Construction of Quaternary Stereocenters: New Perspectives through Enantioselective Michael Reactions

Abstract

Stereoselective C À C bond-forming reactions are of particular importance for the preparation of enantiopure natural compounds and pharmaceuticals. The wide variety of available chiral auxiliaries, reagents, and catalysts nowadays enables the generation of tertiary stereocenters in most cases without any difficulty. However, the approach to complex compounds with quaternary stereocenters is still a challenge for synthetic organic chemists, and every enantioselective procedure in which a fully substituted carbon center is constructed is of value.[1]The Michael reaction, the conjugated or 1,4-addition of enolates to acceptor-substituted olefins, is a fundamental CÀC-coupling reaction, which is catalyzed not only by Brønsted bases, [2] but also by a number of metal compounds. [3] An important breakthrough in the field of extensively investigated metal-catalyzed, enantioselective Michael reactions [4] was recently reported by Sodeoka and co-workers.[5] For the first time, quaternary stereocenters could be generated with > 90 % ee at a relatively high temperature of À20 8C in a Michael reaction of b-dicarbonyl compounds 1 with enones 2 using palladium(ii) as the catalytically active transition metal. The Pd II -diaquadiphosphane complexes 3 a and 3 b are formed with chiral ligands (R)-tol-binap and (R)-binap, respectively. Scheme 1 shows four typical reaction products 4 a-d as examples.Both cyclic (1 a) and acyclic (1 b) bketoesters react with methyl vinyl ketone (2 a) at À20 8C in a Michael reaction promoted by 3 a to give the corresponding products 4 a and 4 b in moderate to good yields with > 90 % ee. Noteworthy is the use of the sterically demanding tert-butyl and phenyl b-ketoesters. The conversion of b-diketone 1 c (X = CH 2 Ar) in the presence of catalyst 3 b (10 mol %) at À10 8C is an outstanding example of an asymmetric Michael addition to acyclic triketones such as 4 c with very high stereoselectivity, an achievement thus far not reported in the literature. Furthermore, the reaction of 1 d with substituted enone 2 b catalyzed by 3 b at À20 8C affords a mixture of two diastereoisomers (d.r. = 8:1), and an enantiomeric excess of 99 % for the major diastereomer of 4 d.The high efficiency of the method developed by Sodeoka and co-workers is clearly demonstrated in view of the work published so far: In 1975 Wynberg and co-workers applied cinchona alkaloids in base-catalyzed, enantioselective Michael reactions.[6] About 10 years later, the first of these reactions catalyzed by a transition metal, cobalt, was published by Brunner and Hammer.[7] Both methods result in quaternary stereocenters with up to 68 % ee, which was improved in the following years by Desimoni et al. to 75 % ee.

Acid-Base Catalysis of Chiral Pd Complexes: Development of Novel Catalytic Asymmetric Reactions and Their Application to Synthesis of Drug Candidates

“…Although the enantioselectivity varied according to the steric size of the substrates, the Michael adducts 7 were obtained with up to 90% ee. In contrast to the ordinary basic conditions under which no desired product was obtained due to instability of the 1,3-diketones and the produced triketones, this reaction system gave the desired triketones in good yield, 20) indicating the utility of these mild reaction conditions.…”

“…17,18) The most frequently examined Michael donors are readily enolizable compounds such as b-ketoesters and malonates, and construction of chiral tertiary carbon centers at the acceptor side has been well investigated. In contrast, as for the synthesis of chiral quaternary carbon centers, 19,20) which are created at the donor side, the number of general methods has been quite limited, and expanding the scope of available substrates was difficult until recently. Therefore the development of an efficient method applicable to various 1,3-dicarbonyl compounds would be extremely useful.…”

A Greener, Efficient Approach to Michael Addition of Barbituric Acid to Nitroalkene in Aqueous Diethylamine Medium

“…The Michael reaction is one of the most powerful tools for the formation of carbon-carbon bonds in organic synthesis [1][2][3][4][5][6]. The addition of various active methines compounds to nitroalkenes has received increased attention since the conjugated addition products are aliphatic nitro compounds.…”

Phosphonium Salts as Chiral Phase‐Transfer Catalysts: Asymmetric Michael and Mannich Reactions of 3‐Aryloxindoles

“…[4] Specifically, the construction of a quaternary chiral center in 3-aryloxindoles is of crucial importance and has received extensive attention as this structural motif is a prominent feature in a number of biologically and pharmaceutically active natural products and molecules.…”