https://pubs.rsc.org/en/content/articlepdf/2019/NP/C8NP00092A

Marine natural products.

The design and implementation of cascade reactions is a challenging facet of organic chemistry, yet one that can impart striking novelty, elegance, and efficiency to synthetic strategies. The application of cascade reactions to natural products synthesis represents a particularly demanding task, but the results can be both stunning and instructive. This Review highlights selected examples of cascade reactions in total synthesis, with particular emphasis on recent applications therein. The examples discussed herein illustrate the power of these processes in the construction of complex molecules and underscore their future potential in chemical synthesis.

Cascade reactions in total synthesis.

Dynamic combinatorial chemistry.

Dehydrogenation as a substrate-activating strategy in homogeneous transition-metal catalysis.

The Golden Age of Transfer Hydrogenation

A tantalum-catalyzed addition of N-alkyl arylamine α-C−H bonds across olefins is reported. These reactions occur with mono- and 2,2-disubstituted olefins to form the branched insertion products in high yield and regioselectivity. The reactions encompass additions of the α-C−H bonds of cyclic and acyclic amines, as well as intramolecular additions. NMR studies indicate that the starting homoleptic, Ta(NMe2)5 precatalyst converts to bis- and tris(N-methylanilide) complexes (among others) in solution. Deuterium-labeling studies suggest that reversible ortho-metalation of the arene substituent occurs under the reaction conditions. However, several experiments imply that this ortho-metalation does not lie on the reaction pathway. Instead, these complexes are proposed to eliminate amine to form N-aryl imine complexes, which insert olefins into the Ta−C bond and undergo protonolysis to regenerate the active catalyst and eliminate the addition product.

Direct, Catalytic Hydroaminoalkylation of Unactivated Olefins with N-Alkyl Arylamines

We describe an enantioselective synthetic route to the antiproliferative alkaloid stephacidin B (1) proceeding in 18 steps and 4.0% yield from 4,4-(ethylenedioxy)-2,2-dimethylcyclohexanone (3). Key features of the synthetic sequence include the use of the Corey-Bakshi-Shibata (CBS) reduction to introduce asymmetry early in the synthetic route, use of the novel electrophile N-(tert-butoxycarbonyl)-5-(isopropylsulfonyloxymethyl)-2,3-dihydropyrrole in a stereoselective enolate alkylation, a diastereoselective Strecker-type addition of hydrogen cyanide to an N-Boc enamine substrate in the solvent hexafluoroisopropanol, platinum-catalyzed nitrile hydrolysis under neutral conditions, cyclization of an acylamino radical intermediate to form the diketopiperazine core of stephacidin B, and implementation of a convergent procedure for introduction of the key 3-alkylidene-3H-indole 1-oxide functional group in the final stage of the route to prepare the structure 2, previously proposed to be the fungal metabolite avrainvillamide (17 steps, 4.2% yield). We observed that synthetic (-)-2 dimerized in the presence of triethylamine to form (+)-stephacidin B (>95%). We also obtained evidence that 2 can form 1 under mild conditions, and that 2 reacts with nucleophiles, such as methanol, by conjugate addition.

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Enantioselective synthesis of stephacidin B.

The intermolecular addition of the α-C−H bonds of unactivated dialkylamines to unactivated olefins in the presence of the chloro amido complex [TaCl3(NEt2)2]2 (2) is described. This process forms the branched insertion products in high yields (up to 96%) and selectivities, and represents a rare example of an intermolecular amine-olefin coupling reaction that does not require preactivation of either substrate. The reaction is shown to encompass the addition of the primary C−H bonds in linear- and branched-methylamines, as well as secondary C−H bonds in higher dialkylamines. The related chloroanilido complex [TaCl3(NMePh)2]2 (4) is also shown to catalyze the addition of N-alkyl-arylamines to olefins at temperatures as low as 90 °C. 1H NMR spectroscopy, identification of the catalyst structure, and deuterium-labeling experiments all suggest that reactions catalyzed by 2 and 4 occur by turnover-limiting generation of an η2-imine complex. These labeling studies also imply that more favorable partitioning of th...

Hydroaminoalkylation of Unactivated Olefins with Dialkylamines

A general method for the hydropyridylation of unactivated alkenes is described. The transformation connects metal-mediated hydrogen atom transfer to alkenes and Minisci addition reactions. The reaction proceeds under mild conditions with high site-selectivities and allows for the construction of tertiary and quaternary centers from simple alkene starting materials.

Intermolecular Hydropyridylation of Unactivated Alkenes.

A general method for the selective hydrogenation of alkenyl halides to alkyl halides is described. Fluoro, chloro, bromo, iodo, and gem-dihaloalkenes are viable substrates for the transformation. The selectivity of the hydrogenation is consistent with reduction by a hydrogen atom transfer pathway.

Seth B. Herzon

Papers

Direct, Catalytic Hydroaminoalkylation of Unactivated Olefins with N-Alkyl Arylamines

Enantioselective synthesis of stephacidin B.

Hydroaminoalkylation of Unactivated Olefins with Dialkylamines

Intermolecular Hydropyridylation of Unactivated Alkenes.

A Method for the Selective Hydrogenation of Alkenyl Halides to Alkyl Halides