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C, 78.15; H, 6.90; N, 9.59. Found: C, 77.93; H, 6.87; N, 9.35. Reaction of N,N-Diethylphenylethynylamine with Tetraphenyl- cyc1opentadienone.
Prosp. Peremohy 37, Kyiv 03056 (Ukraine) c Department of Chemistry, National Taras Shevchenko University of Kyiv, Volodymyrska 64, Kyiv 01033 (Ukraine).
18 May 1973 - C. G. Kruse,3 56and Clarisse L. Habraken*. Gorlaeus Laboratories, University of Leiden, Leiden,The Netherlands. Received October IS, 1978.
ABSTRACT: The reaction of terminal alkynes with n-BuLi, and then with aldehydes, followed by the treatment with molecular iodine, and subsequently hydrazines or hydroxylamine provided the corresponding 3,5-disubstituted pyrazoles or isoxazoles in good yields with high regioselectivity, through the formations of propargyl secondary alkoxides and α-alkynyl ketones. The present reactions are one-pot preparation of 3,5disubstituted pyrazoles from terminal alkynes, aldehydes, molecular iodine, and hydrazines, and 3,5-disubstituted isoxazoles from terminal alkynes, aldehydes, molecular iodine, and hydroxylamine.
ciceptive40 activities. Although the major route to isoxazoles is the 1,3-dipolar cycloaddition of nitrile oxides to alkynes,23 other methods were also reported recently.41−44 On the other hand, terminal alkynes and aldehydes are easily available, and therefore, the preparation of pyrazoles and isoxazoles with those compomds is a very attractive option. However, to the best of our knowledge, the one-pot preparation of pyrazoles and isoxazoles using terminal alkynes and aldehydes with hydrazine and hydroxylamine is little studied. Here, as part of our investigation of the synthetic use of molecular iodine in organic synthesis,45 we would like to report the one-pot preparation of pyrazoles by reacting a terminal alkyne with nBuLi and then aldehydes, followed by the treatment with molecular iodine and K2CO3 and the subsequent reaction with hydrazines, and the preparation of isoxazoles by reacting a terminal alkyne with n-BuLi and then aldehydes, followed by the treatment with molecular iodine and K2CO3 and the subsequent reaction with hydroxylamine.
t-BuOH (3 mL) was added without removal of THF. bReaction time was 4 h at third step. cOverall yield from isolated ketone (II), and the mixture was treated with NH2NH2·H2O (1.1 equiv) in t-BuOH (3 mL) at reﬂuxing temperature for 1 h. dReaction time was 18 h at third step.
a After fourth step reaction, TsOH·H2O (2.5 equiv) was added, and the mixture was stirred for 1 h at rt.
ing 5-t-butyl-3-phenylisoxazole 4n in 90% yield in one pot. On the other hand, the same treatment of phenylacetylene 1a with n-BuLi in THF at 0 °C, followed by the addition of propionaldehyde 2l and cyclohexanecarboxaldehyde 2m, and the subsequent treatment with Fe(NO3)3 in the presence of TEMPO in toluene (Method C) gave α-alkynyl ketones (II) bearing ethyl and cyclohexyl groups in good yields. Treatment of the isolated alkynyl ketones (II) bearing ethyl and cyclohexyl groups with hydroxylamine·HCl under reﬂuxing conditions, followed by treatment with p-TsOH at room temperature provided 5-ethyl-3-phenylisoxazole 4l and 5-cyclohexy-3phenylisoxazole 4m in 68 and 62% yields, respectively. In contrast, the one-pot preparation of 5-ethyl-3-phenylisoxazole 4l and 5-cyclohexy-3-phenylisoxazole 4m through the oxidation of alkoxide (I) to alkynyl ketone (II) by Fe(NO3)3 in the presence of TEMPO (Method C), and the subsequent treatment with hydroxylamine·HCl under reﬂuxing conditions was not eﬀective again, and the yields were 26 and 30%, respectively. The same treatment of 4-methoxyphenylacetylene 1b, 4-methylphenylacetylene 1c, 4-chlorophenylacetylene 1d, 2-pyridylacetylene 1e, and 1-octyne 1f with n-BuLi in THF at 0 °C, followed by the addition of 4-methylbenzaldehyde 2b, and the subsequent treatment with molecular iodine and K2CO3 in t-BuOH (Method A) and then hydroxylamine·HCl under reﬂuxing conditions generated the corresponding 3-aryl-5-(4′methylphenyl)isoxazoles 4o−4q and 3-hexyl-5-(4′methylphenyl)isoxazole 4s in good yields with high regioselectivity. However, 5-(4′-methylphenyl)-3-(2′-pyridyl)isoxazole 4r was not obtained, and precursor IIIr containing a hydroxy group at 5-position, using Method B for oxidation, was obtained in 56% yield. It is known that the aromatic resonance energy of isoxazoles is lower than that of pyrazoles,48 and therefore, we believe that the formation of isoxazoles through the dehydration of intermediates (III) is not eﬃcient. Therefore, the addition of p-TsOH to cyclization intermediates (III) promoted the dehydration to give isoxazoles smoothly, as shown in Table 3. The structure of 5-(4′-chlorophenyl)-3phenylisoxazole 4f was supported by X-ray analysis. A plausible reaction mechanism for the formation of pyrazoles and isoxazoles is shown in Scheme 2. The formed lithium acetylide reacts with aldehyde to form propargyl secondary alkoxide (I), which is further oxidized to α-alkynyl ketone (II) by molecular iodine in the presence of K2CO3 (Method A) mainly, and in some cases, DIB in the presence of TEMPO (Method B) or Fe(NO3)3 in the presence of TEMPO (Method C). Once α-alkynyl ketone (II) is formed, it smoothly reacts with hydrazine to provide pyrazole, mainly through the Michael-type addition of hydrazine to α-alkynyl ketone (II), the 5-exo-trig cyclization onto the ketone group, and the subsequent dehydration. As a related reaction, treatment of 4methylphenyl phenylethynyl ketone, α-alkynyl ketone, with benzylamine (1.2 equiv) as amine nucleophile, in 1,2dichloroethane for 11 h at 80 °C gave 2-benzylamino-2phenylethenyl 4′-methylphenyl ketone, β-aminovinyl ketone, in 93% yield. For isoxazoles, α-alkynyl ketone (II) smoothly reacts with hydroxylamine through the Michael-type addition by hydroxylamine to form β-(N-hydroxyamino)vinyl ketone (IV). Then, the 5-exo-trig cyclization onto the ketone group and subsequent dehydration occur. The addition of p-TsOH at the ﬁfth step promotes the dehydration to form 3,5-disubstituted isoxazoles.
a Reaction time was 4 h at third step. bTBAI, H2O, and TsOH·H2O were not added. cOverall yield is from isolated ketone (II), and the mixture was treated with NH2OH·HCl (1.1 equiv), K2CO3 (0.6 equiv), and TBAI (0.1 equiv) in t-BuOH (3 mL) at reﬂuxing conditions for 5 h. Then, TsOH·H2O (1.0 equiv) was added, and the mixture was stirred at rt for 1 h. dTHF was not removed. eTsOH·H2O was not added. fReaction time was 18 h at third step.
CONCLUSION 3,5-Disubstituted pyrazoles and isoxazoles were prepared in good yields with high regioselectivity in one pot by the treatment of terminal alkynes with aromatic aldehydes, molecular iodine (in some cases, DIB or Fe(NO3)3 in the presence of TEMPO), and hydrazines, and of terminal alkynes with aromatic aldehydes, molecular iodine (in some cases, DIB or Fe(NO3)3 in the presence of TEMPO), and hydroxylamine, respectively. The present reaction is a simple and practical method for the preparation of various 1,3-disubstituted pyrazoles and isoxazoles from easily available compounds.
5-(tert-Butyl)-3-phenylisoxazole (4n). White solid (361 mg, 90% yield): mp 49−50 °C (lit.,63 mp 41 °C); IR (ATR) 2971, 1594, 1576, 1465, 1438, 1402, 1275 cm−1; 1H NMR (500 MHz, CDCl3) δ = 1.40 (s, 9H), 6.25 (s, 1H), 7.40−7.46 (m, 3H), 7.78−7.80 (m, 2H); 13 C NMR (125 MHz, CDCl3) δ = 28.9, 32.8, 96.4, 126.7, 128.8, 129.5, 129.7, 162.1, 181.7; HRMS (ESI) Calcd for C13H16ON (M + H)+ 202.1226, found 202.1226. 3-(4′-Methoxyphenyl)-5-(4″-methylphenyl)isoxazole (4o). White solid (456 mg, 86% yield): mp 149 °C (lit.,64 mp 150−150.5 °C). 3,5-Di(4′-methylphenyl)isoxazole (4p). White solid (431 mg, 86% yield): mp 149−150 °C (lit., mp 152−152.5 °C). 3-(4′-Chlorophenyl)-5-(4″-methylphenyl)isoxazole (4q). White solid (407 mg, 75% yield): mp 193−194 °C (lit., mp 199.5− 200 °C); IR (ATR) 3111, 1601, 1497, 1428, 1379, 1262, 1093 cm−1; 1 H NMR (500 MHz, CDCl3) δ = 2.42 (s, 3H), 6.74 (s, 1H), 7.29 (d, J = 8.0 Hz, 2H), 7.45 (d, J = 8.5 Hz, 2H), 7.72 (d, J = 8.0 Hz, 2H), 7.80 (d, J = 8.5 Hz, 2H); 13C NMR (125 MHz, CDCl3) δ = 21.5, 96.7, 124.6, 125.8, 127.7, 128.0, 129.2, 129.7, 135.9, 140.7, 161.9, 170.9; HRMS (ESI) Calcd for C16H13ONCl (M + H)+ 270.0680, found 270.0681. 5-Hydroxy-5-(4′-methylphenyl)-3-(2″-pyridyl)isoxazoline (IIIr). White solid (285 mg, 56% yield): mp 123−124 °C; IR (ATR) 3198, 1573, 1476, 1443, 1286, 1179 cm−1; 1H NMR (400 MHz, CDCl3) δ = 2.36 (s, 3H), 3.56 (d, J = 18.3 Hz, 1H), 3.85 (d, J = 18.3 Hz, 1H), 4.21 (s, 1H), 7.18 (d, J = 8.0 Hz, 2H), 7.28 (ddd, J = 7.8, 5.0, 1.1 Hz, 1H), 7.51 (d, J = 8.0 Hz, 2H), 7.72 (td, J = 7.8, 1.8 Hz, 1H), 8.03 (dt, J = 7.8, 1.1 Hz, 1H), 8.55 (dd, J = 5.0, 1.8 Hz, 1H), ; 13C NMR (100 MHz, CDCl3) δ = 21.1, 48.2, 108.6, 121.7, 124.4, 125.5, 129.1, 136.5, 137.7, 138.6, 149.0, 149.2, 159.0; HRMS (ESI) Calcd for C15H15O2N2 (M + H)+ 255.1128, found 255.1127. 3-Hexyl-5-(4′-methylphenyl)isoxazole (4s). White solid (335 mg, 69% yield): mp 38 °C; IR (ATR) 2927, 1602, 1516, 1468, 1421, 1421, 1264 cm−1; 1H NMR (500 MHz, CDCl3) δ = 0.89 (t, J = 7.0 Hz, 3H), 1.29−1.35 (m, 4H), 1.36−1.42 (m, 2H), 1.70 (quin, J = 7.7 Hz, 2H), 2.38 (s, 3H), 2.69 (t, J = 7.7 Hz, 2H), 6.31 (s, 1H), 7.24 (d, J = 8.2 Hz, 2H), 7.65 (d, J = 8.2 Hz, 2H),; 13C NMR (125 MHz, CDCl3) δ = 14.0, 21.4, 22.5, 26.1, 28.3, 28.9, 31.5, 98.5, 125.0, 125.6, 129.5, 140.1, 164.7, 169.6; HRMS (ESI) Calcd for C16H22ON (M + H)+ 244.1696, found 244.1695.
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1 H NMR and 13C NMR spectra of all pyrazoles and isoxazoles, and X-ray analysis data of pyrazoles 6 and 7, and isoxazole 4f. This material is available free of charge via the Internet at http://pubs.acs.org.
ACKNOWLEDGMENTS Financial support in the form of a Grant-in-Aid for Scientiﬁc Research (No. 25105710) from the Ministry of Education, Culture, Sports, Science, and Technology in Japan, and Iodine Research Project in Chiba University is gratefully acknowledged.
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