Reagent for the Formation of Alkenylboronic Acid Esters

　The Takai reaction using 2-(dichloromethyl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (1) and aldehydes affords the corresponding alkenylboronic acid esters under mild conditions. The reaction proceeds under neutral conditions at room temperature to afford the corresponding E-alkenylboronic acid esters in high yield and stereoselectivity. Although the reaction requires an excess amount of chromium(II) chloride, this can be decreased to a catalytic amount by using manganese as a reducing agent. It has been reported that bi-functionalized polyenes, which are difficult to synthesize using other methods, can be produced under mild conditions and applied to the synthesis of natural products.²⁾