Chirality transmission

The enantioselective incorporation of these amino acids into the serine octamers represents an example of chiral transmission to elementary biomolecules and a possible way of chirality amplification on primitive earth. 

The trick used in asyrmnetric inclusion polymerization is to perform the reaction in a rigid and chiral environment. With more specific reference to chirality transmission, the choice between the two extreme hypotheses, influence of the starting radical (which is chiral because it comes from a PHTP molecule), or influence of the chirality of the channel (in which the monomers and the growing chain are included), was made in favor of the second by means of an experiment of block copolymerization. This reaction was conducted so as to interpose between the starting chiral radical and the chiral polypentadiene block a long nonchiral polymer block (formed of isoprene units) (352), 93. The iso-prene-pentadiene block copolymer so obtained is still optically active and the... 

K.J. Koch et al., Chiral transmission between amino acids Chirally selective amino acid substitution in the serine octamer as a possible step in homochirogenesis. Angew. Chem. Int. 

DFT calculations of polar charge distributions of the bitartrate-Ni complex [24] show the chiral transmission from the adsorbate to the surface is not restricted to a geometrical reorganization. In fact, the delocalised nature of the molecular... 

Diastereoselective allylzincations of achiral cyclopropenone acetals, addressing the control of the relative configuration at the newly formed C-C bond (centers C3 and C4) and of chiral derivatives such as 20 (R = H), addressing chirality transmission through a spiro carbon center, have been investigated. In the presence of bis(oxazoline) ligands, enantioselective reactions of this kind can also be performed. ... 

The transfer of chirality is expressed as % chirality transfer (% chiral transmission) and calculated as % major enantiomer in product vs. % major enantiomer in substrate times 100. 

Some loss of optical purity (82% chiral transmission) in the conversion of (R)-l to 3 with bis(4-bromo-2,6-di-rert-butylphenoxide)methylaluminum has been ascribed to the participation to some extent of an ionic mechanism. 

The stereoselective ketene-Claisen rearrangements of optically active cyclic allyl sulfides, which can be run slightly above room temperature, result in chirality transfer from a C-S bond to a C-C bond. Due to the mild reaction conditions, the yields often prove to be superior to the oxygen counterparts and result in nearly complete chiral transmission. No loss of optical purity is observed during the dechlorination step, e.g., rearrangement of 1 and 2 and subsequent dechlorination to give 3 and 4429. 

Similarly, rearrangement of alcohol 6 b with Z,Z geometry produces the Cope product 7 b with 72% de and 82% ee969. Although the observed levels of diastereoselection and chiral transmission are lower than those observed for other rearrangements, the results suggest that the 1,3-diaxial interaction concerned allows a high level of, but not complete, stereocontrol. 

A high degree of chiral transmission is observed in the synthesis of iodo acetal 19, a key intermediate in the total synthesis of ( + )-faranal (a trail pheromone of the Pharaoh s ant). The [2,3] Wittig process ot 16 followed by cis hydrogenation gives oxy-Cope substrate 17 with 100% E and Z and >99% threo. The oxy-Cope process of 17 affords en7//ro-aldehyde 18 as a single product with 91 % ee966. 

Self-irnmolative chirality transfer for rr-sLabilized [2,3] Wittig systems derived from nonracemic secondary alcohols follows a stereochemical course consistent with rearrangement through the conformation (vide supra). For bisallylic ethers 46, the fidelity of chiral transmission is larger than 90%34. 

Chiral transmission for isolated 1,4 chirality transfer in the rearrangement of propargyl ethers 54 ranges from 80-93%3 3. 

More selective are rearrangements of propargyl ethers of secondary alcohols 56. for which levels of chiral transmission approach 100%48 51. 

A reliable predictive model for simple diastereosclcction and high levels of chiral transmission have contributed to the utility of the propargyl ether rearrangement as an important tool for the elaboration of remote stereochemical relationships in steroidal systems52 >4. The stereochemical trends observed for acyclic substrates arc maintained in these rigid systems of note is the reversal of simple diastereoselectivity and 1,4 chirality transfer for trimcthylsilylpropynyl ethers 59 and 61, which parallels the behavior of similarly functionalized acyclic substrates52 53. 

Sulfur-to-carbon chirality transmission is also observed for the rearrangement of ylides derived from sulfonium salts 3111. While chirality transfer for the rearrangement is apparently very efficient, absolute configurations were not assigned to either starting materials or products in this study and further details regarding the nature of asymmetric induction for these systems are unavailable. 

The same laboratory has also shown that the Claisen rearrangement also proceeds with almost complete chiral transmission. ... 

Z)-Allylic alcohols (162) can be converted into /3y-unsaturated amides (163) with virtually complete chiral transmission by a high-temperature reaction with dimethylformamide dimethyl acetal, the crucial step being a stereospecific [2,3]sigmatropic rearrangement. By contrast, the corresponding (S)-(E )-alco-hols give rise to mixtures. 

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