Optical isomerism stereoselectivity

Yamaguchi, T. Nakamura, Y. Stereoselective metabolism of 2-phenyl-propionic acid in rat. II. Studies on the organs responsible for the optical isomerization of 2-phenylpropionic acid in rat in vivo. Drug. Metab. Disp. 1987, 15, 535-539. 

Alkoxyallylstannanes are also available by boron trifluoride-diethyl ether complex induced isomerization of their 1-alkoxy isomers. This isomerization proceeds in an antarafacial manner with excellent stereoselectivity to give (Z)-3-alkoxyallylstannanes possibly via an intermolecu-lar exchange process119. Coupled with the asymmetric reduction of acylstannanes (see Section 1.3.3.3.2.3.1) this provides access to 1-alkyl-3-alkoxyallylstannanes of useful optical purity106. 

To date, direct asymmetric synthesis of optically active chiral-at-metal complexes, which by definition leads to a mixture of enantiomers in unequal amounts thanks to an external chiral auxiUary, has never been achieved. The most studied strategy is currently indirect asymmetric synthesis, which involves (i) the stereoselective formation of the chiral-at-metal complex thanks to a chiral inductor located either on the ligand or on the counterion and then (ii) removal of this internal chiral auxiliary (Fig. 4). Indeed, when the isomerization of the stereogenic metal center is possible in solution, in-... 

In contrast to the isomerization of 39, the retention of optical activity for the conversion of 41 to 42 is very small. However, the loss of activity cannot be explained by a degenerate vinylcyclopropane rearrangement, since the recovered starting material ([(X]54g = +32 ) shows essentially complete retention of chirality. An alternative explanation involves competing stereoselective processes in a chiral intermediate (41a ). For 41a a hydride shift from the allylic position, C2, might compete with the migration from C6 to Cl. This possibility can be probed by... 

Rearrangement of allylic alcohols. RuClj in combination with NaOH is a useful catalyst for isomerization of allylic alcohols of the type RCIIOHClI=CH2 to saturated ketones, RCOCH2CH3. In the isomerization of a chiral allylic alcohol such as I, the product is optically active. The 1,3-hydrogen transfer is 37% stereoselective. 

In the hydroformylation of optically active alkenes, all aldehydes were shown to be formed with predominant retention of configuration.53,54 This was interpreted to prove, in accordance with the transformation of deuterium labeled alkenes,55 56 that stereoselective hydrogen shifts of the coordinated olefins occur without dissociation of the isomeric alkenes.30... 

For example, the ene reaction proceeds stereoselectively in a suprafacial manner specifically, oxygen attack and hydrogen removal take place on the same side of the double bond. As a result, neither racemization of optically active com-pounds nor cis-trans isomerization occurs. Diradical or dipolar intermediates are not consistent with these observations. The lack of a Markovnikov-type directing effect also rules out dipolar intermediates.360 399 Isotope effects observed in the reaction of different deuterium-labeled 2,3-dimethyl-2-butenes indicated that... 

Stereoselective Friedel-Crafts alkylation. 4 Alkylation of benzene with methyl (S)-2-(mesyloxy)propionate, derived from (S)-lactic acid, under Friedel-Crafts conditions (2 equiv. of A1C13) affords methyl (S)-phenylpropionate in 50-80% chemical yield and as high as 97% optical yield. Unfortunately extension to other aromatics results in mixtures of isomeric products. 

Enantioselective isomerization of olefins for preparation of optically active olefins has great synthetic potential with a long tradition [1] and the non-stereoselective version is probably the most intensively investigated reaction in transition metal catalysis [2]. In particular, stereoselective hydrogen migration in a-functionalized olefins, for example allyl alcohols and allylamines [3], affording optically active aldehydes, ketones and amines, is part of the standard repertoire of enantioselec-... 

Changing the transition-metal from palladium to rhodium (equation 62) makes possible, in addition to the straight-chain alkylation product (243), the regio- and stereoselective synthesis of amino acid derivatives with a terminal double bond (242), starting from optically active branched allylic substrates 241 (Table 21)" . Remarkably, the substitution products were obtained with high enantiomeric excesses, what might result from a slow isomerization of the intermediary formed allyl rhodium complexes ". 

Not only Cc,-0 and rarely Cg-O bond breaking but C -Cg bond splitting may be observed. A consequence of this is the very stereoselective epimerization of benzylidenecyclohexanone oxide.The stereochemistry of photolysis of optically active dypnone oxide has been studied and the possibility of C-C bond cleavage has likewise been proposed.C-C bond splitting has also been observed during examinations of chalcone oxides transformation to dibenzoylmethane, trans-cis isomerization, and photofragmentation occur via ylides and acylcarbene intermediates (Eq. 341). ... 

Hydroformylation of ( + )-(S)-3-methyl-l-hexene gives 3% of (7 )-3-ethylhexanal (14) with 70% optical purity130 168. Interestingly, isotopic labelling reveals that no direct insertion of carbon monoxide into the C-H bond occurs, but a stereoselective double bond or metal alkyl isomerization preceeds hydroformylation with only partial loss of optical information34-122. 

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