Dibutylboryl triflate

Chiral 2-oxazolidones are useful recyclable auxiliaries for carboxylic acids in highly enantioselective aldol type reactions via the boron enolates derived from N-propionyl-2-oxazolidones (D.A. Evans, 1981). Two reagents exhibiting opposite enantioselectivity ate prepared from (S)-valinol and from (lS,2R)-norephedrine by cyclization with COClj or diethyl carbonate and subsequent lithiation and acylation with propionyl chloride at — 78°C. En-olization with dibutylboryl triflate forms the (Z)-enolates (>99% Z) which react with aldehydes at low temperature. The pure (2S,3R) and (2R,3S) acids or methyl esters are isolated in a 70% yield after mild solvolysis. 

Ideal starting materials for the preparation of. svn-aldols are ketones that can be readily deprotonated to give (Z)-enolates which are known to give predominantly yyu-adducts. Thus, when (5,)-1-(4-methylphenyl)sulfonyl-2-(l-oxopropyl)pyrrolidine is treated with dibutylboryl triflate in the presence of diisopropylethylamine, predominant generation of the corresponding (Z)-boron enolate occurs. The addition of this unpurified enolate to 2-methylpropanal displays not only simple diastereoselectivity, as indicated by a synjanti ratio of 91 9, but also high induced stereoselectivity, since the ratio of syn- a/.vyn-lb is >97 3. 

A solution of 1 equivalent of the oxazolidinone in diethyl ether is cooled to —78 C. To the resultant suspension are added 1.4 equivalents of triethylamine. followed by 1.1 equivalents of dibutylboryl triflate. The cooling bath is removed and the reaction mixture is stirred at 25 °C for 1.5 h. The resultant two-phase mixture is cooled to — 78 "C with vigorous stirring. After 1 equivalent of aldehyde is added, the reaction is stirred at —78 °C Tor 0.5 h, and 0 "C for 1 to 2 h. The solution is diluted with diethyl ether, washed with 1 N aq sodium bisulfate, and concentrated. Following oxidation with 30% aq hydrogen peroxide (10 equivalents, 1 1 methanol/water, 0 C. 1 h), extractive workup and chromatographic purification, the aldol adduct is obtained with >99% diastcrcomeric purity. 

Boron enolates of thioglycolates. Esters do not form boron enolates because of the low acidity of the a-protons. However, methyl phenylthioacetate (2) forms a boron enolate on treatment with Hunig s base and dibutylboryl triflate, and this enolate undergoes aldol reactions with aldehydes with high syn-diastereoselectiv-ity.1... 

Dialkylboryl trifluoromethanesulfonates, 7, 91-92 9, 131, 140 10, 118 11, 159 Macrolactonization. Trimethylsilyl w-trimethylsilyloxycarboxylates cyclizc to macrolidcs in the presence of 1 equiv. of a dialkylboryl triflate. Dipropylboryl triflate is more effective than diethyl- or dibutylboryl triflate (equation I). 

A metal exchange reaction between an alkenyloxysilane (silyl enolate) and a dialkylboryl triflate has already been described (Scheme 11). When a mixture of a silyl enolate and dibutylboryl triflate is allowed to react with an aldehyde, aldol product diastereoselectivity is negligible, presumably because trimethylsilyl triflate promotes an aldol reaction of the silyl enolate. When trimethylsilyl triflate is com-... 

The mixture was then stirred for further 3 h at 25 °C. Distillation under reduced pressure gave pure dibutylboryl triflate 8 (19.15 g, 84% bp 60 °C/... 

To a solution of dibutylboryl triflate (301 mg, 1.1 mmol) and diisopropyl-ethylamine (142 mg, 1.1 mmol) in ether (1.5 mL) tvas added droptvise 2-methyl-4-pentanone 24 (100 mg, 1.0 mmol) in ether (1.5 mL) at —78 °C, under argon, tvith stirring. After stirring of the mixture for 30 min, 3-phenylpropanal (134 mg, 1.0 mmol) in ether (1.5 mL) tvas added at the same temperature. The reaction mixture vas allo ved to stand for 1 h, then added to pH 7 phosphate buffer at room temperature and extracted vith ether. After removal of ether, the mixture vas treated vith 30% H2O2... 

To a 0.2-0.5 m solution of chiral imide 26 in CH2CI2 under argon (0 °C) was added 1.1 equiv. dibutylboryl triflate followed by 1.2 equiv. diisopropylethyl-amine. After 30 min the reaction mixture was cooled (—78 °C) and 1.1 equiv. isobutyraldehyde was added and stirred for 0.5 h at —78 °C and then for 1.5 h at room temperature. The reaction was quenched with pH 7 phosphate buffer and the boron aldolate complex was oxidized with 30% hydrogen peroxide-methanol (0 °C, 1 h). The aldol product 34 was then isolated by ether extraction (syn/anti = 497 1). 

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