Organoaluminum reagents chiral

In contrast, modest enantioselection has been observed in the asymmetric Diels-Alder reaction between cyclopentadiene (18) with methylacrylate and methylpropiolate catalyzed by chiral organoaluminum reagents 58 [59] (Equation 3.15) prepared from trimethylaluminum and (R)-(+)-3,3 -bis(triphenylsi-lyl)-l,l -bi-2-naphthol [60]. The reaction was highly cnJo-diastereoselective. 

These two reagents react to form a chiral organoaluminum reagent formulated as (R)- and (S)-l. 

The chiral trialkylaluminum reagent used in the majority of asymmetric reduction studies is (+ )-tris[(S)-2-methylbutyl]aluminum (119) or its etherates. This reagent is readily prepared from (S)-( + )-2-methyl-l-chlorobutane (152,153). Results of the reductions of prochiral ketones with these reagents and other chiral organoaluminum reagents are shown in Table 15. 

Asymmetric Reduction of Ketones with Chiral Organoaluminum Reagents... 

Yamamoto and Maruoka investigated the reaction of chiral acetals with organoaluminum reagents. Unprecedented regio- and stereochemical control was observed in the addition of trialkylaluminums to chiral a,/3-unsaturated acetals derived from optically pure tartaric acid diamide [83]. The course of the reaction seemed to be highly influenced by the nature of substrates, solvents, and temperature. These findings provide easy access to optically active a-substituted aldehydes (84), /3-substituted aldehydes (85), a-substituted carboxylic acids (86), or allylic alcohols (87). Because optically pure RJi)- and (5,5)-tartaric acid diamides are both readily available, this method enables the predictable synthesis of both enantiomers of substituted aldehydes, carboxylic acids, and allylic alcohols from a,/3-unsaturated aldehydes (Sch. 54). 

Kinetic resolution of chiral acetals has been effected by use of some organoaluminum reagents [84], On treating a chiral acetal 88, derived from (2, 4/ )-(-)-pentanediol, with -Bu3A1 at room temperature, one diastereomer was found to react much faster than the other, and the residu enol ether is transformed into optically pure ketone. The efficiency of this method is demonstrated by a concise synthesis of (5)-(-)-5-hexadecan-l,5-lactone (89), the pheromone of Vespa orientalis, as shown in Sch. 56. 

K. Maruoka, K. Nonoshita, M. Banno and H. Yamamoto, J. Am. Chem. Soc., 1988, 110, 7922. Most recently the first asymmetric Claisen rearrangement catalyzed by a chiral organoaluminum reagent has been reported K. Maruoka, M. Banno and H. Yamamoto, J. Am. Chem. Soc., 1990, 112, 7791. 

Maruoka, K., Hoshino, Y., Shirasaka, T., Yamamoto, H. Asymmetric ene reaction catalyzed by chiral organoaluminum reagent. Tetrahedron Lett. 1988, 29, 3967-3970. 

Based on the concept of the diastereoseiective activation of carbonyl groups with MAD or MAT, the bulky, chiral organoaluminum reagent, (/ )-BINAL or (5)-BINAL has been devised for enantioselective activation of carbonyl groups. 

The modified organoaluminum reagent, BINAL, can be used as a chiral Lewis acid catalyst in the asymmetric hetero-Diels-Alder reaction [41]. Reaction of various aldehydes with activated dienes under the influence of catalytic BINAL (5-10 mol%) at -20 °C gave, after exposure of the resulting hetero-Diels-Alder adducts to tri-fluroacetic acid, predominantly c/v-dihydropyrone in high yield with excellent enan-tioselectivity. 

Chloral. Yamamoto has found that the organoaluminum reagent prepared from MeaAl and (/ )-(+)-3,3 -bis(triphenylsilyl)binaphthol catalyzes the ene reaction of chloral and pentafluorobenzaldehyde with 1,1-disubstituted alkenes at -78 °C, giving the expected ene adducts in 40-90% yield and 60-90% enantiomeric excess. Use of the sterically hindered chiral auxiliary is necessary. Low yields of racemic products are obtained with 3,3 -diphenylbinaphthol. 

An excellent catalytic system is based on (R)- and (5)-2,2 -dihydroxybinaphthoPThe newly designed enantiomerically pure ( + )-(/ )-3,3 -bis(triarylsilyl)binaphthol reacts with trimethyl-aluminum to produce a chiral organoaluminum reagent 15 which is a very efficient catalyst for the hetero-Diels-Alder reaction. [4 + 2] Cycloaddition of aliphatic and aromatic aldehydes 2 to dienes 14 in the presence of 10 mol% of 15 yields m-dihydropyrones 3 together with a few percent of the trans-isomers with a diastereoselectivity as high as d.r. 98.5 1.5. 

An enantioselcctive activation of prostereogenic ether substrates is achieved with chiral organoaluminum reagents of type 1, which arc able to discriminate, for example, between the two chairlike transition states 2 A and 2B of the -configurated substrate ether 2. The method is chirally flexible, since both antipodal rearranged products can be synthesized by use of either (R)-l or (S)- 137" 139-653. 

Asymmetric Claisen Rearrangement of Allylic Vinyl Ethers with Chiral Organoaluminum Reagent (/f)-l or (5)-l (Ar = C6H5) General Procedure138 ... 

Recently, Maruoka reported a new chiral bimetallic organoaluminum reagent 10 for asymmetric Claisen rearrangement (Scheme 2.3) [9]. This reagent activates the allyl vinyl ether to promote rearrangement with double coordination to oxygen. 

ABSTRACT. With the aid of selected examples an overview is given of the development trends in highly discriminative reactions using novel Lewis acid catalysts. 1) Bulky organoaluminum reagent, methylaluminum 3,3 -bis(triphenylsilyl)-l,r-bi-2-naphthoxide, has been successfully utilized for enantioselective activation of carbonyl moiety. 2) A novel reactivity of acyloxyborane derivatives and their usefulness in organic synthesis is demonstrated. As an example, asymmetric Diels-Alder reaction catalyzed by new acyloxyboranes derived from chiral acids is described. 

Asymmetric Hetero-Diels Alder Reaction Catalyzed by Chiral Organoaluminum Reagent. Hetero-Diels-Alder reaction, an important organic transformation for the synthesis of a variety of heterocycles, was found to be catalyzed by the optically pure bulky aluminum reagent. ... 

Treatment of pentafluorobenzaldehyde and 2-(phenylthio)propene with chiral organoaluminum reagent at -78 C for 1 h gave the ene adduct in 90% yield. The enantiomeric excess was determined by HPLC analysis to be 88% ee after converting to its MTPA ester. Here use of stoichiometric amounts of the chiral Lewis acid is indispensable. Attempted reaction of pentafluorobenzaldehyde and 2-(phenylthio)propene under the influence of catalytic amounts of (R)-l (10-20 mol%) gave no reproducible result in chemical as well as optical yields. However, the presence of 4A molecular sieves (activated powder) is capable of making the reaction ca ytic. Thus, the ene reaction of... 

Generation of Chiral Organoaluminum Reagent by Discrimination of the Racemates with Chiral Ketone In the previous section, we have demonstrated a new, chiral organoaluminum catalyst which is highly effective for the introduction of chirality into... 

The racemic organoaluminum reagent was prepared in dichloromethane. Sequential treatment of this catalyst with chiral ketone, diene and benzaldehyde at low temperature and stirring of the mixture for 3 h afford hetero-Diels-Alder adduct after acidic workup. The optical purity of the major cis adduct was determined by HPLC analysis after conversion to the MTPA ester. 

Among several terpene derived chiral ketones examined, 3-d-bromocamphor was found to be most satisfactory and 3d-bromocamphor is responsible for the generation of (S)-catalyst. Combination of the racemic catalyst and chiral ketone in a 1 1 ratio gave a better results, suggesting that decomplexation of one enantiomeric organoaluminum reagent and the chiral ketone is more readily facilitated by the addition of aldehyde, thereby allowing the enantioselective activation of the aldehyde for the asymmetric hetero-Diels-Alder reaction. 

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