Nickel-Electrocatalytic Decarboxylative Arylation to Access Quaternary Centers

There is a pressing need, particularly in the field of drug discovery, for general methods that will enable direct coupling of tertiary alkyl fragments to (hetero)aryl halides. Herein a uniquely powerful and simple set of conditions for achieving this transformation with unparalleled generality and chemoselectivity is disclosed. This new protocol is placed in context with other recently reported methods, applied to simplify the routes of known bioactive building blocks molecules, and scaled up in both batch and flow. The role of pyridine additive as well as the mechanism of this reaction are interrogated through Cyclic Voltammetry studies, titration experiments, control reactions with Ni(0) and Ni(II)-complexes, and ligand optimization data. Those studies indicate that the formation of a BINAPNi(0) is minimized and the formation of an active pyridine-stabilized Ni(I) species is sustained during the reaction. Our preliminary mechanistic studies ruled out the involvement of Ni(0) species in this electrochemical cross-coupling, which is mediated by Ni(I) species via a Ni(I)-Ni(II)-Ni(III)-Ni(I) catalytic cycle.

Complex molecule synthesis by electrocatalytic decarboxylative cross-coupling

Modern retrosynthetic analysis in organic chemistry is based on the principle of polar relationships between functional groups to guide the design of synthetic routes1. This method, termed polar retrosynthetic analysis, assigns partial positive (electrophilic) or negative (nucleophilic) charges to constituent functional groups in complex molecules followed by disconnecting bonds between opposing charges2-4. Although this approach forms the basis of undergraduate curriculum in organic chemistry5 and strategic applications of most synthetic methods6, the implementation often requires a long list of ancillary considerations to mitigate chemoselectivity and oxidation state issues involving protecting groups and precise reaction choreography3,4,7. Here we report a radical-based Ni/Ag-electrocatalytic cross-coupling of substituted carboxylic acids, thereby enabling an intuitive and modular approach to accessing complex molecular architectures. This new method relies on a key silver additive that forms an active Ag nanoparticle-coated electrode surface8,9 in situ along with carefully chosen ligands that modulate the reactivity of Ni. Through judicious choice of conditions and ligands, the cross-couplings can be rendered highly diastereoselective. To demonstrate the simplifying power of these reactions, concise syntheses of 14 natural products and two medicinally relevant molecules were completed.

Convergent Total Synthesis of (-)-Cyclopamine

A concise and enantioselective total synthesis of the Veratrum alkaloid cyclopamine is disclosed. This highly convergent synthesis with a 16-step longest linear sequence (LLS) was enabled by a de novo synthesis of the trans-6,5-heterobicycle via a strain-inducing halocyclization process, a key Tsuji-Trost cyclization to construct the fully substituted, spirocyclic THF motif with exquisite diastereocontrol, and a late-stage ring-closing metathesis (RCM) reaction to forge the central tetrasubstituted olefin.

Overcoming Energy Transfer for the Metallophotoredox Catalyzed Decarboxylative Alkenylation between Alkylcarboxylic Acids and Enol Triflates

Herein we report on the decarboxylative alkenylation between alkyl carboxylic acids and enol triflates. The reaction is mediated by a dual catalytic nickel and iridium system, operating under visible light irradiation. Two competing catalytic pathways, from the excited state iridium photocatalyst, are identified. One is energy transfer from the excited state, resulting in formation of an undesired enol ester. The desired pathway involves electron transfer, resulting in decarboxylation to ultimately give the target product. The use of a highly oxidizing iridium photocatalyst is essential to control the reactivity. A diverse array of enol triflates and alkyl carboxylic acids are investigated, providing both scope and limitations of the presented methodology.

Overcoming Limitations in Decarboxylative Arylation via Ag-Ni Electrocatalysis

A useful protocol for achieving decarboxylative cross-coupling (DCC) of redox-active esters (RAE, isolated or generated in situ) and halo(hetero)arenes is reported. This pragmatically focused study employs a unique Ag-Ni electrocatalytic platform to overcome numerous limitations that have plagued this strategically powerful transformation. In its optimized form, coupling partners can be combined in a surprisingly simple way: open to the air, using technical-grade solvents, an inexpensive ligand and Ni source, and substoichiometric AgNO3, proceeding at room temperature with a simple commercial potentiostat. Most importantly, all of the results are placed into context by benchmarking with state-of-the-art methods. Applications are presented that simplify synthesis and rapidly enable access to challenging chemical space. Finally, adaptation to multiple scale regimes, ranging from parallel milligram-based synthesis to decagram recirculating flow is presented.

Erythrina Alkaloid Analogues as nAChR Antagonists-A Flexible Platform for Leads in Drug Discovery

Erythrina alkaloids and their central nervous system effects have been studied for over a century, mainly due to their potent antagonistic actions at β2-containing nicotinic acetylcholine receptors (nAChRs). In the present work, we report a synthetic approach giving access to a diverse set of Erythrina natural product analogues and present the enantioselective total synthesis of (+)-Cocculine and (+)-Cocculidine, both found to be potent antagonists of the β2-containing nAChRs.

Development of a Divergent Route to Erythrina Alkaloids

Erythrina alkaloids were identified at the end of the 19th century and today, more than 100 members of the erythrinane family have been isolated. They are characterized by a unique tetracyclic, α-tertiary spiroamine scaffold. Herein we detail our efforts towards the development of a divergent enantioselective synthesis of (+)-dihydro-β-erythroidine (DHβE) – one of the most prominent members of this intriguing family of natural products.

1 Introduction

2 Synthetic Strategy

2.1 First Generation

2.2 Second Generation

2.3 Third Generation

2.3.1 Radical Endgame

2.3.2 Completion of the Total Synthesis

3 Conclusion

Strain-Release Driven Cycloadditions for Rapid Construction of Functionalized Pyridines and Amino Alcohols

This paper describes the development of a new variant of stereoselective strain-release driven reactions (formal homo [3 + 2] dipolar cycloadditions) which utilize housane (1) to construct functionalized amino alcohols and pyridine-substituted cyclopentanes in two to three steps from simple and easily available building blocks (nitrones and pyridine N-oxides respectively).

Enantioselective Total Synthesis of (+)-Dihydro-β-erythroidine

Erythrina alkaloids represent a rich source of complex polycyclic, bioactive natural products. In addition to their sedative and hypotensive effect, their curare-like activity and structural framework have made them attractive targets for synthetic and medicinal chemists. (+)-Dihydro-β-erythroidine (DHβE), the most potent nicotine acetylcholine receptor antagonist (nAChR) of the Erythrina family, is synthesized for the first time in 13 steps from commercially available material.