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Oxazaborolidinone catalysts

The polymer-supported chiral oxazaborolidinone catalyst 5 prepared from valine was found by Ituno and coworkers to be a practical catalyst of the asymmetric Diels-Alder reaction [7] (Scheme 1.12). Of the several cross-linked polymers with a... [Pg.10]

In an analogous study by Meske, the impact of various oxazaborolidinone catalysts for the 1,3-dipolar cycloaddition reactions between acyclic nitrones and vinyl ethers was studied [31]. Both the diastereo- and the enantioselectivities obtained in this work were low. The highest enantioselectivity was obtained by the application of 100 mol% of the tert-butyl-substituted oxazaborolidinone catalyst 3d [27, 32] in the 1,3-dipolar cycloaddition reaction between nitrone la and ethyl vinyl ether 8a giving endo-9a and exo-9a in 42% and 27% isolated yield, respectively, with up to 20% ee for endo-9a as the best result (Scheme 6.10). [Pg.219]

The reactions of nitrones constitute the absolute majority of metal-catalyzed asymmetric 1,3-dipolar cycloaddition reactions. Boron, aluminum, titanium, copper and palladium catalysts have been tested for the inverse electron-demand 1,3-dipolar cycloaddition reaction of nitrones with electron-rich alkenes. Fair enantioselectivities of up to 79% ee were obtained with oxazaborolidinone catalysts. However, the AlMe-3,3 -Ar-BINOL complexes proved to be superior for reactions of both acyclic and cyclic nitrones and more than >99% ee was obtained in some reactions. The Cu(OTf)2-BOX catalyst was efficient for reactions of the glyoxylate-derived nitrones with vinyl ethers and enantioselectivities of up to 93% ee were obtained. [Pg.244]

Entry 9 uses the oxaborazolidine catalysts discussed on p. 505 with 2-bromopropenal as the dienophile. The aldehyde adopts the exo position in each case, which is consistent with the proposed TS model. Entry 10 illustrates the use of a cationic oxaborazolidine catalyst. The chirality is derived from trans-1,2-diaminocyclohcxanc. Entry 12 shows the use of a TADDOL catalyst in the construction of the steroid skeleton. Entry 13 is an intramolecular D-A reaction catalyzed by a Cu-Ws-oxazoline. Entries 14 and 15 show the use of the oxazaborolidinone catalyst with more complex dienes. [Pg.518]

Development of highly enantioselective oxazaborolidinone catalysts for the reactions of acyclic a,p-unsaturated ketones 07SL1823. [Pg.85]

The Diels-Alder reaction was performed with cyclopentadiene and methacroleine (Scheme 56) in presence of 15 mol% of the oxazaborolidine and oxazaborolidinone catalysts derived respectively from supported aminoalcohols and from V-sulfonylamino acid polymers. The oxazaborolidine and oxazaborolidinone catalysts were formed in situ by action of BH3, BH2Br, BHBr2 or BBrs. The diastereoselectivity was excellent in favour of the exo adduct and yields from 65 to 99%. It is noteworthy that higher loading of chiral catalyst site in the polymer, lower exo selectivity and enantioselectivity. The diastereoselection depended not only on the nature of the supported ligand, the crosslinker but also on the borane and the solvent. Results are summarized in Table 6. [Pg.88]

A vinylogous Mukaiyama of an aldehyde with an aldehyde-derived dienolate gives access to a-substituted S-hydroxy-a./S-unsaturated aldehydes in up to 94% ee, using chiral oxazaborolidinone catalysts. ... [Pg.23]

An optically active polymer has been prepared by repetitive Mukaiyama aldol reaction, using a bis(silyl ketene acetal) and a dialdehyde, plus a chiral oxazaborolidinone catalyst,and the role of Lewis basicity of aldehydes in oxazaborolidinone-catalysed Mukaiyama aldols has been examined." ... [Pg.21]

Scheeren et al. reported the first enantioselective metal-catalyzed 1,3-dipolar cycloaddition reaction of nitrones with alkenes in 1994 [26]. Their approach involved C,N-diphenylnitrone la and ketene acetals 2, in the presence of the amino acid-derived oxazaborolidinones 3 as the catalyst (Scheme 6.8). This type of boron catalyst has been used successfully for asymmetric Diels-Alder reactions [27, 28]. In this reaction the nitrone is activated, according to the inverse electron-demand, for a 1,3-dipolar cycloaddition with the electron-rich alkene. The reaction is thus controlled by the LUMO inone-HOMOaikene interaction. They found that coordination of the nitrone to the boron Lewis acid strongly accelerated the 1,3-dipolar cycloaddition reaction with ketene acetals. The reactions of la with 2a,b, catalyzed by 20 mol% of oxazaborolidinones such as 3a,b were carried out at -78 °C. In some reactions fair enantioselectivities were induced by the catalysts, thus, 4a was obtained with an optical purity of 74% ee, however, in a low yield. The reaction involving 2b gave the C-3, C-4-cis isomer 4b as the only diastereomer of the product with 62% ee. [Pg.218]

In an extension of this work Scheeren et al. studied a series of derivatives of N-to-syl-oxazaborolidinones as catalysts for the 1,3-dipolar cycloaddition reaction of 1 with 2b [29]. The addition of a co-solvent appeared to be of major importance. Catalyst 3b was synthesized from the corresponding amino acid and BH3-THF, hence, THF was present as a co-solvent. In this reaction (-)-4b was obtained with 62% ee. If the catalyst instead was synthesized from the amino acid and... [Pg.218]

In a more recent work the same research group has applied cyclic and acyclic vinyl ethers in the oxazaborolidinone-catalyzed 1,3-dipolar cycloaddition reaction with nitrones [30]. The reaction between nitrone 5 and 2,3-dihydrofuran 6 with 20 mol% of the phenyl glycine-derived catalyst 3c, gave the product 7 in 56% yield as the sole diastereomer, however, with a low ee of 38% (Scheme 6.9). [Pg.219]

Enantioselective Catalysis of the Aldol Addition Reaction. There are also several catalysts that can effect enantioselective aldol addition. The reactions generally involve enolate equivalents, such as silyl enol ethers, that are unreactive toward the carbonyl component alone, but can react when activated by a Lewis acid. The tryptophan-based oxazaborolidinone 15 has proven to be a useful catalyst.148... [Pg.125]

An oxazaborolidinone derived from valine is also an effective catalyst. In one case, the two enantiomeric catalysts were completely enantioselective for the newly formed... [Pg.126]

Scheme 2.9 gives some examples of use of enantioselective catalysts. Entries 1 to 4 are cases of the use of the oxazaborolidinone-type of catalyst with silyl enol ethers and silyl ketene acetals. Entries 5 and 6 are examples of the use of BEMOL-titanium catalysts, and Entry 7 illustrates the use of Sn(OTf)2 in conjunction with a chiral amine ligand. The enantioselectivity in each of these cases is determined entirely by the catalyst because there are no stereocenters adjacent to the reaction sites in the reactants. [Pg.131]

A number of other chiral catalysts can promote enantioselective conjugate additions of silyl enol ethers, silyl ketene acetals, and related compounds. For example, an oxazaborolidinone derived from allothreonine achieves high enantioselectivity in additions of silyl thioketene acetals.323 The optimal conditions for this reaction also include a hindered phenol and an ether additive. [Pg.195]

Other effective enantioselective catalysts include Yb(OTf)3 with BINOL,160 Mg2+-to-oxazolines,161 and oxazaborolidinones.162... [Pg.537]

Figure 39 The enantioselective polymer-supported catalysts (61) of chiral oxazaborolidinone with cross-linking structures for use in the Diels-Alder reaction of methacrolein with cyclopentadiene. (Adapted from ref. 85.)... Figure 39 The enantioselective polymer-supported catalysts (61) of chiral oxazaborolidinone with cross-linking structures for use in the Diels-Alder reaction of methacrolein with cyclopentadiene. (Adapted from ref. 85.)...
The same reaction has also been catalyzed by chiral oxazaborolidinones derived from amino acids and boranes. They proved to be efficient catalysts for enantioselec-tive Diels-Alder reaction [91,92]. The polymer-supported chiral oxazaborolidinones 75 were reported to be efficient catalysts [93]. These polymer-supported chiral oxaza-borolidinone ligands were prepared both by chemical modification and by the copolymerization shown in Sch. 5 [94]. The polymer-supported chiral ligands were then reacted with borane to give the oxazaborolidines which were used as catalysts in Diels-Alder reaction of cyclopentadiene with methacrolein. [Pg.968]

Chiral oxazaborolidinones supported on silica gel 77 have been prepared as shown in Sch. 6 [87]. Although high conversion was attained with these catalysts, enantioselectivity was low (8 % ee). [Pg.969]

Itsuno et al. have reported the use of polymer-supported oxazaborolidinone 102 as a chiral catalyst for asymmetric Diels-Alder reactions (Scheme 17)... [Pg.483]

Prolinol derivatives of boron tribromide 4 give excellent results in the [4 + 2] cycloaddition of methacrolein (8a) to cyclopentadiene (exojendo ratio 99 1, 97% ee)21. Several boron catalysts derived from /V-sulfonamides of a-amino acids 7a-d have been developed. In the cycloaddition of methacrolein (8a) or croton aldehyde (8b) to cyclopentadiene these catalysts produce enan-tioselectivities ranging from medium to good8-22 23. Chiral oxazaborolidinone 6 is an excellent catalyst for the addition of 2-bromoacrolein (8e) to various dienes24 (see table overleaf)-... [Pg.661]

Most recent research has been focused on the application of polymers as chiral auxiliaries in enantioselective Lewis-acid-catalyzed reactions. Studies of Itsuno and co-workers [44] culminated in the development of a polymer-supported catalyst containing a chiral oxazaborolidinone with oxyethylene crosslinkages which gave the Diels-Alder adduct of cyclopentadiene and methacrolein in 88 % isolated yield with an exotendo ratio of 96 4 and 95 % e. e. for the exo adduct. A variety of polymer-supported chiral Lewis acids was also investigated by Mayoral et al. [45]. Some supported catalysts were more active than their homogeneous analogs, but enantioselectivity was always lower. [Pg.291]

A variety of solid Lewis and Br0nsted acids has been shown to catalyze Diels-Alder reactions. In several instances the results obtained with heterogeneous catalysts were better than those with homogeneous Lewis acid catalysts. Most of the reported reactions of interest in the synthesis of fine chemicals were catalyzed by (modified) zeolites, clays, alumina, or silica. Catalysts with interesting properties were obtained when support materials such as zeolites, alumina, or silica were treated with Lewis acids. These catalysts were moderately selective in diastereo-selective Diels-Alder reactions with chiral dienophiles and induced enantioselec-tivity (up to 31 % e. e.) in the reaction of cyclopentadiene with methacrolein after treatment with chiral derivatives. Excellent enantioselectivity in this reaction (up to 95 % e. e.) was observed with a polymer-supported chiral oxazaborolidinone. Because of their facile recovery and recycling, we expect that solid-acid catalysts will find increasing use in Diels-Alder reactions in the future. [Pg.292]

Another chiral catalyst that was examined is an oxazaborolidinone derived from aZZo-threonine. The reaction proceeded in 80-95% yield and 80-95% ee. These conditions were apphcable to both 1- and 2-methyl indole, as well [247]. [Pg.82]

From all the oxazaborolidine catalyst, the L-valine derived catalyst 115a gave the highest enantioselectivity and an excellent ratio endo/exo. Moreover oxazaborolidinone polymeric catalysts having oxyethylene crosslinkages (116) present better performance. [Pg.88]

Recently, Harada and co-workers have developed aZZo-threonine-derived oxazaborolidinone (OXB) 127 as an efficient chiral Lewis acid catalyst for the activation of a,p-unsaturated ketones. They found that 127 could catalyze the AFC alkylation of furans and indoles with simple acyclic d,p-unsaturated ketones in high yields and excellent enantioselectivity (furans up to 99% yield, 91% ee indoles up to 96% yield, 94% ee) (Scheme 6.52). In this catalytic system, employing AZ,ZV-dimethylaniline as an additive was crucial to the reactivity and enantioselectivity of the reaction. [Pg.251]

Kiyooka, S. and Maeda, H. (1997) Enantioselective acyclic stereoselection under catalyst control. III. A very short asymmetric synthesis of the bryostatin C1-C9 segment using the chiral oxazaborolidinone-promoted aldol reaction. Tetrahedron Asymmetry,... [Pg.1929]

Corey designed and studied a ti7ptophan-derived oxazaborolidinone 158 as a powerful chiral catalyst for enantioselective Diels-Alder reactions (Scheme 17.23) [85, 86]. It was proposed that a r-stacking interaction between the... [Pg.567]


See other pages where Oxazaborolidinone catalysts is mentioned: [Pg.968]    [Pg.478]    [Pg.494]    [Pg.146]    [Pg.568]    [Pg.968]    [Pg.478]    [Pg.494]    [Pg.146]    [Pg.568]    [Pg.46]    [Pg.256]    [Pg.866]    [Pg.867]    [Pg.712]    [Pg.713]    [Pg.1997]    [Pg.23]    [Pg.25]    [Pg.402]    [Pg.316]    [Pg.369]   
See also in sourсe #XX -- [ Pg.23 , Pg.25 ]




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