Asymmetric induction, with

Under sonication conditions, the reaction of perfluoroalkyl bromides or iodides with zinc can be used to effect a variety of functionalization reactions [39, 40, 41, 42] (equation 30) Interestingly, the ultrasound promoted asymmetric induction with the perfluoroalkyl group on the asymmetric carbon was achieved by the reaction of perfluoroalkyl halides with optically active enamines in the presence of zinc powder and a catalytic amount of dichlorobisftc-cyclopenta-dienyl)titanium [42] (equation 31)... 

After 12 hours at 4 kbar. this reaction provided only 35% of a 63 27 mixture of 22 and a compound which was tentatively assigned structure 23. It is assumed that 23 derives from epimerization of 21 prior to reaction with (aS,S,S)-5l0b. Whether this stereochemical assignment is correct or not, this result shows that 5 may have problems with configurationally labile aldehydes in demanding cases of mismatched double diastereosclcction. For further examples of double asymmetric induction with 5 or related reagents, see refs 31, 34 and 47. 

So far, the methods described here and in the text are the only ones which work optically active starting materials must be used, or asymmetric induction with an optically active substituent or reagent, or the product or an intermediate must be resolved. 

In 1994, the scope of this p-hydroxy sulfoximine ligand was extended to the borane reduction of ketimine derivatives by these workers. The corresponding chiral amines were formed with enantioselectivities of up to 72% ee, as shown in Scheme 10.57. It was found that the A -substituent of the ketimine had a major influence on the asymmetric induction, with a ketoxime thioether (SPh) being the most successful substrate. 

It is of particular value from a synthetic viewpoint that, under Wacker-type conditions the cyclization is catalytic [Pd(OAc)2/Cu(OAc)2/MeOH,H20/ air/55°C]121 albeit not by a Wacker-type mechanism.123 Addition of the chiral alkene, (- )-/l-pinene, to the catalyzed cyclization of 2-(but-2-enyl)phenol causes asymmetric induction with an optical yield of 12%... 

Pentadienyltrimethylstannanes undergo regioselective conjugate additions to aldehydes, catalysed by Lewis acids. The dominant product obtained depends on the catalyst used, as shown in reaction 46. In the case of titanium tetrachloride catalysis the reaction is also stereoselective and only one diasteroisomer is obtained297. Reaction with chiral aldehydes leads to asymmetric induction with similar organotin compounds298. 

Lewis acids of chiral metal aryloxides prepared from metal reagents and optically active binaphthol derivatives have played a significant role in asymmetric synthesis and have been extensively studied.23 However, in Diels-Alder reactions, the asymmetric induction with chiral metal aryloxides is, in most cases, controlled by steric interaction between a dienophile and a chiral ligand. This kind of interaction is sometimes insufficient to provide a high level of enantioselectivity. 

The degree of asymmetric induction with 119 increased in the order CF3 — Me < Et < Bu < Pr As with the alkoxyaluminum dichloride reagent (Sect. II1-A-2) the cyclic transition state model (Scheme 20) would predict a continuous... 

Catalyst activity - can the activity of catalysts be increased to achieve similar levels of yield and asymmetric induction with reduced catalyst loading ... 

Intermolecular Stereoselective Aryl Coupling 2.1.1. Internal Asymmetric Induction with Chiral Substrates 2.1.1.1. Dimerization of Chiral Aryl Compounds... 

External Asymmetric Induction with Chiral Auxiliaries or Catalysts... 

Asymmetric induction with aliphatic aldehydes was moderate (18 to 71 %). It was also found that the activity of the chiral catalyst was maximum when it was first purified by nonaqueous solvents. Other cyclic dipeptides containing a histidine moiety (e.g., cyclo[(S)-Ala-(S)-His] cyclo[(S)-His-(S)-His]) cyclo[(S)-Pro-(S)-His]) gave low ee values (0-10%). The linear dipeptide Z-(S )-Phe-(S)-His-OMe showed no asymmetric induction. 

This procedure, which is based entirely on commercially available reagents, is very easy to reproduce. Table 6.12 shows different aryl- and alkyl-substituted enones that can be epoxidised with high asymmetric induction with the in situ formed (/ )-BINOL-zinc-catalyst in diethyl ether, with cumene hydroperoxide as the terminal oxidant. 

The asymmetric halolactonization reactions of unsaturated L-proline amides, developed by Terashima and coworkers,184 has been extended to a-alkyl acrylic acid derivatives (equation 75 and Table 21).185 This allows for the synthesis of either enantiomer of an a-methyl-a-hydroxy acid using L-proline as the auxiliary. Less successful approaches to asymmetric induction with a chiral auxiliary include iodolac-... 

The chiral boron complex prepared in situ from chiral binaphthol and B(OPh)3 is utilized for the asymmetric aza-Diels-Alder reaction of Danishefsky s diene and imines [67] (Eq. 8A.43). Although the asymmetric reaction of prochiral imine affords products with up to 90% ee, the double asymmetric induction with chiral imine by using oc-benzylamine as a chiral auxiliary has achieved almost complete diastereoselectivity for both aliphatic and aromatic aldimines. This method has been successfully applied to the efficient asymmetric synthesis of anabasine and coniine of piperidine alkaloides. 

Double asymmetric induction with terpenes as chiral ene components Terada, M. Sayo, N. Mikami. K. Synlett 1995, 411. 

Although the use of cinchona alkaloids as chiral ligands does provide high asymmetric induction with a number of types of alkene, the search for better systems has resulted in better catalysts (Scheme 9.21).15/W58... 

A frontier which remains to be developed is catalytic asymmetric induction in radical addition to C=N bonds, although there are promising results which suggest asymmetric catalysis of broad scope may be on the horizon (for the first examples of asymmetric induction with catalyst turnover [171-173]). Hopefully this review will inspire new explorations in this direction. 

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