Download PDF
Synlett 2009(12): 1990-1996  
DOI: 10.1055/s-0029-1217517
LETTER
© Georg Thieme Verlag Stuttgart ˙ New York

Gold(III) Bromide Catalyzed Furannulation of 2-Alkynylcycloalk-2-enols: An Expedient Route to Fused Furans

C. Praveen, P. Kiruthiga, P. T. Perumal*
Organic Chemistry Division, Central Leather Research Institute, Adyar, Chennai 600020, India
Fax: +91(44)24911589; e-Mail: ptperumal@gmail.com;
Further Information

Publication History

Received 15 February 2009
Publication Date:
25 June 2009 (online)

    Permissions and Reprints All articles of this category

Abstract

An efficient synthesis of fused furans from 2-alkynyl­cycloalk-2-enols via gold(III) bromide catalyzed cycloisomerization was achieved. The reaction condition is moderate and amenable to structurally diverse substrates, leading to good yield of products.

Key words

2-alkynylcycloalk-2-enol - gold(III) - cycloisomerization - fused furans

14

General Procedure for the Synthesis of 2-Alkynyl­cyclo-alk-2-enol 3a-l Representative Procedure for 2-(Phenylethynyl)cyclohex-2-enol (3f, Table 1, Entry 6)
2-Iodocyclohex-2-enol (1b, 1.0 mmol), Pd(PPh3)Cl2 (5 mol%), and CuI (5 mol%) were placed in an oven-dried flask under N2. Dry Et3N was added, and the resulting suspension was magnetically stirred. Upon dropwise addition of phenylacetylene 2a (1.2 mmol), the mixture was stirred at r.t. until TLC showed the disappearance of the starting 2-iodo-cyclohex-2-enol (ca. 6 h). H2O (50 mL) was added to the reaction mixture, and the residue was extracted into EtOAc (4 × 15 mL), and the extract was dried over anhyd Na2SO4. Removal of the solvent under reduced pressure gave the crude product, which was further purified by column chromatography on silica gel using EtOAc-PE (1:9) as eluent to afford pure product 3f (91%) as a brown oil. IR (CH2Cl2): 3383, 2928, 1592, 1053, 755 cm-¹. ¹H NMR (500 MHz, CDCl3): δ = 1.60-1.62 (m, 1 H), 1.75-1.78 (m, 3 H), 1.87-1.90 (m, 1 H), 2.12-2.21 (m, 2 H), 4.26 (br s, 1 H), 6.30 (t, J = 4.6 Hz, 1 H), 7.28-7.29 (m, 3 H), 7.43-7.44 (m, 2 H). ¹³C NMR (125 MHz, CDCl3): δ = 18.1, 26.0, 30.6, 66.9, 88.4, 89.3, 123.2, 124.2, 128.2, 128.3, 131.6, 137.8. MS (EI): m/z = 198 [M+]. Anal. Calcd for C14H14O: C, 84.81; H, 7.12. Found: C, 84.91; H, 7.09.

15

General Procedure for the Synthesis of Fused Furans 4a-lRepresentative Procedure for 2-Phenyl-4,5,6,7-tetrahydrobenzofuran (4f, Table 3, Entry 6)
To a soln of 2-(phenylethynyl)cyclohex-2-enol (3f, 1.0 mmol) in DCE (1 mL) under N2 was added AuBr3 (5 mol%) and heated the reaction mixture at 70 ˚C for 30 min. After completion of the reaction as indicated by TLC, the reaction mixture was concentrated under reduced pressure and purified by column chromatography over silica gel (100-200 mesh) to afford pure product 4f (86%) as a colourless liquid. R f  = 0.82 (EtOAc-PE, 1:9). IR (CH2Cl2): 2934, 2815, 2354, 1669, 1600, 1247, 760 cm-¹. ¹H NMR (500 MHz, CDCl3): δ = 1.75-1.78 (m, 2 H), 1.84-1.89 (m, 2 H), 2.46 (t, J = 6.1 Hz, 2 H), 6.47 (s, 1 H), 7.20 (t, J = 7.6 Hz, 1 H), 7.35 (t, J = 7.6 Hz, 2 H), 7.62 (d, J = 6.9 Hz, 2 H). ¹³C NMR (125 MHz, CDCl3): δ = 22.2, 23.1, 23.2, 23.4, 106.1, 119.0, 123.3, 126.6, 128.6, 131.5, 150.9. MS (EI): m/z = 221 [M+ + Na+]. Anal. Calcd for C14H14O: C, 84.81; H, 7.12. Found: C, 84.75; H, 7.15.