Triflates dibutylboryl, reaction with

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... 

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. 

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. 

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-... 

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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