A kinetic model of spontaneous amplification of enantiomeric excess in the autocatalytic addition of diisopropylzinc to prochiral pyrimidine carbaldehydes is extended by a negative feedback process. ...Simulations based on the extended model result in large-amplitude oscillations both in a continuous-flow stirred tank reactor (CSTR) and in a semibatch configuration under optimized initial conditions. When sustained oscillations are maintained in a CSTR, no enantiomeric product distribution could be observed in the calculated series; the system keeps its initial enantiomeric ratio endlessly. During damped oscillations, or steady-state conditions, however, chiral amplification from a very small initial enantiomeric excess to more than 99% occurs in a semibatch configuration. Calculations indicated spontaneous enantiomeric product enrichment (i.e., accumulation of one of the enantiomers at the cost of the other one) from strictly achiral starting conditions in a semibatch configuration due to the inherent numerical error of the integrator method, which can be regarded as a model of the statistical fluctuation in the numbers of enantiomeric molecules.
Absolute asymmetric synthesis (AAS) is the preparation of pure (or excess of one) enantiomer of a chiral compound from achiral precursor(s) by a chemical reaction, without enantiopure chiral additive ...and/or without applied asymmetric physical field. Only one well-characterized example of AAS is known today: the Soai-autocatalysis. In an attempt at clarification of the mechanism of this particular reaction we have undertaken
empirical
and
stochastic analysis
of several parallel AAS experiments. Our results show that the
initial steps
of the reaction might be controlled by simple
normal distribution
(“coin tossing”) formalism. Advanced stages of the reaction, however, appear to be of a more complicated nature.
Symmetric beta distribution
formalism could not be brought into correspondence with the experimental observations. A
bimodal beta distribution
algorithm provided suitable agreement with the experimental data. The parameters of this bimodal beta function were determined by a Pólya-urn experiment (simulated by computer). Interestingly, parameters of the resulting bimodal beta function give a
golden section ratio
. These results show, that in this highly interesting autocatalysis two or even perhaps three catalytic cycles are cooperating. An attempt at constructing a “designed” Soai-type reaction system has also been made.
Amino acids are attractive sources of chirality in stoichiometric or catalytic transition metal/organic chemistry. In spite of easy availability and other advantages, the application of these ligands ...is hindered by several problems. Now, at the dawn of emerging d‐amino acid biochemistry, efforts in this direction are becoming increasingly important. The results of research on application of amino acid ligands for transition‐metal reagents in organic syntheses are reviewed in the present work.
Enantiomeric excesses obtained in absolute enantioselective synthesis by chiral autocatalysis (Soai-reaction) were statistically analyzed. Two sets of parallel experiments, which were performed under ...chemically different conditions, are available. One group contains 37, while the other contains 84 preparative results. The former group shows some interesting tendencies but does not give conclusive statistical results. The sample of 84 parallel experiments, providing 39 R- and 45 S-excesses have shown that these data represent two distinct, non-symmetric sets with different non-Gaussian distributions. Clear S preference was found.
Natural-abundance isotopic substitution in isotopically prochiral groups of otherwise achiral molecules can provide stochastically formed enantiomeric excesses which exceed the sensitivity threshold ...of sensitive asymmetric autocatalytic (Soai-type) reactions. This kind of induction of chirality should be taken into consideration in in vitro model experiments and offer a new kind of entry into primary prebiotic or early biotic enantioselection in the earliest stages of molecular evolution.
ee
prod
(
i
+
1
)
=
ee
max
ee
prod,
i
B
+
ee
prod,
i
A simple empirical formula enables the quantitative description of chiral autocatalysis. This formula was used for the prediction of the number of ...consecutive autocatalytic reaction cycles needed to obtain a high enantiomeric excess without chiral auxiliary or an asymmetric physical field (absolute enantioselective synthesis). The results show, that even less selective Soai-type systems can be used for absolute enantioselective synthesis, which, therefore, appears to be a fairly general phenomenon.
Stoichiometric reduction of the C,N double bond of oxime precursors of α-amino acids was performed in aqueous media by Cr(II) complexes of natural amino acids. The reduction of oximes of ...α-ketophenylacetic, α-keto-β-phenylpropionic and α-ketopropionic acids proceeded up to >90% conversion and 2–30% enantiomeric excess. 1:2 complexes of Cr(II) with
l-alanine,
l-valine,
l-aspartic acid,
l-histidine and
l-phenylalanine were used as reducing agents. The reduction of α-(oximino)phenylacetic acid showed increasing e.e. (and decreasing conversion) with increasing temperature.
Stoichiometric reduction of the C,N double bond of oxime precursors of α-amino acids was performed in aqueous media by Cr(II) complexes of natural amino acids.
Anomeric carbon–sulfur bonds in 1-thioglycosides and glycosyl sulfones can be cleaved by chromium(II) complexes in water–DMF medium. Anomeric radicals as well as sugar–chromium(III) complex ...intermediates can be generated in these reactions, leading in some cases, to the exclusive formation of the corresponding glycals.
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