Tuning Photoenolization-Driven Cycloadditions Using Theory and Spectroscopy

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.

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.

Photocyclization of Fluorinated Acetophenones Unlocks an Efficient Way to Solar Energy Storage

The ability to store and release energy efficiently is crucial for advancing sustainable energy technologies, and light-driven molecular isomerization presents a promising solution. However, a persistent challenge in this field is achieving both high stability of the energy-storing photoisomer and establishing efficient catalysis for back-isomerization, a critical process for releasing the stored energy as heat. In this work, we introduce a conceptually new molecular system designed for long-term energy storage, which is based on the reversible isomerization of ortho-methylacetophenone ⇄ benzocyclobutenol. Key to the success of this system is the strategic placement of a trifluoromethyl group, which enhances the overall performance by preventing unwanted side reactions during photochemical cyclization and by increasing the stability of the benzocyclobutenol moiety. Back isomerization is established using simple organic bases as catalysts, taking advantage of significant rate differences between normal and anionic electrocyclic ring-openings. This approach allows for controlled and predictable heat release under ambient conditions, positioning this molecular pair as a promising candidate for practical energy storage solutions.

Ultrafast Processes in Upper Excited Singlet States of Free and Caged 7-Diethylaminothiocoumarin

The photochemistry and photophysics of thiocarbonyl compounds, analogues of carbonyl compounds with sulfur, have long been overshadowed by their counterparts. However, recent interest in visible light reactions has reignited attention toward these compounds due to their unique excited-state properties. This study delves into the ultrafast dynamics of 7-diethylaminothiocoumarin (TC1), a close analogue of the well-known probe molecule coumarin 1 (C1), to estimate intersystem crossing rates, understand the mechanisms of fluorescence and phosphorescence, and evaluate TC1's potential as a solvation dynamics probe. Enclosing TC1 within an organic capsule indicates its potential applications, even in aqueous environments. Ultrafast studies reveal a dominant subpicosecond intersystem crossing process, indicating the importance of upper excited singlet and triplet states in the molecule's photochemistry. The distinct fluorescence and phosphorescence origins, along with the presence of closely spaced singlet excited states, support the observed efficient intersystem crossing. The sulfur atom alters the excited-state behavior, shedding light on reactive triplet states and paving the way for further investigations.

A (TD-)DFT study on photo-NHC catalysis: photoenolization/Diels-Alder reaction of acid fluorides catalyzed by N-heterocyclic carbenes

A comprehensive mechanistic study on the N-heterocyclic carbene (NHC) catalyzed photoenolization/Diels-Alder (PEDA) reaction of acid fluorides was performed in the framework of (time-dependent) density functional theory ((TD)-DFT). The 1,5-hydrogen atom transfer (1,5-HAT) during photoenolization of an ortho-toluoyl azolium salt was found to be feasible via, first, singlet excitation and photoenolization, and then, after crossing to the triplet manifold, populating a biradical dienol which allows for the formation of two ortho-quinodimethane (o-QDM) isomers due to a low rotational barrier. The (Z)-isomer is mostly unproductive through sigmatropic rearrangement back to the starting material while the (E)-isomer reacts in a subsequent concerted Diels-Alder reaction likely as the deprotonated dienolate. The experimentally observed diastereoselectivity is correctly predicted by theory and is determined by a more favorable endo trajectory in the cycloaddition step. These findings demonstrate that ortho-toluoyl azolium species exhibit similar photophysical properties as structurally related benzophenones, highlighting the unique ability of the NHC organocatalyst to transiently alter the excited state properties of an otherwise photoinactive carboxylic acid derivative, thereby expanding the scope of classical carbonyl photochemistry.

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.

Mechanism of the Aza-Piancatelli Reaction: Scope and Limitations of Furan Substitution in Donor-Acceptor Stenhouse Adduct Synthesis

The aza-Piancatelli reaction has been widely used to synthesize donor-acceptor Stenhouse adducts (DASAs), a new class of molecular photoswitches with unique properties. However, the substitution pattern of furan cores has been limited to position 3, as 3,4-disubstituted furans remain unreactive. Herein, we explore the aza-Piancatelli reaction mechanism using density functional theory (DFT) calculations to understand the influence of the different substituents on the reactivity. We found that all the reaction pathways are kinetically accessible, but the driving force of the reaction is lost in disubstituted furans due to the loss of conjugation in the DASA products. Finally, a simple model is proposed to guide the design of synthetic routes using this reaction.

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.

Total Synthesis of Streptovertidione and Bioinspired Transformation to Streptovertidine A and Formicapyridine A

We report herein a concise total synthesis of streptovertidione, and its transformation to streptovertidine A and formicapyridine A through a bioinspired pyridination. This stratage features: 1) a one-pot Ti(O-iPr)4-mediated photoenolization/Diels-Alder...