Big Chemical Encyclopedia

Chemical substances, components, reactions, process design ...

Articles Figures Tables About

Reduction conjugate, enantioselective

After the initial two reports of Rh- and Co-catalyzed reductive aldol couplings, further studies did not appear in the literature until the late 1990s. Beyond 1998, several stereoselective and enantioselective reductive aldol reactions were developed, which are catalyzed by a remarkably diverse range of metal complexes, including those based upon Pd, Cu, Ir, and In. In this chapter, transition metal-catalyzed aldol, Michael, and Mannich reactions that proceed via transition metal hydride-promoted conjugate reduction are reviewed. [Pg.116]

Radical chemistry has seen tremendous progress in the past two decades and can now be considered as an eminent sub discipline in synthetic organic chemistry [1-6]. Diastereoselective radical chemistry is well established and many examples of enantioselective radical reactions have appeared in the recent literature. For reviews on diastereoselective radical chemistry see [7-11] for reviews on enantioselective radical chemistry see [12-16] and for reviews on conjugate additions, see [17,18]. This review will detail different ways to introduce asymmetry during a radical reaction. These transformations can be broadly classified into atom transfer reactions, reductive alkylations, fragmentations, addition and trapping experiments, and electron transfer reactions. [Pg.119]

Vinyl phosphine oxides and phosphonates are highly electron-deficient and can undergo conjugate reduction much like their carbonyl counter parts. The group of Andersson has reported an enantioselective reduction of both to yield chiral phosphine oxides and phosphonates (Table 6) [67]. [Pg.55]

Zhu and Burgess have reported an asymmetric conjugate reduction of 1,3-enol ether esters (Table 9) and 1,3-enol ether alcohols (Table 10) [72]. Initial reaction conditions reached full conversion of E-l-methoxy-l-phenylethene using ligand 9 albeit with a very low enantioselectivity of 29%. [Pg.61]

Subsequent ion exchange of the metal cation with the quaternary ammonium ion catalyst provides a lipophilic ion pair (step 2), which either reacts with the requisite alkyl electrophile at the interface (step 3) or is partitioned into the electrophile-containing organic phase, whereupon alkylation occurs and the catalyst is reconstituted. Enantioselective PTC has found apphcation in a vast number of chemical transformations, including alkylations, conjugate additions, aldol reactions, oxidations, reductions, and C-X bond formations." ... [Pg.336]

Cordova has described a reductive Mannich protocol that proceeds with high chemo-, diastereo- and enantioselectivity [179]. Conjugate reduction of p,p-disub-stituted enal 118 with Hantzsch ester 119 in the presence of 30 (10 mol%) and benzoic acid (10 mol%) (63 h, -20 °C) followed by addition of a-iminoglyoxylate 120 and stirring for a further 24 h gave the product (121) with excellent levels of relative and absolute stereocontrol (10 1-50 1 dr 95-99% ee) (Scheme 49). [Pg.319]

Stephen L. Buchwald at MIT has reported (Org. Lett. 2004, 6, 4809) a complementary approach. Enantioselective conjugate reduction of the inexpensive 3-methylcyclopentenone 4 led to the silyl enol ether 5, Pd-mediated coupling of which with the aryl halide gave the product 6. 3-Methylcyclohexenone gave the analogous product in 84% . [Pg.87]

Asymmetric conjugate addition of hydride to W-enoylbomane- 10,2-sultams (28) was used to prepare homochiral p-branched alkanoic acids.282-283 Another method involves the preparation of such compounds via a catalytic enantioselective 1.4-reduction 284 in the presence of only 1 mol % of a chiral... [Pg.231]

Grignard additions, 9, 66 radical addition of zincs, 2, 401 zinc-containing reagent additions, 2, 398 Nitroolefins, enantioselective conjugate additions, 10, 382 (Nitrophenylthio)osmocenes, preparation, 6, 634 (Nitrophenylthio)ruthenocenes, preparation, 6, 634 Nitropyridines, reductive aminocarbonylation, 11, 543 Nitroso aldol reaction... [Pg.157]

The boron atom dominates the reactivity of the boracyclic compounds because of its inherent Lewis acidity. Consequently, there have been very few reports on the reactivity of substituents attached to the ring carbon atoms in the five-membered boronated cyclic systems. Singaram and co-workers developed a novel catalyst 31 based on dicarboxylic acid derivative of 1,3,2-dioxaborolane for the asymmetric reduction of prochiral ketones 32. This catalyst reduces a wide variety of ketones enantioselectively in the presence of a co-reductant such as LiBH4. The mechanism involves the coordination of ketone 32 with the chiral boronate 31 and the conjugation of borohydride with carboxylic acid to furnish the chiral borohydride complex 34. Subsequent transfer of hydride from the least hindered face of the ketone yields the corresponding alcohol 35 in high ee (Scheme 3) <20060PD949>. [Pg.620]

The Schollkopf nucleophile was also used in the enantioselective synthesis of -methyl-tryptophan. In this case the dihydropyrazine nucleophile undergoes conjugate addition to the sulfone 35a, which yields afl -methyltryptophan after reductive desulfonylation and hydrolysis <95JOC4978>... [Pg.110]


See other pages where Reduction conjugate, enantioselective is mentioned: [Pg.111]    [Pg.251]    [Pg.384]    [Pg.394]    [Pg.92]    [Pg.404]    [Pg.783]    [Pg.798]    [Pg.798]    [Pg.938]    [Pg.88]    [Pg.240]    [Pg.184]    [Pg.251]    [Pg.59]    [Pg.397]    [Pg.317]    [Pg.432]    [Pg.184]    [Pg.251]    [Pg.125]    [Pg.537]    [Pg.38]    [Pg.139]    [Pg.86]    [Pg.363]    [Pg.287]    [Pg.638]    [Pg.1434]   


SEARCH



Conjugate reduction

Conjugated enantioselectivity

Enantioselectivity conjugation

Reduction enantioselective

© 2024 chempedia.info