Diastereo- and Enantioselective Synthesis of (<i>E</i>)-2-Methyl-1,2-<i>syn</i>- and (<i>E</i>)-2-Methyl-1,2-<i>anti</i>-3-pentenediols via Allenylboronate Kinetic Resolution with (^<i>d</i>Ipc)₂BH and Aldehyde Allylboration

Abstract

Enantioselective hydroboration of racemic allenylboronate (±)-1 with 0.48 equiv of ((d)Ipc)(2)BH at -25 °C proceeds with efficient kinetic resolution and provides allylborane (R)-Z-4. When heated to 95 °C, allylborane (R)-Z-4 isomerizes to the thermodynamically more stable allylborane isomer (S)-E-7. Subsequent allylboration of aldehydes with (R)-Z-4 or (S)-E-7 at -78 °C followed by oxidative workup provides 1,2-syn- or 1,2-anti-diols, 2 or 3, respectively, in 87-94% ee.