Directed C–H Bond Functionalization: A Unified Approach to Formal Syntheses of Amorfrutin A, Cajaninstilbene Acid, Hydrangenol, and Macrophyllol

Concise Syntheses of Alternariol, Alternariol-9-monomethyl Ether and Their D3-Isotopologues

Alternariol (AOH) and alternariol-9-monomethyl ether (AME) are two secondary metabolites of Alternaria fungi which can be found in various foodstuffs like tomatoes, nuts, and grains. Due to their toxicity and potential mutagenic activity the need for the development of high-throughput methods for the supervision of AOH and AME levels is of increasing interest. As the availability of both native and labeled AOH and AME analytical standards is very limited, we herein present a novel and concise approach towards their synthesis by employing a ruthenium-catalyzed ortho-arylation as the key step. Finally, we demonstrate their suitability as internal standards in stable-isotope dilution assay (SIDA)-HPLC-MS/MS analysis, a technique commonly used for the quantification of natural products in food and feed.

Synthesis and biological evaluation of cajaninstilbene acid and amorfrutins A-D as cytotoxic agents against human pancreatic carcinoma PANC-1 cells

A concise synthesis of cajaninstilbene acid was achieved in 7 steps from (E)-3,5-dimethoxystilbene in 8.6% overall yield via the Claisen rearrangement of an aryl reverse-prenyl ether as the key step. Cytotoxic activities against human pancreatic carcinoma PANC-1 cells of cajaninstilbene acid and amorfrutins A-D were also evaluated.

Free Amine, Hydroxyl and Sulfhydryl Directed C-H Functionalization and Annulation: Application to Heterocycle Synthesis

Transition metal-catalyzed direct C-H bond functionalization is recognized as an efficient strategy to assemble heterocyclic frameworks. For this purpose, directing groups (DGs) installation on an organic molecule has remained a widely exploited strategy for the years. The installation of directing groups, especially for the amine, alcohol and thiol containing reactants and their removal after the reaction need additional steps. In this regard, the use of free amine, hydroxyl and sulfhydryl as directing groups in native form is advantageous and in recent times, these transformations have stirred undisputable advancements for applications to heterocycle synthesis. In this review, the aromatic sp² -C attached free amine, hydroxyl and sulfhydryl as native functionalities are shown to be useful for the construction of five to seven-membered N-, O- and S-heterocycles.

Syntheses of Amorfrutins and Derivatives via Tandem Diels-Alder and Anionic Cascade Approaches

We describe two complementary approaches based on a convergent [4+2] logic toward the synthesis of amorfrutins, cannabinoids, and related plant metabolites. An anionic cascade cyclization employing β-methoxycrotonates and β-chloro-α,β-unsaturated esters yielded amorfrutins in four linear steps and demonstrated utility of β-alkoxycrotonate-derived nucleophiles as functional equivalents of β-ketoester-derived dianions. Analogously, tandem Diels-Alder/retro-Diels-Alder cycloaddition of dimedone-derived bis(trimethylsiloxy)-dienes and α,β-alkynyl ester dienophiles provided facile access to resorcinol precursors of amorfrutins and cannabinoids, avoiding late-stage installation of prenyl or geranyl moieties as in previous approaches.

Ru-Catalyzed (E)-Specific ortho-C-H Alkenylation of Arenecarboxylic Acids by Coupling with Alkenyl Bromides

In the presence of [p-cymene)RuCl₂]₂, (E)-configured alkenyl bromides couple with aromatic carboxylates to form ortho-vinylbenzoic acids. This C-H vinylation proceeds in high yields without any activating phosphine ligands and has an excellent functional group tolerance. Starting from commonly available (E/Z )-mixtures of alkenyl bromides, (E)-configured vinyl arenes or dienes are formed exclusively. Mechanistic studies show that this selectivity is achieved because the (E)-configured alkenyl bromides undergo a smooth coupling, whereas the (Z)-isomers are rapidly eliminated with the formation of alkynes.

Cross-Dehydrogenative Coupling/Annulation of Arene Carboxylic Acids and Alkenes in Water with Ruthenium(II) Catalyst and Air

A cross-dehydrogenative coupling of arene carboxylic acids with olefins is reported with ruthenium(II) catalyst employing air and water as green oxidant and solvent, respectively. It offers a robust synthesis of valuable phthalide molecules. A one-pot sequential strategy is also disclosed to access Heck-type products that are apparently difficult to make directly from arene carboxylic acids.

Multiple Catalytic C-H Bond Functionalization for Natural Product Synthesis

In the past decade, multiple catalytic C-H bond functionalization has been successfully applied in natural product synthesis as a strategy to reduce the number of steps, increase overall yield and employ more easily available starting materials. This minireview presents selected examples making use of multiple C-H bond functionalization in conceptually different ways. First, linear syntheses are discussed, wherein multiple C-H functionalization is employed either from simple (hetero)cyclic cores, at a late stage, or to build polycyclic systems. Second, the use of multiple C-H functionalization as a strategic tool in convergent synthesis to access and couple complex fragments is discussed. Information on the scalability of the employed methods is provided when available. The presented cases indicate that multiple C-H functionalization strategies should play a great role to shape the future synthesis of functional complex molecules with improved sustainability.

Bidentate Directing Groups: An Efficient Tool in C-H Bond Functionalization Chemistry for the Expedient Construction of C-C Bonds

During the past decades, synthetic organic chemistry discovered that directing group assisted C-H activation is a key tool for the expedient and siteselective construction of C-C bonds. Among the various directing group strategies, bidentate directing groups are now recognized as one of the most efficient devices for the selective functionalization of certain positions due to fact that its metal center permits fine, tunable, and reversible coordination. The family of bidentate directing groups permit various types of assistance to be achieved, such as N,N-dentate, N,O-dentate, and N,S-dentate auxiliaries, which are categorized based on the coordination site. In this review, we broadly discuss various C-H bond functionalization reactions for the formation of C-C bonds with the aid of bidentate directing groups.

Unified total synthesis of amorfrutins A and C via the Claisen rearrangement

A concise, unified total synthesis of the two prenylated aromatic polyketides amorfrutins A and C, which exhibit various medicinally important biological profiles such as antimicrobial, PPARγ modulating and quorum sensing inhibitory activities, has been achieved from commercially available 3,5-dimethoxybenzaldehyde in 38% and 10% overall yields through nine and ten steps, respectively. The key transformation for the synthesis of amorfrutin A was the Claisen rearrangement of a mono-O-(1,1-dimethylallyl)resorcinol derivative to install the C3-prenyl substituent, while that for the synthesis of amorfrutin C was the double Claisen rearrangement of a di-O-(1,1-dimethylallyl)resorcinol derivative to introduce the two prenyl groups at the C3 and C5 positions all at once.