The Total Synthesis of (±)-Fortucine and a Revision of the Structure of Kirkine

Asymmetric Synthesis of α-Arylcyclohexenones Catalyzed by Diphenylprolinol Silyl Ether

A general methodology for the asymmetric synthesis of α-arylcyclohexeneones from arylacetones and α,β-unsaturated aldehydes catalyzed by diphenylprolinol silyl ether followed by p-TSA-mediated cyclization is developed. A variety of arylacetones and α,β-unsaturated aldehydes were successfully converted to α-arylcyclohexeneones in 34-67% yield, 10:1-100:0 dr, and 81-99% ee. The scalability of this methodology by a gram-scale synthesis and their utility by converting the product to the corresponding epoxide, alcohol, and diol are demonstrated.

Visible-Light-Induced Cascade Difunctionalization of Indoles Enabled by the Synergy of Photoredox and Photoexcited Ketones: Direct Access to Alkylated Pyrrolophenanthridones

Herein, we describe a methodology to construct polycyclic pyrrolophenanthridones with an (amino)alkyl side chain that involves visible-light-induced decarboxylative radical addition for the intermolecular dearomatization of indoles and subsequent photoinduced C(sp²)-X bond activation via photoexcited ketones for an intramolecular cyclization cascade. Carboxylic acids serve both as a radical source toward indole dearomatization and as reductants to initiate an electron transfer with photoexcited N-acylindole derivatives in the reaction toward pyrrolophenantridone skeletons, which occurs under mild reaction conditions with good functional group tolerance.

Nitrogen-Centered Radicals in Functionalization of sp2 Systems: Generation, Reactivity, and Applications in Synthesis

The chemistry of nitrogen-centered radicals (NCRs) has plentiful applications in organic synthesis, and they continue to expand as our understanding of these reactive species increases. The utility of these reactive intermediates is demonstrated in the recent advances in C-H amination and the (di)amination of alkenes. Synthesis of previously challenging structures can be achieved by efficient functionalization of sp² moieties without prefunctionalization, allowing for faster and more streamlined synthesis. This Review addresses the generation, reactivity, and application of NCRs, including, but not limited to, iminyl, aminyl, amidyl, and aminium species. Contributions from early discovery up to the most recent examples have been highlighted, covering radical initiation, thermolysis, photolysis, and, more recently, photoredox catalysis. Radical-mediated intermolecular amination of (hetero)arenes can occur with a variety of complex amine precursors, generating aniline derivatives, an important class of structures for drug discovery and development. Functionalization of olefins is achievable in high anti-Markovnikov regioselectivity and allows access to difunctionalized structures when the intermediate carbon radicals are trapped. Additionally, the reactivity of NCRs can be harnessed for the rapid construction of N-heterocycles such as pyrrolidines, phenanthridines, quinoxalines, and quinazolinones.

Elaboration of Functionalized Organophosphates

In this study, the rapid transformation of inexpensive phenols into polyfunctionalized cyclohexenones containing a phosphonate in one pot is described. Such systems readily obtained from simple aromatic compounds could open up a multitude of synthetic possibilities. For example, this scaffold was easily and stereoselectively transformed into the corresponding enol functionality in the same pot by the addition of sodium borohydride.

Synthetic and Computational Study of Tin-Free Reductive Tandem Cyclizations of Neutral Aminyl Radicals

5- exo, 5- exo Cyclizations of conformationally unbiased propargylic aminyl radicals proceed with excellent yield, chemoselectivity, and diastereoselectivity under tin-free reductive cyclization conditions, regardless of the electronic environments and intermediate radical stabilization resulting from various olefin substituents. These conditions avoid the need for slow addition of initiator and reductant. By contrast, analogous 6- exo, 5- exo cyclizations require substituents capable of intermediate radical stabilization to avoid premature reduction products. These experimental results are corroborated by computations that further establish the reactivity of these aminyl radicals upon exposure to tin-free cyclization conditions.

A Gold-Catalyzed Domino Cyclization Enabling Rapid Construction of Diverse Polyheterocyclic Frameworks

We report herein an efficient gold(I)-catalyzed post-Ugi domino dearomatization/ipso-cyclization/Michael sequence that enables access to libraries of diverse (hetero)arene-annulated tricyclic heterocycles. This process affords novel complex polycyclic scaffolds in moderate to good yields from readily available acyclic precursors with excellent chemo-, regio-, and diastereoselectivity. The power of this strategy has been demonstrated by the rapid synthesis of 40 highly functionalized polyheterocycles bearing indole, pyrrole, (benzo)furan, (benzo)thiophene, pyrazole, and electron-rich arene groups in two operational steps.

Some aspects of radical cascade and relay reactions

The ability to create carbon-carbon bonds is at the heart of organic synthesis. Radical processes are particularly apt at creating such bonds, especially in cascade or relay sequences where more than one bond is formed, allowing for a rapid assembly of complex structures. In the present brief overview, examples taken from the authors' laboratory will serve to illustrate the strategic impact of radical-based approaches on synthetic planning. Transformations involving nitrogen-centred radicals, electron transfer from metallic nickel and the reversible degenerative exchange of xanthates will be presented and discussed. The last method has proved to be a particularly powerful tool for the intermolecular creation of carbon-carbon bonds by radical additions even to unactivated alkenes. Various functional groups can be brought into the same molecule in a convergent manner and made to react together in order to further increase the structural complexity. One important benefit of this chemistry is the so-called RAFT/MADIX technology for the manufacture of block copolymers of almost any desired architecture.

Total synthesis of natural products using hypervalent iodine reagents

We present a review of natural product syntheses accomplished in our laboratory during the last 5 years. Each synthetic route features a phenol dearomatization promoted by an environmentally benign hypervalent iodine reagent. The dearomatizations demonstrate the "aromatic ring umpolung" concept, and involve stereoselective remodeling of the inert unsaturations of a phenol into a highly functionalized key intermediate that may contain a quaternary carbon center and a prochiral dienone system. Several new oxidative strategies were employed, including transpositions (1,3-alkyl shift and Prins-pinacol), a polycyclization, an ipso rearrangement, and direct nucleophilic additions at the phenol para position. Several alkaloids, heterocyclic compounds, and a polycyclic core have been achieved, including sceletenone (a serotonin reuptake inhibitor), acetylaspidoalbidine (an antitumor agent), fortucine (antiviral and antitumor), erysotramidine (curare-like effect), platensimycin (an antibiotic), and the main core of a kaurane diterpene (immunosuppressive agent and stimulator of apoptosis). These concise and in some cases enantioselective syntheses effectively demonstrate the importance of hypervalent iodine reagents in the total synthesis of bioactive natural products.

Total synthesis of lycorine-type alkaloids by cyclopropyl ring-opening rearrangement

Anhydrocaranine, γ-lycorane and putative amarbellisine were synthesized based on a series reactions including a novel cyclopropyl ring-opening rearrangement.

Computational study of the rate constants and free energies of intramolecular radical addition to substituted anilines

The intramolecular radical addition to aniline derivatives was investigated by DFT calculations. The computational methods were benchmarked by comparing the calculated values of the rate constant for the 5-exo cyclization of the hexenyl radical with the experimental values. The dispersion-corrected PW6B95-D3 functional provided very good results with deviations for the free activation barrier compared to the experimental values of only about 0.5 kcal mol⁻¹ and was therefore employed in further calculations. Corrections for intramolecular London dispersion and solvation effects in the quantum chemical treatment are essential to obtain consistent and accurate theoretical data. For the investigated radical addition reaction it turned out that the polarity of the molecules is important and that a combination of electrophilic radicals with preferably nucleophilic arenes results in the highest rate constants. This is opposite to the Minisci reaction where the radical acts as nucleophile and the arene as electrophile. The substitution at the N-atom of the aniline is crucial. Methyl substitution leads to slower addition than phenyl substitution. Carbamates as substituents are suitable only when the radical center is not too electrophilic. No correlations between free reaction barriers and energies (ΔG^‡ and ΔG_R) are found. Addition reactions leading to indanes or dihydrobenzofurans are too slow to be useful synthetically.

Some aspects of radical chemistry in the assembly of complex molecular architectures

This review article describes briefly some of the radical processes developed in the authors' laboratory as they pertain to the concise assembly of complex molecular scaffolds. The emphasis is placed on the use of nitrogen-centred radicals, on the degenerate addition-transfer of xanthates, especially on its potential for intermolecular carbon-carbon bond formation, and on the generation and capture of radicals through electron transfer processes.