Total synthesis of tropoloisoquinolines: imerubrine, isoimerubrine, and grandirubrine

Total Synthesis and Cytotoxicity Evaluation of Pareitropone and Analogues

A concise synthesis of pareitropone by oxidative cyclization of a phenolic nitronate is delineated. The use of TMSOTf as an additive to promote the facile formation of a strained norcaradiene intermediate provides convenient access to highly condensed multicyclic tropones in high yields. This synthesis is modular, efficient, and scalable, highlighting the synthetic utility of radical anion coupling reactions in annulation reactions. This work is discussed in the context of total syntheses of the tropoloisoquinoline alkaloids. Also included are the preparation of several congeners and a brief description of their biological activities.

Carbocycloaddition Strategies for Troponoid Synthesis

Tropone is the prototypical aromatic 7-membered ring, and can be found in virtually any undergraduate textbook as a key example of non-benzenoid aromaticity. Aside from this important historical role, tropone is also of high interest as a uniquely reactive synthon in complex chemical synthesis as well as a valuable chemotype in drug design. More recently, there has been growing interest in the utility of tropones for catalysis and material science. Thus, synthetic strategies capable of synthesizing functional tropones are key to fully exploiting the potential of this aromatic ring system. Cycloaddition reactions are particularly powerful methods for constructing carbocycles, and these strategies in turn have proven to be powerful for generating troponoids. The following review article provides an overview of strategies for troponoids wherein the 7-membered carbocycle is generated through a cycloaddition reaction. Representative examples of each strategy are also provided.

Regioselective approach to colchiceine tropolone ring functionalization at C(9) and C(10) yielding new anticancer hybrid derivatives containing heterocyclic structural motifs

The influence of base type, temperature, and solvent on regioselective C(9)/C(10) “click” modifications within the tropolone ring of colchiceine (2) is investigated. New ether derivatives of 2, bearing alkyne, azide, vinyl, or halide aryl groups enable assembly of the alkaloid part with heterocycles or important biomolecules such as saccharides, geldanamycin or AZT into hybrid scaffolds by dipolar cycloaddition (CuAAC) or Heck reaction. Compared to colchicine (1) or colchiceine (2), ether congeners, as e.g. 3e [IC₅₀s_(3e) ∼ 0.9 nM], show improved or similar anticancer effects, whereby the bulkiness of the substituents and the substitution pattern of the tropolone proved to be essential. Biological studies reveal that expanding the ether arms by terminal basic heterocycles as quinoline or pyridine, decreases the toxicity in HDF cells at high anticancer potency (IC₅₀s ∼ 1–2 nM). Docking of ether and hybrid derivatives into the colchicine pocket of α_GTP/β tubulin dimers reveals a relationship between the favourable binding mode and the attractive anticancer potency.

Intramolecular Diels–Alder Reactions of Oxazoles, Imidazoles, and Thiazoles

The development of the intramolecular Diels–Alder cycloaddition­ of azole heterocycles, i.e. oxazoles (IMDAO), imidazoles (IMDAI), and thiazoles (IMDAT), has had a significant impact on the efficient preparation of heterocyclic intermediates and natural products. In particular, highly efficient and versatile IMDAO reactions have been utilized as a key step in several synthetic schemes to provide alkaloids and terpenoid target molecules. More limited studies have been performed on IMDAI and IMDAT cycloadditions. Some drawbacks, such as the occasionally­ challenging preparation of IMDA precursors, are also highlighted in this review. Perspectives are provided on how IMDAI and IMDAT­ transformations can be further expanded for target-directed syntheses.

1 Introduction

2 Oxazoles

2.1 IMDAO Approaches to Furanosesquiterpenes and Furanosteroids

2.1.1 Syntheses of Highly Oxygenated Sesquiterpenes

2.1.2 Syntheses of (±)-Gnididione and (±)-Isognididione

2.1.3 Synthesis of (±)-Stemoamide

2.1.4 Synthesis of (±)-Paniculide A

2.1.5 Syntheses of (+)- and (–)-Norsecurinine

2.1.6 Synthesis of Evodone

2.1.7 Syntheses of (±)-Ligularone and (±)-Petasalbine

2.1.8 Syntheses of Imerubrine, Isoimerubrine, and Grandirubrine

2.1.9 Syntheses of Furanosteroids

2.1.10 Syntheses of Substituted Indolines and Tetrahydroquinolines

2.2 IMDAO Approaches to Pyridines: the Kondrat’eva Reaction

2.2.1 Syntheses of Suaveoline and Norsuaveoline

2.2.2 Synthesis of Eupolauramine

2.2.3 Syntheses of (–)-Plectrodorine and (+)-Oxerine

2.2.4 Synthesis of Amphimedine

2.2.5 Synthetic Approach to the Western Segment of Haplophytine

2.2.6 Synthesis of Marinoquinoline A

2.2.6.1 IMDAO Approach to Marinoquinoline A

2.2.6.2 Scope of Allenyl IMDAO Cycloaddition

2.3 Lewis Acid Catalysis in IMDAO Reactions

2.3.1 Effects of Europium Catalysts on IMDAO Reactions

2.3.2 Effects of Copper Catalysts on IMDAO Reactions

3 Imidazoles

4 Thiazoles

4.1 Syntheses of Menthane and Eremophilane

4.2 Further Comments on the Intramolecular Cycloadditions of Thiocarbonyl Ylides

5 Conclusions and Outlook

Asymmetric Total Synthesis of Cerorubenic Acid-III

The first asymmetric total synthesis of the highly strained compound cerorubenic acid-III is reported. A type II intramolecular [5 + 2] cycloaddition allowed efficient and diastereoselective construction of the synthetically challenging bicyclo[4.4.1] ring system with a strained bridgehead (anti-Bredt) double bond in the final product. A unique transannular cyclization installed the vinylcyclopropane moiety with retention of the desired stereochemistry.

Construction of seven- and eight-membered carbocycles by Lewis acid catalyzed C(sp3)–H bond functionalization

Concise construction of seven- or eight-membered carbocycles was accomplished by Lewis acid catalyzed C(sp³)–H bond functionalization.

Synthesis of 1,2-Oxazinanes via Hydrogen Bond Mediated [3 + 3] Cycloaddition Reactions of Oxyallyl Cations with Nitrones

Reported herein is the development of [3 + 3] cycloaddition reactions between oxyallyl cations and nitrones to yield 1,2-oxazinane heterocycles. Oxyallyl cation intermediates, generated in situ from α-tosyloxy ketones in the presence of hexafluoro-2-propanol (HFIP), a cosolvent, and a base, are found to react with a range of nitrones to afford 1,2-oxazinanes in good to high yields. The reactions are catalyzed by hydrogen-bond donors such as phenols and squaramides, and dramatically higher diastereoselectivities are observed with 4-nitrophenol.

Asymmetric Total Synthesis of Cyclocitrinol

The first and asymmetric total synthesis of cyclocitrinol, an unusual C25 steroid, has been accomplished in a linear sequence of 18 steps from commercially available compound 11. The synthetically challenging bicyclo[4.4.1] A/B ring system with a strained bridgehead (anti-Bredt) double bond of cyclocitrinol was constructed efficiently and diastereoselectively via a type II intramolecular [5 + 2] cycloaddition.

Application of (4+3) cycloaddition strategies in the synthesis of natural products

This review summarizes the applications of (4+3) cycloadditions, both classical and formal, in the syntheses of natural products in the last two decades.

Rh-catalyzed sequential oxidative C–H activation/annulation with geminal-substituted vinyl acetates to access isoquinolines

The concise synthesis of 3-substituted or non-C3-substituted isoquinolines through Rh-catalyzed sequential oxidative C–H activation/annulation with geminal-substituted vinyl acetates was developed.

Synthesis of Naturally Occurring Tropones and Tropolones

Tropones and tropolones are an important class of seven-membered non-benzenoid aromatic compounds. They can be prepared directly by oxidation of seven-membered rings. They can also be derived from cyclization or cycloaddition of appropriate precursors followed by elimination or rearrangement. This review discusses the types of naturally occurring tropones and tropolones and outlines important methods developed for the synthesis of tropone and tropolone natural products.

(4+3) cycloaddition reactions of nitrogen-stabilized oxyallyl cations

The use of heteroatom-substituted oxyallyl cations in (4+3) cycloadditions has had a tremendous impact on the development of cycloaddition chemistry. Extensive efforts have been exerted toward investigating the effect of oxygen, sulfur, and halogen substituents on the reactivity of oxyallyl cations. Most recently, the use of nitrogen-stabilized oxyallyl cations has gained prominence in the area of (4+3) cycloadditions. The following article will provide an overview of this concept utilizing nitrogen-stabilized oxyallyl cations.

Total synthesis of pareitropone via radical anion coupling

A concise (9-step) synthesis of the tropoloisoquinoline alkaloid pareitropone has been achieved starting from 2-bromoisovanillin. The key step features oxidative cyclization of a readily available phenolic nitronate for the convenient construction of the fused tropone ring. This work underscores the synthetic utility of intramolecular oxidative coupling reactions of phenolic nitronates.

Concise synthesis of the oxapentacyclic core of cortistatin A

A concise synthetic approach for constructing the oxapentacyclic framework of cortistatin A is described. The synthesis features a furan-oxyallyl [4 + 3] cycloaddition and double-intramolecular aldol reactions. In addition, an interesting core structure was obtained in 11 steps from furan by using our method.

The Cycloaddition Strategy for the Synthesis of Natural Products Containing Carbocyclic Seven-Membered Rings

Highly substituted carbocyclic seven-membered rings are frequently found in natural products and their synthesis represents a significant challenge to the synthetic chemist. Direct intramolecular cyclization of these systems often proves difficult and this fact has catalyzed the development of a variety of strategies based on a convergent intermolecular cycloaddition strategy. This concept article discusses the major cycloaddition approaches utilized to access these types of structures and primarily focuses on examples employed in the synthesis of natural products.

Alkynyliodonium Salts in Organic Synthesis. Application to the Total Synthesis of the Tropoloisoquinoline Alkaloid Pareitropone

The synthesis of the tropoloisoquinoline alkaloid pareitropone has been accomplished in 14 steps from 2,3,4-trimethoxybenzoic acid. The key transformations include the generation of an alkylidenecarbene intermediate through intramolecular addition of a tosylamide anion to an alkynyliodonium salt, and the cycloaddition of that carbene to a peri positioned aromatic ring to afford a cycloheptatrienylidene product featuring the intact pareitropone skeleton.