Chiral acyloxyboranes , catalysts

The Diels Alder reaction of aldehydes 74 with dienes 75 in the presence of chiral acyloxyborane (CAB) catalysts 76 provides enantioselectively chiral 4-dihydropyranones (Equation 3.22) after CF3CO2H treatment of the cycloadducts [74]. 

The coupling of the achiral stannane 20 and aldehyde 21 was achieved with fair to good enantioselectivity and fair yield using chiral catalysts. Ti-BINOL gave 52% e.e. and 31% yield, whereas an acyloxyborane catalyst (see p. 127) gave 90% e.e. and 24% yield.189... 

The first example of catalytic, enantioselective Sakurai reaction was effected by means of Yamamoto s chiral acyloxyborane (CAB) catalyst 96 (Table 11). The CAB catalysts are derived from tartartic acid, and 97 gave a better yield than 96. ... 

Yamamoto and coworkers have reported that the chiral acyloxyborane (CAB) catalyst (5) is efficient for the enantioselective Diels-Alder reaction between... 

Recently, Yamamoto et al. have shown that the chiral acyloxyborane complex 31 is an excellent catalyst for the asymmetric Mukaiyama condensation of simple silyl enol ethers (Scheme 8B1.19 Table 8B1.11 entries 1-7) [43], The syn-aldol adducts are formed preferentially with high enantiomeric excess regardless of the stereochemistry (EI7) of the silyl enol ethers, suggesting an extended transition state (entries 4, 7). This methodology has been... 

Acyloxyboranes. Yamamoto et a/.1 have used the known reactivity of borane with carboxylic acids to activate acrylic acids for Diels-Alder reactions. Thus addition of BH3-THF to acrylic acids at 0° furnishes an acyloxyborane formulated as 1, which undergoes cycloaddition (equations I and II). The reaction proceeds satisfactorily even when borane is used in catalytic amounts. A chiral acyloxyborane, BL, prepared from a tartaric acid derivative, can serve as a catalyst for an asymmetric Diels-Alder reaction (equation III). 

Most chiral organoboron Lewis acids reported to date are based on an organoborane that is attached to a chiral organic moiety such as a diol, aminoalcohol, or other readily available chiral substrates.Organoboron derivatives recently used as catalysts in enantioselective Diels-Alder reactions include the family of chiral acyloxyboranes (CAB) with (196) and (197) as representative examples and various cyclic boronic esters such as (198) and (199). An interesting system that combines the favorable Lewis acid properties of fluorinated arylboranes with a chiral Bronsted acid has been developed by Ishihara and Yamamoto. The Bronsted acid-assisted chiral Lewis acids (BLA) (200) was found to be highly effective in enantioselective cycloadditions of Q ,jS-enals with various dienes. The presence of the Bronsted acid functionality leads to significant acceleration of the reaction. 

Asymmetric Diels-Alder Reaction of Unsaturated Carboxylic Acids. A chiral acyloxyborane (CAB) complex (1) prepared from mono(2,6-dimethoxybenzoyl)tartaric acid and 1 equiv of borane is an excellent catalyst for the Diels-Alder reaction of a,p-unsaturated carboxylic acids and dienes. In the CAB-catalyzed Diels-Alder reaction, adducts are formed in a highly diastereo- and enantioselective manner under mild reaction conditions (eq 2). The reaction is catalytic 10 mol % of catalyst is sufficient for efficient conversion, and the chiral auxiliary can be recovered and reused. 

Chelation cf prochiral dienophiles by chiral boron, aluminum and titanium Lewis acids 4.13,33 Tartrate-derived chiral acyloxyborane catalysts... 

So far the most promising chiral Diels-Alder catalyst has been obtained in situ by treatment of monoacylated tartaric acid (470) with BHs-THF (1 mol equiv.)." The resulting non-isolated acyloxyborane was assumed to feature a five-membeied ring derived from the a-hydroxy acid moiety of (470) with die boron atom bound to the carboxylate and (Za-positioned oxygen atoms cf. formula 471) (Scheme 111, Table 32). 

Two related chiral Lewis acids are the acyloxyborane 68 [46] and the oxazabo-rolidine 69 [47] which are derived from (/ ,/ )-tartrate and L-valine, respectively. Each uses borane to generate the active catalyst, and they promote a variety of... 

Uncomplexed acrolein, methacrolein, and crotonaldehyde all favor the s-trans conformer, and this preference is enhanced upon complexation to a Lewis acid. For example, Corey showed that the BFj-methacrolein complex adopts the s-trans conformation in the solid state as well as in solution by crystallographic and NMR spectroscopic methods (Fig. 8B) [27], while Denmark and Almstead found that methacrolein adopts the s-trans geometry upon complexation with SnCl4 (Fig. 8C) [28]. Yamamoto demonstrated that methacrolein is also observed in the s-trans conformation upon complexation to his chiral acyloxybo-rane (CAB) catalyst (Fig. 14A and Sect. 3.1.2) [40]. Interestingly, with the same CAB system, crotonaldehyde exhibited varying preferences for the two possible conformers depending on the exact substituents on the boron. On the basis of NOE enhancements, the s-trans conformer was observed exclusively with a hydrogen substituent on boron (Fig. 14B) the s-cis conformer was the only one detected in the case of the aryl-substituted acyloxyborane (Fig. 14C). 

Chiral boronales are generated m situ by reaction of binaphthols 3.7 (R = H, Ph) [231] with BH3 in the presence of acetic acid [778], with H BBr [781] or with B(OPh)3 [782, 783], Chiral borates are formed by reactions of substituted (S)-prolinol derivative 2.13 (R =- CPl OH) and BBr3 [784], These boronates and borates are valuable catalysts in asymmetric Diels-Alder reactions [73, 231, 601, 780], Tartaric acid derivatives, such as borate 3.8 and acyloxyboranes 3.9 recommended by Yamamoto and coworkers [73,601,778,780,785-791], are very efficient catalysts in asymmetric Diels-Alder reactions and in condensations of aldehydes with allylsilanes, enoxysilanes or ketene acetals. These catalysts are generated in situ from substituted monobenzoates of (RJl)- or (S -tartaric acid and BH3 (R = H) or an arylboric acid (R = Ar). The best asymmetric inductions are observed with catalysts 3.9, R = /-Pr. 1,3,2-OxazaboroMnes 3.10, prepared from a-aminoacids [44, 601, 780, 792, 793], are efficient catalysts in asymmetric Diels-Alder reactions. The catalyst generated from A -tosyltrytophan 3.11 is more efficient than borolidines 3.10 (R = Et, /-Pr). The catalysts 3.10 prepared from 3.11, 3.12 and 3.13 are also useful in asymmetric condensations of aldehydes with ketene acetals [794-797]. 

The chiral acyloxyborane 7 (CAB) has also been found to be an excellent catalyst for asymmetric Mukaiyama-Michael type aldol reaction between silyl enol ethers and aldehydes (Scheme 8). Yamamoto et al. [27] have used 20 mol % of CAB in propionitrile at -78 °C as a highly efficient catalyst for the condensation of several E and Z silyl enol ethers and ketene acetals with a variety of aldehydes (yields 49-97 %, 80-97 % ee). 

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. 

With these experimental results at hand, it became of interest to evaluate the inducing ability of appropriate chiral auxiliary by introducing them into the acyloxyborane intermediate. Reaction of the monacylated tartaric acid with one equiv of borane should result in the formation of chiral acyloxyborane intermediate. Acrylic acid was then added to this catalyst and the mixture was treated with cyclopentadiene at -78 C After being stirred for 24 h, the product was isolated in high yield. The product showed a specific rotation of -86 which corresponded to 78% optical yield. 

Hisashi Yamamoto has developed several chiral acyloxyboranes (CABs) as catalysts in various organic transformations, such as allylsilation (55) Diels-Alder reaction (56), aldol reaction (57) (Figure 13), etc. 

Since our group (22) and Hehnchen s (23) independently announced a new class of chiral acyloxyboranes derive from iV-sulfonylamino acids and borane THF, chiral 1,3 -oxazaborolidines, their utility as chiral Lewis acid catalysts in enantioselective synthesis has been convincingly demonstrated (2(5). In particular, Corey s tryptophan-derived chiral oxazaborolidines 10a and 10b are highly effective for not only Mukaiyama aldol reactions (24) but also Diels-Alder reactions (25). More than 20 mol% of 10b is required for the former reaction, however. Actually, the reaction of the trimethylsilyl enol ether derived from cyclopentanone with benzaldehyde afforded the aldoI products in only 71% yield even in the presence of 40 mol%of 10b (24). We recently succeed in renewing 10b as a new and extremely active catalyst lOd using arylboron dichlorides as Lewis acid components (2(5). 

Fig. 5.2 Chiral acyloxyborane catalysts used in Diels-Alder cycloaddition...

Reetz and coworkers introduced the cyclic chlorodialkylboron Lewis acid (75) (Equation 48) [46], and Kiyooka and coworkers made use of acyloxyborane (76) (Equation 49) [47] in enantioselective Mukaiyama-aldol reactions that employ stoichiometric amounts of the respective boron Lewis acids. Both species give high enantioselectivity in the formation of the desired aldol adducts. After Kiyooka s report of (76), various boron catalysts derived from chiral amino acids appeared in the literature. As such, Masamune and coworkers introduced (77) and (78) [48], Kiyooka and co workers introduced (79) [49], and Corey and co workers introduced (80) [50] as chiral acyloxy borane catalysts for enantioselective aldol reactions (Figure 5.7). 

More than a decade ago, Yamamoto and coworkers demonstrated that the chiral acyloxyborane (6) is an excellent catalyst for the Sakurai-Hosomi allylation of aldehydes to furnish the homoallylic alcohols in good yields and enantioselectivities (Equation 60) [59]. This system remains to date, the most effective catalytic enantioselective allylation of aldehydes based on boron Lewis acids. 

The chiral acyloxyboranes (CABs) can also be employed as catalysts for the addition of allylstannanes to aldehydes. Marshall described an approach to kallolide A, in which regio-, diastereo-, and enantioselective addition of stan-nane 192 to aldehyde 191 takes place to afford 194 in 90% ee (Equation 12) [121]. 

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