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Synthetic Study on Lactonamycins, Part 2: Stereoselective Access to ABCD-Ring SystemThis work was supported by Grants-in-Aid from the Japan Society for the Promotion of Science (JSPS) (Nos. JP16H06351 and JP18K06548).
Received: 28 May 2020
Accepted after revision: 18 June 2020
21 July 2020 (online)
Dedicated to the memory of Prof. Hidetoshi Yamada
Toward a stereoselective total synthesis of the lactonamycins, we recently reported an approach to the DEF-ring system. Here we report a model study for constructing the ABCD-ring system, revealing a viable approach through (1) construction of the C-ring by asymmetric benzoin cyclization, (2) introduction of an angular hydroxy group through oxidation of an isoxazolium salt, and (3) construction of the AB rings through a ring-opening/closing sequence.
Key wordsnatural products - total synthesis - lactonamycins - aromatic polyketides - isoxazoles - benzoin cyclization
References and Notes
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- 28 These two steps could be carried out in one pot and, after the ring opening of 17, direct addition of PhSH and Et3N gave 19 in 83% yield (two steps from 17), a better yield than that of the stepwise protocol (see above; 51% yield, two steps).Sulfide 19; One-Pot ProtocolEster 17 (504 mg, 0.790 mmol) was azeotropically dried with toluene and dissolved in CH2Cl2 (24 mL). Et3N (1.32 mL, 9.39 mmol) and Me3SiOTf (850 μL, 4.70 mmol) were added at 0 °C and the mixture was stirred for 17 h at rt. PhSH (160 μL, 1.57 mmol) was then added at 0 °C, and the resulting mixture was stirred for 1 h at rt. Additional portions of Et3N (110 μL, 0.783 mmol) and PhSH (40 μL, 0.392 mmol) were added at 0 °C, and the mixture was again stirred for 1 h at rt. The reaction was quenched by adding sat. aq NaHCO3, and the product was extracted with CH2Cl2 (×3). The combined organic extracts were washed with brine, dried (Na2SO4), and concentrated in vacuo. The residue was purified chromatographically by using a Smart Flash EPCLC W-Prep 2XY system [Ultra Pack Diol-40B, hexane–EtOAc (77:23 to 56:44)] to give a yellow amorphous solid; yield: 538 mg (83%); mp 64–66 °C; Rf = 0.43 (hexane–EtOAc, 2:1).IR (ATR): 2934, 1755, 1727, 1593, 1470, 1370, 1273, 1254, 1149, 1039 cm–1. 1H NMR (600 MHz, CDCl3): δ = –0.09 (s, 9 H), 2.79–2.96 (m, 3 H), 3.11 (br ddd, J = 16.8, 10.8, 3.8 Hz, 1 H), 3.80 (br d, J = 10.5 Hz, 1 H), 3.91 (s, 3 H), 4.20 (br d, J = 10.5 Hz, 1 H), 4.65 (br d, J = 9.9 Hz, 1 H), 4.75 (br d, J = 9.9 Hz, 1 H), 4.90 (br d, J = 10.2 Hz, 1 H), 4.93 (br d, J = 10.2 Hz, 1 H), 5.30 (s, 1 H), 6.96–7.11 (m, 7 H), 7.21–7.33 (m, 10 H), 7.54 (dd, J = 8.1, 7.8 Hz, 1 H), 8.17 (d, J = 17.4 Hz, 2 H), 8.31 (d, J = 17.4 Hz, 2 H). 13C NMR (150 MHz, CDCl3): δ = –0.1, 27.2, 42.1, 56.4, 60.6, 67.5, 74.7, 77.8, 84.2, 92.7, 112.5, 120.8, 123.7, 125.6, 127.8, 128.1, 128.17, 128.19, 128.4, 128.5, 128.6, 128.75, 128.79, 131.3, 135.1, 135.3, 136.4, 137.5, 138.2, 140.8, 150.8, 160.3, 163.6, 188.6, 200.6. HRMS (ESI): m/z [M + Na]+ calcd for C45H45NNaO10SSi: 842.2426; found: 842.2388.
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For a review, see: