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Aromatization of IMDAF cycloadducts to  1,3-dihydro-2-benzofuran-5-ol  and 1,3-dihydro-2-benzothiophene-5-ol.; izobenzofuranol and izobenzothiophenol

SyntheticPage 318
DOI: 10.1039/SP318
Submitted Mar 08, 2010, published Mar 09, 2010
Aydin Demircan (ademircan@nigde.edu.tr)
A contribution from Demircan Group, Nigde


			Reaction Scheme: Aromatization of IMDAF cycloadducts to&nbsp;&nbsp;<SPAN class=csm-chemical-name id=csm1287696913915 title=1,3-dihydro-2-benzofuran-5-ol>1,3-dihydro-2-benzofuran-5-ol</SPAN>&nbsp;&nbsp;and <SPAN class=csm-chemical-name id=csm1287696959448 title=1,3-dihydro-2-benzothiophene-5-ol>1,3-dihydro-2-benzothiophene-5-ol</SPAN>.

Chemicals Used

BF3•Et2O (From Aldrich)
DCM (bulk) dried over CaH2

Procedure

To a solution of and bromo-oxa and thio heterotricycles (1) (0.80 mmol) in dry DCM (5 mL) cooled to -78 °C under N2 was added BF3•Et2O (0.11 mL, tech 50%, 0.88 mmol) in DCM (5 mL) over 3 min. After being stirred at same temperature for 15 min, the reaction mixture was warmed to 0 °C over period of 3 h. The cooling bath was removed and the reaction mixture was stirred at room temperature for another 4 h. The reaction mixture was then diluted with water (5 mL) and the layers were separated. The aqueous layer was further extracted with DCM (10 mL) and the combined organic layers were washed with water (2 x 4 mL), dried (anhydrous MgSO4) and concentrated in vacuo. The residue was subjected to flash column chromatography to afford following compounds.

Author's Comments

Very short and easy reaction, The reaction could be performed under 1 g scale. Temprerature should be kept below -65 oC. The mechanism presumably involves the Lewis Acid (BF3.Et2O) catalysed ring opening of (breaking oxa bridge first) to form allylic cation(2) at the intermediate which upon proton exchange and decomplexation affords the enol, which probably undergoes hydrogen bromide elimination to generate  fused aromatic derivatives (3). IMDAF cycloadducts (1) could easily be aromatisized by Lewis acid like BF3.Et2O. Utility of thermal IMDAF reaction was shown by transforming heteroaromatic compounds to the bicycle-benzoaromatic compounds.

Data

1,3-dihydro-2-benzofuran-5-ol (3a): As white solid, (76, %); m.p: 67-69 ºC, t.l.c., (Hexane: EtOAc (6:2)): Rf : 0.38; νmax (thin film)/cm-1: 3226 (s, O-H), 2929 (s, C-H), 2867 (s, C-H), 1030 (s, C-O); δH (300 MHz, CDCl3): 7.08 (d, 1H, J 8.1 Hz, AB), 6.74 (d, 1H, J 8.1 Hz, AB), 6.71 (s, 1H), 5.06 (s, 4H), 3.61 (s, OH); δC (125 MHz, CDCl3): 155.4 (q), 140.9 (q), 130.9 (q), 121.8, 114.6, 106.4, 73.4, 73.2.

1,3-dihydro-2-benzothiophene-5-ol
(3b): As yellow solid, (72 %); m.p: 62-64 ºC t.l.c., (Hexane: EtOAc (6.2)): Rf : 0.43; νmax (thin film)/cm-1: 3365 (s, O-H), 2920 (s, C-H), 2851 (s, C-H), 1100 (s, C-O), 1026 (s, C-S); δH (300 MHz, CDCl3): 6.90 (d, 1H, J 8.4 Hz, AB), 6.72 (d, 1H, J 8.4 Hz, AB), 6.51 (s, 1H), 4,97 (s, 4H), 4,25 (s, OH); δC (125 MHz, CDCl3): 154.2 (q), 138.2 (q), 131,0 (q), 122.6, 115.0, 104.3, 73.4, 73.3.

Lead Reference

M. Karaarslan and E. Gokturk, A. Demircan*, J. Chem. Res., 2, 117-120 (2007)

Other References

I. N. N. Namboothiri,  M. Ganesh, S. M. Mobin, M. Cojocaru, J. Org. Chem., 2005, 70, 2235.

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Keywords: alcohols, aromatics/arenes, Aromatization by a Lewis Acid, elimination

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