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DOI: 10.1055/a-2192-4044
Ring Transformation of Annulated Benzofuran Derivatives to Medium-Sized Lactones
We gratefully acknowledge support by the Deutsche Forschungsgemeinschaft.
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Dedicated to Prof. Karl Heinz Dötz on the occasion of his 80th birthday
Abstract
Benzannulated eight- and nine-membered-ring lactones were prepared by ring transformation of annulated benzofuran derivatives. The reaction sequence starts with a Brønsted acid catalyzed hydration of the enol ether moiety of the starting materials furnishing hemiacetals as intermediate products which underwent retro-Claisen-type C–C bond cleavage under the reaction conditions. The use of 15 mol% TfOH and two equivalents of water in CHCl3 at 85 °C turned out to be optimal. Products with exocyclic carboxylate function could be further derivatized by amide formation. In total, eleven medium-sized lactones were prepared. Furthermore, three benzannulated spirolactones were obtained from starting materials with exocyclic carboxamide moiety.
Supporting Information
- Supporting information for this article is available online at https://doi.org/10.1055/a-2192-4044.
- Supporting Information
Publikationsverlauf
Eingereicht: 11. September 2023
Angenommen nach Revision: 16. Oktober 2023
Accepted Manuscript online:
16. Oktober 2023
Artikel online veröffentlicht:
22. November 2023
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References and Notes
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- 8 Typical Procedure: Ethyl 2-Oxo-3,4,5,6-tetrahydro-2H-benzo[b]oxocine-6-carboxylate (9a) Benzofuran 7a (115 mg, 0.500 mmol) and H2O (18 mg, 1.0 mmol) were added to a solution of TfOH (11 mg, 0.075 mmol) in CHCl3 (1 mL) in a reaction vial. More CHCl3 (3 mL) was added, the vial was tightly closed, and the mixture stirred at 85 °C for 16 h. After cooling to ambient temperature, all volatiles were removed in vacuum and the residue submitted to column chromatography (SiO2, hexanes/MTBE 3:1, Rf = 0.22) to furnish product 9a (97 mg, 0.39 mmol, 78%) as pale-yellow oil. 1H NMR (500 MHz, CDCl3): δ = 1.25 (t, J = 7.1 Hz, 3 H), 1.76 (pent, J = 7.6 Hz, 2 H), 1.98–2.08 (m, 2 H), 2.34 (t, J = 7.3 Hz, 2 H), 3.73 (t, J = 6.4 Hz, 1 H), 4.12 (q, J = 7.1 Hz, 2 H), 7.10 (d, J = 8.0 Hz, 1 H), 7.15 (t, J = 7.5 Hz, 1 H), 7.26–7.32 (m, 2 H) ppm. 13C{1H} NMR (125 MHz, CDCl3): δ = 14.18 (CH3), 21.27 (CH2), 30.34 (CH2), 33.75 (CH2), 43.14 (CH), 60.42 (CH2), 110.74 (CH), 124.14 (CH), 124.17 (CH), 126.94 (C), 128.86 (CH), 153.71 (C), 172.81 (C), 176.89 (C) ppm. IR (ATR): 2980 (w), 2933 (w), 2872 (w), 1804 (s), 1727 (s), 1479 (m), 1463 (s), 1230 (m), 1197 (m), 1179 (m), 1152 (m), 1123 (m), 1064 (m), 1040 (s), 1029 (s), 891 (m), 751 (vs) cm–1. HRMS (ESI, pos. mode): m/z calcd 249.1127 (for C14H17O4 +); found: 249.1121 [M + H+]; C14H16O4 (248.28 g mol–1).
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