Chiral transference

Enamines derived from ketones are allylated[79]. The intramolecular asymmetric allylation (chirality transfer) of cyclohexanone via its 5-proline ally ester enamine 120 proceeds to give o-allylcyclohexanone (121) with 98% ee[80,8l]. Low ee was observed in intermolecular allylation. Similarly, the asymmetric allylation of imines and hydrazones of aldehydes and ketones has been carried out[82]. 

Complete chirality transfer has been observed in the intramolecular allyla-tion of an alcohol with the activated allylic ester of 2,6-dichlorobenzoic acid 338 to give the 2-substituted tetrahydrofuran 339[208]. 

BROOK Silaketone rearrangement Rearrangement of sila ketones to silyi ethers (with chirality transfer). 

I.3.3.1.4, With Reagent-Induced Stereoselectivity I.3.3.I.4.I. 1,3-Chirality Transfer... 

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. 

Lewis acid catalyzed carbonyl addition reactions of nonracemic chiral allylsilanes34 were shown to proceed with anti-S E attack, thus also enabling 1,3-chirality transfer in an opposite direction (Section D.l. 3.3.3.5.2.3.). 

With 1,3-Chirality Transfer from Enantiomerically Enriched Allyllithium Derivatives... 

Enantiomerically enriched l-(diisopropylaminocarbonyloxy)allyllithium derivatives (Section 1.3.3.3.1.2.) add to carbonyl compounds with syn-l,3-chirality transfer21, giving good evidence for a pericyclic transition state in the main reaction path (Section 1.3.3.1.). However, since the simple diastereoselectivity and the degree of chirality transfer are low, for synthetic purposes a metal exchange with titanium reagents or trialkyltin halides (Section D.1.3.3.3.8.2.3.) is recommended. 

The sense of chirality transfer from chiral titanium derivatives takes the expected... 

The enantiomeric excess which is achieved for a given ally carbamate is independent of the carbonyl compound used it reflects the skill of the operator in the crystallization procedure. The high degree of reagent-controlled chirality transfer is also obvious from the reaction with either enantiomer of 2-benzyloxypropanal103a 107a. 

The stannanes (-)-ent-12 and ( + )-ent- 3 (R = CH3) are obtained with >80% ee from the alkenyllithium (-)-sparteine complex105,107a (Section 1.3.3.3.1.1.). Hence, their titanium(IV) chloride mediated carbonyl additions are accompanied by chirality transfer and enantioface selection of opposite sense. This was demonstrated for the reaction with (5)-2-benzyloxy-propanal107b the d.r. (88 12) roughly reflects the enantiomeric composition of the stannanes. 

A chirality transfer from the allenic sulfone 154 to the Diels-Alder adducts is observed in the reaction of (+)-(S)-l-(p-tolylsulfonyl)buta-l,2-diene with cyclopentadiene (equation 112), in which the diastereomers 155 and 156 are formed predominantly107. 

Elimination gave also here the azirines 32. The use of chiral base in these cases did not result in a chirality transfer. Attempts were also made to prepare the corresponding 2-sulfinyl-2ff-azirines. It turned out that these compounds... 

Palacios et al. utilized the modified Neber reaction for the preparation of 2ff-azirine-2-phosphonates 33 as shown in Scheme 18 [28 a]. The use of quini-dine and dihydroquinidine as the chiral base resulted in moderate chirality transfer (20-52% ee). Similarly, 2-phosphinoyl-2H-azirines could be obtained by the Neber 1,3-elimination reaction [28bj. 

Fig. 24 Conformational equilibrium for 48 caused by chirality transfer to the sulfonylated W-atom...

The [Fe-Cp]-fragment does not only play the role of an additional steric element introducing planar chirality into the otherwise flat pyridine system. Substitution at the pyridine 2-position usually cuts the nucleophilicity of the nitrogen atom thus limiting the possibilities to achieve efficient chirality transfer using nucleophilic pyridine catalysts [84]. Ferrocene, however, functions as a strong electron donor (see Sect. 1) and thus restores the nucleophilicity impaired by substitution. 

It may be of interest to note that the stereospecific transformation shown in equation 15 has been cited as the first reported observation of an 1 - 3 chirality transfer. It is evident that on rearrangement of optically active 6d to 7d, the chiral center at C-a is eliminated and a new one created at C-y. The term self-immolative asymmetric synthesis has also been used to describe syntheses of this kind. As pointed out by Hoffmann , quantitative 1 - 3 chirality transfer will follow from the suprafacial - course of rearrangement, provided the reactant has a uniform configuration at the j8, y-double bond. This stereochemical prediction has also been confirmed by the results obtained in several other [2,3]sigmatropic rearrangements, subsequently reported " . 

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