Trimethylsilyl)methyl carbamates were prepared from (trimethylsilyl)methanol and N,N-dialkylcarbamoyl chlorides and were metalated by s-BuLi/TMEDA. The configurationally labile lithiated carbamates were reacted with borates derived from (+)-pinane-2,3-diol to give boronates diastereoselectively (yields up to 84%, dr up to 17:1). The absolute configurations at the boron-bearing carbon atoms of three boronates were assigned by single crystal X-ray structure analyses.Boronic acids and their esters are versatile compounds in organic chemistry. 2 Their various applications range from the Suzuki-Miyaura coupling 3 and the use of allylboronates for the stereoselective allylation 4 of aldehydes to the asymmetric syntheses based on a-halo boronates 5 elegantly developed by Matteson et al. Borylation of lithium and magnesium organic compounds with trialkyl borates and hydroboration are very general approaches to their synthesis. In a project aiming at the direct chemical synthesis of chiral, nonracemic [ 2 H 1 , 3 H]methanol we got interested in the preparation of suitable precursors of type 1 (Figure 1). Here we report on their synthesis from metalated (trimethylsilyl)methyl carbamates and borates and the determination of their structures. Furthermore, we studied the influence of the solvent and the temperature on the diastereoselectivity of the reaction, which so far has not been addressed. Figure 1Three (trimethylsilyl)methyl carbamates, 4a and (R,R)-and (S,S)-4b derived from C 2 symmetric amines 6 (R,R)-and (S,S)-5 (>99% ee), were prepared to study the diastereoselectivity of borylation (Scheme 1). Carbamate 4a was formed by esterification of trimethylsilylmethanol 7(2) with commercially available carbamoyl chloride 3 in 81% yield. 8 The amines (R,R)-and (S,S)-5 were first transformed into the intermediate carbamoyl chlorides using trichloromethyl chloroformate, which were converted in situ to the (trimethylsilyl)methyl esters (R,R)-and (S,S)-4b in 87% and 82% yield, respectively. The (trimethylsilyl)methanol was obtained by a modified literature procedure 7 in two steps from (chloromethyl)(trimethyl)silane. Nucleophilic substitution of chloride for acetate in dry DMF with NaOAc and a substoichiometric amount of NaI at 100°C for 24 hours gave the (trimethylsilyl)methyl acetate in 76% yield (see experimental part), which was reduced with LiAlH 4 . Scheme 1To obtain hydrolytically stable boronates, 5 four mixed boric acid esters containing a 1,2-diol were used for borylation, isopropanol pinacol boric acid ester 9 and three borates 8 derived from (+)-pinane-2,3-diol (7, 99% ee) and trialkyl borates 6 by transesterification in high yields (>90%) (Scheme 2). The methoxyethyl borate 8c was selected because of the additional ether oxygen atom in the leaving group. Matteson and Man found that boronic acid esters of pinane-2,3-diol are highly resistant to hydrolysis, which was desirable for our studies. 10
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