Pd-Catalyzed Activation of Carbon-Carbon Bonds in Hydroxymethylfurfural Derivatives

Palladium-catalyzed activation of C-C bonds in organic molecules is a powerful tool for the synthesis of value-added compounds. 5-Hydroxymethylfurfural (HMF) derivatives are a promising class of biomass-derived chemicals that have received considerable attention due to their potential applications in the synthesis of biologically active molecules and materials. However, the selective activation of unstrained C-C bonds is a challenging task, mainly due to their relatively high bond dissociation energies. Herein, we report a palladium-catalyzed method for the efficient C-C bond activation of HMF derivatives, enabling their arylation with iodobenzenes. Mechanistic studies, including reaction-profile analysis, competition experiments and head-space IR spectroscopy suggest a decarboxylative mechanism.

Pyrrole-2-carboxaldehydes: Origins and Physiological Activities

Pyrrole-2-carboxaldehyde (Py-2-C) derivatives have been isolated from many natural sources, including fungi, plants (roots, leaves, and seeds), and microorganisms. The well-known diabetes molecular marker, pyrraline, which is produced after sequential reactions in vivo, has a Py-2-C skeleton. Py-2-Cs can be chemically produced by the strong acid-catalyzed condensation of glucose and amino acid derivatives in vitro. These observations indicate the importance of the Py-2-C skeleton in vivo and suggest that molecules containing this skeleton have various biological functions. In this review, we have summarized Py-2-C derivatives based on their origins. We also discuss the structural characteristics, natural sources, and physiological activities of isolated compounds containing the Py-2-C group.

Total Synthesis of Hemerocallisamine I Paved by Gram-Scale Synthesis of (2S,4S)-4-Hydroxyglutamic Acid Lactone

Total synthesis of the 2-formylpyrrole alkaloid hemerocallisamine I is presented, both in racemic and enantiopure form. Our synthetic strategy involves (2S,4S)-4-hydroxyglutamic acid lactone as the key intermediate. Starting from an achiral substrate, the target stereogenic centers were introduced by means of crystallization-induced diastereomer transformation (CIDT) in a highly stereoselective fashion. A Maillard-type condensation was crucial to constructing the desired pyrrolic scaffold.

Comprehensive Review of Recent Advances in Chiral A-Ring Flavonoid Containing Compounds: Structure, Bioactivities, and Synthesis

Flavonoids are a group of natural polyphenolic substances that are abundant in vegetables, fruits, grains, and tea. Chiral A-ring-containing flavonoids are an important group of natural flavonoid derivatives applicable in a wide range of biological activities such as, cytotoxic, anti-inflammatory, anti-microbial, antioxidant, and enzyme inhibition. The desirable development of chiral A-ring-containing flavonoids by isolation, semi-synthesis or total synthesis in a short duration proves their great value in medicinal chemistry research. In this review, the research progress of chiral A-ring-containing flavonoids, including isolation and extraction, structural identification, pharmacological activities, and synthetic methods, is comprehensively and systematically summarized. Furthermore, we provide suggestions for future research on the synthesis and biomedical applications of flavonoids.

Phytochemistry, Biological Activities, Therapeutic Potential, and Socio-Economic Value of the Caper Bush (Capparis spinosa L.)

Capparis spinosa L., commonly known as the caper bush, is an aromatic plant growing in most of the Mediterranean basin and some parts of Western Asia. C. spinosa L. has been utilized as a medicinal plant for quite a long time in conventional phytomedicine. Polyphenols and numerous bioactive chemicals extracted from C. spinosa L. display various therapeutic properties that have made this plant a target for further research as a health promoter. This review is meant to systematically summarize the traditional uses, the phytochemical composition of C. spinosa L., and the diverse pharmacological activities, as well as the synthetic routes to derivatives of some identified chemical components for the improvement of biological activities and enhancement of pharmacokinetic profiles. This review also addresses the benefits of C. spinosa L. in adapting to climate change and the socio-economic value that C. spinosa L. brings to the rural economies of many countries.

Recent advances in cascade reactions and their mechanistic insights: a concise strategy to synthesize complex natural products and organic scaffolds

The beauty of cascade reactions to bestow us with cumbersome organic scaffolds has made them a cutting-edge area of research. Although the planning of cascades may require intuition, their results can be highly impactful. The development of cascades to provide specific targeted molecules of an appropriate structural and stereochemical framework poses a significant challenge but can serve as one of the most impressive tools in organic synthesis. This review shares a broad interest in compiling cascade transformations towards the construction of polycyclic frameworks, induction of chirality/asymmetry in the protocol, etc. to solve diverse challenges in organic synthesis pursuits, as cascades enable the rapid and efficient construction of complex architectures from simple molecules. The studies highlighted herein manifest the utilization of a range of cascade reactions under various classifications for generating natural product skeletons such as palau'amine, benzosimuline, arcutinine, and others from simple building blocks, with emphasis on breakthroughs and potential for asymmetric synthesis. The exquisite synthetic designs of recently completed total synthesis of natural products with a focus on strategic concerns are also highlighted in this review.

Two Dipeptide-Bound Pyrralines with Ile or Ala: A Study on Their Synthesis, Transport across Caco-2 Cell Monolayers, and Interaction with Aminopeptidase N

In this study, pyrralylisoleucine (Pyrr-Ile) and pyrralylalanine (Pyrr-Ala), two dipeptide-bound pyrralines with different C-termini were synthesized as the representatives of dietary advanced glycation end products (dAGEs). The structures of Pyrr-Ile and Pyrr-Ala were characterized by high-resolution mass spectrometry, nuclear magnetic resonance, and Fourier transform infrared spectroscopy. Then, the transport of Pyrr-Ile and Pyrr-Ala across intestinal epithelial cells was investigated using Caco-2 cell monolayers, and their interaction with aminopeptidase N (APN) was analyzed. The results showed that the apparent permeability coefficient (P_app) of Pyrr-Ala was (14.1 ± 2.26) × 10^-7 cm·s^-1 calculated by free pyrraline, while the P_app values of Pyrr-Ile were (32.4 ± 5.35) × 10^-7 and (19.1 ± 1.46) × 10^-7 cm·s^-1 when they were, respectively, calculated according to their dipeptide-bound or free form. Both Pyrr-Ala and Pyrr-Ile were potential substrates of APN, and their hydrolysis by APN may make the intact transmembrane transport of Pyrr-Ala and Pyrr-Ile more difficult, especially for Pyrr-Ala. Besides, the occurrence of product inhibition in hydrolysis of Pyrr-Ile was possible. Pyrr-Ile and Pyrr-Ala were different in P_app values and transport forms, which suggested that the C-terminus may play an important role in their transport across the Caco-2 cell monolayers. In addition, the results highlight the intact transmembrane transport of dipeptide-bound pyrraline.

Recent progress in the total synthesis of pyrrole-containing natural products (2011–2020)

This review discusses total syntheses of pyrrole-containing natural products over the last ten years, highlighting recent advances in the chemistry of pyrroles in the context of their innate reactivity, and their preparation in complex settings.

The Increasing Value of Biomass: Moving From C6 Carbohydrates to Multifunctionalized Building Blocks via 5-(hydroxymethyl)furfural

Recent decades have been marked by enormous progress in the field of synthesis and chemistry of 5-(hydroxymethyl)furfural (HMF), an important platform chemical widely recognized as the "sleeping giant" of sustainable chemistry. This multifunctional furanic compound is viewed as a strong link for the transition from the current fossil-based industry to a sustainable one. However, the low chemical stability of HMF significantly undermines its synthetic potential. A possible solution to this problem is synthetic diversification of HMF by modifying it into more stable multifunctional building blocks for further synthetic purposes.

2-Formylpyrrole natural products: origin, structural diversity, bioactivity and synthesis

2-Formylpyrroles constitute a large and growing family of bioactive Maillard reaction products found in food, traditional medicine and throughout nature.

3-Formylpyrazolo[1,5- a]pyrimidines as Key Intermediates for the Preparation of Functional Fluorophores

A one-pot route for the regioselective synthesis of 3-formylpyrazolo[1,5- a]pyrimidines 4a-k in good yields through a microwave-assisted process is provided. The synthesis proceeds via a cyclocondensation reaction between β-enaminones 1 with NH-3-aminopyrazoles 2, followed by formylation with an iminium salt moiety (Vilsmeyer-Haack reagent). These N-heteroaryl aldehydes 4 were successfully used as strategic intermediates for the preparation of novel functional fluorophores with yields up to 98%. The structures of the products obtained and regioselectivity of the reactions were determined on the basis of NMR measurements and X-ray diffraction analysis. Since pyrazolo[1,5- a]pyrimidines (PPs) 3 have shown an important fluorescence, photophysical properties of four 2-methylderivatives substituted at position 7 with different acceptor (A) or donor (D) groups were investigated. The compounds evaluated exhibited large Stokes shift in different solvents, but only the substituted p-methoxyphenyl (4-An) showed a strong fluorescence intensity with quantum yields up to 44% due to its greater ICT. Therefore, hybrid systems based on pyrazolo[1,5- a]pyrimidines could be used as fluorescent probes to detect biologically or environmentally relevant species.

Synthetic approaches to access acortatarins, shensongines and pollenopyrroside; potent antioxidative spiro-alkaloids with a naturally rare morpholine moiety

Pyrrole spiroketal alkaloids (PSAs) are a class of novel natural products that have been recently disclosed. Acortatarin A and acortatarin B, two potent antioxidative spiroalkaloids with a naturally rare morpholine moiety, are important members of this class. These spiroalkaloids are isolated from Acorus tatarinowii, Brassica campestris, Capparis spinose, bread crust, Xylaria nigripes and medicine Shensong Yangxin and could inhibit significantly the reactive oxygen species (ROS) production in high-glucose-induced mesangial cells in a time- and dose-dependent manner. Hence, these natural products are promising starting points for the formation of new therapeutics to medicate cardiovascular diseases, cancer, diabetic complications, and other diseases in which ROS are implicated. The impressive structure combined with an interesting pharmacological activity prompted synthetic chemists to construct an asymmetric synthetic strategy that could be used to access structural derivatives in addition to the larger quantities of natural products required for further biological investigations. This review summarizes the current state of the literature regarding with the synthesis of acortatarin A and B and its other family members viz. shensongine A, B and C, and pollenopyrroside A. The present review discusses the pros and cons of synthetic methodologies, which would be beneficial for further developments in the synthetic methodologies. Hopefully, this struggle pushes the reader's mind to consider new perspectives, think differently and forge new connections.

Synthesis of α-Formylated N-Heterocycles and Their 1,1-Diacetates from Inactivated Cyclic Amines Involving an Oxidative Ring Contraction

A novel synthesis of pyrrolidine-2-carbaldehydes or tetrahydropyridine-2-carbaldehydes from the cascade reactions of N-arylpiperidines or N-arylazepanes is presented. Mechanistically, the formation of the title compounds involves an unprecedented oxidative ring contraction of inactivated cyclic amines via Cu(OAc)₂/KI/O₂-promoted oxidative cleavage and reformation of the C-N bond. Interestingly, when PhI(OAc)₂ was used in place of KI, 1,1-diacetates of the corresponding aldehydes were directly obtained with good efficiency. To the best of our knowledge, this is the first example of regioselective C(sp³)-H bond functionalization and C(sp³)-N bond activation of saturated cyclic amines using copper salt and oxygen.

Stereocontrolled Synthesis of Spiroketals: An Engine for Chemical and Biological Discovery

Spiroketals are key structural motifs found in diverse natural products with compelling biological activities. However, stereocontrolled synthetic access to spiroketals, independent of their inherent thermodynamic preferences, is a classical challenge in organic synthesis that has limited in-depth biological exploration of this intriguing class. Herein, we review our laboratory's efforts to advance the glycal epoxide approach to the stereocontrolled synthesis of spiroketals via kinetically controlled spirocyclization reactions. This work has provided new synthetic methodologies with applications in both diversity- and target-oriented synthesis, fundamental insights into structure and reactivity, and efficient access to spiroketal libraries and natural products for biological evaluation.

Total Synthesis and Stereochemical Revision of the 2-Formylpyrrole Alkaloid Hemerocallisamine I

The first total synthesis of the 2-formylpyrrole alkaloid hemerocallisamine I is reported. The convergent synthesis features a key Maillard-type condensation of a complex amine derived from cis-4-hydroxy-l-proline with a dihydropyranone, to directly furnish the 2-formylpyrrole ring system. The absolute configuration of hemerocallisamine I has been revised on the basis of optical rotation data obtained for the synthesized compound.

Family-level stereoselective synthesis and biological evaluation of pyrrolomorpholine spiroketal natural product antioxidants

The pyranose spiroketal natural products pollenopyrroside A and shensongine A (also known as xylapyrroside A, ent-capparisine B) have been synthesized by stereoselective spirocyclizations of a common C1-functionalized glycal precursor. In conjunction with our previously reported syntheses of the corresponding furanose isomers, this provides a versatile family-level synthesis of the pyrrolomorpholine spiroketal natural products and analogues. In rat mesangial cells, hyperglycemia-induced production of reactive oxygen species, which is implicated in diabetic nephropathy, was inhibited by pollenopyrroside A and shensongine A with mid-μM IC50 values, while unnatural C2-hydroxy analogues exhibited more potent, sub-μM activity.