A facile procedure for acetalization under aprotic conditions

Total Synthesis of (-)-Elodeoidins A and B

Biomimicry has long been a valuable approach for designing efficient synthetic strategies in complex natural product synthesis. However, abiotic yet powerful transforms can significantly streamline the synthesis by introducing greater convergence to the synthetic route. Herein, we delineate a convergent total synthesis of elodeoidins A and B, enabled by a cross-dehydrogenative coupling (CDC) reaction between an aldehyde and an electron-deficient olefin. The CDC reaction operating under the newly discovered reaction conditions proceeds via distinct concerted deprotonation within the formal Cu(III) catalytic complex. Furthermore, the total synthesis of both structural candidates of elodeoidin B revealed that the natural product exists as a mixture of epimers at the C8 stereocenter.

Aldehyde-Assisted Alkoxysilane Condensation to Form Siloxane Bond: A New Process for Curing Alkoxy-Functional Silicone Resins

The formation of the siloxane bond is one of the most important reactions used in silicone chemistry and technology. In this paper, a new process for the condensation of alkoxy-functional silanes to form a siloxane bond is presented. The new reaction is catalyzed by a Ge(II)+ complex stabilized by pentamethylcyclopentadiene with a weakly coordinated anion, tetrakis(pentafluorophenyl)borane. A mechanistic study of this new condensation process using model alkoxy-functional silanes and propionaldehyde is completed. It is established that the quantitative conversion of alkoxysilanes to the siloxane bond requires stoichiometric amounts of aldehyde. It is also found that paraldehyde can serve as a convenient, higher boiling source of acetaldehyde for the condensation of alkoxysilanes. The results obtained, supported by DFT calculations, allow for us to formulate the mechanism of this reaction.

Recent advances in the total synthesis of alkaloids using chiral secondary amine organocatalysts

Since the early 21st century, organocatalytic reactions have undergone significant advancements. Notably, numerous asymmetric reactions utilizing chiral secondary amine catalysts have been developed and applied in the total synthesis of natural products. In this review, we provide an overview of alkaloid syntheses reported since 2017, categorized by scaffold, with a focus on key steps involving asymmetric reactions catalyzed by secondary amine organocatalysts.

The Latest Progress in the Chemistry of Daphniphyllum Alkaloids

Daphniphyllum alkaloids (DAs) are interesting molecules with rich molecular skeletons and diverse biological activities. Since their discovery, phytochemists have isolated, purified, and identified more than 350 DAs. Synthetic chemists, attracted by the structure and activity of DAs, have accomplished many elegant synthetic jobs. Herein, we summarize work on the isolation, structural identification, bioactivity testing, and synthesis of DAs from 2018 to 2023, with the aim of providing a reference for future studies.

Total synthesis of (-)-securingine G

In this study, we present the first total synthesis of (-)-securingine G. Diverging from the proposed biosynthetic pathway, our approach involves the addition of nucleophilic pyridine anion species to the electrophilic menisdaurilide congener. Crucially, incorporating a weakly basic yet nucleophilic tri(2-pyridinyl)lanthanum complex proved essential to circumvent undesired base-mediated pathways during the key coupling reaction. Notably, we introduce n-Bu3La·5LiCl as a new exchange reagent, facilitating efficient halide/lanthanum exchange of (hetero)aryl halides.

MSTFA as an Effective TMS Source for the TMSOTf-Catalzyed Synthesis of Cyclic Acetals

Commercially and readily available MSTFA [2,2,2-trifluoro-N-methyl-N-(trimethylsilyl)acetamide] was identified as a highly effective TMS (trimethylsilyl) source for the convenient preparation of cyclic acetals under modified Noyori's conditions. The reactions proceeded smoothly under mild conditions, affording a wide range of the corresponding cyclic acetals with excellent yields in the presence of catalytic TMSOTf (trimethylsilyl trifluoromethanesulfonate). The present method does not require a large excess of diols that can be valuable and does not require presynthesized silylated diols. In contrast to other silylating reagents such as BSA [N,O-bis(trimethylsilyl)acetamide] and BSTFA [N,O-bis(trimethylsilyl)trifluoroacetamide], the application of MSTFA avoided the inhibition of catalytic acetalization by the side product 2,2,2-trifluoro-N-methylacetamide.

Enantioselective Total Syntheses of (-)-Silicine and (-)-20-Episilicine

The enantioselective total syntheses of (-)-silicine and (-)-20-episilicine, which contain a chiral piperidine with three contiguous chiral centers (D-ring) and a strained seven-membered ring (C-ring) attached to an indole, were achieved. The key steps of these syntheses included a chiral secondary amine-catalyzed formal aza-[3 + 3] cycloaddition reaction and Lewis acid-mediated irreversible ring-closing reaction. In addition, the stereochemistry at C20 was controlled at a later stage in the syntheses.

General Synthetic Approach to Diverse Taxane Cores

Chemical synthesis of natural products is typically inspired by the structure and function of a target molecule. When both factors are of interest, such as in the case of taxane diterpenoids, a synthesis can both serve as a platform for synthetic strategy development and enable new biological exploration. Guided by this paradigm, we present here a unified enantiospecific approach to diverse taxane cores from the feedstock monoterpenoid (S)-carvone. Key to the success of our approach was the use of a skeletal remodeling strategy which began with the divergent reorganization and convergent coupling of two carvone-derived fragments, facilitated by Pd-catalyzed C-C bond cleavage tactics. This coupling was followed by additional restructuring using a Sm(II)-mediated rearrangement and a bioinspired, visible-light induced, transannular [2 + 2] photocycloaddition. Overall, this divergent monoterpenoid remodeling/convergent fragment coupling approach to complex diterpenoid synthesis provides access to structurally disparate taxane cores which have set the stage for the preparation of a wide range of taxanes.

Accurate Substrate-Like Probes for Trapping Late-Stage Intermediates in Nonribosomal Peptide Synthetase Condensation Domains

Nonribosomal peptide synthetases (NRPSs) are a family of multidomain enzymes dedicated to the production of peptide natural products. Central to NRPS function are condensation (C) domains, which catalyze peptide bond formation and a number of specialized transformations including dehydroamino acid and β-lactam synthesis. Structures of C domains in catalytically informative states are limited due to a lack of clear strategies for stabilizing C domain interactions with their substrates and client domains. Inspired by a β-lactam forming C domain, we report herein the synthesis and application of 1, which forms irreversible cross-links with engineered thiol nucleophiles in a C domain active site. Deployment of 1 demonstrates the synthetic tractability of trapping late-stage nascent peptides in C domains and provides a readily adaptable tactic for stabilizing C domain interactions in multidomain NRPS fragments.

Total Synthesis of Dolabriferol C by a Highly Stereoselective One-Pot Coupling of a meso-3,7-Diketone with Two Chiral Aldehydes

Total synthesis of the noncontiguous polypropionate dolabriferol C was achieved by retro-Claisen fragmentation of its putative contiguous precursor under mild conditions, thus establishing the former as a plausible isolation artifact. The precursor was prepared by a novel one-pot three-component bisaldol coupling of a meso (Z,Z)-bisenolate (generated in situ from a 3,7-diketone) with two enantioenriched aldehydes to set the absolute configuration of seven stereocenters in one step. The first aldol reaction proceeded with enantioselective desymmetrization of the bisenolate to produce an enantiomerically pure enolate-aldolate. Quenching at this stage enabled a streamlined synthesis of dolabriferol. Addition of a racemic aldehyde to the enolate-aldolate resulted in aldol coupling with kinetic resolution of the "matched" aldehyde; overall, a sequential enantiotopic-group-selective (SEGS) bisaldol reaction. Because the desired adduct results from the "mismatched" aldol reaction, use of enantioenriched aldehyde was required.

Development of a Chiral N-Alkoxyamide Strategy and Application to the Asymmetric Total Synthesis of Fasicularin

The asymmetric total synthesis of fasicularin by a chiral N-alkoxyamide strategy is reported. Incorporation of the chiral alkoxy group onto an amide nitrogen changes the original reactivity of the amide, enabling two key transformations: aza-spirocyclization and the reductive Strecker reaction. DFT calculations indicate that pyramidalization of the N-alkoxyamide nitrogen is crucial to produce a cyclic hemiaminal in equilibrium, which undergoes aza-spirocyclization. The chiral alkoxy group is also used as a stereocontrol element to establish two consecutive stereocenters. The iridium-catalyzed reductive Strecker reaction of the N-alkoxylactam provides the aminonitrile with high diastereoselectivity.

Total Synthesis of ent-Callilongisin B

The first enantioselective total synthesis of tricyclic diterpenoid callilongisin B, which was isolated from Callicarpa longissima, has been achieved. The synthetic method includes a diastereoselective 1,4-addition and Hosomi-Sakurai allylation followed by Wacker oxidation, intramolecular aldol reaction to construct a six-membered ring, and oxidative dearomatization accompanied by diastereoselective δ-lactonization.

Synthesis of Lignans Based on a Borate-mediated One-pot Sequential Suzuki-Miyaura Coupling of Cyclic Boranes

Lignans are a group of polyphenolic phytochemicals that possess a large spectrum of chemical structures and biological activities. Here the syntheses of lignans - anwulignan, burseran, dehydroxycubebin, ruburisandrin B, and sesamin - are achieved based on a borate-mediated one-pot sequential Suzuki-Miyaura coupling of cis- and trans-fused bicyclic boranes, which were prepared by diastereoselective cyclic hydroboration of exo-cyclic diene with cyclopentyl- and thexylboranes, respectively. A one-pot sequential Suzuki-Miyaura coupling of each cyclic borate with various aryl bromides initiated by activation of the cyclic borane with the carbon nucleophile provided 2,3-dibenzylbutane derivatives with different aromatic substituents. Finally, the syntheses of naturally occurring lignans were accomplished in several steps from the products of Suzuki-Miyaura coupling.

Stereoselective Synthesis of the I-L Fragment of the Pacific Ciguatoxins

The I-L ring system found in all the Pacific ciguatoxins has been prepared from a tricyclic precursor in a highly stereoselective manner. Subtle differences in the reactivity of the enones present in the seven- and eight-membered rings of the tricyclic ether starting material have been exploited to allow selective protection of the enone in the eight-membered ring. Subsequent distereoselective allylation of the seven-membered ring has been accomplished by a palladium-mediated Tsuji-Trost reaction. The K-ring methyl and hydroxyl groups have been installed in a highly stereoselective manner by sequential conjugate reduction and enolate oxidation reactions. Ring L has been constructed by a use of a novel relay ring-closing metathesis reaction to complete the tetracyclic framework, which possesses the functionality necessary for elaboration of rings I and L and the introduction of ring M.

Total Synthesis of (±)-Thebainone A by Intramolecular Nitrone Cycloaddition

Using an intramolecular nitrone cycloaddition and a Heck cyclization as the crucial transformations, a total synthesis of the racemic morphine alkaloid thebainone A was accomplished in 22 steps commencing with isovanillin.

Construction of all-carbon quaternary stereocenters by catalytic asymmetric conjugate addition to cyclic enones in natural product synthesis

This review discusses the construction of all-carbon quaternary stereocenters using catalytic asymmetric conjugate addition and its application in natural product synthesis.

Scandium(III) Triflate Catalyzed Direct Synthesis of N-Unprotected Ketimines

N-Unprotected ketimines are useful substrates and intermediates for synthesizing valuable nitrogen-containing compounds, but their potential applicability is limited by the available synthetic methods. To address this issue, we report a scandium(III) triflate catalyzed direct synthesis of N-unprotected ketimines. Using commercially available reagents and Lewis acid catalysts, ketones were directly transformed into the corresponding N-unprotected ketimines in high yields with broad functional group tolerance, even in multigram scales. The reactions were readily applicable for one-pot synthesis of important compounds such as a glycine Schiff base without isolation of N-unprotected ketimine intermediates. Preliminary mechanistic studies to clarify the reaction mechanism are also described.

Intramolecular [2+2] Photocycloaddition of Cyclic Enones: Selectivity Control by Lewis Acids and Mechanistic Implications

The intramolecular [2+2] photocycloaddition of 3‐alkenyl‐2‐cycloalkenones was performed in an enantioselective fashion (nine representative examples, 54–86 % yield, 76–96 % ee) upon irradiation at λ=366 nm in the presence of an AlBr₃‐activated oxazaborolidine as the Lewis acid. An extensive screening of proline‐derived oxazaborolidines showed that the enantioface differentiation depends strongly on the nature of the aryl group at the 3‐position of the heterocycle. DFT calculations of the Lewis acid–substrate complex indicate that attractive dispersion forces may be responsible for a change of the binding mode. The catalytic [2+2] photocycloaddition was shown to proceed on the triplet hypersurface with a quantum yield of 0.05. The positive effect of Lewis acids on the outcome of a given intramolecular [2+2] photocycloaddition was illustrated by optimizing the key step in a concise total synthesis of the sesquiterpene (±)‐italicene.

"One-Pot" Protection, Glycosylation, and Protection-Glycosylation Strategies of Carbohydrates

Carbohydrates, which are ubiquitously distributed throughout the three domains of life, play significant roles in a variety of vital biological processes. Access to unique and homogeneous carbohydrate materials is important to understand their physical properties, biological functions, and disease-related features. It is difficult to isolate carbohydrates in acceptable purity and amounts from natural sources. Therefore, complex saccharides with well-defined structures are often most conviently accessed through chemical syntheses. Two major hurdles, regioselective protection and stereoselective glycosylation, are faced by carbohydrate chemists in synthesizing these highly complicated molecules. Over the past few years, there has been a radical change in tackling these problems and speeding up the synthesis of oligosaccharides. This is largely due to the development of one-pot protection, one-pot glycosylation, and one-pot protection-glycosylation protocols and streamlined approaches to orthogonally protected building blocks, including those from rare sugars, that can be used in glycan coupling. In addition, new automated strategies for oligosaccharide syntheses have been reported not only for program-controlled assembly on solid support but also by the stepwise glycosylation in solution phase. As a result, various sugar molecules with highly complex, large structures could be successfully synthesized. To summarize these recent advances, this review describes the methodologies for one-pot protection and their one-pot glycosylation into the complex glycans and the chronological developments associated with automated syntheses of oligosaccharides.

Evolution of a Strategy for the Enantioselective Total Synthesis of (+)-Psiguadial B

(+)-Psiguadial B is a diformyl phloroglucinol meroterpenoid that exhibits antiproliferative activity against the HepG2 human hepatoma cancer cell line. This full account details the evolution of a strategy that culminated in the first enantioselective total synthesis of (+)-psiguadial B. A key feature of the synthesis is the construction of the trans-cyclobutane motif by a Wolff rearrangement with in situ catalytic, asymmetric trapping of the ketene. An investigation of the substrate scope of this method to prepare enantioenriched 8-aminoquinolinamides is disclosed. Three routes toward (+)-psiguadial B were evaluated that featured the following key steps: (1) an ortho-quinone methide hetero-Diels-Alder cycloaddition to prepare the chroman framework, (2) a Prins cyclization to form the bridging bicyclo[4.3.1]decane system, and (3) a modified Norrish-Yang cyclization to generate the chroman. Ultimately, the successful strategy employed a ring-closing metathesis to form the seven-membered ring and an intramolecular O-arylation reaction to complete the polycyclic framework of the natural product.

A bio-based pro-antimicrobial polymer network via degradable acetal linkages

The synthesis of a fully degradable, bio-based, sustained release, pro-antimicrobial polymer network comprised of degradable acetals (PANDA) is reported. The active antimicrobial agent - p-anisaldehyde (pA) (an extract from star anise) - was converted into a UV curable acetal containing pro-antimicrobial monomer and subsequently photopolymerized into a homogenous thiol-ene network. Under neutral to acidic conditions (pH < 8), the PANDAs undergo surface erosion and exhibit sustained release of pA over 38 days. The release of pA from PANDAs was shown to be effective against both bacterial and fungal pathogens. From a combination of confocal microscopy and transmission electron microscopy, we observed that the released pA disrupts the cell membrane. Additionally, we demonstrated that PANDAs have minimal cytotoxicity towards both epithelial cells and macrophages. Although a model platform, these results point to promising pathways for the design of fully degradable sustained-release antimicrobial systems with potential applications in agriculture, pharmaceuticals, cosmetics, household/personal care, and food industries.

STATEMENT OF SIGNIFICANCE

With the increasing number of patients prescribed immunosuppressants coupled with the rise in antibiotic resistance - life-threatening microbial infections are a looming global threat. With limited success within the antibiotic pipeline, nature-based essential oils (EOs) are being investigated for their multimodal effectiveness against microbes. Despite the promising potential of EOs, difficulties in their encapsulation, limited water solubility, and high volatility limit their use. Various studies have shown that covalent attachment of these EO derivatives to polymers can mitigate these limitations. The current study presents the synthesis of a fully-degradable, sustained release, cytocompatible, pro-antimicrobial acetal network derived from p-anisaldehyde. This polymer network design provides a pathway toward application-specific EO releasing materials with quantitative encapsulation efficiencies, sustained release, and broad-spectrum antimicrobial activity.

Scalable Synthesis of a Key Intermediate for the Production of Pleuromutilin-Based Antibiotics

An improved synthesis of an eneimide, which is a useful precursor to pleuromutilin-based antibiotics, is reported. This synthesis proceeds in six steps and 17% overall yield (27% based on recovery of a key hydrindenone intermediate) and requires two fewer chromatography steps and five fewer days of reaction time than the previously reported route. The use of expensive, acutely toxic, and precious metal reagents or catalysts has been minimized.

Total Synthesis of TAN1251C via Diastereoselective Construction of the Azaspiro Skeleton

An efficient total synthesis of TAN1251C was accomplished by employing a Ugi four-component accumulation reaction and a Dieckmann condensation to construct the spiro-fused cyclohexanone and γ-lactam ring. Diastereoselective reduction by side-chain-controlled hydrogenation of enamide 15 or Zn reduction of oxime 23 enabled construction of the amino group with the desired stereochemistry.

A modular and enantioselective synthesis of the pleuromutilin antibiotics

The tricyclic diterpene fungal metabolite (+)-pleuromutilin has served as a starting point for antibiotic development. Semisynthetic modification of its glycolic acid subunit at C14 provided the first analogs fit for human use, and derivatization at C12 led to 12-epi-pleuromutilins with extended-spectrum antibacterial activity, including activity against Gram-negative pathogens. Given the inherent limitations of semisynthesis, however, accessing derivatives of (+)-pleuromutilin with full control over their structure presents an opportunity to develop derivatives with improved antibacterial activities. Here we disclose a modular synthesis of pleuromutilins by the convergent union of an enimide with a bifunctional iodoether. We illustrate our approach through synthesis of (+)-12-epi-mutilin, (+)-11,12-di-epi-mutilin, (+)-12-epi-pleuromutilin, (+)-11,12-di-epi-pleuromutilin, and (+)-pleuromutilin itself in 17 to 20 steps.

Carbon-Nitrogen Bond Formation via the Vanadium Oxo Catalyzed Sigmatropic Functionalization of Allenols

A vanadium catalyzed 1,3-rearrangement of allenols to form transient vanadium enolates that selectively couple with electrophilic nitrogen sources is reported even in the presence of competing simple protonation and Alder-ene pathways. Hydrazine products can be cyclized in a 6-endo-trig fashion which, upon reductive cleavage of the N-N bond, yield 1,4-diamines. Additionally, cleavage of the N-N bond before cyclization can be achieved to form β-hydroxy amines, a common structural motif of biologically active compounds.

Alcohol, Aldehyde, and Ketone Liberation and Intracellular Cargo Release through Peroxide-Mediated α-Boryl Ether Fragmentation

α-Boryl ethers, carbonates, and acetals, readily prepared from the corresponding alcohols that are accessed through ketone diboration, react rapidly with hydrogen peroxide to release alcohols, aldehydes, and ketones through the collapse of hemiacetal intermediates. Experiments with α-boryl acetals containing a latent fluorophore clearly demonstrate that cargo can be released inside cells in the presence of exogenous or endogenous hydrogen peroxide. These experiments show that this protocol can be used for drug activation in an oxidative environment without generating toxic byproducts.