Chiral Lewis adds

D. A. Evans, P. H. Carter, E M. Carreira, J. A. Pmnet, A. B. Charette, M. Lautens Asymmetric Synthesis of Bryosta-tin 2 , Angew. Chem, Int. Ed. Engl. 1998,37,2354-2359. For methodological studies on asymmetric Cu-catalyzed aldol addition, see D. A. Evans, J. Murry, M. C. Koz-lowski C2-Symmetric Cu(II) Complexes as Chiral Lewis Adds. Catalytic Enantiosdective Aldol Additions of Silylketene Acetals to (Benzyloxy)acetaldehyde , J. Am Chem. Soc 1996,118,5814-5815. 

Hoshino et al. [33] reported the first example of an enantioselective radical reaction employing a chiral Lewis add complex. The enantioselective reduction of a-methoxy-methyl-a-iodolactone 41 with tributyltin hydride (BusSnH) in the presence of stoichiometric amounts of the chiral complex of a chiral diamine 42 and Mgl2, gave the reduced product 43 in 88 % yield with 62 % ee (Sch. 17). Reaction using Mg(C104)2, TiCU, Znl2,... 

To achieve asymmetric allylation with allylsilanes, the control of absolute configuration by use of chiral auxiliaries was extensively studied in the nineteen-eighties. Recently, much effort has been directed toward the use of optically active a-chiral allylsilanes and catalytic asymmetric allylation using chiral Lewis adds and bases. 

The additions of allylsilanes and -stannanes to carbonyl groups are usually catalyzed by a Lewis add. Therefore, chirality has been introduced either on the Lewis add [786, 816] or on the reagent [569, 698, 737, 739, 740, 1213-1217]. The use of a chiral Lewis add has been proposed by Yamamoto and coworkers [777a, 786, 791], Acyloxyboranes (R,R)- or (S,S)-3.9 (R = /-Pr, R = H or 3,5-(CF3)2C5H3) catalyze the asymmetric addition of allylsilanes and -stannanes to aldehydes at -78°C. Provided that R" H, the chemical yields and the selectivities of the reactions of allylsilanes are good only with PhCHO or unsaturated aldehydes [791], Better results are obtained from allylstannanes and any aldehyde [816] (Figure 6.53). 

Scheme 8.42. Effect of pressure on the equilibrium of the chiral Lewis add derived from TADDOL and on the Diels-Alder reaction.

Using Mgl2 ligand 406 as the chiral Lewis add, enantioselective radical alkylation of an MBH adduct furnished aldol produets 407 in good yields and selectivities without protection of the hydroxyl group. An unexpected reversal in enantioselectivity is observed between methyl and tert-butyl esters, that is, the selectivity in the hydrogen atom transfer is dependent on the size of the ester substituent, with smaller substituents providing better enantioselectivity (Scheme 3.179). ... 

High enantioselectivities are obtained using tartaric acid-derived boronate ester 31 in combination with lithium borohydride or sodium borohydride for asymmetric reduction of alkyl or aryl ketones. The chiral Lewis add is easUy prepared in one hour, and the resulting alcohols are obtained in enantiomeric excesses of 88-99% (Equation 46) [44]. 

Kobayashi, S., Ogino, T., Shimizu, H., Ishikawa, S., Hamada, T. and Manabe, K., Bismuth trillate hiral bipyridine complexes as water-compatible chiral Lewis adds, Org. Lett, 2005,7,4729—4731. 

Catalytic Asymmetric Induction with Chiral Lewis Adds... 

For a long period, chiral Lewis adds were the most studied catalysts. Only recently have chiral bases, Br0nsted acids, and N-heterocycUc carbenes been utilized as effective organocatalysts for the oxa-hetero-Diels-Alder reaction. 

Chiral Lewis Add-Catalyzed Cycloaddition-Dual Activation... 

Several chiral Lewis add catalysts have been used for asymmetric hetero Diels-Alder reactions. J0igensen applied diiral bis(oxazcdine)/Cn(n) catalyst to asymmetric hetero Diels-Alder... 

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