Asymmetric Diels-Alder reactions catalyzed

Table 1.2 Asymmetric Diels-Alder reactions catalyzed by CAB catalyst 3 [5a,b ...

Effect of water additive was examined in the asymmetric Diels-Alder reactions catalyzed by the J ,J -DBF0X/Ph-Ni(C104)2 complex. After addition of an appropriate amount of water to the anhydrous complex A, the reaction with an excess amount of cyclopentadiene was performed at room temperature. Enantioselectivity was as high as 93% ee for the endo cycloadduct up to five equivalents of water added and the satisfactory level of 88% ee was maintained when 10 equivalents were added. However, enantioselectivity gradually decreased with the increased amounts of water added 83 and 55% ee from 15 and 50 equivalents, respectively (Scheme 7.11). When the reaction temperature went down to -40 °C, the enantioselectivity as high as 98% ee resulted up to 15 equivalents of water additive. The effect of methanol at room temperature was even more surprising. In the presence of 15 and 100 equivalents of methanol, high levels of enantioselectivities of 88% and 83% ee, respectively, were recorded for the reactions at room temperature. 

Keywords Lewis acids, asymmetric Diels-Alder reactions catalyzed by chiral... 

Kiindig EP, Bourdin B, Bemardinelli G (1994) Asymmetric Diels-Alder reactions catalyzed by a chiral iron Lewis acid. Angew Chem Int Ed 33 1856-1858... 

Corey, E. J. Shibata, T. Lee, T. W. Asymmetric Diels-Alder Reactions Catalyzed by a Triflic Acid Activated Chiral Oxazaborolidine. J. Am. Chem. Soc. 2002,124,3808-3809. 

Kobayashi reported an asymmetric Diels-Alder reaction catalyzed by a chiral lanthanide(III) complex 24, prepared from ytterbium or scandium triflate [ Yb(OTf)3 or Sc(OTf)3], (Zf)-BINOL and tertiary amine (ex. 1,2,6-trimethylpiperidine) [30], A highly enantioselective and endose-lective Diels-Alder reaction of 3-(2-butenoyl)-l,3-oxazolidin-2-one (23) with cyclopentadiene (Scheme 9.13) takes place in the presence of 24. When chiral Sc catalyst 24a was used, asymmetric amplification was observed with regard to the enantiopurity of (/ )-BINOL and that of the endoadduct [31 ]. On the other hand, in the case of chiral Yb catalyst 24b, NLE was affected by additives, that is, when 3-acetyl-l,3-oxazolidin-2-one was added, almost no deviation was observed from linearity, whereas a negative NLE was observed with the addition of 3-pheny-lacetylacetone. 

The stability of (BIPHEP)PtX2 compounds with respect to racemization over several hours at room temperature suggested that these compounds could be employed as catalysts at room temperature or below. To explore this possibility, [BlPHEP]Pt(OTf)2 was employed in the asymmetric Diels-Alder reaction as illustrated in equation (6). Freshly generated [(l )-BlPHEP]Pt(OTf)2 promoted the asynunetric Diels Alder reaction forming the product in 94 6 endo exo ratio with the ee of the major diastereomer of 92 94%. The enantiomeric excess of the catalyst [(l )-BlPHEP]Pt(OTf)2 was not diminished over the course of the reaction, as determined by quenching the reaction at >90% conversion with (5, 5 )-DPEN and subsequent P NMR analysis of the resulting mixture. The facial selectivity of the asymmetric Diels-Alder reaction catalyzed by [(/f)-BlPHEP]Pt(OTf)2 was the same as observed with [(/ )-BINAP]Pt(OTf)2 with the same... 

Asymmetric catalysis by bimetallic catalysts is currently a research field of great interest. There have been few studies of the interaction between bidentate Lewis acids and carbonyl groups [22a]. Reilly and Oh [22c] have reported the only example of such an asymmetric Diels-Alder reaction catalyzed by a 1 1 complex of N-tosyltrypto-phan [17-19] with l,8-naphthalenediylbis(dichloroborane), in which the two Lewis-acidic sites work in a cooperative manner (Eq. 22). 

Several researchers have reported synthetic approaches based on asymmetric Diels-Alder reactions catalyzed by TADDOL-Ti complexes [117-120]. Dendritic [121] and polymer-supported TADDOL-Ti complexes [122] have also been employed as recoverable and reusable catalysts to give comparatively high enantioselectivity. Transition-state models have been proposed independently by several groups for TADDOL-type titanium catalysis [121,123]. 

The Diels-Alder reaction of methacrolein with 1,3-dienol derivatives can also be catalyzed by the BINOL-derived titanium complex, although the catalyst must be freed from molecular sieves (MS) to give the endo adduct with high enantioselectivity (Sch. 50) [131], because MS act as achiral catalysts in the Diels-Alder reaction. The asymmetric Diels-Alder reaction catalyzed by the MS-free (MS-(-)) BINOL-Ti complex (L) can be applied naphthoquinone derivatives as dienophiles to provide entry to the asymmetric synthesis of tetra- and anthracyclinone [132] aglycones (Sch. 51). The sense of asymmetric induction is exactly the same as that observed in the presence of MS in the asymmetric catalytic reactions described above. 

Efficient asymmetric Diels-Alder reactions catalyzed by chiral Lewis acids have recently been reported [34]. Although rare earth compounds were expected to be promising Lewis acid reagents, few asymmetric reactions catalyzed by chiral rare earth Lewis acids were reported [35], although rare earth triflates, especially Yb(OTf)3 and Sc(OTf)3, are good catalysts in the Diels-Alder reactions of a variety of dienophiles with cyclic and acyclic dienes (as mentioned in Section 19.2.6). 

Recently, some efficient asymmetric Diels-Alder reactions catalyzed by chiral Lewis acids have been reported [67]. The chiral Lewis acids employed in these reactions are generally based on traditional acids such as titanium, boron, or aluminum reagents, and they are well modified to realize high enantioselectivi-ties. Although lanthanide compounds were expected to be Lewis acid reagents, only a few asymmetric reactions catalyzed by chiral lanthanide Lewis acids were reported. Pioneering work by Danishefsky et al. demonstrated that Eu(hfc)3 (an NMR shift reagent) catalyzed hetero-Diels-Alder reactions of aldehydes with si-loxydienes, but enantiomeric excesses were moderate [68]. 

Corey et al. have shown the asymmetric Diels-Alder reactions catalyzed by a triflic acid activated chiral oxazaborolidine (eq 45). Triflic acid has also been found to be an efficient catalyst (1 mol %) for the hetero-Diels-Alder reaction between aromatic aldehydes and unactivated dienes. ... 

The term aminocatalysis has been coined [4] to designate reactions catalyzed by secondary and primary amines, taking place via enamine and iminium ion intermediates. The field of asymmetric aminocatalysis, initiated both by Hajos and Parrish [5] and by Eder, Sauer, and Wiechert [6] in 1971, has experienced a tremendous renaissance in the past decade [7], triggered by the simultaneous discovery of proline-catalyzed intermolecular aldol [8] and Mannich [9] reactions and of asymmetric Diels-Alder reactions catalyzed by chiral imidazolidinones [10]. Asymmetric enamine and iminium catalysis have been influential in creating the field of asymmetric organocatalysis [11], and probably for this reason aminocatalytic processes have been the object of the majority of mechanistic smdies in organocatalysis. 

Proposed transition state for the asymmetric Diels-Alder reaction catalyzed by a Cu-box complex... 

Chiral Catalysts Containing Group 3 (Sc and Y) Metals Including Lanthanides and Actinides. Although lanthanide compounds were expected to be Lewis acid reagents and efficient asymmetric Diels-Alder reactions catalyzed by Lewis acids (containing aluminum and boron) were reported, only few asymmetric Diels-Alder reactions are known. 

ABSTRACT. With the aid of selected examples an overview is given of the development trends in highly discriminative reactions using novel Lewis acid catalysts. 1) Bulky organoaluminum reagent, methylaluminum 3,3 -bis(triphenylsilyl)-l,r-bi-2-naphthoxide, has been successfully utilized for enantioselective activation of carbonyl moiety. 2) A novel reactivity of acyloxyborane derivatives and their usefulness in organic synthesis is demonstrated. As an example, asymmetric Diels-Alder reaction catalyzed by new acyloxyboranes derived from chiral acids is described. 

Some efficient asymmetric Diels-Alder reactions catalyzed by chiral Y(OTf)3 complexes have been reported. Treatment of 3-acryloxyloxazolidin-2-one and eye lopentadiene in the presence of a chiral ferrocene-yttrium complex gave the Diels-Alder adduct in 99% conversion, 74 26 (endo.exo), smd 50% ee(endo) (eq 11). ... 

Fig. 8.147 Examples of asymmetric Diels-Alder reaction catalyzed by chiral ln(lll)-BINOL complex.

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