Asymmetric silyl enol ethers

A useful catalyst for asymmetric aldol additions is prepared in situ from mono-0> 2,6-diisopropoxybenzoyl)tartaric acid and BH3 -THF complex in propionitrile solution at 0 C. Aldol reactions of ketone enol silyl ethers with aldehydes were promoted by 20 mol % of this catalyst solution. The relative stereochemistry of the major adducts was assigned as Fischer- /ir o, and predominant /i -face attack of enol ethers at the aldehyde carbonyl carbon atom was found with the (/ ,/ ) nantiomer of the tartaric acid catalyst (K. Furuta, 1991). 

Another useful method for the asymmetric oxidation of enol derivatives is osmium-mediated dihydroxylation using cinchona alkaloid as the chiral auxiliary. The oxidation of enol ethers and enol silyl ethers proceeds with enantioselectivity as high as that of the corresponding dihydroxylation of olefins (vide infra) (Scheme 30).139 It is noteworthy that the oxidation of E- and Z-enol ethers gives the same product, and the E/Z ratio of the substrates does not strongly affect the... 

Hagiwara et al.107 reported the chiral Pd(II) complex-catalyzed asymmetric addition of enol silyl ethers to imines, based on the belief that Pd(II) enolate was involved in the reaction. They found that with compound 171a as the catalyst, very low enantioselectivity was obtained in the asymmetric reactions between silyl enol ether and imine compounds (Scheme 3-58). However, in the... 

Ferraris et al.108 demonstrated an asymmetric Mannich-type reaction using chiral late-transition metal phosphine complexes as the catalyst. As shown in Scheme 3-59, the enantioselective addition of enol silyl ether to a-imino esters proceeds at —80°C, providing the product with moderate yield but very high enantioselectivity (over 99%). 

Following their success with chiral ketone-mediated asymmetric epoxidation of unfunctionalized olefins, Zhu et al.113 further extended this chemistry to prochiral enol silyl ethers or prochiral enol esters. As the resultant compounds can easily be converted to the corresponding a-hydroxyl ketones, this method may also be regarded as a kind of a-hydroxylation method for carbonyl substrates. Thus, as shown in Scheme 4-58, the asymmetric epoxidation of enol silyl... 

SCHEME 120. Lewis acid-catalyzed asymmetric aldol-type reaction of enol silyl ethers. 

Enantiotopos discrimination, 93, 128, 142, 234, 235, 331 Ene reactions asymmetric, 223 binaphthol, 222 chiral metal complexes, 222 intramolecular, 226 methyl glyoxylate, 290 Enol silyl ether substrates, 128, 228, 230 Enol substrates, 28 Enolates ... 

Further examination of the fluoride ion-catalyzed asymmetric aldol reaction of the enol silyl ethers prepared from acetophenones and pinacolone with benzaldehyde using 4b and its pseudoenantiomer 4c revealed the dependence of the stereochemistry of the reactions on the hydroxymethyl-quinudidine fragment of the catalyst (Table 9.3) [10,15]. 

Table 9.3 Asymmetric aldol reactions of enol silyl ethers with benzaldehyde catalyzed by chiral quaterna7 ammonium fluorides 4b or 4c.

Asymmetric aldol reactions.4 The borane complex 3 can also serve as the Lewis acid catalyst for the aldol reaction of enol silyl ethers with aldehydes (Mukaiyama reactions).5 Asymmetric induction is modest (80-85% ee) in reactions of enol ethers of methyl ketones, but can be as high as 96% ee in reactions of enol ethers of ethyl ketones. Moreover, the reaction is syn-selective, regardless of the geometry of the enol. However, the asymmetric induction is solvent-dependent, being higher in nitroethane than in dichloromethane. 

As discussed in Section III J, in general, catalytic asymmetric aldol reactions have been studied using enol silyl ethers, enol methyl ethers, or ketene silyl acetals as a starting material. So far several types of chiral catalysis have been reported.75-85 The chiral lanthanoid complex prepared from Ln(OTf)3 and a chiral sulfonamide ligand was effective in promoting an asymmetric Mukaiyama aldol reaction with a ketene silyl acetal.86 The preparation of the catalyst and a representative reaction are shown in Figure 45. 

Table 4.2 Asymmetric Michael addition of silyl nitronate 16 to a,/ -unsaturated aldehydes and cyclohexenone catalyzed by chiral quaternary ammonium bifluorides (/ ,/ )-15. Isolation of optically active enol silyl ethers 20 and 21.

Recent success was achieved in carrying out direct catalytic asymmetric aldol reactions of aldehydes with unmodified ketones [22]. No preconversion of the ketone moiety to a more reactive species such as an enol silyl ether or enol methyl ether is necessary. 

Stereoselective additions to chiral a- and -alkoxy aldehydes. Lewis-acid-catalyzed additions of enol silyl ethers to chiral ct-alkoxy or (3-alkoxy aldehydes can proceed with high 1,2- and 1,3-asymmetric induction. Moreover, the sense of induction can be controlled by the Lewis acid. Thus BF, which is nonchelating, can induce diastereo-... 

Asymmetric Aldol-Type Reaction. CAB complex (2) is an excellent catalyst for the Mukaiyama condensation of simple achiral enol silyl ethers of ketones with various aldehydes. The CAB-catalyzed aldol process allows the formation of adducts in a highly diastereo- and enantioselective manner (up to 96% ee) under mild reaction conditions (eqs 4 and 5). The reactions are catalytic 20 mol % of catalyst is sufficient for efficient conversion, and the chiral auxiliary can be recovered and reused. 

Analogous with the previous results of enol silyl ethers of ketones, nonsubstituted ketene silyl acetals are found to exhibit lower levels of stereoregulation, while the propionate-derived ketene silyl acetals display a high level of asymmetric induction. The reactions with aliphatic aldehydes, however, resulted in a slight reduction in optical and chemical yields. With phenyl ester-derived ketene silyl acetals, syn adducts predominate, but the selectivities are moderate in most cases in comparison with the reactions of ketone-derived silyl enol ethers. Exceptions are a,p-unsaturated aldehydes, which revealed excellent diastereo- and enantioselectivities. The observed syn selectivity and re-face attack of nucleophiles on the carbonyl carbon of aldehydes are consistent with the aforementioned aldol reactions of ketone-derived enol silyl ethers. 

Asynunetric Deprotonation/Protonation of Ketones. Lithium amides of chiral amines have been used for performing asymmetric deprotonations of symmetrically substituted (prochiral) ketones. The resulting optically active enols orenol derivatives (most frequently enol silanes) are highly versatile synthetic intermediates. Particularly useful for this purpose are chiral amines possessing Cj symmetry, such as (1). For example, reaction of 4-r-butylcyclohexanone with the lithium amide of (R,R)-(1) (readily prepared in situ by treatment of (1) with n-Butyllithium) is highly stereoselective the resulting enol silyl ether possesses an 88% ee (eq 4). ... 

The asymmetric aldol reaction of enol silyl ethers of thioesters with aldehydes is performed in high enantiomeric excess by employing a chiral promoter, tin(II) trifluoromethanesulfonate coordinated with chiral diamine 1 and tri-n-butyltin fluoride (eqs 20 and 21). Highly enantioselective aldol reactions of achiral ketene silyl acetals with achiral aldehydes are carried out by means of the same chiral promoter (eq 22). ... 

Optically active 1,2-diol units are often observed in nature as carbohydrates, macrolides or polyethers, etc. Several excellent asymmetric dihydroxylation reactions of olefins using osmium tetroxide with chiral ligands have been developed to give the optically active 1,2-diol units with high enantioselectivities. However, there still remain some problems, for example, preparation of the optically active anti-1,2-diols and so on. The asymmetric aldol reaction of an enol silyl ether derived from a-benzyloxy thioester with aldehydes was developed in order to introduce two hydroxyl groups simultaneously with stereoselective carbon-carbon bond formation by using the chiral tin(II) Lewis acid. For example, various optically active anti-a,p-dihydroxy thioester derivatives are obtained in good yields with excellent diastereo-... 

A stoichiometric amount of 3f catalyzed the asymmetric aldol reaction of aldehydes with enol silyl ethers and subsequent asymmetric reduction, in one pot, to afford syn 1,3-diols with high enantioselectivity (Eq. 49) [43b]. With a variety of aldehydes, 1,3-diols were obtained in moderate yields (53-70 %) with high syn diastereoselectivity. The syn 1,3-diols prepared from aliphatic aldehydes in the reaction (in EtCN as sol-... 

The silatropic ene pathway, i.e. direct silyl transfer from an enol silyl ether to an aldehyde, might be a possible mechanism in the Mukaiyama aldol-type reaction. Indeed, ab initio calculations show the silatropic ene pathway, involving the cyclic (boat and chair) transition states for the BHs-promoted aldol reaction of the trihydro-silyl enol ether derived from acetaldehyde with formaldehyde, to be favored [94], We recently reported the possible intervention of a silatropic ene pathway in the asymmetric catalytic aldol-type reaction of silyl enol ethers of thioesters [95]. The chloro and amino compounds thus obtained are useful intermediates in the synthesis of carnitine and GABOB (Sch. 34) [96]. 

Having developed an efficient catalytic asymmetric nitroaldol reaction, we next applied our attention to a direct catalytic asymmetric aldol reaction. The aldol reaction is generally regarded as one of the most powerful carbon-carbon bond-forming reactions. The development of a range of catalytic asymmetric aldol-type reactions has proven to be a valuable contribution to asymmetric synthesis. In all these catalytic asymmetric aldol-type reactions, however, preconversion of the ketone moiety to a more reactive speeies such as an enol silyl ether, enol methyl ether or ketene silyl... 

Copper(II) complexes of two imino nitrogen atoms belonging to chiral oxazoline and sulfoximine moieties (70) are able to elicit asymmetric consequences in the Mukaiyama-aldol reaction of enol silyl ethers and a-keto esters/ ... 

Scheme 5.13. Asymmetric addition of acetate enol silyl ethers to aldehydes [79].

---