Enantioselective desymmetrization of cyclopropenes by hydroacylation

We report an enantioselective desymmetrization of cyclopropenes by intermolecular Rh-catalyzed hydroacylation. Cyclopropylketones, bearing quaternary stereocenters, are produced with diastereocontrol (up to >20:1) and excellent enantiomeric excess (up to >99 ee).

A Unifying Synthesis Approach to the C18-, C19-, and C20-Diterpenoid Alkaloids

The secondary metabolites that comprise the diterpenoid alkaloids are categorized into C18, C19, and C20 families depending on the number of contiguous carbon atoms that constitute their central framework. Herein, we detail our efforts to prepare these molecules by chemical synthesis, including a photochemical approach, and ultimately a bioinspired strategy that has resulted in the development of a unifying synthesis of one C18 (weisaconitine D), one C19 (liljestrandinine), and three C20 (cochlearenine, paniculamine, and N-ethyl-1α-hydroxy-17-veratroyldictyzine) natural products from a common intermediate.

Recognition and site-selective transformation of monosaccharides by using copper(II) catalysis

We demonstrate copper(II)-catalyzed acylation and tosylation of monosaccharides. Various carbohydrate derivatives, including glucopyranosides and ribofuranosides, are obtained in high yields and regioselectivities. Using this versatile strategy, the site of acylation can be switched by choice of ligand. Preliminary mechanistic studies support nucleophilic addition of a copper-sugar complex to the acyl chloride to be turnover limiting.

Rh-Catalyzed Desymmetrization of α-Quaternary Centers by Isomerization-Hydroacylation

We describe a Rh-catalyzed desymmetrization of all-carbon quaternary centers from α,α-bis(allyl)aldehydes by a cascade featuring isomerization and hydroacylation. This desymmetrization competes with two other novel olefin functionalizations that are triggered by C-H bond activation, including carboacylation and bisacylation. A BIPHEP ligand promotes enantioselective formation of α-vinylcyclopentanones. Mechanistic studies support irreversible and enantioselective olefin-isomerization followed by olefin-hydroacylation.

A Benzyne Insertion Approach to Hetisine-Type Diterpenoid Alkaloids: Synthesis of Cossonidine (Davisine)

The hetisine-type natural products exhibit one of the most complex carbon skeletons within the diterpenoid alkaloid family. The use of network analysis has enabled a synthesis strategy to access alkaloids in this class with hydroxylation on the A-ring. Key transformations include a benzyne acyl-alkylation to construct a key fused 6-7-6 tricycle, a chemoselective nitrile reduction, and sequential C-N bond formations using a reductive cyclization and a photochemical hydroamination to construct an embedded azabicycle. Our strategy should enable access to myriad natural and unnatural products within the hetisine-type.

Syntheses of Denudatine Diterpenoid Alkaloids: Cochlearenine, N-Ethyl-1α-hydroxy-17-veratroyldictyzine, and Paniculamine

The denudatine-type diterpenoid alkaloids cochlearenine, N-ethyl-1α-hydroxy-17-veratroyldictyzine, and paniculamine have been synthesized for the first time (25, 26, and 26 steps from 16, respectively). These syntheses take advantage of a common intermediate (8) that we have previously employed in preparing aconitine-type natural products. The syntheses reported herein complete the realization of a unified strategy for the preparation of C20, C19, and C18 diterpenoid alkaloids.

Dynamic kinetic resolution of allylic sulfoxides by Rh-catalyzed hydrogenation: a combined theoretical and experimental mechanistic study

A dynamic kinetic resolution (DKR) of allylic sulfoxides has been demonstrated by combining the Mislow [2,3]-sigmatropic rearrangement with catalytic asymmetric hydrogenation. The efficiency of our DKR was optimized by using low pressures of hydrogen gas to decrease the rate of hydrogenation relative to the rate of sigmatropic rearrangement. Kinetic studies reveal that the rhodium complex acts as a dual-role catalyst and accelerates the substrate racemization while catalyzing olefin hydrogenation. Scrambling experiments and theoretical modeling support a novel mode of sulfoxide racemization which occurs via a rhodium π-allyl intermediate in polar solvents. In nonpolar solvents, however, the substrate racemization is primarily uncatalyzed. Computational studies suggest that the sulfoxide binds to rhodium via O-coordination throughout the catalytic cycle for hydrogenation.

Bioinspired chemical synthesis of monomeric and dimeric stephacidin A congeners

Stephacidin A and its congeners are a collection of secondary metabolites that possess intriguing structural motifs. They stem from unusual biosynthetic sequences that lead to the incorporation of a prenyl or reverse-prenyl group into a bicyclo[2.2.2]diazaoctane framework, a chromene unit or the vestige thereof. To complement biosynthetic studies, which normally play a significant role in unveiling the biosynthetic pathways of natural products, here we demonstrate that chemical synthesis can provide important insights into biosynthesis. We identify a short total synthesis of congeners in the reverse-prenylated indole alkaloid family related to stephacidin A by taking advantage of a direct indole C6 halogenation of the related ketopremalbrancheamide. This novel strategic approach has now made possible the syntheses of several natural products, including malbrancheamides B and C, notoamides F, I and R, aspergamide B, and waikialoid A, which is a heterodimer of avrainvillamide and aspergamide B. Our approach to the preparation of these prenylated and reverse-prenylated indole alkaloids is bioinspired, and may also inform the as-yet undetermined biosynthesis of several congeners.

Tandem rhodium catalysis: exploiting sulfoxides for asymmetric transition-metal catalysis

Sulfoxides are uncommon substrates for transition-metal catalysis due to their propensity to inhibit catalyst turnover. In a collaborative effort with Ken Houk, we developed the first dynamic kinetic resolution (DKR) of allylic sulfoxides using asymmetric rhodium-catalyzed hydrogenation. A detailed mechanistic analysis of this transformation using both experimental and theoretical methods revealed rhodium to be a tandem catalyst that promoted both hydrogenation of the alkene and racemization of the allylic sulfoxide. Using a combination of deuterium labelling and DFT studies, a novel mode of allylic sulfoxide racemization via a Rh(III)-π-allyl intermediate was identified.

Rhodium(I)-Catalyzed Intermolecular Hydroacylation of α-Keto Amides and Isatins with Non-Chelating Aldehydes

The application of the bidentate, electron-rich bisphosphine ligand, 1,3-bis(dicyclohexyl)phosphine-propane (dcpp), in rhodium(I)-catalyzed intermolecular ketone hydroacylation is herein described. Isatins and α-keto amides are shown to undergo hydroacylation with a variety of non-chelating linear and branched aliphatic aldehydes. Also reported is the synthesis of new bidentate chiral phosphine ligands, and their application in hydroacylation is discussed.

A Copper-Mediated Conjugate Addition Approach to Analogues of Aconitine-Type Diterpenoid Alkaloids

A copper-mediated conjugate addition of electron-rich aryl groups into a complex vinyl nitrile using arylmagnesium bromides is reported. The conjugate addition adducts were advanced toward the synthesis of designed aconitine-type analogues. The variation in oxygenation patterns on the arene coupling partner, introduced through the current conjugate addition approach, may ultimately provide insight into structure-activity relationships of the diterpenoid alkaloids.

The biology and total syntheses of bisbenzylisoquinoline alkaloids

This mini-review provides a concise overview of the biosynthetic pathway and pharmacology of bisbenzylisoquinoline alkaloid (bisBIA) natural products. Additional emphasis is given to the methodologies in the total syntheses of both simpler acyclic diaryl ether dimers and their macrocyclic counterparts bearing two diaryl ether linkages.

Synthesis of Protoberberine Alkaloids by C-H Functionalization and Anionic Aza-6π-Electrocyclization: Dual Activity as AMPK Activators and Inhibitors

5'-Adenosine monophosphate-activated protein kinase (AMPK) plays a critical role in maintaining cellular energy homeostasis, and its activation has garnered attention for treating chronic metabolic diseases. Inhibitors of AMPK are underdeveloped but bear implications in treating cancers, controlling autophagy, and elderly wasting. Protoberberine alkaloids are typically regarded as AMPK activators. Herein, we report a modular synthesis strategy to access a collection of oxyberberine alkaloids, including the first synthesis of stepharotudine. In vitro assays reveal how subtle structural modifications can negate AMPK activation while conferring unprecedented inhibitory properties within the same class of compounds, which was previously unknown. Key steps in the synthesis include an oxidative Rh(III)-catalyzed C-H functionalization using electron-rich alkenes, NaH-mediated reductive N-O bond cleavage, and a rare example of an anionic aza-6π-electrocyclization. Additionally, we provide mechanistic support for nucleophilic hydride transfer reactivity with NaH in DMF.

A Fenton Approach to Aromatic Radical Cations and Diarylmethane Synthesis

Manipulating carbon-centered radicals to add to electron-deficient systems is a well-precedented process. By coupling the Fe(II)-mediated Fenton reaction with the Fe(III)-mediated single-electron oxidation of anisolic compounds, we demonstrate how electron-rich carbon-centered radicals can react with electron-rich arenes through a radical-polar cascade pathway. This bioinspired approach produces diarylmethane derivatives from simple unfunctionalized precursors.

Synergistic Brønsted/Lewis acid catalyzed aromatic alkylation with unactivated tertiary alcohols or di-tert-butylperoxide to synthesize quaternary carbon centers

Dual Brønsted/Lewis acid catalysis involving environmentally benign, readily accessible protic acid and iron promotes site-selective tert-butylation of electron-rich arenes using di-tert-butylperoxide. This transformation inspired the development of a synergistic Brønsted/Lewis acid catalyzed aromatic alkylation that fills a gap in the Friedel-Crafts reaction literature by employing unactivated tertiary alcohols as alkylating agents, leading to new quaternary carbon centers. Corroborated by DFT calculations, the Lewis acid serves a role in enhancing the acidity of the Brønsted acid. The use of non-allylic, non-benzylic, and non-propargylic tertiary alcohols represents an underexplored area in Friedel-Crafts reactivity.

Semisynthesis of bersavine and berbamine derivatives that target the CaMKIIγ:cMyc axis for lymphoma therapy

Berbamine, a bisbenzylisoquinoline alkaloid (bisBIA), is a promising lead for developing novel therapeutics to treat aggressive cancers such as lymphoma, by targeting the CaMKIIγ:cMyc axis. Herein, we report an aza-Friedel-Crafts method for ortho-aminoalkylation of berbamine's phenolic motif, enabling the semisynthesis of the natural product bersavine and analogs that complement current methods focusing on modifying the phenolic oxygen. Several new analogs synthesized by this method exhibit potent cytotoxicity against lymphoma-associated cell line H9 exceeding the naturally occurring berbamine (1) and bersavine (3a). A molecular docking analysis was used to devise a model that rationalizes the structure-activity relationship between the novel bisBIA analogs and CaMKIIγ inhibition.