Catalytic substitution/cyclization sequences of: O -substituted Isocyanates: Synthesis of 1-alkoxybenzimidazolones and 1-alkoxy-3,4-dihydroquinazolin-2(1 H)-ones

Article abstract of DOI:10.1039/c7cc07926e

O-Substituted isocyanates (O-isocyanates) have rarely been used in organic synthesis, given their tendency to undergo side reactions (e.g., trimerization). Herein, we show that masked (blocked) O-isocyanate precursors allow one-pot or cascade reaction sequences featuring base-catalyzed substitution with 2-iodoanilines and 2-iodobenzylamines followed by copper-catalyzed cyclization, to form benzimidazolones and 3,4-dihydroquinazolin-2(1H)-ones. This work shows that O-isocyanates can serve as efficient building blocks for the synthesis of hydroxylamine-containing heterocycles.

Full text of DOI:10.1039/c7cc07926e

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Catalytic substitution/cyclization sequences
of O-substituted Isocyanates: synthesis of
1-alkoxybenzimidazolones and 1-alkoxy-3,4-
dihydroquinazolin-2(1H)-ones†
a
Cite this:DOI: 10.1039/c7cc07926e
Received 13th October 2017,
Accepted 14th November 2017
Qiang Wang,ab Jing An,b Howard Alper,ab Wen-Jing Xiao
and
DOI: 10.1039/c7cc07926e
ab
Andre M. Beauchemin
*
´
rsc.li/chemcomm
O-Substituted isocyanates (O-isocyanates) have rarely been used in In contrast to the ubiquitous C-isocyanates, the chemistry of highly
organic synthesis, given their tendency to undergo side reactions reactive O-isocyanates remains largely unexplored due to side
(e.g., trimerization). Herein, we show that masked (blocked) O-iso- reactions (e.g. trimerization).15 Our recent results showed that
cyanate precursors allow one-pot or cascade reaction sequences controlled formation and reactivity is possible from blocked
featuring base-catalyzed substitution with 2-iodoanilines and 2-iodo- O-isocyanate precursors,14,16 allowing them to be suitable com-
benzylamines followed by copper-catalyzed cyclization, to form ponents of cascade reactions. Herein, we expand this approach to
benzimidazolones and 3,4-dihydroquinazolin-2(1H)-ones. This work reaction sequences involving the substitution of 2-iodoanilines
shows that O-isocyanates can serve as efficient building blocks for and 2-iodobenzylamines onto in situ generated O-isocyanates,
the synthesis of hydroxylamine-containing heterocycles.
followed by a copper-catalyzed cyclization that is compatible with
the weak N–O bond,17 to form complex hydroxylamine-containing
Hydroxylamines and their derivatives are an important family heterocycles: 1-alkoxy-benzimidazolones18 and -3,4-dihydro-
of compounds in organic chemistry, which have many quinazolin-2(1H)-ones19 (Scheme 1B).
applications1 and are often found in bioactive natural products
To our knowledge O-isocyanates have not been used in
and pharmaceuticals.2 The weak N–O s bond has an average combination with metal-catalyzed reactions, a strategy that would
energy of B57 kcal molꢀ1, which is significantly lower than the enable access to various complex hydroxylamine derivatives.
energies of sC–X bonds (X = C, N, O; 69–91 kcal molꢀ1).1b For
this reason, hydroxylamine derivatives can be difficult to form
as the weakness of the N–O bond can lead to rearrangements
and other side reactions.3 Over the past decade, our group has
developed Cope-type hydroaminations of hydroxylamines, forming
various types of N–O bond containing molecules through intra-
or intermolecular reactions.4 This work made us aware of the
limitations of many hydroxylamine syntheses. In addition, recent
synthetic developments exploiting complex hydroxylamine
derivatives in amination5,6 and photoredox catalysis7 and also
the frequent use of such derivatives in medicinal chemistry
efforts (e.g., Scheme 1A)8–11 suggest that the development of
new methods for their synthesis is needed. Given this and as
part of our efforts on amphoteric N-isocyanate intermediates,12
we were drawn to O-isocyanates, a rare class of isocyanates13
with excellent potential to assemble N–O bond containing motifs.14
a CCNU-uOttawa Joint Research Centre, Key Laboratory of Pesticide & Chemical
Biology, Ministry of Education, College of Chemistry, Central China Normal
University, 152 Luoyu Road, Wuhan, Hubei 430079, China
b Centre for Catalysis Research and Innovation, Department of Chemistry and
Scheme 1 (A) Biologically active heterocycles containing a hydroxylamine
motif. (B) This work: catalytic heterocyclic synthesis using rare O-isocyanates
as key intermediates.
Biomolecular Sciences, University of Ottawa, 10 Marie-Curie, Ottawa, ON,
K1N 6N5, Canada. E-mail: andre.beauchemin@uottawa.ca
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7cc07926e
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To address this void and expand the synthetic reach of O-iso- reactions, K2CO3 and K3PO4, only led to the decomposition of the
cyanates, it was necessary to synthesize suitable cyclization starting material (entries 5 and 6). This was not surprising, given
precursors. From a blocked isocyanate perspective, phenol our recent observation that mixed ureas from anilines can act as
is a common blocking group that typically provides stable N-isocyanate blocking (masking) groups.12d,e In contrast, the
isocyanate precursors, which can be deblocked upon heating organic base Et3N gave a comparable result to DABCO (99%
or in the presence of a base as a catalyst.12b,c,e,f We thus selected yield, entry 7) but other organic bases such as i-Pr2NEt and
the readily accessible phenyl methoxycarbamate 1a (R1 = Me) DMAP gave inferior results (entries 8 and 9). Further exploration
and 2-iodoaniline 2a as model substrates to optimize the of solvent effects using DMF, MeCN and 1,4-dioxane only led to
O-isocyanate substitution reaction (see ESI† for details and low reaction efficiency and moderate yields (entries 10–12).
discussion). Gratifyingly, substitution proceeded in the presence Control experiments revealed that both CuI and ligand were
of several bases to afford the desired mixed N-methoxyurea 3a necessary for this reaction (entries 13–15). Given the efficiency
upon heating at 100 1C (Table S1 (ESI†), entries 1–5). Further observed for each individual step, a one-pot approach was
optimization led to the cyclization precursor 3a being isolated attempted by: (1) performing the substitution step in p-xylene
in 92% yield. With a robust procedure available to form the in the presence of DABCO, (2) concentrating in vacuo, (3) adding
cyclization precursors, the identification of suitable conditions CuI, ligand L2 and DMSO: this gave the desired product 4a in
to convert mixed urea 3a into N-methoxybenzimidazolone 4a 87% isolated yield (entry 2). When 2-bromoaniline was used,
began (Table 1).
product 4a0 was formed in 43% yield (Table 2). With the optimal
To our delight, the copper-catalyzed cyclization proceeded reaction conditions in hand, we then examined the scope of
smoothly in the presence of CuI (10 mol%), 1,10-phenanthroline 2-iodoanilines for this one-pot reaction (Table 2).
(L1, 20 mol%), DABCO as a base and DMSO as a solvent, providing
The conditions identified for the one-pot sequence proved
the desired product 4a in 96% yield (entry 1). These conditions versatile for the synthesis of various 1-alkoxy-imidazolones (Table 2).
were adapted from our recent study of related cascade reactions For example, a variety of para-substituted 2-iodo-anilines showed
using N-isocyanate derivatives.12f However, investigation of ligand good reactivity and the corresponding products 4b–4g were
effects revealed that N,N0-dimethyl-1,2-ethanediamine (L2) was the obtained in 70–92% yields over two steps. This included substrates
best ligand for the cyclization (entries 1–4), as a quantitative yield with an electron-withdrawing group (CO2Me, 4g, 70% yield)
of product 4a was detected by NMR. The effect of the base on the and with fluoro, chloro, bromo and trifluoromethyl groups.
coupling reaction was further explored (entries 5–9). Interestingly, 2-Iodoanilines bearing methyl group(s) at different positions were
inorganic bases commonly used in copper-catalyzed C–N coupling explored, and as expected sterically hindered anilines afforded
the desired product in lower yields (4i–j, 45–49% yields). Then
different substitutions with different O-isocyanate precursors
Table 1 Optimization of reaction conditions for cyclizationa
were investigated. Precursors bearing benzyl, allyl, and cyclohexyl
functional groups exhibited good compatibility under the present
conditions, forming products 4k–m efficiently (75–82% yields). A
synthetically useful alkyne moiety was also well-tolerated under
the one-pot conditions, affording product 4n in 84% yield.
Then we sought to extend the approach to form 6-membered
heterocycles bearing a hydroxylamine functionality.19,20 This implied
Entry
Ligand
Base
Solvent
Yieldb (%)
1
2
3
4
5
6
7
8
L1
L2
L3
L4
L2
L2
L2
L2
L2
L2
L2
L2
L2
—
DABCO
DABCO
DABCO
DABCO
K2CO3
K3PO4
Et3N
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMF
MeCN
1,4-Dioxane
DMSO
DMSO
DMSO
96
498 (87)c
Table 2 Substrate scope of 1-alkoxybenzimidazolonesa
76
70
0
0
99
90
51
52
32
66
19
63
o5
i-Pr2NEt
DMAP
9
10d
11e
12e
13f
14g
15h
DABCO
DABCO
DABCO
DABCO
DABCO
DABCO
—
a
Conditions: 3a (0.10 mmol), base (0.20 mmol), solvent (1.0 mL).
b
Determined by 1H NMR using 1,3,5-trimethoxybenzene as an internal
standard. Isolated yield of one-pot reaction in parentheses. d Reaction
time: 12 h. Reaction time: 36 h. Without CuI. Without L2. Without
CuI and L2.
c
e
f
g
h
a
Conditions: 1 (0.20 mmol), 2 (0.30 mmol), DABCO (0.40 mmol),
p-xylene (2.0 mL), 100 1C, 12 h, and then concentrated; adding CuI
(0.02 mmol), L2 (0.04 mmol), and DMSO (2.0 mL), then stirred at 100 1C
b
for 5 h. 2-Bromoaniline and L1 were used, 12 h for cyclization.
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using the substitution of 2-iodobenzylamines on the O-isocyanate
derivative, which we felt would benefit from the higher nucleo-
philicity of such amines. In contrast, the copper-catalyzed
cyclization would likely require some reoptimization. Indeed,
the substitution reaction of O-isocyanate precursor 1a and
2-iodobenzylamine 5a did occur under the standard conditions.
In contrast, the copper-catalyzed cyclization failed under the
standard conditions for benzimidazolone synthesis (see ESI†
Scheme S1). Fortunately, after a brief screen of the effect of the
ligand, base and solvent on this reaction, optimal reaction
conditions were identified and the desired 6-membered hetero-
cycle 6a was formed in 95% yield following a two-step, one-pot
sequence (see footnote, Table 3). The substrate scope of this
one-pot reaction sequence was examined, and the results are
summarized in Table 3.
Scheme 2 A comparison of cascade vs. one-pot reaction for three
substrate types.
Generally, O-isocyanate precursors bearing different functional
groups and various N-substituted-2-iodo-benzylamines engaged in
productive reaction sequences (Table 3). For example, O-isocyanate
precursors with benzyl and allyl functional groups gave the
corresponding products 7b and 7c in good yields (70% and 83%,
respectively). A precursor with an O-cyclohexyl group gave the
desired product 7d in moderate yield (7d, 49% yield). The
formation of product 7e illustrates the possibility of rapidly
assembling molecular complexity with this sequence: a product
with N-cyclopropyl and O-propargyl substituents was isolated in
84% yield under standard conditions. The impact of the nitrogen
Scheme 3 Removal of the benzyl group to form the free hydroxyamic
acid heterocycles.
substituent on the 2-iodobenzylamine 5 was examined. Substrates the reaction and produced the desired product 7k in 90% yield.
with hexyl, phenyl and allyl groups reacted smoothly under the The iodo substituent in product 7k provides a convenient handle
standard conditions and were converted to products 7f–h in for further incorporation of complexity, for example using a
excellent yields (7f–7h, 89–95% yields). When the ortho-bromo cross-coupling reaction.
substrate was used, product 7i0 was isolated in comparable yield
In order to further simplify the process, we investigated
(7i, 94% vs. 7i0, 88%). Even a sterically hindered substrate such as cascade reactions of O-isocyanate precursors with 2-iodoaniline
the N-cyclohexyl benzylic amine also showed high reaction or 2-iodobenzylamines (Scheme 2). In the presence of a copper
efficiency, with product 7j being isolated in 85% yield. Moreover, catalyst and DMSO as the solvent, the cascade reaction of
the substrate bearing an iodo substituent was untouched during phenyl methoxycarbamate 1a and 2-iodoanilines 2a did indeed
occur albeit in relatively moderate yield compared with the one-
pot reaction sequence (Scheme 2a). When the cascade reaction
Table 3 Substrate scope of 1-alkoxy-3,4-dihydroquinazolin-2(1H)-onesa
of 1a and 2-iodobenzylamine 5a was examined, the corresponding
product 7a was isolated in lower yield (47% vs. 95%) (Scheme 2b).
In contrast, the cascade reaction of 1a and N-(2-iodobenzyl)prop-2-
en-1-amine 5c produced the desired product 7h in comparable
efficiency to the one-pot sequence (Scheme 2c). We believe the
differences above are consistent with the ability of anilines to act
as blocked isocyanate precursors, leading to the degradation of
intermediate 3a (Scheme 2a).12d,e The cyclization of ureas derived
from secondary 2-iodobenzylic amines (Scheme 2c) is significantly
more favourable than the cyclization of the parent primary amine
adducts (Scheme 2b), due to the strong conformational preferences
of s-cis urea present that disfavour the latter cyclization.
To expand the synthetic utility of this strategy, we removed
the benzyl group from products 4k and 7b using TiCl4 under
mild conditions. This afforded the free hydroxamic acid products
a
Conditions: 1 (0.20 mmol), 5 (0.30 mmol), DABCO (0.01 mmol), PhCF3 8 and 9 in moderate to good yields (Scheme 3, 8: 87%; 9: 61%).
(2.0 mL), 100 1C, 12 h, and then concentrated; adding CuI (0.02 mmol),
L1 (0.04 mmol), Cs2CO3 (0.4 mmol) and DMSO (2.0 mL), then stirred at
100 1C for 6 h. Corresponding o-bromo substrate was used, 12 h for
In summary, we have developed a new reaction sequence for
the rapid synthesis of complex 5- and 6-membered hydroxylamine-
b
cyclization.
derived heterocycles: benzimidazolones and 3,4-dihydroquinazolin-
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O-isocyanates, using masked O-isocyanate precursors in a base-
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cyclization. The cyclization is not inhibited by the phenol released
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We are grateful to the University of Ottawa, NSERC, CFI, and
the Ontario MRI, Q. W. thanks the China Scholarship Council
and excellent doctoral dissertation cultivation grant from Central
China Normal University for generous financial support.
Conflicts of interest
There are no conflicts to declare.
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