Dicyclohexylboron

Entry 2 is an example of the polar (3-oxy directing effect. Entries 3 and 4 involve formation of E-enolates using dicyclohexylboron chloride. The stereoselectivity is consistent with a cyclic TS in which a polar effect orients the benzyloxy group away from the enolate oxygen. 

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. 

When using method B (slow addition of Lewis acid) the uncatalysed reaction leading to 101 tends to compete. For this reason is best to use dicyclohexylboron enolate, since it is less reactive than the dibutylboron enolate under uncatalysed conditions. 

A stock solution of dicyclohexylboron trifluoromethanesulfonate in hexane is reasonably stable, if stored at 4°C under nitrogen. It may be purified by storing the hexane solution at -20°C, collecting the crystals that form, and redissolving this material in a calculated amount of dry hexane to give a 1 M solution. 

Dicyclohexylboron trifluoromethanesulfonate Methanesulfonic acid, trifluoro-, anhydride with dicyclohexylborinic acid (13) (145412-54-0)... 

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. 

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. 

Boron-mediated ketone-ketone aldol reactions have been described, using boron enolates formed with dicyclohexylboron chloride and triethylamine.124 Following addition of the acceptor ketone to form a boron aldolate, oxidation with peroxide yields the aldol product. 

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... 

In the synthesis of the marine macrolide leucascandrolide A (54), Kozmin used the samarium-catalyzed diastereoselective ketone reduction method in a highly stereocontrolled synthesis of the C1-C15 fragment 5814 (Scheme 4.2n). Generation of the dicyclohexylboron enolate of the ketone 55 followed by addition... 

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. ... 

Since the reactions of lithium (El-iOl-enolates are not stereoselective (see Table 6.3b), formation of 2,3-anti aldol products is conveniently accomplished by ketone enolization using dicyclohexylboron chloride. Reaction of the resultant boron... 

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