Chirality Transfer from Allylic Position

As a consequence of the pericyclic reaction path, the addition of a-stereogenic allylmctals to carbonyl compounds is accompanied by an effective 1,3-chirality transfer in the allylic moiety combined with 1,4-chira induction at the prostereogenic carbonyl group3032. The scheme also demonstrates the importance of the orientation of the substituent X in the six-membered transition state. By changing from a pseudo-axial to a pseudo-equatorial position, the cation-induced sy/i-attack addresses opposite faces of both prostereogenic moieties, leading to a Z-and an -isomer, these being enantiomeric in respect to the chiral moiety. 

The [2,3]- or [3,3]-sigmatropic rearrangements (Scheme 24) provide a means to introduce either the protected amine or the carbon atom which will become the carboxylic acid, while also positioning the double bond in the correct position for the alkene isosteres. Moreover, when starting from homochiral allyl alcohols, a very effective chirality transfer assures the stereospecific construction of the R1 and R2 side-chain stereochemistries. 

Due to the concerted, suprafacial nature of the Claisen rearrangement a new stereogenic center (stereocenter) is created in the C-6 position in those cases where the, vp -hybridized C-4 carbon and the terminus of the allylic moiety is asymmetrically substituted, while the stereogenic center at C-4 is destroyed. What is transferred in "1.3-chirality transfer processes" is stereogenicity from C-4 to C-6. These processes have been called self-immolative processes372. 

The classical approach for the introduction of chirality is the rearrangement of chiral allylic esters. If these esters are used, chirahty transfer occurs not only from the ester moiety (C-4) to C-6 but also to the a-position (C-1) of the carboxylic acid subunit (1,4-chirality transfer). 

If the Claisen rearrangement is carried out with peptide allylic esters, the transfer of an allylic side chain to the a-position of the C terminal amino acid results in a modification of the peptide chain. This concept is comparable to the alkylations of peptide enolates described by Seebach et al. [93]. If it is possible to carry out the rearrangement not only with amino acids but also with peptide esters, the question arises if it is possible to transfer the chiral information from the peptide chain to the new chiral center formed during the rearrangement process, prohahly via some peptide metal enolate complexes. 

Starting from homoallylic alcohol 13, one can transfer the chirality from the hydroxylated carbon to the allylic position of allylic arenes (Scheme 5.20) [18]. In contrast, alcohol 15 bearing a (Z)-disubstituted double bond is not a suitable... 

Table 17) with two substituents in position C3 the oxygen transfer by the chiral hydroperoxides occurred from the same enantioface of the double bond, while epoxidation of the (ii)-phenyl-substituted substrates 142c,g,i resulted in the formation of the opposite epoxide enantiomer in excess. In 2000 Hamann and coworkers reported a new saturated protected carbohydrate hydroperoxide 69b , which showed high asymmetric induction in the vanadium-catalyzed epoxidation reaction of 3-methyl-2-buten-l-ol. The ee of 90% obtained was a milestone in the field of stereoselective oxygen transfer with optically active hydroperoxides. Unfortunately, the tertiary allylic alcohol 2-methyl-3-buten-2-ol was epoxidized with low enantioselectivity (ee 18%) with the same catalytic system . 

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