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Synlett 2011(8): 1151-1153  
DOI: 10.1055/s-0030-1259938
LETTER
© Georg Thieme Verlag Stuttgart ˙ New York

Electrophilic Cyclization of 1,6-Enynes

Tobias Harschneck, Stefan F. Kirsch*, Michael Wegener
Department Chemie, Catalysis Research Center, Technische Universität München, Lichtenbergstr. 4, 85747 Garching, Germany
Fax: +49(89)28913315; e-Mail: stefan.kirsch@ch.tum.de;
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Publication History

Received 25 January 2011
Publication Date:
07 April 2011 (online)

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Abstract

The NIS-mediated iodocyclization of 1,6-enynes is described. While 1,6-enynes with a cation-stabilizing substituent at C2 position undergo 6-exo cyclization in poor yields, 1,6-enynes with donor substituents at C1 position favor the 5-exo mode of cyclization. The resulting five-membered carbocycles are obtained in moderate to good yields, thus demonstrating another facet in the ­iodocyclization of enynes.

Key words

enynes - cyclizations - iodine - alkynes - carbocycles

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15

Representative Experimental Procedure for 6f
Enyne 5f (32.0 mg, 105 µmol) was dissolved in CH2Cl2 (1 mL) and NIS (47.0 mg, 209 µmol, 2 equiv) were added. The solution was stirred at r.t. in the dark until TLC indicated full consumption of the starting material. The reaction mixture was diluted with CH2Cl2 (10 mL), and a sat. aq Na2S2O3 solution (10 mL) was added. The phases were separated, and the aqueous phase was extracted with CH2Cl2 (2 × 10 mL). The combined organic phases were washed with brine and dried over Na2SO4. The solvent was evaporated, and the residue was purified by flash chromatography on silica gel (pentane-Et2O = 98:2). Carbocycle 6f was obtained as a yellow oil (30.4 mg, 70.3 µmol, 67%); R f  = 0.18 (pentanes-EtOAc = 95:5) [UV, CAM]. ¹H NMR (500 MHz, CDCl3): δ = 1.26 (t, J = 7.1 Hz, 6 H), 1.48-1.56 (m, 2 H), 1.59-1.69 (m, 2 H), 1.81-1.86 (m, 2 H), 1.98-2.05 (m, 2 H), 2.23 (t, J = 12.5 Hz, 1 H), 2.57 (dd, J = 12.8, 7.4 Hz, 1 H), 2.78 (dt, J = 18.1, 2.4 Hz, 1 H), 3.08 (d, J = 18.1 Hz, 1 H), 3.17-3.21 (m, 1 H), 4.20 (virt. pent, J = 6.7 Hz, 4 H), 5.53-5.57 (s, 1 H), 5.79-5.83 (m, 1 H). ¹³C NMR (91 MHz, CDCl3): δ = 14.2, 22.6, 23.1, 24.3, 25.5, 39.2, 44.9, 53.5, 58.0, 61.8, 71.9, 126.2, 135.6, 153.5, 171.3, 171.5. LRMS (EI): m/z (%) = 432 (5) [M+], 387 (5), 305 (21), 231 (100), 157 (39). HRMS (EI): m/z calcd for C18H25O4I [M+]: 432.0792; found: 432.0788.

16

When 2,2-dimethyl-5-(3-methylbut-2-en-1-yl)-5-(3-phenylprop-2-yn-1-yl)-1,3-dioxane (as an internal alkyne derived from 5a) was reacted with NIS in CH2Cl2 at r.t., the cyclization product was not observed. Instead, at least two compounds were formed that could not be unequivocally identified while, after 24 h, the bulk was unreacted starting material.