Asymmetric cycloaddition reaction catalyzed

Asymmetric Cycloaddition Reactions Catalyzed by Cinchona-Based Primary Amines 313... 

Jprgensen and co-workers (247) investigated the asymmetric 1,3-dipolar cycloaddition reaction catalyzed by bis(oxazoline)-copper(II) complexes. In the presence of 25 mol% 269c, nitrone (401) reacts with ethyl vinyl ether and methoxypropene to afford the [3 + 2] adducts in modest diastereoselectivity and high enantioselectivity, Eq. 217. Ethyl vinyl ether preferentially forms the exo adduct while methoxypropene prefers the endo mode for reasons that are unclear. 

In recent years, much work has been done on catalyzed and asymmetric cycloaddition reactions. In the presence of 5 mol% bismuth trichloride, the simple dienes 10 (R1 = R" = H R1 = H, R2 = Me or R1 = Me, R2 = H) react with diethyl mesoxalate to afford mixtures of the cycloadducts 11 and the products 12 of an ene-reaction (equation 13)11 12. 1,3-Cyclohexadiene and ethyl glyoxylate give solely the endo adduct 13 in 50% yield (equation 14)12. 

The [4-1-3] cycloaddition between 2-aminofuran and oxyallyl cations, followed by base-induced elimination of the resulting adducts, was used in the synthesis of 3-aminotropones <05TL8475>. A chiral Lewis acid-catalyzed [4t-3] cycloaddition between furans and nitrogen-stabilized oxyallyl cations derived from allenamides was developed. As depicted below, the Cj-symmetric salen-based ligand is the most effective in promoting this asymmetric cycloaddition reaction <05JA50>. 

The other recent examples of asymmetric syntheses involving Danishefsky s diene focused on hetero Diels-Alder reactions. Shibasaki and Feng separately reported asymmetric reactions with carbonyl dienophiles. Shibasaki demonstrated successful asymmetric reactions of ketones using a chiral Cu(I)-Walphos catalyst. Feng used a chiral A,iV -dioxide/In(OTf)3 catalyst in asymmetric cycloaddition reactions of aldehydes. Imine dienophiles are also amenable to asymmetric Diels-Alder reactions with Danishefsky s diene. Wulff reported enantioselective reactions using a VAPOL-B(OPh)3 catalyst system, while Snapper and Hoveyda disclosed silver-catalyzed enantioselective aza Diels-Alder reactions. ... 

A Pd-catalyzed asymmetric cycloaddition reaction of a chiral (/3-snlfinyl)vinylcyclo-propane derivative with acrylonitrile provides an optically active cyclopentane deriva-tiveJ" The asymmetric cycloaddition reaction of (5s)-15 with acrylonitrile was carried out under heating in THF for 3 h in the presence of Pd(PPh3)4 (0.1 equiv) and PPhs (0.2 equiv) to give stereoselectively (3/ ,4/ ,5s)-16 with 66% de (Scheme 6). 

Asymmetric Catalytic [2+2] Cycloaddition Reaction Using a Chiral Aluminum Complex In 2007, Canales and Corey provided the first enantioselective [2+2] cycloadditions of sUyl enol ethers with a, 3-unsaturated esters by catalytic amounts of aluminum bromide complex [21]. Aluminum catalyst 18 is conveniently generated in situ by the addition of a commercially available solution of aluminum bromide in CH2Br2 to a solution of the known oxazaborohdine component [22,23]. The results of enantioselective [2+2] cycloaddition reaction catalyzed by aluminum complex are summarized in Table 4.6. 

TABLE 7.20 Asymmetric Cycloaddition Reactions of 2-Benzopyrylinm-4-olate with Pyruvates Catalyzed by Sc(III)>PyBOX Complexes in the Presence of Additives"... 

TABLE 7.21 Asymmetric Cycloaddition Reactions of 2-Benzopyrylium-4-olate with a-Ketoesters Catalyzed by Sc(III)-PyBOX-i-Pr-TFA Complex"... 

SCHEME 7.27 Asymmetric cycloaddition reactions of 2-benzopyrylium-4-olate with 3-acryloyl-2-oxazolidinone catalyzed by PyBOX-Ph-Yb(IH) complex. 

S.2.2 Inverse Electron Demand Cycloaddition Chiral Lewis acid-catalyzed asymmetric cycloaddition reactions of carbonyl yhdes with electron-dehcient dipolarophiles described up to this point could be clas sified by the reaction controlled by the strongest interaction between highest occupied molecular orbital (HOMO) of the carbonyl ylides and LUMO of the dipolarophiles. It is known that inverse electron demand type cycloadditions of carbonyl ylides, which are controlled by the strongest interaction between the dipolarophile HOMO and the carbonyl ylide LUMO, also occur. In 2007, Suga et al. also reported that high enantioselectivities were obtained for the inverse electron... 

SCHEME 7.28 Asymmetric cycloaddition reactions of 2-benzopyrylium-4-olates with 3-(2-alkenoyl)-2-oxazolidinones catalyzed by (4S,5S)-PyBOX-4,5-Ph2-Yb(in) complex. 

TABLE 7.23 Asymmetric Cycloaddition Reactions of 3-Acyl-2-benzopyrylium-4-olates with Vinyl Ethers Catalyzed hy (R)-BINIM-4Me-2QN-Ni(II) Complex"... 

Catalytic asymmetric Diels-Alder reactions are presented by Hayashi, who takes as the starting point the synthetically useful breakthrough in 1979 by Koga et al. The various chiral Lewis acids which can catalyze the reaction of different dieno-philes are presented. Closely related to the Diels-Alder reaction is the [3-1-2] carbo-cyclic cycloaddition of palladium trimethylenemethane with alkenes, discovered by Trost and Chan. In the second chapter Chan provides some brief background information about this class of cycloaddition reaction, but concentrates primarily on recent advances. The part of the book dealing with carbo-cycloaddition reactions is... 

Chiral boron(III) Lewis acid catalysts have also been used for enantioselective cycloaddition reactions of carbonyl compounds [17]. The chiral acyloxylborane catalysts 9a-9d, which are also efficient catalysts for asymmetric Diels-Alder reactions [17, 18], can also catalyze highly enantioselective cycloaddition reactions of aldehydes with activated dienes. The arylboron catalysts 9b-9c which are air- and moisture-stable have been shown by Yamamoto et al. to induce excellent chiral induction in the cycloaddition reaction between, e.g., benzaldehyde and Danishefsky s dienes such as 2b with up to 95% yield and 97% ee of the cycloaddition product CIS-3b (Scheme 4.9) [17]. 

Asymmetric Metal-catalyzed 1,3-Dipolar Cycloaddition Reactions... 

The 1,3-dipoles consist of elements from main groups IV, V, and VI. The parent 1,3-dipoles consist of elements from the second row and the central atom of the dipole is limited to N or O [10]. Thus, a limited number of structures can be formed by permutations of N, C, and O. If higher row elements are excluded twelve allyl anion type and six propargyl/allenyl anion type 1,3-dipoles can be obtained. However, metal-catalyzed asymmetric 1,3-dipolar cycloaddition reactions have only been explored for the five types of dipole shown in Scheme 6.2. 

Finally, there is the enantioselectivity of the 1,3-dipolar cycloaddition reactions. This chapter is limited to describing only the metal-catalyzed asymmetric 1,3-dipolar cycloaddition reactions that involve non-chiral starting materials. The only fac-... 

Scheeren et al. reported the first enantioselective metal-catalyzed 1,3-dipolar cycloaddition reaction of nitrones with alkenes in 1994 [26]. Their approach involved C,N-diphenylnitrone la and ketene acetals 2, in the presence of the amino acid-derived oxazaborolidinones 3 as the catalyst (Scheme 6.8). This type of boron catalyst has been used successfully for asymmetric Diels-Alder reactions [27, 28]. In this reaction the nitrone is activated, according to the inverse electron-demand, for a 1,3-dipolar cycloaddition with the electron-rich alkene. The reaction is thus controlled by the LUMO inone-HOMOaikene interaction. They found that coordination of the nitrone to the boron Lewis acid strongly accelerated the 1,3-dipolar cycloaddition reaction with ketene acetals. The reactions of la with 2a,b, catalyzed by 20 mol% of oxazaborolidinones such as 3a,b were carried out at -78 °C. In some reactions fair enantioselectivities were induced by the catalysts, thus, 4a was obtained with an optical purity of 74% ee, however, in a low yield. The reaction involving 2b gave the C-3, C-4-cis isomer 4b as the only diastereomer of the product with 62% ee. 

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