Recent developments in the total synthesis of natural products using the Ugi multicomponent reactions as the key strategy

The total syntheses of selected natural products using different versions of the Ugi multicomponent reaction is reviewed on a case-by-case basis. The revision covers the period 2008-2023 and includes detailed descriptions of the synthetic sequences, the use of state-of-the-art chemical reagents and strategies, as well as the advantages and limitations of the transformation and some remedial solutions. Relevant data on the isolation and bioactivity of the different natural targets are also briefly provided. The examples clearly evidence the strategic importance of this transformation and its key role in the modern natural products synthetic chemistry toolbox. This methodology proved to be a valuable means for easily building molecular complexity and efficiently delivering step-economic syntheses even of intricate structures, with a promising future.

Preparation of pyridopyrazines through tandem Pd-catalyzed C-N/C-C coupling reactions of Ugi adducts

A Pd-catalyzed selective tandem cyclization of the Ugi adduct via Buchwald-Hartwig/C-H bond functionalization reactions has been reported. This sequence offers an interesting approach for synthesizing a wide range of pyrido[1,2-a]pyrazine-3,6-dione scaffolds under mild reaction conditions in moderate to excellent yields. The scope and limitations of the protocol are discussed.

Post-Ugi Cyclizations Towards Polycyclic N-Heterocycles

The Ugi reaction has become one of the highly explored reactions for the formation of multifunctional adducts, due to the mild reaction conditions, wide scope and high variability. By carefully selecting the starting four components, Ugi-adducts could undergo different kinds of post-transformations for the synthesis of bioactive heterocycles, natural products and macrocycles. Considering the significance of polycycles, diverse post-Ugi transformations have been developed over the years for constructing structurally novel polycycles. In this account, we summarize important efforts for the synthesis of polycyclic N-heterocycles via post-Ugi cyclizations from the Van der Eycken laboratory onwards 2016. With the aid of transition metal catalysis from gold, rhodium, silver and palladium, as well as metal-free strategies, versatile polyheterocycles are prepared with high efficiency and step-economy.

Asymmetric Dearomatization of Phenols via Ligand-Enabled Cooperative Gold Catalysis

By employing a chiral bifunctional phosphine ligand, a gold(I)-catalyzed efficient and highly enantioselective dearomatization of phenols is achieved via versatile metal-ligand cooperation. The reaction is proven to be remarkably general in scope, permitting substitutions at all four remaining benzene positions, accommodating electron-withdrawing groups including strongly deactivating nitro, and allowing carbon-based groups of varying steric bulk including tert-butyl at the alkyne terminus. Moreover, besides N-(o-hydroxyphenyl)alkynamides, the corresponding ynoates and ynones are all suitable substrates. Spirocyclohexadienone-pyrrol-2-ones, spirocyclohexadienone-butenolides, and spirocyclohexadenone-cyclopentenones are formed in yields up to 99 % and with ee up to 99 %.

Synthesis of tetrahydrofuro[3,2-c]pyridines via Pictet-Spengler reaction

A semi-one-pot method for the synthesis of 4-substituted tetrahydrofuro[3,2-c]pyridines by the Pictet-Spengler reaction was developed. The method is based on the condensation of easily accessibly 2-(5-methylfuran-2-yl)ethanamine with commercially available aromatic aldehydes followed by acid-catalyzed Pictet-Spengler cyclization. Using this approach, we synthesized a range of 4-substituted tetrahydrofuro[3,2-c]pyridines in reasonable yields. The reactivity of some of the products was investigated and selected synthetic transformations of the obtained tetrahydrofuro[3,2-c]pyridines were shown.

Advancements in Gold-Catalyzed Cascade Reactions to Access Carbocycles and Heterocycles: An Overview

This review summarizes recent developments (from 2006 to 2022) in numerous important and efficient carbo- and heterocycle generations using gold-catalyzed cascade protocols. Herein, methodologies involve selectivity, cost-effectiveness, and ease of product formation being controlled by the ligand as well as the counter anion, catalyst, substrate, and reaction conditions. Gold-catalyzed cascade reactions covered different strategies through the compilation of various approaches such as cyclization, hydroarylation, intermolecular and intramolecular cascade reactions, etc. This entitled reaction is also useful for the synthesis of spiro, fused, bridged carbo- and heterocycles.

Regioselective Synthesis of Functionalized Pyrrolo[1,2-a]pyrazine-3,6(2H,4H)-diones via Tandem Post-Ugi Cyclization and Gold(I)-Catalyzed Annulation

A convenient synthesis of less explored pyrrolo[1,2-a]pyrazine-3,6(2H,4H)-diones is described in two steps from Ugi adducts. The method involves acid-mediated cyclization of Ugi adducts to form dihydropyrazinones followed by gold(I)-catalyzed regioselective annulation. The generality of the transformation was established by reacting a variety of substituted dihydropyrazinones under the optimized reaction conditions to form densely functionalized pyrrolo[1,2-a]pyrazine-3,6(2H,4H)-diones in good-to-excellent yields. It was also observed some of the acetone-derived Ugi adducts furnish 7-acyl-pyrroloimidazolones as a byproduct during TFA-mediated cyclization via alkyne-carbonyl metathesis and condensation.

Palladium-catalyzed post-Ugi arylative dearomatization/Michael addition cascade towards plicamine analogues

A palladium-catalyzed intramolecular cyclization of Ugi-adducts via a cascade dearomatization/aza-Michael addition process has been developed. Diverse plicamine analogues are constructed in a rapid, highly efficient and step-economical manner, through the combination of an Ugi-4CR and a palladium-catalyzed dearomatization. The synthetic utility of this approach is illustrated by further functional group transformations.

Palladium-Catalyzed Arylative Dearomatization and Subsequent Aromatization/Dearomatization/Aza-Michael Addition: Access to Zephycarinatine and Zephygranditine Skeletons

We have developed a novel palladium-catalyzed arylative dearomatization and subsequent aromatization/dearomatization/aza-Michael addition process of Ugi adducts, enabling the rapid construction of diverse zephycarinatine and zephygranditine scaffolds containing two adjacent quaternary carbon stereocenters with excellent chemoselectivity and stereoselectivity in a rapid, step-economical, and highly efficient manner. This approach shows broad substrate scope and excellent functional-group tolerance with diverse electron-rich and electron-deficient aromatic substrates. The synthetic utility of this method is further demonstrated by versatile transformations of the products.

Strategies and methodologies for the construction of spiro-γ-lactams: an update

This review gives an insight into the most recent synthetic methodologies towards spiro-γ-lactams, a class of compounds that are present in a wide range of synthetic bioactive and naturally occurring molecules.

Sequential and direct multicomponent reaction (MCR)-based dearomatization strategies

Dearomatization strategies in a multicomponent fashion often result in complex heterocyclic frameworks, which have attracted the attention of chemists due to their natural product-like structures. The combination of these two processes can easily achieve extended molecular complexity and diversity from simple starting materials with high atom economy. Thus, this field has attracted extensive interest owing to its potential significance in both asymmetric catalysis and convenient build-up of libraries of molecules with novel three-dimensional scaffolds, which may find application in medicinal chemistry. Accordingly, a systematic review on this topic will provide the synthetic organic community with a conceptual overview and comprehensive understanding of the different multicomponent reaction (MCR) cascades involving dearomatization as the characteristic step. In addition, this review will help researchers to look at this promising area from a different perspective with respect to drug discovery, new MCR-based disconnections and often hidden opportunities.

Gold-catalyzed domino cyclization to diverse polyheterocyclic frameworks: mechanism, origin of the cooperative hydrogen bond, and role of π-stacking interactions

The detailed mechanism and origins of gold-catalyzed domino cyclization to diverse fused polyheterocyclic frameworks by cooperative catalysis and cascade catalysis were studied systematically.

Gold-catalyzed domino cyclization enabling construction of diverse fused azaspiro tetracyclic scaffolds: a cascade catalysis mechanism due to a substrate and counterion

The detailed mechanism and origins of gold-catalyzed domino cyclization to diverse fused azaspiro tetracyclic scaffolds by cooperative dual catalysis and cascade catalysis are systematically studied.

A gold-triggered dearomative spirocarbocyclization/Diels–Alder reaction cascade towards diverse bridged N-heterocycles

An efficient chemo- and diastereoselective gold-triggered post-Ugi non-oxidativeortho-dearomative spirocarbocyclization/Diels–Alder reaction cascade sequence has been developed to deliver diverse bridged polycyclic N-heterocycles.