Arynes as synthetic linchpins towards the construction of diversely functionalized natural product skeletons

Arynes are a privileged class of reactive intermediates in synthetic organic chemistry, and their unusual reactivities have been the subject of engrossing research interest. Recently, there are many reports on novel aryne-based synthetic innovations as a linchpin approach to accomplish the total synthesis of structurally diverse natural products or their derivatives in a racemic and enantiopure fashion. This review provides an overview of the literature on synthetic strategies, employing arynes as crucial intermediates to construct architecturally intriguing bioactive natural products/derivatives in a period of 2019 to 2022. This study highlights the need to investigate the effective synthesis and search for new biological uses of highly functionalized natural product skeletons.

A world tour in the name of natural products

BACKGROUND

Names of natural products (NP) are usually given depending on the species of origin, be it a plant, a marine organism or a microbial species. In some cases, names have been given with reference to people, animals, music, foods or places. Many NP refer to countries, cities or specific places such as mountains, deserts, seas and oceans.

PURPOSE

On the basis of NP names, a world tour has been imagined referring to more than one hundred NP with names evocative of over 50 countries and regions.

RESULTS

The world tour goes from UK (britannin) to Italy (vaticanol) in Europe, from Uganda (ugandoside) to Senegal (senegalene, senegalenines) in Africa, from Brazil (brasilin) to Chile (santiaguine) in South America, from Utah (utahin) to Florida (floridanolide) in the US. It includes Central America (mexicanin, panamine) and the Caribbean islands (jamaicin, bahamaolides). It also crosses Alaska (alaskene) and Canada (quebecol, canadaline). The tour continues throughout Asia, from Thailand (thailandine) to China (Chinaldine) and Pakistan (pakistanamine), to finally reaches Oceania with Australia (australigenin) and Vanuatu (vanuatine), among other countries. This virtual journey, without bordure or wall, brings us to the highest mountains (himalayamine), the deepest oceans (pacificins) and the largest deserts (desertomycin).

CONCLUSION

In the current period of COVID-19 pandemia, with restricted opportunities for international travels, this NP name-based virtual journey offers a world tour to learn more from nature and to inspire scientists to contribute to the field of NP discovery and drug design. There are also limitations associated with the use of trivial names for NP. NP names can be further exploited for teaching and learning.

Design, synthesis, and evaluation of cytotoxic activities of arylnaphthalene lignans and aza-analogs

A concise and versatile synthetic strategy for the total synthesis of arylnaphthalene lignans and aza-analogs was developed. The main objective was to develop synthetic tactics for the creation of the lactone and lactam unit that would give access to an array of synthetic, natural, and/or bioactive compounds through rather simple chemical manipulation. The flexibility and potentiality of these new processes were further illustrated by the total synthesis of retrojusticidin B (13b), justicidin C (14b), and methoxy-vitedoamine A (22a). In this study, a series of novel aryl-naphthalene lignans and aza-analogs were synthesized, and the cytotoxic activities of all compounds on cancer cell growth were evaluated. The target compounds were structurally characterized by ¹ H NMR (nuclear magnetic resonance), ¹³ C NMR, infrared, high-resolution mass spectrometry, and X-ray crystallography. The IC₅₀ values of these compounds on five tumor cell lines (A549, HS683, MCF-7, SK-MEL-28, and B16-F1) were obtained by MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) colorimetric assay. Five of the compounds exhibited excellent activity compared to 5-fluorouracil and etoposide against the five cell lines tested, with IC₅₀ values ranging from 1 to 10 μM.

Silyl Tosylate Precursors to Cyclohexyne, 1,2-Cyclohexadiene, and 1,2-Cycloheptadiene

Transient strained cyclic intermediates have become valuable intermediates in modern synthetic chemistry. Although silyl triflate precursors to strained intermediates are most often employed, the instability of some silyl triflates warrants the development of alternative precursors. We report the syntheses of silyl tosylate precursors to cyclohexyne, 1,2-cyclohexadiene, and 1,2-cycloheptadiene. The resultant strained intermediates undergo trapping in situ to give cycloaddition products. Additionally, the results of competition experiments between silyl triflates and silyl tosylates are reported.

Stereoselective Total Synthesis of the Dimeric Naphthoquinonopyrano-γ-lactone (-)-Crisamicin A: Introducing the Dimerization Site by a Late-Stage Hartwig Borylation

The first stereoselective total synthesis of the dimeric naphthoquinonopyrano-γ-lactone (-)-crisamicin A was realized (13 steps, 5% overall yield). 1,4,5-Trimethoxynaphthalene, reached in five known steps, was brominated at C-3 to install a but-3-enoic ester by an ensuing Heck coupling. An asymmetric Sharpless dihydroxylation followed and gave a β-hydroxy-γ-lactone with >99.9% ee. Its OH substituent and acetaldehyde established the dihydropyran ring in a completely diastereoselective oxa-Pictet-Spengler cyclization. The 2,3-fused anisole moiety allowed the C⁵-H bond under Hartwig's conditions to be borylated. This set the stage for engaging the resulting C⁵-B bond in an oxidative dimerization, which led to a binaphthohydroquinon-5-yl. The latter was advanced to synthetic crisamicin A by a double CAN oxidation (→ a binaphthoquinon-5-yl) and a double demethylation.

Safety Assessment of Benzyne Generation from a Silyl Triflate Precursor

Silyl triflate precursors to benzyne and related intermediates have emerged as valuable synthetic building blocks. However, data addressing the safety of employing these silyl triflate precursors are lacking. We report the calorimetric analysis of a typical Kobayashi procedure for forming and trapping benzyne using a silyl triflate precursor. Our findings suggest that, unlike benzenediazonium carboxylate precursors to benzyne, silyl triflates may be employed under mild conditions without severe concern for runaway reaction.

Cycloadditions of Oxacyclic Allenes and a Catalytic Asymmetric Entryway to Enantioenriched Cyclic Allenes

The chemistry of strained cyclic alkynes has undergone a renaissance over the past two decades. However, a related species, strained cyclic allenes, especially heterocyclic derivatives, have only recently resurfaced and represent another class of valuable intermediates. We report a mild and facile means to generate the parent 3,4-oxacyclic allene from a readily accessible silyl triflate precursor, and then trap it in (4+2), (3+2), and (2+2) reactions to provide a variety of cycloadducts. In addition, we describe a catalytic, decarboxylative asymmetric allylic alkylation performed on an α-silylated substrate, to ultimately permit access to an enantioenriched allene. Generation and trapping of the enantioenriched cyclic allene occurs with complete transfer of stereochemical information in a Diels-Alder cycloaddition through a point-chirality, axial-chirality, point-chirality transfer process.

Concise Approach to Cyclohexyne and 1,2-Cyclohexadiene Precursors

Silyl triflate precursors to cyclic alkynes and allenes serve as valuable synthetic building blocks. We report a concise and scalable synthetic approach to prepare the silyl triflate precursors to cyclohexyne and 1,2-cyclohexadiene. The strategy involves a retro-Brook rearrangement of an easily accessible cyclohexanone derivative, followed by triflation protocols. This simple, yet controlled, method should enable the further study of strained alkynes and allenes in chemical synthesis.

Arynes and Cyclic Alkynes as Synthetic Building Blocks for Stereodefined Quaternary Centers

We report a facile method to synthesize stereodefined quaternary centers from reactions of arynes and related strained intermediates using β-ketoester-derived substrates. The conversion of β-ketoesters to chiral enamines is followed by reaction with in situ generated strained arynes or cyclic alkynes. Hydrolytic workup provides the arylated or alkenylated products in enantiomeric excesses as high as 96%. We also describe the one-pot conversion of a β-ketoester substrate to the corresponding enantioenriched α-arylated product. Computations show how chirality is transferred from the N-bound chiral auxiliary to the final products. These are the first theoretical studies of aryne trapping by chiral nucleophiles to set new stereocenters. Our approach provides a solution to the challenging problem of stereoselective β-ketoester arylation/alkenylation, with formation of a quaternary center.

Aryne-based strategy in the total synthesis of naturally occurring polycyclic compounds

This review has outlined the strategies and tactics of using arynes in the total syntheses of polycyclic natural products.