Stereoselective Total Synthesis of Formosanol, Tsugacetal, and Methyl β-Conidendral

Highly Stereoselective [2 + 4] Annulation of Phosphenes and Enones with β-Electron-Donating Groups

Enones with β-electron-donating groups can exist in their zwitterionic resonance forms, which favor nucleophilic attack to phosphenes generated from phosphoryl (aryl)diazomethanes under blue light irradiation to yield zwitterionic intermediates. The intermediates cyclize to afford highly stereoselective trans-3,4-dihydro-1,2-oxaphosphinine 2-oxides in good to excellent yields. In the cyclic transition state of the final cyclization, the stereoelectronic effect and the Coulomb force control the high stereoselectivity. The electron-donating groups of enones play double roles in both nucleophilic attack and cyclization. The reaction features readily available starting materials, high atom-economy, no catalyst, a fast reaction rate, broad functional group-tolerance and substrate scope, high yields and stereoselectivity, and mild conditions.

Palladium-Catalyzed (3 + 2) Annulation of Azaborines with Vinyl Epoxides for Constructing Polycyclic Oxazaborolidines

A palladium-catalyzed (3 + 2) annulation of azaborines with vinyl epoxides has been established. By this strategy, various polycyclic oxazaborolidines with structural diversity were synthesized in generally high yields (up to 99%). The annulation can be scaled up and the polycyclic oxazaborolidines can be further functionalized through olefin metathesis and Heck reaction, which demonstrated good feasibility for downstream transformations. Moreover, the catalytic asymmetric version of this (3 + 2) annulation has been accomplished under the catalysis of palladium/chiral phosphoramidite ligand, producing chiral oxazaborolidines in overall good enantioselectivities (up to 98:2 er). This work not only represents the first catalytic asymmetric (3 + 2) annulation of 1,2-azaborines with vinyl epoxides but also offers an efficient strategy for constructing benzooxazaborolidine skeletons, particularly those in an enantioenriched fashion.

Major specialized natural products from the endangered plant Heptacodium miconioides, potential medicinal uses and insights into its longstanding unresolved systematic classification

A comprehensive phytochemical investigation of the flower buds and leaves/twigs of Heptacodium miconioides, a cultivated ornamental plant native to China and categorized as 'vulnerable', has led to the isolation of 45 structurally diverse compounds, which comprise 18 phenylpropanoids (1-4, 7-20), 11 pentacyclic triterpenoids (5, 6, 21-29), eight secoiridoid glycosides (30-37), three quinic acid derivatives (38-40), and a few miscellaneous components (41-45). Among them, (+)-α-intermedianol (1), (+)-holophyllol A (2), and (-)-pseudolarkaemin A (3) represent previously unreported enantiomeric lignans, while (+)-7'(R)-hydroxymatairesinol (4) is an undescribed naturally occurring lignan. Heptacoacids A (5) and B (6) are undescribed 24-nor-urs-28-oic acid derivatives. Their chemical structures were determined by 2D-NMR, supplemented by evidence from specific rotations and circular dichroism spectra. Given the uncertainty surrounding the systematic position of Heptacodium, integrative taxonomy (ITA), a method utilized to define contentious species, is applied. Chemotaxonomy, a vital aspect of ITA, becomes significant. By employing hierarchical clustering analysis (HCA) and syntenic pattern analysis methods, a taxonomic examination based on the major specialized natural products from the flower buds of H. miconioides and two other Caprifoliaceae plants (i.e., Lonicera japonica and Abelia × grandiflora) could offer enhanced understanding of the systematic placement of Heptacodium. Additionally, compounds 39 and 40 displayed remarkable inhibitory activities against ATP-citrate lyase (ACL), with IC50 values of 0.11 and 1.10 μM, respectively. In summary, the discovery of medical properties and refining systematic classification can establish a sturdy groundwork for conservation efforts aimed at mitigating species diversity loss while addressing human diseases.

Recent updates on vinyl cyclopropanes, aziridines and oxiranes: access to heterocyclic scaffolds

This present review delineates the repertoire of vinyl cyclopropanes and their stuctural analogues to accomplish a wide array of oxa-cycles, aza-cycles, and thia-cycles under transition metal catalysis and metal-free approaches from early 2019 to the present date. The generation of electrophilic π-allyl intermediates and 1-3/1-5-dipolarophile chemistry originating from VCPs are always green, step- and atom-economical and sustainable strategies in comparsion with prefunctionalized and/or C-H activation protocols. Here, the strained ring-system extends its applicability by relieving the strain to undergo a ring-expansion reaction to accomplish 5-9 membered carbo- and heterocyclic systems. The availability of chiral ligands in the ring-expansion reaction of VCPs and their analogues has paved the way to realizing asymmetric synthetic transformations.

Intramolecular Ni-catalyzed reductive coupling enables enantiodivergent synthesis of linoxepin

A nickel-catalyzed reductive tandem cyclization of the elaborated β-bromo acetal with a dibenzoxepin scaffold was invented to strategically construct the remaining two rings in linoxepin. The generated diasterodivergent intermediates could be easily converted to both enantiomers of this unique cyclolignan molecule via facile oxidations, thus realizing enantiodivergent total synthesis of linoxepin for the first time.

Chemistry and Biology of Podophyllotoxins: An Update

Podophyllotoxin is an aryltetralin lignan lactone derived from different plants of Podophyllum. It consists of five rings with four chiral centers, one trans-lactone and one aryl tetrahydronaphthalene skeleton with multiple modification sites. Moreover, podophyllotoxin and its derivatives showed lots of bioactivities, including anticancer, anti-inflammatory, antiviral, and insecticidal properties. The demand for podophyllotoxin and its derivatives is rising as a result of their high efficacy. As a continuation of our previous review (Chem. Eur. J., 2017, 23, 4467-4526), herein, total synthesis, biotransformation, structural modifications, bioactivities, and structure-activity relationships of podophyllotoxin and its derivatives from 2017 to 2022 are summarized. Meanwhile, a piece of update information on the origin of new podophyllotoxin analogues from plants from 2014 to 2022 was compiled. We hope that this review will provide a reference for future high value-added applications of podophyllotoxin and its analogues in the pharmaceutical and agricultural fields.

Studies on the [4 + 2] cycloaddition and allylic substitution of indole-fused zwitterionic π-allylpalladium

The zwitterionic π-allylpalladium species, also known as dipoles, are important synthons widely used in various reactions including cycloaddition and allylic substitution. This study reported the development of a new indole-fused zwitterionic π-allylpalladium precursor compound and its application in [4 + 2] cycloaddition and allylic substitution reactions. As a result, the synthesis of pyrrolo[3,2,1-ij]quinazolin-3-one and 7-vinyl indole compounds was achieved with moderate to good yields. Notably, the allylic substitution reaction exhibited excellent regio- and stereoselectivity.

Synthesis of Chiral 2,3-Dihydrofurans via One-Pot Pd-Catalyzed Asymmetric Allylic Cycloaddition and a Retro-Dieckmann Fragmentation Cascade

An efficient method for the enantioselective synthesis of 2,3-dihydrofurans bearing a quaternary stereocenter has been developed via Pd-catalyzed asymmetric allylic cycloaddition and a retro-Dieckmann Fragmentation cascade. The asymmetric allylic cycloaddition of vinylethylene carbonates with 3-cyanochromone followed by base-assisted retro-Dieckmann fragmentation proceeded smoothly via a one-pot process to produce chiral 3,4-disubstituted 2,3-dihydrofurans in high yields with excellent enantioselectivities.