Synthesis of α‑Borylmethyl‑(<i>E</i>)‑allylborons via Cu-Catalyzed Diboration of 1‑Substituted Allenols and Their Application in Stereoselective Aldehyde Allylation

1,2-Diborons with one boron atom each in the allyl and homoallyl positions are of great utility, especially as double-allylation reagents. However, only a few synthetic methods have been reported to date and have a limited substrate scope. Herein, we developed the Cu-catalyzed regio- and stereoselective synthesis of α-borylmethyl-(<i>E</i>)-allylborons from easily accessible 1-substituted allenols and bis(pinacolato)diboron. Importantly, this method allowed the highly efficient and regioselective formation of double-allylating diborons with diverse substituents, which would be otherwise cumbersome to synthesize, and could be successfully performed on a gram scale. The synthetic application of α-borylmethyl-(<i>E</i>)-allylborons was demonstrated by the enantio- and (<i>Z</i>)-selective allylation of aldehydes via Brønsted acid catalysis. Furthermore, (<i>E</i>)-allyl and (<i>E</i>)-homoallyl diols with excellent diastereoselectivity were generated by the Lewis acid catalyzed diastereo- and (<i>E</i>)-selective allyl transfer of (<i>E</i>)-allyldiborons to aldehydes. Using this strategy, the key intermediate in the construction of the C₇–C₁₂ fragment of (−)-discodermolide was also synthesized.