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DOI: 10.1055/s-0030-1259938
Electrophilic Cyclization of 1,6-Enynes
Publication History
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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References and Notes
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.
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.