Organic Syntheses, Vol. 76, 263
Checked by Scott A. Frank and William R. Roush.
1. Procedure
Caution! Because of the corrosive and toxic nature of the reagents, steps A and B should be conducted in an efficient fume hood. Eye protection and protective clothing should be worn while performing these experiments.
2. Notes
1. The salt is crushed and then azeotropically dried with
benzene three times, filtered, dried under vacuum for 2 days, and stored under
argon over
phosphorus pentoxide.
2. The flask is removed from the
ice bath, wiped dry, and weighed every 3-5 min until the desired weight is achieved. Also, to relieve pressure, the flask is vented with an exit needle connected to tygon tubing placed in the back of the hood.
3.
1-Octyn-3-ol (96%) was purchased from Aldrich Chemical Company, Inc. It can be fractionally distilled to remove minor impurities.
4. TLC analysis is performed on E. Merck silica gel 60F-254 glass plates of 0.25-mm thickness purchased from EM Reagents. The eluting solvent was
15% ethyl acetate-hexane.
6. Spectral analysis for
2-bromo-1-octen-3-ol is as follows: IR (film) cm
−1: 3358, 2930, 1626, 1465, 896;
1H NMR (400 MHz, CDCl
3) δ: 0.87 (t, 3 H, J = 6.8), 1.24-1.37 (m, 6 H), 1.57-1.69 (m, 2 H), 1.86 (d, 1 H, J = 6.0), 4.06 (q, 1 H, J = 6.3), 5.54 (d, 1 H, J = 1.9), 5.85 (dd, 1 H, J = 0.74, J = 1.9);
13C NMR (100 MHz, CDCl
3) δ: 14.0, 22.5, 24.8, 31.5, 35.5, 76.1, 116.9, 137.6.
8.
(Ph3P)2PdCl2 was purchased from the Aldrich Chemical Company, Inc..
9.
CuI purchased from the Aldrich Chemical Company, Inc. was washed with
tetrahydrofuran in a
Soxhlet extractor overnight, dried under vacuum overnight, and stored under
argon over
calcium sulfate.
10. The reaction mixture turned bright yellow after addition of the
palladium catalyst. The color changed to greenish brown after the CuI was added and became yellow upon addition of the
1-nonyne.
11. Extended reaction times cause colored by-products to form, that are difficult to separate. TLC analysis was performed on E. Merck silica gel 60F-254 glass plates of 0.25-mm thickness purchased from EM Reagents. The eluting solvent was 20%
ethyl acetate-
hexane; in this solvent system, the R
f's of
7-methylene-8-hexadecyn-6-ol and
2-bromo-1-octen-3-ol are 0.68 and 0.55, respectively.
12. If the reaction was not complete within 2 hr, the checkers added an additional 0.25 equiv of
1-nonyne. The reaction was typically complete within 30-45 min following this addition.
13. A
70 × 370-mm column packed with 500 g of silica gel was used.
14. Spectral analysis for
7-methylene-8-hexadecyn-6-ol is as follows: IR (film) cm
−1: 3363, 2929, 2858, 2225, 1614, 1465, 902;
1H NMR (400 MHz, CDCl
3) δ: 0.75-0.98 (m, 6 H), 1.09-1.40 (m, 14 H), 1.41-1.66 (m, 5 H), 2.31 (t, 2 H, J = 6.9), 4.03 (q, 1 H, J = 6.3 Hz), 5.33 (d, 2 H, J = 7.5);
13C NMR (100 MHz, CDCl
3) δ: 14.0, 14.0, 19.2, 22.5, 22.6, 25.0, 28.6, 28.7, 28.8, 31.6, 31.7, 35.9, 74.8, 77.7, 92.7, 119.2, 135.2.
15. The checkers obtained a
92% yield for this reaction.
16. The
10% silver nitrate on silica gel (200 mesh) was purchased from the Aldrich Chemical Company, Inc.
17. The flask was wrapped with
aluminum foil.
18. A 70 × 370-mm column packed with 300 g of silica gel was eluted with
hexane; in this solvent system, the product has R
f = 0.65.
20. Spectral analysis for
2-pentyl-3-methyl-5-heptylfuran is as follows: IR (film) cm
−1: 2927, 2856, 1577, 1467, 792;
1H NMR (300 MHz, CDCl
3) δ: 0.93 (t, 3 H, J = 6.8), 0.94 (t, 3 H, J = 6.8), 1.23-1.45 (m, 12 H), 1.57-1.67 (m, 4 H), 1.94 (s, 3 H), 2.54 (t, 2 H, J = 7.3), 2.57 (t, 2 H, J = 7.3), 5.77 (s, 1 H);
13C NMR (100 MHz, CDCl
3) δ: 9.8, 14.0, 14.0, 22.4, 22.6, 25.9, 28.0, 28.2, 28.4, 29.1, 29.6, 31.4, 31.8, 107.6, 113.7, 149.3, 153.4. Anal. Calcd for C
17H
30O: C, 81.53; H, 12.07. Found: C, 81.42; H, 11.99.
All toxic materials were disposed of in accordance with "Prudent Practices in the Laboratory"; National Academy Press; Washington, DC, 1995.
3. Discussion
The present procedure evolved from our previous work on the conversion of allenals, allenones, and allenylcarbinols to furans and 2,5-dihydrofurans with catalytic
silver nitrate (AgNO
3) in
acetone.
5,6,7,8,9,10 It has also been shown that allenylcarbinols can be converted to
2,5-dihydrofuran under these conditions.
11 β- and γ-Alkynyl
allylic alcohols can also be isomerized to furans under strongly basic conditions with
potassium tert-butoxide in
tetrahydrofuran-tert-butyl alcohol-18-crown-6 or
hexamethylphosphoramide (KO-t-Bu in THF-t-BuOH-18-crown-6 or HMPA).
12 The AgNO
3/silica gel method is milder, faster, and more efficient than the previously reported procedures.
13 Moreover, it offers the potential advantage of catalyst recovery and possible applicability to a flow system in which a packed column, protected from light, could serve as the reactor.
4
Appendix
Chemical Abstracts Nomenclature (Collective Index Number);
(Registry Number)
2-Pentyl-3-methyl-5-heptylfuran: Furan, 5-heptyl-3-methyl-2-pentyl- (13); (170233-67-7)
2-Bromo-1-octen-3-ol: 1-Octen-3-ol, 1-bromo-. (E)- (9); (52418-90-3)
Tetraethylammonium bromide: Ammonium, tetraethyl-, bromide (8); Ethaniminium, N,N,N-triethyl-, bromide (9); (71-91-0)
Hydrogen bromide: Hydrobromic acid (8,9); (10035-10-6)
1-Octyn-3-ol (8,9); (818-72-4)
7-Methylene-8-hexadecyn-6-ol: 8-Hexadecyn-6-ol, 7-methylene- (13); (170233-66-6)
Dichlorobis(triphenylphosphine)palladium(II); Palladium, dichlorobis(triphenylphosphine)- (8,9); (13965-03-2)
Copper(I) iodide: Copper iodide (8,9); (7681-65-4)
1-Nonyne (8,9); (3452-09-3)
Silver nitrate
10 wt, % on silica gel: Nitric acid silver(+) salt (9); (7761-88-8)
Phosphorus pentoxide: Phosphorus oxide (8,9); (1314-56-3)
(Z)-1-Bromo-1-octen-3-ol: 1-Octen-3-ol, 1-bromo-, (Z)- (11); (87937-09-5)
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