Application of Relay C−H Oxidation Logic to Polyhydroxylated Oleanane Triterpenoids

Synthesis of Quillaic Acid through Sustainable C-H Bond Activations

To meet the demand for quillaic acid, a multigram synthesis of quillaic acid was accomplished in 14 steps, starting from oleanolic acid, leading to an overall yield of 3.4%. Key features include C-H activation at C-16 and C-23. Through Pd-catalyzed C-H acetoxylation, the oxidation at C-23 was observed as the major product, as opposed to at C-24. A copper-mediated C-H hydroxylation using O2 successfully afforded the single isomer, 16β-ol triterpenoid, followed by configuration inversion to the desired 16α-ol compound. In summary, with steps optimized and conducted on a multigram scale, quillaic acid could be feasibly acquired through C-H activation with inexpensive copper catalysts, promoting a more sustainable approach.

Enantioselective Divergent Syntheses of Diterpenoid Pyrones

Capitalizing a synergy between late-stage C(sp3)-H alkynylation and a series of transition metal-catalyzed alkyne functionalization reactions, we reported herein enantioselective divergent synthesis of 10 diterpenoid pyrones within 14-16 steps starting from chiral pool enoxolone, including the first enantioselective synthesis of higginsianins A, B, D, E, and metarhizin C. Our synthesis also highlights an unprecedented biomimetic oxidative rearrangement of α-pyrone into 3(2H)-furanone, as well as applications of Echavarren C(sp3)-H alkynylation reaction and Toste chiral counterion-mediated Au-catalyzed intramolecular allene hydroalkoxylation in natural product synthesis.

Strategic application of C-H oxidation in natural product total synthesis

The oxidation of unactivated C-H bonds has emerged as an effective tactic in natural product synthesis and has altered how chemists approach the synthesis of complex molecules. The use of C-H oxidation methods has simplified the process of synthesis planning by expanding the choice of starting materials, limiting functional group interconversion and protecting group manipulations, and enabling late-stage diversification. In this Review, we propose classifications for C-H oxidations on the basis of their strategic purpose: type 1, which installs functionality that is used to establish the carbon skeleton of the target; type 2, which is used to construct a heterocyclic ring; and type 3, which installs peripheral functional groups. The reactions are further divided based on whether they are directed or undirected. For each classification, examples from recent literature are analysed. Finally, we provide two case studies of syntheses from our laboratory that were streamlined by the judicious use of C-H oxidation reactions.

Reductive Aldol Approach to Natural Products: Bioinspired Synthesis of abeo-11(12 → 13)-Oleanane Triterpenoids

A synthesis of alstoscholarinoid B (1) and 3β-acetoxy-11α-hydroxy-11(12 → 13)abeooleanan-12-al (2) has been accomplished in 7-9 steps and 10%-16% overall yield from oleanolic acid. This synthesis featured a bioinspired SmI2-mediated reductive aldol reaction to establish the abeo-11(12 → 13)-oleanane framework of both 1 and 2 and a retro-aldol/aldol/lactonization cascade to fully construct the skeleton of 1. Moreover, the investigation of the bioinspired aldol reaction also sheds light on the potential biogenesis of natural products.

A simple monoselective C-H oxygenation approach for the synthesis of ursane triterpenoids

Herein, we presented a simple approach for C-H oxidation in the C23 or/and C24 of ursane triterpenoids without any protection of a Δ^12,13 double bond. As a result, from commercial ursolic acid (UA), six naturally occurring ursane triterpenoids were synthesized in overall yields of 3.4% to 36.8%, which implied the importance of this approach for the derivation of natural products and their application in biological activity.

Stereoselective Epoxidation of Triterpenic Allylic Alcohols and Cytotoxicity Evaluation of Synthesized Compounds

The epoxidation process of semi-synthetic triterpenoids 2-methyl-3-oxo-19β,28-epoxy- 18α-olean-1-ene, and its allylic alcohol derivatives were examined. 1,2α-epoxide, as the main product, was found to be formed from the starting enone exposed to m-chloroperbenzoic acid (mCPBA). In the case of hydroxy-directed mCPBA-oxidation of triterpenic allyl alcohols and their 3α-alkyl-substituted derivatives, inversion of C1 and C2 asymmetric centers with the formation of 1,2β-epoxyalcohols took place. The synthesis of 2,3α-epoxides was fulfilled from 2,3-dialkyl-substituted C(3) allyl alcohols by the action of pyridinium chlorochromate under [1,3]-oxidative rearrangement conditions. The transformations brought about enabled chiral oleanane derivatives with an oxygen-containing substituent at the C1, C2, and C3 atoms to be obtained. The study also provides information on in silico PASS prediction of pharmacological effects and in vitro evaluation of the cytotoxic activity of the synthesized compounds.

Strategy Evolution in a Skeletal Remodeling and C-H Functionalization-Based Synthesis of the Longiborneol Sesquiterpenoids

Detailed herein are our synthesis studies of longiborneol and related natural products. Our overarching goals of utilizing a "camphor first" strategy enabled by skeletal remodeling of carvone, and late-stage diversification using C-H functionalizations, led to divergent syntheses of the target natural products. Our initial approach proposed a lithiate addition to unite two fragments followed by a Conia-ene or Pd-mediated cycloalkylation reaction sequence to install the seven-membered ring emblematic of the longibornane core. This approach was unsuccessful and evolved into a revised plan that employed a Wittig coupling and a radical cyclization to establish the core. A reductive radical cyclization, which was explored first, led to a synthesis of copaborneol, a structural isomer of longiborneol. Alternatively, a metal-hydride hydrogen atom transfer-initiated cyclization was effective for a synthesis of longiborneol. Late-stage C-H functionalization of the longibornane core led to a number of hydroxylated longiborneol congeners. The need for significant optimization of the strategies that were employed as well as the methods for C-H functionalization to implement these strategies highlights the ongoing challenges in applying these powerful reactions. Nevertheless, the reported approach enables functionalization of every natural product-relevant C-H bond in the longibornane skeleton.

Aza-Heterocycles via Copper-Catalyzed, Remote C-H Desaturation of Amines

The majority of medicines contain a nitrogen atom within a five- or six- membered ring. To rapidly access both such aza-heterocycles, we sought to develop a remote C-H desaturation of amines. Inspired by the Hofmann-Löffler-Freytag synthesis of five-membered pyrrolidines, we tackled the century-old challenge of synthesizing six-membered piperidines by H-atom transfer. We present herein a double, vicinal C-H oxidation by dual catalysis, entailing Ir photocatalytic initiation of 1,5-HAT by an N-centered radical and Cu-catalyzed interception of the C-centered radical to facilitate desaturation. By this mechanism, two C-H bonds (δ and ε to N) are regioselectively removed from unbiased, remote positions of an alkyl chain. Over 50 examples illustrate efficiency, selectivity, functional group tolerance, and medicinal utility of this synthesis of both internal and terminal δ vinylic amines and aza-heterocycles. Mechanistic experiments probe the alkylcopper intermediate, as well as kinetics and regioselectivity of the HAT and elimination steps.

Advances in the Semi-Synthesis of Triterpenoids

Recent achievements in triterpenoid semi-synthesis are discussed in this short review, which is divided into three parts according to the type of synthetic strategy being employed. These strategies include functionalization, modification of the carbon skeleton, and glycosylation. In the section on functionalization strategies, both functional group interconversions and new functional group installations on triterpenoid starting materials are described. The section on modification of the carbon skeleton is divided into three parts according to the tactic being applied, and incorporates rearrangement of the carbon skeleton, ring scission, and introduction of an additional heterocyclic ring. Meanwhile, in the section on glycosylation, notable achievements in the semi-synthesis of both natural and artificial triterpene saponins are discussed. Overall, the pivotal transformations that have brought about striking chemical structure variations of triterpenoid starting materials are highlighted herein, and it is hoped that this short review will provide inspiration to both established and new investigators engaged in this field of research.

1 Introduction

2 Semi-Synthesis of Triterpenoids via Functionalization Strategies

2.1 Functionalization of Rings with Functional Groups

2.2 Functionalization of a Side Chain

2.3 Functionalization of Rings without Existing Functional Groups

2.4 Functionalization of Angular Methyl Groups

2.5 Functionalization of Angular Methyl Groups and Functional-Group-Free Rings

2.6 Multisite Modifications

3 Semi-Synthesis of Triterpenoids via C-Skeleton Modification Strategies

3.1 Rearrangement Tactics

3.2 Ring-Opening Tactics

3.3 Additional Ring Introduction Tactics

4 emi-Synthesis of Triterpenoids via a Glycosylation Strategy

5 Conclusions and Outlook

Synthesis of a Biomimetic Tetracyclic Precursor of Aspochalasins and Formal Synthesis of Trichoderone A

Aspochalasins are leucine-derived cytochalasins. Their complexity is associated with a high degree of biosynthetic oxidation, herein inspiring a two-phase strategy in total synthesis. We thus describe the synthesis of a putative biomimetic tetracyclic intermediate. The constructive steps are an intramolecular Diels-Alder reaction to install the isoindolone core of cytochalasins, whose branched precursor was obtained from a stereoselective Ireland-Claisen rearrangement performed from a highly unsaturated substrate. This also constitutes a formal synthesis of trichoderone A.

Innovation of hypervalent(iii) iodine in the synthesis of natural products

The focus article discusses the innovation of hypervalent(iii) iodine regarding skeletal rearrangement, cycloaddition and cyclization, and sp³ C–H functionalization in natural product synthesis.

Insights into Ruthenium(II/IV)-Catalyzed Distal C-H Oxygenation by Weak Coordination

C-H hydroxylation of aryl acetamides and alkyl phenylacetyl esters was accomplished via challenging distal weak O-coordination by versatile ruthenium(II/IV) catalysis. The ruthenium(II)-catalyzed C-H oxygenation of aryl acetamides proceeded through C-H activation, ruthenium(II/IV) oxidation and reductive elimination, thus providing step-economical access to valuable phenols. The p-cymene-ruthenium(II/IV) manifold was established by detailed experimental and DFT-computational studies.

Site-selective C-H hydroxylation of pentacyclic triterpenoids directed by transient chiral pyridine-imino groups

Pentacyclic triterpenoids (PTs) constitute one of the biggest families of natural products, many with higher oxidation state at the D/E rings possess a wide spectrum of biological activties but are poorly accessible. Here we report a site-selective C-H hydroxylation at the D/E rings of PTs paving a way toward these important natural products. We find that Schönecker and Baran's Cu-mediated aerobic oxidation can be applied and become site-selective on PT skeletons, as being effected unexpectedly by the chirality of the transient pyridine-imino directing groups. To prove the applicability, starting from the most abundant triterpenoid feedstock oleanane, three representative saponins bearing hydroxyl groups at C16 or C22 are expeditiously synthesized, and barringtogenol C which bears hydroxyl groups at C16, C21, and C22 is synthesized via a sequential hydroxylation as the key steps.