Symmetric catalytic synthesis

Chiral allylic amines constitute key synthetic intermediates, auxiliaries, and resolving agents in the synthesis of both natural and nonnatural products. In 2004, Jamison and Patel reported the first highly enantioselective catalytic synthesis of allylic amines from alkynes, imines, and organobo-ranes such as triethylborane. Catalysed by a chiral complex derived from Ni(cod)2 and chiral ferrocenylphosphine [R)-27, this novel three-component process provided chiral tetrasubstituted allylic amines in good yields in one-pot (Scheme 4.27). These products were obtained in moderate to very good enantioselectivities of up to 89% ee. Both symmetrical and... 

Example 9.4 Complete asymmetric synthesis of —)-chloramphenicol, (l/ ,2/ )-TM 9.2 is outlined in Scheme 9.6. An exemplary application of chiral organocatalysts is present in symmetric catalytic cyclization to aziridine 4 in the key step of the synthesis [11, 12]. Scheme 9.7 presents the detailed mechanism of chemo- and stereoselective rearrangement of chiral aziridine daivative 4 in the final intermediate 5. 

Ghosh et al. [70] reviewed a few years ago the utihty of C2-symmetric chiral bis(oxazoline)-metal complexes for catalytic asymmetric synthesis, and they reserved an important place for Diels-Alder and related transformations. Bis(oxazoline) copper(II)triflate derivatives have been indeed described by Evans et al. as effective catalysts for the asymmetric Diels-Alder reaction [71]. The bis(oxazoline) Ugand 54 allowed the Diels-Alder transformation of two-point binding N-acylimide dienophiles with good yields, good diastereos-electivities (in favor of the endo diastereoisomer) and excellent ee values (up to 99%) [72]. These substrates represent the standard test for new catalysts development. To widen the use of Lewis acidic chiral Cu(ll) complexes, Evans et al. prepared and tested bis(oxazoHnyl)pyridine (PyBOx, structure 55, Scheme 26) as ligand [73]. 

More recently, catalytic asymmetric allylations of imines and imine derivatives in aqueous media have been studied. An /V-spiro C2-symmetrical chiral quaternary ammonium salt (5,5)-I-Br (,S, .S )-()-Np-NAS-Br] has been evaluated in the allylation of glycine tert-Bu ester benzophenone Schiff base [Ph2C=NCH2COOCMe3] for synthesis of both natural and unnatural a-amino acids (Eq. 11,45).76... 

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. 

Tridehydro[18]annulene here and in the sequel refers to the symmetrical isomer shown in the formula, admixed with smaller quantities of an unsymmetrical isomer and tetradehydro[18]annulene.6 These can be separated by chromatography on alumina coated with 20% silver nitrate, but this is unnecessary for the synthesis of [18]annulene sinceallthree substances give this annulene on catalytic hydrogenation.8... 

Vaska s complex ([IrCl(CO)(PPh3)2]) also catalyzed the carbonylative coupling of diynes, which provided bicyclic cyclopentadienones (Scheme 11.23) [35]. Due to the instability of the products, the substrates are limited to symmetrical diynes with aromatic groups on their termini nonetheless, this reaction still serves as the catalytic and practical procedure for the synthesis of cyclopentadienones, which are anti-aromatic with a 47t system and serve as active synthetic intermediates. 

For a recent review on C2-symmetric chiral bis(oxazoUne)—metal complexes in catalytic asymmetric synthesis see Ghosh, A. K. Mathivanan, P CappieUo, J. Tetrahedron Asymmetry 1998, 9, 1-45. 

This enantioselective preparation of allylic alcohols has been applied to the synthesis of the side chain of prostaglandins . The addition to functionalized aldehydes, such as 483, allows the synthesis of C2-symmetrical 1,4-diols, such as 484, with excellent diastereoselectivity and enantioselectivity . An extension of this method allows the synthesis of C3-symmetrical dioF . Aldol-type products result from the catalytic enantioselective addition of functionalized dialkylzincs to 3-TIPSO-substituted aldehydes, such as 485, followed by a protection-deprotection and oxidation sequence affording 486 in 70% yield and 91% ee (Scheme 118) . The addition to a-alkoxyaldehydes provides a... 

---