One-pot, regiospecific assembly of (E)-benzamidines from δ- and γ-amino acids via an intramolecular aminoquinazolinone rearrangement

Article abstract of DOI:10.1039/c5ob02378e

The efficient generation of novel, N-linked benzamidines resulting from a regiospecific rearrangement of quinazolinones is described. This methodology study explored reaction parameters including the effect of changing solvent and temperature, as well as varying electronic substituents on the structural core. The transformation was extensively optimized in terms of reaction conditions and scope, resulting in a protocol that consistently affords diversely functionalized amidines in high yield. The process permits regional structural derivatization that was previously inaccessible, and the multistep process was also reduced to a telescoped, five-step sequence that efficiently affords pharmacologically unique (E)-benzamidoamidines from N-BOC protected γ- and δ-amino acids.

Full text of DOI:10.1039/c5ob02378e

Organic &
Biomolecular Chemistry
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One-pot, regiospecific assembly of
(E)-benzamidines from δ- and γ-amino acids via an
intramolecular aminoquinazolinone
rearrangement†
Cite this: DOI: 10.1039/c5ob02378e
Chad E. Schroeder, Sarah A. Neuenswander, Tuanli Yao, Jeffrey Aubé‡ and
Jennifer E. Golden*§
The efficient generation of novel, N-linked benzamidines resulting from a regiospecific rearrangement of
quinazolinones is described. This methodology study explored reaction parameters including the effect of
changing solvent and temperature, as well as varying electronic substituents on the structural core. The
transformation was extensively optimized in terms of reaction conditions and scope, resulting in a proto-
col that consistently affords diversely functionalized amidines in high yield. The process permits regional
structural derivatization that was previously inaccessible, and the multistep process was also reduced to a
telescoped, five-step sequence that efficiently affords pharmacologically unique (E)-benzamidoamidines
from N-BOC protected γ- and δ-amino acids.
Received 18th November 2015,
Accepted 25th March 2016
DOI: 10.1039/c5ob02378e
www.rsc.org/obc
Introduction
The 4-quinazolinone core has been extensively derivatized
chemically and shown to possess diverse pharmacological
activity. With a spectrum of activity that includes antimicro-
bial,1 antiinflammatory,2 antiproliferative, sedative/hypnotic,
and anticonvulsant effects,3 this structural framework is con-
sidered a privileged structure.3,4 In the pursuit of our own
Fig. 1 Highlighted 4-quinazolinone core of hit scaffold 1.
anti-infective program, we discovered
a 4-quinazolinone-
derived hit scaffold 1 that was worthy of additional structure–
activity relationship (SAR) studies (Fig. 1).5,6 As such, we
sought to explore the pharmacophoric elements of this chemo-
type for our project through synthetic manipulation.
The synthesis of 2-alkylquinazolinones had been previously
described, so our preliminary efforts focused on modifying
components accessible within the scope of published
protocols.7–10
A
three step procedure11 was employed
to prepare piperazine-containing analogs of 1 (Scheme 1).
Scheme 1 Synthetic route to quinazolinone analogs of 1.
University of Kansas Specialized Chemistry Center, Del Shankel Structural Biology
Center, 2034 Becker Drive, Lawrence, KS 66047, USA.
E-mail: jennifer.golden@wisc.edu
†Electronic supplementary information (ESI) available: General methods, syn-
thetic procedures and characterization for compounds 12a–m, 18a–b, 19a, and
24a–d as well as 1H and 13C NMR for final compounds. See DOI: 10.1039/
Substituted anthranilic acids 2 were treated with chloroacetyl
chloride in the presence of triethylamine to generally afford
2-(2-chloroacetamido)benzoic acids 3. While reported proto-
cols11,12 alternatively describe the intermediacy of 2-(chloro-
methyl)-4H-benzo[d][1,3]oxazin-4-ones 4, these were rarely
obtained in our hands. Nonetheless, dehydrative amidation of
c5ob02378e
‡Present address: School of Pharmacy, University of North Carolina,
3012 Marsico Hall, 125 Mason Farm Road, Chapel Hill, NC 27599.
§Present address: School of Pharmacy, Department of Pharmaceutical Sciences,
University of Wisconsin-Madison, 777 Highland Avenue, Madison, WI 53705.
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significant. Additionally, it was biologically advantageous for
our amidines to bear a C-5 nitro group on the core; however, it
was unclear if the presence of a strongly electron withdrawing
group was required for the rearrangement to work. Perhaps
most importantly, the rearrangement also featured a note-
worthy limitation in that modification of the cyclic amino-
amidine portion of the scaffold was not feasible based on the
functional need of a diamine component as the reacting
partner. Therefore, diversification of this structural region was
impossible, thus severely restricting the level of pharmacologi-
cal optimization that our team could pursue. Given these
questions and our need to pursue our antiviral program, we
launched a methodology study around this rearrangement in
order to better understand and expand its efficiency and
scope.
Scheme 2 Synthetic exploration of open chain analogs.
3 or 4 with selected anilines and POCl3 led to chloromethyl-
quinazolinones 5 which were subsequently aminated with
N-alkylpiperazines to give substituted quinazolinones 6.
Results and discussion
While this stage of the medicinal chemistry effort pro-
ceeded as expected, we were intrigued by a series of failed
chemistry experiments when our focus shifted to compounds
bearing acyclic diamine variants of the piperazine moiety of 1.
Our initial attempts to generate these analogs involved
treatment of 6-nitro-2-chloromethylquinazolinone 7a6,13 with
mono-N-BOC protected 1,2-ethanediamine or mono-N-BOC
protected 1,3-propanediamine to afford the corresponding qui-
nazolinones 8 or 9 with the incorporated BOC-protected alkyl-
amine appendage (Scheme 2).6 Intermediates 8 and 9 were
stable and characterized by NMR and LCMS/UV. Notably, upon
treatment with TFA to remove the BOC group and subsequent
basic workup, the open chain 1,2-ethanediamine product 10
could not be isolated. In cases employing a mono-N-BOC pro-
tected 1,3-propanediamine, the desired open chain analog 11
was obtained in 98% yield as the TFA salt. Analysis of the reac-
tion employing 8 revealed the formation of a rearranged
(E)-amidine 12a in 50% yield that was structurally confirmed
by 1H and 13C NMR, NOESY, and 2D NMR spectroscopy
(Scheme 2). A diagnostic benzamide N-H signal was observed
at 11.0 ppm in the proton spectrum, and clear nuclear Over-
hauser effects (NOEs) were observed between (a) the aryl 3-CH
proton and the methylene protons of the newly formed ami-
noamidine ring, as well as between (b) the amidine N-methyl
protons and the benzamide ortho-CH protons (see Scheme 2,
12a). A crystalline, halogenated analog was later assessed by
X-ray crystallography, thus confirming the initial NMR assign-
ments (data not shown).
With the structure and configuration of the rearranged
product established, attention was shifted to optimizing the
yield of the process and mechanistically accounting for
amidine formation. Amidine 12a was the only product isolated
from the reaction with N1,N2-dimethylethane-1,2-diamine, and
the two step process from 7a modestly yielded 18% overall (cf.,
Scheme 2). Our studies revealed that potassium iodide was not
required and that thermal conditions afforded better yields.
It was also determined that the BOC-protecting group was
unnecessary for those reactions utilizing
a symmetrical
diamine and that the rearrangement was successful under
basic conditions used to install the diamine linker onto the
quinazolinone core. Thus, a one-step conversion from 7a to
12a was achieved with an improved 86% overall yield
(Scheme 3).
Isolation and characterization of the BOC-protected inter-
mediates 8 and 9 indicated that the rearrangement did not
occur until after the terminal amine was exposed. Interest-
ingly, amidine 12a was generated when chloromethyl-
quinolinone 7a was treated with a two-carbon linked diamine,
The rearrangement of nitroquinazolinones to nitrobenz-
amidines was unprecedented, and fortuitously afforded com-
pounds of significant pharmacological value that were
inaccessible by other methods. Nonetheless, several consider-
able chemistry issues remained unresolved. Reaction con-
ditions used in the rearrangement were un-optimized and
exploited reagents that were deemed unnecessary and subopti-
mal. Notably, it was also not understood if the rearrangement
was productive once TFA was introduced to remove the BOC-
protecting group, or if the “work-up” with aqueous base was
Scheme 3 Modified reaction conditions and mechanistic consider-
ations for (E)-amidine formation.
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but a three-carbon tethered diamine produced the expected nitro group with a hydrogen atom afforded amidine 12e in
quinazolinone 11. It was reasoned that amidine formation was 83% when DMF was employed (entry 5). The effect of installing
favored when the reaction proceeded through a six-membered, an electron-donating methoxy group on the core was also
spirocyclic intermediate such as 14, resulting from an intra- examined. Yields were especially poor for those substrates
molecular attack of the terminal amine, followed by carbon– bearing methoxy groups at the R1 or R3 positions (entries 6
nitrogen bond cleavage. Exclusive isolation of the (E)-config- and 8, respectively), while R2 and R4 substitution was tolerated
ured double bond amidine was presumed to result from a when the reaction was performed in DMF (entries 7 and 9,
more favorable transition state that did not result in a steric respectively). This effect was likely due to the increased
clash between the N-methylamidine moiety and the nitro- electron density contributed at the reacting alkyl chloride via
phenyl component of the scaffold (Scheme 3).
resonance of methoxy groups at the R1 or R3 positions.
It was unknown if the presence of the quinazolinone Installation of alternative electron withdrawing substituents
C6-nitro group of 7a was essential for the rearrangement to such as fluorine and nitrile groups was found to produce the
occur or if the electronic nature of the substituent had any corresponding amidines analogously to the nitro group
effect on the reaction. Consequently, variants were prepared bearing substrates.
that surveyed the effect of quinazolinone core substitution on
Within the scope of our medicinal chemistry program, it
the efficiency and scope of the reaction (Table 1). The reaction was desirable to also modulate the character of the cyclo-
could be carried out efficiently in acetonitrile at 50 °C or at amino-amidine moiety itself; however, we were initially unable
room temperature in DMF, though in the former case we to explore any changes in this structural space using this or
observed in nearly half of the examples the formation of several other methods. Therefore, focus shifted to exploiting
dimers that were otherwise absent when DMF was used at a the rearrangement to expand our structural interrogation,
milder temperature. Under either set of conditions, the specifically permitting the generation of carbon- and oxygen-
(Z)-amidine was not observed for any of the alkyl chlorides we containing cycloamidines. It was initially anticipated that
examined. In the case of reactions using 7b, the desired analogs with structural permutations of the amidine portion
(E)-amidine 12b was isolated along with an unidentified, of the scaffold could be prepared by incorporating alternative
minor, and unstable compound with the same mass; however, chloroalkyl appendages off of the quinazolinone core. To this
it was clearly not the (Z)-isomer by 1H NMR analysis.
end, chloroacylalkylchlorides 16a–b were used to construct
Overall, yields of the amidine products were reasonably chloroalkylquinzalinones 17a–c (Scheme 4). Subsequent treat-
high except in a few select cases. Migration of the nitro group ment of 17a–c with an amine was expected to generate
to any of the available quinazolinone C5-, C7-, or C8-positions appended amines in situ and trigger the rearrangement to
afforded amidines 12b–d in high yields (entries 2–4, Table 1). afford the corresponding amidines 18a–c. Feasibility of this
The presence of a strong electron-withdrawing group was not approach was explored using n-butylamine due to its lower
required for the rearrangement to occur, as replacement of the volatility at higher temperatures versus methylamine.
Table 1 Effect of reaction conditions and chloromethylquinazolinone substituents on product yield
Entry
sm
Prdt
R1
R2
R3
R4
Condition A, yield (%)
Condition B, yield (%)
1
2
3
4
5
6
7
8
7a
7b
7c
7d
7e
7f
7g
7h
7i
12a
12b
12c
12d
12e
12f
12g
12h
12i
H
NO2
H
H
H
OCH3
H
H
H
H
H
H
NO2
H
H
H
OCH3
H
H
F
NO2
H
H
H
H
H
H
OCH3
H
H
H
H
H
NO2
H
H
H
H
OCH3
H
86
77
97a
85
82
83
34
85
NA
98
72
65
71
89
71a
94
74
58b
13b
55b
16b
44b
64b
80
9
10
11
12
13
7j
7k
7l
12j
12k
12l
H
H
H
H
H
F
F
CN
F
H
H
H
72
94
7m
12m
a Overall yield of a mixture containing 12b and an unidentified, unstable minor isomer, ratio = 1.0 : 0.3–0.5. b Dimeric product was also isolated
and characterized (yields not shown); NA = not available due to decomposition of product.
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Scheme 4 Structural diversity from chloroalkylquinazolinones.
Scheme 5 One-pot synthesis of benzamidoamidines from N-BOC pro-
tected δ- or γ-amino acids.
The successful conversion of chloroalkyl intermediates
17a–c to the desired amidines 18a–c was determined to hinge
on multiple factors including substitution pattern of the
quinazolinone core, nucleophilic amine, solvent, temperature,
and duration of the reaction (data not shown). For example,
preliminary conditions scouted with 17b encouragingly
afforded the desired amidine 18b exclusively in a reasonable
62% yield; however, the same conditions employed with
chloroalkylquinzalinone 17a afforded only novel cyclopropyl-
quinazolinone 19a (65% yield), likely formed from intra-
molecular displacement of the chloride after deprotonation of
the allylic-like carbon of the alkyl appendage. While interest-
ing in their own right, the formation of these and other by-
products, along with the variability in amidine yield as a func-
tion of multiple parameters, ultimately motivated us to investi-
gate a more efficient route to modified amidines. Based on
these results, we reasoned that the conversion of an anthrani-
lic acid to the desired amidine could be achieved in a one-pot,
multicomponent assembly. To study this approach, requisite
carboxylic acids 20a–c were prepared bearing a BOC protected
amine (Scheme 5).
Use of the stepwise protocol in hand with 20a led to
amidine 24a in 18% yield while the same conditions applied
to 20b led to significant decomposition. While the initial
yields were not ideal, we were encouraged that the multi-step
sequence delivered moderate quantities of desired products on
the first pass and that the individual steps could be optimized
further to accommodate new substrates. Moreover, scrutiny of
each individual reaction step revealed that insightful modifi-
cations of the reaction conditions would likely permit an
efficient, one-pot assembly. Thus, treatment of acids 20a–c
with an anthranilic acid in the presence of pyridine and tri-
phenylphosphite9 (not POCl3) generated benzoxazinones
21a–d which were then transformed to the corresponding
quinazolinones 22a–d by the addition of aniline under micro-
wave irradiation. Removal of the pyridine in vacuo, followed by
the addition of TFA revealed the appended amines 23a–d
in situ. The addition of aqueous base to quench the TFA and
Table 2 Yields of (E)-benzamidoamidines from BOC-protected amino
acids
N-BOC-amino
acid
Amidine
product
Amidine
yield (%)
Entry
X
R1
1
2
3
4
20a
20b
20c
20a
24a
24b
24c
24d
CH2
NO2
NO2
NO2
H
80
62
(CH2)2
OCH2
CH2
24 (77%)a
89
a 77% yield obtained when a two-pot sequence was employed.
adjust the pH to 10 at elevated temperature generally afforded
the desired (E)-benzamidoamidines 24a–d in good yield
(Table 2).
The reaction was unaffected by the presence or absence of a
core nitro group substituent, as both anthranilic acid sub-
strates, when treated with N-BOC-amino acid 20a, led to five-
membered, carbocyclic amidine ring products 24a and 24d in
high yields (80–89%). The six-membered, carbocyclic amidine
24b was prepared in good yield as well. Introduction of an
oxygen atom in the amine linker led to a suboptimal yield of
amidine 20c (24%). A stepwise analysis of the reaction
sequence for 20c revealed that desired amidine was generated
efficiently; however, the product was unstable under one-pot
conditions, as determined from the isolation and characteriz-
ation of cleavage products 25 and 26 (Scheme 6). Thus, we
found that the penultimate N-BOC aminoalkoxy quinazolinone
22c was isolated in 77% overall yield from a telescoped
sequence that started with N-BOC protected amino acid 20c.
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and 4-((tert-butoxycarbonyl)(methyl)amino)butanoic acid (20a,
217 mg, 1.00 mmol) were dissolved in dry pyridine (1.0 mL).
Triphenyl phosphite (0.40 mL, 1.5 mmol) was added, and the
resulting mixture was heated at 100 °C under MWI for 1 min
and slowly allowed to cool down to rt over a period of 10 min.
This MWI heating process (100 °C for 1 min, followed by slow
cool-down to rt over 10 min) was repeated two more times.
TLC analysis (50% EtOAc/hexanes) showed the disappearance
of starting material (5-nitroanthranilic acid) at Rf = 0.2 and the
presence of the corresponding benzoxazinone at Rf = 0.5.
Aniline (0.18 mL, 2.0 mmol) was added to the mixture and the
reaction mixture was heated at 100 °C with MWI for 1 min and
slowly allowed to cool down to rt over a period of 10 min. TLC
analysis showed the disappearance of the benzoxazinone (Rf =
0.5) and the presence of the corresponding quinazolinone
(Rf = 0.4). Pyridine was removed in vacuo and the resulting
clear, yellow oil was dissolved in dry CH3CN (9 mL) and TFA
(5.8 mL). The mixture was stirred at rt for 1–2 hours until the
BOC-protected quinazolinone (Rf = 0.4) was consumed. The
mixture was diluted with CH3CN (30 mL) and slowly quenched
to pH 10 with saturated aq. Na2CO3 (50 mL). The reaction
mixture was stirred at 50 °C for 1 hour. The mixture was
allowed to reach rt and water (30 mL) was added. The product
was extracted with CH2Cl2 (3 × 100 mL), with minimal shaking
in order to avoid an emulsion. The separated, organic extracts
were combined and concentrated in vacuo to give a clear,
yellow oil, which was purified by reverse-phase chromato-
graphy (10–100% CH3CN/water) yielding 24a (135 mg, 80%) as
a yellow solid, mp 180–182 °C. 1H NMR (400 MHz, CDCl3)
δ 11.62 (s, 1H), 9.17 (d, J = 2.8 Hz, 1H), 8.16 (dd, J = 8.8, 2.9 Hz,
1H), 7.69–7.64 (m, 2H), 7.38–7.32 (m, 2H), 7.15–7.09 (m, 1H),
6.85 (d, J = 8.8 Hz, 1H), 3.56 (t, J = 7.0 Hz, 2H), 3.22 (s, 3H),
2.64 (t, J = 7.8 Hz, 2H), 2.10 (p, J = 7.4 Hz, 2H). 13C NMR
(101 MHz, CDCl3) δ 164.3, 162.8, 155.8, 142.7, 138.5, 129.1,
127.5, 126.4, 126.3, 124.1, 123.2, 120.2, 51.7, 31.9, 28.8, 20.0.
HRMS (m/z): calcd for C18H19N4O3 (M + H)+ 339.1452; found
339.1449.
Scheme 6 Two-step modified procedure for oxygen-containing
cycloamidine 24c.
Isolation and purification of 22c, followed by submission to
the TFA and base-catalyzed rearrangement conditions afforded
amidine 24c quantitatively.
Conclusions
This effort was driven by a necessity to efficiently synthesize
structural architecture that was otherwise inaccessible and to
address multiple limitations and unknowns associated with
our discovery of a new quinazolinone rearrangement. Inessen-
tial reagents were excluded, the most favorable temperature
and solvent conditions were defined leading to optimal yields
for a range of substrates of various electronic character, and
we determined that the key reaction was triggered under basic
conditions. Importantly, the process has been modified to
permit augmentation of the cyclic amidine, thereby improving
the overall scope and use of the transformation. As a result, we
have now developed divergent methodology that permits the
efficient, regiospecific synthesis of structurally and pharmaco-
logically novel (E)-amidines from either chloroalkylquinazoli-
nones or N-BOC-protected δ- and γ-amino acids. This
unprecedented intramolecular rearrangement tolerates a good
range of substitution on the benzamide core as well as
changes in the amidine-forming linker. Furthermore, the
transformation was optimized to generate the benzamidoami-
dine framework in a one-pot assembly that comprises at least
5 distinct chemical operations. This methodology is currently
being leveraged to examine amidine-related structure–activity
relationships in our virology program; however, we are also
studying the use of this reaction in combination with other
cascade reactions to generate unique, structurally diverse
species as templates for further pharmacological exploration
and new chemical methodology development.
Acknowledgements
This work was funded by the National Institutes of Health
(J. Aubé, U54HG005031 and J. Golden, 1R01AI118814-01). We
thank Dr. Victor Day for X-ray crystallography assistance and
acknowledge the following support for KU NMR instrumenta-
tion: NIH S10RR024664 and NSF 0320648. J. Golden thanks
postdoctoral research associate Dr. Xufeng Cao at UW-Madison
for the resynthesis and characterization of 17a and 22c.
Experimental
General procedure for the one pot synthesis of
(E)-benzamidines
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