Tartarate derived boronates

High enantioselectivities (up to 94%) are obtained in the sodium borohydride reduction of aliphatic ketones using a tartaric acid-derived boronic ester (TarB-N02) as a chiral catalyst. A mechanism (Scheme 14) involving an acyloxyborohydride intermediate has been postulated.319... 

High enantioselectivities are obtained using tartaric acid-derived boronate ester 31 in combination with lithium borohydride or sodium borohydride for asymmetric reduction of alkyl or aryl ketones. The chiral Lewis add is easUy prepared in one hour, and the resulting alcohols are obtained in enantiomeric excesses of 88-99% (Equation 46) [44]. 

The directive effect of allylic hydroxy groups can be used in conjunction with chiral catalysts to achieve enantioselective cyclopropanation. The chiral ligand used is a boronate ester derived from the (VjA jA N -tetramethyl amide of tartaric acid.186 Similar results are obtained using the potassium alkoxide, again indicating the Lewis base character of the directive effect. 

Stable aryl boronates derived from tartaric acid catalyze the reaction of cyclo-pentadiene with vinyl aldehyde with high selectivity. Chiral acyloxy borane (CAB), derived from tartaric acid, has proved to be a very powerful catalyst for the enantioselective Diels-Alder reaction and hetero Diels-Alder reaction. Scheme 5 23 presents an example of a CAB 73 (R = H) catalyzed Diels-Alder reaction of a-bromo-a,/i-cnal 74 with cyclopentadiene. The reaction product is another important intermediate for prostaglandin synthesis. In the presence of... 

Yamamoto and colleagues developed achiral boron catalysts 379 and 380a-b derived from monoacylated tartaric acid and BH3 -THF as shown for 379 in equation 112. The cycloaddition of cyclopentadiene to acrylic acid (381) afforded endo 382 with 78% ee and 93% yield when catalyst 379 was employed (equation 113)239. 

The characteristic feature of the aforementioned oxazaborolidine catalyst system consists of a-sulfonamide carboxylic acid ligand for boron reagent, where the five-membered ring system seems to be the major structural feature for the active catalyst. Accordingly, tartaric acid-derived chiral (acyloxy)borane (CAB) complexes can also catalyze the asymmetric Diels-Alder reaction of a,P-unsaturated aldehydes with a high level of asymmetric induction [10] (Eq. 8A.4). Similarly, a chiral tartrate-derived dioxaborolidine has been introduced as a catalyst for enantioselective Diels-Alder reaction of 2-bromoacrolein [11] (Eq. 8A.5). 

The B-alkylated CAB catalyst (3) is easily prepared in situ by mixing a 1 1 molar ratio of tartaric acid derivative and phenyl-boronic acid in dry propionitrile at room temperature for 0.5 h. The hetero Diels-Alder reaction of aldehydes with Danishefsky dienes is promoted by 20 mol % of this catalyst solution at —78 °C for several hours to produce dihydropyranone derivatives of high optical purity (eq 9). 

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). 

We have developed a stable CAB 2 (R = aryl) complex that can be prepared in situ by mixing tartaric acid derivative and arylboronic acid at room temperature. In contrast with 2, R = H, which is both air- and moisture-sensitive, the B-alkylated catalyst 2, R = aryl or alkyl, is stable and can be stored in closed containers at room temperature (Eq. 39). A solution of the catalyst (20 mol %) is effective in catalyzing the hetero Diels-Alder reaction of aldehydes with a Danishefsky diene to produce dihydro-pyrone derivatives of high optical purity (up to 98 % ee) (Eq. 40) [39]. The extent of asymmetric induction is largely dependent on the structure of the boronic acid. In general, bulky phenylboronic acid (R = 2,4,6-Me3CeH2, o-MeOC6H4) results in excellent asymmetric induction. 

CAB 2, R = H, derived from monoacyloxytartaric acid and diborane is also an excellent catalyst (20 mol %) for the Mukaiyama condensation of simple enol silyl ethers of achiral ketones with various aldehydes. The reactivity of aldol-type reactions can, furthermore, be improved, without reducing the enantioselectivity, by use of 10-20 mol % of 2, R = 3,5-(CF3)2C6H3, prepared from 3,5-bis(trifluoromethyl)phenyl-boronic acid and a chiral tartaric acid derivative. The enantioselectivity could also be improved, without reducing the chemical yield, by using 20 mol % 2, R = o-PhOCgH4, prepared from o-phenoxyphenylboronic acid and chiral tartaric acid derivative. The CAB 2-catalyzed aldol process enables the formation of adducts in a highly diastereo- and enantioselective manner (up to 99 % ee) under mild reaction conditions [47a,c]. These reactions are catalytic, and the chiral source is recoverable and re-usable (Eq. 62). 

Several arylboronic acids have been examined in place of borane-THF to improve the Lewis acidity of 2 and the stereoselectivity [49b]. The boron substituent of 2 has a large effect on the chemical yield and the enantiomeric excess of the allylation adduct, and 3,5-bistrifluoromethylbenzeneboronic acid results in the greatest reactivity— when a complex which is easily prepared from a tartaric acid derivative and 3,5-bistri-fluoromethylbenzeneboronic acid in propionitrile at room temperature is used, the reactivity is improved without reducing the enantioselectivity. For instance, the reaction of l-trimethylsilyl-2-methyl-2-propene with benzaldehyde in the presence of only 10 mol % 2 proceeds to give 99 % yield and 88 % ee (Fig. 19). 

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]. 

Finally, a heterocyclic derivative of tartaric acid should be mentioned, w hich has been used for the construction of boron enolates useful for allyl additions to carbonyl compounds (Section D. 1,3.3.3.). The eight-membered ring is formed from 2,3-O-benzylidenetartaric acid and lV,./V -dibenzyl-l,2-ethanediamine with the help of. V-methyl-2-chloropyridinium iodide as the condensing agent43. 

Chiral boron complexes derived from tartaric acid derivatives and from P-aminoalcohols act as mild Lewis acids and asymmetric catalysts for the reaction of aldehydes (94T979) and methyl glyoxalate (94CC1563) with Danishefsky dienes. The resulting 2,3-dihydropyran-4-ones are obtained in high optic purity. 

The use of chiral boronic esters in the Petasis borono-Mannich reaction has been reported to result in low levels of enantioselectivity of the adducts at room temperature (6-15% ee) [48]. Auxiliaries used in this study by Scobie and co-workers included pinanediol and tartaric acid derived alkenylboronates. Morpholine was the only secondary amine used, with the primary amine ethyl glycinate failing to react. 

Example 4.6 In the previous example catalytic complex VIII is a derivative of tartaric acid. More easily accessible is boronate X, derived from V-tosyl-L-tyrosine IX. This catalyst proved highly effective in the synthesis of (35)-a-hydroxy ketones 26-30 starting from silyl enolates and aldehydes (Scheme 4.14) [14]. 

Other enantiomerically pure B-allylboranes also show excellent stereoselectivity in these reactions." Allyl and 2-butenyl derivatives of the cyclic boronate ester 3, derived from tartaric acid, also give enantioselective additions to aldehydes. ... 

Yamamoto developed a remarkable boron-derived catalyst for enantioselec-tive Diels-Alder reactions which is easily assembled from monoacylated tartaric acid and borane. Spectroscopic data provided evidence that supports the proposed catalyst structure 144 depicted in Equation 16 [79, 80]. Such chiral (acyloxy)borane (CAB) catalysts have been employed in numerous cyclo-additions with unsaturated aldehydes to afford the corresponding products, such as 145, with high selectivity (98% ee, endo exo > 99 1) [80]. 

---