eq 66) with a chiral C-N axis, the racemization barrier for which was found to be 16.0 kcal mol. ... 

As with many asymmetric processes, there are three ways to control absolute stereochemistry in the Nazarov cyclization Asymmetry transfer, the use of chiral auxiliaries, or asymmetric catalysis. It is important to realize, however, that there are two distinct processes operating that determine the stereochemistry of the product. To control the absolute stereochemistry of the p-carbon atom(s), it is necessary to control the sense of conrotation, clockwise or counterclockwise (torquoselectivity, see Section 3.4.3). To control the absolute stereochemistry of the a-carbon atom however, it is necessary to control the facial selectivity for enol protonation. 

In terms of asymmetry transfer, several effective means of controlling the absolute asymmetry of the product have emerged. Denmark and coworkers have published extensively on the use of silicon substituents to aid selectivity in Nazarov cyclizations (see Section 3.4.5.1). In one example of asymmetry transfer, they used a stereogenic trimethylsilyl-bearing carbon atom to control the sense of conrotation. Treatment of ketone 63 with ferric chloride gave product 64 in excellent yield and with complete transfer of asymmetry (see Section 3.4.5.1 for the mechanism of the silicon-directed... 

SCHEME 19.30. Axial to tetrahedral asymmetry transfer in the Nazarov cyclization. 

Charge asymmetry can be associated with a particular bond in a molecule and gives rise to what is called a bond dipole moment or, simply, bond moment. One use of bond moments is that they can be transferred, to a fair degree of approximation, from one molecule to another. In this way the dipole moment of a molecule can sometimes be estimated from a vector sum... 

While being very similar in the general description, the RLT and electron-transfer processes differ in the vibration types they involve. In the first case, those are the high-frequency intramolecular modes, while in the second case the major role is played by the continuous spectrum of polarization phonons in condensed 3D media [Dogonadze and Kuznetsov 1975]. The localization effects mentioned in the previous section, connected with the low-frequency part of the phonon spectrum, still do not show up in electron-transfer reactions because of the asymmetry of the potential. 

In the elucidation of retention mechanisms, an advantage of using enantiomers as templates is that nonspecific binding, which affects both enantiomers equally, cancels out. Therefore the separation factor (a) uniquely reflects the contribution to binding from the enantioselectively imprinted sites. As an additional comparison the retention on the imprinted phase is compared with the retention on a nonimprinted reference phase. The efficiency of the separations is routinely characterized by estimating a number of theoretical plates (N), a resolution factor (R ) and a peak asymmetry factor (A ) [19]. These quantities are affected by the quality of the packing and mass transfer limitations, as well as of the amount and distribution of the binding sites. 

Consequently, the peak will exhibit the asymmetry shown in figure 1. It is seen that the relative values of the resistance to mass transfer... 

It would seem that, in practice, the inequality defined in (3) can frequently occur but the converse does not appear to be true. Thus, peak asymmetry (in part or whole) resulting from inequality in mass transfer between the two phases manifests itself in the form shown in figure 1. 

The mechanisms involved in the establishment of lipid asymmetry are not well understood. The enzymes involved in the synthesis of phospholipids are located on the cytoplasmic side of microsomal membrane vesicles. Translocases (flippases) exist that transfer certain phospholipids (eg, phosphatidylcholine) from the inner to the outer leaflet. Specific proteins that preferentially bind individual phospholipids also appear to be... 

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