Dicyclohexylboron triflate

Rq = Pr 1 Equivalents Lewis acid per equivalent of aldehyde Product ratios and yields were determined by integration of the H NMR spectra of the product mixture using an internal standard. The yield given is the total yield of aldol mixture The yield in this run was not determined In this run the enolate was formed with dicyclohexylboron triflate and Huning s base 1.5 equivalents of aldehyde were used. 

The present procedure is a modification of that originally reported by the submitter and co-workers. 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 anf/-selective aldol reactions with dicyclohexylboron triflate and triethylamine as enolization reagents. ... 

A. (1 R)-Phenyl-(2 S)-[(phenylmethyl)[(2,4,6-trimethylphenyl)sulfonyl]amino]-propyl (3R)-hydroxy-(2R),4-dimethylpentanoate An oven-dried, 500-mL, round-bottomed flask is charged with (1R, 2S)-(-h)-1 4.80 g, 10 mmol) (Note 1) and dichloromethane (50 mL) (Note 2) under nitrogen. To this solution is added triethylamine (3.40 mL, 24 mmol) (Note 2) via syringe. The solution is cooled to -78°C and a solution of dicyclohexylboron triflate (1.0 M in hexane, 22 mL, 22 mmol) (Note 3) is added dropwise over 20 min. The resulting solution is stirred at -78°C for 30 min. To the -78°C enolate solution is then added isobutyraldehyde (1.08 mL, 12 mmol, freshly distilled) dropwise. 

In 1997, Masamune et al. disclosed another anti -selective aldol reaction method, using a readily available chiral ligand, norephedrine2 (Scheme 2.2c). The ester 3 was treated with 2 equivalents of dicyclohexylboron triflate (Cy2BOTf) and 2.4 equivalents of triethylamine at -78°C for 2 hours. When aldehyde was... 

As the boron enolate from dicyclohexylboron triflate and triethylamine at -78° C was determined to be an -isomer, the predominant formation of anti -aldol product can be explained via the classic Zimmerman-Traxler six-membered chairlike transition state3 (Scheme 2.2d). In transition state A, the norephedrine unit of the boron enolate presumably arranges itself in such a way that the phenyl group directs the approach of the aldehyde.4... 

The Masamune aldol condensation, in common with the Evans aldol condensation, involves a boron enolate of an ester containing a norephedrine derived chiral auxiliary however, unlike the latter, the Masamune aldol delivers a 3-hydroxy-2-methyl carbonyl moiety with the an/z-stereochemistry. Crucial to the success of this reaction is the use of dicyclohexylboron triflate to generate the boron enolate. Note in the Evans aldol condensation, dibutylboron triflate is utilized. 

A stock solution (1 M) of dicyclohexylboron trifluromethanesulfonate was prepared according to the accompanying procedure (Abiko, A. Org. Synth. 2002, 79, 103). Two equivalents of the boron triflate are necessary for complete enolization of the ester. When one equivalent is used, the enolization proceeds only to 50% conversion. 

Although stereoselective formation of enolates from acyclic ketones with bases such as LDA is rather difficult, stereodefined boron enolates are more readily accessible. In the Mukaiyama method, an ethyl ketone is treated with a dialkylboron triflate and a tertiary amine, usually i-Pr2NEt. The resultant Z-(0) boron enolates (also known as enol borinates) are believed to be formed under kinetic control by deprotonation of the Lewis acid-complexed substrate. Brown and co-workers have shown that E- 0) boron enolates may be prepared by treatment of ethyl ketones with dicyclohexylboron chloride in the presence of Et3N. ... 

It is all very well knowing what enolates give rise to what aldol products, but this is not much use unless we can choose to have cis or trans enolates. Clearly with cyclopentanone there is no option but to form a trans enolate. But with an acyclic ketone we will need a little more care. Boron enol ethers provide an answer. Two common boron reagents used are 9-BBN chloride 46 (derived from 9-BBN 45) and another is dicyclohexylboron chloride 47. Other reagents include the corresponding triflates. There are also the chiral boron reagents that we shall meet later chapters. 

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