Dearomative Synthetic Entry into the Altemicidin Alkaloids

Azine Dearomatization in Natural Product Total Synthesis

Since antiquity, alkaloid natural products have served as medicinal ingredients that still contribute as an inspiration for the development of novel therapeutics. For the synthetic chemist, much of the importance of natural products lies in their acting as a forcing-function for the invention of new synthetic strategies and tactics for molecular assembly. With this rich history in mind, it remains an important goal for chemists to build nitrogenous structures with greater efficiency, abiding by economies of synthesis. Nitrogenous aromatic feedstocks have been an intriguing starting point for the functionalization and construction of alkaloids for several decades, but recent advances in reaction design have opened new doors for leveraging their abundance in concise synthesis. Herein, advances in this area of synthetic ingenuity will be summarized with the aim of instructing chemists towards considering dearomatization as a strategic avenue for both target-oriented and diversity-oriented synthetic campaigns. Overall, syntheses are evaluated, compared, and contrasted to give a systematic overview of this continued area of research.

Cinchona Organocatalyzed Enantioselective Amination for Quaternized Serines as Tertiary Amides

Herein, we describe a Cinchona-aminocatalyzed enantioselective α-hydrazination of an α-formyl amide for the production of protected quaternized serines as tertiary amides with ee's of generally >98% and ≤99% yields. The proposed TS model supported by density functional theory calculations involves a quinuclidinium ion Brønsted acid-assisted delivery of DtBAD, which occurs from the Re face of an H-bonded enaminone when using a 9S-cinchonamine catalyst, resulting in a hydrazide with the R-configuration as determined by X-ray analysis.

Enantioselective Transfer Hydrogenation of α-Methoxyimino-β-keto Esters

α-Methoxyimino-β-keto esters are reported to undergo highly enantioselective catalytic transfer hydrogenation using the Noyori-Ikariya complex RuCl(p-cymene)[(S,S)-Ts-DPEN] in a mixture of formic acid-triethylamine and dimethylformamide at 25 °C. The experimental study performed on over 25 substrates combined with computational analysis revealed that a Z-configured methoxyimino group positioned alpha to a ketone carbonyl leads to higher reactivity and mostly excellent enantioselectivity within this substrate class. Density functional theory calculations of competing transition states were used in rationalizing the origins of enantioselectivity and the possible role of the methoxyimino group in the reaction outcome.

Synthesis of Phenol-Pyridinium Salts Enabled by Tandem Electron Donor-Acceptor Complexation and Iridium Photocatalysis

Herein, we describe a dual photocatalytic system to synthesize phenol-pyridinium salts using visible light. Utilizing both electron donor-acceptor (EDA) complex and iridium(III) photocatalytic cycles, the C-N cross-coupling of unprotected phenols and pyridines proceeds in the presence of oxygen to furnish pyridinium salts. Photocatalytic generation of phenoxyl radical cations also enabled a nucleophilic aromatic substitution (SNAr) of a fluorophenol with an electron-poor pyridine. Spectroscopic experiments were conducted to probe the mechanism and reaction selectivity. The unique reactivity of these phenol-pyridinium salts were displayed in several derivatization reactions, providing rapid access to a diverse chemical space.

Recent Strategies in the Nucleophilic Dearomatization of Pyridines, Quinolines, and Isoquinolines

Dearomatization reactions have become fundamental chemical transformations in organic synthesis since they allow for the generation of three-dimensional complexity from two-dimensional precursors, bridging arene feedstocks with alicyclic structures. When those processes are applied to pyridines, quinolines, and isoquinolines, partially or fully saturated nitrogen heterocycles are formed, which are among the most significant structural components of pharmaceuticals and natural products. The inherent challenge of those transformations lies in the low reactivity of heteroaromatic substrates, which makes the dearomatization process thermodynamically unfavorable. Usually, connecting the dearomatization event to the irreversible formation of a strong C-C, C-H, or C-heteroatom bond compensates the energy required to disrupt the aromaticity. This aromaticity breakup normally results in a 1,2- or 1,4-functionalization of the heterocycle. Moreover, the combination of these dearomatization processes with subsequent transformations in tandem or stepwise protocols allows for multiple heterocycle functionalizations, giving access to complex molecular skeletons. The aim of this review, which covers the period from 2016 to 2022, is to update the state of the art of nucleophilic dearomatizations of pyridines, quinolines, and isoquinolines, showing the extraordinary ability of the dearomative methodology in organic synthesis and indicating their limitations and future trends.

Brønsted acid-catalyzed annulation reaction of hydroxamic acids: synthesis of cyclopentane-fused isoxazolidines and their benzilic amide rearrangement

Readily available hydroxamic acids were leveraged to access challenging nitrones in the presence of H3PO4 as a Brønsted acid catalyst and engaged in an intramolecular (3+2) annulation reaction to make valuable cyclopentane-fused isoxazolidines with high yields and excellent diastereoselectivity. The products were further utilized in a unique base-promoted benzilic amide rearrangement to provide cyclopentane-fused γ-lactams bearing three contiguous stereocenters as a single diastereomer.

Total Synthesis of Altemicidin: A Surprise Ending for a Monoterpene Alkaloid

Monoterpene alkaloids encompass distinct chemical diversity and wide-ranging bioactivity. Their compact complexity has made them popular as synthetic targets and has inspired many distinct strategies and tactics in the field of heterocyclic chemistry. This article documents the evolution of a synthetic program aimed at accessing the unusual sulfonamide-containing natural product altemicidin, which was generally believed to be a monoterpene alkaloid throughout our entire synthetic investigations but has recently been found to originate through an unexpected and quite disparate biosynthetic pathway. By leveraging a pyridine dearomatization/cycloaddition strategy, we developed a concise pathway to the 5,6-fused bicyclic azaindane core and, after significant experimentation, an ultimate synthesis of altemicidin itself. Tactics to productively manipulate the multiple functional groups present on this highly polar scaffold proved challenging but were eventually realized via several carefully orchestrated and chemoselective transformations-investments that paid dividends in the form of significantly shorter chemical synthesis. Surprisingly, the bond-forming logic between our presumed abiotic synthetic strategy to this alkaloid class and its subsequently identified biosynthetic pathway is eerily similar.

Noncanonical Functions of Enzyme Cofactors as Building Blocks in Natural Product Biosynthesis

Enzymes involved in secondary metabolite biosynthetic pathways have typically evolutionarily diverged from their counterparts functioning in primary metabolism. They often catalyze diverse and complex chemical transformations and are thus a treasure trove for the discovery of unique enzyme-mediated chemistries. Besides major natural product classes, such as terpenoids, polyketides, and ribosomally or nonribosomally synthesized peptides, biosynthetic investigations of noncanonical natural product biosynthetic pathways often reveal functionally distinct enzyme chemistries. In this Perspective, we aim to highlight challenges and opportunities of biosynthetic investigations on noncanonical natural product pathways that utilize primary metabolites as building blocks, otherwise generally considered as enzyme cofactors. A focus is made on the discovered chemical and enzymological novelties.

Concise Enantioselective Total Synthesis of Daphenylline Enabled by an Intramolecular Oxidative Dearomatization

Daphenylline is a structurally unique member of the triterpenoid Daphniphyllum natural alkaloids, which exhibit intriguing biological activities. Six total syntheses have been reported, five of which utilize aromatization approaches. Herein, we report a concise protecting-group-free total synthesis by means of a novel intramolecular oxidative dearomatization reaction, which concurrently generates the critical seven-membered ring and the quaternary-containing vicinal stereocenters. Other notable transformations include a tandem reductive amination/amidation double cyclization reaction, to assemble the cage-like architecture, and installation of the other two chiral stereocenters via a highly enantioselective rhodium-catalyzed challenging hydrogenation of the diene intermediate (90% e.e.) and an unprecedented remote acid-directed Mukaiyama-Michael reaction of the complex benzofused cyclohexanone (13:1 d.r.).

β-NAD as a building block in natural product biosynthesis

ABSTRATCT

β-Nicotinamide adenine dinucleotide (β-NAD) is a pivotal metabolite for all living organisms and functions as a diffusible electron acceptor and carrier in the catabolic arms of metabolism^1,2. Furthermore, β-NAD is involved in diverse epigenetic, immunological and stress-associated processes, where it is known to be sacrificially utilized as an ADP-ribosyl donor for protein and DNA modifications, or the generation of cell-signalling molecules^3,4. Here we report the function of β-NAD in secondary metabolite biosynthetic pathways, in which the nicotinamide dinucleotide framework is heavily decorated and serves as a building block for the assembly of a novel class of natural products. The gatekeeping enzyme of the discovered pathway (SbzP) catalyses a pyridoxal phosphate-dependent [3+2]-annulation reaction between β-NAD and S-adenosylmethionine, generating a 6-azatetrahydroindane scaffold. Members of this novel family of β-NAD-tailoring enzymes are widely distributed in the bacterial kingdom and are encoded in diverse biosynthetic gene clusters. The findings of this work set the stage for the discovery and exploitation of β-NAD-derived natural products.