Asymmetric Intramolecular Hydroalkoxylation of 2-Vinylbenzyl Alcohols with Chiral Boro-Phosphates

Asymmetric intramolecular hydroalkoxylation of alkenes represents a very important approach to access optically active cyclic ethers. It was found that B(C6F5)(3) could catalyze the cyclization of 2-vinylbenzyl alcohols with only 0.05 mol % catalyst loading, affording the desired product in high yields. To accomplish the asymmetric reaction, a novel type of chiral boro-phosphates was developed by treating chiral phosphoric acid with Piers' borane, in which the oxygen atom of P=O and the boron atom act as Lewis base and acid centers, respectively. A highly enantioselective hydroalkoxylation was successfully realized to give optically active 1,3-dihydroisobenzofuran derivatives in 78-99 % yields with 60-97 % ee's, in which an activation of O-H bond of alcohols by the boro-phosphate species is hypothesized.