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.

Orthogonal sp3-Ge/B Bimetallic Modules: Enantioselective Construction and Enantiospecific Cross-Coupling

In this study, a series of enantioenriched sp3-Ge/B bimetallic modules were successfully synthesized via an enantioselective copper-catalyzed hydroboration of carbagermatrane (Ge)-containing alkenes. Orthogonal cross-coupling selectivity under different Pd-catalyzed conditions was achieved in an enantiospecific manner. Notably, the chiral secondary Ge exhibited a remarkable transmetallation ability prior to primary or secondary Bpin. The effectiveness of this Ge/B bimetallic strategy was further demonstrated through the development of new functional small molecules with Aggregation-Induced Emission (AIE) and Circularly Polarized Luminescence (CPL) performance. This represents the first successful example of synthesis of enantioenriched alkylgermanium reagents that permit enantiospecific cross-coupling reactions.

Recent Developments with Organogermanes: their Preparation and Application in Synthesis and Catalysis

Within the sphere of traditional Pd0 /PdII cross coupling reactions, organogermanes have been historically outperformed both in terms of scope and reactivity by more conventional transmetalating reagents. Subsequently, this class of compounds has been largely underutilized as a coupling partner in bond-forming strategies. Most recent studies, however, have shown that alternative modes of activation of these notoriously robust building blocks transform organogermanes into the most reactive site of the molecule-capable of outcompeting other functional groups (such as boronic acids, esters and silanes) for both C-C and C-heteroatom bond formation. As a result, over the past few years, the literature has increasingly featured methodologies that explore the potential of organogermanes as chemoselective and orthogonal coupling partners. Herein we highlight some of these recent advances in the field of organogermane chemistry both with respect to their synthesis and applications in synthetic and catalytic transformations.

An asymmetric metal-templated route to amino acids with an isoquinolone core via a Rh(III)-catalyzed coupling of aryl hydroxamates with chiral propargylglycine Ni(II) complexes

A general protocol for the asymmetric synthesis of artificial amino acids (AAs) comprising an isoquinolone skeleton was successfully elaborated via a straightforward Rh(III)-catalyzed C-H activation/annulation of various aryl hydroxamates with a series of robust chiral propargylglycine Ni(II) complexes derived from glycine (Gly), alanine (Ala) and phenylalanine (Phe) in a green solvent (methanol) under mild conditions (at room temperature under air). Notably, in the case of phenylalanine-derived complexes, the formation of unfavorable 4-substituted isoquinolone regioisomers was achieved by a catalyst control for the first time. The subsequent acidic decomposition of the obtained Ni(II) complexes provides the target unnatural α- and α,α-disubstituted AAs with an isoquinolone core in an enantiopure form.

Germatranes and carbagermatranes: (hetero)aryl and alkyl coupling partners in Pd-catalyzed cross-coupling reactions

In the past few decades, palladium-catalyzed cross-coupling reactions have taken root in the construction of a complex synthetic community. The development of organometallics has been an important objective in this field. Our group has focused on exploiting new germanium-based reagents and the corresponding catalytic processes. In the past three years, we have established new methods for the synthesis of structure-modified (hetero)aryl germatranes and alkyl carbagermatranes. Particularly for alkyl carbagermatranes, the stability to be compatible with various derivatization reactions and the high activity for transmetallation (e.g. base/additive-free for primary alkyl carbagermatranes) distinguish them from many reported nucleophiles. In this article, we would introduce (1) the development process of organogermanium reagents in palladium-catalyzed cross-couplings; (2) the history of germatrane-type systems and the breakthrough we have made in the field; (3) the outlook for (carba)germatranes and alkyl-GeMe₃.

Tertiary cyclopropyl carbagermatranes: synthesis and cross-coupling

The construction of the cyclopropyl quaternary carbon center can afford a series of 1,1-olefin bioisosteres. Here, we report tertiary cyclopropyl carbagermatranes, which can be easily obtained by the zinc-mediated decarboxylation of NHP esters. In addition, they exhibit efficient reactivity in the palladium-catalyzed cross-coupling reaction and orthogonal reactivity with boron reagents, therefore acting as robust nucleophiles for the synthesis of tertiary cyclopropane and efficient intermediates for the formation of quaternary centers.

Synthesis of Dialkyl-Substituted Monofluoroalkenes via Palladium-Catalyzed Cross-Coupling of Alkyl Carbagermatranes

An unprecedented cross-coupling reaction of alkyl carbagermatranes with bromofluoroolefins to deliver dialkyl-substituted monofluoroalkenes was achieved. This cross-coupling reaction was performed under base/additive-free conditions with excellent functional group tolerance, therefore offering an opportunity for challenging dialkyl-substituted monofluoroalkenes. The preparation of bioactive agent analogues including an antitubercular agent mimic and a COX-2 inhibitor analogue and the late-stage fluoroalkenylation of drug-molecule derivatives proved the utility of this strategy.

Synthesis of C3-sulfone substituted naphthols via rhodium(III)-catalyzed annulation of sulfoxonium ylides with alkynylsulfones

C-H activation of sulfoxonium ylides catalyzed by rhodium(iii) with subsequent annulation by alkynylsulfones was accomplished. This methodology offers a step-economical approach for assembling C3-sulfone-substituted naphthols with a high level of regioselectivity that is complementary to previous protocols. The approach has an extensive substrate spectrum and broad functional group tolerance.

Ni-Catalyzed Reductive and Merged Photocatalytic Cross-Coupling Reactions toward sp3/sp2-Functionalized Isoquinolones: Creating Diversity at C-6 and C-7 to Address Bioactive Analogues

Naturally occurring isoquinolones have gained considerable attention over the years for their bioactive properties. While the late-stage introduction of various functionalities at certain positions, namely, C-3, C-4, and C-8, has been widely documented, the straightforward introduction of challenging sp3 carbon-linked acyclic aminoalkyl or aza- and oxacyclic appendages at C-6 and C-7 remains largely underexplored. Interest in 6-substituted azacyclic analogues has recently garnered attention in connection with derivatives exhibiting anticancer activity. Reported here is the first application of the versatile and recently emerging field of Ni-catalyzed reductive cross-coupling reactions to the synthesis of 6- and 7- hetero(cyclo)alkyl-substituted isoquinolones. In a second and complementary approach, a new set of C-6- and C-7-substituted positional isomers of hetero(cyclo)alkyl appendages were obtained from the merging of photocatalytic and Ni-catalyzed coupling reactions. In both cases, 6- and 7-bromo isoquinolones served as dual-purpose reacting partners with readily available tosylates and carboxylic acids, respectively.

Ruthenium-catalyzed cascade C-H activation/annulation of N-alkoxybenzamides: reaction development and mechanistic insight

A highly selective ruthenium-catalyzed C–H activation/annulation of alkyne-tethered N-alkoxybenzamides has been developed. In this reaction, diverse products from inverse annulation can be obtained in moderate to good yields with high functional group compatibility. Insightful experimental and theoretical studies indicate that the reaction to the inverse annulation follows the Ru(ii)–Ru(iv)–Ru(ii) pathway involving N–O bond cleavage prior to alkyne insertion. This is highly different compared to the conventional mechanism of transition metal-catalyzed C–H activation/annulation with alkynes, involving alkyne insertion prior to N–O bond cleavage. Via this pathway, the in situ generated acetic acid from the N–H/C–H activation step facilitates the N–O bond cleavage to give the Ru-nitrene species. Besides the conventional mechanism forming the products via standard annulation, an alternative and novel Ru(ii)–Ru(iv)–Ru(ii) mechanism featuring N–O cleavage preceding alkyne insertion has been proposed, affording a new understanding of transition metal-catalyzed C–H activation/annulation.

A highly selective ruthenium-catalyzed C–H activation/annulation through a pathway involving N–O bond cleavage prior to alkyne insertion is developed.