Synthesis of spirocyclic indolines by interruption of the Bischler-Napieralski reaction

Ir-Catalyzed Chemoselective Reductive Condensation Reactions of Tertiary Amides with Active Methylene Compounds

The catalytic reductive condensation reactions of tertiary amides with active methylene compounds leading to multifunctionalized non-N-containing products is described. The reactions proceed through sequential iridium-catalyzed hydrosilylation of the amides followed by acid-mediated condensation with the active methylene compounds. This scalable method is broad in scope and shows remarkable chemoselectivity for the amide group in the presence of several sensitive or even more reactive functionalities such as ester, cyano, nitro, silyl dienol ether, and ketone.

Direct, four-step synthetic pathway to iheyamine A and several analogues

A novel synthetic route toward the pentacyclic azepinobisindole alkaloid iheyamine A and its several analogues has been developed in four steps from commercially available isatins and tryptamines. This crucial transformation involves the Bischler-Napieralski cyclization to deliver the characteristic seven-membered framework. Then the ester intermediate undergoes a hydrolyzation-decarboxylation-dehydrogenation cascade to yield the final product.

Chemoselektive γ-Oxidation von β,γ-ungesättigten Amiden mit TEMPO

Ein chemoselektives und robustes Protokoll zur γ‐Oxidation von β,γ‐ungesättigten Amiden wird dargelegt. Bei dieser Methode ermöglicht elektrophile Amidaktivierung eine bei ungesättigten Amiden bisher selten angewendete regioselektive Reaktion mit TEMPO, die zu γ‐aminoxylierten α,β‐ungesättigten Amiden führt. Radikalische Zyklisierungen und Oxidationen der synthetisierten Produkte untermauern die Nützlichkeit der hergestellten Verbindungen.

Interrupted reactions in chemical synthesis

Interrupted reactions reroute established processes to new and often unanticipated end points. Of particular interest are the cases in which a known reactive intermediate takes on a new reaction pathway, either because this pathway is lower in energy or because the conventional pathway is no longer available. Through analysis of documented cases, we aim to dissect the known interrupted reactions and trace their mechanistic origins. As new chemical processes are being discovered at a seemingly ever-increasing pace, it is likely that new interrupted reactions will continue to emerge. Our hope is that the cases considered in this Review will help identify new classes of these fascinating transformations.

Chemoselective γ-Oxidation of β,γ-Unsaturated Amides with TEMPO

A chemoselective and robust protocol for the γ‐oxidation of β,γ‐unsaturated amides is reported. In this method, electrophilic amide activation, in a rare application to unsaturated amides, enables a regioselective reaction with TEMPO resulting in the title products. Radical cyclisation reactions and oxidation of the synthesised products highlight the synthetic utility of the products obtained.

Chemoselective α-Sulfidation of Amides Using Sulfoxide Reagents

The direct α-sulfidation of tertiary amides using sulfoxide reagents under electrophilic amide activation conditions is described. Employing convenient and readily available reagents, selective functionalization takes place to generate isolable sulfonium ions en route to α-sulfide amides. Mechanistic studies identified activated sulfoxides as promoters of the desired transformation and enabled the extension of the methodology from benzylic to aliphatic amide substrates.

Amide activation: an emerging tool for chemoselective synthesis

It is textbook knowledge that carboxamides benefit from increased stabilisation of the electrophilic carbonyl carbon when compared to other carbonyl and carboxyl derivatives. This results in a considerably reduced reactivity towards nucleophiles. Accordingly, a perception has been developed of amides as significantly less useful functional handles than their ester and acid chloride counterparts. However, a significant body of research on the selective activation of amides to achieve powerful transformations under mild conditions has emerged over the past decades. This review article aims at placing electrophilic amide activation in both a historical context and in that of natural product synthesis, highlighting the synthetic applications and the potential of this approach.

An Interrupted Schmidt Reaction: C-C Bond Formation Arising from Nitrilium Ion Capture

The rerouting of the nitrilium ion formed in the Schmidt reaction of ketones and TMSN₃ to encompass C-C bond formation with an electron-rich aromatic group is reported. Thus, when the reaction is carried out in HFIP using AlCl₃ or AlBr₃ as the promoter, imines, iminium ions, or enamide derivatives are obtained through one-pot procedures. The scope and possible mechanisms of these new transformations are considered.

Enantioselective and Divergent Syntheses of Alstoscholarisines A, E and Their Enantiomers

Concise, enantioselective, and divergent syntheses of alstoscholarisines A and E are presented in 8 and 9 steps, respectively; alstoscholarisine E has never been accessed before. A boron-mediated aldol reaction and Rh-catalyzed cycloisomerization were exploited to access stereoisomers 8 and 9 as key intermediates. The challenging sterically congested alstoscholarisine core was furnished by a reductive transannular cyclization in the final steps. This strategy was also used for the syntheses of enantiomers of alstoscholarisines A and E.

Enantioselective Synthesis of (-)-Vallesine: Late-Stage C17-Oxidation via Complex Indole Boronation

The first enantioselective total synthesis of (-)-vallesine via a strategy that features a late-stage regioselective C17-oxidation followed by a highly stereoselective transannular cyclization is reported. The versatility of this approach is highlighted by the divergent synthesis of the archetypal alkaloid of this family, (+)-aspidospermidine, and an A-ring-oxygenated derivative, (+)-deacetylaspidospermine, the precursor to (-)-vallesine, from a common intermediate.

Enantioselective Total Synthesis of (-)-Deoxoapodine

The first enantioselective total synthesis of (-)-deoxoapodine is described. Our synthesis of this hexacyclic aspidosperma alkaloid includes an efficient molybdenum-catalyzed enantioselective ring-closing metathesis reaction for the desymmetrization of an advanced intermediate that introduces the C5-quaternary stereocenter. After C21-oxygenation, the pentacyclic core was accessed by electrophilic C19-amide activation and transannular spirocyclization. A biogenetically inspired dehydrative C6-etherification reaction proved highly effective to secure the F-ring and the fourth contiguous stereocenter of (-)-deoxoapodine with complete stereochemical control.

Concise Total Syntheses of (+)-Haplocidine and (+)-Haplocine via Late-Stage Oxidation of (+)-Fendleridine Derivatives

We report the first total syntheses of (+)-haplocidine and its N1-amide congener (+)-haplocine. Our concise synthesis of these alkaloids required the development of a late-stage and highly selective C-H oxidation of complex aspidosperma alkaloid derivatives. A versatile, amide-directed ortho-acetoxylation of indoline amides enabled our implementation of a unified strategy for late-stage diversification of hexacyclic C19-hemiaminal ether structures via oxidation of the corresponding pentacyclic C19-iminium ions. An electrophilic amide activation of a readily available C21-oxygenated lactam, followed by transannular cyclization and in situ trapping of a transiently formed C19-iminium ion, expediently provided access to hexacyclic C19-hemiaminal ether alkaloids (+)-fendleridine, (+)-acetylaspidoalbidine, and (+)-propionylaspidoalbidine. A highly effective enzymatic resolution of a non-β-branched primary alcohol (E = 22) allowed rapid preparation of both enantiomeric forms of a C21-oxygenated precursor for synthesis of these aspidosperma alkaloids. Our synthetic strategy provides succinct access to hexacyclic aspidosperma derivatives, including the antiproliferative alkaloid (+)-haplocidine.

Direct Observation of Intermediates Involved in the Interruption of the Bischler-Napieralski Reaction

The first mechanistic investigation of electrophilic amide activation of α,α-disubstituted tertiary lactams and the direct observation of key intermediates by in situ FTIR, (1)H, (13)C, and (19)F NMR in our interrupted Bischler-Napieralski-based synthetic strategy to the aspidosperma alkaloids, including a complex tetracyclic diiminium ion, is discussed. The reactivity of a wide range of pyridines with trifluoromethanesulfonic anhydride was systematically examined, and characteristic IR absorption bands for the corresponding N-trifluoromethanesulfonylated pyridinium trifluoromethanesulfonates were assigned. The reversible formation of diiminium ether intermediates was studied, providing insight into divergent mechanistic pathways as a function of the steric environment of the amide substrate and stoichiometry of reagents. Importantly, when considering base additives during electrophilic amide activation, more hindered α-quaternary tertiary lactams require the use of non-nucleophilic pyridine additives in order to avoid deactivation via a competing desulfonylation reaction. The isolation and full characterization of a tetracyclic iminium trifluoromethanesulfonate provided additional correlation between in situ characterization of sensitive intermediates and isolable compounds involved in this synthetic transformation.

Electrophilic Carbonyl Activation: Competing Condensative Cyclizations of Tryptamine Derivatives

A series of tryptamine derived bisindole substrates were subject to electrophilic activation of the functional grouping at their alpha-nitrogen in the form of iminium ions to enable cyclization onto the sterically hindered indole substructure. Our observations regarding divergent cyclization outcomes using electronically distinct bisindole substrates are described. Surprising preference for Friedel-Crafts alkylation reaction and evidence for an intriguing reversible spirocyclization are discussed.

Direct transformation of amides: a one-pot reductive Ugi-type three-component reaction of secondary amides

An efficient reductive Ugi-type reaction employing common secondary amides as starting materials has been established. The reaction exhibited a broad substrate scope, good chemoselectivity and functional group tolerance.

Aza-Knoevenagel-type condensation of secondary amides: direct access to N-monosubstituted β,β-difunctionalized enamines

An efficient approach toN-monosubstituted β,β-difunctionalized enamines, a class of versatile building blocks for the synthesis of bioactive compounds, is reported.

C7-derivatization of C3-alkylindoles including tryptophans and tryptamines

A versatile strategy for C7-selective boronation of tryptophans, tryptamines, and 3-alkylindoles by way of a single-pot C2/C7-diboronation-C2-protodeboronation sequence is described. The combination of a mild iridium-catalyzed C2/C7-diboronation followed by an in situ palladium-catalyzed C2-protodeboronation allows efficient entry to valuable C7-boroindoles that enable further C7-derivatization. The versatility of the chemistry is highlighted by the gram-scale synthesis of C7-boronated N-Boc-L-tryptophan methyl ester and the rapid synthesis of C7-halo, C7-hydroxy, and C7-aryl tryptophan derivatives.

Concise and enantioselective total synthesis of (-)-mehranine, (-)-methylenebismehranine, and related Aspidosperma alkaloids

We report an efficient and highly stereoselective strategy for the synthesis of Aspidosperma alkaloids based on the transannular cyclization of a chiral lactam precursor. Three new stereocenters are formed in this key step with excellent diastereoselectivity due to the conformational bias of the cyclization precursor, leading to a versatile pentacyclic intermediate. A subsequent stereoselective epoxidation followed by a mild formamide reduction enabled the first total synthesis of the Aspidosperma alkaloids (-)-mehranine and (+)-(6S,7S)-dihydroxy-N-methylaspidospermidine. A late-stage dimerization of (-)-mehranine mediated by scandium trifluoromethanesulfonate completed the first total synthesis of (-)-methylenebismehranine.

Silver triflate and triflic anhydride-promoted expedient synthesis of acylated 1-aminoisoquinolines

A practical and convergent synthesis of biologically active 1-(N-acyl)-1-aminoisoquinolines from the reaction of 2-alkynylbenzaldoximes with amides has been realized. The readily available amides could be activated with triflic anhydride (Tf2O) and could efficiently participate in the domino reaction of 2-alkynylbenzaldoximes when catalyzed by AgOTf, thus providing various acylated 1-aminoisoquinolines with up to 98% yields.