Zwitterionic biphenyl quinone methides in photodehydration reactions of 3-hydroxybiphenyl derivatives: laser flash photolysis and antiproliferation study

Different Reaction Mechanisms Triggered by the Meta Effect: Photoinduced Generation of Quinone Methides from Hydroxybiphenyl Derivatives

A series of sterically congested quinone methides (QMs) exhibit photoinduced antiproliferative activity against some human cancer cell lines. To elucidate the structure-reactivity relationship and details of mechanisms of the photogeneration of sterically congested QMs, we chose phenylphenol derivatives 1-3 as QM precursors and investigated their photodehydration processes in aqueous solutions using ultrafast spectroscopy and theoretical computations. We found that meta derivatives 1 and 2 undergo water-mediated excited-state proton transfer (ESPT) from the phenol OH, followed by expulsion of the OH- to form QMs. By comparison, para derivative 3 proceeds via water-mediated ESPT from H2O to benzyl alcohol coupled with dehydration as the first step, delivering a cation intermediate, which further deprotonates to yield QM. Such results would help chemists understand more about the meta effects in photochemistry and about ESPT and would help synthetic chemists design sterically congested QM precursors with extraordinary reactivities and expand applications of QMs in biological and medical systems.

Excited-state antiaromaticity relief drives facile photoprotonation of carbons in aminobiphenyls

A combined computational and experimental study reveals that ortho-, meta- and para-aminobiphenyl isomers undergo distinctly different photochemical reactions involving proton transfer. Deuterium exchange experiments show that the ortho-isomer undergoes a facile photoprotonation at a carbon atom via excited-state intramolecular proton transfer (ESIPT). The meta-isomer undergoes water-assisted excited-state proton transfer (ESPT) and a photoredox reaction via proton-coupled electron transfer (PCET). The para-isomer undergoes a water-assisted ESPT reaction. All three reactions take place in the singlet excited-state, except for the photoredox process of the meta-isomer, which involves a triplet excited-state. Computations illustrate the important role of excited-state antiaromaticity relief in these photoreactions.

Photogeneration of quinone methide from adamantylphenol in an ultrafast non-adiabatic dehydration reaction

The ultrafast photochemical reaction of quinone methide (QM) formation from adamantylphenol was monitored in real time using femtosecond transient absorption spectroscopy and fluorescence upconversion in solution at room temperature. Experiments were complemented by theoretical studies simulating the reaction pathway and elucidating its mechanism. Excitation with sub-20 fs UV pulses and broadband probing revealed ultrafast formation of the long-lived QM intermediate directly in the ground state, occurring with a time constant of around 100 fs. UV-vis transient absorption data covering temporal dynamics from femtoseconds to hundreds of milliseconds revealed persistence of the absorption band assigned to QM and partially overlapped with other contributions tentatively assigned to triplet excited states of the adamantyl derivative and the phenoxyl radical that are clearly distinguished by their evolution on different time scales. Our data, together with the computations, provide evidence of a non-adiabatic photodehydration reaction, which leads to the formation of QM in the ground state via a conical intersection, circumventing the generation of a transient QM excited state.

Synthesis of Highly Substituted Biaryls by the Construction of a Benzene Ring via In Situ Formed Acetals

Herein, we present an interesting method for the construction of a benzene ring using propargylic alcohols and 1,3-dicarbonyls, which involves three new C-C bond formations via cascade alkylation, formylation, annulation, and aromatization to make substituted biaryls. This one-pot Brønsted acid-promoted protocol utilizes the unique reactivity of the acetal formed under the reaction conditions. Alkynyl methyl ketones could be employed instead of 1,3-dicarbonyls as they are converted to 1,3-dicarbonyls by hydration under the reaction conditions.

Photochemical Reactivity of Naphthol-Naphthalimide Conjugates and Their Biological Activity

Quinone methide precursors 1a–e, with different alkyl linkers between the naphthol and the naphthalimide chromophore, were synthesized. Their photophysical properties and photochemical reactivity were investigated and connected with biological activity. Upon excitation of the naphthol, Förster resonance energy transfer (FRET) to the naphthalimide takes place and the quantum yields of fluorescence are low (Φ_F ≈ 10⁻²). Due to FRET, photodehydration of naphthols to QMs takes place inefficiently (Φ_R ≈ 10⁻⁵). However, the formation of QMs can also be initiated upon excitation of naphthalimide, the lower energy chromophore, in a process that involves photoinduced electron transfer (PET) from the naphthol to the naphthalimide. Fluorescence titrations revealed that 1a and 1e form complexes with ct-DNA with moderate association constants K_a ≈ 10⁵–10⁶ M⁻¹, as well as with bovine serum albumin (BSA) K_a ≈ 10⁵ M⁻¹ (1:1 complex). The irradiation of the complex 1e@BSA resulted in the alkylation of the protein, probably via QM. The antiproliferative activity of 1a–e against two human cancer cell lines (H460 and MCF 7) was investigated with the cells kept in the dark or irradiated at 350 nm, whereupon cytotoxicity increased, particularly for 1e (>100 times). Although the enhancement of this activity upon UV irradiation has no imminent therapeutic application, the results presented have importance in the rational design of new generations of anticancer phototherapeutics that absorb visible light.

Competing photochemical reactions of bis-naphthols and their photoinduced antiproliferative activity

The photophysical properties and photochemical reactivities of a series of bis-naphthols 4a-4e and bis-anthrols 5a and 5e were investigated by preparative irradiation in CH3OH, fluorescence spectroscopy and laser flash photolysis (LFP). Methanolysis taking place via photodehydration (bis-naphthols: ΦR = 0.04-0.05) is in competition with symmetry breaking charge separation (SB-CS). The SB-CS gave rise to radical ions that were detected for 4a and 4e by LFP. Photodehydration gave quinone methides (QMs) that were also detected by LFP (λmax = 350 nm, τ ≈ 1-2 ms). In the aqueous solvent, excited state proton transfer (ESPT) competes with the abovementioned processes, giving rise to naphtholates, but the process is inefficient and can only be observed in the buffered aqueous solution at pH > 7. Since the dehydration of bis-naphthols delivers QMs, their potential antiproliferative activity was investigated by an MTT test on three human cancer cell lines (NCI-H1299, lung carcinoma; MCF-7, breast adenocarcinoma; and SUM159, pleomorphic breast carcinoma). Cells were treated with 4 or 5 with or without irradiation (350 nm). An enhancement of the activity (up to 10-fold) was observed upon irradiation, which may be associated with QM formation. However, these QMs do not cross-link DNA. The activity is most likely associated with the alkylation of proteins present in the cell cytoplasm, as evidenced by photoinduced alkylation of bovine and human serum albumins by 4a.

Ultrafast Adiabatic Photodehydration of 2-Hydroxymethylphenol and the Formation of Quinone Methide

The photochemical reactivity of 2-hydroxymethylphenol (1) was investigated experimentally by photochemistry under cryogenic conditions, by detecting reactive intermediates by IR spectroscopy, and by using nanosecond and femtosecond transient absorption spectroscopic methods in solution at room temperature. In addition, theoretical studies were performed to facilitate the interpretation of the experimental results and also to simulate the reaction pathway to obtain a better understanding of the reaction mechanism. The main finding of this work is that photodehydration of 1 takes place in an ultrafast adiabatic photochemical reaction without any clear intermediate, delivering quinone methide (QM) in the excited state. Upon photoexcitation to a higher vibrational level of the singlet excited state, 1 undergoes vibrational relaxation leading to two photochemical pathways, one by which synchronous elimination of H₂ O gives QM 2 in its S₁ state and the other by which homolytic cleavage of the phenolic O-H bond produces a phenoxyl radical (S₀ ). Both are ultrafast processes that occur within a picosecond. The excited state of QM 2 (S₁ ) probably deactivates to S₀ through a conical intersection to give QM 2 (S₀ ), which subsequently delivers benzoxete 4. Elucidation of the reaction mechanisms for the photodehydration of phenols by which QMs are formed is important to tune the reactivity of QMs with DNA and proteins for the potential application of QMs in medicine as therapeutic agents.

Photochemical formation of quinone methides from peptides containing modified tyrosine

We have demonstrated that quinone methide (QM) precursors can be introduced in the peptide structure and used as photoswitchable units for peptide modifications. QM precursor 1 was prepared from protected tyrosine in the Mannich reaction, and further used as a building block in peptide synthesis. Moreover, peptides containing tyrosine can be transformed into a photoactivable QM precursor by the Mannich reaction which can afford monosubstituted derivatives 2 or bis-substituted derivatives 3. Photochemical reactivity of modified tyrosine 1 and dipeptides 2 and 3 was studied by preparative irradiation in CH₃OH where photodeamination and photomethanolysis occur. QM precursors incorporated in peptides undergo photomethanolysis with quantum efficiency Φ_R = 0.1-0.2, wherein the peptide backbone does not affect their photochemical reactivity. QMs formed from dipeptides were detected by laser flash photolysis (λ_max ≈ 400 nm, τ = 100 μs-20 ms) and their reactivity with nucleophiles was studied. Consequently, QM precursors derived from tyrosine can be a part of the peptide backbone which can be transformed into QMs upon electronic excitation, leading to the reactions of peptides with different reagents. This proof of principle showing the ability to photochemically trigger peptide modifications and interactions with other molecules can have numerous applications in organic synthesis, materials science, biology and medicine.

Photochemical Formation of Anthracene Quinone Methide Derivatives

Anthrols 2-7 were synthesized and their photochemical reactivity investigated by irradiations in aq CH₃OH. Upon excitation with visible light (λ > 400 nm) in methanolic solutions, they undergo photodehydration or photodeamination and deliver methyl ethers, most probably via quinone methides (QMs), with methanolysis quantum efficiencies Φ_R = 0.02-0.3. Photophysical properties of 2-7 were determined by steady-state fluorescence and time-correlated single photon counting. Generally, anthrols 2-7 are highly fluorescent in aprotic solvents (Φ_F = 0.5-0.9), whereas in aqueous solutions the fluorescence is quenched due to excited-state proton transfer (ESPT) to solvent. The exception is amine 4 that undergoes excited-state intramolecular proton transfer (ESIPT) in neat CH₃CN where photodeamination is probably coupled to ESIPT. Photodehydration may take place via ESIPT (or ESPT) that is coupled to dehydration or via a hitherto undisclosed pathway that involves photoionization and deprotonation of radical cation, followed by homolytic cleavage of the alcohol OH group from the phenoxyl radical. QMs were detected by laser flash photolysis and their reactivity with nucleophiles investigated. Biological investigation of 2-5 on human cancer cell lines showed enhancement of antiproliferative effect upon exposure of cells to irradiation by visible light, probably due to formation of electrophilic species such as QMs.

Photoinduced electron transfer interaction of anthraquinones with aniline quenchers: Influence of methyl substitution in aniline donors

Photoinduced electron transfer between triplet state of 9,10-anthraquinone (AQ) and its two derivatives: 2-chloro-9,10-anthraquinone (CAQ) and sodium anthraquinone-2-sulfonate (AQS) and ground state aniline (AN) and its dimethyl substitutions: 2,3-dimethylaniline (2,3-DMA), 2,6-dimethylaniline (2,6-DMA), 3,5-dimethylaniline (3,5-DMA) and N,N-dimethylaniline (N,N-DMA) is studied using nanosecond laser flash photolysis at room temperature. Detection of radical bands of quinone anions and aniline cations along with their formation and/or decay kinetics are used to confirm the electron transfer (ET) process. In MeCN medium, AN quenches the triplet state of CAQ (CAQ^T) but not the triplets AQ^T or AQS^T. However in aqueous medium, AN quenches AQS^T and forms radical ion pair. All the DMAs can react through ET with all the triplet quinones at different degrees of efficiency in MeCN medium. Noticeably, the ring substituted DMAs are less efficient in electron donation to AQ^T or AQS^T while the N,N-DMA shows high efficiency in donating electron to all triplet quinones in MeCN medium. Charge distribution of donor molecules, in MeCN medium is calculated using density functional theory (DFT), and shows an enhancement of electron density of the ring of N,N-DMA, making it an ideal electron donor for ET studies compared to other DMAs. This systematic selection and usage of anilines with electrochemically tunable quinones can be viewed as a working model of donor-acceptor system that can be utilized in photoinduced ET applications.

Enhancement of antiproliferative activity by phototautomerization of anthrylphenols

An antiproliferative investigation was conducted on 3 human cancer cell lines, HCT 116 (colon), MCF-7 (breast), and H 460 (lung), on a series of 4 anthrylphenols in the dark and upon exposure to light (350 nm). 9-(2-Hydroxyphenyl)anthracene (1) moderately inhibited proliferation, but irradiation considerably enhanced the effect. The other anthracenes 4–6 exhibited antiproliferative activity in the dark, which was not enhanced upon irradiation. The enhancement of the antiproliferative effect on the irradiation of 1 was rationalized as being due to the formation of quinone methide (QM 2) by excited state proton transfer. QM 2 acts as an electrophilic species capable of reacting with biological molecules. Although QM 2 reacts with nucleotides, the adducts could not be isolated. On the contrary, cysteine adduct 8 was isolated and characterized, whereas the adducts with glycine, serine and tripeptide glutathione were characterized by MS. Non-covalent binding of 1 to DNA and bovine serum albumin was demonstrated by UV-vis, fluorescence and CD spectroscopy. However, a straightforward conclusion regarding the DNA or protein alkylating (damaging) ability of 2 could not be drawn. The results obtained by the irradiation of 1 in the presence of DNA, amino acids and peptides, cell cycle perturbation analysis, and in vitro translation of GFP suggest that the effect is not only due to the damage of DNA but also due to the impact on the cellular proteins. Considering that to date all QM agents were assumed to target DNA dominantly, this is an important finding with an impact on the further development of anticancer agents based on QMs.

Reactivity of Cations and Zwitterions Formed in Photochemical and Acid-Catalyzed Reactions from m-Hydroxycycloalkyl-Substituted Phenol Derivatives

Three m-substituted phenol derivatives, each with a labile benzylic alcohol group and bearing either protoadamantyl 4, homoadamantyl 5, or a cyclohexyl group 6, were synthesized and their thermal acid-catalyzed and photochemical solvolytic reactivity studied, using preparative irradiations, fluorescence measurements, nanosecond laser flash photolysis, and quantum chemical calculations. The choice of m-hydroxy-substitution was driven by the potential for these phenolic systems to generate m-quinone methides on photolysis, which could ultimately drive the excited-state pathway, as opposed to forming simple benzylic carbocations in the corresponding thermal route. Indeed, thermal acid-catalyzed reactions gave the corresponding cations, which undergo rearrangement and elimination from 4, only elimination from 5, and substitution and elimination from 6. On the other hand, upon photoexcitation of 4-6 to S1 in a polar protic solvent, proton dissociation from the phenol, coupled with elimination of the benzylic OH (as hydroxide ion) gave zwitterions (formal m-quinone methides). The zwitterions exhibit reactivity different from the corresponding cations due to a difference in charge distribution, as shown by DFT calculations. Thus, protoadamantyl zwitterion has a less nonclassical character than the corresponding cation, so it does not undergo 1,2-shift of the carbon atom, as observed in the acid-catalyzed reaction.

Photodeamination Reaction Mechanism in Aminomethyl p-Cresol Derivatives: Different Reactivity of Amines and Ammonium Salts

Derivatives of p-cresol 1-4 were synthesized, and their photochemical reactivity, acid-base, and photophysical properties were investigated. The photoreactivity of amines 1 and 3 is different from that for the corresponding ammonium salts 2 and 4. All compounds have low fluorescence quantum yields because the excited states undergo deamination reactions, and for all cresols the formation of quinone methides (QMs) was observed by laser flash photolysis. The reactivity observed is a consequence of the higher acidity of the S1 states of these p-cresols and the ability for excited-state intramolecular proton transfer (ESIPT) to occur in the case of 1 and 3, but not for salts 2 and 4. In aqueous solvent, deamination depends largely on the prototropic form of the molecule. The most efficient deamination takes place when monoamine is in the zwitterionic form (pH 9-11) or diamine is in the monocationic form (pH 7-9). QM1, QM3, and QM4 react with nucleophiles, and QM1 exhibits a shorter lifetime when formed from 1 (τ in CH3CN = 5 ms) than from 2 (τ in CH3CN = 200 ms) due to the reaction with eliminated dimethylamine, which acts as a nucleophile in the case of QM1. Bifunctional QM4 undergoes two types of reactions with nucleophiles, giving adducts or new QM species. The mechanistic diversity uncovered is of significance to biological systems, such as for the use of bifunctional QMs to achieve DNA cross-linking.

Near-visible light generation of a quinone methide from 3-hydroxymethyl-2-anthrol

Excitation of 2-hydroxy-3-(diphenylhydroxymethyl)anthracene (7) to S1 initiates photodehydration, giving the corresponding quinone methide (QM) that was detected by laser flash photolysis (LFP) in 2,2,2-trifluoroethanol (λ = 580 nm, τ = 690 ± 10 ns). The QM decays by protonation, giving a cation (λ = 520 nm, τ = 84 ± 3 μs), which subsequently reacts with nucleophiles. The rate constants in the reactions with nucleophiles were determined by LFP, whereas the adducts were isolated via preparative photolyses. The photogeneration of QMs in the anthrol series is important for potential use in biological systems since the chromophore absorbs at wavelengths > 400 nm. Antiproliferative investigations conducted with 2-anthrol derivative 7 on three human cancer cell lines showed higher activity for irradiated cells.

Sterically congested adamantylnaphthalene quinone methides

Five new (2-adamantyl)naphthol derivatives (5-9, quinone methide precursors, QMP) were synthesized and their photochemical reactivity was investigated by preparative photolyses, fluorescence spectroscopy, and laser flash photolysis (LFP). Excitation of QMP 5 to S(1) leads to efficient excited state intramolecular proton transfer (ESIPT) coupled with dehydration, giving quinone methide QM5 which was characterized by LFP (in CH(3)CN-H(2)O, λ(max) = 370 nm, τ = 0.19 ms). On irradiation of QMP 5 in CH(3)OH-H(2)O (4:1), the quantum yield of methanolysis is Φ = 0.70. Excitation of naphthols QMP 6-8 to S(1) in CH(3)CN leads to photoionization and formation of naphthoxyl radicals. In a protic solvent, QMP 6-8 undergo solvent-assisted PT giving QM6 or zwitterion QM8 that react with nucleophiles delivering adducts, but with a significantly lower quantum efficiency. QMP 9 in a protic solvent undergoes two competitive processes, photosolvolysis via QM9 and solvent-assisted PT to carbon atom of the naphthalene giving zwitterion. QM9 has been characterized by LFP (in CH(3)CN-H(2)O, λ(max) > 600 nm, τ = 0.9 ms). In addition to photogenerated QMs, two stable naphthalene QMs, QM10 and QM11 were synthesized thermally and characterized by X-ray crystallography. QM10 and QM11 do not react with H(2)O but undergo acid-catalyzed fragmentation or rearrangement. Antiproliferative activity of 5-9 was investigated on three human cancer cell lines. Exposure of MCF-7 cells treated with 5 to 300 nm irradiation leads to an enhanced antiproliferative effect, in accordance with the activity being due to the formation of QM5.