A new chiral dirhodium tetracarboxylate catalyst, Rh( S-2-Cl-5-BrTPCP), has been developed for C-H functionalization reactions by means of donor/acceptor carbene intermediates. The dirhodium catalyst contains four ( S)-1-(2-chloro-5-bromophenyl)-2,2-diphenylcyclopropane-1-carboxylate ligands, in which all four 2-chloro-5-bromophenyl groups are on the same face of the catalyst, leading to a structure, which is close to C symmetric. The catalyst induces highly site selective functionalization of remote, unactivated methylene C-H bonds even in the presence of electronically activated benzylic C-H bonds, which are typically favored using earlier established dirhodium catalysts, and the reactions proceed with high levels of diastereo- and enantioselectivity. This C-H functionalization method is applicable to a variety of aryl and heteroaryl derivatives. Furthermore, the potential of this methodology was illustrated by sequential C-H functionalization reactions to access the macrocyclic core of the cylindrocyclophane class of natural products.
An enantioselective formal synthesis
of (−)-aflatoxin B2 from 4-methoxyphenylacetic acid
has been achieved by an approach
that produces a key carbon–carbon bond, a benzylic stereocenter,
and two arene carbon–oxygen bonds in the course of three site-selective
C–H functionalizations. The carbonyl-directed acetoxylation
of two arene C–H bonds described herein is unprecedented in
natural product synthesis and occurs under mild conditions that preserve
the configuration of a sensitive benzylic stereocenter.
Readily available propargyl alcohols were found to be useful substrates for the copper(I)-catalyzed synthesis of β,γ-unsaturated amides. Nucleophilic attack by the alcohol on the in situ generated ketenimine followed by base-catalyzed elimination and subsequent ring opening yields the desired products under mild conditions.
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