Synthesis of the Acutumine Spirocycle via a Radical−Polar Crossover Reaction

Chemo- and Enantioselective Reduction of α-Keto Amides to α-Hydroxy Amides using Reusable CuO-Nanoparticles as Catalyst

An efficient, commercially available, and reusable copper-oxide nanoparticle (CuO-NPs) and (R)-(-)-DTBM SEGPHOS catalyzed chemo- and enantioselective reduction of α-keto amides to α-hydroxy amides has been developed. The scope of the reaction has been studied with various α-keto amides containing electron-donating and electron-withdrawing groups affording the enantiomerically enriched α-hydroxy amides in good yields with excellent enantioselectivity. The CuO-NPs catalyst has been recovered and reused up to four catalytic cycles without any significant change in particle size, reactivity, and enantioselectivity.

Asymmetric Total Synthesis of Hasubanan Alkaloids: Periglaucines A-C, N,O-Dimethyloxostephine and Oxostephabenine

We report herein the asymmetric total synthesis of periglaucines A-C, N,O-dimethyloxostephine and oxostephabenine. The key strategies used include: 1) a Rh^I -catalyzed regio- and diastereoselective Hayashi-Miyaura reaction to connect two necessary fragments; 2) an intramolecular photoenolization/Diels-Alder (PEDA) reaction to construct the highly functionalized tricyclic core skeleton bearing a quaternary center; 3) a bio-inspired intramolecular Michael addition and transannular acetalization to generate the aza[4.4.3]propellane and the tetrahydrofuran ring.

Enantioselective Synthesis of (-)-10-Hydroxyacutuminine

An enantioselective synthesis of (-)-10-hydroxyacutuminine is reported. Central to our strategy is a photochemical [2+2] cycloaddition that forges two of the quaternary stereocenters present in the acutumine alkaloids. A subsequent retro-aldol/Dieckmann sequence furnishes the spirocyclic cyclopentenone. Efforts to chlorinate the acutumine scaffold at C10 under heterolytic or radical deoxychlorination conditions led to the synthesis of an unexpected cyclopropane-containing pentacycle.

Applications of Halogen-Atom Transfer (XAT) for the Generation of Carbon Radicals in Synthetic Photochemistry and Photocatalysis

The halogen-atom transfer (XAT) is one of the most important and applied processes for the generation of carbon radicals in synthetic chemistry. In this review, we summarize and highlight the most important aspects associated with XAT and the impact it has had on photochemistry and photocatalysis. The organization of the material starts with the analysis of the most important mechanistic aspects and then follows a subdivision based on the nature of the reagents used in the halogen abstraction. This review aims to provide a general overview of the fundamental concepts and main agents involved in XAT processes with the objective of offering a tool to understand and facilitate the development of new synthetic radical strategies.

Et3 B/Et2 AlCl/O2 -Mediated Radical Coupling Reaction between α-Alkoxyacyl Tellurides and 2-Hydroxybenzaldehyde Derivatives

A newly devised radical-based strategy enabled coupling between multiply oxygenated α-alkoxyacyl tellurides and 2-hydroxybenzaldehyde derivatives. A reagent combination of Et₃ B, Et₂ AlCl, and O₂ promoted the formation of the α-alkoxy carbon radical from the α-alkoxyacyl telluride and the addition of the radical to the carbonyl group of 2-hydroxybenzaldehyde. The reaction chemo- and stereoselectively forged the hindered C-C bond between two oxygen-functionalized carbons at ambient temperature. The method was applied to the preparation of 12 coupling adducts with three to six contiguous stereocenters and to the concise synthesis of an antitumor compound, LLY-283.

Synthetic, Mechanistic, and Biological Interrogation of Ginkgo biloba Chemical Space En Route to (-)-Bilobalide

Here we interrogate the structurally dense (1.64 mcbits/Å3) GABAA receptor antagonist bilobalide, intermediates en route to its synthesis, and related mechanistic questions. 13C isotope labeling identifies an unexpected bromine migration en route to an α-selective, catalytic asymmetric Reformatsky reaction, ruling out an asymmetric allylation pathway. Experiment and computation converge on the driving forces behind two surprising observations. First, an oxetane acetal persists in concentrated mineral acid (1.5 M DCl in THF-d8/D2O); its longevity is correlated to destabilizing steric clash between substituents upon ring-opening. Second, a regioselective oxidation of des-hydroxybilobalide is found to rely on lactone acidification through lone-pair delocalization, which leads to extremely rapid intermolecular enolate equilibration. We also establish equivalent effects of (-)-bilobalide and the nonconvulsive sesquiterpene (-)-jiadifenolide on action potential-independent inhibitory currents at GABAergic synapses, using (+)-bilobalide as a negative control. The high information density of bilobalide distinguishes it from other scaffolds and may characterize natural product (NP) space more generally. Therefore, we also include a Python script to quickly (ca. 132 000 molecules/min) calculate information content (Böttcher scores), which may prove helpful to identify important features of NP space.

Convergent Total Synthesis of Hikizimycin Enabled by Intermolecular Radical Addition to Aldehyde

Hikizimycin (1), which exhibits powerful anthelmintic activity, has the most densely functionalized structure among nucleoside antibiotics. A central 4-amino-4-deoxyundecose of 1 possesses 10 contiguous stereocenters on a C1-C11 linear chain and is decorated with a cytosine base at C1 and a 3-amino-3-deoxyglucose at C6-OH. These distinctive structural features of 1 make it an extremely challenging target for de novo construction. Herein, we report a convergent total synthesis of 1 from four known components: 3-azide-3-deoxyglucose derivative 4, bis-TMS-cytosine 5, d-mannose 9, and d-galactose derivative 10. We first designed and devised a novel radical coupling reaction between multiply hydroxylated aldehydes and α-alkoxyacyl tellurides. The generality and efficiency of this process was demonstrated by the coupling of 7c and 8, which were readily accessible from two hexoses, 9 and 10, respectively. Et₃B and O₂ rapidly induced decarbonylative radical formation from α-alkoxyacyl telluride 8, and intermolecular addition of the generated α-alkoxy radical to aldehyde 7c yielded 4-amino-4-deoxyundecose 6-α with installation of the desired C5,6-stereocenters. Subsequent attachments of the cytosine with 5 and of the 3-azide-3-deoxyglucose with 4 were realized through selective activation of the C1-acetal and selective deprotection of the C6-hydroxy group. Finally, the 3 amino and 10 hydroxy groups were liberated in a single step to deliver the target 1. Thus, the combination of the newly developed radical-coupling and protective-group strategies minimized the functional group manipulations and thereby enabled the synthesis of 1 from 10 in only 17 steps. The present total synthesis demonstrates the versatility of intermolecular radical addition to aldehyde for the first time and offers a new strategic design for multistep target-oriented syntheses of various nucleoside antibiotics and other bioactive natural products.

Enantioselective total synthesis of pyrrolo-[2,1-c][1,4]-benzodiazepine monomers (S)-(−)-barmumycin and (S)-(+)-boseongazepine B

An efficient enantioselective total synthesis of pyrrolo-[2,1-c][1,4]benzodiazepine (PBD) monomers (S)-(−)-barmumycin and (S)-(+)-boseongazepine B was achieved through a stereocontrolled strategy, which relies on a proline catalysed asymmetric α-amination and ester α-ethylenation.

Synthetic Strategies toward Natural Products Containing Contiguous Stereogenic Quaternary Carbon Atoms

Strategies for the total synthesis of complex natural products that contain two or more contiguous stereogenic quaternary carbon atoms in their intricate structures are reviewed with 12 representative examples. Emphasis has been put on methods to create quaternary carbon stereocenters, including syntheses of the same natural product by different groups, thereby showcasing the diversity of thought and individual creativity. A compendium of selected natural products containing two or more contiguous stereogenic quaternary carbon atoms and key reactions in their total or partial syntheses is provided in the Supporting Information.

An Enantioselective Approach to 4-O-Protected-2-cyclopentene-l,4-diol Derivatives via a Rhodium-Catalyzed Redox-Isomerization Reaction

Kinetic resolution of a series of cyclopentene-1,4-diol derivatives has been successfully achieved with enantiomeric excess up to 99.4% and a kf/ks ratio of 55 by a rhodium-catalyzed redox-isomerization reaction in a noncoordinating solvent.

A highly convergent synthesis of the C1-C31 polyol domain of amphidinol 3 featuring a TST-RCM reaction: confirmation of the revised relative stereochemistry

The concise enantioselective synthesis of the revised C1-C31 fragment of the polyketide amphidinol 3 was accomplished in 16 steps and 12.8% overall yield. Salient features of the strategy include chemoselective Weinreb amide coupling and concomitant CBS reduction for the preparation of the C1-C15 tris-syn-1,5-diol motif and a temporary silicon-tethered ring-closing metathesis (TST-RCM) reaction in combination with a diastereoselective hydroboration for the construction of the C16-C31 polypropionate fragment. The union of the fragments was accomplished by a regioselective ring-opening of the terminal epoxide with a phenyl sulfone stabilized carbanion, which upon reduction and deprotection permits a comparison of the relative configuration with the natural product.

Progress and developments in the turbo Grignard reagent i-PrMgCl·LiCl: a ten-year journey

The structural and kinetic perspectives of i-PrMgCl·LiCl help to rationalize the trends of its unique reactivity and selectivity.

Substrate-modified functional group reactivity: hasubanan and acutumine alkaloid syntheses

Functional group taxonomy provides a powerful conceptual framework to classify and predict the chemical reactivity of molecular structures. These principals are most effective in monofunctional settings, wherein individual functional groups can be analyzed without complications. In more complex settings, the predictive value of these analyses decreases as alternative reaction pathways, promoted by neighboring substituents and aggregate molecular properties, emerge. We refer to this phenomenon as substrate-modified functional group reactivity. In this Perspective, we explain how substrate-modified functional group reactivity molded our synthetic routes to the hasubanan and acutumine alkaloids. These investigations underscore the potential for discovery and insight that can only be gained by studying the reactivity of complex multifunctional structures.

Total synthesis of the congested propellane alkaloid (-)-acutumine

The enantioselective total synthesis of (-)-acutumine is described. The synthetic strategy was inspired by the premise that the cyclohexenone ring could be derived from an aromatic precursor. After successful construction of a propellane model system, an initial attempt to prepare the spirocyclic subunit was thwarted by incorrect regioselectivity in a radical cyclization. A second-generation approach involving a radical-polar crossover reaction was successful, and the chemistry developed in the aforementioned model system was then applied to synthesize the natural product. Key reactions included a phenolic oxidation, a diastereoselective ketone allylation utilizing Nakamura's chiral allylzinc reagent, an anionic oxy-Cope rearrangement, an acid-promoted cyclization of a secondary amine onto an α,β-unsaturated ketal, and a regioselective methyl enol etherification of a 1,3-diketone.

The Hasubanan and Acutumine Alkaloids

Research in the hasubanan and acutumine alkaloid fields up to 1970 was discussed under "morphine alkaloids" in Volume 13 of this chapter. Advances in the field of hasubanan alkaloids from 1971 to 1975 were reviewed in Volume 16 and from 1976 to 1986 in Volume 33. This chapter extends the information in the three preceding reviews to hasubanan alkaloid literature published from 1987 to June 2013. This chapter covers acutumine alkaloid literature since (-)-acutumine (3) was isolated in 1929. This chapter includes occurrence and physical constants, new alkaloids, synthesis, biosynthesis, and pharmacology. Section 1 introduces the foremost alkaloids, (-)-hasubanonine (1) and (-)-acutumine (3), and the numbering systems of the hasubanan (2) and acutumine (4) skeletons. Section 2 details the occurrence and physical constants of 29 new hasubanan and 15 acutumine alkaloids. The isolation and structural determination of these new alkaloids are described in Section 3. Section 4 summarizes total syntheses and synthetic studies toward hasubanan and acutumine alkaloids. Completed syntheses of the hasubanan alkaloids (+)-cepharamine (ent-71), (-)-hasubanonine (1), (-)-runanine (8), (-)-delavayine (6), (+)-periglaucine B (19), and (-)-8-demethoxyrunanine (12) are reviewed. Completed syntheses of (-)-acutumine (3) and (-)-dechloroacutumine (52) are also described. Section 5 details biosyntheses of (-)-acutumine (3) advanced by Barton, Wipf, and Sugimoto. Section 6 summarizes pharmacological studies of hasubanan and acutumine alkaloids. Opioid receptor affinity, anti-HBV activity, and antimicrobial activity of hasubanan alkaloids are reported. Antiamnesic properties, cytotoxicity, and anti-HBV activity of acutumine alkaloids are described.

Stereodivergent synthesis of enantioenriched 4-hydroxy-2-cyclopentenones

Protected 4-hydroxycyclopentenones (4-HCPs) constitute an important class of intermediates in chemical synthesis. A route to this class of compound has been developed. Key steps include Noyori reduction (which establishes the stereochemistry of the product), ring-closing metathesis, and simple functional group conversions to provide a set of substituted 4-HCPs in either enantiomeric form.

Development of enantioselective synthetic routes to the hasubanan and acutumine alkaloids

We describe a general strategy to prepare the hasubanan and acutumine alkaloids, a large family of botanical natural products that display antitumor, antiviral, and memory-enhancing effects. The absolute stereochemistry of the targets is established by an enantioselective Diels-Alder reaction between 5-(trimethylsilyl)cyclopentadiene (36) and 5-(2-azidoethyl)-2,3-dimethoxybenzoquinone (24). The Diels-Alder adduct 38 is transformed to the tetracyclic imine 39 by a Staudinger reduction-aza-Wittig sequence. The latter serves as a universal precursor to the targets. Key carbon-carbon bond constructions include highly diastereoselective acetylide additions to the N-methyliminium ion derived from 39 and Friedel-Crafts and Hosomi-Sakurai cyclizations to construct the carbocyclic skeleton of the targets. Initially, this strategy was applied to the syntheses of (-)-acutumine (4), (-)-dechloroacutumine (5), and four hasubanan alkaloids (1, 2, 3, and 8). Herein, the synthetic route is adapted to the syntheses of six additional hasubanan alkaloids (12, 13, 14, 15, 18, and 19). The strategic advantage of 5-(trimethylsilyl)cyclopentadiene Diels-Alder adducts is demonstrated by site-selective functionalization of distal carbon-carbon π-bonds in the presence of an otherwise reactive norbornene substructure. Evaluation of the antiproliferative properties of the synthetic metabolites revealed that four hasubanan alkaloids are submicromolar inhibitors of the N87 cell line.

Enantioselective total synthesis of (-)-acutumine

An account of the total synthesis of the tetracyclic alkaloid (-)-acutumine is presented. A first-generation approach to the spirocyclic subunit was unsuccessful as a result of incorrect regioselectivity in a radical cyclization. However, this work spawned a second-generation strategy in which the spirocycle was fashioned via a radical-polar crossover reaction. This process merged an intramolecular radical conjugate addition with an enolate hydroxylation and created two stereocenters with excellent diastereoselectivity. The reaction was promoted by irradiation with a sunlamp, and a ditin reagent was required for aryl radical formation. These facts suggest that the substrate may function as a sensitizer, thereby facilitating homolytic cleavage of the ditin reagent. The propellane motif of the target was then installed via annulation of a pyrrolidine ring onto the spirocycle. The sequence of reactions used included a phenolic oxidation, an asymmetric ketone allylation mediated by Nakamura's chiral allylzinc reagent, an anionic oxy-Cope rearrangement, a one-pot ozonolysis-reductive amination, and a Lewis acid promoted cyclization of an amine onto an alpha,beta-unsaturated dimethyl ketal. Further studies of the asymmetric ketone allylation demonstrated the ability of the Nakamura reagent to function well in a mismatched situation. A TiCl(4)-catalyzed regioselective methyl enol etherification of a 1,3-diketone completed the synthesis.

Kinetics of bromine-magnesium exchange reactions in substituted bromobenzenes

Competition experiments have been performed to determine the relative reactivities of substituted bromobenzenes, bromonaphthalenes, and 9-bromoanthracene toward i-PrMgCl x LiCl in THF at 0 degrees C. The rates of the bromine-magnesium exchange reactions are accelerated by electron-acceptor substituents, the activating efficiency of which increases in the order para < meta << ortho. The activation free enthalpies of the bromine-magnesium exchange reactions correlate fairly (r(2) = 0.83) with the proton affinities of analogously substituted aryllithiums (slope 0.8). The kinetics of two representative bromoarenes with i-PrMgCl x LiCl were found to be first-order in both bromoarene and i-PrMgCl x LiCl. Combination of the resulting second-order rate constants with the k(rel) values from competition experiments allowed us to calculate reaction times for the bromine-magnesium exchange reactions of a large variety of bromoarenes.

Synthesis of the common propellane core structure of the hasubanan alkaloids

The racemic synthesis of the common propellane core structure found in various hasubanan alkaloids is reported. The successful completion hinged upon the stereocontrolled construction of the cis-substituted heterobicycle as a precursor for the intramolecular Dieckmann condensation. A novel strategy is introduced for the facile hydrolysis of a sterically demanding carboxamide under a mild condition. The 2-nitroanilide obtained by the Goldberg arylation of a carboxamide with 2-iodonitrobenzene was readily converted to the corresponding ester derivative by way of N-acylbenzotriazole. We expect that the reported synthetic route will allow the synthesis of a series of hasubanan alkaloids starting from the correspondingly functionalized 2-tetralone derivatives.