Carbon-carbon bond formation via catalytic hydrogenation and transfer hydrogenation : application in the total synthesis of bryostatin 7
Under the conditions of transfer hydrogenation employing ortho-cyclometallated iridium C,O-benzoate catalysts, two protocols of iterative chain elongation of 1,3-diols to furnish 1,3-polyols were developed. First, one-directional chain elongation employing mono-protected 1,3-diols as starting materials was achieved. In all cases, high levels of catalyst-directed enantioselectivity and diastereoselectivity were observed. Then, double asymmetric allylation of 1,n-glycols to deliver C₂-symmetric adducts with exceptional level of enantioselectivity was devised. Iterative two-directional elongation of 1,3-diols to furnish 1,3-polyols with high level of catalyst-directed diastereoselectivity was then achieved. Implementation of this methodology and other hydrogenative C-C bond formations proved to be effective means for the preparation of a known bryostatin A-ring fragment and the total synthesis of bryostatin 7.