Gold-catalyzed cyclization of N-alkynyl carbamates

Article abstract of DOI:10.1055/s-2007-982562

Six different N-alkynyl carbamates containing Boc groups were prepared. Their gold(I)-catalyzed cyclization to oxazolinones proceeded radily at room temperature or even lower temperatures. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-2007-982562

CLUSTER
1763
Gold-Catalyzed Cyclization of N-Alkynyl Carbamates
1
A
u-CatalyzedCyc
.
lizationof
N
-
S
Alkynyl Carb
t
amate
s
ephen K. Hashmi,* Ralph Salathé, Wolfgang Frey
Institut für Organische Chemie, Universität Stuttgart, Pfaffenwaldring 55, 70569 Stuttgart, Germany
Fax +49(711)68564330; E-mail: hashmi@hashmi.de
Received 1 March 2007
Table 1 Alkynyl Carbamates
Abstract: Six different N-alkynyl carbamates containing Boc
Boc
groups were prepared. Their gold(I)-catalyzed cyclization to
oxazolinones proceeded radily at room temperature or even lower
temperatures.
N
N
R2
R1
KHMDS
1.2 equiv)
3a–d
Boc
Ph
(
+
I
R2
+
Key words : alkynes, carbamates, gold catalysis, iodonium salts,
oxazolinones
N
H
+
R1
toluene
–
OTf
0
°C to r.t.
Boc
R1
(
1.2 equiv)
H
1
a–c
2a,b
Among gold-catalyzed reactions2 the intramolecular
oxyauration of C–C multiple bonds is an excellent tool for
the formation of oxygen-containing heterocycles. The
first example was the cycloisomerization of allenyl ke-
4a,b
Entry Carbamate
Iodonium salt
Product Yield (%)
1
2
1
2
3
4
1a; R = Ts
2a; R = TMS
3a/4a
4b
24/46
23a
51
3
tones or propargyl ketones, similar results were obtained
1
1b; R = Boc
2a
2a
4
with 2-alkynyl-2-alken-1-ones.
Subsequently, the
isomerization of N-propargylcarboxamides to oxazoles or
even the rare alkylidene oxazolines was investigated.5
Propargylic and homopropargylic trichloroacetimidates
1b
1b
3b
2
2b; R = TBDMS
3c
27
6
react analogously. Carboxylates and tert-butyl carboxy-
1
5
1c; R = Piv
2a
3d
27
7
lates also react. A study of the cyclization of tert-butyl al-
a
Cs CO (1.3 equiv) instead of KHMDS; DMF instead of toluene.
2
3
lenoates showed that the tert-butyl group is eliminated
8
and a butenolide is formed. This principle was then
extended to tert-butyl carbonates, where for example a Gold catalysis with Gagosz’s catalyst12 [Ph PAu]NTf2
9
3
propargylic carbonate was shown to cyclize faster than a readily delivered the corresponding oxazolinones 5 in 65–
9
a
homopropargylic one. A similar behavior was reported 93% yield. As shown in Table 2, the terminal (entries 1
for propargylic and homopropargylic tert-butyl carb- and 3), the TMS (entries 2, 4, and 6), and the TBDMS (en-
9
c,d
amates.
try 5) carbamates all react well. With the combination Ts/
Boc it was very clear that the Boc carbonyl oxygen atom
is the more nucleophilic partner (entries 1 and 2), but in
the combination Piv/Boc also a high selectivity towards
We now wanted to investigate N-alkynylcarbamates as
substrates, where instead of a 5-exo-dig or 6-exo-dig, a 5-
endo-dig cyclization would lead to oxazolinones. To the
best of our knowledge, so far only one reference has de-
scribed the use of derivatives of alkynylamines in gold- Table 2 Gold-Catalyzed Oxazolinone Synthesis
catalyzed reactions.10
O
O
Boc
Ph3PAuNTf2 (5 mol%)
R2
The synthesis of the substrates started from the Boc-pro-
R2
N
N
tected amines 1, using the phenyliodonium salts 2,11 the
tert-butyl N-alkynylcarbamates 3 and 4 were obtained
CDCl , r.t.
R1
R1
3
3
4
a–d
a,b
5
a–f
(
Table 1). For the ynamine synthesis two acceptors are
needed on the amine, the Boc group is either accompanied
by a second Boc group, a Ts group, or a pivaloyl group.
When the terminal position of the alkyne was occupied by
the TMS group, partial desilylation was observed. Chang-
ing to the more robust TBDMS group avoided this prob-
lem. Overall, the yields of these substrates were not high,
but still allowed the investigation of the cyclization of 3
and 4.
Entry
Alkynyl carbamate
Product
5a
Yield (%)
1
a
2
1
2
3
4
5
4a; R = Ts, R = H
71
67
65
90
93
87
1
a
2
3a; R = Ts, R = TMS
5b
1
2
4b; R = Boc, R = H
5c
1
2
3b; R = Boc, R = TMS
5d
1
2
3c; R = Boc, R = TBDMS
5e
1
2
SYNLETT 2007, No. 11, pp 1763–1766
0
7
.0
3
.2
0
0
7
6
3d; R = Piv, R = TMS
instead of CDCl , 0 °C.
5f
Advanced online publication: 25.06.2007
DOI: 10.1055/s-2007-982562; Art ID: Y00507ST
a
C
H
Cl
2
4
2
3
©
Georg Thieme Verlag Stuttgart · New York

1
764
A. S. K. Hashmi et al.
CLUSTER
cyclization via the Boc carbonyl oxygen atom was ob-
served, the less nucleophilic Piv carbonyl oxygen atom
did not compete (entry 6).
Mechanistically, it is interesting to note that the nucleo-
philic oxygen atom reaches the distal position of the
alkyne. A mechanistic proposal basing on the knowledge
from the mechanistic investigation of reactions mentioned
in the introduction is shown in Scheme 1. While in 3 the
distance between the carbonyl oxygen atom and the distal
alkyne carbon atom is relatively large, the coordination of
1
3
14
the catalyst on the alkyne should bend the latter in the
sense shown by both donation and back donation and thus
reduce this distance (intermediate A).
O
R2
O
N
R1
[AuI]
3
A
Figure 1 Solid state structure of 2b
H
O
O
[
AuI]
R2
B
N
R1
[
AuI]
Representative Experimental Procedure for the Synthesis of
Alkynyl Carbamates
C
KHMDS (4.44 mL, 2.22 mmol) was added to a solution of carbam-
ate 1b (402 mg, 1.85 mmol) in 8 mL toluene under argon at 0 °C.
The reaction mixture was allowed to warm to r.t. and trimethylsilyl-
ethynyl(phenyl)iodonium triflate (2a, 1.00 g, 2.22 mmol) was add-
ed. The mixture was stirred for 12 h. The solvent was removed
under reduced pressure and the residue was purified by column
5
O
O
R2
N
R1
AuI]
[
+
H+
H+
chromatography on silica gel (hexane–EtOAc–Et N) to yield the
3
desired product 3b (300 mg, 51%) as a white solid.
Scheme 1
tert-Butyl (4-Methylphenyl)sulfonyl[(trimethylsilyl)ethy-
nyl]carbamate (3a)
Mp 60 °C.
After the nucleophilc attack, intermediate B fragments
–
1
and delivers C, isobutene and the proton, which is neces- IR (neat): 2959, 2172, 1744, 1381, 1244, 1143 cm .
1
sary for the final protodeauration step to set free the cata-
lyst and produced product 5.
H NMR (300 MHz, CD Cl ): d = 0.20 (s, 9 H), 1.39 (s, 9 H), 2.44
2 2
(s, 3 H), 7.36 (d, J = 8.2 Hz, 2 H), 7.84 (d, J = 8.2 Hz, 2 H).
1
3
C NMR (126 MHz, CD Cl ): d = 0.00 (q, 3 C), 21.98 (q), 27.89 (q,
2
2
3
1
C), 79.82 (s), 86.78 (s), 88.78 (s), 128.97 (d, 2 C), 130.09 (d, 2 C),
35.40 (s), 146.50 (s), 149.49 (s).
{
[tert-Butyl(dimethyl)silyl]ethynyl}(phenyl)iodonium Trifluo-
+
romethanesulfonate (2b)
MS (EI, 70 eV): m/z = 367 (2) [M ], 267 (23), 155 (94), 91 (48), 57
Compound 2b was synthesized in analogy to the corresponding
(100).
TMS derivative.15 A crystal structure analysis of the TBDMS deriv-
Anal. Calcd for C H NO SSi: C, 55.55; H, 6.86; N, 3.81. Found:
C, 55.77; H, 6.87; N, 3.68.
ative 2b could be obtained (Figure 1).16
17 25
4
Yield 63%.
Mp 138 °C.
tert-Butyl Ethynyl[(4-methylphenyl)sulfonyl]carbamate (4a)
IR (neat): 3281, 2934, 1751, 1371, 1142 cm .
–
1
–
1
IR (neat): 2929, 2859, 1285, 1213, 1168, 985 cm .
1
H NMR (500 MHz, CDCl ): d = 1.41 (s, 9 H), 2.45 (s, 3 H), 3.19
3
1
H NMR (300 MHz, CDCl ): d = 0.17 (s, 6 H), 0.91 (s, 9 H), 7.51–
3
(s, 1 H), 7.35 (d, J = 8.3 Hz, 2 H), 7.89 (d, J = 8.3 Hz, 2 H).
7
.56 (m, 2 H), 7.64–7.69 (m, 1 H), 8.06–8.09 (m, 2 H).
1
3
C NMR (126 MHz, CDCl ): d = 21.86 (q), 27.76 (q, 3 C), 65.19
3
1
3
C NMR (126 MHz, CDCl ): d = 5.08 (q, 3 C), 16.93 (s), 26.00 (q,
3
(d), 69.84 (s), 86.75 (s), 128.65 (d, 2 C), 129.74 (d, 2 C), 134.99 (s),
45.93 (s), 149.12 (s).
2
1
C), 44.46 (s), 117.36 (s), 119.01 (s), 119.91 (q, J = 320 Hz, 1 C),
1
32.68 (d, 2 C), 132.79 (d), 133.95 (d, 2 C).
+
+
MS (ESI): m/z = 318 [M + Na] (100), 262 [M + Na – t-Bu] (23).
Anal. Calcd for C H F IO SSi: C, 36.59; H, 4.09. Found: C,
15
20
3
3
+
HRMS (ESI): calcd for C H NO S: 318.0770 [M + Na] ; found
1
4
17
4
3
6.71; H, 4.12.
+
3
18.0768 [M + Na] .
MS (ESI): m/z = 343 [M – OTf]+.
Synlett 2007, No. 11, 1763–1766 © Thieme Stuttgart · New York

CLUSTER
Au-Catalyzed Cyclization of N-Alkynyl Carbamates
1765
1
3
Di-tert-butyl Ethynylimidodicarbonate (4b)
IR (neat): 3276, 2981, 2360, 1808, 1768, 1347, 1242 cm .
C NMR (126 MHz, CD Cl ): d = 21.93 (q), 113.86 (d), 128.80 (d,
2
2
–
1
2 C), 129.42 (d), 130.50 (d, 2 C), 133.45 (s), 147.37 (s), 150.24 (s).
1
+
H NMR (500 MHz, C D ): d = 1.33 (s, 18 H), 2.78 (s, 1 H).
MS (ESI): m/z = 262 [M + Na] (100), 166 (2), 155 (2).
6
6
1
3
+
C NMR (126 MHz, C D ): d = 27.54 (q, 6 C), 62.86 (d), 72.42 (s),
HRMS (ESI): calcd for C H NO S: [M + Na] 262.0144; found
[M + Na] 262.0142 .
6
6
10
9
4
+
8
4.17 (s, 2 C), 149.97 (s, 2 C).
+
MS (CI, 70 eV): m/z = 242 (10) [M + NH ], 186 (35), 103 (32), 57
4
3
2
-[(4-Methylphenyl)sulfonyl]-5-(trimethylsilyl)-1,3-oxazol-
(
100).
(3H)-one (5b)
HRMS (CI): m/z calcd: 242.1392; found: 242.1391.
–1
IR (neat): 2960, 2360, 1776, 1381, 1177 cm .
1
H NMR (500 MHz, CD Cl ): d = 0.20 (s, 9 H), 2.44 (s, 3 H), 7.06
2
2
Di-tert-butyl (Trimethylsilyl)ethynylimidodicarbonate (3b)
(
s, 1 H), 7.39 (d, J = 8.3 Hz, 2 H), 7.91 (d, J = 8.3 Hz, 2 H).
Mp 30 °C.
13C NMR (126 MHz, CDCl
d), 128.62 (d, 2 C), 128.91 (s), 130.09 (d, 2 C), 133.29 (s), 146.50
(s), 152.19 (s).
): d = –2.68 (q, 3 C), 21.84 (q), 120.06
3
–
1
IR (neat): 2981, 2195, 1811, 1774, 1370, 1248, 1137 cm .
(
1
H NMR (300 MHz, CD Cl ): d = 0.16 (s, 9 H), 1.49 (s, 18 H).
2
2
1
3
C NMR (126 MHz, CD Cl ): d = 0.00 (q, 3 C), 27.89 (q, 6 C),
+
+
2
2
MS (ESI): m/z = 334 [M + Na] , 262 [M + Na – TMS] .
7
6.61 (s), 84.93 (s, 2 C), 91.21 (s), 149.90 (s, 2 C).
+
HRMS (ESI): calcd for C H NO SSi: [M + Na] 334.0540;
found [M + Na] 334.0533.
1
3
17
4
+
+
MS (ESI): m/z = 336 [M + Na] , 236 [M + Na – Boc] .
+
+
HRMS (ESI): m/z calcd for C H NO Si: 336.1602 [M + Na] ;
found 336.1592 [M + Na] .
1
5
27
4
+
tert-Butyl 2-Oxo-1,3-oxazole-3(2H)-carboxylate (5c)
Mp 84 °C.
Di-tert-butyl [tert-butyl(dimethyl)silyl]ethynylimidodicarbon-
ate (3c)
Mp 27 °C.
IR (neat): 3162, 2980, 2359, 1807, 1371, 1254 cm–1.
1
H NMR (300 MHz, CD Cl ): d = 1.55 (s, 9 H), 6.72 (d, J = 2.3 Hz,
2
2
1
H), 6.95 (d, J = 2.3 Hz, 1 H).
–
1
IR (neat): 2931, 2193, 1810, 1773, 1245, 1137 cm .
1
3
C NMR (126 MHz, CD Cl ): d = 27.96 (q, 3 C), 85.91 (s), 113.50
2
2
1
H NMR (300 MHz, CD Cl ): d = 0.10 (s, 6 H), 0.93 (s, 9 H), 1.48
2
2
(d), 128.46 (d), 146.98 (s), 150.53 (s).
(
s, 18 H).
+
+
MS (ESI): m/z = 208 [M + Na] , 152 [M + Na – t-Bu] .
1
3
C NMR (126 MHz, CD Cl ): d = –4.38 (q, 2 C), 17.06 (s), 26.37
2
2
+
HRMS (ESI): m/z calcd for C H NO : 208.0580 [M + Na] ; found:
8
11
4
(
(
q, 3 C), 28.05 (q, 6 C), 75.15 (s), 84.98 (s, 2 C), 91.58 (s), 149.95
s, 2 C).
+
2
08.0586 [M + Na] .
+
+
MS (ESI): m/z = 378 [M + Na] (100), 278 [M + Na – Boc] (21).
tert-Butyl 2-Oxo-5-(trimethylsilyl)-1,3-oxazole-3(2H)-carboxy-
late (5d)
Mp 80 °C.
Anal. Calcd for C H NO Si: C, 60.81; H, 9.36; N, 3.89. Found: C,
18
33
4
6
0.73; H, 9.26; N, 3.84.
–
1
IR (neat): 3126, 2957, 1772, 1747, 1337, 1252, 1100 cm .
tert-Butyl 2,2-Dimethylpropanoyl[(trimethylsilyl)ethynyl]car-
1
H NMR (500 MHz, CDCl ): d = 0.24 (s, 9 H), 1.59 (s, 9 H), 6.96
3
bamate (3d)
IR (neat): 2962, 2179, 1748, 1297, 1115 cm .
–
1
(s, 1 H).
13C NMR (126 MHz, CDCl
5.44 (s), 120.22 (d), 144.04 (s), 146.84 (s), 152.62 (s).
): d = –2.66 (q, 3 C), 27.90 (q, 3 C),
1
3
H NMR (300 MHz, CD Cl ): d = 0.17 (s, 9 H), 1.36 (s, 9 H), 1.48
2
2
8
(
s, 9 H).
+
+
1
3
MS (ESI): m/z = 280 [M + Na] (100), 224 [M + Na – t-Bu] (22),
C NMR (62.9 MHz, CD Cl ): d = 0.00 (q, 3 C), 27.68 (q, 3 C),
2
2
+
1
80 [M + Na – Boc] (32).
2
1
8.01 (q, 3 C), 43.61 (s), 79.52 (s), 85.09 (s), 92.74 (s), 151.56 (s),
79.09 (s).
+
HRMS (ESI): calcd for C H NO Si: 280.0976 [M + Na] ; found
1
1
19
4
+
+
280.0970 [M + Na] .
MS (ESI): m/z = 320 [M + Na] (100), 224 [M + Na – TMSacet-
ylene] (12).
tert-Butyl 5-[tert-Butyl(dimethyl)silyl]-2-oxo-1,3-oxazole-
3(2H)-carboxylate (5e)
Mp 71 °C.
+
HRMS (ESI): calcd for C H NaNO Si: 320.1652 [M + Na] ;
found 320.1643 [M + Na] .
1
5
27
3
+
–
1
IR (neat): 3121, 2929, 1794, 1335, 1282, 1094 cm .
Representative Experimental Procedure for the Synthesis of
Oxazolinones
1
H NMR (500 MHz, CDCl ): d = 0.19 (s, 6 H), 0.94 (s, 9 H), 1.59
3
[
Ph PAuNTf ] (3.50 mg, 4.79 mmol) was added to 3d (28.5 mg, 95.8
(s, 9 H), 6.98 (s, 1 H).
3
2
1
mmol) in 700 mL CDCl . The reaction was monitored by H NMR
13
3
C NMR (126 MHz, CDCl ): d = –7.03 (q, 2 C), 16.59 (s), 26.28
3
spectroscopy. After the reaction was finished the solvent was re-
moved under reduced pressure and the residue was purified by col-
umn chromatography on silica gel (hexane–EtOAc) to yield the
desired product 5f (20.0 mg, 87%) as a white solid.
(q, 3 C), 28.02 (q, 3 C), 85.63 (s), 121.41 (d), 142.94 (s), 146.97 (s),
1
52.71 (s).
+
+
MS (ESI): m/z = 322 [M + Na] (58), 266 [M + Na – t-Bu] (44),
+
2
22 [M + Na – Boc] (100).
3
-[(4-Methylphenyl)sulfonyl]-1,3-oxazol-2(3H)-one (5a)
+
HRMS (ESI): calcd for C H NO Si: 322.1445 [M + Na] ; found
1
4
25
4
–
1
IR (neat): 3154, 2361, 1788, 1381, 1178 cm .
+
3
22.1443 [M + Na] .
1
H NMR (300 MHz, CD Cl ): d = 2.44 (s, 3 H), 6.76 (d, J = 2.3 Hz,
2
2
3
-(2,2-Dimethylpropanoyl)-5-(trimethylsilyl)-1,3-oxazol-2(3H)-
1
H), 7.05 (d, J = 2.3 Hz, 1 H), 7.39 (d, J = 8.7 Hz, 2 H), 7.91 (d,
one (5f)
J = 8.7 Hz, 2 H).
–
1
IR (neat): 3281, 2982, 2144, 1751, 1371, 1252, 1142 cm .
Synlett 2007, No. 11, 1763–1766 © Thieme Stuttgart · New York

1
766
A. S. K. Hashmi et al.
CLUSTER
1
H NMR (500 MHz, CDCl ): d = 0.27 (s, 9 H), 1.38 (s, 9 H), 7.07
(6) (a) Kang, J.-E.; Kim, H.-B.; Lee, J.-W.; Shin, S. Org. Lett.
2006, 8, 3537. (b) Hashmi, A. S. K.; Rudolph, M.;
3
(
s, 1 H).
1
3
Schymura, S.; Visus, J.; Frey, W. Eur. J. Org. Chem. 2006,
C NMR (126 MHz, CDCl ): d = –1.64 (q, 3 C), 28.80 (q, 3 C),
3
4905.
3
3.92 (s), 135.73 (d), 135.73 (s), 154.33 (s), 175.15 (s).
(
7) (a) Genin, E.; Tullec, P. Y.; Antoniotti, S.; Brancour, C.;
+
MS (EI, 70 eV): m/z = 242 (25) [M – H ], 157 (11), 57 (100) [M –
H – t-Bu].
Genêt, J.-P.; Michelet, V. J. Am. Chem. Soc. 2006, 128,
+
3112. (b) Shin, S. Bull. Korean Chem. Soc. 2005, 26, 1925.
(
c) Kang, J.-E.; Shin, S. Synlett 2006, 717.
(
(
8) Kang, J.-E.; Lee, E.-S.; Park, S.-I.; Shin, S. Tetrahedron
Lett. 2005, 46, 7431.
9) (a) Buzas, A.; Gagosz, F. Org. Lett. 2006, 8, 515. (b) Kang,
J.-E.; Shin, S. Synlett 2006, 717. (c) Buzas, A.; Gagosz, F.
Synlett 2006, 2727. (d) Robles-Machin, R.; Adrio, J.;
Carretero, J. C. J. Org. Chem. 2006, 71, 5023.
Acknowledgment
We are grateful to the DFG and Umicore AG & Co. KG for their
steady support of our research.
References and Notes
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(
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4
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