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Synlett 2010(7): 1027-1032  
DOI: 10.1055/s-0029-1219779
LETTER
© Georg Thieme Verlag Stuttgart ˙ New York

One-Pot, Three-Component Synthesis of 4-Aryl-5,6-dihydropyran via Prins-Friedel-Crafts Reaction

Udagandla C. Reddy, Anil K. Saikia*
Department of Chemistry, Indian Institute of Technology Guwahati, Guwahati 781039, India
Fax: +91(361)2690762; e-Mail: asaikia@iitg.ernet.in;
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Publication History

Received 20 January 2010
Publication Date:
17 March 2010 (online)

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Abstract

An efficient method for the synthesis of 4-aryldihydropyrans has been developed from the reaction of aldehydes or epoxides with homopropargylic alcohols in the presence of arenes promoted by boron trifluoride etherate

Key words

Lewis acid - multicomponent - Prins cyclization - Friedel-Crafts reaction - dihydropyran

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Synthesis of 2,4-Diphenyl-5,6-dihydro-2 H -pyran (1b)
To a mixture of benzaldehyde (0.10 mL, 1.0 mmol) and homopropagyl alcohol (140 mg, 2.0 mmol) in dry benzene (2.0 mL) was added freshly distilled BF3˙OEt2 (0.15 mL, 1.2 mmol) under nitrogen atmosphere. The reaction mixture was stirred at 40 ˚C for 8 h. The progress of the reaction was monitored by TLC with EtOAc and hexane as eluents. After completion of the reaction, the reaction mixture was treated with aq NaHCO3, the mixture was extracted with Et2O, and the combined organic extracts washed with brine and water. The organic layer was dried (Na2SO4) and evaporated to leave the crude product, which was purified by short column chromatography over silica gel to give 2,4-diphenyl-5,6-dihydro-2H-pyran 1b (106 mg, 45% yield) as a liquid. ¹H NMR (400 MHz, CDCl3): δ = 2.45-2.52 (m, 1 H), 2.70-2.80 (m, 1 H), 3.92 (ddd, J = 11.6, 8.8, 4.0 Hz, 1 H), 4.18 (ddd, J = 11.6, 5.2, 3.6 Hz, 1 H), 5.30 (d, J = 2.4 Hz, 1 H), 6.22 (d, J = 1.2 Hz, 1 H), 7.23-7.45 (m, 10 H). ¹³C NMR (100 MHz, CDCl3): δ = 27.3, 63.6, 72.4, 124.3, 124.4, 125.1, 127.9, 128.6 (2 C), 129.6, 133.4, 135.5, 136.9, 139.9, 148.5. IR: 3029, 2923, 2854, 1600, 1493, 1453, 1273, 1119, 1075, 1029, 886, 749, 698 cm-¹. HRMS (APCI): m/z calcd for C17H16O [M + H]+: 237.1279; found: 237.1258.
6-Isopropyl-4-phenyl-5,6-dihydro-2 H -pyran (22)
To a mixture of 1,2-epoxy-2-methylpropane (0.09 mL, 1.0 mmol) and homopropagyl alcohol (140 mg, 2.0 mmol) in dry benzene (3.0 mL) was added freshly distilled BF3˙OEt2 (0.15 mL, 1.2 mmol) under nitrogen atmosphere. The reaction mixture was stirred at r.t. for 5 h. The progress of the reaction was monitored by TLC with EtOAc and hexane as eluents. After completion of reaction, the reaction mixture was treated with aq NaHCO3, and the product was extracted with Et2O, and then the combined organic extracts washed with brine and H2O. The organic layer was dried (Na2SO4) and evaporated to leave the crude product, which was purified by short column chromatography over silica gel to give 6-isopropyl-4-phenyl-5,6-dihydro-2H-pyran 25 (90 mg, 45% yield) as a liquid. ¹H NMR (400 MHz, CDCl3): δ = 0.99 (t, J = 6.8 Hz, 6 H), 1.82-1.93 (m, 1 H), 2.27-2.40 (m, 1 H), 2.60-2.71 (m, 1 H), 3.73 (dt, J = 10.4, 3.2 Hz, 1 H), 4.00-4.06 (m, 1 H), 4.16 (ddd, J = 10.8, 5.6, 1.6 Hz, 1 H), 6.10 (m, 1 H), 7.24-7.28 (m, 1 H), 7.30-7.36 (m, 2 H), 7.38-7.42 (m, 2 H). ¹³C NMR (100 MHz, CDCl3): δ = 18.1, 18.6, 27.7, 33.3, 64.2, 79.4, 125.0 (2C), 127.5, 127.7, 135.6, 140.8. IR: 2960, 2853, 1645, 1494, 1363, 1115, 1089, 880, 751, 699 cm-¹. HRMS (APCI): m/z calcd for C14H18O [M + H]+: 203.1436; found: 203.1431.

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The product 26 was sensitive to silica gel. While purifying the compound by column chromatography, it was decomposed. Therefore, the silica gel of the column chromatography was impregnated with 3% Et3N in hexane during the purification