Photocatalytic [2,3]-Sigmatropic Rearrangement Reactions of Ethyl Diazoacetate

Abstract

We describe a photocatalytic reaction of diazo compounds with allyl sulfides under visible-light reaction conditions. In the presence of Ru(bpy)₃Cl₂ as a photocatalyst, a [2,3]-sigmatropic rearrangement reaction occurs that leads to the formation of homoallylic sulfides. This reaction proceeds in acetone as the solvent, which is unusual in carbene-transfer reactions, and it shows a broad substrate scope in the rearrangement reaction of allylic sulfides.

Carbene-transfer reactions play a key role in organic synthesis, and find a broad spectrum of applications, ranging from cycloaddition reactions, C–H or X–H functionalization reactions, and rearrangement reactions to multicomponent reactions (Scheme [1a]).[1] [2] To achieve such carbene-transfer reactions, metal catalysts are commonly employed to access the carbene fragment, to control its reactivity, and to enable chemoselective transformations.[1,2] Diazo compounds are among the most versatile reagents to access the carbene fragment through loss of nitrogen and formation of a metal-bound carbene intermediate.

In recent years, however, visible-light-mediated transformations have attracted significant interest in organic synthesis as a means of conducting green and sustainable synthesis (Scheme [1b]).[3] [4] Among these, the photolysis reaction of diazo compounds plays a central role in the reaction of strongly colored donor/acceptor diazo compounds.[3] When it comes to less strongly colored acceptor-only diazo compounds, however, direct photolysis becomes less attractive; instead, triplet sensitization with an appropriate photosensitizer is more favorable.[4–12] Recent reports by several research groups appear to show that such triplet sensitization leads to the formation of a triplet carbene intermediate,[5] [6] [7] [8] [9] [10] [11] [12] as witnessed, for example, in stereoconvergent cyclopropanation reactions[11] and gem-difluoroolefination reactions.[12] A recent report by Koenigs and Gryko provided details on the solvent dependency of this photosensitization and showed that aprotic solvents result in favorable triplet carbene formation, whereas protic solvents result in a more-favorable proton-coupled electron transfer and radical generation.[10]

Table 1 Reaction Optimization^a

Entry	Photocatalyst (mol%)	Solvent	Yieldb (%) of 9a
1	4-CzIPN (3)	CH2Cl2	NRc,d
2	eosin Y (3)	CH2Cl2	NRd
3	fluorescein (3)	CH2Cl2	NRd
4	Ir(ppy)3 (3)	CH2Cl2	traced
5	Ru(bpy)3Cl2 (3)	CH2Cl2	87
6	Ru(bpy)3Cl2 (1)	CH2Cl2	58
7	Ru(bpy)3Cl2 (1)	DCE	58
8	Ru(bpy)3Cl2 (1)	CHCl3	62
9	Ru(bpy)3Cl2 (1)	THF	traced
10	Ru(bpy)3Cl2 (1)	MeCN	traced
11	Ru(bpy)3Cl2 (1)	acetone	60
12	Ru(bpy)3Cl2 (3)	acetone	96

^a Reaction conditions: ethyl diazoacetate (6a; 0.2 mmol, 1.0 equiv), allyl phenyl sulfides (7a; 0.6 mmol, 3.0 equiv), solvent (2.0 mL), 2 × 40 W blue LEDs (λ = 467 nm) from a distance of 4 cm, under Ar, r.t., 16 h.

^b Isolated yield.

^c NR = no reaction.

^d Analyzed by ¹H NMR spectroscopy with mesitylene as an internal standard.

Against this background, we considered that the influence of heavy atoms might result in a distinct pathway and lead to singlet carbene reactivity (Scheme [1c]). Specifically, we hypothesized that in the presence of a heavy atom, the intersystem crossing from the intermittently formed triplet carbene to a singlet carbene would be favored. For this purpose, we considered organosulfur compounds, which are a well-studied class of substrates in rearrangement reactions with carbenes or arynes.[2] [13] [14] [15] Herein, we report on our studies on the photocatalytic reaction of acceptor-only diazo alkanes in Doyle–Kirmse rearrangement reactions.

To assess the above hypothesis, we first evaluated the influence of various archetypal photocatalysts in the reaction of allyl phenyl sulfide (7a) with ethyl diazoacetate (6a) in CH₂Cl₂ as the solvent. Most catalysts, however, proved inefficient, and the desired reaction product 9a was not obtained (Table [1], entries 1–4). A surprising and unexpected observation was made when Ru(bpy)₃Cl₂ was used as the catalyst. In this case, the [2,3]-sigmatropic rearrangement reaction proceeded in high yield (entry 5). Further optimization involved the evaluation of various solvents. In line with previous reports on photochemical or photocatalytic carbene-transfer reactions, chlorinated solvents proved compatible (entries 6–8), whereas nucleophilic solvents, such as THF or acetonitrile, were found to be incompatible (entries 9 and 10). A very surprising observation, however, was made when acetone was used as the solvent (entry 11). In this case, the rearrangement reaction proceeded in good yield and, given the safety and environmental hazards associated with chlorinated solvents, we decided to further evaluate this photocatalytic Doyle–Kirmse rearrangement reaction in acetone solvent.

With the optimized conditions in hand, we next studied the substrate scope, and examined the compatibility of various allylic sulfides (Scheme [2]).[16] We first examined allyl para-substituted-aryl sulfides and we obtained the corresponding rearrangement products 9b–f in good yields. Similarly, meta- or ortho-substitution of the aromatic ring was tolerated (9g–r). In this context, various simple aliphatic substituents such as methyl, tert-butyl, or halo groups were well tolerated. The presence of an ether or ester substituent had a slightly detrimental effect on the product yield, which we assume to be related to competitive ylide formation. Similarly, the trifluoromethyl group was found to be less well tolerated, probably due to reduced nucleophilicity of the sulfur atom. In the course of further evaluation, we observed that the mesityl-substituted allylic sulfide 9s similarly underwent rearrangement with a comparable yield to mono-ortho-substituted aromatic allylic sulfides. The limitation of this method lies in its compatibility toward aromatic heterocycles. A low yield of the rearrangement product 9t was obtained from allyl 2-thienyl sulfide, whereas N-heterocycles were found to be incompatible. Next, the influence of the rearranging group was studied. In the case of a propargyl sulfide and a methylallyl sulfide, the desired rearrangement products were obtained in good yields.

We next examined the scope of various diazo compounds in this photocatalytic rearrangement reaction (Scheme [3]). Various aralkyl diazoacetates gave the desired rearrangement products, but in significantly lower yields. When comparing the reaction of benzyl diazoacetate with that of 2-phenylethyl diazoacetate, a significant difference in the yield of the rearrangement product was observed, which may be related to an undesired C–H functionalization reaction in the benzylic position. Further exploration involved the evaluation of an example of a diazoalkane bearing a protected sugar (9z), various donor–acceptor diazoalkanes (10a and 10b), and an example of a diazo ketone (11). In each case, a moderate yield of the desired rearrangement product was obtained.

For applications of this protocol in rearrangement reactions, we explored the [1,2]-sigmatropic rearrangement of benzyl phenyl sulfide (12) and the Sommelet–Hauser rearrangement of ethyl 2-(phenylsulfanyl)acetate (14). In the photocatalytic reaction of 12 with ethyl diazoacetate, however, only a low yield of the [1,2]-rearrangement product 13 was obtained. The Sommelet–Hauser rearrangement of 14 did not take place; in this case, only the decomposition of the ethyl diazoacetate occurred.

Finally, we examined control reactions to assess the mechanism of this photocatalytic rearrangement reaction (Scheme [4]). In the case of the cinnamyl phenyl sulfide (7b), the [2,3]-sigmatropic rearrangement reaction occurred in good yield to give product 9a in a 1:1 diastereomeric ratio. This suggests that this photocatalytic reaction occurs through a mechanism similar to metal-catalyzed or photochemical [2,3]-sigmatropic rearrangement reactions.[13] [15] Further control experiments involved a competition reaction with allyl phenyl sulfide (7a) and β-methylstyrene (15). The latter can serve as a probe to assess the spin state of the carbene intermediate. In this case, however, we only observed the product of the rearrangement reaction, which might be explained by a heavy-atom-induced rapid intersystem crossing after triplet sensitization, leading to selective ylide formation and rearrangement.

In summary, we report the photocatalytic reaction of various diazo compounds with organosulfur compounds. In the presence of a simple and commercially available ruthenium catalyst, [2,3]-sigmatropic rearrangement reactions can be achieved with moderate to high yields.

Conflict of Interest

The authors declare no conflict of interest.

• References and Notes

• 1a Doyle MP, Duffy R, Ratnikov M, Zhou L. Chem. Rev. 2010; 110: 704
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• 1b Davies HM. L, Morton D. Chem. Soc. Rev. 2011; 40: 1857
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• 1c Lu H, Zhang XP. Chem. Soc. Rev. 2011; 40: 1899
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• 1d Ford A, Miel H, Ring A, Slattery CN, Maguire AR, McKervey MA. Chem. Rev. 2015; 115: 9981
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• 1e Davies HM. L, Liao K. Nat. Rev. Chem. 2019; 3: 347
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• 1f Empel C, Jana S, Koenigs RM. Molecules 2020; 25: 880
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• 1g Wang J, Qiu D. Recent Developments of Diazo Compounds in Organic Synthesis. World Scientific; Singapore: 2021
  Search in Google Scholar
• 2a Vanecko JA, Wan H, West FG. Tetrahedron 2006; 62: 1043
  CrossrefSearch in Google Scholar
• 2b Zhang Y, Wang J. Coord. Chem. Rev. 2010; 254: 941
  CrossrefSearch in Google Scholar
• 2c West TH, Spoehrle SS. M, Kasten K, Taylor JE. Smith A. D. ACS Catal. 2015; 5: 7446
  Search in Google Scholar
• 2d Sheng Z, Zhang Z, Chu C, Zhang Y, Wang J. Tetrahedron 2017; 73: 4011
  CrossrefSearch in Google Scholar
• 2e Zhang X.-M, Tu Y.-Q, Zhang F.-M, Chen Z.-H, Wang S.-H. Chem. Soc. Rev. 2017; 46: 2272
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• 2f Jana S, Guo Y, Koenigs RM. Chem. Eur. J. 2021; 27: 1270
  Search in Google Scholar
• 2g Empel C, Jana S, Koenigs RM. Synthesis 2021; 53: 4567
  Thieme ConnectSearch in Google Scholar
• 3a Ciszewski ŁW, Rybicka-Jasińska K, Gryko D. Org. Biomol. Chem. 2019; 17: 432
  CrossrefPubMedSearch in Google Scholar
• 3b Yang Z, Stivanin ML, Jurberg ID, Koenigs RM. Chem. Soc. Rev. 2020; 49: 6833
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• 3c Durka J, Turkowska J, Gryko D. ACS Sustainable Chem. Eng. 2021; 9: 8895
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• 3d Chen Z, Xie Y, Xuan J. Eur. J. Org. Chem. 2022; e202201066
  Crossref
• 4 Empel C, Pei C, Koenigs RM. Chem. Commun. 2022; 58: 2788
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• 5 Rybicka-Jasińska K, Shan W, Zawada K, Kadish KM, Gryko D. J. Am. Chem. Soc. 2016; 138: 15451
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• 6 Rybicka-Jasinska K, Ciszewski ŁW, Gryko D. Adv. Synth. Catal. 2016; 358: 1671
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• 7 Ye H.-B, Zhou X.-Y, Li L, He X.-K, Xuan J. Org. Lett. 2022; 24: 6018
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• 8 Ye H.-B, Bao Y.-P, Liu T.-Y, Wei T, Yang C, Liu Q.-A, Xuan J. Tetrahedron Chem 2023; 7: 100040
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• 9 Li W, Li S, Empel C, Koenigs RM, Zhou L. Angew. Chem. Int. Ed. 2023; 62: e202309947
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• 10 Empel C, Jana S, Ciszewski ŁW, Zawada K, Pei C, Gryko D, Koenigs RM. Chem. Eur. J. 2023; 29: e202300214
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• 11 Langletz T, Empel C, Jana S, Koenigs RM. Tetrahedron Chem 2022; 3: 100024
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• 12 Li F, Pei C, Koenigs RM. Angew. Chem. Int. Ed. 2022; 61: e202111892
  CrossrefPubMedSearch in Google Scholar

For selected references on [2,3]-sigmatropic rearrangement reactions of light chalcogenonium ylides, see:
• 13a Kirmse W, Kapps M. Chem. Ber. 1968; 101: 994
  CrossrefSearch in Google Scholar
• 13b Doyle MP, Tamblyn WH, Bagheri VJ. J. Org. Chem. 1981; 46: 5094
  CrossrefSearch in Google Scholar
• 13c McMillen DW, Varga N, Reed BA, King C. J. Org. Chem. 2000; 65: 2532
  CrossrefPubMedSearch in Google Scholar
• 13d Simmoneaux G, Galardon E, Paul-Roth C, Gulea M, Masson S. J. Organomet. Chem. 2001; 617–618: 360
  CrossrefSearch in Google Scholar
• 13e Liao M, Wang J. Green Chem. 2007; 9: 184
  CrossrefSearch in Google Scholar
• 13f Li Z, Boyarskikh V, Hansen JH, Autschbach J, Musaev DG, Davies HM. L. J. Am. Chem. Soc. 2012; 134: 15497
  CrossrefPubMedSearch in Google Scholar
• 13g Holzwarth MS, Alt I, Plietker B. Angew. Chem. Int. Ed. 2012; 51: 5351
  CrossrefPubMedSearch in Google Scholar
• 13h Xu X, Li C, Tao Z, Pan Y. Green Chem. 2017; 19: 1245
  CrossrefSearch in Google Scholar
• 13i Hock KJ, Mertens L, Hommelsheim R, Spitzner R, Koenigs RM. Chem. Commun. 2017; 53: 6577
  CrossrefPubMedSearch in Google Scholar
• 13j Dairo TO, Woo LK. Organometallics 2017; 36: 927
  CrossrefSearch in Google Scholar
• 13k Zhang Z, Sheng Z, Yu W, Zhang R, Chu W.-D, Zhang Y, Wang J. Nat. Chem. 2017; 9: 970
  Search in Google Scholar

For sigmatropic rearrangement reactions with arynes, see:
• 14a Tan J, Zheng T, Xu K, Liu C. Org. Biomol. Chem. 2017; 15: 4946
  CrossrefPubMedSearch in Google Scholar
• 14b Xu X.-B, Lin Z.-H, Liu Y, Guo J, He Y. Org. Biomol. Chem. 2017; 15: 2716
  CrossrefPubMedSearch in Google Scholar
• 14c Gaykar RN, George M, Guin A, Bhattacharjee S, Biju AT. Org. Lett. 2021; 23: 3447
  CrossrefPubMedSearch in Google Scholar
• 14d Reddy RS, Lagishetti C, Kiran IN. C, You H, He Y. Org. Lett. 2016; 18: 3818
  CrossrefPubMedSearch in Google Scholar
• 15 Hock KJ, Koenigs RM. Angew. Chem. Int. Ed. 2017; 56: 13566
  CrossrefPubMedSearch in Google Scholar
• 16 Photocatalytic Rearrangement Reactions: General Procedure The appropriate diazoalkane (0.2 mmol, 1.0 equiv) and allyl aryl sulfide (0.6 mmol, 3.0 equiv) were dissolved in acetone (2 mL), and the solution was irradiated with a module containing two blue LEDs (2 × 40 W, λ = 467 nm) overnight under Ar. When the reaction was complete, the solvent was removed under reduced pressure, and the product was purified by column chromatography (silica gel, hexane–EtOAc). Ethyl 2-(Phenylsulfanyl)pent-4-enoate (9a) Prepared according to the general procedure and purified by column chromatography [silica gel, hexane–EtOAc (80:1 to 60:1 to 40:1)] as a colorless oil; yield: 45.4 mg (96%). ¹H NMR (600 MHz, CDCl₃): δ = 7.33–7.23 (m, 2 H), 7.19–7.03 (m, 3 H), 5.66 (ddt, J = 27.3, 17.1, 10.2, 6.9 Hz, 1 H), 5.00–4.84 (m, 2 H), 3.93 (qd, J = 7.1, 1.7 Hz, 2 H), 3.52 (dd, J = 8.7, 6.4 Hz, 1 H), 2.49–2.41 (m, J = 14.4, 8.4, 7.0, 1.3 Hz, 1 H), 2.38–2.29 (m, 1 H), 0.98 (t, J = 7.1 Hz, 3 H). ¹³C NMR (151 MHz, CDCl₃): δ = 171.6, 133.9, 133.1, 128.9, 128.0, 118.0, 61.1, 50.2, 35.8, 14.0. These data agree with those reported in the literature.^13h

Corresponding Authors

Publication History

Received: 30 September 2023

Accepted after revision: 30 October 2023

Accepted Manuscript online:
30 October 2023

Article published online:
20 December 2023

© 2023. Thieme. All rights reserved

Georg Thieme Verlag KG
Rüdigerstraße 14, 70469 Stuttgart, Germany

• References and Notes

• 1a Doyle MP, Duffy R, Ratnikov M, Zhou L. Chem. Rev. 2010; 110: 704
  CrossrefPubMedSearch in Google Scholar
• 1b Davies HM. L, Morton D. Chem. Soc. Rev. 2011; 40: 1857
  CrossrefPubMedSearch in Google Scholar
• 1c Lu H, Zhang XP. Chem. Soc. Rev. 2011; 40: 1899
  CrossrefPubMedSearch in Google Scholar
• 1d Ford A, Miel H, Ring A, Slattery CN, Maguire AR, McKervey MA. Chem. Rev. 2015; 115: 9981
  CrossrefPubMedSearch in Google Scholar
• 1e Davies HM. L, Liao K. Nat. Rev. Chem. 2019; 3: 347
  CrossrefPubMedSearch in Google Scholar
• 1f Empel C, Jana S, Koenigs RM. Molecules 2020; 25: 880
  CrossrefPubMedSearch in Google Scholar
• 1g Wang J, Qiu D. Recent Developments of Diazo Compounds in Organic Synthesis. World Scientific; Singapore: 2021
  Search in Google Scholar
• 2a Vanecko JA, Wan H, West FG. Tetrahedron 2006; 62: 1043
  CrossrefSearch in Google Scholar
• 2b Zhang Y, Wang J. Coord. Chem. Rev. 2010; 254: 941
  CrossrefSearch in Google Scholar
• 2c West TH, Spoehrle SS. M, Kasten K, Taylor JE. Smith A. D. ACS Catal. 2015; 5: 7446
  Search in Google Scholar
• 2d Sheng Z, Zhang Z, Chu C, Zhang Y, Wang J. Tetrahedron 2017; 73: 4011
  CrossrefSearch in Google Scholar
• 2e Zhang X.-M, Tu Y.-Q, Zhang F.-M, Chen Z.-H, Wang S.-H. Chem. Soc. Rev. 2017; 46: 2272
  CrossrefPubMedSearch in Google Scholar
• 2f Jana S, Guo Y, Koenigs RM. Chem. Eur. J. 2021; 27: 1270
  Search in Google Scholar
• 2g Empel C, Jana S, Koenigs RM. Synthesis 2021; 53: 4567
  Thieme ConnectSearch in Google Scholar
• 3a Ciszewski ŁW, Rybicka-Jasińska K, Gryko D. Org. Biomol. Chem. 2019; 17: 432
  CrossrefPubMedSearch in Google Scholar
• 3b Yang Z, Stivanin ML, Jurberg ID, Koenigs RM. Chem. Soc. Rev. 2020; 49: 6833
  CrossrefPubMedSearch in Google Scholar
• 3c Durka J, Turkowska J, Gryko D. ACS Sustainable Chem. Eng. 2021; 9: 8895
  CrossrefSearch in Google Scholar
• 3d Chen Z, Xie Y, Xuan J. Eur. J. Org. Chem. 2022; e202201066
  Crossref
• 4 Empel C, Pei C, Koenigs RM. Chem. Commun. 2022; 58: 2788
  CrossrefPubMedSearch in Google Scholar
• 5 Rybicka-Jasińska K, Shan W, Zawada K, Kadish KM, Gryko D. J. Am. Chem. Soc. 2016; 138: 15451
  CrossrefPubMedSearch in Google Scholar
• 6 Rybicka-Jasinska K, Ciszewski ŁW, Gryko D. Adv. Synth. Catal. 2016; 358: 1671
  CrossrefSearch in Google Scholar
• 7 Ye H.-B, Zhou X.-Y, Li L, He X.-K, Xuan J. Org. Lett. 2022; 24: 6018
  CrossrefPubMedSearch in Google Scholar
• 8 Ye H.-B, Bao Y.-P, Liu T.-Y, Wei T, Yang C, Liu Q.-A, Xuan J. Tetrahedron Chem 2023; 7: 100040
  CrossrefSearch in Google Scholar
• 9 Li W, Li S, Empel C, Koenigs RM, Zhou L. Angew. Chem. Int. Ed. 2023; 62: e202309947
  CrossrefPubMedSearch in Google Scholar
• 10 Empel C, Jana S, Ciszewski ŁW, Zawada K, Pei C, Gryko D, Koenigs RM. Chem. Eur. J. 2023; 29: e202300214
  CrossrefPubMedSearch in Google Scholar
• 11 Langletz T, Empel C, Jana S, Koenigs RM. Tetrahedron Chem 2022; 3: 100024
  CrossrefSearch in Google Scholar
• 12 Li F, Pei C, Koenigs RM. Angew. Chem. Int. Ed. 2022; 61: e202111892
  CrossrefPubMedSearch in Google Scholar

For selected references on [2,3]-sigmatropic rearrangement reactions of light chalcogenonium ylides, see:
• 13a Kirmse W, Kapps M. Chem. Ber. 1968; 101: 994
  CrossrefSearch in Google Scholar
• 13b Doyle MP, Tamblyn WH, Bagheri VJ. J. Org. Chem. 1981; 46: 5094
  CrossrefSearch in Google Scholar
• 13c McMillen DW, Varga N, Reed BA, King C. J. Org. Chem. 2000; 65: 2532
  CrossrefPubMedSearch in Google Scholar
• 13d Simmoneaux G, Galardon E, Paul-Roth C, Gulea M, Masson S. J. Organomet. Chem. 2001; 617–618: 360
  CrossrefSearch in Google Scholar
• 13e Liao M, Wang J. Green Chem. 2007; 9: 184
  CrossrefSearch in Google Scholar
• 13f Li Z, Boyarskikh V, Hansen JH, Autschbach J, Musaev DG, Davies HM. L. J. Am. Chem. Soc. 2012; 134: 15497
  CrossrefPubMedSearch in Google Scholar
• 13g Holzwarth MS, Alt I, Plietker B. Angew. Chem. Int. Ed. 2012; 51: 5351
  CrossrefPubMedSearch in Google Scholar
• 13h Xu X, Li C, Tao Z, Pan Y. Green Chem. 2017; 19: 1245
  CrossrefSearch in Google Scholar
• 13i Hock KJ, Mertens L, Hommelsheim R, Spitzner R, Koenigs RM. Chem. Commun. 2017; 53: 6577
  CrossrefPubMedSearch in Google Scholar
• 13j Dairo TO, Woo LK. Organometallics 2017; 36: 927
  CrossrefSearch in Google Scholar
• 13k Zhang Z, Sheng Z, Yu W, Zhang R, Chu W.-D, Zhang Y, Wang J. Nat. Chem. 2017; 9: 970
  Search in Google Scholar

For sigmatropic rearrangement reactions with arynes, see:
• 14a Tan J, Zheng T, Xu K, Liu C. Org. Biomol. Chem. 2017; 15: 4946
  CrossrefPubMedSearch in Google Scholar
• 14b Xu X.-B, Lin Z.-H, Liu Y, Guo J, He Y. Org. Biomol. Chem. 2017; 15: 2716
  CrossrefPubMedSearch in Google Scholar
• 14c Gaykar RN, George M, Guin A, Bhattacharjee S, Biju AT. Org. Lett. 2021; 23: 3447
  CrossrefPubMedSearch in Google Scholar
• 14d Reddy RS, Lagishetti C, Kiran IN. C, You H, He Y. Org. Lett. 2016; 18: 3818
  CrossrefPubMedSearch in Google Scholar
• 15 Hock KJ, Koenigs RM. Angew. Chem. Int. Ed. 2017; 56: 13566
  CrossrefPubMedSearch in Google Scholar
• 16 Photocatalytic Rearrangement Reactions: General Procedure The appropriate diazoalkane (0.2 mmol, 1.0 equiv) and allyl aryl sulfide (0.6 mmol, 3.0 equiv) were dissolved in acetone (2 mL), and the solution was irradiated with a module containing two blue LEDs (2 × 40 W, λ = 467 nm) overnight under Ar. When the reaction was complete, the solvent was removed under reduced pressure, and the product was purified by column chromatography (silica gel, hexane–EtOAc). Ethyl 2-(Phenylsulfanyl)pent-4-enoate (9a) Prepared according to the general procedure and purified by column chromatography [silica gel, hexane–EtOAc (80:1 to 60:1 to 40:1)] as a colorless oil; yield: 45.4 mg (96%). ¹H NMR (600 MHz, CDCl₃): δ = 7.33–7.23 (m, 2 H), 7.19–7.03 (m, 3 H), 5.66 (ddt, J = 27.3, 17.1, 10.2, 6.9 Hz, 1 H), 5.00–4.84 (m, 2 H), 3.93 (qd, J = 7.1, 1.7 Hz, 2 H), 3.52 (dd, J = 8.7, 6.4 Hz, 1 H), 2.49–2.41 (m, J = 14.4, 8.4, 7.0, 1.3 Hz, 1 H), 2.38–2.29 (m, 1 H), 0.98 (t, J = 7.1 Hz, 3 H). ¹³C NMR (151 MHz, CDCl₃): δ = 171.6, 133.9, 133.1, 128.9, 128.0, 118.0, 61.1, 50.2, 35.8, 14.0. These data agree with those reported in the literature.^13h

• Supplementary Material