Formation of C-C bonds via transfer hydrogenation : from methodology development to natural product synthesis
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Under the conditions of transfer hydrogenation employing ortho-cyclometallated iridium C,O-benzoate catalysts, selective silylallylation and CF₃-allylation were developed. In both cases, high levels of catalyst-directed enantioselectivity and diastereoselectivity were observed. Column chromatography was then tested as a new protocol to purify the iridium precatalyst; this single component precatalyst was proved to be more efficient to promote carbonyl crotylation reactions, both diastereo- and enantioselectivity were increased. Then, double asymmetric crotylation of 1,3-diols to deliver (pseudo-)C₂-symmetric adducts with exceptional level of enantioselectivity was devised. Implementation of this methodology and other hydrogenative C-C bond formations proved to be effective means for the preparation of two known polypropionate natural product fragments of C19-C25 of scytophycin C, C19-C27 of rifamycin S and the total synthesis of 6-deoxyerythronolide B.