Transformation of masked benzyl alcohols to: O -aminobenzaldehydes through C-H activation: A facile approach to quinazolines

Article abstract of DOI:10.1039/c6cc05560e

Direct transformation of a directing group to important synthetic units would provide a high atom efficiency synthetic approach in synthetic chemistry. Herein, a convenient protocol for the synthesis of o-aminobenzaldehyde and benzoxazole derivatives from benzyl alcohols has been developed by employing (N,N-dimethyl)oxamoyl amide as a directing group in a palladium-catalyzed intramolecular amination. Furthermore, the attached directing center may not only be transformed into the product, but may also be further applied to generate synthetically important quinazoline and quinoline units. Finally, a high atom efficiency one-pot, two-step approach to form quinazolines from benzyl alcohol derivatives has been achieved in good yields, thus demonstrating its high utility.

Full text of DOI:10.1039/c6cc05560e

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Transformation of Masked Benzyl Alcohols to o-
Aminobenzaldehydes through C-H Activation: A Facile Approach
to Quinazolines
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Xiaolan Chen, Jian Han, Yan Zhu, Chunchen Yuan, Jingyu Zhang* and Yingsheng Zhao*
www.rsc.org/
Direct Transformation of directing group to important synthetic units dimethyl)oxamoyl amide-directed intramolecular amination to
would provide a high atom efficiency synthetic approach in synthetic generate o-aminobenzaldehyde and benzoxazole derivatives from
chemistry. Herein,
a
convenient protocol in synthesis of o- benzyl alcohols. The newly formed directing center, (N,N-
aminobenzaldehyde and benzoxazole derivatives from benzyl alcohols has dimethyl)oxamoyl amide, can not only be partly transformed into
been developed by employing (N,N-dimethyl)oxamoyl amide as a directing the products, but also further applied in the synthesis of important
group in a palladium-catalyzed intramolecular amination. Furthermore, synthetic units (Figure 1), such as quinoline and quinazoline
the attached directing center may not only be transformed into product, derivatives. We further demonstrate that the (N,N-
but may also be further applied to generate synthetically important dimethyl)oxamoyl amide-protected benzyl alcohols may be transfo-
quinazoline and quinoline units. Finally, a high atom efficiency one-pot, med into highly important quinazoline building blocks through a
two-step approach to form quinazolines from benzyl alcohol derivatives two-pot, four-step strategy in moderate to good yields.
has been achieved in good yields, thus demonstrating its high utility.
Figure 1. Drugs that contain the quinazoline unit.
MeO
Me
Highly regioselective functionalization of a C-H bond to construct
Me
N
NH2
N
N
Br
MeO
complex organic molecules is an important and attractive process in
synthetic chemistry.1 The use of a directing group to form a
coordination center has become a successful strategy for activating
specific C-H bonds, which provides a straightforward method for
the synthesis of functionalized organic molecules.2 Thus, various
directing groups, such as oximes,3 pyridines,4 acetanilides,5
oxazolines,6 amides,7 esters,8 and thioethers,9 have been developed
to accomplish site-selective C-H functionalization of different types
of compounds. However, this approach usually requires both the
installation of a directing group and its subsequent removal, which
necessitates several additional synthetic steps and erodes the high
atom economy.
Me
N
O
N
O
OMe
N
N
O
N
I
N
N
NH
O
Me
NH2
N
N
HN
N
Me
O
Vandetanib
Linagliptin
Alfuzosin
Me
O
O
OEt
N
O
N
Me
N
N
N
H
N
N
N
N
O
N
N
N
H
N
N
F
NH
H
N
H
I: Inhibitor of BAZ2B
In view of this shortcoming, the development of C-H
functionalization strategies whereby the introduced directing group
can not only assist C-H functionalization, but also be partly or
completely transformed into another functional group is highly
Idelalisib
II: Rho-kinase inhibitor
Highly regioselective functionalization of alcohols is particularly
attractive, because the corresponding products are omnipresent
components in many natural products and bioactive compounds,
and are sufficiently reactive to be converted into versatile synthetic
precursors. To date, only a few approaches have been devised to
directly functionalize alcohols. One successful approach is the use
of hydroxyl as a directing group, which has been developed by the
groups of Yu10 and Hartwig11, respectively. However, limited
substrate scope hindered further application. Besides, Dong’s
group12 disclosed an “exo-oxime-directing mode” to realize
acetoxylation reactions of masked alcohols. Later, our group
showed the monodentate acetoxime directing group to be very
appealing. Herein, we report
a palladium-catalyzed (N,N-
Key Laboratory of Organic Synthesis of Jiangsu Province, College of Chemistry,
Chemical Engineering and Materials ScienceSoochow University, Suzhou 215123,
PR China. E-mail: yszhao@suda.edu.cn.
College of Physics, Optoelectronics and Energy & Collaborative Innovation Center
of Suzhou Nano Science and Technology, Soochow University, Suzhou 215006 (PR
China)
E-mail: jyzhang@suda.edu.cn
†Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See DOI:
10.1039/x0xx00000x
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suitable for activating ortho-arene C-H bonds of aromatic alcohols, with a view to improve the selectivity between 3a and 2a
affording the ortho-arylated13 and olefinated14 aromatic alcohols Sodium acetate was found to increase the yield of product 2a
(Scheme 1a). to 91%, while suppressing that of cyclized 3a to 3%. In order to
Although these directing group strategies for the selectively obtain cyclized product 3a, the use of two different
.
functionalization of aromatic alcohols have been well explored, the
oxidants was screened. To our delight, a better yield of 3a was
achieved when we combined the two oxidants K2S2O8 and
H2O2. Further screening revealed that the additive 3,5-
dichlorobenzoic acid had a more beneficial effect than PivOH
or Ac-Gly-OH. It is probable that a weakly acidic system
stabilizes the cyclized product 3a. Further control reactions
revealed the palladium catalyst to be essential for the
transformation.
a) General approach in functionalization of benzyl alcohol
ODG
OH
+ DG
OH
ODG
R
- DG
O
[Pd]
FG
FG
FG
FG
FG
H
R
OAc
R
CO2Et
O
O
N
O
N
O
FG
N
Me
Me
Having established the optimal reaction conditions, various
(N,N-dimethyl)oxamoyl amide-protected benzyl alcohols were
evaluated with NFSI as oxidant to form various substituted o-
aminobenzaldehyde derivatives (Table 1). Generally, ortho-
O
FG
Reported by Ferreira
R = olefin & aryl
Reproted By Zhao
Reported by Dong
b) This work: Decorating strategy for transformation benzyl alcohol into important
synthetic units
(
2b–d), meta- (2e–h), and para-substituted (2j–m) benzyl
R
R
alcohols were all transformed to the o-aminobenzaldehyde
derivatives in moderate to excellent yields, along with small
amounts of cyclized product 3a (< 10%). It is worth mentioning
that the meta-substituted benzyl alcohols were all selectively
R
O
FG
OH
O
FG
O
Pd(OAc)2
FG
N
O
K2S2O8, H2O2
one pot
HN
O
NMe2
FG
O
R
O
R
O
aminated at the less hindered position (2e
substituted benzyl alcohols also reacted well, affording the
corresponding products in good yields (2m 2n 2p). To our
–h). Multiply-
FG
NMe2
NH
FG
Pd(OAc)2
NFSI
NH3
NH2
O
O
Functional group
,
,
I
Key intermediate
for Oxime synthesis
N
NMe2
NH3
One pot two steps synthesis of quinazolines
delight, α-substituted benzyl alcohols could also be
transformed to 2′-aminoacetophenone derivatives in good
yields under the standard conditions.
N
Pd(OAc)2, NFSI
O
R = H or Me
Me2N
Scheme 1. Transform benzyl alcohols to synthetic units.
Table 1. Scope of Pd-catalyzed amination using NFSI as an oxidant.[a]
attached directing centers need to be removed after C-H
functionalization to provide synthetic units. Thus, a more
appealing strategy for selective C-H transformation of benzyl
alcohols would involve the introduced directing center both
assisting C-H functionalization and being utilized in the
synthesis of other complex compounds as a key functional
group.
Our group has developed a one-pot, three-step acet-
oxime synthesis. The substrate
I
is the key intermediate for
might be easily
oximes. We speculated that substrate
I
transformed into the well-known amide directing center,
which might offer a unique advantage in the synthesis of
important building blocks. With these conditions in hand, we
firstly treated various amides, derived from o-
benzylhydroxylamine, with Pd(OAc)2 (5 mol%) and PhI(OAc)2 (2
o
equiv) in either toluene or HFIP at 100 C for 10 h to form the
[a] Reaction conditions: 1 (0.2 mmol), Pd(OAc)2 (5 mol%), NFSI (0.5 mmol),
NaOAc (0.1 mmol), HFIP (3 mL), 100 oC, 24 h; [b] NFSI (0.3 mmol), 16 h; [c]
NFSI (0.6 mmol).
intramolecular amination products. (N,N-dimethyl)oxamoyl
amide-protected benzyl alcohol gave the cyclized product 3a in
12% yield (see the SI). Next, we explored various oxidants,
solvents, and additives. The oxidants K2S2O8, TBPB, DTBP, and
H2O2 all gave the cyclized product 3a in moderate yield, along
with recovered starting material. Interestingly, the F+ oxidant
NFSI afforded a mixture of 3a and 2a in yields of 56% and 43%,
respectively (entry 3). It is probable that F+ can oxidize both
Pd(II) to Pd(IV) and 3a to 2a in the catalytic cycle. We carried
out the reaction with NFSI (2.5 equiv) for 24 h (entry 5).
Gratifyingly, the important synthetic unit 2a was obtained in
80% yield, along with 10% of 3a. Several additives were tested
We next explored the substrate scope of the cyclization reaction.
Various substituted benzyl alcohols proved to be compatible with
the transformation, affording the corresponding products in
moderate to good yields (Table 2). Excellent regioselectivity in favor
of the less hindered position was also observed in the cyclization
reaction. A bromo substituent was preserved, which could be used
to introduce different functional groups through cross-coupling
reactions.
2 | J. Name., 2012, 00, 1-3
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Encouraged by the unique features of N-(2-formyl-phenyl) Therefore, we tried to develop more effective steps whereby
acetamide in the synthesis of various heterocyclic compounds, we quinazolines (See the SI, Table 3, 8a–f) could be obtained by a one-
set out to ascertain whether the product 2a could show similar pot, two-step strategy. Previously, these quinazolines have been
reactivity (Scheme 2). Thus, we treated 2a with various substrates obtained from benzyl alcohols through a three-pot, five-step
having multiple reaction centers, such as ethyl acetoacetate,15 ethyl sequence in good to excellent yields. The products could be further
cyanoacetate,16 and cyanamide.16 The corresponding N-containing converted into derivatives functionalized at the 4-position through
heterocycles (5–7) were obtained in excellent yields. It is worth
Table 2. Scope of Pd-catalyzed amination using two oxidants. [a]
transition-metal-catalyzed cross-coupling reactions,18e the final
products belonging to a novel class of potent ligands of the
translocator protein,19 antiproliferative agents,20 and so on.21
Compared to the previous procedure,22 this work offers a concise
and highly effective approach to prepare quinazolines from benzyl
alcohols, highlighting the convenience of C-H functionalization in
the synthesis of complex molecules.
Pd(OAc)2 (5 mol%)
FG
O
O
H2O2 (4 equiv)
K2S2O8 (2 equiv)
DCB (0.5 equiv)
O
FG
N
HN
O
NMe2
O
O
1
HFIP, 100 oC, 4-16 h
NMe2
3
Br
Cl
F
Table 3. Synthesis of quinazolines in one pot.
O
O
O
N
O
N
N
N
O
O
O
O
O
O
O
O
NMe2
NMe2
NMe2
NMe2
3a, 81%
Me
3b, 76%
3d, 74%
3c, 71%
F3C
O
Br
Cl
O
O
N
O
N
O
N
O
N
O
O
O
O
O
O
NMe2
NMe2
NMe2
NMe2
3f[c], 70%
3g[c], 58%
3h[c], 75%
3e[b], 65%
O
N
O
N
O
N
O
Cl
Br
N
F
O
O
Me
O
O
O
O
O
O
NMe2
NMe2
NMe2
NMe2
3i[b], 73%
3k, 78%
3j, 72%
3l, 68%
Reaction conditions: a) 1 (0.3 mmol), Pd(OAc)2 (5 mol%), NFSI (0.75 mmol),
NaOAc (0.15 mmol), HFIP (4.5 mL); b) NH3 (2 M in MeOH) (0.5 mL, 20 equiv),
80 oC, over night.
[a] Reaction conditions: 1 (0.2 mmol), Pd(OAc)2 (5 mol%), K2S2O8 (0.4 mmol),
H2O2 (0.8 mmol), DCB (0.1 mmol), HFIP (3 mL), 100 oC, 10 h. [b] 4 h, without
K2S2O8. [c] 110 oC, 16 h. DCB = 3,5-dichlorobenzoic acid
Several control reactions were performed to better understand
the reaction path. As shown in Scheme 3, when we directly treated
product 3a with NFSI without NaOAc, the oxidized product 2a was
obtained in 67% yield along with several over-oxidized products.
Thus, NFSI apparently oxidized the benzylic C-H bonds, transforming
the product 3a to 2a during the catalytic cycle. This result explained
why 2.5 equivalents of NFSI were needed in the transformation. In
the presence of sodium acetate as an additive, the product 2a was
obtained in 80% yield. Though the precise role of NaOAc is not clear,
it may serve as a pH buffer to stabilize the sensitive product 2a.
Based on pioneering studies and the aforementioned results, a
plausible mechanism is proposed (see the SI and Scheme 3). The
mentioning that the directing group was directly removed during
aromatization. Moreover, the directing group could also be easily
removed
under
basic
conditions,
affording
the
o-
aminobenzaldehyde in excellent yield.
attached (N,N-dimethyl)oxamoyl amide firstly takes part in
a
Pd(II)/Pd(IV) catalytic cycle, involving sequential C-H activation,
Pd(II)/Pd(IV) oxidation, and C-N reductive elimination. The benzylic
C-H bonds could be oxidized by NFSI, affording the ring-opened o-
aminobenzaldehyde products.
Scheme 2. Transformation of product 2a into various synthetic units.
To further explore the synthetic simplicity and versatility of the
(N,N-dimethyl)oxamoyl amide group, the product 2a was treated
with ammonia in methanol at 80 oC overnight.17 The quinazoline
derivative 8a was isolated in 92% yield. Here, the directing group
acted as a functional group, being transformed to a carboxylic
amide. Importantly, product 8a may potentially be converted into
various bioactive compounds through well-developed methods.18
Scheme 3. Preliminary mechanistic study: role of NFSI.
In summary, we have developed a decorating strategy for the
functionalization of benzyl alcohols through C-H activation. The
newly attached functional group serves as both a directing center
and a reaction partner, affording the important synthetic units of o-
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Ahmed, K. Weise, N. G. Akhmedov, J. L. Petersen and X. Shi,
Chem. Sci. 2013, , 3712-3716; (d) M. Wasa and J.-Q. Yu, J.
Am. Chem. Soc. 2008, 130, 14058-14059.
aminobenzaldehyde and benzoxazole derivatives with different
oxidants. Various transformations of (N,N-dimethyl)oxamoylamide-
protected o-aminobenzaldehyde to N-containing heterocycles have
also been achieved. Further exploration of the obtained o-
aminobenzaldehydes has led to an approach of high atom efficiency
(one-pot, two-step) to generate important quinazolines in good
yields through use of the newly attached directing center. These
4
8
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Acknowledgements
We are grateful to the Natural Science Foundation of China (NO.
21572149) and Young National Natural Science Foundation of China
(NO. 21402133). The PAPD is also gratefully acknowledged.
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NO2
NO2
NH2
NO2
NO2
a
b
c
NH2
d
OH
e
NH2
f
O
N
N
3
4
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