Tryptophane-derived catalysts, addition with

The iminium activation strategy with the trifunctional chiral primary amine 28, which was used in the y-selective conjugate addition of y-butenolides (Scheme 28), also proved highly effective for promoting reaction with the y-butyrolactam (Scheme 32, first line) [52]. Although various chiral diamines, amino thioureas, and amino sulfonamides were evaluated during the optimization study, none of these catalysts gave rise to impressive levels of diastereoselectivity. Eventually, a bulky acid additive, such as the protected tryptophan derivative (V-Boc-L-Trp-OH),... 

Oxamborolidenes. There are noteworthy advances in the design, synthesis, and study of amino acid-derived oxazaborolidene complexes as catalysts for the Mukaiyama aldol addition. Corey has documented the use of complex 1 prepared from A-tosyl (S)-tryptophan in enantioselective Mukaiyama aldol addition reactions [5]. The addition of aryl or alkyl methyl ketones 2a-b proceeded with aromatic as well as aliphatic aldehydes, giving adducts in 56-100% yields and up to 93% ee (Scheme 8B2.1, Table 8B2.1). The use of 1-trimethylsilyloxycyclopentene 3 as well as dienolsilane 4 has been examined. Thus, for example, the cyclopentanone adduct with benzaldehyde 5 (R = Ph) was isolated as a 94 6 mixture of diastereomers favoring the syn diastereomer, which was formed with 92% ee, Dienolate adducts 6 were isolated with up to 82% ee it is important that these were shown to afford the corresponding dihydropyrones upon treatment with trifuoroacetic acid. Thus this process not only allows access to aldol addition adducts, but also the products of hetero Diels-Alder cycloaddition reactions. 

In recent years, catalytic asymmetric Mukaiyama aldol reactions have emerged as one of the most important C—C bond-forming reactions [35]. Among the various types of chiral Lewis acid catalysts used for the Mukaiyama aldol reactions, chirally modified boron derived from N-sulfonyl-fS)-tryptophan was effective for the reaction between aldehyde and silyl enol ether [36, 37]. By using polymer-supported N-sulfonyl-fS)-tryptophan synthesized by polymerization of the chiral monomer, the polymeric version of Yamamoto s oxazaborohdinone catalyst was prepared by treatment with 3,5-bis(trifluoromethyl)phenyl boron dichloride ]38]. The polymeric chiral Lewis acid catalyst 55 worked well in the asymmetric aldol reaction of benzaldehyde with silyl enol ether derived from acetophenone to give [i-hydroxyketone with up to 95% ee, as shown in Scheme 3.16. In addition to the Mukaiyama aldol reaction, a Mannich-type reaction and an allylation reaction of imine 58 were also asymmetrically catalyzed by the same polymeric catalyst ]38]. 

Chiral boranes have been recommended as Lewis acids catalysts by Reetz [689], Yamamoto [787, 788], Kiyooka [795, 1302], Masamune and their coworicers [796, 797], These groups used, respectively, boranes 2.61, 3.9 (R = H, R = /-Pr), 3.10 (R = i-Pr or tert-Bu, R = H) and derivatives of 3.12 and 3.13. These boranes are very efficient catalysts in asymmetric additions of symmetrically substituted ketene silylacetals 6.113 to aldehydes (Figure 6.94). Similar reactions can also be conducted with enoxysilanes derived from methylketones or from tert-Bu thiolacetate [787, 794, 796], Oxazaborolidine 3.10 derived from tryptophan 3.11 is also a very potent catalyst [794],... 

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