Transfer of chiral information

Tius and co-workers elegantly applied a variant of the Nazarov reaction to the preparation of cyclopentenone prostaglandins (Scheme 19.39) [46]. Moreover, it was demonstrated that the chirality of non-racemic allenes is transferred to an sp3-hybridized carbon atom. Preparation of allenic morpholinoamide 214 and resolution of the enantiomers by chiral HPLC provided (-)- and (+)-214. Compound (-)-214 was exposed to the vinyllithium species 215 to afford a presumed intermediate which was not observed but spontaneously cyclized to give (+)- and (—)-216 as a 5 1 mixture. Compound (+)-216 was obtained with an 84% transfer of chiral information and (-)-216 was obtained in 64% ee. The lower enantiomeric excess of (—)-216 indicates that some Z to E isomerization took place. This was validated by the conversion of 216 to 217, where the absolute configuration was established. The stereochemical outcome of this reaction has been explained by conrotatory cyclization of 218 in which the distal group on the allene rotates away from the alkene to give 216. 

Abstract An overview of the area of organocatalytic asymmetric acyl transfer processes is presented inclnding O- andiV-acylation. The material has been ordered according to the structnral class of catalyst employed rather than reaction type with the intention to draw mechanistic parallels between the manner in which the varions reactions are accelerated by the catalysts and the concepts employed to control transfer of chiral information from the catalyst to the substrates. 

As a consequence of the restricted jump-rope rotation around the trans double bond, (i j-cyclo-octene 38E is chiral. Optically active (E)-cyclo-octene has long been known, but the conventional multistep synthesis is rather tedious [138-140]. In contrast, direct-preparation of optically active (Ej-cyclo-octene through asymmetric photosensitization is an attractive alternative. The first enantiodifferentiating Z-E photoisomerization of cyclo-octene 38Z sensitized by simple chiral alkyl benzenecarboxylates was reported in 1978 to give low enantiomeric excesses (ee s) of <6% [141] a variety of systems and conditions have been examined since then to raise the product ee. For an efficient transfer of chiral information... 

The transfer of chiral information from the nanoscale adsorbed motif to the macroscale organisation of adsorbates leads to another consequence, namely, that switching molecular chirality should also flip the organisational chirality. When enantiopure (5, 5)-TA is adsorbed on Cu(llO), the RAIRS spectra are identical to those of the (R,R) enantiomer, showing that the chemical nature of the adsorbed species is the same for both enantiomers. However, STM and TF.F.D data show that the mirror organisation is now created, yielding a (9 0,-1 2) overlayer (Fig. 5.4). A change in... 

ABSTRACT. Stereocontrol based on the principle illustrated in 1 allows the 1,5 transfer of chiral information in some S 2 reactions, as shown in Schemes 5 and 6, and, to some extent, the control of exo-cyclic double bond geometry, as shown in Schemes 15 and 16. The latter device is used in a highly stereocontrolled synthesis of the carbacychn analogue 49. 

We have already reported our first work in this area, when we described the remarkably high level of transfer of chiral information in the reaction shown in Scheme 5. The major product 8 in this... 

The intramolecular transfer of chiral information in [(alkoxycarbonyl)methyl]-cobalt tricarbonyl triphenylphosphine complexes was investigated by semi-... 

The group of Li and Chan [22] reported that metal complexes of bipyridyl-based diphosphane, such as P-Phos, were good catalysts in Rh(I)-catrJyzed 1,4-addition of boronic acids to enones. The rigidity of the bipyridyl backbone allows good transfer of chiral information (Scheme 5.7). The obtained results using a variety of a,p-unsaturated ketones as well as arylboronic acids led the authors to... 

Photochemical asymmetric synthesis ( photochirogenesis ) uses photoexcited short-lived, weakly interacting molecular states. Their short lifetimes makes it challenging to control electron transfer and subsequent product formation and asymmetric induction in particular [30]. Until recently the transfer of chiral information was restricted to photoreactions where the chirality transfer was assured... 

Enantiomeric (E)-cyclooctene (20E) was first resolved in 1963 through its diastereomeric platinum(II) complex. Synthesis of optically active 20E has been the subject of intensive study since 1968. The first preparation involves the treatment of enantiopure (E)-cyclooctane-l,2-thiocarbonate with triisooctyl phosphate or of (E)-cyclooatane-l,2-trithiocarbonate with l,3-dibenzyl-2-methyl-l,3,2-diazaphospholi-dine. Following analogous synthetic routes, enantiomeric (E)-cycloheptene (18E) can be produced and trapped by 2,5-diphenyl-3,4-isobenzofuran as an optically active adduct. In 1973, the circular dichroism spectrum of enantiopure 20E vapor was recorded in the vacuum UV region down to 150 nm. The first enantiodifferentiating Z-E photoisomerization of cyclooctene sensitized by chiral benzenecarboxylates appeared in 1978. Transfer of chiral information from sensitizer to substrate occurs within the exciplex intermediate. ... 

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