BF₃•Et₂O (From Aldrich)
DCM (bulk) dried over CaH^₂

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.

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.

1,3-dihydro-2-benzofuran-5-ol (3a): As white solid, (76, %); m.p: 67-69 ºC, t.l.c., (Hexane: EtOAc (6:2)): R_f : 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)): R_f : 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.

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

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