References
For some recent examples, see:
1a
Padwa A.
Kissell WS.
Eidell CK.
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1b
Kende AS.
Martin Hernando JI.
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1c
Velázquez F.
Olivo HF.
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1d
Paintner FF.
Allmendinger L.
Bauschke G.
Polborn K.
Synlett
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1308
1e
Brueggemann M.
McDonald AI.
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Scott JP.
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1f
Kemmler M.
Herdtweck E.
Bach T.
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2004,
4582
1g
Schobert R.
Urbina-González JM.
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2005,
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3657
For reviews on the synthesis and chemistry of tetronic acids, see:
2a
Pattenden G.
Fortschr. Chem. Org. Naturst.
1978,
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133
2b
Tejedor D.
García-Tellado F.
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3a
Ley SV.
Wadsworth DJ.
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3b
Ley SV.
Trudell ML.
Wadsworth DJ.
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47:
8285
4
Wadsworth DJ.
PhD Thesis
Imperial College, University of London;
London:
1989.
5 Lithium 4-alkoxy 2-furanolates generated by deprotonation of 5-unsubstituted 4-O-alkyl tetronates, e.g. 6a, have been reported to react with electrophiles exclusively at C-5, see: Pelter A.
Al-Bayati RIH.
Ayoub MT.
Lewis W.
Pardasani P.
Hänsel R.
J. Chem. Soc., Perkin Trans. 1
1987,
717
6 For the regioselective deprotonation of 5-monoalkyl-substituted 4-O-methyl tetronates at C-3 under kinetically controlled conditions and subsequent trapping of the 3-lithiated intermediates with electrophiles, see: Miyata O.
Schmidt RR.
Angew. Chem., Int. Ed. Engl.
1982,
21:
637
7
Paintner FF.
Allmendinger L.
Bauschke G.
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8 Quenching the reaction with deuterated MeOH followed by phosphate buffer (pH 7.5) led to a mixture of (2H)-6a and 3-deuterated 4-methoxy-2-triisopropylsilyloxyfuran. Our attempts to isolate the 2-triisopropylsilyloxyfuran by chromatography failed due to rapid hydrolysis to the corresponding tetronate (2H)-6a. This is in accordance with previous findings showing 4-O-alkyl 2-trialkylsilyloxy-furans to be very prone to hydrolysis (ref. 5)
The facile deprotonation of 2-alkoxy- or 2-trialkylsilyloxy furans in 5-position with t-BuLi at low temperature has previously been reported, see for example:
9a
Kraus GA.
Sugimoto H.
J. Chem. Soc., Chem. Commun.
1978,
30
9b
Jefford CW.
Rossier J.-C.
Boukouvalas J.
Huang P.
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77:
661
10 4-O-Methyl tetronate (6a) is commercially available, e.g. from Acros Organics BVBA, Janssen Pharmaceuticalaan 3a, B-2440 Geel, Belgium
11
Paintner FF.
Allmendinger L.
Bauschke G.
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12
Campos PJ.
Tan C.-Q.
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1995,
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5257
13
Typical Procedure.
A solution of iodine (10.15 g, 40 mmol) in DMF (20 mL) was added to an ice-cold solution of 6a (1.14 g, 10 mmol) and pyridine (807 µL, 10 mmol) in DMF (20 mL). The mixture was allowed to warm to r.t. and stirred under the exclusion of light for 22 h, at which time it was poured into sat. aq NaHCO3 (100 mL). The mixture was extracted with CH2Cl2. The combined organic extracts were washed with aq Na2S2O3 and H2O, dried (MgSO4) and evaporated under reduced pressure. The resulting residue was recrystallized from EtOAc to give 7a (2.06 g, 86%) as pale yellow crystals; mp 158-160 °C. IR (KBr): 3001, 2950, 1732, 1643, 1621 cm-1. 1H NMR (500 MHz, CDCl3): δ = 4.14 (s, 3 H), 4.81 (s, 2 H). 13C NMR (100 MHz, CDCl3): δ = 50.5, 58.4, 68.4, 170.7, 178.5. Anal. Calcd for C5H5IO3: C, 25.02; H, 2.10; I, 52.88. Found: C, 25.01; H, 2.05; I, 52.84.
14
Typical Procedure.
TIPSOTf (867 µL, 3.15 mmol) was slowly added to an ice-cold solution of 7a (720 mg, 3.0 mmol) and Et3N (481 µL, 3.45 mmol) in CH2Cl2 (3 mL). After stirring for 1 h at 0 °C the mixture was poured into ice-cold half-sat. aq NaHCO3 and extracted with Et2O. The combined organic extracts were washed with ice-cold half-sat. aq NaHCO3 and brine, dried (MgSO4) and evaporated under reduced pressure to afford a mixture of 7a and 8a. To separate the product from starting material the residue was dissolved in n-hexane, filtered and evaporated under reduced pressure to leave 8a (1.13 g, 95%, ÷97% pure as determined by 1H NMR spectroscopy) as a pale yellow oil. 1H NMR (500 MHz, CD2Cl2): δ = 1.09 (d, J = 7.3 Hz, 18 H), 1.27 (sept, J = 7.3 Hz, 3 H), 3.68 (s, 3 H), 6.56 (s, 1 H).
15
Typical Procedure.
A solution of t-BuLi (1.5 M in hexane, 666 µL, 1.0 mmol) was added dropwise to a solution of 8a (198 mg, 0.5 mmol) in THF (5 mL) at -78 °C. After stirring for 15 min at -78 °C, benzyl bromide (121 µL, 1.0 mmol) was added. The reaction mixture was stirred at -78 °C for 1 h and then was allowed to warm to r.t. during 2 h. Phosphate buffer (pH 5.5, 5 mL) was added and the resulting mixture was stirred for another 30 min at r.t. before it was extracted with Et2O. The combined organic extracts were dried (MgSO4) and evaporated under reduced pressure. The resulting residue was purified by flash chromatography (n-hexane-CH2Cl2-Et2O, 20:20:60) to give 1e
[17]
(90 mg, 88%) as a colorless oil. 1H NMR (500 MHz, CDCl3): δ = 3.60 (s, 2 H), 3.93 (s, 3 H), 4.69 (s, 2 H). 13C NMR (100 MHz, CDCl3): δ = 27.8, 57.7, 65.4, 102.7, 126.4, 128.3, 128.5, 139.1, 173.0, 174.6. HRMS: m/z calcd for C12H12O3 [M+]: 204.0787. Found: 204.0774.
16
Clemo NG.
Pattenden G.
J. Chem. Soc., Perkin Trans. 1
1985,
2407
17a
Calam CT.
Todd AR.
Waring WS.
Biochem. J.
1949,
45:
520
17b
Wengel AS.
Reffstrup T.
Boll PM.
Tetrahedron
1979,
35:
2181