Catalytic asymmetric synthesis and Stereocenter assignment of naturally occurring Sesquiterpenoid, (−)-Baccharisketone

Catalytic asymmetric total synthesis of either enantiomer of naturally occurring sesquiterpenoid, i.e. (−)-baccharisketone (1a) and (+)-baccharisketone (ent-1a) has been achieved via a key catalytic asymmetric p-tolyl boronic acid addition onto E-ester. Thus, an efficient five-step approach to both enantiomers of baccharisketone has been shown using 2 mol% Rh(COD)₂BF₄-(R)-BINAP as catalyst to achieve the product with a stereogenic center at the pseudobenzylic position (up to 93 % ee). This also confirms the assignment of the stereogenic center present at the pseudobenzylic position.