Further Investigation of the Intermolecular Diels-Alder Cycloaddition for the Synthesis of Bicyclo[2.2.2]diazaoctane Alkaloids

The Mutually Inspiring Biological and Chemical Synthesis of Fungal Bicyclo[2.2.2]diazaoctane Indole Alkaloids

Fungal indole alkaloids bearing a bicyclo[2.2.2]diazaoctane (BCDO) core structure are a fascinating family of natural products that exhibit a wide spectrum of biological activities. These compounds also display remarkable structural diversity, with many different diastereomers and enantiomers produced by specific fungal strains. The biogenesis of the unique BCDO moiety has long been proposed to involve an intramolecular [4+2] hetero-Diels-Alder (IMDA) reaction, but the exact mechanisms for this hypothetical transformation have remained elusive until recently. This review aims to summarize the whole history of synthetic and biosynthetic studies of fungal BCDO indole alkaloids, by covering the discovery, biomimetic syntheses, total syntheses, biosynthetic pathway elucidation, and biological activities of representative compounds. We highlight the mutual inspiration and corroboration between biological and synthetic chemists in exploring the intriguing biosynthetic mysteries of this family of natural products. We also provide perspectives and clues for the remaining biosynthetic problems.

Total Synthesis of (+)-Brevianamides A and B

(+)-Brevianamides A (1a) and B (1b) are distinguished by their unique bicyclo[2.2.2]diazaoctane structure and have captured the interest of synthetic chemists due to their fascinating array of biological activities. The biosynthetic proposal of these classes of alkaloids led to the discovery of a number of interesting strategies. We present a biomimetic synthesis of these alkaloids starting from naturally occurring 4-hydroxy-l-proline and L-tryptophan. Gratifyingly, we emulate an alternative biosynthetic process through a unique elimination-isomerization sequence triggered by a dual-base system to generate the key aza-diene required for the Diels-Alder reaction to craft the bicyclo[2.2.2]diazaoctane structure.

A comprehensive understanding of the mechanism of the biomimetic total synthesis of brevianamide A

Recently, several studies on the chemical synthesis of brevianamide A (BA) were reported. In particular, a highly efficient and remarkably selective synthetic strategy was reported by Lawrence's group. However, a unified mechanistic understanding of these results is still lacking. We have carried out a DFT study and proposed a unified mechanism to understand these experimental results. Starting from intermediate 2, the most favorable reaction sequence is a fast tautomerization, followed by a σ-migration of the base moiety, and a final inverse-electron demanding Diels-Alder reaction, resulting in the formation of the BA product stereoselectively. This reaction mechanism can also be applied to understand the biosynthesis of BA that involves enzymatic catalysis.

Asymmetric Intramolecular Amination Catalyzed with Cp*Ir-SPDO via Nitrene Transfer for Synthesis of Spiro-Quaternary Indolinone

An asymmetric intramolecular spiro-amination to high steric hindering α-C-H bond of 1,3-dicarbonyl via nitrene transfer using inactive aryl azides has been carried out by developing a novel Cp*Ir(III)-SPDO (spiro-pyrrolidine oxazoline) catalyst, thereby enabling the first successful construction of structurally rigid spiro-quaternary indolinone cores with moderate to high yields and excellent enantioselectivities. DFT computations support the presence of double bridging H-F bonds between [SbF6]- and both the ligand and substrate, which favors the plane-differentiation of the enol π-bond for nitrenoid attacking. These findings open up numerous opportunities for the development of new asymmetric nitrene transfer systems.

Unified total synthesis of the brevianamide alkaloids enabled by chemical investigations into their biosynthesis

The bicyclo[2.2.2]diazaoctane alkaloids are a vast group of natural products which have been the focus of attention from the scientific community for several decades. This interest stems from their broad range of biological activities, their diverse biosynthetic origins, and their topologically complex structures, which combined make them enticing targets for chemical synthesis. In this article, full details of our synthetic studies into the chemical feasibility of a proposed network of biosynthetic pathways towards the brevianamide family of bicyclo[2.2.2]diazaoctane alkaloids are disclosed. Insights into issues of reactivity and selectivity in the biosynthesis of these structures have aided the development of a unified biomimetic synthetic strategy, which has resulted in the total synthesis of all known bicyclo[2.2.2]diazaoctane brevianamides and the anticipation of an as-yet-undiscovered congener.

The photoredox alkylarylation of styrenes with alkyl N-hydroxyphthalimide esters and arenes involving C-H functionalization

The In(OTf)₃-promoted three-component photoredox alkylarylation of styrenes with alkyl NHP esters and arenes to access alkylated arene derivatives through C-C bond cleavage and C-H functionalization is reported. By utilizing visible-light photoredox catalysis, alkyl N-hydroxyphthalimide esters serving as alkyl carbon-centered radicals and a wide range of arenes (e.g., indoles, pyrrole, and electron-rich arenes) as nucleophiles were used to enable the introduction of various alkyl groups and aryl groups across the C[double bond, length as m-dash]C bonds with excellent selectivity and functional group tolerance.