Studies toward the Total Synthesis of Sorangicins: A Shortened Synthesis of the Dioxabicyclo[3.2.1]octane Core

Abstract

An access to the dioxabicyclo[3.2.1]octane core of sorangicin A was developed, using a keto lactone formation, a Mukaiyama–Michael reaction and an epoxide opening as the key steps.

Key words

sorangicin A - antibiotic activity - asymmetric synthesis - dioxabicyclo[3.2.1]octane core - Mukaiyama–Michael reaction

• References and Notes

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• 16 General Procedure to Obtain 20: Compound 17 (3.291 g, 5.41 mmol) was dissolved in THF (40 mL) and a solution of TBAF in THF (6.5 mL, 1 M, 6.5 mmol). After 15 min stirring at r.t., MeOH (40 mL) and K₂CO₃ (7.5 g, 54.27 mmol) were added. After 48 h, H₂O was added, phases were separated and the aqueous phase was extracted with EtOAc. The combined organic phases were dried over Na₂SO₄ and the residue, after evaporation of the solvent, was purified by flash chromatography (EtOAc–pentane, 1:1) to yield 20 (0.866 g, 3.33 mmol, 62%) as a colorless oil; [α]_D ²⁰ – 55° (c = 1,0 in CH₂Cl₂). ¹H NMR (400 MHz, CDCl₃): δ = 7.32–7.33 (m, 2 H), 7.29–7.33 (m, 2 H), 7.21–7.26 (m, 1 H), 6.56–6.60 (d, ³ J = 15.8 Hz, 1 H), 6.09–6.15 (dd, ³ J = 15.8, 7.5 Hz, 1 H), 4.44–4.46 (m, 1 H), 4.31–4.33 (d, ³ J = 6.6 Hz, 1 H), 4.04–4.10 (m, 3 H), 3.93–3.98 (m, 1 H), 2.05–2.10 (ddd, ² J = 11.7 Hz, ³ J = 6.6, 2.7 Hz, 1 H), 1.93–1.96 (dd, ² J = 11.7 Hz, ³ J = 1.5 Hz, 1 H), 1.50–1.57 (dq, ³ J = 9.1, 6.8 Hz, 1 H), 1.24–1.28 (t, ³ J = 7.16 Hz, <1 H), 0.94–0.95 (d, ³ J = 6.7 Hz, 3 H). ¹³C NMR (100 MHz, CDCl₃): δ = 136.4, 132.1, 129.0, 128.5, 127.7, 126.5, 82.9, 79.8, 79.0, 74.4, 60.9, 41.6, 38.8, 15.2. IR (KBr): 3247 (m), 2976 (m), 2934 (m), 2888 (m), 1740 (w), 1496 (w), 1449 (m), 1371 (w), 1360 (w), 1239 (m), 1143 (s), 1078 (m), 1061 (s), 1037 (s), 962 (s), 929 (w), 746 (m), 693 (m), 544 (w) cm^–1. MS (EI): m/z (%) = 260.1 (100) [M⁺], 245.1 (17), 211.1 (42), 160.1 (32), 131.1 (78), 115.1 (36), 104.1 (64), 91.1 (49), 69.1 (25), 55 (14). HRMS: m/z [M] calcd for C₁₆H₂₀O₃: 260.1412; found: 260.1412. The NMR and mass spectroscopic data matched the values published by Crimmins (ref. 4) and Smith (ref. 3c).
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