Fig. 3. Stereoselective synthesis of 1,2-bis(boronic esters) from primary benzoates (A), secondary benzylic carbamates (B) and secondary dialkyl benzoates (C). Footnotes: (a) enantiomeric ratios determined by HPLC, GC or SFC analysis using chiral stationary phases. (b) Diastereomeric ratios determined by 1H NMR analysis. (c) Prepared from the corresponding carbamate (d) using (–)-sparteine in place of (+)-sparteine. (e) 2.2 equivalents of (+)-sparteine, sBuLi and 1. Abbreviations: Cb = N,N-diisopropyl carbamoyl, MOM = methoxymethyl, PMP = para-methoxyphenyl, TBS = tert-butyldimethylsilyl, TIB = 2,4,6-triisopropylbenzoyl, TMEDA = N,N,N′,N′-tetramethylethylenediamine, pin = pinacolato.
