Visible-Light-Induced Oxidative Generation of o-Quinone Methides for Inverse-Electron-Demand [4+2] Cycloaddition Reactions

Abstract

Organophotoredox-catalyzed oxidative generation of o-quinone methides (o-QMs) for inverse-electron-demand [4+2] cycloaddition reactions has been developed. One-electron oxidation of 2-(sulfanylmethyl)phenols by thioxanthylium photoredox catalyst generated o-QMs, which reacted with various styrenes to produce chromanes with high regioselectivity. This reaction offers a valuable approach for in situ generating o-QMs via one-electron oxidation process under irradiation with mild green light.

o-Quinone methides (o-QMs) are highly reactive intermediates widely used in organic synthesis.[1] Among the various methods for generating o-QMs (Figure [1]), oxidative processes have been of interest and attempted over the past few decades.[2] In 1989, Sato and co-authors reported that treatment of 2-(isopropylsulfanylmethyl)phenols with Ag₂O generated o-QMs in situ, which were trapped with dienophiles to yield chromanes via [4+2] cycloaddition (Scheme [1a]).[2a] Furthermore Chiba developed a method for generating o-QMs via photosensitized electrochemical oxidation of sulfides under visible-light irradiation (Scheme [1b]).[2`] [c] [d] [e] Recently, Ishihara reported a transition-metal-free oxidative generation of o-QMs from o-alkylarenols via hypoiodite catalysis (Scheme [1c]).[2f] While oxidative methods for generating o-QMs are a promising approach, studies of visible-light-induced oxidative generation of o-QMs by photoredox catalysis are rare and have not yet been fully Investigated.[3]

Over the past decade, light-induced chemical transformations have attracted much attention due to their potential in sustainable energy-conversion systems. Previous methods of generating o-QMs via photoirradiation process require the use of high-energy irradiation, such as ultraviolet light, which often leads to decomposition, dimerization, and undesired side reactions.[4] On the other hand, visible-light-induced generation of o-QMs may offer an efficient method to circumvent these issues. In 2019, Xiao reported a carbon-radical-mediated strategy for in situ generation of o-QMs from 2-vinyl phenols using an iridium photoredox catalyst via oxidative quenching cycle under irradiation with blue light.[5] More recently, Audebert and co-authors reported 2,5,8-tris(TFE)heptazine-catalyzed oxidative [4+2] cycloaddition of o-(sulfanylmethyl)phenol as one example.[3]

Meanwhile, we have developed various types of synthetic reactions using o-QMs.[6] Recently, we have developed several one-electron oxidation reactions using thioxanthylium organophotoredox catalysts (TXT), which have relatively high excited-state reduction potentials (E _1/2 (C^*/C^•–) = +1.76 to +1.94 V vs SCE).[7] Based on the above, we assumed that when 2-(sulfanylmethyl)phenols, which have easily oxidizable atoms such as sulfur, are used as substrates, one-electron oxidation by TXT photocatalyst occurs to generate o-QMs with the elimination of thiyl radicals (Scheme [1d]). Subsequently, o-QMs may react with styrenes to give chromanes via inverse-electron-demand [4+2] cycloaddition.[8] Thus, to expand the utility of o-QMs, we report herein the visible-light-induced oxidative generation of o-QMs for inverse-electron-demand [4+2] cycloaddition reactions via organophotoredox catalysis.

We initially examined the reaction of 2-(sulfanylmethyl)phenol (1a) with styrene (2a) in the presence of thioxanthylium photoredox catalyst (TXT) under irradiation of green light (Table [1]). When a nonpolar solvent, toluene, was used, the reaction proceeded to afford the desired cycloadduct 3a with high regioselectivity (entry 1).[9] Acetonitrile and trifluoroethanol (TFE) were somewhat more effective, but a good increase in yield was observed using dichloroethane (DCE) and nitromethane as the solvent (entries 2–5). After some trials, the reaction in a mixed solvent of DCE/TFE (1:1 volume ratio) was found to afford the desired chroman in 74% yield (cis/trans = 2:1, entry 6). Blank experiments reveal that the reaction effectively proceeds in the presence of photocatalyst, light source, and air (entries 7–9). However, since the yield of the reaction under Ar atmosphere is only slightly less than the yield under optimal conditions (entry 9), it is suggested that photocatalytic and/or radical chain mechanism without O₂ would be underway to furnish the desired cycloadduct.[10] When oxygen was used instead of air, product yield did not improve (entry 10). Reducing the amount of TXT catalyst (from 5.0 to 1.0 mol%) decreased the yield (entry 11). Finally, the optimum reaction conditions were decided as follows: 1a (0.2 mmol), 2a (0.6 mmol), TXT (5.0 mol%) in an 1:1 mixture of DCE/TFE (4.0 mL) at room temperature for 24 h under irradiation with green light.[11]

Table 1 Reaction of o-Sulfanylphenols with Styrenes in the Presence of TXT Photocatalyst^a

Entry	Solvent	Yield (%)	cis/trans b
1	toluene	48	5:1
2	CH3CN	50	5:2
3	CF3CH2OH (TFE)	54	2:1
4	CH2ClCH2Cl (DCE)	69	2:1
5	CH3NO2	69	3:1
6	TFE/DCEc	74	2:1
7d	TFE/DCE	N.R.
8e	TFE/DCE	N.R.
9f	TFE/DCE	57	2:1
10g	TFE/DCE	72	2:1
11h	TFE/DCE	30	2:1

^a All reactions were carried out with 1a (0.2 mmol), 2a (0.6 mmol), TXT (5.0 mol%) in solvent (4.0 mL) at room temperature for 24 h under irradiation with green light.

^b Determined by ¹H NMR spectroscopy.

^c TFE/DCE = 1:1.

^d No catalyst.

^e No light.

^f Under Ar.

^g O₂ was used instead of air.

^h 1 mol% of TXT.

With the optical conditions in hand, we investigated the scope of styrenes and 2-(sulfanylmethyl)phenols (Scheme [2]). The reactions of styrene with electron-donating group afforded the desired products in high yields (3b,c). Styrene bearing acetoxy group also gave the product in high yield (3d). Halogen functionality was suitable for the reaction (3e). In addition, 2- and 3-methylstyrenes can be applied to this reaction (3f and 3g). Subsequently, 2-(sulfanylmethyl)phenols substituted with electron-donating groups afford the corresponding products in high yields (3h and 3i). 2-(Sulfanylmethyl)phenols that bear halogen functionality on the phenol ring, such as fluoro, chloro, and bromo group, were good substrates for the reaction (3j–l). Phenyl group having methyl or chloro substituent at the para position also afforded the desired products in high yields (3m and 3n). Although methoxy group on the phenyl ring would be expected to help oxidize 2-(sulfanylmethyl)phenols by TXT catalyst, the o-QM could not be sufficiently stable, resulting in lower yield (3o). These reactions give the cis-rich products, which is consistent with the trend of inverse-electron-demand [4+2] cycloaddition of o-QM with styrenes.[8] Disubstituted olefin 1,1-diphenylethene is also a good reactant to afford triphenylchroman 3p in good yield (80%).

The following plausible reaction mechanisms can be inferred (Scheme [3]). According to Stern–Volmer experiments shown in Figure [2], the single-electron transfer (SET) from 2-(sulfanylmethyl)phenol 1 to the photocatalyst should occur smoothly. Thus, excitation of the photocatalyst (TXT; E _1/2 (C*/C^•−) = +1.76 V vs SCE) under irradiation with green light enables the oxidation of 2-(sulfanylmethyl)phenol 1 (E _p/2 = +1.55 V vs SCE).[12] [13] Then, the elimination of thiyl radical and proton from intermediate A generated o-QM, which would react with styrenes 2 to afford the cycloadduct 3. According to the blank experiment (Table [1], entry 9), the reduced photocatalyst (PC^•−) should be regenerated into the original photocatalyst (PC) via single-electron transfer with O₂ or thiyl radical.

In summary, we have developed an efficient method of organophotoredox-catalyzed in situ generation of o-QMs under visible-light irradiation for inverse-electron-demand [4+2] cycloadditions. The o-QMs were generated through one-electron oxidation of 2-(sulfanylmethyl)phenol by the thioxanthylium photoredox catalyst, which reacted with styrenes to produce polysubstituted chromanes with high regioselectivity and good yields. We believe that this reaction provides a promising tool for the oxidative generation of o-QMs and may have important applications in synthetic organic chemistry.

Conflict of Interest

The authors declare no conflict of interest.

Acknowledgment

We thank Prof. Mahito Atobe of Yokohama National University for cyclic voltammetry measurements. We also thank Center for Analytical Instrumentation, Chiba University for HRMS measurements.

• References and Notes

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  Crossref
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  Crossref
• 2d Chiba K, Yamaguchi Y, Tada M. Tetrahedron Lett. 1998; 39: 9035
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• 2e Chiba K, Hirano T, Kitano Y, Tada M. Chem. Commun. 1999; 691
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• 6a Tanaka K, Ueno K, Tanaka Y, Ohtsuka N, Asada Y, Kishimoto M, Sunaga S, Hoshino Y, Honda K. Synlett 2020; 31: 1197
  Thieme ConnectSearch in Google Scholar
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  Thieme ConnectSearch in Google Scholar
• 6d Tanaka K, Hoshino Y, Honda K. J. Synth. Org. Chem. Jpn. 2018; 76: 1341
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• 6e Tanaka K, Kishimoto M, Hoshino Y, Honda K. Tetrahedron Lett. 2018; 59: 1841
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• 6g Tanaka K, Hoshino Y, Honda K. Heterocycles 2017; 95: 474
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• 6h Tanaka K, Hoshino Y, Honda K. Tetrahedron Lett. 2016; 57: 2448
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• 6i Tanaka K, Shigematsu Y, Sukekawa M, Hoshino Y, Honda K. Tetrahedron Lett. 2016; 57: 5914
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• 6j Tanaka K, Sukekawa M, Hoshino Y, Honda K. Chem. Lett. 2018; 47: 440
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• 6k Tanaka K, Sukekawa M, Kishimoto M, Hoshino Y, Honda K. Heterocycles 2019; 99: 145
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• 7a Tanaka K, Asada Y, Hoshino Y. Chem. Commun. 2022; 58: 2476
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• 7b Tanaka K, Kishimoto M, Tanaka Y, Kamiyama Y, Asada Y, Sukekawa M, Ohtsuka N, Suzuki T, Momiyama N, Honda K, Hoshino Y. J. Org. Chem. 2022; 87: 3319
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• 7d Tanaka K, Asada Y, Hoshino Y, Honda K. Org. Biomol. Chem. 2020; 18: 8074
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• 7e Tanaka K, Hoshino Y, Honda K. J. Jpn. Soc. Colour Mater. 2020; 93: 49
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• 7f Tanaka K, Omata D, Asada Y, Hoshino Y, Honda K. J. Org. Chem. 2019; 84: 10669
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• 7g Tanaka K, Tanaka Y, Kishimoto M, Hoshino Y, Honda K. Beilstein J. Org. Chem. 2019; 15: 2105
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• 7h Tanaka K, Sukekawa M, Kishimoto M, Hoshino Y, Honda K. Tetrahedron Lett. 2018; 59: 3361
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• 8a Selenski C, Pettus TR. R. J. Org. Chem. 2004; 69: 9196
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• 8c Marsini MA, Huang Y, Lindsey CC, Wu K.-L, Pettus TR. R. Org. Lett. 2008; 10: 1477
  CrossrefPubMedSearch in Google Scholar
• 9 Since it was difficult to separate the diastereomers in our system, the diastereomeric ratio of 3 was determined by ¹H NMR analysis comparing the benzylic peaks at the C2 position of chroman. Each of the diastereomers of 3a was confirmed by comparison with the spectral data described in the literature, see: Pagar VV, Tseng C.-C, Liu R.-S. Chem. Eur. J. 2014; 20: 10519
  CrossrefPubMedSearch in Google Scholar
• 10 Yamaguchi T, Sugiura Y, Yamaguchi E, Tada N, Ito A. Asian J. Org. Chem. 2017; 6: 432
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• 11 2,4-Diphenylchoroman (3a): Typical Procedure 2-(Sulfanylmethyl)phenol (1a, 58.3 mg, 0.20 mmol), styrene (2a, 62.5 mg, 0.60 mmol), TXT (5.0 mg, 9.0 μmol, 5.0 mol%), DCE (2.0 mL), and TFE (2.0 mL) were added into an 8 mL borosilicate vial. The resulting solution was stirred at room temperature under air and green LED irradiation for 24 h. The desired cycloadduct 3a was isolated by column chromatography on silica gel (hexane/ethyl acetate = 50:1). (2S*,4R*)- and (2R*,4R*)-2,4-Diphenylchromane (3a) White solid (42.4 mg, 74% yield). ¹H NMR (500 MHz, CDCl₃): δ = 7.52–7.48 (m, 2 H) (major), 7.42–7.38 (m, 2 H) (mixture), 7.37–7.29 (m, 8 H) (mixture), 7.29–7.19 (m, 6 H) (mixture), 7.17–7.12 (m, 2 H) (mixture), 7.02 (ddd, J = 14.1, 7.9, 1.5 Hz, 2 H) (minor), 6.97–6.95 (m, 1 H) (major), 6.90 (td, J = 7.4, 1.1 Hz, 1 H) (minor), 6.83–6.76 (m, 2 H) (major), 5.23 (dd, J = 11.2, 2.1 Hz, 1 H) (major), 5.05 (dd, J = 10.5, 2.3 Hz, 1 H) (minor), 4.37 (q, J = 6.1 Hz, 1 H) (major), 4.25 (q, J = 2.9 Hz, 1 H) (mixture), 2.51–2.46 (m, 1 H) (minor), 2.42 (ddd, J = 13.7, 5.7, 2.1 Hz, 1 H) (major), 2.33–2.24 (m, 1 H) (major). ¹³C{¹H} NMR (126 MHz, CDCl₃) (mixture): δ = 155.5, 155.4, 146.0, 144.5, 141.4, 141.2, 130.8, 129.8, 128.6, 128.6, 128.4, 128.1, 128.0, 127.7, 126.8, 126.4, 126.1, 126.0, 125.7, 123.1, 120.6, 120.5, 117.0, 117.0, 78.1, 73.2, 43.5, 40.6, 40.2, 38.3.
• 12 Lyu J, Claraz A, Vitale MR, Allain C, Masson G. J. Org. Chem. 2020; 85: 12843
  CrossrefPubMedSearch in Google Scholar
• 13 See the Supporting Information.

Corresponding Authors

Publication History

Received: 20 June 2023

Accepted after revision: 22 August 2023

Accepted Manuscript online:
22 August 2023

Article published online:
09 October 2023

© 2023. Thieme. All rights reserved

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• References and Notes

• 1a Bai W.-J, David JG, Feng Z.-G, Weaver MG, Wu K.-L, Pettus TR. R. Acc. Chem. Res. 2014; 47: 3655
  CrossrefPubMedSearch in Google Scholar
• 1b Sharma A, Hazarika H, Gogoi P. J. Org. Chem. 2021; 86: 4883
  CrossrefPubMedSearch in Google Scholar
• 1c Barta P, Fülöp F, Szatmári I. Beilstein J. Org. Chem. 2018; 14: 560
  CrossrefPubMedSearch in Google Scholar
• 1d Singh MS, Nagaraju A, Anand N, Chowdhury S. RSC Adv. 2014; 4: 55924
  CrossrefSearch in Google Scholar
• 2a Inoue T, Inoue S, Sato K. Chem. Lett. 1989; 18: 653
  CrossrefSearch in Google Scholar
• 2b Chiba K, Sonoyama J, Tada M. J. Chem. Soc., Chem. Commun. 1995; 1381
  Crossref
• 2c Chiba K, Sonoyama J, Tada M. J. Chem. Soc., Perkin Trans. 1 1996; 1435
  Crossref
• 2d Chiba K, Yamaguchi Y, Tada M. Tetrahedron Lett. 1998; 39: 9035
  CrossrefSearch in Google Scholar
• 2e Chiba K, Hirano T, Kitano Y, Tada M. Chem. Commun. 1999; 691
  Crossref
• 2f Uyanik M, Nishioka K, Kondo R, Ishihara K. Nat. Chem. 2020; 12: 353
  CrossrefPubMedSearch in Google Scholar
• 3a Le T, Galmiche L, Masson G, Allaina C, Audebert P. Chem. Commun. 2020; 56: 10742
  CrossrefPubMedSearch in Google Scholar
• 3b Lyu J, Claraz A, Retailleau P, Masson G. Org. Biomol. Chem. 2022; 20: 9593
  CrossrefPubMedSearch in Google Scholar
• 4a Arumugam S, Popik VV. J. Am. Chem. Soc. 2011; 133: 5573
  CrossrefPubMedSearch in Google Scholar
• 4b Fujiwara M, Sakamoto M, Komeyama K, Yoshida H, Takaki K. J. Heterocycl. Chem. 2015; 52: 59
  CrossrefSearch in Google Scholar
• 4c Nakatani K, Higashida N, Saito I. Tetrahedron Lett. 1997; 38: 5005
  CrossrefSearch in Google Scholar
• 4d Škalamera Đ, Mlinarić-Majerski K, Martin-Kleiner I, Kralj M, Wan P, Basarić N. J. Org. Chem. 2014; 79: 4390
  CrossrefPubMedSearch in Google Scholar
• 5 Zhou F, Cheng Y, Liu X.-P, Chen J.-R, Xiao W.-J. Chem. Commun. 2019; 55: 3117
  CrossrefPubMedSearch in Google Scholar
• 6a Tanaka K, Ueno K, Tanaka Y, Ohtsuka N, Asada Y, Kishimoto M, Sunaga S, Hoshino Y, Honda K. Synlett 2020; 31: 1197
  Thieme ConnectSearch in Google Scholar
• 6b Tanaka K, Kishimoto M, Asada Y, Hoshino Y, Honda K. J. Org. Chem. 2019; 84: 13858
  CrossrefPubMedSearch in Google Scholar
• 6c Tanaka K, Kishimoto M, Ohtsuka N, Iwama Y, Wada H, Hoshino Y, Honda K. Synlett 2019; 30: 189
  Thieme ConnectSearch in Google Scholar
• 6d Tanaka K, Hoshino Y, Honda K. J. Synth. Org. Chem. Jpn. 2018; 76: 1341
  CrossrefSearch in Google Scholar
• 6e Tanaka K, Kishimoto M, Hoshino Y, Honda K. Tetrahedron Lett. 2018; 59: 1841
  CrossrefSearch in Google Scholar
• 6f Tanaka K, Sukekawa M, Shigematsu Y, Hoshino Y, Honda K. Tetrahedron 2017; 73: 6456
  CrossrefSearch in Google Scholar
• 6g Tanaka K, Hoshino Y, Honda K. Heterocycles 2017; 95: 474
  CrossrefSearch in Google Scholar
• 6h Tanaka K, Hoshino Y, Honda K. Tetrahedron Lett. 2016; 57: 2448
  CrossrefSearch in Google Scholar
• 6i Tanaka K, Shigematsu Y, Sukekawa M, Hoshino Y, Honda K. Tetrahedron Lett. 2016; 57: 5914
  CrossrefSearch in Google Scholar
• 6j Tanaka K, Sukekawa M, Hoshino Y, Honda K. Chem. Lett. 2018; 47: 440
  CrossrefSearch in Google Scholar
• 6k Tanaka K, Sukekawa M, Kishimoto M, Hoshino Y, Honda K. Heterocycles 2019; 99: 145
  CrossrefSearch in Google Scholar
• 7a Tanaka K, Asada Y, Hoshino Y. Chem. Commun. 2022; 58: 2476
  CrossrefPubMedSearch in Google Scholar
• 7b Tanaka K, Kishimoto M, Tanaka Y, Kamiyama Y, Asada Y, Sukekawa M, Ohtsuka N, Suzuki T, Momiyama N, Honda K, Hoshino Y. J. Org. Chem. 2022; 87: 3319
  CrossrefPubMedSearch in Google Scholar
• 7c Tanaka K, Iwama Y, Kishimoto M, Ohtsuka N, Hoshino Y, Honda K. Org. Lett. 2020; 22: 5207
  CrossrefPubMedSearch in Google Scholar
• 7d Tanaka K, Asada Y, Hoshino Y, Honda K. Org. Biomol. Chem. 2020; 18: 8074
  CrossrefPubMedSearch in Google Scholar
• 7e Tanaka K, Hoshino Y, Honda K. J. Jpn. Soc. Colour Mater. 2020; 93: 49
  CrossrefSearch in Google Scholar
• 7f Tanaka K, Omata D, Asada Y, Hoshino Y, Honda K. J. Org. Chem. 2019; 84: 10669
  CrossrefPubMedSearch in Google Scholar
• 7g Tanaka K, Tanaka Y, Kishimoto M, Hoshino Y, Honda K. Beilstein J. Org. Chem. 2019; 15: 2105
  CrossrefPubMedSearch in Google Scholar
• 7h Tanaka K, Sukekawa M, Kishimoto M, Hoshino Y, Honda K. Tetrahedron Lett. 2018; 59: 3361
  CrossrefSearch in Google Scholar
• 8a Selenski C, Pettus TR. R. J. Org. Chem. 2004; 69: 9196
  CrossrefPubMedSearch in Google Scholar
• 8b Jones RM, Selenski C, Pettus TR. R. J. Org. Chem. 2002; 67: 6911
  CrossrefPubMedSearch in Google Scholar
• 8c Marsini MA, Huang Y, Lindsey CC, Wu K.-L, Pettus TR. R. Org. Lett. 2008; 10: 1477
  CrossrefPubMedSearch in Google Scholar
• 9 Since it was difficult to separate the diastereomers in our system, the diastereomeric ratio of 3 was determined by ¹H NMR analysis comparing the benzylic peaks at the C2 position of chroman. Each of the diastereomers of 3a was confirmed by comparison with the spectral data described in the literature, see: Pagar VV, Tseng C.-C, Liu R.-S. Chem. Eur. J. 2014; 20: 10519
  CrossrefPubMedSearch in Google Scholar
• 10 Yamaguchi T, Sugiura Y, Yamaguchi E, Tada N, Ito A. Asian J. Org. Chem. 2017; 6: 432
  CrossrefPubMedSearch in Google Scholar
• 11 2,4-Diphenylchoroman (3a): Typical Procedure 2-(Sulfanylmethyl)phenol (1a, 58.3 mg, 0.20 mmol), styrene (2a, 62.5 mg, 0.60 mmol), TXT (5.0 mg, 9.0 μmol, 5.0 mol%), DCE (2.0 mL), and TFE (2.0 mL) were added into an 8 mL borosilicate vial. The resulting solution was stirred at room temperature under air and green LED irradiation for 24 h. The desired cycloadduct 3a was isolated by column chromatography on silica gel (hexane/ethyl acetate = 50:1). (2S*,4R*)- and (2R*,4R*)-2,4-Diphenylchromane (3a) White solid (42.4 mg, 74% yield). ¹H NMR (500 MHz, CDCl₃): δ = 7.52–7.48 (m, 2 H) (major), 7.42–7.38 (m, 2 H) (mixture), 7.37–7.29 (m, 8 H) (mixture), 7.29–7.19 (m, 6 H) (mixture), 7.17–7.12 (m, 2 H) (mixture), 7.02 (ddd, J = 14.1, 7.9, 1.5 Hz, 2 H) (minor), 6.97–6.95 (m, 1 H) (major), 6.90 (td, J = 7.4, 1.1 Hz, 1 H) (minor), 6.83–6.76 (m, 2 H) (major), 5.23 (dd, J = 11.2, 2.1 Hz, 1 H) (major), 5.05 (dd, J = 10.5, 2.3 Hz, 1 H) (minor), 4.37 (q, J = 6.1 Hz, 1 H) (major), 4.25 (q, J = 2.9 Hz, 1 H) (mixture), 2.51–2.46 (m, 1 H) (minor), 2.42 (ddd, J = 13.7, 5.7, 2.1 Hz, 1 H) (major), 2.33–2.24 (m, 1 H) (major). ¹³C{¹H} NMR (126 MHz, CDCl₃) (mixture): δ = 155.5, 155.4, 146.0, 144.5, 141.4, 141.2, 130.8, 129.8, 128.6, 128.6, 128.4, 128.1, 128.0, 127.7, 126.8, 126.4, 126.1, 126.0, 125.7, 123.1, 120.6, 120.5, 117.0, 117.0, 78.1, 73.2, 43.5, 40.6, 40.2, 38.3.
• 12 Lyu J, Claraz A, Vitale MR, Allain C, Masson G. J. Org. Chem. 2020; 85: 12843
  CrossrefPubMedSearch in Google Scholar
• 13 See the Supporting Information.

• Supplementary Material