Concise synthesis of yingzhaosu C and epi-yingzhaosu C by peroxyl radical cyclization. Assignment of relative configuration

Peroxygenase-Enabled Reductive Kinetic Resolution for the Enantioenrichment of Organoperoxides

Enantiomerically pure organoperoxides serve as valuable precursors in organic transformations. Herein, we present the first examples of unspecific peroxygenase catalyzed kinetic resolution of racemic organoperoxides through asymmetric reduction. Through meticulous investigation of the reaction conditions, it is shown that the unspecific peroxygenase from Agrocybe aegerita (AaeUPO) exhibits robust catalytic activity in the kinetic resolution reactions of the model substrate with turnover numbers up to 60000 and turnover frequency of 5.6 s-1. Various aralkyl organoperoxides were successfully resolved by AaeUPO, achieving excellent enantioselectivities (e.g., up to 99 % ee for the (S)-organoperoxide products). Additionally, we screened commercial peroxygenase variants to obtain the organoperoxides with complementary chirality, with one mutant yielding the (R)-products. While unspecific peroxygenases have been extensively demonstrated as a powerful oxidative catalysts, this study highlights their usefulness in catalyzing the reduction of organoperoxides and providing versatile chiral synthons.

Research Progress on Sesquiterpene Compounds from Artabotrys Plants of Annonaceae

Artabotrys, a pivotal genus within the Annonaceae family, is renowned for its extensive biological significance and medicinal potential. The genus's sesquiterpene compounds have attracted considerable interest from the scientific community due to their structural complexity and diverse biological activities. These compounds exhibit a range of biological activities, including antimalarial, antibacterial, anti-inflammatory analgesic, and anti-tumor properties, positioning them as promising candidates for medical applications. This review aims to summarize the current knowledge on the variety, species, and structural characteristics of sesquiterpene compounds isolated from Artabotrys plants. Furthermore, it delves into their pharmacological activities and underlying mechanisms, offering a comprehensive foundation for future research.

A Catalytic Asymmetric Hydrolactonization

Despite recent advancements in the development of catalytic asymmetric electrophile induced lactonization reactions of olefinic carboxylic acids, the archetypical hydrolactonization has long remained an unsolved and well-recognized challenge. Here, we report the realization of a catalytic asymmetric hydrolactonization using a confined imidodiphosphorimidate (IDPi) Brønsted acid catalyst. The method is operationally simple, scalable, and compatible with a wide variety of substrates. Its potential is showcased with concise syntheses of the sesquiterpenes (-)-boivinianin A and (+)-gossonorol. Through in-depth physicochemical and DFT analyses, we derive a nuanced picture of the mechanism and enantioselectivity of this reaction.

Advocacy for the Medicinal Plant Artabotrys hexapetalus (Yingzhao) and Antimalarial Yingzhaosu Endoperoxides

The medicinal plant Artabotrys hexapetalus (synonyms: A.uncinatus and A. odoratissimus) is known as yingzhao in Chinese. Extracts of the plant have long been used in Asian folk medicine to treat various symptoms and diseases, including fevers, microbial infections, ulcers, hepatic disorders and other health problems. In particular, extracts from the roots and fruits of the plant are used for treating malaria. Numerous bioactive natural products have been isolated from the plant, mainly aporphine (artabonatines, artacinatine) and benzylisoquinoline (hexapetalines) alkaloids, terpenoids (artaboterpenoids), flavonoids (artabotrysides), butanolides (uncinine, artapetalins) and a small series of endoperoxides known as yingzhaosu A-to-D. These natural products confer antioxidant, anti-inflammatory and antiproliferative properties to the plant extracts. The lead compound yingzhaosu A displays marked activities against the malaria parasites Plasmodium falciparum and P. berghei. Total syntheses have been developed to access yingzhaosu compounds and analogues, such as the potent compound C14-epi-yingzhaosu A and simpler molecules with a dioxane unit. The mechanism of action of yingzhaosu A points to an iron(II)-induced degradation leading to the formation of two alkylating species, an unsaturated ketone and a cyclohexyl radical, which can then react with vital parasitic proteins. A bioreductive activation of yingzhaosu A endoperoxide can also occur with the heme iron complex. The mechanism of action of yingzhaosu endoperoxides is discussed, to promote further chemical and pharmacological studies of these neglected, but highly interesting bioactive compounds. Yingzhaosu A/C represent useful templates for designing novel antimalarial drugs.

Harnessing the Power of the Asymmetric Grignard Synthesis of Tertiary Alcohols: Ligand Development and Improved Scope Exemplified by One-Step Gossonorol Synthesis

A series of N-substituted cyclohexyldiaminophenolic ligands for the asymmetric Grignard synthesis of tertiary alcohols is reported. The 2,5-dimethylpyrrole-decorated ligand led to improved enantioselectivities and broadened the scope of the methodology. As an exemplar, we report an unprecedented highly selective one-step synthesis of gossonorol in 93% ee, also constituting the shortest formal syntheses of natural products boivinianin B and yingzhaosu C.

Peroxidation of 3,4-dihydro-1,4-benzoxazin-2-ones

The sp³-C-H peroxidation of 3,4-dihydro-1,4-benzoxazin-2-ones was achieved under mild and simple catalyst-free reaction conditions. A range of biologically important alkylated benzoxazinone peroxides are synthesized in high yield with a good functional group tolerance. The C(sp³)-OO bond was constructed efficiently and could be further converted into C(sp³)-C(sp³), C(sp³)-C(sp²), C(sp³)-C(sp), C-P and C[double bond, length as m-dash]O bonds by late-stage functional group transformations.

Very short highly enantioselective Grignard synthesis of 2,2-disubstituted tetrahydrofurans and tetrahydropyrans

Phenones with elongated chains are shown to be excellent substrates for ligand-promoted asymmetric Grignard synthesis of tertiary alcohols. In turn this enables the simple, short and highly enantioselective (up to 96% ee) preparation of chiral 2,2-disubstituted THFs and THPs. Thus, asymmetric addition of Grignard reagents to γ-chlorobutyrophenones and δ-chlorovalerophenones takes place in the presence of a chiral diaminocyclohexyl-derived tridentate ligand and subsequent base-promoted intramolecular cyclisation occurs with complete retention of asymmetry. As examples of the methodology, we report the shortest syntheses of gossonorol, γ-ethyl-γ-phenylbutyrolactone and δ-methyl-δ-tolylvalerolactone, the joint-shortest and flexible synthesis of boivinianin A and the shortest formal syntheses of boivinianin B and yingzhaosu C.

Enantioselective Organocatalyzed Bromolactonizations: Applications in Natural Product Synthesis

Asymmetric bromolactonization reactions of δ-unsaturated carboxylic acids have been investigated in the presence of 10 chiral squaramide hydrogen-bonding organocatalysts. The best catalyst enabled the cyclization of several 5-arylhex-5-enoic acids into the corresponding bromolactones with up to 96% ee and in high to excellent chemical yields. The reported catalysts are prepared in a straightforward manner in two steps from dimethyl squarate. The utility of the developed protocol was demonstrated in highly enantioselective syntheses of the sesquiterpenoids (-)-gossoronol and (-)-boivinianin B. Both natural products were obtained in ≥99% enantiomeric excess.

Hyperisampsins H-M, Cytotoxic Polycyclic Polyprenylated Acylphloroglucinols from Hypericum sampsonii

Six new polycyclic polyprenylated acylphloroglucinols (PPAPs), named hyperisampsins H-M (1-6), were isolated from the aerial parts of Hypericum sampsonii, together with five known analogs (7-11). The structures of 1-6 were established by extensive spectroscopic analyses, including HRESIMS and NMR. In addition, the absolute configurations of these new compounds were determined by electronic circular dichroism (ECD) calculations. Compounds 1 and 2 represent the first examples of PPAPs possessing a unique γ-lactone ring at C-23, while 3-6 differed from normal PPAPs with an unprecedented 1,2-dioxane ring. Compounds 1-7 were evaluated for their cytotoxic activities against a panel of human cancer cell lines in vitro, of which 3, 4, and 6 exhibited significant cytotoxic activities with IC50 values ranging from 0.56 to 3.00 μM. Moreover, compound 3 induces leukemia cell apoptotic death, evidenced by activation of caspase-3, degradation of PARP, up-regulation of Bax, and down-regulation of Bcl-2 and Bcl-xl.

Four-step synthesis of the antimalarial cardamom peroxide via an oxygen stitching strategy

A four-step synthesis of the antimalarial terpene cardamom peroxide, a 1,2-dioxepane-containing natural product, is reported from (-)-myrtenal and molecular oxygen. This highly concise route was guided by biosynthetic logic and enabled by an unusual manganese-catalyzed, tandem hydroperoxidation reaction. The absolute configuration of the cardamom peroxide is reported, and its mode of fragmentation following Fe(II)-mediated endoperoxide reduction is established. These studies reveal the generation of reactive intermediates distinct from previously studied endoperoxide natural products.

Stereoselective nucleophilic fluoromethylation of aryl ketones: dynamic kinetic resolution of chiral α-fluoro carbanions

Although many methods are available for the synthesis of optically enriched monofluoromethyl secondary alcohols, synthesizing optically enriched monofluoromethyl tertiary alcohols remains a challenge. An efficient and easy-to-handle nucleophilic fluoromethylation protocol was developed. The current monofluoromethylation showed much higher facial selectivity than the corresponding difluoromethylation and proceeded via a different type of transition state. Excellent stereoselective control at the fluorinated carbon chiral center was found, an effect believed to be facilitated by the dynamic kinetic resolution of the chiral α-fluoro carbanions.

Palladium(II)-catalyzed cyclization of unsaturated hydroperoxides for the synthesis of 1,2-dioxanes

The cyclization of gamma,delta-unsaturated tertiary hydroperoxides in the presence of a palladium(II) catalyst afforded 1,2-dioxanes resembling biologically active natural products. A variety of substrates were screened, and synthetic manipulations were accomplished to construct compounds with structural similarity to antimalarial targets.

Identification of carbon-centred radicals derived from linalyl hydroperoxide, a strong skin sensitizer: a possible route for protein modifications

Few studies are reported on the formation of reactive carbon-centred radical species from toxic xenobiotics. In this paper the formation of carbon radicals derived from the skin sensitizer linalyl hydroperoxide is described using radical trapping and EPR studies. Radical trapping used TMIO as scavenger agent and light, heat or TPP-Fe(3+) as radical inducers. EPR spin trapping was based on the use of the parent alcohol, generating the same allyloxyl radical than the hydroperoxide by photolysis of the corresponding nitrite formed with t-BuONO, also playing the role of the spin trap. It is suggested that the generation of these carbon radical species could play an important role for the binding of the hydroperoxide with skin proteins to form antigenic structures, the first step of the skin sensitization mechanism.

Synthesis of antimalarial yingzhaosu A analogues by the peroxidation of dienes with Co(II)/O2/Et3SiH

Co(II)-catalyzed peroxidation of dienes including (S)-limonene in the presence of molecular oxygen and triethylsilane provided in each case the corresponding 2,3-dioxabicyclo[3.3.1]nonane derivatives via the intramolecular cyclization of the unsaturated peroxy radical intermediates. The product composition was remarkably influenced by the structure of the dienes, the nature of the solvents, and the concentration of the substrates and the catalyst. Some of the yingzhaosu A analogues obtained in this study showed notable antimalarial activities in vitro.

beta-Methoxy-gamma-methylene-alpha,beta-unsaturated-gamma-butyrolactones from Artabotrys hexapetalus

The dichloromethane extract of the aerial parts of Artabotrys hexapetalus afforded three beta-methoxy-gamma-methylene-alpha,beta-unsaturated-gamma-butyrolactones, which are proposed to be derived from a C(18) unsaturated fatty acid by a biosynthetic route similar to that proposed for the Annonaceous acetogenins. The structure of the unique beta-methoxy-gamma-methylene-substituted, alpha,beta-unsaturated-gamma-butyrolactone ring of artapetalins A-C (1-3) was determined by 2D-NMR spectroscopic analyses. Two unusual simple butyrolactones, (+)-tulipalin B and (2R,3R)- 3-hydroxy-2-methylbutyrolactone were also isolated from this species.

Halonium ion-mediated reaction of unsaturated hydroperoxy acetals. Competition between the formation of cyclic peroxides and the migration of the methoxy (or hydroxy) group

Monoozonolyses of dienes 2 in methanol gave in each case the corresponding unsaturated alpha-methoxy hydroperoxides 3. Capture of 2-alkyl-substituted cyclohexanone oxides by methanol was highly diastereoselective, thereby providing exclusively the hydroperoxides derived from attack by methanol from the less hindered face of the carbonyl oxide intermediates. Halonium ion-mediated reactions of the hydroperoxides 3 gave the novel methoxy- or hydroxy-migrated products, together with the expected halogen-substituted 1, 2-dioxanes and/or 1,2-dioxepanes, the composition of the product mixture being a function of the halogenating agent utilized and the structure of 3.