Chiral boronates—versatile reagents in asymmetric synthesis

The new, axially chiral borates 2, 9, and 11 and boronates 3–6 and 12 are synthesized in good yields. They are representatives of three different structural types. The bicyclic borates 2 and 9 are homochiral propeller compounds; their exclusive formation is used to increase the enantiomerical purity of 1. Especially the borates are efficient Lewis acidic catalysts for stereoselective Diels–Alder reactions. The new seven-membered boron compounds 4, 5, 6, and 12 are interesting reagents for different asymmetric synthetic steps. Activated vinylboronates 31 can serve as efficient cyclophiles in [3+2] cycloaddition reactions with methylenecyclopropane 27 giving borylated methylenecyclopentanes 32.