Total synthesis of natural products using photocycloaddition reactions of arenes

Catalytic Asymmetric Dearomative [2 + 2] Photocycloaddition/Ring-Expansion Sequence of Indoles with Diversified Alkenes

Developing novel strategies for catalytic asymmetric dearomatization (CADA) reactions is highly valuable. Visible light-mediated photocatalysis is demonstrated to be a powerful tool to activate aromatic compounds for further synthetic transformations. Herein, a catalytic asymmetric dearomative [2 + 2] photocycloaddition/ring-expansion sequence of indoles with simple alkenes was reported, providing a facile access to enantioenriched cyclopenta[b]indoles with good to high yields and enantioselectivities by means of chiral lanthanide photocatalysis. This protocol exhibited a broad substrate scope and good functional group tolerance, as well as potential applications in the synthesis of bioactive molecules. Mechanistic studies, including control experiments, UV-vis absorption spectroscopy, emission spectroscopy, and DFT calculations, were carried out, shedding insights into the reaction mechanism and the origin of enantioselectivity.

Dearomative Construction of 2D/3D Frameworks from Quinolines via Nucleophilic Addition/Borate-Mediated Photocycloaddition

Dearomative construction of multiply-fused 2D/3D frameworks, composed of aromatic two-dimensional (2D) rings and saturated three-dimensional (3D) rings, from readily available quinolines has greatly contributed to drug discovery. However, dearomative cycloadditions of quinolines in the presence of photocatalysts usually afford 5,6,7,8-tetrahydroquinoline (THQ)-based polycycles, and dearomative access to 1,2,3,4-THQ-based structures remains limited. Herein, we present a chemo-, regio-, diastereo-, and enantioselective dearomative transformation of quinolines into 1,2,3,4-THQ-based 6-6-4-membered rings without any catalyst, through a combination of nucleophilic addition and borate-mediated [2+2] photocycloaddition. Detailed mechanistic studies revealed that the photoexcited borate complex, generated from quinoline, organolithium, and HB(pin), accelerates the cycloaddition and suppresses the rearomatization that usually occurs in conventional photocycloaddition. Based on our mechanistic analysis, we also developed further photoinduced cycloadditions affording other types of 2D/3D frameworks from isoquinoline and phenanthrene.

Stereodivergent, Kinetically Controlled Isomerization of Terminal Alkenes via Nickel Catalysis

Because internal alkenes are more challenging synthetic targets than terminal alkenes, metal-catalyzed olefin mono-transposition (i.e., positional isomerization) approaches have emerged to afford valuable E- or Z- internal alkenes from their complementary terminal alkene feedstocks. However, the applicability of these methods has been hampered by lack of generality, commercial availability of precatalysts, and scalability. Here, we report a nickel-catalyzed platform for the stereodivergent E/Z-selective synthesis of internal alkenes at room temperature. Commercial reagents enable this one-carbon transposition of terminal alkenes to valuable E- or Z-internal alkenes via a Ni-H-mediated insertion/elimination mechanism. Though the mechanistic regime is the same in both systems, the underlying pathways that lead to each of the active catalysts are distinct, with the Z-selective catalyst forming from comproportionation of an oxidative addition complex followed by oxidative addition with substrate and the E-selective catalyst forming from protonation of the metal by the trialkylphosphonium salt additive. In each case, ligand sterics and denticity control stereochemistry and prevent over-isomerization.

Construction of olefinic coordination polymer single crystal platforms: precise organic synthesis, in situ exploration of reaction mechanisms and beyond

Olefin [2+2] photocycloaddition reactions based on coordination-bond templates provide numerous advantages for the selective synthesis of cyclobutane compounds. This review outlines the recent advances in the design and construction of single crystal platforms of olefinic coordination polymers for precise organic synthesis, in situ exploration of reaction mechanisms, and possible developments as comprehensively as possible. Numerous examples are presented to illustrate how the arrangements of the olefin pairs influence the solid-state photoreactivity and examine the types of cyclobutane products. Furthermore, the photocycloaddition reaction mechanisms are investigated by combining advanced techniques such as single crystal X-ray diffraction, powder X-ray diffraction, nuclear magnetic resonance, infrared spectroscopy, fluorescence spectroscopy, laser scanning confocal microscopy and theoretical calculations. Finally, potential applications resulting from promising physicochemical properties before and after photoreactions are discussed, and existing challenges and possible solutions are also proposed.

Enantioselective Intramolecular ortho Photocycloaddition Reactions of 2-Acetonaphthones

2-Acetonaphthones, which bear an alkenyl group tethered to its C1 carbon atom via an oxygen atom, were found to undergo an enantioselective intramolecular ortho photocycloaddition reaction. A chiral oxazaborolidine Lewis acid leads to a bathochromic absorption shift of the substrate and enables an efficient enantioface differentiation. Visible light irradiation (λ=450 nm) triggers the reaction which is tolerant of various groups at almost any position except carbon atom C8 (16 examples, 53-99 % yield, 80-97 % ee). Consecutive reactions were explored including a sensitized rearrangement to tetrahydrobiphenylenes, which occurred with full retention of configuration. Evidence was collected that the catalytic photocycloaddition occurs via triplet intermediates, and the binding mode of the acetonaphthone to the chiral Lewis acid was elucidated by DFT calculations.

Anionic photochemical rearrangement of 3-hydroxypyran-4-ones bearing oxazol-2-one fragment

The photochemical behavior of in situ generated anions of 3-hydroxypyran-4-ones containing an oxazol-2-one moiety was studied. For the first time, it was demonstrated that blue LED light irradiation (450 nm) of substituted 3-hydroxypyran-4-ones in the presence of a base leads regiospecifically to the formation of isomeric 3-hydroxypyran-2-ones. Transformation of the starting 3-hydroxypyran-4-ones into the corresponding anions is necessary for the presented photoprocess. Based on the considered visible light induced rearrangement, a general method for the synthesis of 3-hydroxypyran-2-ones with an oxazol-2-one moiety was elaborated. The structure of one of the synthesized compounds was confirmed by X-ray diffraction.

Chemoselective Nozaki-Hiyama-Takai-Kishi and Grignard reaction: short synthesis of some carbahexopyranoses

A common, divergent, efficient, stereoselective and short approach for the total syntheses of some carbahexopyranoses namely, MK7607, (-)-gabosine A, (-)-conduritol E, (-)-conduritol F, 6a-carba-β-d-fructopyranose and other carbasugars using chemoselective Grignard or Nozaki-Hiyama-Takai-Kishi (NHTK) reactions and RCM. Herein, the Grignard and NHTK reactions are able to differentiate the reactivity difference between lactol or lactolacetate and aldehyde of 2 & 6 under given conditions to give the desired skeleton chemoselectivity.

Synthesis of substituted 8H-benzo[h]pyrano[2,3-f]quinazolin-8-ones via photochemical 6π-electrocyclization of pyrimidines containing an allomaltol fragment

For the first time, we elaborated a method for the synthesis of pyrimidines containing an allomaltol unit. The suggested approach is based on the reaction of 2-(1-(dimethylamino)-3-oxo-3-arylprop-1-en-2-yl)-3-hydroxy-6-methyl-4H-pyran-4-ones with cyanamide. The photochemical behavior of the obtained pyrimidines was investigated. It was shown that for the hydroxy derivatives the main pathway of phototransformation is a 6π-electrocyclization of the 1,3,5-hexatriene system and subsequent [1,9]-H sigmatropic shift leading to dihydrobenzo[h]pyrano[2,3-f]quinazolines. At the same time, for methylated analogues the photoreaction proceeds in two directions resulting in the formation of a mixture of the corresponding dihydrobenzo[h]pyrano[2,3-f]quinazolines and polyaromatic products. The obtained dihydro derivatives are stable compounds and do not undergo aromatization upon further UV irradiation. The structures of two of the dihydrobenzo[h]pyrano[2,3-f]quinazolines were confirmed by X-ray diffraction analysis. Based on the performed studies, a two-stage telescopic method for the synthesis of polyaromatic benzo[h]pyrano[2,3-f]quinazolines including the initial photocyclization of the starting pyrimidines and the final dehydration was proposed.

4a,7a-Dihydroxy-1-(2-hydroxyethyl)-5-methyl-2′,3′,4a,5′,6′,7a-hexahydrospiro[cyclopenta[b]pyridine-4,4′-pyran]-2,7(1H,3H)-dione

An environment-friendly photochemical approach to the synthesis of 4a,7a-dihydroxy-1-(2-hydroxyethyl)-5-methyl-2′,3′,4a,5′,6′,7a-hexahydrospiro[cyclopenta[b]pyridine-4,4′-pyran]-2,7(1H,3H)-dione from 2-(4-(3-hydroxy-6-methyl-4-oxo-4H-pyran-2-yl)tetrahydro-2H-pyran-4-yl)-N-(2-hydroxyethyl)acetamide was elaborated. The suggested method is based on the ESIPT-promoted contraction of 3-hydroxypyran-4-one fragment followed by intramolecular cyclization of generated in situ α-hydroxy-1,2-diketone intermediate. The distinctive feature of the presented protocol is the employment of water as a solvent for the considered photoreaction. The structure of the obtained photoproduct was confirmed by 1H, 13C-NMR, IR spectroscopy and high-resolution mass spectrometry.

Arene Activation through Iminium Ions: Product Diversity from Intramolecular Photocycloaddition Reactions

While 2-alk-ω-enyloxy-sustituted benzaldehydes do not display any photochemical reactivity at the arene core, the respective iminium perchlorates were found to undergo efficient reactions either upon direct irradiation (λ=366 nm) or under sensitizing conditions (λ=420 nm, 2.5 mol% thioxanthen-9-one). Three pathways were found: (a) Most commonly, the reaction led to benzoxacyclic products in which the olefin in the tether underwent a formal, yet unprecedented carboformylation (13 examples, 44-99 % yield). The cascade process occurred with high diastereoselectivity and was found to be stereoconvergent. (b) If a substituent resides in the 3-position of the benzene ring, a meta photocycloaddition was observed which produced tetracyclic skeletons with five stereogenic centers in excellent regio- and diastereoselectivity (2 examples, 58-79 % yield). (c) If the tether was internally substituted at the alkene, an arene photocycloaddition was avoided and an azetidine was formed in an aza Paternò-Büchi reaction (2 examples, 95-98 % yield).

A study of the photochemical behavior of terarylenes containing allomaltol and pyrazole fragments

The photochemical properties behavior of 3-hydroxy-4-pyranone containing terarylenes with a pyrazole bridge fragment were studied. It was shown that UV-induced 6π-electrocyclization of the 1,3,5-hexatriene system was not observed for the considered objects molecules. At the same time, the phototransformation of such systems proceeds exclusively in the direction of the contraction of the pyranone ring leading to unstable α-hydroxydiketones. For the first time the possibility of isolation of the resulting α-hydroxydiketones in pure form was demonstrated. Wherein, it was shown that relatively low stable α-hydroxydiketones can be trapped by reaction with 1,2-phenylenediamine. The general method for the preparation of the corresponding quinoxalines on the basis of the aforementioned condensation was implemented. It was demonstrated that the studied photoreaction does not depend on the type of pyrazole bridge. The structures of three of synthesized products were established by X-ray diffraction.

Facile access to fused 2D/3D rings via intermolecular cascade dearomative [2 + 2] cycloaddition/rearrangement reactions of quinolines with alkenes

Hybrid fused two-dimensional/three-dimensional (2D/3D) rings are important pharmacophores in drugs owing to their unique structural and physicochemical properties. Preparation of these strained ring systems often requires elaborate synthetic effort and exhibits low efficiency, thus representing a limiting factor in drug discovery. Here, we report two types of energy-transfer-mediated cascade dearomative [2 + 2] cycloaddition/rearrangement reactions of quinoline derivatives with alkenes, which provide a straightforward avenue to 2D/3D pyridine-fused 6−5−4−3- and 6−4−6-membered ring systems. Notably, this energy-transfer-mediated strategy features excellent diastereoselectivity that bypasses the general reactivity and selectivity issues of photochemical [2 + 2] cycloaddition of various other aromatics. Tuning the aza-arene substitutions enabled selective diversion of the iridium photocatalysed energy transfer manifold towards either cyclopropanation or cyclobutane-rearrangement products. Density functional theory calculations revealed a cascade energy transfer scenario to be operative.

C-H deuteration of organic compounds and potential drug candidates

C-H deuteration has been intricately developed to satisfy the urgent need for site-selectively deuterated organic frameworks. Deuteration has been primarily used to study kinetic isotope effects of reactions but recently its significance in pharmaceutical chemistry has been discovered. Deuterium labelled compounds have stolen the limelight since the inception of the first FDA-approved deuterated drug, for the treatment of chorea-associated Huntington's disease, and their pharmacological importance was realised by chemists, although surprisingly very late. Various approaches were developed to carry out site-selective deuteration. However, the most common and efficient method is hydrogen isotope exchange (HIE). This review summarises deuteration methods of various organic motifs containing C(sp²)-H and C(sp³)-H bonds utilizing C-H bond functionalisation as a key step along with a variety of catalysts, and exemplifies their biological relevance.

Technological Innovations in Photochemistry for Organic Synthesis: Flow Chemistry, High-Throughput Experimentation, Scale-up, and Photoelectrochemistry

Photoinduced chemical transformations have received in recent years a tremendous amount of attention, providing a plethora of opportunities to synthetic organic chemists. However, performing a photochemical transformation can be quite a challenge because of various issues related to the delivery of photons. These challenges have barred the widespread adoption of photochemical steps in the chemical industry. However, in the past decade, several technological innovations have led to more reproducible, selective, and scalable photoinduced reactions. Herein, we provide a comprehensive overview of these exciting technological advances, including flow chemistry, high-throughput experimentation, reactor design and scale-up, and the combination of photo- and electro-chemistry.

Visible light enabled [4+2] annulation reactions for anthracenone-furans from 2,3-dibromonaphthoquinone and phenylbenzofurans

A facile visible light promoted approach to anthracenone-furans from readily available 2,3-dibromonaphthoquinones and phenylbenzofurans via a formal Diels Alder reaction is reported. This reaction involves wavelength-selective agitation of 4CzIPN, energy transfer to quinones, recombination of 1,6-biradicals and elimination to give anthracenone-furans in good to excellent yields in one pot.

A facile visible light promoted [4 + 2] annulation reaction from readily available starting materials using an organo-photocatalyst gave anthracenone-furans with up to 95% yield in one-pot.

Photoinduced 6π-Electrocyclization of a 1,3,5-Hexatriene System Containing an Allomaltol Fragment

For the first time, the possibility of photocyclization of the 1,3,5-hexatriene system containing a fragment of allomaltol was demonstrated. A preparative method for the synthesis of previously unknown benzo[5,6]chromeno[8,7-d]oxazole-2,7(3H)-diones was developed based on the investigated photoreaction. A distinctive feature of this approach is the modification of the starting terarylenes aimed at blocking the competitive process leading to side reactions of the pyranone fragment. It was shown that the proposed photocyclization of substituted oxazol-2-ones can be used for the photogeneration of biologically active alcohols and various acids. The structure of one of the cyclization products was determined by X-ray diffraction.

Concise Total Synthesis of Agarozizanol B via a Strained Photocascade Intermediate

The prezizane-type sesquiterpene agarozizanol B was synthesized employing a photochemical cascade reaction as the key step. Starting from a readily available 1-indanone with a tethered olefin, a strained tetracyclic skeleton was assembled which contained all carbon atoms of the sesquiterpene with the correct relative configuration. The conversion into the tricyclic prezizane skeleton was accomplished by a strategic cyclopropane bond cleavage. Prior to the cyclopropane ring opening an adaption of the oxidation state was required, which could be combined with a reductive resolution step. After removal of two functional groups, the natural product was obtained both in racemic form or, if resolved, as the (+)-enantiomer which was shown to be identical to the natural product.

Construction of Spiro-γ-butyrolactone Core via Cascade Photochemical Reaction of 3-Hydroxypyran-4-one Derivatives

We elaborate a novel one-step photochemical method for the synthesis of spiro-γ-butyrolactone derivatives from 3-hydroxypyran-4-ones. The suggested approach is based on a cascade process including initial photoinduced contraction of 4-pyranone ring followed by intramolecular cyclization leading to the final spiro system. A distinctive feature of the proposed method is intramolecular trapping of unstable α-hydroxydiketone intermediate formed in situ as a result of a photochemical reaction. The structures of two synthesized 1-oxaspiro[4.4]non-8-ene-2,6,7-triones were determined by X-ray diffraction.

Synthesis of the Hexahydropyrrolo-[3,2-c]-quinoline Core Structure and Strategies for Further Elaboration to Martinelline, Martinellic Acid, Incargranine B, and Seneciobipyrrolidine

Natural products are rich sources of interesting scaffolds possessing a plethora of biological activity. With the isolation of the martinella alkaloids in 1995, namely martinelline and martinellic acid, the pyrrolo[3,2-c]quinoline scaffold was discovered. Since then, this scaffold has been found in two additional natural products, viz. incargranine B and seneciobipyrrolidine. These natural products have attracted attention from synthetic chemists both due to the interesting scaffold they contain, but also due to the biological activity they possess. This review highlights the synthetic efforts made for the preparation of these alkaloids and formation of analogues with interesting biological activity.