Dicyclohexylboron triflate-triethylamine

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... 

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