Silylated β-enaminones as precursors in the regioselective synthesis of silyl pyrazoles

Article abstract of DOI:10.1016/j.tet.2005.10.001

Silyl β-enaminones have been synthesized by reductive cleavage of 5-silyl, 3-, 4- and 5-silylmethylisoxazoles.These versatile synthons bearing different silyl groups in various positions of the enaminoketonic system are of great interest in the regioselec

Full text of DOI:10.1016/j.tet.2005.10.001

Tetrahedron 62 (2006) 611–618
Silylated b-enaminones as precursors in the regioselective
synthesis of silyl pyrazoles
*
´ ´
Mariola Calle, Luis A. Calvo, Alfonso Gonzalez-Ortega and Ana M. Gonzalez-Nogal
´
´
Departamento de Quımica Organica, Facultad de Ciencias, Universidad de Valladolid, Dr. Mergelina s/n, 47011 Valladolid, Spain
Received 10 August 2005; revised 5 October 2005; accepted 6 October 2005
Available online 2 November 2005
Abstract—Silyl b-enaminones have been synthesized by reductive cleavage of 5-silyl, 3-, 4- and 5-silylmethylisoxazoles.These versatile
synthons bearing different silyl groups in various positions of the enaminoketonic system are of great interest in the regioselective synthesis
of 3- or 5-silylpyrazoles and 3-, 4- or 5-silylmethylpyrazoles, which can serve as building blocks in heterocyclic chemistry.
q 2005 Elsevier Ltd. All rights reserved.
1. Introduction
depends on the nature of the hydrazine substituents
(H, Me, Ph) and silyl group (Me3, Me2Ph, tBuPh2). When
hydrazine or methyl hydrazine were used the 4-trimethyl-
silyl- and 4-dimethylphenylsilyl-3-butyn-2-one gave the
corresponding 5-silylpyrazoles. Nevertheless, heating of the
hindered 4-tert-butyldiphenylsilyl-3-butyn-2-one with
hydrazine or methyl hydrazine afforded the respective
hydrazones. On the other hand, when phenylhydrazine was
used as a 1,2-dinucleophile (free, in the form of
hydrochloride or with acid catalysis) cyclization did not
take place. The reaction with trimethyl-, dimethylphenyl-
and tert-butyldiphenyl silyl alkynones led to the appropriate
phenyl hydrazones. Interestingly, the trimethylsilyl-
and dimethylphenylsilyl N-phenyl hydrazones, but not the
tert-butyldiphenylsilyl derivative, could be cyclized to
4-functionalized 5-trimethylsilyl- and 5-dimethylphenyl-
silylpyrazoles by reaction with electrophiles.
In a preliminary communication1 we described the synthesis
of silylated b-enaminoketones by catalytic hydrogenation of
4- and 5-silylisoxazoles, as well as those resulting from the
reductive cleavage of their 4- and 5-homologues. Moreover,
we used these synthons as substrates in the synthesis of
silylated pyrroles, pyrazoles, pyrimidines and pyridinones.
Following on with our current research in the synthesis and
applications of silylated azoles, we were especially
interested in the regioselective synthesis of azoles bearing
silyl groups in different positions of the heterocyclic ring.
Previously, we had reported2 the synthesis of regioisomeric
silicon and tin 4- or 5-metalated pyrazoles by silyl- or
stannylcupration of 4-halopyrazoles or lithiation with LDA
of 5-unsubstituted pyrazoles and the subsequent treatment
with chlorosilanes or chlorostannanes, respectively. Further-
more, starting from 5-unsubstituted 4-halopyrazoles
using both procedures, we were able to synthesize a variety
of 4,5-dimetalated pyrazoles bearing different silyl and tin
groups. This methodology could be applied to the
preparation of 4-silyl- and 4-stannylisoxazoles, but not to
the synthesis of 5-silyl- or 5-stannylisoxazoles, because the
5-lithio intermediate is opened by fission of N–O and C3–C4
bonds.3 Fortunately, we synthesized 5-silylisoxazoles by
reaction of b-silylalkynones with hydroxylamine.4 The
reaction of these substrates with hydrazines is more
complex. The cyclization to the 5-silylpyrazole or the
exclusive formation of the corresponding hydrazone
In this paper, we describe the full results in the preparation
of b-enaminones bearing different silyl groups in distinct
positions of the enaminoketonic system5 and their reaction
with various hydrazines with the aim of knowing the scope
of this methodology in the regioselective synthesis of a
variety of new silylpyrazoles impossible or difficult to
synthesize by other procedures.
2. Results and discussion
Differently silylated b-aminoenones have been synthesized
by catalytic hydrogenation of 5-silyl-, 3-, 4-, and
5-silylmethylisoxazoles (Scheme 1).
Keywords: Silicon isoxazoles; Silicon b-enaminones; Silicon pyrazoles.
*
Corresponding author. Tel.: C34 983 423213; fax: C34 983 423013;
e-mail: agn@qo.uva.es
0040–4020/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2005.10.001

612
M. Calle et al. / Tetrahedron 62 (2006) 611–618
R1
Me
Me
NH2
R2
R1
R2
R3
R2NHNH2
EtOH, rt
H2 [Ni]
EtOH
N
NH2
R31Si
R3
N
N
O
R31Si
O
O
R2
4
1
2
2
1a; R1 = Me, R2 = H, R3 = SiMe3
1b; R1 = Me, R2 = H, R3 = SiMe2Ph
1c; R1 = Me, R2 = H, R3 = SiPh2But
2a (91%)
2b (95%)
4d (91%); R13 = Me3, R2 = Et
4e (85%); R13 = Me3, R2 = Ph
4f (80%); R13 = Me3 R2 = CONH2
2a; R13 = Me3
2a
2a
2c (92%)
2d (89%)
,
1d; R1 = Me, R2 = CH2SiMe3, R3 = Me
2b; R13 = Me2Ph 4g (<5%); R13 = Me2Ph, R2 = But
1e; R1 = Me, R2 = H, R3 = CH2SiMe3
1f; R1 = Me, R2 = H, R3 = CH2SiMePh2
2e (90%)
2f (90%)
2b
4h (69%); R13 = Me2Ph, R2 = Ph
4i (80%); R13 = Ph2But, R2 = Bn
2c; R13 = Ph2But
1g; R1 = Me, R2 = H, R3 = CH2SiPh2But
2g (95%)
4j (<5%); R13 = Ph2But, R2 = But
4k (75%); R13 = Ph2But, R2 = Ph
2c
2c
1h; R1 = Pri, R2 = H, R3 = CH2SiPh2But
1i; R1 = Ph, R2 = H, R3 = CH2SiMe3
2h (96%)
2i (91%)
2j (98%)
Scheme 3.
1j; R1 = Ph, R2 = H, R3 = CH2SiPh2But
1k; R1 = CH2SiMe3, R2 = H, R3 = Ph
1l; R1 = CH2SiPh2But, R2 = H, R3 = Ph
The low reactivity (!5%) of b-enaminoacylsilanes 2b and
2c with tert-butylhydrazine may be due to the steric
hindrance between the bulky tert-butyl and silyl groups.
2k (95%)
2l (96%)
Although we had previously synthesized 5-silylpyrazoles by
reaction of b-silylalkynones with hydrazines,4 the cycli-
zation of N-phenylhydrazones and all types of hydrazones
bearing the bulky tert-butyldiphenylsilyl group had not
taken place. Therefore, this is a complementary method for
preparing, with acceptable yields, 5-silylated N-phenyl-
pyrazoles 4e, 4h and N-alkyl- or N-phenyl 5-tert-butyl-
diphenylsilylpyrazoles 4i or 4k, respectively.
Scheme 1.
The catalytic hydrogenation of 5-silylisoxazoles 1a–c
afforded stable acylsilane derivatives 2a–c, which reacted
regioselectively with hydrazines to give 3- or 5-silyl
pyrazoles. The formation of the 3- or 5-regioisomer depends
exclusively on the nature of the hydrazine. When
methylhydrazine was used as a nucleophile, the trimethyl,
dimethylphenyl and tert-butyldiphenylsilyl b-enaminones
2a–c led to 3-silylpyrazoles 3a–c with acceptable regio-
selectivity (Scheme 2).
The silylated b-aminoenone 2d, obtained by reductive
cleavage of 4-trimethylsilylmethylisoxazole 1d, is very
stable despite containing an allylsilane moiety and it was
shown to be reactive toward phenylhydrazine and semi-
carbazide giving 4-trimethylsilylmethylpyrazoles 6a and
6b, respectively (Scheme 4).
Me
O
SiR3
+
Me
MeNHNH2
EtOH,
NH2
N
N
R3Si
Me
N
Me
N
Me
reflux
Me3Si
R3Si
Me
Me
RNHNH2
Me3Si
NH2
3
4
2
N
Me
N
R
EtOH, rt
O
3a (55%) + 4a (18%)
3b (50%) + 4b (15%)
Me
2a; R3 = Me3
2
6
2b; R3 = Me2Ph
2d
2d
6a (72%); R = Ph
2c; R3 = Ph2But
3c (75%) + 4c (16%)
6b (78%); R = CONH2
Scheme 2.
Scheme 4.
This result is very interesting because, as far as we know,
general procedures for synthesizing N-methyl-3-silyl
pyrazoles have not been previously described.
The formation of 4-trimethylsilylmethylpyrazoles 6a and
6b should take place through the respective 5-hydroxy-
pyrazoline, because the stabilized intermediate 5 resulting
from the reaction of 2d with semicarbazide was identified
by 1H NMR spectroscopy from the reaction mixture
(Scheme 5).
However, the reaction of these substrates with other
hydrazines (ethyl, tert-butyl or phenylhydrazine) afforded
only the respective 5-silylpyrazoles 4d–k (Scheme 3).

M. Calle et al. / Tetrahedron 62 (2006) 611–618
613
R2
SiMe3
R2
Me
NH2
R3NHNH2
EtOH, rt
Me
NH2
NH2CONHNH2
EtOH, rt
Me
Me3Si
Me
N
N
3 R3
N
O
SiR1
N
C
O
H
SiR1
O
3
2
7
NH2
2d
O
5
3
1
2
7d (93%); R = Et
2f; R = MePh , R = Me
3
2
Me
3
Me3Si
2f
7e(79%); R = CONH
2
H2O
N
Me
N
1
t
2
3
t
2g; R = Ph Bu , R = Me
7f (85%); R = Bu
3
2
CONH2
3
7g (91%); R = Et
2g
2g
6b
3
Scheme 5.
7h (89%); R = Ph
It is interesting to point out that this methodology has
allowed us to prepare 4-silylmethylpyrazoles, for the first
time, impossible to obtain by the procedure used to
synthesize the analogous isoxazoles (starting from 4-chloro-
methylisoxazoles by treatment with chlorosilanes) because,
in contrast to the isoxazole ring, which can be chloro-
methylated in the 4-position, the pyrazole nucleus does not
undergo chloromethylation reaction.
1
t
2
i
3
2h; R = Ph Bu , R = Pr
7i (60%); R = Ph
3
2
______
______
2i; R13 = Me3, R2 = Ph
1
t
2
2j; R = Ph Bu , R = Ph
3
2
Scheme 7.
into its 3-silylmethyl regioisomer 8c (R13ZPh2But, R2Z
CONH2) (Scheme 6).
Conversely, the stability of the silyl b-enaminones 2e–j
resulting from catalytic hydrogenolysis of 5-silylmethyl
isoxazoles 1e–j, depends on the nature of the silyl group
(Schemes 6 and 7). When this is trimethylsilyl, the a0-silyl-
b-enaminoketones 2e and 2i were unstable in the acidic
reaction medium. In their treatment with hydrazine
hydrochlorides or oxalates, desilylated pyrazoles were
obtained. Nevertheless, b-aminoenones bearing silyl groups
of minor nucleofugacity such as diphenylmethyl 2f and
tert-butyldiphenyl 2g,h furnished, in general, the
corresponding 5-silylmethylpyrazoles. The unstable
a0-trimethylsilyl enaminone 2e reacted only with free
methylhydrazine but unfortunately the regioselectivity was
lost and a mixture almost equimolecular of 3-trimethyl-
silylmethyl- 7a and 5-trimethylsilylmethyl pyrazole 8a was
obtained. The same behavior was observed in the reaction of
this hydrazine with the tert-butyldiphenylsilyl derivative 2g
and a mixture of the two possible regioisomers 7b and 8b
was again isolated, in which the first was the majority
product. On the other hand, when 2g was treated with
semicarbazide only the 5-silylmethylpyrazole 7c was
obtained which, left to stand at rt, was slowly transformed
The reaction of b-enaminones 2f–h with other hydrazines
was regioselective leading to 5-silylmethylpyrazoles 7d–i
exclusively. The reactivity of b-aryl enaminones 2i,j
toward hydrazines at reflux from ethanol was low.
Attempts to increase the yields by heating the reaction
mixture at higher temperatures led to decomposition of the
substrate (Scheme 7).
Finally, the presence of the phenyl group in the g-silyl-b-
enaminones 2k,l, resulting from catalytic hydrogenation of
3-silylmethyl-5-phenylisoxazoles 1k,l, decreases their reac-
tivity toward hydrazines. Moreover, the trimethylsilyl
derivative 2k is not very stable, affording desilylated
pyrazoles. Fortunately, the tert-butyldiphenylsilyl counter-
part 2l could be condensed with different hydrazines,
yielding regioselectively the corresponding 3-tert-butyl-
diphenylsilylmethylpyrazoles 8d,e (Scheme 8).
SiPh2But
SiPh2But
RNHNH2
Me
NH2
Me
SiR1
N
NH2
R2NHNH2
3
Ph
N
R
EtOH, rt
N
N
Me
Ph
8
O
N
N
R2
2
EtOH,
reflux
O
SiR1
R2
SiR1
3
3
8d (45%); R = But
8e (34%); R = Ph
2
8
2l
2l
7
2
1
7a (40%) + 8a (38%); R = Me
2e; R = Me
3
3
Scheme 8.
1
t
2
2g; R = Ph Bu
7b (51%) + 8b (28%); R = Me
3
2
The regioselectivity observed when ethyl, tert-butyl and
phenylhydrazine or semicarbazide were used, can be
explained by initial 1,4-addition of the NH2 group to
2
1
t
7c (75%); R =CONH
2g; R = Ph Bu
2
3
2
Scheme 6.

614
M. Calle et al. / Tetrahedron 62 (2006) 611–618
the enaminoketone, followed by 1,2-addition of the NH
group to give a 5-hydroxy-2-pyrazoline intermediate 5 (R1,
R3ZMe; R2ZCH2SiMe3; R4ZCONH2), which was ident-
ified by 1H NMR in the reaction of the enaminone 2d with
semicarbazide (Scheme 9).
3. Experimental
3.1. General
THF was distilled from sodium benzophenone ketyl in a
recycling still and ethanol was dried by refluxing with Mg.
All chromatographic and workup solvents were distilled
prior to use. Reactions involving organometallic reagents
R1
R1
O
R2
R3
R4NHNH2
A1,4
NH2
NH
R2
R3
1
were carried out under N2. H and 13C NMR spectra were
NH2
recorded at 300 and 75.4 MHz, respectively in CDCl3 as an
internal standard. Carbon multiplicities were assigned by
DEPT experiments. Reactions were monitored by TLC on a
precoated plate of silica gel 60 (nano-SIL-20, Macheray-
Nagel). Flash chromatography was performed on silica gel
60 (230–240 mesh, M-N).
NHR4
O
R4 = Et, But, Ph, CONH2
R1
R2
HO
R1
R2
R1
R2
R3
A1,2
NHR4
N
N
N
R3
3.2. Preparation of silylisoxazoles
N
R4
R3
N
R4
O
5
5-Silylisoxazoles 1a, 1b and 1c have been prepared by a
procedure described by us.3 The 4-silylmethylisoxazole 1d
and 5-silylmethylisoxazole 1e have also been previously
described.8 The remaining 5-silylmethyl- 1h–j and 3-silyl-
methylisoxazoles 1k,l were synthesized by the same
procedure8 starting from the corresponding 5-methyl- or
3-methylisoxazoles by hydrogen–lithium exchange and
quenching with the suitable chlorosilane. The following
products were obtained:
R1, R3 = Me;
R2 =CH2SiMe3;
R4 = CONH2
Scheme 9.
On the other hand, the loss of regioselectivity in the
reactions with methylhydrazine could be due to the
competitive 1,4-addition of the more nucleophilic NHMe
group to the enaminone and subsequent 1,2-addition of the
NH2 group to afford the corresponding regioisomer
(Scheme 10).
3.2.1. 3-Methyl-5-(methyldiphenylsilylmethyl)isoxazole
(1f). Yield 83%; bp 132–134 8C/0.1 mmHg; IR (film)
1596, 1427, 1114, 818, 798, 731, 699 cmK1 1H NMR
;
(CDCl3) d 0.65 (s, 3H), 2.20 (s, 3H), 2.74 (s, 2H), 5.49
(s, 1H), 7.39–7.55 (m, 10H); 13C NMR (CDCl3) d K4.42,
11.33, 15.44, 100.99, 127.93, 129.69, 134.36, 134.91,
159.79, 170.49. Anal. Calcd for C18H19NOSi: C, 73.68;
H, 6.53; N, 4.77. Found: C, 73.91; H, 6.49; N, 4.91.
R1
R1
NH2
R2
R3
R2
R3
MeNHNH2
A1,4
NH2
NMe
NH2
O
O
3.2.2.
5-(tert-Butyldiphenylsilylmethyl)-3-methyl-
isoxazole (1g). Yield 75%; bp 164–167 8C/0.5 mmHg; IR
(film) 1595, 14.27, 1108, 734, 700 cmK1; 1H NMR (CDCl3)
d 1.09 (s, 9H), 2.08 (s, 3H), 2.82 (s, 2H), 5.21 (s, 1H),
7.35–7.63 (m, 10H); 13C NMR (CDCl3) d 11.30, 11.66,
18.38, 27.49, 101.40, 127.68, 129.52, 133.01, 135.79,
159.67, 170.50. Anal. Calcd for C21H25NOSi: C, 75.18;
H, 7.51; N, 4.17. Found: C, 74.95; H, 7.62; N, 4.20.
R1
R2
R1
NMe
R2
R3
R2
R3
HO
NMe
NH2
N
R1
N
H
A1,2
R3
N
O
Me
Scheme 10.
In conclusion, we have easily prepared b-enamino acyl-
silanes, a-silylmethyl-, a0-silyl- and g-silyl-b-enaminones by
reductive cleavage of 5-silyl-, 4-silylmethyl, 5-silylmethyl
and 3-silylmethylisoxazoles, respectively. Moreover, we
demonstrated1 that they are interesting synthons in the
creation of pyrroles, pyrazoles, pyrimidines and pyridinones
bearing arylsilane, arylmethylsilane and a-silylketone
moieties. In this paper, we have used these substrates for
synthesizing regioselectively a variety of 3- or 5-silyl-
pyrazoles, 3-silylmethyl-, 4-silylmethyl- and 5-silylmethyl-
pyrazoles, of which no general method of synthesis is known.
The easy electrophilic ipso-substitution in heteroarylsilanes2,6
may permit the introduction, through the silyl group, of
carbon or heteoatomic groups. Likewise, arylmethylsilanes7
are synthetic equivalents of silicon-stabilized carbanions with
wide synthetic possibilities.
3.2.3. 5-(tert-Butyldiphenylsilylmethyl)-3-isopropyl-
isoxazole (1h). Yield 62%; mp 59–60 8C (from hexane);
IR (KBr) 1588, 1467, 1100, 794, 731, 700 cmK1; 1H NMR
(CDCl3) d 1.08 (s, 9H), 1.10 (d, JZ7.11 Hz, 6H), 2.80
(s, 2H), 2.84 (m, JZ7.11 Hz, 1H), 5.17 (s, 1H), 7.33–7.65
(m, 10H); 13C NMR (CDCl3) d 11.97, 18.36, 21.57, 26.27,
27.53, 98.81, 127.65, 129.49, 133.14, 135.88, 169.30,
170.27. Anal. Calcd for C23H29NOSi: C, 75.98; H, 8.04;
N, 3.85. Found: C, 76.10; H, 7.99; N, 3.82.
3.2.4. 5-(Trimethylsilylmethyl)-3-phenylisoxazole (1i).
Yield 95%; mp 51 8C (from hexane); IR (KBr) 1594,
1
1575, 1469, 1251, 846, 769, 696 cmK1; H NMR (CDCl3)
d 0.13 (s, 9H), 2.29 (s, 2H), 6.12 (s, 1H), 7.43–7.82 (m, 5H);
13C NMR (CDCl3) d K1.75, 17.90, 97.21, 126.62, 128.72,
129.52, 129.60, 162.36, 172.89. Anal. Calcd for

M. Calle et al. / Tetrahedron 62 (2006) 611–618
615
C13H17NOSi: C, 67.49; H, 7.41; N, 6.05. Found: C, 67.61;
H, 7.35; N, 5.99.
13C NMR (CDCl3) d 0.48, 22.16, 27.97, 105.77, 127.55,
129.15, 133.83, 136.33, 159.31, 220.75. Anal. Calcd for
C20H25NOSi: C, 74.25; H, 7.79; N, 4.33. Found: C, 74.38;
H, 7.60; N, 4.51.
3.2.5. 5-(tert-Butyldiphenylsilylmethyl)-3-phenyl-
isoxazole (1j). Yield 82%; mp 103 8C (from hexane); IR
1
(KBr) 1592, 1575, 1467, 1109, 767, 700 cmK1; H NMR
3.3.4. 4-Amino-3-(trimethylsilylmethyl)pent-3-en-2-one
(2d). Yield 89%; mp 58 8C (from hexane); IR (KBr) 3338,
2953, 1602, 1480, 1247, 851, 838 cmK1; 1H NMR (CDCl3)
d 0.02 (s, 9H), 1.63 (s, 2H), 1.90 (s, 3H), 2.11 (s, 3H);
13C NMR (CDCl3) d K0.67, 17.20, 22.24, 28.77, 101.80,
156.96, 197.13. Anal. Calcd for C9H19NOSi: C, 58.32;
H, 10.33; N, 7.56. Found: C, 58.56; H, 10.31; N, 7.40.
(CDCl3) d 1.13 (s, 9H), 2.92 (s, 2H), 5.66 (s, 1H), 7.38–7.67
(m, 15H); 13C NMR (CDCl3) d 12.09, 18.41, 27.55, 98.71,
126.56, 127.76, 128.65, 129.42, 129.54, 129.63, 133.95,
135.87, 162.23, 171.47. Anal. Calcd for C26H27NOSi:
C, 78.54; H, 6.84; N, 3.52. Found: C, 78.43; H, 6.82; N, 3.61.
3.2.6. 3-(Trimethylsilylmethyl)-5-phenylisoxazole (1k).
Yield 61%; mp 59–60 8C (from hexane); IR (KBr) 1613,
3.3.5. 4-Amino-1-(trimethylsilyl)pent-3-en-2-one (2e).
Yield 90%; mp 69 8C (from hexane); IR (KBr) 3300,
1590, 1573, 1423, 1248, 1150, 819, 767, 695 cmK1
;
1H NMR (CDCl3) d 0.20 (s, 9H), 2.21 (s, 2H), 6.31
(s, 1H), 7.40–7.85 (m, 5H); 13C NMR (CDCl3) d K1.90,
15.91, 99.57, 125.39, 128.62, 128.77, 129.49, 162.22,
168.80. Anal. Calcd for C13H17NOSi: C, 67.49; H, 7.41;
N, 6.05. Found: C, 67.38; H, 7.46; N, 6.12.
3138, 1605, 1530, 1249, 855 cmK1 1H NMR (CDCl3)
;
d 0.07 (s, 9H), 1.87 (s, 3H), 1.97 (s, 2H), 4.86 (s, 1H), 4.87
(br, 1H), 9.56 (br, 1H); 13C NMR (CDCl3) d K1.20, 22.28,
37.08, 96.13, 159.15, 198.26. Anal. Calcd for C8H17NOSi:
C, 56.09; H, 10.00; N, 8.18. Found: C, 55.91; H, 9.83;
N, 8.30.
3.2.7. 3-(tert-Butyldiphenylsilylmethyl)-5-phenyl-
isoxazole (1l). Yield 31%; mp 90–91 8C (from hexane);
IR (KBr) 1686, 1574, 1425, 1415, 1111, 765, 740, 702,
3.3.6. 4-Amino-1-(methyldiphenylsilyl)pent-3-en-2-one
(2f). Yield 90%; mp 76 8C (from hexane); IR (KBr) 3303,
1
691 cmK1; H NMR (CDCl3) d 1.12 (s, 9H), 2.79 (s, 2H),
3134, 1606, 1530, 1288, 1126, 1062, 777, 698 cmK1
;
5.49 (s, 1H), 7.40–7.69 (m, 15H); 13C NMR (CDCl3)
d 10.70, 18.52, 27.65, 100.00, 125.52, 127.61, 127.74,
128.71, 129.53, 129.65, 133.40, 136.07, 161.93, 168.62.
Anal. Calcd for C26H27NOSi: C, 78.54; H, 6.84; N, 3.52.
Found: C, 78.65; H, 6.75; N, 3.59.
1H NMR (CDCl3) d 0.69 (s, 3H), 1.75 (s, 3H), 2.54 (s, 2H),
4.76 (s, 1H), 4.93 (br, 1H), 7.26–7.61 (m, 10H), 9.56
(br, 1H); 13C NMR (CDCl3) d K4.04, 22.05, 34.29, 96.65,
127.68, 129.25, 134.49, 136.28, 159.51, 196.72. Anal. Calcd
for C18H21NOSi: C, 73.17; H, 7.16; N, 4.77. Found: C,
72.95; H, 7.22; N, 4.91.
3.3. Synthesis of silyl b-aminoenones. Typical procedure
3.3.7. 4-Amino-1-(tert-butyldiphenylsilyl)pent-3-en-2-
one (2g). Yield 95%; mp 117 8C (from hexane); IR (KBr)
A mixture of silylated isoxazole (20 mmol) and 1.0 g of
Raney nickel in 10 mL of dry ethanol was stirred under
400 psi of hydrogen at rt for 24 h. Then, the catalyst was
removed by filtration and the solvent was evaporated under
reduced pressure. The concentrate was chromatographed on
silica gel using CH2Cl2–Et2O (20/1) as eluent. The
following silyl b-aminoenones were obtained:
1
3310, 3073, 1613, 1471, 1205, 789, 701 cmK1; H NMR
(CDCl3) d 1.01 (s, 9H), 1.6 (s, 3H), 2.6 (s, 2H), 4.5 (s, 1H),
4.65 (br, 1H), 7.35–7.50 (m, 6H), 7.55–7.7 (m, 4H), 9.40
(br, 1H); 13C NMR (CDCl3) d 18, 22, 27, 31, 97, 126, 129,
135, 137, 149, 197. Anal. Calcd for C21H27NOSi: C, 74.73;
H, 8.06; N, 4.15. Found: C, 75.00; H, 7.92; N, 4.18.
3.3.1. 3-Amino-1-(trimethylsilyl)but-2-en-1-one (2a).
Yield 91%; mp 84 8C (from hexane); IR (KBr) 3255,
3097, 1618, 1522, 1385, 1269, 1246, 867, 840, 752,
3.3.8. 4-Amino-1-(tert-butyldiphenylsilyl)-5-methylhex-
3-en-2-one (2h). Yield 96%; mp 63–64 8C (from hexane);
IR (KBr) 3300, 3105, 1599, 1520, 1277, 1109 cmK1
;
679 cmK1; H NMR (CDCl3) d 0.12 (s, 9H), 1.90 (s, 3H),
1H NMR (CDCl3) d 0.85 (d, JZ6.9 Hz, 6H), 1.09 (s, 9H),
1.97 (m, JZ6.9 Hz, 1H), 2.60 (s, 2H), 4.54 (s, 1H), 4.71
(br, 1H), 7.27–7.41 (m, 6H), 7.66–7.69 (m, 4H), 9.30
(br, 1H); 13C NMR (CDCl3) d 18.59, 20.71, 27.55, 31.38,
34.41, 94.01, 127.32, 129.00, 134.35, 136.11, 168.37,
197.45. Anal. Calcd for C23H31NOSi: C, 75.56; H, 8.55;
N, 3.83. Found: C, 75.63; H, 8.61; N, 3.81.
1
5.46 (s, 1H), 5.62 (br, 1H), 10.76 (br, 1H); 13C NMR
(CDCl3) d 2.95, 22.00, 102.19, 159.78, 225.08. Anal. Calcd
for C7H15NOSi: C, 53.45; H, 9.61; N, 8.91. Found: C, 53.38;
H, 9.73; N, 9.00.
3.3.2. 3-Amino-1-(dimethylphenylsilyl)but-2-en-1-one
(2b). Yield 95%; mp 88–89 8C (from hexane); IR (KBr)
3213, 1590, 1427, 1253, 1118, 830, 797, 700 cmK1
;
3.3.9. 4-Amino-1-(trimethylsilyl)4-phenylbut-3-en-2-one
(2i). Yield 91%; mp 64 8C (from hexane); IR (KBr) 3324,
1H NMR (CDCl3) d 0.48 (s, 6H), 1.89 (s, 3H), 5.17 (br, 1H),
5.49 (s, 1H), 7.27–7.61 (m, 5H), 10.85 (br, 1H). Anal. Calcd
for C12H17NOSi: C, 65.71; H, 7.81; N, 6.39. Found:
C, 65.50; H, 7.71; N, 6.48.
1
3153, 1609, 1534, 850 cmK1; H NMR (CDCl3) d 0.13
(s, 9H), 2.13 (s, 2H), 5.11 (br, 1H), 5.30 (s, 1H), 7.42–7.55
(m, 5H), 9.83 (br, 1H); 13C NMR (CDCl3) d K1.14, 37.89,
95.73, 126.18, 128.81, 130.23, 137.64, 159.42, 198.90.
Anal. Calcd for C13H19NOSi: C, 66.90; H, 8.21; N, 6.00.
Found: C, 67.10; H, 8.02; N, 6.20.
3.3.3. 3-Amino-1-(tert-butyldiphenylsilyl)but-2-en-1-one
(2c). Yield 92%; mp 155–156 8C (from hexane); IR (KBr)
1
3310, 1624, 1507, 1383, 1288, 1102, 702 cmK1; H NMR
(CDCl3) d 1.20 (s, 9H), 1.80 (s, 3H), 5.24 (br, 1H), 5.39
(s, 1H), 7.35–7.42 (m, 6H), 7.70–7.73 (m, 4H), 10.94 (br, 1H);
3.3.10. 4-Amino-1-(tert-butyldiphenylsilyl)-4-phenylbut-
3-en-2-one (2j). Yield 98%; mp 88–89 8C (from hexane);

616
M. Calle et al. / Tetrahedron 62 (2006) 611–618
IR (KBr) 3456, 1602, 1571, 1520, 1484, 700 cmK1
;
137.47, 146.23. Anal. Calcd for C21H26N2Si: C, 75.40;
H, 7.83; N, 8.37. Found: C, 75.21; H, 7.70; N, 8.51.
1H NMR (CDCl3) d 1.14 (s, 9H), 2.73 (s, 2H), 4.90 (br, 1H),
4.94 (s, 1H), 7.03–7.76 (m, 15H), 9.60 (br, 1H); 13C NMR
(CDCl3) d 18.71, 27.63, 32.05, 96.89, 126.04, 127.54,
128.50, 129.14, 130.07, 134.29, 136.19, 137.19, 158.79,
197.81. Anal. Calcd for C26H29NOSi: C, 78.15; H, 7.31;
N, 3.51. Found: C, 78.32; H, 7.02; N, 3.50.
3.4.4. 1,3-Dimethyl-5-(trimethylsilyl)pyrazole (4a). Yield
18%; oil, Rf 0.27 (hexane/ethyl acetate, 10:1); IR (film)
1
1589, 1500, 1470, 1260 cmK1; H NMR (CDCl3) d 0.29
(s, 9H), 2.31 (s, 3H), 3.87 (s, 3H), 6.08 (s, 1H); 13C NMR
(CDCl3) d K1.02, 12.80, 39.14, 114.07, 143.51, 147.63.
Anal. Calcd for C8H16N2Si: C, 57.09; H, 9.58; N, 16.64.
Found: C, 57.40; H, 9.22; N, 16.28.
3.3.11. 3-Amino-4-(trimethylsilyl)-1-phenyl-but-2-en-1-
one (2k). Yield 95%; mp 89 8C (from hexane); IR (KBr)
3430, 1594, 1569, 1523, 852, 704 cmK1; 1H NMR (CDCl3)
d 0.15 (s, 9H), 1.79 (s, 2H), 5.17 (br, 1H), 5.57 (s, 1H), 7.40
(m, 3H), 7.87 (m, 2H), 10.43 (br, 1H); 13C NMR (CDCl3) d
K1.52, 29.42, 91.24, 126.95, 128.12, 130.47, 140.60,
167.17, 188.20. Anal. Calcd for C13H19NOSi: C, 66.90;
H, 8.21; N, 6.00. Found: C, 66.75; H, 8.30; N, 5.91.
3.4.5. 1,3-Dimethyl-5-(dimethylphenylsilyl)pyrazole
(4b). Yield 15%; oil, Rf 0.28 (hexane/ethyl acetate, 10:1);
1
IR (film) 1580, 1500, 1470, 1270, 1100 cmK1; H NMR
(CDCl3) d 0.58 (s, 6H), 2.29 (s, 3H), 3.68 (s, 3H), 6.21,
(s, 1H), 7.39–7.73 (m, 5H). Anal. Calcd for C13H18N2Si: C,
67.77; H, 7.88; N, 12.16. Found: C, 67.43; H, 7.52; N, 12.49.
3.3.12. 3-Amino-4-(tert-butyldiphenylsilyl)-1-phenylbut-
2-en-1-one (2l). Yield 96%; mp 138 8C; IR (KBr): 3270,
1593, 1521, 1324, 1107, 769, 701 cmK1; 1H NMR (CDCl3)
d 1.17 (s, 9H), 2.42 (s, 2H), 4.99 (br, 1H), 5.61 (s, 1H),
7.35–7.50 (m, 9H), 7.65–7.70 (m, 6H), 10.17 (br, 1H);
13C NMR (CDCl3) d 18.60, 22.75, 27.54, 93.01, 126.88,
127.87, 129.69, 130.34, 133.09, 135.89, 140.36, 165.98,
188.95. Anal. Calcd for C26H29NOSi: C, 78.15; H, 7.31; N,
3.51. Found: C, 78.43; H, 7.10; N, 3.60.
3.4.6. 5-(tert-Butyldiphenylsilyl)-1,3-dimethylpyrazole
(4c). Yield 16%; oil, Rf 0.33 (hexane/ethyl acetate, 3:1);
IR (film) 1516, 1471, 1106 cmK1; 1H NMR (CDCl3) d 1.14
(s, 9H), 2.37 (s, 3H), 3.28 (s, 3H), 6.60 (s, 1H), 7.35–7.59
(m, 10H); 13C NMR (CDCl3) d 13.11, 18.43, 27.63, 40.03,
116.78, 127.94, 129.61, 133.01, 135.84, 137.50, 147.56.
Anal. Calcd for C21H26N2Si: C, 75.40; H, 7.83; N, 8.37.
Found: C, 75.68; H, 7.72; N, 8.51.
3.4. Synthesis of silylpyrazoles. General procedure
3.4.7. 1-Ethyl-3-methyl-5-(trimethylsilyl)pyrazole (4d).
Yield 91%; oil, bp 108–110 8C/1 mmHg; 1H NMR (CDCl3)
d 0.31 (s, 9H), 1.44 (t, 3H, JZ7.13 Hz), 2.28 (s, 3H), 4.13
(q, 2H, JZ7.13 Hz), 6.09 (s, 1H). Anal. Calcd for
C9H18N2Si: C, 59.28; H, 9.95; N, 15.36. Found: C, 59.42;
H, 10.01; N, 15.15.
A mixture of silylated b-aminoenone (2 mmol) and free
methylhydrazine, tert-butyl- and phenylhydrazine hydro-
chloride, ethylhydrazine oxalate or semicarbazide hydro-
chloride (3 mmol) in 8 mL of dry ethanol was stirred at rt
or at reflux. At the end of the reaction (monitored by TLC)
the solvent was evaporated under reduced pressure and
the residue was extracted with CH2Cl2/H2O. The organic
layer was dried and concentrated. The residue was purified
by flash chromatography on silica gel using as eluent
CH2Cl2–Et2O (20/1–10/1) to give the following products:
3.4.8. 3-Methyl-1-phenyl-5-(trimethylsilyl)pyrazole (4e).
Yield 85%; oil, bp 175–180 8C/1.5 mmHg; IR (film) 1600,
1501, 1252, 844, 762, 695 cmK1; 1H NMR (CDCl3) d 0.10
(s, 9H), 2.34 (s, 3H), 6.33 (s, 1H), 7.33–7.41 (m, 5H);
13C NMR (CDCl3) d K0.62, 12.84, 115.65, 125.74, 127.83,
128.53, 142.28, 144.26, 148.74. Anal. Calcd for
C13H18N2Si: C, 67.77; H, 7.88; N, 12.16. Found:
C, 67.45; H, 8.00; N, 12.21.
3.4.1. 1,5-Dimethyl-3-(trimethylsilyl)pyrazole (3a). Yield
60%; oil, bp 95–98 8C/0.5 mmHg; IR (film) 1247, 842,
1
757 cmK1; H NMR (CDCl3) d 0.26 (s, 9H), 2.25 (s, 3H),
3.80 (s, 3H), 6.13 (s, 1H); 13C NMR (CDCl3) d 1.13, 10.86,
35.90, 111.60, 138.16, 151.36. Anal. Calcd for C8H16N2Si:
C, 57.09; H, 9.58; N, 16.64. Found: C, 58.21; H, 9.60;
N, 16.41.
3.4.9. 3-Methyl-5-(trimethylsilyl)pyrazole-1-carbox-
amide (4f). Yield 80%; mp 78–80 8C (from hexane); IR
(KBr) 3462, 3233, 1725, 1584, 1438, 1371, 947 cmK1
;
H NMR (CDCl3) d 0.31 (s, 9H), 2.27 (s, 3H), 5.57 (br, 1H),
6.30 (s, 1H), 7.27 (br, 1H); 13C NMR (CDCl3) d K0.90,
13.11, 118.99, 146.17, 151.43, 152.36. Anal. Calcd for
C8H15N3OSi: C, 48.70; H, 7.66; N, 21.30. Found: C, 48.82;
H, 7.60; N, 21.52.
3.4.2. 1,5-Dimethyl-3-(dimethylphenylsilyl)pyrazole
(3b). Yield 50%; oil, Rf 0.38 (CH2Cl2/Et2O, 20:1); IR
(film) 1537, 1425, 1245, 1109, 831, 814, 776, 698 cmK1
;
1H NMR (CDCl3) d 0.66 (s, 6H), 2.30 (s, 3H), 3.86 (s, 3H),
6.21 (s, 1H), 7.42–7.71 (m, 5H); 13C NMR (CDCl3)
d K2.37, 10.83, 35.93, 112.54, 127.57, 128.83, 133.84,
138.14, 138.21, 149.28. Anal. Calcd for C13H18N2Si: C,
67.77; H, 7.88; N, 12.16. Found: C, 67.99; H, 7.78; N, 12.10.
3.4.10. 3-Methyl-5-(dimethylphenylsilyl)-1-phenyl-
pyrazole (4h). Yield 69%; oil, Rf 0.50 (CH2Cl2); IR (film)
1
1598, 1500, 1250, 1111, 779, 734, 670 cmK1; H NMR
3.4.3. 3-(tert-Butyldiphenylsilyl)-1,5-dimethylpyrazole
(3c). Yield 68%; oil, Rf 0.55 (CH2Cl2); IR (film) 1540,
(CDCl3) d 0.35 (s, 6H), 2.38 (s, 3H), 6.42 (s, 1H), 7.15–7.50
(m, 10H); 13C NMR (CDCl3) d K0.19, 13.31, 117.22,
125.46, 127.75, 127.90, 128.49, 129.15, 133.80, 137.10,
142.00, 142.50, 148.92. Anal. Calcd for C18H20N2Si:
C, 73.92; H, 6.89; N, 9.58. Found: C, 74.10; H, 6.81;
N, 9.72.
1427, 1107, 821, 702 cmK1 1H NMR (CDCl3) d 1.19
;
(s, 9H), 2.27 (s, 3H), 3.89 (s, 3H), 6.04 (s, 1H), 7.35–7.38
(m, 6H), 7.71–7.74 (m, 4H); 13C NMR (CDCl3) d 11.02,
18.51, 27.83, 36.35, 115.15, 127.39, 128.93, 135.10, 136.42,

M. Calle et al. / Tetrahedron 62 (2006) 611–618
617
3.4.11. 1-Benzyl-5-(tert-butyldiphenylsilyl)-3-methyl-
pyrazole (4i). Yield 80%; oil, Rf 0.52 (CH2Cl2); IR (film)
1428, 1107, 820, 731, 702 cmK1; 1H NMR (CDCl3) d 1.16 (s,
9H), 2.44 (s, 3H), 4.84 (s, 2H), 6.71 (s, 1H), 6.98–7.60 (m,
15H); 13C NMR (CDCl3) d 13.37, 18.51, 27.78, 55.51,
116.88, 126.62, 126.77, 127.53, 127.77, 129.39, 132.70,
135.95, 137.23, 137.87, 148.40. Anal. Calcd for C27H30N2Si:
C, 78.97; H, 7.36; N, 6.82. Found: C, 79.10; H, 7.24; N, 6.75.
1107, 700 cmK1; 1H NMR (CDCl3) d 1.11 (s, 9H), 2.04 (s,
3H), 3.37 (s, 2H), 5.21 (br, 1H), 5.50 (s, 1H), 6.81 (br, 1H),
7.30–7.55 (m, 10H); 13C NMR (CDCl3) d 11.07, 13.51,
18.43, 27.58, 109.03, 127.36, 129.13, 133.72, 136.04,
145.37, 150.09, 152.74. Anal. Calcd for C22H27N3OSi: C,
69.99; H, 7.21; N, 11.13. Found: C, 70.20; H, 7.26; N, 10.99.
3.4.19. 1-Ethyl-3-methyl-5-(methyldiphenylsilylmethyl)-
pyrazole (7d). Yield 93%; bp 200–205 8C/1 mmHg; IR
1
3.4.12. 5-(tert-Butyldiphenylsilyl)-3-methyl-1-phenyl-
pyrazole (4k). Yield 75%; mp 145–146 8C (from hexane);
(film) 1540, 1427, 1113, 808, 733, 700 cmK1; H NMR
(CDCl3) d 0.62 (s, 3H), 1.22 (t, JZ7.2 Hz, 3H), 2.20 (s, 3H),
2.52 (s, 2H), 3.71 (q, JZ7.2 Hz, 2H), 5.56 (s, 1H),
7.36–7.52 (m, 10H); 13C NMR (CDCl3) d K4.48, 13.27,
13.50, 15.29, 42.90, 104.06, 127.92, 129.60, 134.39, 135.48,
138.71, 147.00. Anal. Calcd for C20H24N2Si: C, 74.95;
H, 7.55; N, 8.74. Found: C, 75.24; H, 7.50; N, 8.53.
IR (KBr) 1599, 1499, 1108, 817, 704 cmK1 1H NMR
;
(CDCl3) d 1.04 (s, 9H), 2.46 (s, 3H), 6.87 (s, 1H), 6.88–7.49
(m, 15H); 13C NMR (CDCl3) d 13.21, 18.71, 28.08, 118.57,
126.34, 127.27, 127.52, 127.98, 129.18, 133.43, 135.89,
138.56, 141.86, 148.84. Anal. Calcd for C26H28N2Si:
C, 78.74; H, 7.12; N, 7.06. Found: C, 78.46; H, 7.02; N, 7.26.
3.4.20. 3-Methyl-5-(methyldiphenylsilylmethyl)pyra-
zole-1-carboxamide (7e). Yield 79%; mp 113–114 8C; IR
(KBr) 3461, 3304, 3248, 1727, 1562, 1427, 1387, 1113, 732,
3.4.13. 4,5-Dihydro-5-hydroxy-3,5-dimethyl-4-(tri-
methylsilylmethyl)pyrazole-1-carboxamide (5). H NMR
(CDCl3) d K0.20 (s, 9H), 0.85 (dd, 1H, JZ8.3, 14.8 Hz),
1.11 (dd, 1H, JZ6.6, 14.8 Hz), 1.77 (s, 3H), 2.14 (s, 3H),
3.43 (dd, 1H, JZ6.6, 8.3 Hz), 5.62 (br, 1H), 7.05 (br, 1H).
1
1
699 cmK1; H NMR (CDCl3) d 0.54 (s, 3H), 2.15 (s, 3H),
3.23 (s, 2H), 5.24 (br, 1H), 5.62 (s, 1H), 6.98 (br, 1H),
7.33–7.65 (m, 10H); 13C NMR (CDCl3) d K4.62, 13.58,
15.86, 108.55, 127.75, 129.36, 134.57, 135.87, 145.38,
150.50, 152.61. Anal. Calcd for C19H21N3OSi: C, 68.02;
H, 6.31; N, 12.53. Found: C, 67.93; H, 6.40; N, 12.65.
3.4.14. 3,5-Dimethyl-4-(trimethylsilylmethyl)-1-phenyl-
pyrazole (6a). Yield 72%; mp 190–191 8C (from hexane);
IR (KBr) 1598, 1502, 1364, 1246, 857, 760, 696 cmK1
;
1H NMR (CDCl3) d 0.47 (s, 9H), 1.76 (s, 2H), 2.18 (s, 3H),
2.22 (s, 3H), 7.45–7.47 (m, 5H); 13C NMR (CDCl3) d K1.56,
11.31, 11.96, 12.48, 115.22, 124.01, 126.32, 128.51, 133.81,
139.90, 146.67. Anal. Calcd for C15H22N2Si: C, 69.71;
H, 8.58; N, 10.84. Found: C, 70.00; H, 8.41; N, 10.80.
3.4.21. 1-(tert-Butyl-5-(tert-butyldiphenylsilylmethyl))-3-
methylpyrazole (7f). Yield 85%; oil, Rf 0.45 (CH2Cl2); IR
(film) 1653, 1540, 1471, 1427, 1236, 1106, 1011, 820, 734,
1
700 cmK1; H NMR (CDCl3) d 1.10 (s, 9H), 1.61 (s, 9H),
2.01 (s, 3H), 2.85 (s, 2H), 5.46 (s, 1H), 7.35–7.80 (m, 10H);
13C NMR (CDCl3) d 10.73, 13.48, 18.35, 27.79, 30.61,
58.94, 108.11, 127.59, 129.29, 133.60, 135.91, 138.48,
144.47. Anal. Calcd for C25H34N2Si: C, 76.87; H, 8.77;
N, 7.17. Found: C, 76.99; H, 8.73; N, 7.22.
3.4.15. 3,5-Dimethyl-4-(trimethylsilylmethyl)pyrazole-1-
carboxamide 6b. Yield 78%; mp 103–104 8C (from
hexane); IR (KBr) 3431, 3332, 1731, 1700, 1400, 863,
1
575 cmK1; H NMR (CDCl3) d 0.01 (s, 9H), 1.69 (s, 2H),
2.12 (s, 3H), 2.42 (s, 3H), 5.33 (br, 1H), 7.07 (br, 1H);
13C NMR (CDCl3) d K1.32, 12.30, 12.47, 12.86, 118.60,
137.38, 149.69, 152.65. Anal. Calcd for C10H19N3OSi:
C, 53.29; H, 8.50; N, 18.65. Found: C, 53.57; H, 8.49;
N, 18.40.
3.4.22. 5-(tert-Butyldiphenylsilylmethyl)-1-ethyl-3-
methylpyrazole (7g). Yield 91%; mp 45–47 8C; IR (KBr)
1540, 1470, 1427, 1107, 732, 701 cmK1; 1H NMR (CDCl3)
d 1.12 (s, 9H), 1.21 (t, 3H, JZ7.2 Hz), 2.09 (s, 3H), 2.59
(s, 2H), 3.64 (q, 2H, JZ7.2 Hz), 5.44 (s, 1H), 7.31–7.55
(m, 10H); 13C NMR (CDCl3) d 8.70, 13.43, 15.15, 18.23,
27.63, 42.70, 104.69, 127.56, 129.36, 133.34, 135.82,
138.62, 146.65. Anal. Calcd for C23H30N2Si: C, 76.19;
H, 8.34; N, 7.73. Found: C, 76.23; H, 8.40; N, 7.68.
3.4.16. 1,3-Dimethyl-5-(trimethylsilylmethyl)pyrazole
7a. Yield 40%; oil, bp 100–105 8C/0.2 mmHg; IR (film)
1
1542, 1249, 852 cmK1; H NMR (CDCl3) d 0.01 (s, 9H),
1.93 (s, 2H), 2.16 (s, 3H), 3.61 (s, 3H), 5.61 (s, 1H);
13C NMR (CDCl3) d K1.74, 13.31, 15.55, 35.53, 103.10,
141.25, 146.91. Anal. Calcd for C9H18N2Si: C, 59.28;
H, 9.95; N, 15.36. Found: C, 59.42; H, 10.00; N, 15.22.
3.4.23. 5-(tert-Butyldiphenylsilylmethyl)-3-methyl-1-
phenylpyrazole (7h). Yield 89%; oil, bp 250 8C/
0.5 mmHg; IR (film) 1598, 1501, 1107, 732, 699 cmK1
;
1H NMR (CDCl3) d 0.96 (s, 9H), 2.12 (s, 3H), 2.68 (s, 2H),
5.51 (s, 1H), 7.13–7.51 (m, 15H); 13C NMR (CDCl3) d 9.26,
13.48, 18.19, 27.50, 105.92, 126.04, 127.42, 127.59, 128.78,
129.37, 133.47, 135.94, 139.61, 140.36, 148.53. Anal. Calcd
for C27H30N2Si: C, 78.97; H, 7.36; N, 6.82. Found: C, 79.08;
H, 7.29; N, 6.93.
3.4.17. 5-(tert-Butyldiphenylsilylmethyl)-1,3-dimethyl-
pyrazole (7b). Yield 51%; mp 70–71 8C; IR (KBr) 1543,
1426, 1106, 734, 702 cmK1 1H NMR (CDCl3) d 1.11
;
(s, 9H), 2.07 (s, 3H), 2.56 (s, 2H), 3.23 (s, 3H), 5.47 (s, 1H),
7.31–7.52 (m, 10H); 13C NMR (CDCl3) d 9.25, 13.35,
18.24, 27.63, 35.18, 104.50, 127.60, 129.43, 133.28, 135.86,
139.62, 145.61. Anal. Calcd for C22H28N2Si: C, 75.81;
H, 8.10; N, 8.04. Found: C, 75.99; H, 8.00; N, 8.18.
3.4.24. 5-(tert-Butyldiphenylsilylmethyl)-3-isopropyl-1-
phenylpyrazole (7i). Yield 60%; oil, Rf 0.45 (CH2Cl2);
IR (film) 1598, 1501, 1380, 1106, 733, 699 cmK1; 1H NMR
(CDCl3) d 0.97 (s, 9H), 1.13 (d, JZ6.9 Hz, 6H), 2.72
(s, 2H), 2.87 (m, JZ6.9 Hz, 1H), 5.51 (s, 1H), 7.12–7.46
(m, 15H); 13C NMR (CDCl3) d 9.51, 18.12, 22.76, 27.44,
3.4.18. 5-(tert-Butyldiphenylsilylmethyl)-3-methyl-
pyrazole-1-carboxamide (7c). Yield 75%; mp
106–108 8C; IR (KBr) 3401, 3236, 1690, 1590, 1395,

618
M. Calle et al. / Tetrahedron 62 (2006) 611–618
27.65, 102.82, 126.09, 127.29, 127.46, 128.72, 129.26,
133.54, 135.94, 139.65, 139.97, 150.04. Anal. Calcd for
C29H34N2Si: C, 79.40; H, 7.81; N, 6.39. Found: C, 79.71;
H, 7.75; N, 6.22.
1595, 1361, 1109, 970, 868 cmK1; 1H NMR (CDCl3) d 1.08
(s, 9H), 2.41 (s, 2H), 6.34 (s, 1H), 7.22–7.74 (m, 20H);
13C NMR (CDCl3) d 13.51, 18.96, 26.51, 107.69, 125.10,
127.04, 127.59, 128.01, 128.34, 128.57, 128.78, 129.47,
130.67, 134.78, 135.36, 140.04, 143.64, 149.41. Anal. Calcd
for C32H38N2Si: C, 81.31; H, 6.82; N, 5.93. Found: C, 81.63;
H, 6.97; N, 6.10.
3.4.25. 1,5-Dimethyl-3-(trimethylsilylmethyl)pyrazole
(8a). Yield 38%; oil, bp 95–99 8C/0.1 mmHg; IR (film)
1
1550, 1453, 1247, 849 cmK1; H NMR (CDCl3) d 0.01
(s, 9H), 1.96 (s, 2H), 2.19 (s, 3H), 3.66 (s, 3H), 5.64 (s, 1H);
13C NMR (CDCl3) d K1.69, 11.09, 18.07, 35.46, 103.83,
138.61, 148.91. Anal. Calcd for C9H18N2Si: C, 59.28;
H, 9.95; N, 15.36. Found: C, 59.43; H, 10.01; N, 15.28.
Acknowledgements
´
We thank the Ministerio de Ciencia y Tecnologıa of Spain
for supporting this work (Grant BQU2000-0943).
3.4.26. 3-(tert-Butyldiphenylsilylmethyl)-1,5-dimethyl-
pyrazole (8b). Yield 28%; oil, Rf 0.39 (CH2Cl2/Et2O,
30:1); IR (film) 1619, 1549, 1427, 1107, 734, 701 cmK1
;
1H NMR (CDCl3) d 1.07 (s, 9H), 2.00 (s, 3H), 2.72 (s, 2H),
3.62 (s, 3H), 5.15 (s, 1H), 7.35–7.71 (m, 10H); 13C NMR
(CDCl3) d 10.91, 11.85, 18.34, 27.66, 35.28, 104.53, 127.30,
128.87, 134.56, 136.04, 138.31, 147.45. Anal. Calcd for
C22H28N2Si: C, 75.81; H, 8.10; N, 8.04. Found: C, 76.01;
H, 7.95; N, 8.15.
References and notes
´ ´
1. Calvo, L. A.; Gonzalez-Nogal, A. M.; Gonzalez-Ortega, A.;
San˜udo, M. C. Tetrahedron Lett. 2001, 42, 8981–8984.
´
2. Calle, M.; Cuadrado, P.; Gonzalez-Nogal, A. M.; Valero, R.
3.4.27. 3-(tert-Butyldiphenylsilylmethyl)-5-methyl-
pyrazole-1-carboxamide (8c). IR (film) 3430, 3330,
Synthesis 2001, 1949–1958.
´
3. Cuadrado, P.; Gonzalez-Nogal, A. M.; Valero, R. Tetrahedron
1720, 1690, 1100, 763, 695 cmK1 1H NMR (CDCl3)
;
2002, 58, 4975–4980.
¨
4. Hoppe, I.; Schollkopf, U. Liebigs Ann. Chem. 1979, 219.
d 1.06 (s, 9H), 2.37 (s, 3H), 2.65 (s, 2H), 5.00 (br, 1H), 5.45
(s, 1H), 6.81 (br, 1H), 7.30–7.55 (m, 10H). Anal. Calcd for
C22H27N3OSi: C, 69.99; H, 7.21; N, 11.13. Found: C, 70.31;
H, 7.55; N, 11.45.
5. We have only found one paper by Dalpozzo et al. concerning
the synthesis of silyl b-enaminones. These authors prepared
a0- and g-silylenaminones by reaction with chlorosilanes of
a0- and g-dianions of b-enaminones: Bartoli, G.; Bosco, M.;
Dalpozzo, R.; De Nino, A.; Iantorno, E.; Tagarelli, A.; Palmieri,
G. Tetrahedron 1996, 52, 9179–9186.
3.4.28. 1-tert-Butyl-3-(tert-butyldiphenylsilylmethyl)-5-
phenylpyrazole (8d). Yield 45%; oil, Rf 0.47 (CH2Cl2);
IR (film) 1605, 1427, 1110, 821, 738, 700 cmK1; 1H NMR
(CDCl3) d 1.09 (s, 9H), 1.45 (s, 9H), 2.31 (s, 2H), 5.95
(s, 1H), 7.26–7.76 (m, 15H). Anal. Calcd for C30H36N2Si:
C, 79.59; H, 8.02; N, 6.19. Found: C, 79.21; H, 7.90; N, 6.55.
6. Birkofer, L.; Stuhl, O. Top. Curr. Chem. 1980, 88, 62–65.
7. They are easily deprotonated by BuLi or LDA. See, for
example: Konakahara, T.; Tagaki, Y. Tetrahedron Lett. 1980,
21, 2073–2076.
8. Nesi, R.; Ricci, A.; Taddei, M.; Tedeschi, P.; Seconi, G.
J. Organomet. Chem. 1980, 195, 275–283.
3.4.29. 3-(tert-Butyldiphenylsilylmethyl)-1,5-diphenyl-
pyrazole (8e). Yield 34%; oil, Rf 0.44 (CH2Cl2); IR (film)