Organocatalysts spiro

Currently, the chiral phase-transfer catalyst category remains dominated by cinchona alkaloid-derived quaternary ammonium salts that provide impressive enantioselec-tivity for a range of asymmetric reactions (see Chapter 1 to 4). In addition, Maruoka s binaphthyl-derived spiro ammonium salt provides the best results for a variety of asymmetric reactions (see Chapters 5 and 6). Recently, some other quaternary ammonium salts, including Shibasaki s two-center catalyst, have demonstrated promising results in asymmetric syntheses (see Chapter 6), while chiral crown ethers and other organocatalysts, including TADDOL or NOBIN, have also found important places within the chiral phase-transfer catalyst list (see Chapter 8). 

Highly enantioselective Michael addition of silyl nitronates (105) to cyclic o /3-unsaturated ketones (106 n = 0-2) has been accomplished by the utilization of N-spiro C2-symmetric chiral quaternary ammonium bifluoride (108) as an organocatalyst, offering a new route to the enol silyl ethers of scalemic y-nitro ketones (107 70-90% ee).156... 

Maruoka and co-workers developed an elegant solution by creating phase-transfer-catalysts of type 30 [36], For example, the C2-symmetric N-spiro organocatalyst (S,S)-30, which contains a conformationally flexible biphenyl subunit, efficiently catalyzed the alkylation of glycinate 18 with benzyl bromide, with formation of the product (R)-20b in 95% yield and with 92% ee (Scheme 3.10) [36],... 

A spiro-organocatalyst (27) bearing convergent Br0nsted acid and Lewis base moieties catalyses aza-MBH reaction of a./S-unsaturated ketones with A-tosyl benzaldimines at sub-zero temperatures. ... 

A quite elegant example of one-pot three-component reaction via a domino Michael/cyclization sequence was reported by Yuan et al. to provide a wide range of optically active spiro[4 f-pyran-3,3 -oxindoles] 117 [64]. Among all the tested chiral organocatalyst, cupreine (CPN) 116 resulted to be the most effective in terms of yield (up to 99%), enan-tioselectivity (up to 97% ee), and tolerance to the substrate modification (Scheme 2.34). 

The reaction between isatin derivatives 44, malonodini-trile 21, and CH acids 43 led to the formation of spiropyran compounds 46 by the application of a variety of catalysts. Yuan et al. reported the use of 10mol% cupreine 45 as an organocatalyst that led to spiro compounds with enhanced enantiomeric excess (Scheme 13.15) [23]. The reactions are conducted in dichloromethane or 1,2-dichloroethane as solvents at 0 °C, while the addition of 4 A molecular sieves provides shorter reaction times and higher ee s. 

An asymmetric one-pot sequential Mannich/hydroamination sequence involves a three-catalyst system a chiral organocatalyst, BF3 and a gold complex. It converts an indole-imine into privileged spiro[pyrrolidin-3,2 -oxindole] structures in up to 91/97% yield/ee. 

L-Proline can function as an efficient organocatalyst for the multicomponent synthesis of polyfunctionalized spiro[benzo[c]pyrano[3,2-fl]phenazine] derivatives [126]. L-Proline acts as a bifunctional catalyst for the one-pot... 

A. Hasaninejad, S. Firoozi, F. Mandegani, An efficient synthesis of novel spiro[benzo[c]pyrano[3,2-a]phenazines] via domino multi-component reactions using L-proline as a bifunctional organocatalyst. Tetrahedron Lett. 54 (2013) 2791-2794. 

The metal-free asymmetric amination of benzofuranones has been achieved using binol-derived P-spiro quaternary phosphonium salts as organocatalysts (Scheme 3.36) [34]. The amination reactions were carried out under mild conditions using (E)-dibenzyl diazene-l,2-dicarboxylate as the nitrogen source. After stirring for several days, outstanding yields of the aminated compounds were obtained. 

Spirodicycloalkylphenyl derivatives. The optically active l,l -spiro-biindane-7,7 -diol based phosphoric acids (200-206) were employed as organocatalysts for reactions of indoles (195) with aldimines (196) and 3,y-unsaturated-ot-ketoesters (197) in the synthesis of adducts (198) and (199), respectively (Scheme 54). ... 

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