Aldol reactions anri-selective

Ono and Kamimura have found a very simple method for the stereo-control of the Michael addition of thiols, selenols, or alcohols. The Michael addition of thiolate anions to nitroalkenes followed by protonation at -78 °C gives anti-(J-nitro sulfides (Eq. 4.8).11 This procedure can be extended to the preparation of a/jti-(3-nitro selenides (Eq. 4.9)12 and a/jti-(3-nitro ethers (Eq. 4.10).13 The addition products of benzyl alcohol are converted into P-amino alcohols with the retention of the configuration, which is a useful method for anri-P-amino alcohols. This is an alternative method of stereoselective nitro-aldol reactions (Section 3.3). The anti selectivity of these reactions is explained on the basis of stereoselective protonation to nitronate anion intermediates. The high stereoselectivity requires heteroatom substituents on the P-position of the nitro group. The computational calculation exhibits that the heteroatom covers one site of the plane of the nitronate anion.14... 

The present procedure is a modification of that originally reported by the submitter and co-workers.2 This procedure is applicable to a large scale preparation of the title compound in high overall yield (-80%) without purification of the intermediates by chromatography. The title compound is reported to be a useful reagent for anri-selective aldol reactions with dicyclohexylboron triflate and triethylamine as enolization reagents.3... 

In addition to the acetate aldol problem, stereoselective aldol additions of substituted enolates to yield 1,2-anti- or f/treo-selective adducts has remained as a persistent gap in asymmetric aldol methodology. A number of innovative solutions have been documented recently that provide ready access to such products. The different successful approaches to anri-selective propionate aldol adducts stem from the design of novel auxiliaries coupled to the study of metal and base effects on the reaction stereochemistry. The newest class of auxiliaries are derived from A-arylsulfonyl amides prepared from readily available optically active vicinal amino alcohols, such as cw-l-aminoindan-2-ol and norephedrine. 

Additive aldol reaction. An anri-selective synthesis of 5-phenylthio esters of 3-hydroxy-2-phenylthiomethylalkanethiolates from a mixture of acryloyl chloride, PhSLi and aldehydes is mediated by MgBr2 OEt2. 

Aldol reaction. Aldol reaction catalyzed by proline and derivatives has been reviewed. A ball-mill operation on cycloalkanones, ArCHO with (5)-proline leads to predominantly anri-aldol products. The aldol reaction between 4-tetrahydrothiapyrone with the racemic 3-aldehyde based on the same heterocycle shows excellent enantiotopic group-selectivity and thence manifesting dynamic kinetic resolution. ... 

The anri-selective aldol reaction between cyclohexanone and ArCHO reaches >99% ee if it is conducted in the presence of trawi -4-(4-t-butylphenoxy)-L-piDline and sulfated P-cyclodextrin in water at room temperature. Another catalyst is cw-A-fl-adamantane-carboxamido)-(5)-proline (1) in conjunction with P-cyclodextrin. ... 

Under the same conditions, similar anri-selective aldol reactions obtain with several other chiral ethyl ketones such as 3.4... 

It is interesting to note that anri-selectivity of aldolization (with L-proline promotion) involving hydroxyacetone as the donor is switched in the Mannich reaction. Thus syn-2-hydroxy-3-arnino ketones are obtained as major products. 

Aldol reactions. Titanium -ate complexes effect anri-selective aldolization (5 examples, 72-81%). 

Stereoselective reactions with acetals. Noyori et al. (10,438) have used this Lewis acid to promote an aldol-type reaction between enol silyl ethers and acetals and have noted high. syn-selectivity in this process. Molander and Haar report that reaction of acetals with cyanotrimethylsilane promoted by TMSOTf results in a-alkoxy cyanides and that this reaction can be diastereoselective when the acetal is substituted at the 4-position by an alkoxy group. The diastereoselectivity depends on the nature of the acetal and the 4-alkoxy group. Dimethoxy acetals show slight diastereoselectivity, but diisopropoxy and dibenzyl acetals can show diastereoselectivity of 5-10 1. The diastereoselectivity also depends on the type of 4-substituent. Acetoxy and t-butyldimethylsilyloxy groups have no effect on the diastereoselectivity, but methoxy, benzyloxy, and allyloxy groups promote anri-selectivity. Since a metal template is not involved, the diastereoselectivity... 

Reformatsky reactions. As activating agent for a-bromo esters CrClj-Lil shows excellent selectivity for aldehydes over ketones (>50 1 vs. methyl ketones and >200 1 for more bulky ketones). anri-Aldol products can be obtained. 

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