Sequential coupling-isomerization-coupling reactions - A novel three-component synthesis of aryl chalcones

Article abstract of DOI:10.1055/s-2006-956480

Bromoaryl boronates bear a halide and a dormant organometallic functionality and, therefore, are perfectly suited for sequentially Pd-catalyzed reactions. The microwave-accelerated consecutive reaction of bromoaryl boronates, (hetero)aryl propargyl alcoho

Full text of DOI:10.1055/s-2006-956480

LETTER
3469
Sequential Coupling–Isomerization–Coupling Reactions – A Novel Three-
Component Synthesis of Aryl Chalcones
N
W
ovel Three-Componen
e
t
Synthesis
i
of
A
ry
-
l
C
halco
W
ne
s
ei Liao, Thomas J. J. Müller*
Organisch-Chemisches Institut der Ruprecht-Karls-Universität Heidelberg, Im Neuenheimer Feld 270, 69120 Heidelberg, Germany
Fax +49(6221)546579; E-mail: Thomas.J.J.Mueller@oci.uni-heidelberg.de
Received 28 September 2006
the sense of a (hetero)aryl chalcone synthesis in a one-pot
fashion.
Abstract: Bromoaryl boronates bear a halide and a dormant orga-
nometallic functionality and, therefore, are perfectly suited for se-
quentially Pd-catalyzed reactions. The microwave-accelerated The CIR usually proceeds smoothly under mild condi-
consecutive reaction of bromoaryl boronates, (hetero)aryl propar-
tions with sufficiently electron-poor (hetero)aryl halides
gyl alcohols, and (hetero)aryl halides furnishes (hetero)aryl chal-
as coupling partners. Therefore, bromo pinacolyl aryl
cones in good yields in the sense of a one-pot coupling–
boronates should be sufficiently electron-withdrawing to
isomerization–coupling sequence.
trigger the CIR before being consumed in a Suzuki
Keywords: alkynes, catalysis, cross-couplings, isomerizations, mi-
coupling after addition of a strong base.
crowave reactions
OH
NC
Br
+
The use of one catalyst for more than one type of transfor-
mation in a one-pot fashion is generally referred to as se-
quential catalysis. From the perspective of economically
Br
1
1a
2
and ecologically benign processes this concept paves the
way to a plethora of new sequences and becomes a play-
ground for developing and inventing new methodologies.
Among transition-metal-catalyzed sequences, sequential-
ly palladium-catalyzed processes are elegant cascade re-
actions and experience quite recently an increasing
[
(Ph3P)4Pd, CuI]
3
Et N, THF, ∆
O
NC
2
Br
interest. Addressing the issue of synthetic efficiency,
3
3
,4
one-pot multicomponent processes are diversity-orient-
ArB(OH)2
K2CO3, ∆
5
R
ed syntheses that can be often developed into combinato-
4
d,6
O
rial and solid-phase strategies
promising manifold
CO2Me
DMF
K2CO3
O
opportunities for generating novel lead structures of phar-
maceuticals, catalysts, and even novel molecule-based
NC
NC
materials. As part of our program designed to develop
∆
new sequentially transition-metal-catalyzed reactions,7
65–74%
Ar
R
62–66%
we could show that the mild coupling–isomerization reac-
O
8
9
tion (CIR), a Sonogashira coupling of electron-deficient
hetero)aryl halides and (hetero)aryl propargyl alcohols
NC
(
and subsequent base-catalyzed isomerization to chal-
cones, is well suited for sequentially Pd-catalyzed pro-
8
2
CO Me
49%
cesses. By virtue of an intermediate bromo chalcone 3,
generated by a CIR of p-bromobenzonitrile (1a) with the
corresponding propargyl alcohol 2, this en route generated
halide electrophile serves as a substrate in subsequent
Sonogashira, Heck, or Suzuki coupling reactions
Scheme 1 Sequential coupling–isomerization–coupling reactions
with a bromo chalcone as an intermediate
Therefore, we decided to first optimize the formation of
the borylated chalcone 6a by performing the CIR of 4-
bromophenyl boronate (4a) and propargyl alcohol 5a
under various conditions (Table 1).1
1
0
(
Scheme 1). However, an intriguing and highly versatile
expansion of this CI–coupling (CIC) strategy would be a
dormant organometallic functionality that could be easily
1,12
switched on for the subsequent coupling step. Here, we As conductive heating only leads to minimal yields of the
communicate a new type of CIC sequence with halo aryl chalcone 6a (entries 1 and 2), dielectric heating, i.e. mi-
13
boronates, propargyl alcohols, and (hetero)aryl halides in crowave irradiation, which already has been successful
14
in CIR, was applied (entries 3–12). In combination with
a sufficiently strong base such as DBU in slight excess
SYNLETT 2006, No. 20, pp 3469–3473
Advanced online publication: 08.12.2006
DOI: 10.1055/s-2006-956480; Art ID: G28706ST
1
8
.1
2
.2
0
0
6
(
1.05 equiv) and triphenylphosphane as an additive to
stabilize the transition-metal species under microwave
©
Georg Thieme Verlag Stuttgart · New York

3
470
W.-W. Liao, T. J. J. Müller
LETTER
Table 1 Optimization of the CIR of 4a and 5aa
solvent, base
additive, conditions
O
O
O
OH
Ph
O
O
B
Br
[(Ph P) PdCl , CuI]
+
3
2
2
B
Ph
4a
5a
6a
Entry
Solvent
THF
Base
Et N (6.0 M)
Conditions
Reflux, 20 h
Reflux, 20 h
Additive
Yield of 6a (%)b
1
–
–
–
–
–
–
–
4
22
–
3
2
THF
DBU (2.0 equiv)
3
THF
Et N (6.0 M)
MW, 110 °C, 60 min
MW, 110 °C, 60 min
MW, 110 °C, 60 min
MW, 110 °C, 60 min
MW, 110 °C, 60 min
MW, 110 °C, 60 min
MW, 110 °C, 60 min
MW, 110 °C, 60 min
MW, 120 °C, 30 min
MW, 140 °C, 10 min
3
4
DMF
MeCN
THF
Et N (6.0 M)
–
3
5
Et N (6.0 M)
–
3
6
DBU (2.0 equiv)
DBU (2.0 equiv)
DBU (2.0 equiv)
19
22
37
5
7c
8
THF
THF
Ph P (0.2 equiv)
3
9
THF
HNEt (6.0 M)
Ph P (0.2 equiv)
2
3
10
THF
DBU (1.1 equiv)
DBU (1.05 equiv)
DBU (1.05 equiv)
Ph P (0.2 equiv)
39
64
53
3
1
1d
2d
THF
THF
Ph P (0.2 equiv)
3
1
Ph P (0.2 equiv)
3
a
Reaction conditions: 1.0 equiv of the haloaryl boronate 4a, 1.05 equiv of the propargyl alcohol 5a, 0.04 equiv of Pd(PPh ) Cl , 0.04 equiv of
3
2
2
CuI, 0.9 mL of solvent (0.2 M), heating in a sealed vessel in a Smith Synthesizer single-mode microwave cavity.
b
Yields refer to isolated yields of compound 6a after flash chromatography on silica gel to be ≥95% pure as determined by NMR spectroscopy.
c
Carried out at concentrations of 4a and 5a of 0.4 M.
d
Carried out at concentrations of 4a and 5a of 0.3 M.
irradiation the reaction times considerably shorten and the Therefore, after reacting various bromoaryl boronates 4,
yields increase (entries 10–12). These findings now pave and 1-(hetero)aryl propargyl alcohols 5 under optimized
the way to a novel type of CIC sequence where a boronate conditions (Table 1, entry 11) in the microwave cavity,
serves as an electron-withdrawing substituent for the CIR (hetero)aryl halides 1, potassium carbonate and water
and as a dormant organometallic functionality suitable for were added to the vessel and dielectric heating was con-
a subsequent Suzuki coupling.
tinued at 110 °C for 20–30 minutes to give the (hetero)ar-
yl chalcones 7 in good yields (Table 3).1
2,16
For optimizing this projected CIC sequence the boronate
4
a and the propargyl alcohol 5a were reacted under opti- As expected for Suzuki couplings the (hetero)aryl halides
mized conditions (Table 1, entry 11) in the microwave 1 can be broadly varied in their electronic nature giving
cavity, and then, 4-bromobenzonitrile (1), bases and co- rise to a versatile and diversity-oriented synthesis of (het-
solvents were added and the reaction conditions for the ero)aryl chalcones in sequential, one-pot fashion. Advan-
1
2
Suzuki coupling step were varied (Table 2).
tageously, no additional catalyst needs to be added to the
reaction mixture and, therefore, this new three-component
reaction can be considered as another showcase for se-
quentially palladium-catalyzed processes.2
Amine bases and CsF are no suitable bases for the con-
cluding Suzuki coupling (entries 1–3 and 14, 15) and con-
ductive heating in the latter step is less effective than
microwave irradiation. Also the choice of the co-solvent In conclusion, bromoaryl boronates are highly versatile
water is in agreement with the standard conditions of substrates for CIC sequences, where the boronate first
Suzuki reactions (entries 3–5 and 11) and gives rise to the serves as an electron-withdrawing functionality necessary
highest yields for this model reaction. With these opti- to trigger the CIR and then, this dormant organometallic
1
5
mized CIC conditions in hand (entry 5) the stage was set group is activated for Suzuki coupling by the addition of
for developing a new modular three-component synthesis potassium carbonate. Mechanistically, this sequential
of aryl chalcones based upon a CI–Suzuki coupling se- transformation opens potential for the development of
quence.
novel diversity-oriented processes. Studies addressing the
scope of this novel sequence to enhance the molecular di-
versity are currently underway.
Synlett 2006, No. 20, 3469–3473 © Thieme Stuttgart · New York

LETTER
Novel Three-Component Synthesis of Aryl Chalcones
3471
Table 2 Optimization of the Suzuki Coupling Step in the CI–Coupling Sequence of 4a, 5a, and 1aa
O
1
. (Ph3P)2PdCl2 (4 mol%), CuI (4 mol%)
PPh3 (20 mol%), DBU (1.05 equiv)
THF, MW (120 °C), 30 min
4a
+
5a
NC
Ph
2
. p-NCC6H4Br (1a; 1.0 equiv),
base conditions
7a
Entry
Base
Solvent
Conditions
Yield of 7a (%)b
1
2
3
4
5
6
7
8
9
Et N (3.0 equiv)
THF–DMF
THF
MW, 110 °C, 20 min, 150 °C, 10 min
MW, 110 °C, 20 min
MW, 110 °C, 20 min
Reflux, 20 h
–
3
DBU (1.1 equiv)
DBU (1.1 equiv)
K CO (1.5 equiv)
–
THF–H O (15:1)
38
48
76
67
10
14
15
Trace
51
Trace
–
2
THF–H O (3:2)
2
3
2
K CO (1.5 equiv)
THF–H O (3:2)
MW, 110 °C, 20 min
MW, 110 °C, 20 min
MW, 110 °C, 20 min
MW, 110 °C, 20 min
MW, 110 °C, 20 min
MW, 110 °C, 20 min
MW (110 °C), 20 min
MW, 110 °C, 20 min
MW, 110 °C, 20 min
Reflux, 20 h
2
3
2
K CO (1.5 equiv)
THF–MeOH (3:2)
THF–EtOH (3:2)
THF–DME (3:2)
THF–DMF (3:2)
THF–MeCN (3:2)
2
3
K CO (1.5 equiv)
2
3
K CO (1.5 equiv)
2
3
K CO (1.5 equiv)
2
3
1
1
1
1
1
1
0
1
2
3
4
5
6
K CO (1.5 equiv)
2 3
K CO (1.5 equiv)
THF–MeCN–H O (3:2:1)
2
3
2
K CO (1.5 equiv)
THF–toluene (3:2)
THF
2
3
K CO (1.5 equiv)
2
3
KF (2.0 equiv)
CsF (2.0 equiv)
CsF (2.0 equiv)
THF–DME (1:1)
THF–DME (1:1)
THF–DME (1:1)
26
35
17
Reflux, 20 h
1
MW, 110 °C, 20 min
a
Reaction conditions: 1.0 equiv of haloaryl boronate 4a, 1.05 equiv of propargyl alcohol 5a, 0.04 equiv of Pd(PPh ) Cl , 0.04 equiv of CuI, 0.2
3
2
2
equiv of PPh , 1.05 equiv of DBU, and 0.9 mL of THF (0.3 M) heating in a sealed vessel at 120 °C in a Smith Synthesizer single mode micro-
3
wave cavity for 30 min, then after adding (hetero)aryl halide 1a heating was continued.
b
Yields refer to isolated yields of compound 7a after flash chromatography on silica gel to be ≥95% pure as determined by NMR spectroscopy.
Table 3 CI–Coupling Synthesis of Aryl Chalcones 7a
R
1. (Ph3P)2PdCl2 (4 mol%), CuI (4 mol%)
PPh3 (20 mol%), DBU (1.05 equiv)
OH
O
R
Br +
O
Ar1
THF, MW (120 °C), 30 min
Ar1
B
2
2
. Ar Hal (1; 1.0 equiv), K2CO3, (1.5 equiv)
Ar2
O
H2O, MW (110 °C), 20–30 min
7
(49–76 %)
4
5
Entry
1
Boronate 4
Propargyl alcohol 5
(Hetero)aryl bromide 1
Yield of 7 (%)b
O
O
B
O
Br
1
2
NC
5a: Ar = Ph
1a: Ar = p-NCC H
Ph
6
4
7
a (76)
4a
O
2
2
3
4a
5a
5a
1b: Ar = 4-pyridyl
N
Ph
7
b (55)
O
c
2
4a
1c: Ar = Ph
Ph
7
c (72)
Synlett 2006, No. 20, 3469–3473 © Thieme Stuttgart · New York

3
472
W.-W. Liao, T. J. J. Müller
LETTER
a
Table 3 CI–Coupling Synthesis of Aryl Chalcones 7 (continued)
R
1. (Ph3P)2PdCl2 (4 mol%), CuI (4 mol%)
PPh3 (20 mol%), DBU (1.05 equiv)
OH
O
R
Br +
O
Ar1
THF, MW (120 °C), 30 min
Ar1
B
2
2
. Ar Hal (1; 1.0 equiv), K2CO3, (1.5 equiv)
Ar2
O
H2O, MW (110 °C), 20–30 min
7
(49–76 %)
4
5
Entry
4
Boronate 4
Propargyl alcohol 5
(Hetero)aryl bromide 1
Yield of 7 (%)b
O
2
4a
5a
1d: Ar = p-MeC H
Me
6
4
Ph
7
d (65)
O
c
2
5
6
4a
4a
5a
1e: Ar = p-MeOC H
6
4
MeO
Ph
7
e (64)
O
NC
1
5b: Ar = p-MeOC H
1a
6
4
OMe
7
f (63)
Me
Me
O
O
O
7
8
B
Br
5a
1e
1a
1a
MeO
Ph
Me
Me
4
b
7g (49)
O
1
4a
5c: Ar = 2-thienyl
NC
S
7
h (59)
NC
O
O
B
9
Br
5a
Ph
O
7
i (56)
4c
a
Reaction conditions: 1.0 equiv of haloaryl boronate 4, 1.05 equiv of propargyl alcohol 5, 0.04 equiv of Pd(PPh ) Cl , 0.04 equiv of CuI, 0.2
3
2
2
equiv of PPh , 1.05 equiv of DBU, 0.9 mL of THF (0.3 M), heating in a sealed vessel at 120 °C in a Smith Synthesizer single mode microwave
3
cavity for 30 min, then after adding (hetero)aryl halide 1, 1.5 equiv of K CO , and 1 mL of THF–H O (3:2) heating in a sealed vessel at 110 °C
2
3
2
in the microwave cavity for 20 min.
b
Yields refer to isolated yields of compound 7 after flash chromatography on silica gel to be Š95% pure as determined by NMR spectroscopy.
The corresponding iodides were used.
c
Acknowledgment
(3) For a recent monograph, see: Multicomponent Reactions;
Zhu, J.; Bienaymé, H., Eds.; Wiley-VCH: Weinheim, 2005.
Financial support of the Alexander-von-Humboldt foundation
scholarship for W.-W.L.), Deutsche Forschungsgemeinschaft, and
(
4) For reviews, see: (a) Bienaymé, H.; Hulme, C.; Oddon, G.;
Schmitt, P. Chem. Eur. J. 2000, 6, 3321. (b) Dömling, A.;
Ugi, I. Angew. Chem. Int. Ed. 2000, 39, 3168. (c) Ugi, I.;
Dömling, A.; Werner, B. J. Heterocycl. Chem. 2000, 37,
(
the Fonds der Chemischen Industrie are gratefully acknowledged.
We also cordially thank Dr. Oana G. Schramm for helpful discussi-
ons and BASF AG for the generous donation of chemicals.
647. (d) Weber, L.; Illgen, K.; Almstetter, M. Synlett 1999,
366. (e) Armstrong, R. W.; Combs, A. P.; Tempest, P. A.;
Brown, S. D.; Keating, T. A. Acc. Chem. Res. 1996, 29, 123.
References and Notes
(
(
f) Ugi, I.; Dömling, A.; Hörl, W. Endeavour 1994, 18, 115.
g) Posner, G. H. Chem. Rev. 1986, 86, 831.
(
1) For reviews, see: (a) Wasilke, J.; Obrey, S. J.; Baker, R. T.;
Bazan, G. C. Chem. Rev. 2005, 105, 1001. (b) Ajamian, A.;
Gleason, J. L. Angew. Chem. Int. Ed. 2004, 43, 3754.
(
5) For reviews on diversity-oriented syntheses, see:
a) Schreiber, S. L.; Burke, M. D. Angew. Chem. Int. Ed.
004, 43, 46. (b) Burke, M. D.; Berger, E. M.; Schreiber, S.
(
2
(
c) Lee, J. M.; Na, Y.; Han, H.; Chang, S. Chem. Soc. Rev.
004, 33, 302.
2) For a review, see: Müller, T. J. J. Top. Organomet. Chem.
006, 19, 149.
L. Science 2003, 302, 613. (c) Arya, P.; Chou, D. T. H.;
Baek, M. G. Angew. Chem. Int. Ed. 2001, 40, 339. (d) Cox,
B.; Denyer, J. C.; Binnie, A.; Donnelly, M. C.; Evans, B.;
2
(
2
Synlett 2006, No. 20, 3469–3473 © Thieme Stuttgart · New York

LETTER
Novel Three-Component Synthesis of Aryl Chalcones
(13) For recent reviews and monographs on microwave-
3473
Green, D. V. S.; Lewis, J. A.; Mander, T. H.; Merritt, A. T.;
Valler, M. J.; Watson, S. P. Progr. Med. Chem. 2000, 37,
3. (e) Schreiber, S. L. Science 2000, 287, 1964.
accelerated syntheses, see for example: (a) Kappe, C. O.
Angew. Chem. Int. Ed. 2004, 43, 6250. (b) Nüchter, M.;
Ondruschka, B.; Bonrath, W.; Gum, A. Green Chem. 2004,
6, 128. (c) Nüchter, M.; Müller, U.; Ondruschka, B.; Tied,
A.; Lautenschläger, W. Chem. Eng. Technol. 2003, 26,
1207. (d) de la Hoz, A.; D’Waz-Ortis, A.; Moreno, A.;
Langa, F. Eur. J. Org. Chem. 2000, 3659. (e) Xu, Y.; Guo,
Q.-X. Heterocycles 2004, 63, 903. (f) Microwaves in
Organic Synthesis; Loupy, A., Ed.; Wiley-VCH: Weinheim,
2002. (g) Hayes, B. L. Microwave Synthesis: Chemistry at
the Speed of Light; CEM Publishing: Matthews, NC, 2002.
(h) Microwave-Assisted Organic Synthesis; Lidström, P.;
Tierney, J. P., Eds.; Blackwell: Oxford, 2004. (i) Kappe, C.
O.; Stadler, A. Microwaves in Organic and Medicinal
Chemistry; Wiley-VCH: Weinheim, 2005.
8
(6) Kobayashi, S. Chem. Soc. Rev. 1999, 28, 1.
(7) (a) Kressierer, C. J.; Müller, T. J. J. Org. Lett. 2005, 7, 2237.
(
b) Karpov, A. S.; Merkul, E.; Oeser, T.; Müller, T. J. J.
Chem. Commun. 2005, 2581.
(8) (a) Braun, R. U.; Ansorge, M.; Müller, T. J. J. Chem. Eur. J.
2006, 12, 9081. (b) Müller, T. J. J.; Ansorge, M.; Aktah, D.
Angew. Chem. Int. Ed. 2000, 39, 1253.
(
9) For lead reviews on Sonogashira couplings, see for
example: (a) Takahashi, S.; Kuroyama, Y.; Sonogashira, K.;
Hagihara, N. Synthesis 1980, 627. (b) Sonogashira, K. In
Metal-Catalyzed Cross-Coupling Reactions; Diederich, F.;
Stang, P. J., Eds.; Wiley-VCH: Weinheim, 1998, 203.
(
(
c) Sonogashira, K. J. Organomet. Chem. 2002, 653, 46.
d) Negishi, E.-I.; Anastasia, L. Chem. Rev. 2003, 103,
(14) Schramm, O. G.; Müller, T. J. J. Synlett 2006, 1841.
(15) Without isolation, the yield for the first step is apparently
higher than in the optimization (Table 1, entry 11).
1979. (e) Marsden, J. A.; Haley, M. M. In Metal-Catalyzed
Cross-Coupling Reactions, 2nd ed.; de Meijere, A.;
Diederich, F., Eds.; Wiley-VCH: Weinheim, 2004, 317.
(16) Typical Procedure (7a, Table 3, Entry 11).
(
(
10) Braun, R. U.; Müller, T. J. J. Mol. Diversity 2003, 6, 251.
11) Typical Procedure (6a, Table 1, Entry 11).
To a magnetically stirred solution of 86 mg (0.30 mmol) of
4a, 42 mg (0.31 mmol) of 5a, 8 mg (12 mmol) of
To a magnetically stirred solution of 86 mg (0.30 mmol) of
Pd(PPh )Cl , 2 mg (10 mmol) of CuI, and 16 mg (60 mmol)
3
2
4
a, 42 mg (0.31 mmol) of 5a, 8 mg (12 mmol) of
of PPh in 0.9 mL of degassed THF in a microwave vial
3
Pd(PPh )Cl , 2 mg (10 mmol) of CuI, and 16 mg (60 mmol)
of PPh in 0.9 mL of degassed THF in a microwave vial
under nitrogen were added 48 mL (0.32 mmol) of DBU. Then
the sealed vessel was heated by microwave irradiation to 120
°C for 30 min. After cooling to r.t. 55 mg (0.31 mmol) of 1a,
63 mg (0.45 mmol) of K CO , and 0.6 mL of H O were
3
2
3
under nitrogen were added 48 mL (0.32 mmol) of DBU. Then
the sealed vessel was heated by microwave irradiation to 120
2
3
2
°
C for 30 min. After cooling to r.t., H O was added and the
added to the reaction mixture, and then the sealed vessel was
heated by microwave irradiation to 110 °C for 20 min. After
2
aqueous layer was extracted with EtOAc. The combined
organic phases were dried with MgSO . The solvents were
cooling to r.t. H O was added and the aqueous layer was
4
2
removed in vacuo and the residue was chromatographed on
extracted with EtOAc. The combined organic phases were
silica gel to give 64 mg (64%) of the desired product 6a, mp
dried with MgSO . The solvents were removed in vacuo and
4
1
1
44–145 °C. H NMR (300 MHz, CDCl ): d = 1.36 (s, 12 H),
the residue was chromatographed on silica gel to give 70 mg
3
1
7.48–7.62 (m, 6 H), 7.79–7.87 (m, 3 H), 8.01–8.04 (m, 2 H).
(76%) of 7a as a yellow solid, mp 120–122 °C. H NMR
13
C NMR (75.5 MHz, CDCl ): d = 24.9 (CH ), 84.0 (Cquat.),
(300 MHz, CDCl ): d = 7.50–7.67 (m, 6 H), 7.70–7.77 (m, 6
3
3
3
13
122.8 (CH), 127.6 (CH), 128.5 (CH), 128.6 (CH), 132.8
H), 7.85 (d, J = 15.6 Hz, 1 H), 8.05–8.02 (m, 2 H). C NMR
(
CH), 135.3 (CH), 137.3 (Cquat.), 138.2 (Cquat.), 144.6 (CH),
(75.5 MHz, CDCl ): d = 111.4 (Cquat.), 118.7 (Cquat.), 122.7
3
+
1
2
3
90.5 (Cquat.). MS (70 eV, EI): m/z (%) = 334 (100) [M ],
(CH), 127.6 (CH), 127.7 (CH), 128.5 (CH), 128.7 (CH),
129.1 (CH), 132.7 (CH), 132.9 (CH), 135.2 (Cquat.), 138.0
+
07 (32) [M – Bpin]. HRMS: m/z calcd for C H BO :
2
1
23
3
34.1740; found: 334.1731. Anal. Calcd for C H BO
(Cquat.), 140.9 (Cquat.), 143.6 (CH), 144.5 (Cquat.), 190.2
2
1
23
3
+
(
334.2): C, 75.47; H, 6.94. Found: C, 75.36; H, 6.96.
(C ). MS (70 eV, EI): m/z (%) = 309 (100) [M ], 207 (50)
quat.
+
1
(
12) All compounds have been fully characterized by H NMR,
[M – C H CN]. Anal. Calcd for C H NO (309.4): C,
6
4
22 15
13
C NMR and DEPT, COSY, NOESY, HETCOR and
85.41; H, 4.89; N, 4.53. Found: C, 85.49; H, 4.91; N, 4.58.
HMBC NMR experiments, IR, MS, HRMS and/or
combustion analyses.
Synlett 2006, No. 20, 3469–3473 © Thieme Stuttgart · New York