Catalytic asymmetric Nozaki-Hiyama reactions with a tridentate bis(oxazolinyl)carbazole ligand.

Nozaki-Hiyama reactions are powerful Cr(II)-mediated C--C bond-forming reactions conducted under mild conditions that show excellent compatibility with various functional groups. Therefore, Nozaki-Hiyama reactions have been utilized for many total syntheses of complex natural products, but at least two equivalents of Cr(II) salt are required to complete the reaction because Cr(II) salt is a one-electron donor. In 1996, however, the quantity of Cr(II) salts required was successfully reduced by a catalytic redox system reported by Fürstner and Shi. Since the report by Fürstner, the catalytic asymmetric Nozaki-Hiyama reactions have attracted attention because they would allow control over enantioselectivity, thereby further enhancing the versatility of Nozaki-Hiyama reactions. In this review, we describe the development of a tridentate bis(oxazolinyl)carbazole ligand for the catalytic asymmetric Nozaki-Hiyama allylation, methallylation, propargylation, and allenylation. Also described are their successful applications to the highly stereoselective construction of the side chain of calcitriol lactone, as well as structure elucidation and the enantioselective first total synthesis of the potent 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitors, FR901512 and FR901516.