Previous Article in Journal
Simultaneous Photocatalytic CO2 Reduction and Methylene Blue Degradation over TiO2@(Pt, Au, or Pd)
 
 
Search for Articles:
Title / Keyword
Author / Affiliation / Email
Journal
Article Type
 
 
Advanced Search
 
Section
Special Issue
Volume
Issue
Number
Page
 
Journals
Photochem
Volume 5
Issue 4
10.3390/photochem5040031
photochem-logo
Submit to this Journal Review for this Journal Propose a Special Issue
► Article Menu

Article Menu

share Share announcement Help format_quote Cite question_answer Discuss in SciProfiles
first_page
settings
Order Article Reprints
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Article

[2+2]-Photocycloadditions of 2-Acetoxy-1,4-naphthoquinone and Structure Determination of the Main Photoadducts

by
Madyan A. Yaseen
1,2,
Zhifang Guo
1,
Peter C. Junk
1 and
Michael Oelgemöller
1,3,*
1
College of Science and Engineering, James Cook University, Townsville, QLD 4811, Australia
2
College of Education, University of Samarra, Samarra 34010, Iraq
3
Faculty of Chemistry and Biology, Hochschule Fresenius gGmbH—University of Applied Sciences, 65510 Idstein, Germany
*
Author to whom correspondence should be addressed.
Photochem 2025, 5(4), 31; https://doi.org/10.3390/photochem5040031
Submission received: 1 September 2025 / Revised: 30 September 2025 / Accepted: 1 October 2025 / Published: 10 October 2025
Browse Figures
Versions Notes

Abstract

The [2+2]-photocycloaddition of 2-acetoxy-1,4-naphthoquinone with 1,1-diphenylethylene, styrene and cyclopentene was conducted in a conventional batch reactor. Prolonged irradiation selectively produced the corresponding anti and head-to-head cyclobutanes in acceptable to good yields. The batch process was subsequently transferred to continuous-flow operation in a simple capillary device. Likewise, the photocycloaddition with diphenylacetylene gave the corresponding cyclobutene and a benzoanthracenone derivative in acceptable yields. The crystal structures of all main photoproducts were successfully determined.

1. Introduction

1,4-naphthoquinone derivatives play an important role as biologically active natural products in medicinal chemistry and building blocks in organic synthesis [1,2,3,4,5]. The spectroscopic properties and photochemistry of 1,4-naphthoquinones have been likewise widely explored [6,7,8,9]. The [2+2]-photocycloaddition has been studied in particular [10,11], and selected photoadducts have shown promising biological activities [12,13]. In general, 2-substituted 1,4-naphthoquinones can form a variety of regio- (head-to-head vs. head-to-tail) and diastereoisomeric (syn vs. anti) cyclobutanes (Scheme 1).
Traditionally, photochemical transformations are performed under batch conditions using medium-pressure mercury lamps and hazardous solvents [14,15]. The inhomogeneous light distribution and limited mass-transfer within these devices often necessitate exhaustive irradiations that favor photodecomposition and reduce product yields and purities [16]. To overcome these limitations, photochemical transformations are now routinely conducted under continuous-flow conditions [17,18,19,20,21,22]. Thus far, flow-photochemical transformations of quinones are rare and have predominantly focused on the parent, unsubstituted 1,4-naphthoquinone [23,24,25,26]. As part of a PhD study [27], this work investigates [2+2]-photocycloadditions of 2-acetoxy-1,4-naphthoquinone (1) as a model compound [28,29,30].

2. Materials and Methods

2.1. General Information

All starting materials, reagents and solvents were purchased from commercial suppliers (Merck Life Science Pty Ltd., Bayswater, VIC, Australia, or Thermo Fisher Scientific Australia Pty Ltd., Scoresby, VIC, Australia) and were used as received.
Batch irradiations were performed in a Rayonet RPR-200 photochemical chamber reactor (Southern New England Ultraviolet Company, Branford, CT, USA) carrying 16 × 8 W UVB (306 ± 20 nm, G8T5E, Ushio Inc., Tokyo, Japan), UVA (352 ± 20 nm, F4T5BL, Ushio Inc., Tokyo, Japan) or visible light (419 ± 25 nm, RPR-4190A, Southern New England Ultraviolet Company, Brandford, USA) fluorescent tubes. Schlenk flasks equipped with a cold finger and made from Pyrex glass (λ > 300 nm) were used as reaction vessels (capacity: approx. 60 mL). Photoreactions were monitored by thin-layer chromatography (TLC). Continuous-flow operations were conducted using a previously described in-house reactor module equipped with a single 8 W UVA (352 ± 20 nm, F4T5BL, Ushio Inc., Tokyo, Japan) fluorescent tube [31]. Additional technical information and pictures of the reactors can be found in the Supplementary Materials.
Thin-layer chromatography was completed in glass jars on silica gel plates (polygram sil G UV254, Macherey-Nagel GmbH & Co KG, Düren, Germany) and using a 1:4 (vol%) mixture of ethyl acetate and n-hexane as the mobile phase. A CombiFlash® Rf+ LumenTM flash chromatography system (Teledyne Isco, Lincoln, NE, USA) was utilized for product purification. Normal phase cartridges were used as stationery, and a 1:4 (vol%) mixture of ethyl acetate and n-hexane as the mobile phase.
Melting points were measured in open capillaries using a Tathastu or Gallenkamp melting point apparatus and were uncorrected.
Nuclear magnetic resonance (NMR) spectroscopy was conducted on a Bruker 400 AscendTM (1H: 400 MHz and 13C: 100 MHz) equipped with an auto-sampler. Spectra were generated with the MestReNova software (Version 6.0.2-5475, Mestrelab Research S.L., Santiago de Compostela, Spain) using the residual solvent peak as reference. Samples were prepared in CDCl3 (δ = 7.26/77.3 ppm) [32]. The NMR spectra of all photocycloaddition products can be found in the Supplementary Materials.
Mass spectra were recorded using direct injection on a Shimadzu LCMS-2020 equipped with a DUIS ion source. Ions were subsequently detected in positive and/or negative mode within a mass range of m/z 100–500. The mobile phases were aqueous solutions of HPLC-grade methanol or acetonitrile with 0.1% formic acid added. All experimental event sequences were controlled, and processing was performed using LabSolutions for LCMS-2020 software.
Infrared spectra were recorded on a Perkin Elmer Spectrum One FT-IR Spectrometer as thin films. Spectra were recorded in the range of 600–4000 cm−1.
Structure determinations were conducted on the MX1 beamline at the Australian Synchrotron [33]. Crystallographic data (excluding structure factors) for the structures in this paper have been deposited with the Cambridge Crystallographic Data Centre (CCDC) as supplementary numbers CCDC2345860 (3a), CCDC2345861 (anti-3b), CCDC2345862 (anti-3c) and CCDC2345863 (6), respectively. The data files can be obtained free of charge from the CCDC.

2.2. General Procedure for Photocycloadditions Under Batch Conditions

A solution of 2-acetoxy-1,4-naphthoquinone (1 mmol) and alkene or alkyne (5 mmol) in the respected organic solvent (50 mL) was irradiated in a Schlenk flask for 10–20 h while being purged with a slow stream of nitrogen gas. The reaction mixture was evaporated to dryness, and the residue was subjected to automated column chromatography.

2.3. General Procedure for Photocycloadditions Under Continuous-Flow Conditions

The capillary of the continuous-flow reactor was filled with acetone, and the cooling fans and fluorescent tube were started. A solution of 2-acetoxy-1,4-naphthoquinone (0.5 mmol) and alkene or alkyne (2.5 mmol) in acetone (25 mL) was degassed with nitrogen for 5 min, drawn into a glass syringe and pumped through the in-house flow photoreactor. At the end of the delivery, the capillary was flushed with approx. 15 mL of acetone. The reaction mixture and solvent washings were collected in an amber round-bottom flask. The products were isolated as described above.

2.4. Spectroscopic Details

Compound 1 and photoadducts 3a, 3b and 6 are known, and their spectroscopic data matched previously described data [28,29,34,35].
2-Acetoxy-l,4-naphthoquinone (1) [34]. Yellowish prisms. M.p.: 124–126 °C (Lit. 131–132 °C). 1H-NMR (400 MHz, CDCl3): δ (ppm) = 2.39 (s, 3H, CH3), 6.76 (s, 1H, CHquin), 7.78 (m, 2H, CHarom), 8.12 (m, 2H, CHarom). 13C-NMR (100 MHz, CDCl3): δ (ppm) = 20.9, 126.3, 126.8, 127.3, 131.3, 132.2, 134.3, 134.7, 154.6, 167.7, 178.9, 184.8.
3,8-Dioxo-2,2-diphenyl-1,2,2a,3,8,8a-hexahydrocyclobuta[b]naphthalen-2a-yl acetate (3a) [29]. Colorless solid. M.p.: 192–193 °C (Lit. 191–192 °C). 1H-NMR (400 MHz, CDCl3): δ (ppm) = 2.08 (s, 3H, CH3), 3.46 (ddd, J = 12.3, 7.6 Hz, 2H, CH2), 3.57 (dd, J = 10.6, 4.6 Hz, 1H, CH), 6.88 (dd, J = 6.8 Hz, 1H, CHarom), 6.99 (dd, J = 7.7 Hz, 2H, CHarom), 7.22 (dd, J = 7.4, 1.3 Hz, 3H, CHarom), 7.34 (dd, J = 7.7 Hz, 2H, CHarom), 7.49 (dd, J = 8.4, 1.2 Hz, 2H, CHarom), 7.53 (dd, J = 7.6, 1.5 Hz, 1H, CHarom), 7.58 (ddd, J = 7.5, 1.5 Hz, 1H, CHarom), 7.79 (dd, J = 7.4, 1.3 Hz, 1H, CHarom), 7.94 (dd, J = 7.8, 1.7 Hz, 1H, CHarom). 13C-NMR (100 MHz, CDCl3): δ (ppm) = 20.9, 34.0, 49.1, 58.7, 85.5, 126.6, 126.8, 127.1, 127.3, 127.6, 128.1, 128.2, 128.3, 133.9, 134.0, 134.9, 135.3, 139.6, 143.8, 170.5, 192.5, 196.0. MS (DUIS): m/z = 397 [M+ + H], expected: 396 [M+]. IR (neat): ῦ (cm−1) = 1723, 1687, 1593, 1293, 1276, 1217, 1085, 784.
3,8-Dioxo-2-phenyl-1,2,2a,3,8,8a-hexahydrocyclobuta[b]naphthalen-2a-yl acetate (anti-3b) [29]. Colorless crystals. M.p.: 147–148 °C (Lit. 134–136 °C). 1H-NMR (400 MHz, CDCl3): δ (ppm) = 2.00 (s, 3H, CH3), 2.59 (ddd, J = 12.1, 9.3, 4.6 Hz, 1H, CH), 3.11 (ddd, J = 12.0, 11.1, 9.1 Hz, 1H, CH), 3.62 (ddd, J = 11.1, 4.6, 1.2 Hz, 1H, CH), 3.98 (dd, J = 9.1 Hz, 1H, CH), 7.27–7.33 (m, 3H, CHarom), 7.34–7.41 (m, 2H, CHarom), 7.84 (dd, J = 5.8, 3.3 Hz, 2H, CHarom), 8.21 (dd, J = 6.2, 3.3 Hz, 1H, CHarom), 8.28 (dd, J = 5.9, 2.9 Hz, 1H, CHarom). 13C-NMR (100 MHz, CDCl3): δ (ppm) = 20.6, 28.2, 45.6, 49.7, 81.2, 127.5, 127.8, 128.3, 128.5, 133.7, 134.7, 134.8, 135.0, 136.1, 170.5, 192.0, 195.9. MS (DUIS): m/z = 321 [M+ + H], expected: 320 [M+]. IR (neat): ῦ (cm−1) = 1725, 1687, 1593, 1268, 1214, 1122, 971.
4,9-Dioxo-2,3,3a,3b,4,9,9a,9b-octahydro-1H-cyclopenta [3,4]cyclobuta[1,2-b]naphthalen-3b-yl acetate (anti-3c). Colorless crystals. M.p.: 183–184 °C. 1H-NMR (400 MHz, CDCl3): δ (ppm) = 1.51–1.75 (m, 2H, CH2), 1.78–2.01 (m, 3H, CH), 2.11 (s, 3H, CH3), 2.38 (dd, J = 13.8, 6.1 Hz, 1H, CH), 2.70 (dd, J = 12.9, 6.3 Hz, 1H, CH), 2.90 (t, J = 8.0 Hz, 1H, CH), 3.03 (d, J = 6.9 Hz, 1H, CH), 7.73–7.83 (m, 2H, CHarom), 8.08–8.12 (m, 1H, CHarom), 8.14–8.19 (m, 1H, CHarom). 13C-NMR (100 MHz, CDCl3): δ (ppm) = 20.3, 25.5, 26.8, 32.9, 40.2, 43.8, 56.2, 75.7, 127.6, 127.9, 133.4, 133.9, 134.6, 134.8, 170.2, 194.2, 194.3. MS (DUIS): m/z = 285 [M+ + H], expected: 284 [M+]. IR (neat): ῦ (cm−1) = 1730, 1686, 1592, 1243, 1227, 1062, 997.
3,8-Dioxo-1,2-diphenyl-2a,3,8,8a-tetrahydrocyclobuta[b]naphthalen-2a-yl acetate (6) [35]. Yellowish crystals. M.p.: 211–212 °C (Lit. 209–211 °C). 1H-NMR (400 MHz, CDCl3): δ (ppm) = 2.22 (s, 3H, CH3), 4.34 (s, 1H, =CH), 7.37–7.30 (m, 6H, CHarom), 7.60–7.55 (m, 2H, CHarom), 7.73–7.66 (m, 4H, CHarom), 7.96–7.92 (m, 1H, CHarom), 8.14–8.09 (m, 1H, CHarom). 13C-NMR (100 MHz, CDCl3): δ (ppm) = 20.7, 60.2, 80.6, 127.5, 127.6, 127.7, 128.3, 128.7, 128.8, 129.5, 130.1, 131.3, 132.1, 132.9, 133.6, 134.4, 134.8, 137.9, 144.0, 171.0, 192.3, 194.3. MS (DUIS): m/z = 394 [M+], expected: 394 [M+]. IR (neat): ῦ (cm−1) = 1728, 1682, 1584, 1444, 1278, 1265, 1228, 1138, 1055, 902, 762.
7-Benzoyl-4-oxo-4H-benzo[de]anthracen-5-yl-acetate (7). Yellow solid. M.p.: 196–197 °C. 1H-NMR (400 MHz, CDCl3): δ (ppm) = 2.32 (s, 3H, CH3), 7.35 (s, 1H, =CH), 7.48 (dd, J = 7.9 Hz, 2H, CHarom), 7.57 (dd, J = 7.5 Hz, 1H, CHarom), 7.65 (dd, J = 7.7 Hz, 1H, CHarom), 7.69 (d, J = 8.0 Hz, 1H, CHarom), 7.79 (dd, J = 8.3 Hz, 1H, CHarom), 7.91 (d, J = 8.0 Hz, 2H, CHarom), 8.00 (dd, J = 8.0 Hz, 1H, CHarom), 8.74 (d, J = 8.6 Hz, 1H, CHarom), 8.80 (d, J = 8.4 Hz, 1H, CHarom), 9.08 (d, J = 7.4 Hz, 1H, CHarom). MS (DUIS): m/z = 392 [M+], expected: 392 [M+]. IR (neat): ῦ (cm−1) = 1765, 1663, 1646, 1577, 1446, 1223, 1228, 1183, 757.

3. Results and Discussion

3.1. Synthesis of 2-Acetoxy-1,4-naphthoquinone

2-Acetoxy-1,4-naphthoquinone (1) was prepared from 2-hydroxy-1,4-naphthoquinone in a yield of 80% following a procedure by Clark [34]. The synthesis is described in detail in the Supplementary Materials.

3.2. Photocycloadditions Under Batch Conditions

All batch irradiations followed a modified procedure by Maruyama et al. [36]. Experiments were conducted in Pyrex vessels inside a chamber reactor equipped with fluorescent tubes. Excess amounts of alkene or alkyne were used to prevent photodimerization of 1 [37]. The reaction mixture was constantly purged with a stream of N2 to remove air and provide mixing.

3.2.1. Optimization Studies

The photoaddition of 1,1-diphenylethylene (2a) was initially chosen as a model reaction for process optimization (Scheme 2). A series of exposures for 20 h with different light was subsequently performed in acetone or acetonitrile (Table 1). In line with previous studies [28,29,30], the reaction chemoselectively produced the cyclobutane adduct 3a under all experimental conditions, while the formation of spiro-oxetanes was not observed.
Despite the exhaustive reaction time, photocycloadditions of the 1/2a pair remained incomplete when irradiated in either acetone or acetonitrile with UVB or visible light (entries 1, 2 and 5). In contrast, naphthoquinone 1 was completed consumed in both solvents when UVA light was chosen (entries 3 and 4). The absorption spectrum of 2-acetoxy-1,4-naphthoquinone (1) has been previously described and shows a dominant and broad benzenoid transition around 335 nm [38,39,40,41]. A low intensity but masked quinonoid transition has also been suggested in the 330–450 nm region [40]. These absorptions overlap efficiently with the emission spectrum of the UVA lamp, thus enabling effective excitation (see Supplementary Materials). The electron-donating acetoxy group furthermore increases the energy of the n,π* triplet state of 1 above that of its corresponding π,π* counterpart, hence favoring the addition of the alkene at the quinonoid C=C site [10]. The remarkable head-to-head regioselectivity can be explained via a stepwise triplet state mechanism featuring the more stable 1,4-diradical intermediate [42]. For the irradiation in acetone with UVB light (entry 1), competing absorption by the solvent (T10% = 329 nm [43]) and subsequent triplet sensitization cannot be ruled out [44]. In contrast, direct absorption operates with UVA or visible light in this solvent instead (entries 3 and 5).

3.2.2. Preparative Photocycloadditions with Selected Alkenes

Due to its low toxicity [45], photoprotective nature [24], excellent solvation ability, and easy removability, the photocycloaddition of 1 was subsequently conducted in acetone with selected alkenes (2a–c) and utilizing UVA light (Scheme 3). After extended reaction times of 10–20 h, the conversion rates of 1 reached 76–100%. The need to remove unreacted 1 or other minor impurities by automated chromatography reduced isolated yields of the corresponding cyclobutanes 3a–c to 41–69% (Table 2) [46].
Photocycloadditions of 1 with 1,1-diphenylethylene (2a) and styrene (2b) with artificial light have been previously described [28,29]. While similar isolated yields of approx. 50% have been achieved for both reactions, the reported procedure used acetonitrile as reaction medium, and a Pyrex immersion-well reactor equipped with a 125 W medium-pressure mercury lamp as the light source. The developed protocol in this study utilized non-hazardous and volatile acetone in combination with more selective UVA light emitted from low heat-generating fluorescent tubes instead.
Photocycloaddition with 1,1-Diphenylethylene
The reaction between 2-acetoxy-1,4-naphthoquinone (1) and 1,1-diphenylethylene (2a) demanded 20 h for completion and furnished cyclobutane 3a in an isolated yield of 60% (entry 1). The head-to-head structure of 3a was unambiguously confirmed by X-ray crystallographic analysis (Figure 1). The slightly puckered structure of the cyclobutane system resulted in bond angles in the range of 90–92°, except for the carbon carrying the phenyl substituents, which gave a bond angle of 87° instead [47,48]. The two hydrogen atoms H3 and H10b were placed at pseudo-equatorial positions, while H3 and H10a adopted a not quite eclipsed conformation with a dihedral angle of ca 14°.
The 1H-NMR spectrum of cycloadduct 3a in CDCl3 (see Supplementary Materials) showed a sharp singlet at 2.08 ppm for the acetoxy group. The three protons of the cyclobutane system displayed signals between 3.10–4.63 ppm. A pair of doublets at 3.40 and 3.59 ppm with a germinal coupling of 2J = 12.3 Hz was observed for the methylene protons H10a and H10b, whereas a multiplet at 3.57 ppm represented the remaining proton H3. The aromatic protons appeared between 6.86 and 7.94 ppm.
Photocycloaddition with Styrene
The photoaddition of styrene 2b resulted in a conversion of 95% after just 10 h of irradiation. Subsequent automated chromatography produced pure anti-3b in an isolated yield of 69% (entry 2). X-ray structure analysis confirmed the formation of the head-to-head and anti-isomer of 3b (Figure 2). The cyclobutane ring showed a slightly puckered geometry, with bond angles found at 88–91° [47,48]. The shallow ring conformation led to a near eclipsed alignment of H7 and H8A with a torsion angle of around 17°.
The 1H-NMR spectrum of anti-3b in CDCl3 (see Supplementary Materials) showed the four protons of the cyclobutane ring as four separate signals, i.e., three doublets of doublets at 2.59, 3.11, and 3.62 ppm and one doublet of doublet at 3.98 ppm, respectively. A sharp singlet at 2.00 ppm represented the methyl protons of the acetoxy group. Five separate signals were observed for the aromatic protons, two multiplets between 7.28 and 7.39 ppm (phenyl protons) and three doublets of doublets at 7.84, 8.21 and 8.28 ppm, respectively.
Photocycloaddition with Cyclopentene
Photoirradiation of 1 with cyclopentene (2c) for 12 h furnished an incomplete conversion of 76% and solely gave the anti-isomer of 3c in an isolated yield of 41% (entry 3). The crystal structure of anti-3c is depicted in Figure 3. The structure contained two different cycloalkane conformations. The puckered cyclobutane ring remained rather shallow with bond angles of 89–90° [47,48]. The cis-protons H9 and H10 appeared not to be completely eclipsed with a torsion angle of about 6°. The cyclopentane conformation was similar to that found in the corresponding photoadduct with 1,4-naphthoquinone [24].
In the 1H-NMR spectrum of anti-3c in CDCl3 (see Supplementary Materials), the six bridging -CH2- protons of the cyclopentane ring appeared as broad signals at 2.38 and between 1.55 and 1.95 ppm, respectively. A clear singlet at 2.11 ppm represented the acetoxy group. The protons of the cyclobutane moiety gave three signals, which appeared as doublets of doublets at 2.70 and 2.91 ppm and as a doublet at 3.03 ppm, respectively. A set of three multiplets at 7.78, 8.10 and 8.17 ppm represented the four aromatic protons.

3.2.3. Photocycloadditions with Diphenylacetylene

The photoaddition of 1 was furthermore investigated with diphenylacetylene (5). Irradiation in acetone for 10 h with UVA light produced cyclobutene 6 and benzoanthracenone 7 in isolated yields of 15 and 9% (Scheme 4), respectively.
Competing photoinduced electron transfer and direct excitation mechanisms have been proposed for the photoaddition involving alkynes [49]. The formation of 7 may be rationalized via ring-opening of the initial spiro-oxetene 8 to quinone methide 9, followed by successive oxidative photodehydrocyclization [49,50]. The reaction has been described by Pappas et al. [35], who isolated compounds 6 and 9 in yields of 20% each after prolonged irradiation in acetonitrile with UVA light for 31 h. However, the authors did not define the position of the acetoxy group in compound 9.
The structure of photoadduct 6 was confirmed by X-ray crystallographic analysis (Figure 4). The almost planar structure of the cyclobutene system with a C=C bond length of 1.36 Å resulted in bond angles of 85–95°. The two phenyl groups were placed nearly eclipsed with a dihedral angle of just ca 7°.
The 1H-NMR spectrum of 6 in CDCl3 (see Supplementary Materials) showed five multiplets for the aromatic protons between 7.34 and 8.12 ppm. The single cyclobutene proton showed a singlet at 4.34 ppm [51], while a sharp singlet at 2.22 ppm was observed for the acetoxy group. The structure of the benzoanthracenone 7 was likewise supported by 1H-NMR analysis (see Supplementary Materials). In CDCl3, a total of nine signals with a combined integration of twelve protons were found in the aromatic region. Of these, anisotropy effects shifted the inner aromatic protons of the phenanthrene moiety downfield to 8.80 and 9.08 ppm, respectively [52]. The position of the acetoxy group was assigned by comparison with analog photoadditions involving 2-substituted quinones [49,50,53]. The =CH proton was detected as a singlet at 7.35 ppm, while the acetoxy protons appeared as a strong singlet at 2.32 ppm.

3.3. Photocycloadditions with Selected Alkenes Under Continuous-Flow Conditions

As a proof-of-concept, 2-acetyloxy-1,4-naphthoquinone (1) was furthermore reacted with alkenes 3a–c in an in-house continuous-flow reactor (Scheme 3). Flow rates and hence residence times were varied significantly between experiments to determine the consumption of 1 by 1H-NMR analysis (Table 3).
The general reactivity differences observed in batch were also noted during flow operations, although the latter achieved a significant reduction in irradiation times. However, further residence time optimizations are required to determine realistic productivity rates. 1,1-Diphenylethylene (2a) gave complete conversion with a residence time of 300 min, and the cycloadduct 3a was isolated in a yield of 55% (entry 1). In contrast, styrene (2a) reacted readily, showed a near complete conversion of 97% with a residence time of 120 min and furnished cyclobutane anti-3b in an excellent yield of 91% (entry 2). The photoaddition of cyclopentene (2c) gave the lowest conversion of 36% with the shortest residence time of 100 min. Photoproduct anti-3c was subsequently obtained in a yield of 30% (or 83% based on consumed 1). The results from this limited study support previous findings on the general superiority of continuous-flow operations in terms of conversions, yields and irradiation times [17,18,19,20,21,22,23,24,25,26]. In particular, the narrow reaction channels allow for effective light penetration, while flow operation removes the potentially photoactive products from the reaction room [17,18,19,20,21,22]. However, the scale-up, productivity and reproducibility of flow-photochemical processes remain challenging [54,55,56,57].

4. Conclusions

In summary, photocycloadditions of 2-acetoxy-1,4-naphthoquinone with selected alkenes and a model alkyne have been successfully realized. The simple irradiation protocol developed uses acetone and UVA light for the construction of cyclobutane and cyclobutene photoproducts in yields of 15–69%. The photoaddition of diphenylacetylene additionally produced a benzoanthracenone derivative in 9% yield. A limited continuous-flow study conducted with the chosen alkenes gave the corresponding photoproducts in similar or improved yields up to 91% but with significantly shorter irradiation times.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/photochem5040031/s1: Technical details, experimental procedure for the synthesis of 1, crystallographic data and NMR spectra. Figure S1. Rayonet chamber reactor during irradiation. Figure S2. In-house continuous-flow capillary reactor during operation. Figure S3. UV-Vis spectra of 1 (in MeCN) and acetone vs. emissions of UVB, UVA and 419 nm lamps. Table S1. Crystal data and structural refinement for compounds 3a, anti-3b, anti-3c and 6 [58,59,60,61].

Author Contributions

M.A.Y., Z.G. and M.O. conducted the research and collected the data; P.C.J. supervised and assisted with the structure determination; M.O. also secured the funding, supervised the chemical research and wrote the manuscript. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the College of Science and Engineering at James Cook University through a Competitive Research Training Grant.

Data Availability Statement

The original contributions presented in this study are included in the article/Supplementary Material. Further inquiries can be directed to the corresponding author.

Acknowledgments

M.A.Y. thanks the Ministry of Higher Education and Scientific Research of Iraq for a Ph.D. Scholarship. Parts of this research was undertaken on the MX1 beamline at the Australian Synchrotron, part of ANSTO.

Conflicts of Interest

Author M.O. is employed by Hochschule Fresenius gGmbH—University of Applied Sciences. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

References

  1. Jha, R.K.; Kumar, S. Direct Functionalization of para-Quinones: A Historical Review and New Perspectives. Eur. J. Org. Chem. 2024, 27, e202400535. [Google Scholar] [CrossRef]
  2. Angulo-Elizari, E.; Henriquez-Figuereo, A.; Morán-Serradilla, C.; Plano, D.; Sanmartín, C. Unlocking the Potential of 1,4-Naphthoquinones: A Comprehensive Review of Their Anticancer Properties. Eur. J. Med. Chem. 2024, 268, 116249. [Google Scholar] [CrossRef] [PubMed]
  3. Navarro-Tovar, G.; Vega-Rodríguez, S.; de Loera, D.; Leyva, E.; Loredo-Carrillo, S.; López-López, L.I. The Relevance and Insights on 1,4-Naphthoquinones as Antimicrobial and Antitumoral Molecules: A Systematic Review. Pharmaceuticals 2023, 16, 496. [Google Scholar] [CrossRef] [PubMed]
  4. Mone, N.S.; Bhagwat, S.A.; Sharma, D.; Chaskar, M.; Patil, R.H.; Zamboni, P.; Nawani, N.N.; Satpute, S.K. Naphthoquinones and Their Derivatives: Emerging Trends in Combating Microbial Pathogens. Coatings 2021, 11, 434. [Google Scholar] [CrossRef]
  5. Ahmadi, E.S.; Tajbakhsh, A.; Iranshahy, M.; Asili, J.; Kretschmer, N.; Shakeri, A.; Sahebkar, A. Naphthoquinone Derivatives Isolated from Plants: Recent Advances in Biological Activity. Mini-Rev. Med. Chem. 2020, 20, 2019–2035. [Google Scholar] [CrossRef] [PubMed]
  6. Perpète, E.A.; Lambert, C.; Wathelet, V.; Preat, J.; Jacquemin, D. Ab initio Studies of the λmax of Naphthoquinones Dyes. Spectrochim. Acta A 2007, 68, 1326–1333. [Google Scholar] [CrossRef] [PubMed]
  7. Jang, J.; Lee, G.; Cho, E.J. Visible Light Induced Reactions of Quinones. Bull. Korean Chem. Soc. 2024, 45, 966–976. [Google Scholar] [CrossRef]
  8. de Lucas, N.C.; Ferreira, A.B.B.; Netto-Ferreira, J.C. Fotoquímica de Naftoquinonas. Rev. Virtual Quim. 2015, 7, 403–463. [Google Scholar] [CrossRef]
  9. Maruyama, K.; Osuka, A. Recent advances in the photochemistry of quinones. In The Chemistry of Quinonoid Compounds; Patai, S., Rappaport, Z., Eds.; John Wiley & Sons Ltd.: New York, NY, USA, 1988; Volume 2, Part 1, Chapter 13; pp. 759–878. [Google Scholar] [CrossRef]
  10. Gilbert, A. 1,4-Quinone Cycloaddition Reactions with Alkenes, Alkynes, and Related Compounds. In CRC Handbook of Organic Photochemistry and Photobiology, 2nd ed.; Horspool, W.M., Lenci, F., Eds.; CRC Press: Boca Raton, FL, USA, 2004; Chapter 87; pp. 1–12. [Google Scholar] [CrossRef]
  11. Creed, D. 1,4-Quinone Cycloaddition Reactions with Alkene, Alkynes and Related Compounds. In CRC Handbook of Organic Photochemistry and Photobiology; Horspool, W.M., Song, P.-S., Eds.; CRC Press: Boca Raton, FL, USA, 1995; Chapter 59; pp. 737–747. [Google Scholar]
  12. Tan, S.-B.; Huang, C.; Chen, X.; Wu, Y.; Zhou, M.; Zhang, C.; Zhang, Y. Small Molecular Inhibitors of miR-1 Identified from Photocycloadducts of Acetylenes with 2-Methoxy-1,4-naphthalenequinone. Bioorg. Med. Chem. 2013, 21, 6124–6131. [Google Scholar] [CrossRef]
  13. Chen, X.; Huang, C.; Zhang, W.; Wu, Y.; Chen, X.; Zhang, C.-Y.; Zhang, Y. A Universal Activator of micro RNAs Identified from Photoreaction Products. Chem. Comm. 2012, 48, 6432–6434. [Google Scholar] [CrossRef]
  14. Protti, S.; Ravelli, D.; Fagnoni, M. Introduction to Photochemistry for the Synthetic Chemist. In Enabling Tools and Techniques for Organic Synthesis: A Practical Guide to Experimentation, Automation, and Computation; Newman, S.G., Ed.; John Wiley & Sons Ltd.: New York, NY, USA, 2023; Chapter 2; pp. 37–72. [Google Scholar] [CrossRef]
  15. Douglas, P.; Evans, R.C.; Burrows, H.D. The Photochemical Laboratory. In Applied Photochemistry; Evans, R.C., Douglas, P., Burrows, H.D., Eds.; Springer: Dordrecht, The Netherlands, 2013; Chapter 14; pp. 467–531. [Google Scholar] [CrossRef]
  16. Bochet, C.G. On the Sustainability of Photochemical Reactions. Chimia 2019, 73, 720–723. [Google Scholar] [CrossRef] [PubMed]
  17. Crawford, R.; Baumann, M. Continuous Flow Technology Enabling Photochemistry. Adv. Synth. Catal. 2025, 367, e202500133. [Google Scholar] [CrossRef]
  18. Srivastava, V.; Singh, P.P.; Sinha, S.; Singh, P.K.; Kumar, D. Continuous-Flow Photochemistry: The Synthesis of Marketed Pharmaceutical Compounds. ChemistrySelect 2024, 9, e202405020. [Google Scholar] [CrossRef]
  19. Zhang, M.; Roth, P. Flow photochemistry–From Microreactors to Large-scale Processing. Curr. Opin. Chem. Eng. 2023, 39, 100897. [Google Scholar] [CrossRef]
  20. Fukuyama, T.; Kasakado, T.; Hyodo, M.; Ryu, I. Improved Efficiency of Photo-induced Synthetic Reactions Enabled by Advanced Photo Flow Technologies. Photochem. Photobiol. Sci. 2022, 21, 761–775. [Google Scholar] [CrossRef]
  21. Rehm, T.H. Flow Photochemistry as a Tool in Organic Synthesis. Chem. Eur. J. 2020, 26, 16952–16974. [Google Scholar] [CrossRef]
  22. Oelgemöller, M.; Hoffmann, N.; Shvydkiv, O. From ‘Lab & Light on a Chip’ to Parallel Microflow Photochemistry. Austr. J. Chem. 2014, 67, 337–342. [Google Scholar] [CrossRef]
  23. Yaseen, M.A.; Oelgemöller, M. Photochemical Acylation of 1,4-Naphthoquinone with Aldehydes Under Continuous-Flow Conditions. Organics 2025, 6, 9. [Google Scholar] [CrossRef]
  24. Yaseen, M.A.; Guo, Z.; Junk, P.J.; Oelgemöller, M. [2+2]-Photocycloadditions of 1,4-Naphthoquinone Under Batch and Continuous-Flow Conditions. Molecules 2024, 29, 5920. [Google Scholar] [CrossRef]
  25. Khan, H.; Rajesh, V.M.; Ravva, M.K.; Sen, S. Optimization of Blue LED Photo-Flow Synthesis in Continuous Flow Reactors Using Design of Experiments (DoE): Efficient Synthesis of Diverse Diaryl Ketones. Chem. Eng. J. 2024, 501, 157657. [Google Scholar] [CrossRef]
  26. Yaseen, M.A.; Mumtaz, S.; Hunter, R.L.; Wall, D.; Belluau, V.; Robertson, M.J.; Oelgemöller, M. Continuous-Flow Photochemical Transformations of 1,4-Naphthoquinones and Phthalimides in a Concentrating Solar Trough Reactor. Austr. J. Chem. 2020, 73, 1149–1157. [Google Scholar] [CrossRef]
  27. Yaseen, M.A. Photochemical Transformations of Quinones under Batch and Continuous–Flow Conditions. Ph.D. Thesis, James Cook University, Townsville, QLD, Australia, 2019. [Google Scholar]
  28. Cleridou, S.; Covell, C.; Gadhia, A.; Gilbert, A.; Kamonnawin, P. Photocycloaddition of Arylethenes to 2-Substituted-1, 4-naphthoquinones and Reactions of the Cyclobutane Adduct Isomers. J. Chem. Soc. Perkin Trans. 1 2000, 1149–1155. [Google Scholar] [CrossRef]
  29. Covell, C.; Gilbert, A.; Richter, C. Sunlight-induced Regio-and Stereo-specific (2π + 2π) Cycloaddition of Arylethenes to 2-Substituted-1, 4-naphthoquinones. J. Chem. Res. (S) 1998, 316–317. [Google Scholar] [CrossRef]
  30. Senboku, H.; Kajizuka, Y.; Kobayashi, K.; Tokuda, M.; Suginome, H. Furan Annelation of 2-Hydroxynaphthoquinone Involving Photochemical Addition and Radical Fragmentation: Exclusion of the Intermediacy of [2+2] Cycloadduct in a One-pot Formation of Furanoquinones by the Regioselective 3+2 Photoaddition of Hydroxyquinones with Alkenes. Heterocycles 1997, 44, 341–348. [Google Scholar]
  31. Mumtaz, S.; Robertson, M.J.; Oelgemöller, M. Continuous Flow Photochemical and Thermal Multi-step Synthesis of Bioactive 3-Arylmethylene-2,3-dihydro-1H-isoindolin-1-ones. Molecules 2019, 24, 4527. [Google Scholar] [CrossRef] [PubMed]
  32. Fulmer, G.R.; Miller, A.J.; Sherden, N.H.; Gottlieb, H.E.; Nudelman, A.; Stoltz, B.M.; Bercaw, J.E.; Goldberg, K.I. NMR Chemical Shifts of Trace Impurities: Common Laboratory Solvents, Organics, and Gases in Deuterated Solvents Relevant to the Organometallic Chemist. Organometallics 2010, 29, 2176–2179. [Google Scholar] [CrossRef]
  33. Cowieson, N.P.; Aragao, D.; Clift, M.; Ericsson, D.J.; Gee, C.; Harrop, S.J.; Mudie, N.; Panjikar, S.; Price, J.R.; Riboldi-Tunnicliffe, A.; et al. MX1: A Bending-magnet Crystallography Beamline Serving both Chemical and Macromolecular Crystallography Communities at the Australian Synchrotron. J. Synchrotron Radiat. 2015, 22, 187–190. [Google Scholar] [CrossRef]
  34. Clark, N.G. The Fungicidal Activity of Substituted 1, 4-Naphthoquinones. Part II: Alkoxy, Phenoxy and Acyloxy derivatives. Pesticide Sci. 1984, 15, 235–240. [Google Scholar] [CrossRef]
  35. Pappas, S.; Portnoy, N.A. Substituent Effects on the Photoaddition of Diphenylacetylene to 1, 4-Naphthoquinones. J. Org. Chem. 1968, 33, 2200–2203. [Google Scholar] [CrossRef]
  36. Maruyama, K.; Otsuki, T.; Takuwa, A.; Kako, S. Photochemical Reaction of 1, 4-Naphthoquinone with Olefins. Bull. Inst. Chem. Res. Kyoto Univ. 1972, 50, 344–347. [Google Scholar]
  37. Dekker, J.; van Vuuren, P.J.; Venter, D.P. Photodimerization. I. The syn and anti-Photodimers of 1,4-Naphthoquinone. J. Org. Chem. 1968, 33, 464–466. [Google Scholar] [CrossRef]
  38. Daglish, C. The Ultraviolet Absorption Spectra of Some Hydroxynaphthalenes. J. Am. Chem. Soc. 1950, 72, 4859–4864. [Google Scholar] [CrossRef]
  39. Hill, R.R.; Mitchell, G.H. The Ultraviolet and Visible Absorption Spectra of Some Polysubstituted 1,4-Naphthaquinones and 9,10-Anthraquinones. J. Chem. Soc. B 1969, 61–64. [Google Scholar] [CrossRef]
  40. Singh, I.; Ogata, R.T.; Moore, R.E.; Chang, C.W.J.; Scheuer, P.J. Electronic spectra of substituted naphthoquinones. Tetrahedron 1968, 24, 6053–6073. [Google Scholar] [CrossRef]
  41. Macbeth, A.K.; Price, J.R.; Winzor, F.L. The Colouring Matters of Drosera Whittakeri. Part I. The Absorption Spectra and Colour Reactions of Hydroxy-naphthaquinones. J. Chem. Soc. 1935, 325–333. [Google Scholar] [CrossRef]
  42. Crimmins, M.T.; Reinhold, T.L. Enone Olefin [2+2] Photochemistry Cycloaddition. In Organic Reactions, Paquette, L.A., Ed.; John Wiley & Sons, Inc.: New York, NY, USA, 1993; Volume 44, pp. 298–579. [Google Scholar] [CrossRef]
  43. Montalti, M.; Credi, A.; Prodi, L.; Gandolfi, M.T. Solvent Properties. In Handbook of Photochemistry, 3rd ed.; CRC Press: Boca Raton, FL, USA, 2006; Chapter 9; pp. 535–559. [Google Scholar] [CrossRef]
  44. Bunce, N.J.; Ridley, J.E.; Zerner, M.C. On the Excited States of p-Quinones and an Interpretation of the Photocycloaddition of p-Quinones to Alkenes. Theor. Chim. Acta 1977, 45, 283–300. [Google Scholar] [CrossRef]
  45. Capello, C.; Fischer, U.; Hungerbühler, K. What is a Green Solvent? A Comprehensive Framework for the Environmental Assessment of Solvents. Green Chem. 2007, 9, 927–934. [Google Scholar] [CrossRef]
  46. Wernerova, M.; Hudlicky, T. On the Practical Limits of Determining Isolated Product Yields and Ratios of Stereoisomers: Reflections, Analysis, and Redemption. Synlett 2010, 2701–2707. [Google Scholar] [CrossRef]
  47. Cotton, F.A.; Frenz, B.A. Conformations of Cyclobutane. Tetrahedron 1974, 30, 1587–1594. [Google Scholar] [CrossRef]
  48. Dragojlovic, V. Conformational Analysis of Cycloalkanes. ChemTexts 2015, 1, 14. [Google Scholar] [CrossRef]
  49. Bosch, E.; Hubig, S.; Kochi, J. Paterno-Büchi Coupling of (Diaryl) Acetylenes and Quinone via Photoinduced Electron Transfer. J. Am. Chem. Soc. 1998, 120, 386–395. [Google Scholar] [CrossRef]
  50. Farid, S.; Kothe, W.; Pfundt, G. Competitive Photoadditions of Acetylenes to the C=C and C=O Bonds of p-Quinones. Tetrahedron Lett. 1968, 9, 4147–4150. [Google Scholar] [CrossRef]
  51. Farid, S.; Kothe, W.; Pfundt, G. NMR-study of Cyclobutene Derivatives. Tetrahedron Lett. 1968, 9, 4151–4154. [Google Scholar] [CrossRef]
  52. Balo, C.; Fernández, F.; González, C.; Lopez, C. 1H NMR Spectra of Monosubstituted Phenanthrenes. Spectrochim. Acta 1994, 50A, 937–940. [Google Scholar] [CrossRef]
  53. Fadeev, A.A.; Kotora, M. Catalytic vs. Uncatalyzed [2+2] Photocycloadditions of Quinones with Alkynes. Org. Biomol. Chem. 2023, 21, 6174–6179. [Google Scholar] [CrossRef]
  54. Donnelly, K.; Baumann, M. Scalability of Photochemical Reactions in Continuous Flow Mode. J. Flow Chem. 2021, 11, 223–241. [Google Scholar] [CrossRef]
  55. Elliott, L.D.; Knowles, J.P.; Koovits, P.J.; Maskill, K.G.; Ralph, M.J.; Lejeune, G.; Edwards, L.J.; Robinson, R.I.; Clemens, I.R.; Cox, B.; et al. Batch versus Flow Photochemistry: A Revealing Comparison of Yield and Productivity. Chem. Eur. J. 2014, 20, 15226–15232. [Google Scholar] [CrossRef]
  56. Bonfield, H.E.; Mercer, K.; Diaz-Rodriguez, A.; Cook, G.C.; McKay, B.S.J.; Slade, P.; Taylor, G.M.; Ooi, W.X.; Williams, J.D.; Roberts, J.P.M.; et al. The Right Light: DeNovo Design of a Robust Modular Photochemical Reactor for Optimum Batch and Flow Chemistry. ChemPhotoChem 2020, 4, 45–51. [Google Scholar] [CrossRef]
  57. Loubière, K.; Oelgemöller, M.; Aillet, T.; Dechy-Cabaret, O.; Prat, L. Continuous-flow Photochemistry: A Need for Chemical Engineering. Chem. Eng. Process. 2016, 104, 120–132. [Google Scholar] [CrossRef]
  58. McPhillips, T.M.; McPhillips, S.E.; Chiu, H.J.; Cohen, A.E.; Deacon, A.M.; Ellis, P.J.; Garman, E.; Gonzalez, A.; Sauter, N.K.; Phizackerley, R.P.; et al. Blu-Ice and the Distributed Control System: Software for Data Acquisition and Instrument Control at Macromolecular Crystallography Beamlines. J. Synchrotron Radiat. 2002, 9, 401–406. [Google Scholar] [CrossRef]
  59. Kabsch, W. Automatic Processing of Rotation Diffraction Data from Crystals of Initially Unknown Symmetry and Cell Constants. J. Appl. Crystallogr. 1993, 26, 795–800. [Google Scholar] [CrossRef]
  60. Sheldrick, G.M. Crystal Structure Refinement with SHELXL. Acta Cryst. 2015, C71, 3–8. [Google Scholar] [CrossRef]
  61. Dolomanov, O.V.; Bourhis, L.J.; Gildea, R.J.; Howard, J.A.K.; Puschmann, H. OLEX2: A Complete Structure Solution, Refinement and Analysis Program. J. Appl. Crystallogr. 2009, 42, 339–341. [Google Scholar] [CrossRef]
Photochem 05 00031 sch001
Scheme 1. [2+2]-Photocycloadditions of 2-substituted 1,4-naphthoquinones with alkenes to diastereo- and regioisomeric cyclobutanes.
Scheme 1. [2+2]-Photocycloadditions of 2-substituted 1,4-naphthoquinones with alkenes to diastereo- and regioisomeric cyclobutanes.
Photochem 05 00031 sch001
Photochem 05 00031 sch002
Scheme 2. [2+2]-Photocycloaddition of 2-acetoxy-1,4-naphthoquinone with 1,1-diphenylethylene as a model reaction.
Scheme 2. [2+2]-Photocycloaddition of 2-acetoxy-1,4-naphthoquinone with 1,1-diphenylethylene as a model reaction.
Photochem 05 00031 sch002
Photochem 05 00031 sch003
Scheme 3. [2+2]-Photocycloadditions of 2-acetoxy-1,4-naphthoquinone with selected alkenes.
Scheme 3. [2+2]-Photocycloadditions of 2-acetoxy-1,4-naphthoquinone with selected alkenes.
Photochem 05 00031 sch003
Photochem 05 00031 g001
Figure 1. Crystal structure of 3a (dark grey: carbon, light blue: hydrogen, red: oxygen).
Figure 1. Crystal structure of 3a (dark grey: carbon, light blue: hydrogen, red: oxygen).
Photochem 05 00031 g001
Photochem 05 00031 g002
Figure 2. Crystal structure of anti-3b (dark grey: carbon, light blue: hydrogen, red: oxygen).
Figure 2. Crystal structure of anti-3b (dark grey: carbon, light blue: hydrogen, red: oxygen).
Photochem 05 00031 g002
Photochem 05 00031 g003
Figure 3. Crystal structure of anti-3c (dark grey: carbon, light blue: hydrogen, red: oxygen).
Figure 3. Crystal structure of anti-3c (dark grey: carbon, light blue: hydrogen, red: oxygen).
Photochem 05 00031 g003
Photochem 05 00031 sch004
Scheme 4. [2+2]-Photocycloaddition of 2-acetoxy-1,4-naphthoquinone with diphenylacetylene.
Scheme 4. [2+2]-Photocycloaddition of 2-acetoxy-1,4-naphthoquinone with diphenylacetylene.
Photochem 05 00031 sch004
Photochem 05 00031 g004
Figure 4. Crystal structure of 6 (dark grey: carbon, light blue: hydrogen, red: oxygen).
Figure 4. Crystal structure of 6 (dark grey: carbon, light blue: hydrogen, red: oxygen).
Photochem 05 00031 g004
Table 1. Experimental results of the optimization study (Pyrex, 20 h).
Table 1. Experimental results of the optimization study (Pyrex, 20 h).
EntryLightSolventConversion
(%) 1		Composition 1
3a (%)4a (%)
1306 ± 20 nmacetone88≥95n.d. 2
2306 ± 20 nmacetonitrile82≥95n.d. 2
3352 ± 20 nmacetone100≥95n.d. 2
4352 ± 20 nmacetonitrile100≥95n.d. 2
5419 ± 25 nmacetone78≥95n.d. 2
1 Determined by 1H-NMR analysis (±3%). 2 Not determined.
Table 2. Experimental results of selected alkenes (Pyrex, UVA/352 ± 20 nm).
Table 2. Experimental results of selected alkenes (Pyrex, UVA/352 ± 20 nm).
EntryAlkene (2)Time (h)Conversion
(%) 1		Yield (%) 2
R1R2R3
1PhHPh2010060 (3a)
2PhHH109569 (3b)
3-(CH2)3-H127641 (3c)
1 Determined by 1H-NMR analysis (±3%). 2 After automated flash chromatography.
Table 3. Experimental results of photocyclizations with selected alkenes under continuous-flow conditions (Pyrex, UVA/352 ± 20 nm, acetone).
Table 3. Experimental results of photocyclizations with selected alkenes under continuous-flow conditions (Pyrex, UVA/352 ± 20 nm, acetone).
EntryAlkene (2)Residence
Time (mins)		Conversion
(%) 1		Yield (%) 2
R1R2R3
1PhHPh30010055 (3a)
2PhHH1209791 (3b)
3-(CH2)3-H1003630 (3c)/83 3
1 Determined by 1H-NMR analysis (±3%). 2 After automated flash chromatography. 3 Yield based on consumed 1.
Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Share and Cite

MDPI and ACS Style

Yaseen, M.A.; Guo, Z.; Junk, P.C.; Oelgemöller, M. [2+2]-Photocycloadditions of 2-Acetoxy-1,4-naphthoquinone and Structure Determination of the Main Photoadducts. Photochem 2025, 5, 31. https://doi.org/10.3390/photochem5040031

AMA Style

Yaseen MA, Guo Z, Junk PC, Oelgemöller M. [2+2]-Photocycloadditions of 2-Acetoxy-1,4-naphthoquinone and Structure Determination of the Main Photoadducts. Photochem. 2025; 5(4):31. https://doi.org/10.3390/photochem5040031

Chicago/Turabian Style

Yaseen, Madyan A., Zhifang Guo, Peter C. Junk, and Michael Oelgemöller. 2025. "[2+2]-Photocycloadditions of 2-Acetoxy-1,4-naphthoquinone and Structure Determination of the Main Photoadducts" Photochem 5, no. 4: 31. https://doi.org/10.3390/photochem5040031

APA Style

Yaseen, M. A., Guo, Z., Junk, P. C., & Oelgemöller, M. (2025). [2+2]-Photocycloadditions of 2-Acetoxy-1,4-naphthoquinone and Structure Determination of the Main Photoadducts. Photochem, 5(4), 31. https://doi.org/10.3390/photochem5040031

Article Metrics

Back to TopTop