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D. Drikermann et al.
Letter
Synlett
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(16) Methyl 5-Phenyl-1H-pyrazole-3-carboxylate (2a); Typical
Procedure
A mixture of cinnamic alcohol (3.35 g, 25.0 mmol, 1.00 equiv),
formic acid (2.83 mL, 75.0 mmol, 3.00 equiv), acetic anhydride
(7.09 mL, 75.0 mmol, 3.00 equiv), tris(dibenzylideneace-
tone)dipalladium(0) (114 mg, 0.5 mol%) and Xantphos (289 mg,
2.0 mol%) was suspended in toluene (15 mL) and heated to 80 °C
for 12 h.17 The mixture was filtered through a short pad of silica
and washed with ethyl acetate. The volatiles were removed
under reduced pressure to give a crude product. The product
could be purified by column chromatography (dry loading, 10–
20% v/v ethyl acetate/petroleum ether) and isolated as a yellow
solid (3.45 g, 85% yield). A suspension of the crude carboxylic
acid from the previous step (3.00 g, 18.5 mmol, 1.00 equiv),
Cs2CO3 (3.01 g, 9.25 mmol, 0.50 equiv) and MeI (2.30 mL, 37.0
mmol, 2.00 equiv) in acetone (150 mL) was heated at 80 °C for 3
h.[18] Silica gel was added and the solvent was evaporated. After
column chromatography purification (dry loading, 5–20% v/v
ethyl acetate/petroleum ether), methyl (E)-4-phenylbut-3-
enoate (3a) was isolated as a yellow oil (2.29 g, 70% yield). To a
solution of p-ABSA (4.09 g, 17.0 mmol, 1.50 equiv) and ester 3a
(2.00 g, 11.3 mmol, 1.00 equiv) in dry acetonitrile (0.10 M) at
0 °C under a N2 atmosphere was slowly added DBU (3.37 mL,
22.6 mmol, 2.00 equiv) over a period of 30 min. After the addi-
tion was complete, the mixture was allowed to warm to RT
overnight and then quenched by addition of sat. NH4Cl solution.
The aqueous layer was extracted with ethyl acetate (3 × 150
mL), and the combined organic phase was dried over MgSO4,
concentrated and the crude residue purified by column chro-
matography (5–20% v/v ethyl acetate/petroleum ether) to give
methyl (E)-2-diazo-4-phenylbut-3-enoate (1a) as a dark red oil
(2.29 g, 72% yield).[13] A solution of vinyldiazo compound 1a
(101 mg, 0.50 mmol, 1.00 equiv) in benzotrifluoride (0.42 mL,
1.20 M) was heated to reflux until complete consumption of the
vinyldiazo compound was observed (<10 min). Silica gel was
then added and the solvent was evaporated. The product was
purified by column chromatography (dry loading, 50–70% v/v
ethyl acetate/petroleum ether) and pyrazole 2a was isolated as
an off-white solid (95.2 mg, 94%). The analytical data matched
with the literature.[10d]
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J. J. Fluorine Chem. 2007, 128, 507. (d) Supurgibekov, B. M.;
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2011, 52, 341. (e) Bel Abed, H.; Mammoliti, O.; Bande, O.; Van
Lommen, G.; Herdewijn, P. J. Org. Chem. 2013, 78, 7845.
1H NMR (250 MHz, CDCl3): = 7.89–7.76 (m, 2 H), 7.51–7.37 (m,
3 H), 7.14 (s, 1 H), 3.97 (s, 3 H).
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Sewald, N. Org. Lett. 2007, 9, 817.
© 2020. Thieme. All rights reserved. Synlett 2020, 31, A–E