Fig. 4. Synthesis of C 2-symmetric-1,3-bis(boronic esters) from diborylmethane 1 by using diamine-free primary carbenoids (A) and stepwise double homologation with secondary benzylic carbamates (B). Also, the one-pot mixed homologation of diborylmethane to form non-symmetric 1,3-bis(boronic esters) (C). Footnotes: (a) diastereomeric ratios were determined by 1H NMR analysis. (b) Opposite enantiomer of stannane was used. Abbreviations: Cb = N,N-diisopropyl carbamoyl, PMP = para-methoxyphenyl, TBS = tert-butyldimethylsilyl, TIB = 2,4,6-triisopropylbenzoyl, pin = pinacolato.
