Iodine-Atom-Transfer-Type Carbocyclization of 2-(2-Propynyloxy)ethyl Iodides Involving a Lithium Alkylidene Carbenoid Intermediate

Abstract

In the presence of 1-hexynyllithium, 2-(2-propynyl­oxy)ethyl iodides undergo carbocyclization with 1-iodo-1-hexyne to afford 3-(diiodomethylene)tetrahydrofurans in an atom-economical manner with incorporation of the two iodine atoms from the substrates and the reagent.

Key words

alkynes - carbenoids - catalysis - cyclizations - halides

References and Notes

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• For iodine-transfer radical cyclization of hex-5-enyl and hex-5-ynyl iodides; see:
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• 3 Harada T. Mizunashi K. Muramatsu K. Chem. Commun.  2006,  638 
• 4a Harada T. Iwazaki K. Otani T. Oku A. J. Org. Chem.  1998,  63:  9007 
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• 5 For a relevant LDA-catalyzed cycloisomerization of iodoalkynes 6, see: Harada T. Muramatsu K. Fujiwara T. Kataoka H. Oku A. Org. Lett.  2005,  7:  779 

6

Enyne 7 might be formed by the addition of 1-hexynyl-lithium to an alkylidene carbenoid intermediate^4a followed by iodination of the resulting alkenyllithium.

7

Representative Procedure for Carbocyclization (Table 2, Entry 4)
To a solution of 1-iodo-1-hexyne (0.12 mL, 0.75 mmol) in THF (1 mL) under argon atmosphere at 0 °C was added n-BuLi (1.6 M solution in hexane, 0.13 mL, 0.20 mmol). The resulting solution was stirred at 0 °C for 30 min. To the resulting solution of 1-hexynyllithium (0.2 mmol) and 1-iodo-1-hexyne (0.55 mmol) in THF was added iodoalkyne 5d (0.141 g, 0.50 mmol). The resulting solution was stirred at 40 °C for 1.5 h. The mixture was poured into H₂O and extracted twice with EtOAc. The combined organic layers were dried (Na₂SO₄) and concentrated in vacuo. The residue was purified by flash chromatography (SiO₂, 7% EtOAc in hexane) to give 0.165 g (81% yield) of 2d: ¹H NMR (500 MHz, C₆D₆): δ = 1.11 (3 H, t, J = 7.0 Hz), 1.54 (3 H, s), 1.71 (3 H, s), 2.62 (1 H, dd, J = 5.5, 17.4 Hz), 2.88 (1 H, d, J = 17.4 Hz), 3.24 (1 H, m), 3.75 (1 H, m), 4.76 (1 H, d, J = 5.4 Hz). ¹³C NMR (125.8 MHz, C₆D₆): δ = -2.6, 15.2, 26.7, 27.0, 52.5, 62.4, 85.2, 99.0, 161.3. Anal. Calcd for C₉H₁₄O₂I₂: C, 26.49; H, 3.46. Found: C, 26.53; H, 3.59.