solvents for the oxidation of some allylic and benzylic
alcohols by a Pd/C-ethylene system,10 produced the corre-
sponding pyrazoles only in low yield (1-33% yield). It
should be noted that the use of a 1:5 mixture of acetic acid/
CH3CN in the reaction of 1 afforded the product in 81%
yield (18 h). Table 1 exemplifies the conversion of 1,3,5-
(Table 2). It took less time to consume starting pyrazolines
(1.3-2 h), though the yields of pyrazoles were not so high
Table 2. Conversionof 1,3,5-Trisubstituted Pyrazoline to
Pyrazole without Pd/Ca
Table 1. Conversionof 1,3,5-Trisubstituted Pyrazoline to
Pyrazole Catalyzed by Pd/Ca
a All reactions were carried out in a gram scale. b Isolated yield.
(45-78%) compared with those of Pd/C-catalyzed reactions.
This is due to the formation of N-oxides of pyrazolines and
pyrazoles, which was confirmed by HPLC-MS spectra (m/z
314 and 312 for entry 1 in Table 2).11
a All reactions were carried out in a gram scale. b Isolated yield.
Then, we applied a Pd/C-acetic acid system to the
oxidation of Hantzsch 1,4-dihydropyridines to pyridines. This
process has also been conventionally done using an excess
of oxidizing reagents such as HNO3,12 DDQ,13 NaNO2,14
(NH4)2Ce(NO3)6,15 Cu(NO3)2,16 and Bi(NO3)5‚5H2O.17 We
found in this process that the Pd/C-acetic acid system also
trisubstituted pyrazolines (1-6) to pyrazoles (7-12). The
pyrazolines possessing a variety of substituents at the
5-position were treated with 20 wt % 10% Pd/C at 80 °C
for 5.5-19 h to produce the corresponding pyrazoles in high
yield (77__86% yield). We found that when the reactions were
run under aerobic conditions (in the presence of air), slow
conversion was observed even in the absence of Pd/C.
Furthermore, when the oxygen was blown into the reaction
mixture, the reaction proceeded efficiently even without Pd/C
(11) Trace amounts of N-oxides of pyrazoline and pyrazole were observed
even in the reactions under aerobic conditions.
(12) Bo¨cker, R. H.; Guengerich, F. P. J. Med. Chem. 1986, 29, 1596-
1603.
(13) Meyers, A. I.; Natale, N. R. Heterocycles 1982, 18, 13-19.
(14) Loev, B.; Snader, K. M. J. Org. Chem. 1964, 30, 1914-1916.
(15) Pfister, J. R. Synthesis 1990, 689-690.
(16) Maquestiau, A.; Eynde, J.-J. V. Tetrahedron Lett. 1991, 32, 3839-
3840.
(10) (a) Hayashi, M.; Yamada, K.; Arikita, O. Tetrahedron Lett. 1999,
40, 1171-1174. (b) Hayashi, M.; Yamada, K.; Arikita, O. Tetrahedron
1999, 55, 8331-8340. (c) Hayashi, M.; Yamada, K.; Nakayama, S. Synthesis
1999, 1869-1871. (d) Hayashi, M.; Yamada, K.; Nakayama, S. J. Chem.
Soc., Perkin Trans. 1 2000, 1501-1503. (e) Hayashi, M.; Yamada, K.;
Nakayama, S.; Hayashi, H.; Yamazaki, S. Green Chem. 2000, 257-260.
(f) Hayashi, M.; Kawabata, H. J. Synth. Org. Chem. Jpn. 2002, 60, 137-
144.
(17) Mashraqui, S. H.; Karnik, M. A. Synthesis 1998, 713-714. See
also, (NO): Cheng, J.-P.; Zhu, X.-O. J. Org. Chem. 2000, 65, 8158-8163.
(Mn(OAc)3): Varma, R. S.; Kumar, D. Tetrahedron Lett. 1999, 40, 21-
24. (CrO2): Ko, K.-Y.; Kim, J.-Y. Tetrahedron Lett. 1999, 40, 3207-3208.
(RuO3/O2): Mashraqui, S. H.: Karnik, M. A. Tetrahedron Lett. 1998, 39,
4895-4898. (hυ/high pressure): Liu, Z.-L.; Jin, M.-Z. Chem. Commun.
1998, 2451-2452.
3956
Org. Lett., Vol. 4, No. 22, 2002