Molecular shape defines function in both biological and material settings, and chemists have developed an ever-increasing vernacular to describe these shapes. Noncanonical atropisomers—shape-defined molecules that are formally topologically trivial but are interconvertible only by complex, nonphysical multibond torsions—form a unique subset of atropisomers that differ from both canonical atropisomers (e.g., binaphthyls) and topoisomers (i.e., molecules that have identical connectivity but nonidentical molecular graphs). Small molecules, in contrast to biomacromolecules, are not expected to exhibit such ambiguous shapes. Using total synthesis, we found that the peptidic alkaloid tryptorubin A can be one of two noncanonical atropisomers. We then devised a synthetic strategy that drives the atropospecific synthesis of a noncanonical atrop-defined small molecule.

N-nitroso compounds are a versatile class of organic structures that allow expedient access to a diversity of synthetically useful architectures through demonstrated reactivities. We report herein the development of a Rh(III)-catalyzed N-nitroso-directed methodology for the ortho-olefination of arenes. The heightened reactivity endowed by the N-nitroso group translates to mild reaction conditions, high reaction yields, and synthetic compatibility of otherwise elusive substrates (e.g., an unactivated olefin, 1-octene). Comprehensive mechanistic studies on the electronic effect, deuterium exchange, kinetic isotope effect, kinetic profile, and numerous Rh(III) complexes have established [RhCp*](2+) as the catalyst resting state, electrophilic C-H activation as the turnover-limiting step, and a five-membered rhodacycle as a catalytically competent intermediate. The ability to elaborate the N-nitroso moiety to an amine functionality provides a seminal example of the innumerable synthetic possibilities offered by this transformable directing group.

An asymmetric cascade annulation between 2-hydroxystyrenes and 2-alkynylbenaldehyes or 1-(2-alkynylphenyl)ketones has been established with good to excellent enantioselectivities (up to >99.5% ee), on the basis of an enantioselective oxa-Diels-Alder cycloaddition of in situ generated metallo-isochromenylium intermediates, by cooperative binary catalysis of Pd(OAc)2 and (S)-Trip. The developed methodology is workable for a broad spectrum of substrates and shows great efficiency in establishing dense multiple chiral centers including quaternary carbons of variable bridged ring systems. The mechanism study suggests that (S)-Trip plays multiple roles in assembling the reactants and controlling the stereoselectivity.

One of the most oft-employed methods for C-C bond formation involving the coupling of vinyl-halides with aldehydes catalyzed by Ni and Cr (Nozaki-Hiyama-Kishi, NHK) has been rendered more practical using an electroreductive manifold. Although early studies pointed to the feasibility of such a process those precedents were never applied by others due to cumbersome setups and limited scope. Here we show that a carefully optimized electroreductive procedure can enable a more sustainable approach to NHK, even in an asymmetric fashion on highly complex medicinally relevant systems. The e-NHK can even enable non-canonical substrate classes, such as redox-active esters, to participate with low loadings of Cr when conventional chemical techniques fail. A combination of detailed kinetics, cyclic voltammetry, and in situ UV-vis spectroelectrochemistry of these processes illuminates the subtle features of this mechanistically intricate process. File list (2)download file view on ChemRxiv e-NHK_TEXT Final_03192021.pdf (2.63 MiB) download file view on ChemRxiv e-NHK_SI_Final YG03192021.pdf (33.98 MiB)