Ti(Oi-Pr)4-promoted photoenolization Diels-Alder reaction to construct polycyclic rings and its synthetic applications

Total Synthesis of Lugdunomycin via Sequential Photoinduced Spiroketalization and Isobenzofuran Diels-Alder Reactions

The presence of sterically rigid contiguous quaternary stereocenters in natural products imposes conformational constraints with significant effects on their biological activities. However, achieving the direct synthesis of multiple contiguous quaternary stereocenters in a single step remains a formidable challenge. Here, we present the total synthesis of the antibacterial metabolite lugdunomycin (1) in thirteen steps via a sequence of photochemical transformations. A photoenolization, keto-enol tautomerization, and spiroketalization sequence was developed to generate the spiroketal 4 from actinaphthoran B (3). Subsequently, a photoinduced isobenzofuran Diels-Alder reaction between elmonin (4) and iso-maleimycin (5) was developed to construct the polycyclic benzaza[4,3,3]propellane framework bearing three contiguous quaternary stereocenters in the compact C-ring along with a distal hydroxyl group at C19. The mechanism of these photochemical reactions was investigated using synthetic and computational approaches.

Total Synthesis of Polycyclic Natural Products via Photoenolization/Diels-Alder Reaction

ConspectusPolycyclic ring systems represent the most common structural features of drug molecules and natural products. Chemical synthesis of complex polycyclic molecules with multiple stereogenic centers, especially quaternary carbon stereocenters, has been a significant challenge in the field of total synthesis. Due to the low reactivities of the substrates and congested chemical environments, the efficient establishment of polycyclic rings and enantioselective construction of quaternary carbon stereocenters are still ongoing challenges. In our laboratory, we are devoted to developing new methodologies and strategies for the total syntheses of bioactive polycyclic natural products and the exploration of their biological potentials. The photoenolization/Diels-Alder (PEDA) reaction has been recognized as a powerful strategy to increase synthetic efficiency and address the aforementioned issues. Over the past several years, our group systematically reinvestigated this reaction in terms of its reactivity and stereoselectivity and developed a unique dinuclear metal-promoted reaction process for constructing fused or spiro polycyclic rings bearing quaternary carbon stereocenters. During the course of this investigation, we have come to realize how to rationally design the synthetic route based on the PEDA reaction and successfully implement the synthetic projects.In this Account, we summarize our endeavors and journeys in the development and application of the PEDA reaction to the total synthesis of topologically complex natural products in order to draw attention to its broad utility and encourage further uptake. In the first part, we provide the details on the investigation of the PEDA reaction to address the issues of reactivity, diastereoselectivity, and enantioselectivity. An enantioselective PEDA reaction involving Ti(Oi-Pr)4 and TADDOL-type ligands was developed. This reaction enables the sterically bulky dienophiles to interact with the transient photoenolized hydroxy-o-quinodimethanes, delivering a wide range of polycyclic rings with single or vicinal quaternary carbon stereocenters in good yields with excellent enantioselectivities. In the second part, we showcase the synthetic potential of PEDA reaction in total synthesis of natural products. The fused tricyclic ring systems, bearing gem-dimethyl groups or quaternary carbon stereocenters located at the ring junction, were efficiently constructed by Ti(Oi-Pr)4-promoted PEDA reactions, which enabled the syntheses of three different types of natural products, including aromatic polyketides (anthrabenzoxocinones, fasamycins/naphthacemycins, and benastatins), meroterpenoid (oncocalyxone B), and halenaquinones (xestoquinone, adociaquinones A and B). To access structurally more complex triterpenoids, namely, perovskones and hydrangenones, the asymmetric PEDA reaction was developed to build a tricyclic ring along with three contiguous quaternary carbon stereocenters. The asymmetric PEDA reaction was also applied to achieve the total synthesis of aryltetralin lactone lignans. Furthermore, an intramolecular PEDA reaction provides a new pathway for the rapid construction of highly congested hydrophenanthrene with a quaternary carbon stereocenter, facilitating the total synthesis of five hasubanan alkaloids. We anticipate that the development of the PEDA reaction will inspire future innovations and progressions in asymmetric photo reactions, and its synthetic potential will be expanded by further applications in the total synthesis of complex natural and drug molecules.

Visible Light-Mediated [4+2] Annulation of Silylimines with Olefins to 1-Aminotetralins Enabled by Diradical Hydrogen Atom Transfer of C-H Bonds

A facile photochemical, one-pot synthesis of highly functionalized 1-aminotetralins derivatives (>70 examples) from readily accessible o-alkyl and o-formyl aryl silylimines with olefins is described. A diradical-mediated hydrogen atom transfer (DHAT) of primary, secondary, and tertiary C(sp3)-H bonds of o-alkyl arylsilylimines and C(sp2)-H bonds of o-formyl arylsilylimines enabled a [4+2] annulation with olefins in excellent diastereoselectivity. This was accomplished upon irradiation at λ = 420 nm in the presence of thioxanthen-9-one (10 mol %) as the sensitizer via energy transfer. Moreover, sulfur-substituted o-alkyl silylimines can undergo such photochemical process in the absence of an external photosensitizer. This effective protocol is compatible with a variety of functional groups and can be applied to the modification of bioactive molecules. Based on mechanistic evidences and computational studies, it is suggested that the silyl substituent enables an efficient energy transfer leading to the formation of a key C,N-diradical and subsequent [4+2]-cyclization was supported by a better molecular orbital matching between the HSOMO of the 1,4-diradical intermediate and the LUMO of the olefins. Thus, upon irradiation, the excited silylimine unlocks a carbon-to-nitrogen DHAT and subsequent [4+2] cyclization that allows the divergent functionalization of benzylic C(sp3)-H bonds and C(sp2)-H bonds.

Temperature-Dependent Photoreactivity of 2-Azidomethylbenzophenone: Insights into the Triplet Imine Biradical Pathway

Photoenols, formed through photoinduced intramolecular H atom abstraction in o-alkyl-substituted arylketones, typically have limited utility as reactive intermediates owing to fast reversion to the starting material. Herein, we introduced an azido group on the o-alkyl substituent to render the photoreaction irreversible. Irradiation of 2-azidomethylbenzophenone (1) in methanol yielded 2-(hydroxy(phenyl)methyl)benzonitrile (2). Laser flash photolysis of 1 revealed the formation of biradical 3Br1 followed by intersystem crossing to photoenols Z-3 (τ ∼ 3.3 μs) and E-3 (τ > 45 μs), both of which reverted to 1. Alternatively, 3Br1 could lose N2 to form 3Br2 (not detected), which decays to 2. In cryogenic argon matrices, irradiation of 1 yielded nitrene 31N and 2 but no photoenols, likely because Z-3 regenerated 1. Both ESR spectroscopy and absorption analysis in methyltetrahydrofuran (80 K) confirmed 31N formation. Upon prolonged irradiation, the absorbance of 31N decreased, whereas that of 3 remained unchanged and that of 2 increased. Thus, TK of 1 is proposed to form 3Br1 via H atom abstraction, with subsequent intersystem crossing to 3 competing with the loss of N2 to generate 3Br2. DFT calculations revealed a small energy gap (∼2 kcal/mol) between the triplet and singlet configurations of Br2, supporting a mechanism in which 3Br2 intersystem crosses to yield 2.

Synthesis and discovery of simplified pleurotin analogs bearing tricyclic core as novel thioredoxin reductase inhibitors

Pleurotin (1) is a benzoquinone meroterpenoid known for its wide-spectrum antitumor and antibiotic activities, notably acting as natural inhibitors of the thioredoxin reductase (TrxR). Pleurotin (1) has been chemically synthesized, but only in milligram quantities through at least 13 longest linear steps with 0.8 % overall yield due to its complex structure such as fused hexacyclic core with 8 contiguous stereocenters. Therefore, structural simplification strategy is applied to pleurotin natural products for their structure-activity relationship (SAR) study and further therapeutics development. Herein, we judiciously designed pleurotin analogs of tricyclic A/D/E ring core, retaining the putative pharmacophore of para-quinone moiety D and its supportive A and E rings. Thus 16 simplified analogs of pleurotin bearing tricyclic A/D/E core were readily synthesized in only 2 to 6 steps with up to 50 % overall yield from commercially available materials. Significantly, the best analog 14f with benzonitrile substituent exhibited more potent TrxR inhibitory activity with an IC50 of 3.5 μM than the positive control micheliolide (IC50 = 6.23 μM). Furthermore, the mechanism study revealed that compound 14f could induce apoptosis of tumor cells by inducing ROS generation and inhibiting TrxR activities. Our study for the first time showed that the tricyclic A/D/E ring scaffold from the natural product pleurotin (1) with proper substitution can maintain or even improve the TrxR inhibitory and antiproliferative activities, with high synthetic accessibility, affording natural product-derived lead compounds for the further development of TrxR inhibitors as anti-tumor therapeutics.

Development of a Divergent Synthesis of Pleurotinoid Natural Products

Herein, we describe the evolution of our syntheses of the pleurotinoid natural products pleurotin (1), pleurogrisein (3), and 4-hydroxypleurogrisein (4). An approach based on a proximity-induced intramolecular Diels-Alder cycloaddition of a transient ortho-quinone dimethide (e.g., 6, Scheme 1) was inferior to an alternative construction featuring Gao's titanium(IV)-mediated photoenolization Diels-Alder coupling of ortho-tolualdehyde 20 with functionalized hydrindenone 22. While this pairing exhibited the desired stereoface selectivity and produced cis-fused hydrindanone 23, the successful realization of our syntheses of 1, 3, and 4 required a post-Diels-Alder epimerization of the unactivated stereocenter at C-5 in compound 23. Ultimately, it was possible to generate a reactive oxygen-centered radical via a reductive homolytic cleavage of the N-O bond in 23 and capitalize on its ability to break the C5-H bond in an intramolecular 1,5-hydrogen atom transfer (HAT). The carbon radical arising from this pivotal 1,5-HAT was subsequently trapped in situ by an exogenous thiol in a kinetically controlled HAT reaction to establish the natural configuration at C-5. The successful flipping of the cis-hydrindane in 23 to the challenging trans configuration in 24 provided a firm foundation for a formal synthesis of pleurotin (1), as well as syntheses of pleurogrisein (3) and 4-hydroxypleurogrisein (4).

Substituent-Determined Intramolecular Hydrogen Transfer for Photopromoted Intermolecular Cycloaddition of Anthraquinones with Aryl Olefins

The formation of intramolecular hydrogen bonds in anthraquinones makes them inert to photoinduced reactions; therefore, it is a great challenge to phototransform these compounds. Herein, we reported a formal visible-light-induced [4 + 2] cycloaddition of both 1-hydroxyanthraquinones and 1-aminoanthraquinones with olefins under external photocatalyst-free conditions with high regioselectivity. More than 60 substrates are disclosed, demonstrating the reliability of this protocol to construct diverse functionalized anthraquinone derivatives.

Asymmetric Total Synthesis of Hasubanan Alkaloids: Periglaucines A-C, N,O-Dimethyloxostephine and Oxostephabenine

We report herein the asymmetric total synthesis of periglaucines A-C, N,O-dimethyloxostephine and oxostephabenine. The key strategies used include: 1) a Rh^I -catalyzed regio- and diastereoselective Hayashi-Miyaura reaction to connect two necessary fragments; 2) an intramolecular photoenolization/Diels-Alder (PEDA) reaction to construct the highly functionalized tricyclic core skeleton bearing a quaternary center; 3) a bio-inspired intramolecular Michael addition and transannular acetalization to generate the aza[4.4.3]propellane and the tetrahydrofuran ring.

NickelII-catalyzed asymmetric photoenolization/Mannich reaction of (2-alkylphenyl) ketones

A diastereo- and enantioselective photoenolization/Mannich (PEM) reaction of ortho-alkyl aromatic ketones with benzosulfonimides was established by utilizing a chiral N,N'-dioxide/Ni(OTf)2 complex as the Lewis acid catalyst. It afforded a series of benzosulfonamides and the corresponding ring-closure products, and a reversal of diastereoselectivity was observed through epimerization of the benzosulfonamide products under continuous irradiation. On the basis of the control experiments, the role of the additive LiNTf2 in achieving high stereoselectivity was elucidated. This PEM reaction was proposed to undergo a direct nucleophilic addition mechanism rather than a hetero-Diels-Alder/ring-opening sequence. A possible transition state model with a photoenolization process was proposed to explain the origin of the high level of stereoinduction.

A Concise Synthesis of Pleurotin Enabled by a Nontraditional C-H Epimerization

An 8-step synthesis of a known pentacyclic intermediate toward the natural product pleurotin (1) is described. Pleurotin and related benzoquinone natural products are of great interest for their powerful anticancer and antibiotic activities. The route features a regio- and diastereoselective intermolecular photoenolization/Diels-Alder cycloaddition and an alkoxy-radical-induced hydrogen atom transfer-mediated C-H epimerization to construct pleurotin's carbon framework with appropriate relative stereochemical relationships. The synthesis concludes with a ring-forming benzylic C-H oxidation to deliver oxepane 19.

Dearomatization of Benzenoid Arenes Triggered by Triplet Excited State Intramolecular Proton Transfer

The detailed mechanism of photoinduced dearomatization of benzenoid arenes is investigated using both the high-level ab initio method and density functional theory. The results suggest that the optically allowed singlet excited state (S₂) can quickly decay to the lowest triplet excited state (T₁) through a barrierless internal conversion and intersystem crossing. Importantly, we find a triplet excited state intramolecular proton transfer (T-ESIPT) pathway to produce a diradical triplet intermediate (³M_O-H), which can trigger the subsequent [4 + 2] dearomatization reaction. Furthermore, the diastereoselectivity of the reaction was illustrated by the rotation of the O_-H group of ³M_O-H, which could be effectively modulated by the solvent effect (arising from the strength of the intermolecular hydrogen bond) and the substituted effect (arising from the strength of the electron-donation group). This photochemical mechanism can explain well the experimental observations, and the novel T-ESIPT process can open a new door in studying the photoinduced proton transfer reactions.

Regioisomerism in Symmetric Dimethyl Dialdehydes Dictates their Photochemical Reactivity

We herein report the first light-driven selective monoderivatization (desymmetrization) of two chemically equivalent carbonyl groups in a single chromophore. By comparing of four symmetric regioisomers, featuring two equivalent ortho-methylbenzaldehyde units, we identify dimethyltherephtalaldehydes (DMTAs) which can be activated in a dual wavelength-selective fashion. Under visible light and UV-light irradiation, DMTAs undergo two consecutive Diels-Alder reactions exhibiting near-quantitative endo-selectivity (>99%) and provide excellent yields (96-98%). The influence of the regioisomerism of the dialdehydes on their photochemical behavior is profound, evidenced by an in-depth investigation of their photochemical performance. We exemplify the capability of the photosystems via the synthesis of complex Diels-Alder adducts with various dienophiles, including alkynes.

Enantioselective Total Synthesis of (+)-Pepluanol A

Herein, we report an enantioselective and convergent total synthesis of (+)-pepluanol A, a structurally intriguing Euphorbia diterpenoid natural product featuring a 5/6/7/3-fused tetracyclic skeleton, from known building blocks in 11 steps. The successful strategy relies on a phenyl selenide-mediated Morita-Baylis-Hillman type reaction as a connective step, forging the precursor for the key intramolecular Diels-Alder reaction to construct the congested 5/6/7-tricyclic framework. A diastereoconvergent cascade starting with an acid-induced removal of the C1-MOM protecting group followed by a retro-aldol/aldol reaction resulted in the formation of a single diastereomer. This stereoconvergency allowed for the successful substrate-controlled diastereoselective cyclopropanation of an advanced intermediate to establish the full carboskeleton of (+)-pepluanol A (1).

Tuning Photoenolization-Driven Cycloadditions Using Theory and Spectroscopy

The first broad spectrum investigation into the photoenolization/Diels-Alder (PEDA) sequence was carried out using M06-2X/6-31+G(d,p) in conjunction with SMD solvation and supported by experimental UV-vis spectroscopy. A test set of 20 prodienes was chosen to examine the role of the H atom acceptor group (substituted and unsubstituted carbonyl, thiocarbonyl, and imine), the H atom donor group, and bystander ring substituents. As reaction partners for the photogenerated dienes, a diverse test set of 20 dienophiles was examined, comprising electron rich, electron poor, neutral, strain activated, hydrocarbon, and heteroatom-containing molecules including CO₂ and CO. A key finding of this work is the demonstration that the PEDA sequence of carbonyl based prodienes is tolerant of most substitution patterns. Another is that thiocarbonyl derivatives should behave analogously to the carbonyls but are likely to do so much more slowly, due to an inefficient intersystem crossing, an endothermic 1,5-hydrogen atom transfer (HAT) step, and a [1,5] sigmatropic H shift to regenerate the starting material that outcompetes the [4 + 2]cycloaddition. In contrast, the T₁ state of the ortho-alkyl imines displays the incorrect orbital symmetry for 1,5-HAT and is correspondingly accompanied by higher barriers, even in the excited state. However, provided these barriers can be overcome, the remaining steps in the PEDA sequence are predicted to be facile. The Diels-Alder reaction is predicted to be of much broader scope than reported synthetic literature: while electron poor dienophiles are expected to be the most reactive partners, ethylene and electron rich alkenes should react at a synthetically useful rate. CO is predicted to undergo a facile (4 + 1)cheletropic addition instead of the normal [4 + 2]cycloaddition pathway. This unique photoenolization/cheletropic addition (PECA) sequence could provide metal-free access to benzannelated cyclopentanones.

UV light-driven asymmetric vinylogous aldol reaction of isatins with 2-alkylbenzophenones and enantioselective synthesis of 3-hydroxyoxindoles

A UV light-driven enantioselective synthesis of 3-hydroxyoxindoles is reported for the first time.

Assembling the carbon skeleton of A-74528

The 2′-phosphodiesterase inhibitor A-74528, which combines an intriguing biosynthesis with unusual biological activity, is one of the most complex type II polyketides. As a synthetic target, it represents a significant challenge due to its size but also due to a unique carbon skeleton that features a hexacarbocyclic core with an appended pyrone. Here we report our efforts toward the synthesis of A-74528, which culminated in the construction of the full carbon skeleton and the correct installation of all but one stereocenter. Our strategy employs a molybdenum-catalyzed branched allylation to establish the central quaternary carbon and relies on establishing the remaining stereocenters in a substrate-controlled manner. Carbocycles were established using a spiro epoxide annulation, a 1,3-dipolar cycloaddition, followed by an aldol condensation, and a gold-catalyzed hydroarylation. The pyrone was appended to an aldehyde branching off the quaternary stereocenter by a one-carbon homologation and Mukaiyama aldol addition.

We report our efforts toward the synthesis of A-74528, which culminated in the construction of the full carbon skeleton.

Exo-Selective and Enantioselective Photoenolization/Diels-Alder Reaction

An unusual exo-selective photoenolization/Diels-Alder reaction of electron-rich 2-methylbenzaldehydes and dienophiles containing a benzoyl group at its α position was reported herein. The chiral TADDOL-type ligand plays a key role in this process: (1) accelerating the reaction; (2) controlling the enantioselectivity; and (3) improving and tuning the diastereoselectivity of the reaction.

Asymmetric Total Synthesis of Aglacins A, B, and E

An asymmetric photoenolization/Diels-Alder (PEDA) reaction between electron-rich 2-methylbenzaldehydes and unsaturated γ-lactones was developed to directly construct the basic tricyclic core of aryltetralin lactone lignans. This methodology enabled the first asymmetric total synthesis of aglacins A, B, and E and revision of the absolute configuration of these natural lignans. The strategy was also used to prepare the naturally occurring aryldihydronaphthalene-type lignans (-)-7,8-dihydroisojusticidin B and (+)-linoxepin in four and six steps, as well as 27 natural-product-like molecules containing a C8' quaternary center. We believe that the synthetic aglacins and small-molecule library provide new opportunities to carry out the SAR studies of the podophyllotoxin family of natural products.

Asymmetric Total Synthesis and Biosynthetic Implications of Perovskones, Hydrangenone, and Hydrangenone B

Perovskones and hydrangenones are a family of structurally complex triterpenoids that were mainly isolated from the genus Salvia medicinal plants. These isoprenoids exhibit a broad range of biological activities, such as antitumor and antiplasmodial activities. Here, we report the collective total synthesis of perovskone, perovskones C, D, F, hydrangenone, and hydrangenone B. The key strategies in this work include the following: (1) an asymmetric photoenolization/Diels-Alder reaction was developed to construct a tricyclic ring bearing three contiguous quaternary centers, which was used to build the core icetexane skeleton; (2) a bioinspired Diels-Alder reaction of perovskatone D with trans-α-ocimene was applied to stereospecifically generate perovskones; (3) late-stage oxidations and ring forming steps were developed to synthesize perovskones and hydrangenones. Our synthetic work suggests that (1) perovskatone D may serve as the precursor of the biosynthesis of perovskones and (2) the formation of hydrangenone and hydrangenone B, containing a five-membered D ring, may involve an oxidative ring cleavage and ring regeneration process.

Construction of polycyclic structures with vicinal all-carbon quaternary stereocenters via an enantioselective photoenolization/Diels-Alder reaction

All-carbon quaternary stereocenters are ubiquitous in natural products and significant in drug molecules. However, construction of all-carbon stereocenters is a challenging project due to their congested chemical environment. And, when vicinal all-carbon quaternary stereocenters are present in one molecule, they will dramatically increase its synthetic challenge. A chiral titanium promoted enantioselective photoenolization/Diels-Alder (PEDA) reaction allows largely stereohindered tetra-substituted dienophiles to interact with highly active photoenolized hydroxy-o-quinodimethanes, delivering fused or spiro polycyclic rings bearing vicinal all-carbon quaternary centers in excellent enantiomeric excess through one-step operation. This newly developed enantioselective PEDA reaction will inspire other advances in asymmetric excited-state reactions, and could be used in the total synthesis of structurally related complex natural products or drug-like molecules for drug discovery.

Asymmetric total synthesis of (+)-xestoquinone and (+)-adociaquinones A and B

The asymmetric total synthesis of (+)-xestoquinone and (+)-adociaquinones A and B was achieved in 6-7 steps using an easily accessible meso-cyclohexadienone derivative. The [6,6]-bicyclic decalin B-C ring and the all-carbon quaternary stereocenter at C-6 were prepared via a desymmetric intramolecular Michael reaction with up to 97% ee. The naphthalene diol D-E ring was constructed through a sequence of Ti(Oi-Pr)4-promoted photoenolization/Diels-Alder, dehydration, and aromatization reactions. This asymmetric strategy provides a scalable route to prepare target molecules and their derivatives for further biological studies.

Synthesis of polycyclic naphthols and naphthalenes via tandem Ti(Oi-Pr)4-promoted photoenolization Diels–Alder reaction and aromatization

An efficient approach to naphthol and naphthalene scaffolds has been developed using a sequence involving tandem Ti(Oi-Pr)₄-promoted photoenolization Diels–Alder (PEDA) and aromatization reactions.

Light-enabled, AlCl3-catalyzed regioselective intramolecular nucleophilic addition of non-nucleophilic alkyls to alkynes

Light-enabled, AlCl3-catalyzed regioselective intramolecular nucleophilic cyclization of alkynes using non-nucleophilic alkyls as the nucleophile is reported. Upon photoexcitation, o-alkylphenyl alkynyl ketones can be transferred into (E)-photoenols. Thus, a nucleophilic methylene is formed from the non-nucleophilic alkyl. An AlCl3 catalyst can stabilize the (E)-photoenol intermediate and facilitate further intramolecular nucleophilic cyclization. DFT calculations indicated that the AlCl3-catalyzed cyclization is the regioselectivity determining step.

Activation of allylic esters in an intramolecular vinylogous kinetic resolution reaction with synergistic magnesium catalysts

Kinetic resolution (KR) of racemic starting materials is a powerful and practical alternative to prepare valuable enantiomerically enriched compounds. A magnesium-catalyzed kinetic resolution based on a designed intramolecular vinylogous Michael reaction is disclosed. Here we show a synergistic catalytic strategy based on the development of chiral ligands. Substrates containing linear allylic ester structures are designed and synthesized to construct key [6.6.5]-tricyclic chiral skeletons via this kinetic resolution process. Detailed mechanistic studies reveal a rational mechanism for the current intramolecular vinylogous KR reaction. The desired direct intramolecular asymmetric vinylogous Michael reaction of linear allylic esters is realized in high efficiency and enantioselectivity with the synergistic catalytic system.

Kinetic resolution allows to obtain enantioenriched compounds from racemic mixtures. Here, the authors report a synergistic magnesium catalyst promoting kinetic resolution of an intramolecular vinylogous Michael reaction to access [6.6.5]-tricyclic chiral skeletons.

One-Pot Synthesis of Spirocyclopenta[a]indene Derivatives via a Cascade Ring Expansion and Intramolecular Friedel-Crafts-Type Cyclization

A one-pot efficient synthetic approach for the rapid construction of spirocyclopenta[a]indene derivatives has been developed via an iodine-initiated cascade ring expansion and intramolecular Friedel-Crafts-type cyclization from propargyl alcohol-tethered alkylidenecyclobutanes under mild conditions with broad substrate scope. This cascade process can be elegantly conducted on a gram scale. A plausible reaction mechanism has been proposed on the basis of a series of deuterium labeling and control experiments.

Metal-free construction of contiguous quaternary stereocentres with a polycyclic framework

A series of contiguous all-carbon quaternary stereocentres with an arene-annulated polycyclic framework were constructed efficiently by a metal-free and atom economic acid-catalyzed method. The reactions could be performed by acid-catalyzed cationic cyclization and rearrangement under mild conditions. Moreover, the resulting polycyclic products showed highly twisted architectures with two perpendicular planes.

Integrated continuous flow/batch protocol for the photoreduction of ortho-methyl phenyl ketones using water as the hydrogen source

The direct hydrogenation of ketones (RRCO) with water to secondary alcohols under catalyst-free, minimal risk conditions, through the light-driven transfer hydrogenation platform.

Recent advances in the total synthesis of gracilamine

Gracilamine belongs to the Amaryllidaceae alkaloid group and has a unique five-ring skeleton. Its scarcity in plants limits biological studies on it. A number of synthetic chemists have carried out the total synthesis of gracilamine in the past few years. This review will summarize the synthetic studies of gracilamine, and hopes to bring new inspiration to the research of Amaryllidaceae alkaloids.

Recent advances in the application of Diels–Alder reactions involving o-quinodimethanes, aza-o-quinone methides and o-quinone methides in natural product total synthesis

This review summarizes recent advances in Diels–Alder reactions involving o-QDMs, o-QMs and aza-o-QMs. The power and potential of this strategy in organic synthesis and natural product total synthesis is highlighted.

Synthetic studies on daphniglaucins

The synthetic approach to the core framework of daphniglaucin-type Daphniphyllum alkaloids is described herein.