The rapid synthesis of diverse substituted polycyclic quinazolinones was achieved by two orthogonal Ugi four-component reaction (Ugi-4CR)-based protocols: the first two-step approach via an ammonia-Ugi-4CR followed by palladium-catalyzed annulation; in the second approach, cyanamide was used unprecedently as an amine component in Ugi-4CR followed by an AIBN/tributyltin hydride-induced radical reaction. Like no other method, MCR and cyclization could efficiently construct many biologically interesting compounds with tailored properties in very few steps.
The rapid synthesis of diverse substituted polycyclic quinazolinones was achieved by two orthogonal Ugi four-component reaction (Ugi-4CR)-based protocols: the first two-step approach via an ammonia-Ugi-4CR followed by palladium-catalyzed annulation; in the second approach, cyanamide was used unprecedently as an amine component in Ugi-4CR followed by an AIBN/tributyltin hydride-induced radical reaction. Like no other method, MCR and cyclization could efficiently construct many biologically interesting compounds with tailored properties in very few steps.
Quinazolinones are among the very important
class of biologically
active N-fused heterocyclic scaffolds that have been involved in many
marketed drugs or potential candidates.[1] Notably, polycyclic quinazolinones, such as luotonins (1–3), sclerotigenin (4), and rutaecarpine
(5), have been reported to exhibit a broad spectrum of
biological activities.[2] Thus, the large
structural diversity and promising therapeutic potential of polycyclic
quinazolinone analogs have inspired chemists to further explore novel
variants of quinazolinone structures or develop more efficient synthetic
methodologies.Multicomponent reactions (MCRs) are reactions
that employ at least
three starting materials to form one single product, where the majority
of the atoms from starting materials are incorporated into final products.[3] Due to their high efficiency, mild conditions,
and large scaffolds diversity, the MCRs have become a unique tool
for the rapid generation of a great variety of natural products and
pharmaceuticals.[4] For example, β-amino
amides have been successfully synthesized in a one-pot manner by applying
triazenyl alkynes, carboxylic acids, aldehydes, and anilines as the
starting materials.[5] The Ugi reaction is
one of the most well-known and broadly used MCRs.[6] It has been involved in the synthesis of diverse scaffolds
as a key step. For instance, the synthesis of a potent amino acid
antibiotic furanomycin and a naturally cyclic peptide ustiloxin D
used the Ugi reaction as the critical step.[7] In our previous studies, we have introduced Ugi reactions for constructing
important bioactive scaffolds, such as isoquinolone-4-carboxylic acid
and isoquinoline scaffolds.[8]The
rapid construction of polycyclic quinazolinones is an extensively
studied topic. Several metal catalysts, such as copper, rhodium, and
palladium, have been used for the synthesis of the corresponding compounds.[9] The application of radical cyclization as well
as self-catalyzed phototandem perfluoroalkylation/cyclization of unactivated
alkenes also has been reported for the synthesis of the same scaffolds.[10] As shown in Scheme , these approaches suffer from one or many
issues, such as lengthy sequential synthesis, limited scope, and generality.[9−11] Inspired by previous syntheses and based on our deep interest in
MCR chemistry, we envisioned that isoindolo[1,2-b]quinazolinone derivatives could be synthesized in a concise manner
by an Ugi-4CR reaction of o-bromobenzoic acids, o-cyanobenzaldehydes, isocyanides, and ammonia followed
by a metal-catalyzed intramolecular N-arylation to form the desired
products.[12] Alternatively, we figured,
in a second strategy, that it could also be synthesized by an Ugi-4CR
using o-iodobenzaldehyde, benzoic acid, isocyanides,
and cyanamide followed by radical cyclization of the N-acylcyanamide
moiety since the cyano group is a well-established radical acceptor
that has been involved in the build of various heterocycles and carbocycles.[13] We were the first to report cyanamide as an
acid component in the Ugi reaction, reacting with enamines and isocyanides
in the presence of Lewis acids to give the medicinally important scaffold
α-amino-N-cyanoamidines.[14] Here,
we report for the first time cyanamide to react as an amine component
in the Ugi reaction.
Scheme 1
Representatives and Synthesis of Polycyclic
Quinazolinones
Results and Discussion
As the starting point of our
work to elaborate the first strategy,
the model Ugi-4CR with 2-bromobenzoic acid (6a), 2-cyanobenzaldehyde
(7a), cyclohexyl isocyanide (8a), and NH4Cl (9a) was performed in MeOH/H2O
(3:1) under room temperature for 12 h (entry 1) as shown in Table .[15] The precipitated Ugi product 10a formed, and
the solid was filtered and washed with diethyl ether giving a 33%
yield. We then used MeOH as the solvent (entry 2) or increased the
temperature to 55 °C (entry 3), giving yields of 60% and 33%,
respectively. Not satisfied, we tried to use ammonia (7 N in MeOH, 9c) instead of NH4Cl in 2,2,2-trifluoroethanol
(TFE) at 55 °C in a closed vial for 12 h (entry 5).[16] No precipitated Ugi product was observed, but
a yield of 62% was isolated after flash chromatography purification.
Thereafter, the reactions were carried out under room temperature
in TFE (entry 6) or MeOH (entry 7), which both formed the precipitated
Ugi product with yields of 75% and 60%, respectively. When ammonia
(in water, 9b) was used (entry 4), a trace amount of
Ugi product was formed. Though 2,4-dimethoxylbenzylamine (entry 8)
as an ammonia surrogate showed good performance in this reaction,
two steps with a total yield of 59% were not competitive. Finally,
the optimized reaction condition was concluded to be ammonia (in MeOH, 9c) in TFE at room temperature for 12 h (entry 6).
Reaction conditions: 6a (1 mmol), 7a (1 mmol), 8a (1 mmol), 9 (1 mmol), solvent (1 mL).Isolated yields.Followed by HCl-catalyzed cleavage.With optimized conditions for the Ugi-4CR in hand,
we next investigated
the subsequent palladium-catalyzed annulation step by exploring varied
catalysts, ligands, and bases (Table ). When the reaction was performed in the presence
of 5 mol % Pd(OAc)2, 10 mol % 1,1′-bis(diphenylphosphino)ferrocene
(dppf), and 2 equiv of K3PO4 in 1,4-dioxane
at reflux for overnight, the desired product 10b was
obtained in 65% yield (entry 9). When PdCl2 (entry 10)
or PdO (entry 11) has been used as the catalyst, the yield decreased
to 16% and 20%, respectively. Replacing the dppf with Xantphos resulted
in a lower yield (33%, entry 12). To our delight, the desired product 10b was formed in 81% yield by using K2CO3 (entry 15) as the base, compared with K3PO4 (65%, entry 9) and Cs2CO3 (72%, entry 14).
Notably, microwaves did not facilitate the reaction as we expected
(42%, entry 13). Furthermore, we tried different organometallic complexes
giving no product formation or low yields (entry 16–19). Finally,
the optimized condition was concluded to be the Ugi adduct 10a, Pd(OAc)2, dppf, and K2CO3 in 1,4-dioxane
at reflux overnight (entry 15).
Reaction conditions: 10a (0.5 mmol), catalyst (5 mol %), ligand (10 mol %), base (1 mmol),
1,4 dioxane (5 mL), reflux, overnight.Microwave, 120 °C, 30 min.Isolated yields.Not detected.With the optimal conditions in hand, we then set out
to explore
the scope and limitation of these tandem reactions. A set of Ugi-4CR
adducts 10a–33a were efficiently
synthesized in moderate to good yields without column purification
required for most cases. Aromatic/aliphatic isocyanides, versatile o-bromobenzoic acids, and substituted o-cyanobenzaldehydes
were all tolerated well in this ammonia Ugi four-component reaction.
Then, all the Ugi adducts were examined to determine the scope of
the tandem reaction to furnish the corresponding products 10b–33b.As can be seen in Scheme , all the Ugi adducts led to
the expected polycyclic quinazolinones.
We initially replaced cyclohexyl isocyanide with various aromatic
isocyanides, such as benzyl (11b, 41%), phenethyl (12b, 70%), phenyl (13b, 78%), thienylmethyl (14b, 46%), and pyridylmethyl (16b, 56%), all
giving good yields. Benzyl isocyanides bearing diverse functional
groups also performed well in this reaction, as examples, methoxy
(15b, 53%), nitrile (22b, 78%), halide (23b, 37%), and trifluoromethoxy (24b, 69%). Further,
aliphatic isocyanides 1-adamantyl isocyanide (17b, 85%),
tetrahydrofuran-2-ylmethyl isocyanide (18b, 47%), tert-octyl isocyanide (19b, 91%), methyl isocyanoacetate
(20b, 63%), and tert-butyl (2-isocyanoethyl)carbamate
(21b, 70%) gave good yields as well. This is meaningful
since the previously reported methods either have the limited type
of substitutions or are non-substituted in this position.[9−11] Meanwhile, by using our strategy, we opened the substitution possibilities
at this position for all available isocyanides providing the opportunity
for various further chemical modifications.
Scheme 2
Substrate Scope in
the [Pd]-Catalyzed Synthesis of Polycyclic Quinazolinones–
Acid, isocyanide,
aldehyde,
and ammonia components are depicted with red, green, blue, and pink
color, respectively.
Ugi reaction was carried out using 6 (1 mmol), 7 (1 mmol), 8 (1 mmol), and 9c (1
mmol) in TFE (1 M) for 12 h at r.t.
Substrate Scope in
the [Pd]-Catalyzed Synthesis of Polycyclic Quinazolinones–
Acid, isocyanide,
aldehyde,
and ammonia components are depicted with red, green, blue, and pink
color, respectively.Ugi reaction was carried out using 6 (1 mmol), 7 (1 mmol), 8 (1 mmol), and 9c (1
mmol) in TFE (1 M) for 12 h at r.t.[Pd]-catalyzed reaction conditions: 10–34a (0.2 mmol), Pd(OAc)2 (5 mol %), dppf (10 mol
%), K2CO3 (1 mmol), 1,4 dioxane (2 mL), reflux,
overnight.Yield refers
to purified products.Subsequently, we examined
the ability of these tandem reactions
to incorporate a diverse panel of carboxylic acids and aldehydes components.
All the used acid and aldehyde components were well-tolerated in Ugi-4CR,
giving good yields ranging from 45% to 80%. For the subsequent [Pd]-catalyzed
annulation, thiophenecarboxylic acid (26b), nicotinic
acids (29b, 31b), and naphthoic acid (30b) proceeded smoothly. Nicotinic acids significantly increased
yields when compared with benzoic acid (11b), which may
probably benefit from the electron-withdrawing effect of the nitrogen
atom. Additionally, different functional groups at the aromatic ring
of o-bromobenzoic acid were well tolerated, such
as methyl (25b, 27b) and bromine (27b). The relatively lower yields have been observed when
the methyl group attached may be due to its electron-donating effect,
making the acid less reactive. In addition, good substrate tolerance
was also achieved for the aldehyde component. 2-Cyano-4-methoxybenzaldehyde
(32b) and 4-fluoro-3-cyanobenzaldehyde (33b) yielded 71% and 66% of products. A plausible mechanism of this
reaction is that the cyano group is first activated by a transition
metal to form a σ-coordination, which will facilitate further
nucleophilic addition. Then, the following intramolecular cyclization
might be executed by the metal-catalyzed SNAr reaction.Furthermore, the scalability of this method was investigated. A
four-component reaction of 2-bromobenzoic acid (6a),
2-cyanobenzaldehyde (7a), 2-chloro-3-methyl-6-fluorobenzyl
isocyanide (8b), and ammonia (9c) was conducted
on a 10 mmol scale. The participate was filtered and washed with diethyl
ether, affording 2.6 g of Ugi adduct 34a (51%). Then,
Ugi adduct 34a yielded 0.88 g of the final cyclized product 34b (41%) through silica gel flash chromatography as shown
in Scheme .
Scheme 3
Scale-up
Synthesis
luotonins A, B, and E (1–3) are
naturally occurring cytotoxic alkaloids, being demonstrated as human
DNA topoisomerase I (hTopI) poisons.[17] The
current synthesis of luotonin derivatives may suffered from multiple
synthetic procedures and/or a limited substrate scope.[9−11,18]Scheme illustrates our initial success of luotonin
A derivative 35b (51%) synthesis of which the Ugi adduct 35a (43%) was assembled from the appropriate 2-bromobenzoic
acid (6a), 2-quinolinecarbonitrile (7b),
ammonia (9c), and benzyl isocyanide (8c).
Despite this, the Passerini three-component (P-3CR) byproduct was
observed in Ugi-4CR. Our strategy could incorporate a variety of functionalities
from largely available isocyanides into the luotonin scaffold.
Scheme 4
Synthesis of luotonin Derivatives
Encouraged by our successful development, we
continued to explore
the second approach via a cyanamide-Ugi-4CR followed by an AIBN/tributyltin
hydride-induced radical cyclization. The reaction started with benzoic
acid (36), 2-iodobenzaldehyde (37), cyanamide
(38), and three different isocyanides (Scheme ). The reactions afforded the
corresponding Ugi products 39a–41a with moderate yields because P-3CR byproducts were isolated with
a nearly 1:1 ratio from each reaction. The P-3CR product is often
observed when less nucleophilic amines are employed in the Ugi-4CR,
which can be explained by a low formation of the iminium intermediates
in the reaction mixture.[19] To optimize
the reaction, we tried several reaction conditions: (i) using 2 equiv
of cyanamide; (ii) heating to 60 °C; (iii) using microwaves (100
°C, 30 min); (iv) changing the solvent; (v) adding Lewis acids
as the catalyst, such as ZnCl2, TiCl4, HClO4, scandium(III) triflate, ytterbium(III) triflate.[20] Unfortunately, they failed to improve the yields.
Scheme 5
Various Isocyanides in the Synthesis of Polycyclic Quinazolinones
via Radical Cyclization–
Acid, aldehyde,
isocyanide,
and cyanamide components are depicted with red, blue, green, and pink
color, respectively.
Ugi reaction was carried out using 36 (1 mmol), 37 (1 mmol), 8 (1 mmol), and 38 (1
mmol) in TFE (1 M) for 12 h at r.t.
Various Isocyanides in the Synthesis of Polycyclic Quinazolinones
via Radical Cyclization–
Acid, aldehyde,
isocyanide,
and cyanamide components are depicted with red, blue, green, and pink
color, respectively.Ugi reaction was carried out using 36 (1 mmol), 37 (1 mmol), 8 (1 mmol), and 38 (1
mmol) in TFE (1 M) for 12 h at r.t.Radical cyclization conditions: 39–41a (0.15 mmol), tributyltin hydride (0.3 mmol), AIBN (0.15
mmol), benzene (7 mL), reflux, overnight.Yield refers to purified products.Despite achieving moderate yields, we were able to isolate
sufficient
quantities of the Ugi products to examine the subsequent step. The
reactions were performed between Ugi adducts, tributyltin hydride,
and AIBN in benzene at reflux overnight. To our delight, all the three
Ugi adducts yielded the corresponding products 39–41b, giving yields of 40%, 47%, and 33%, respectively. Therefore,
we have successfully introduced cyanamide into the Ugi-4CR as the
amino component that generated the stable N-acylcyanamide
moiety, which could undergo further cyclization leading to the desired
polycyclic quinazolinones. A plausible mechanism for this radical
cascade is that the trapped aromatic radical on the cyanamide triple
bond could further cyclize on an aryl substituent. This cyclization
undergoes direct addition to form the hexadienyl radical, which further
goes with rearomatization.[21]
Conclusions
Two orthogonal Ugi-4CR-based synthesis
strategies for fully substituted
polycyclic quinazolinone derivatives have been developed. Alternative
Ugi-4CRs were instrumental in this approach for introducing the critical
cyano group, leading to potentially bioactive polycyclic quinazolinones
through two different cyclization mechanisms. For this, we introduced
cyanamide for the first time as an amine component in the Ugi-4CR.
Remarkably, cyanamide is thus the first amphoteric building block
that can function as an acid and amine component in Ugi-type transformations.
Significantly, the successfully synthesized N-acylcyanamide
moiety, as an efficient amide-iminyl radical, opens the door for the
rapid and straightforward synthesis of pharmaceutically important
heterocycles by Ugi-4CR. Diverse isocyanides with versatile functional
groups can be introduced into the polycyclic quinazolinone scaffold
allowing the modification of physicochemical properties, further chemical
manipulations, and so on. Our protocol is outperforming all known
strategies toward this important scaffold in terms of utilization
and accessibility of commercially available starting materials, mild
conditions, and simple purification work-up and is of excellent maneuverability
and efficiency. Work aiming at the optimization of cyanamide involved
Ugi-4CR as well as extending the scope of the subsequent annulation
is in progress and will be reported in due course.
Experimental Section
General Information
Nuclear magnetic resonance spectra
were recorded on a Bruker Avance 500 spectrometer. Chemical shifts
for 1H NMR were reported relative to TMS (δ 0 ppm)
or the internal solvent peak (CDCl3 δ 7.26 ppm, DMSO-d6 δ 2.50 ppm or CD3OD δ
3.31 ppm) and coupling constants were in hertz (Hz). The following
abbreviations were used for spin multiplicity: s = singlet, d = doublet,
t = triplet, dt = double triplet, ddd = doublet of double doublet,
m = multiplet, and br = broad. Chemical shifts for 13C
NMR reported in ppm relative to the solvent peak (CDCl3 δ 77.23 ppm, DMSO δ 39.52 ppm, CD3OD δ
49.00 ppm). Flash chromatography was performed on a Grace Reveleris
X2 using Grace Reveleris Silica columns (12 g) and a gradient of petroleum
ether/ethyl acetate (0–100%) or dichloromethane/methanol (0–10%)
was applied. Thin-layer chromatography was performed on Fluka precoated
silica gel plates (0.20 mm thick, particle size 25 μm). Reagents
were available from commercial suppliers and used without any purification
unless otherwise noted. All isocyanides were made in house by performing
the Ugi procedure. Other reagents were purchased from Sigma Aldrich,
ABCR, Acros, Fluorochem, and AK Scientific and were used without further
purification. High-resolution mass spectra (HRMS) were recorded using
a QTOF Bruker Maxis Plus, mass range 100–1500 m/z, spectra rate 2.00 Hz. Yields given refer to
chromatographically purified and spectroscopically pure compounds
unless otherwise stated.
General Experimental Procedure of Ammonia-Ugi-4CR
To
a stirred solution of aldehyde (1 mmol, 1 equiv) in 2,2,2-trifluoroethanol
(1 mL) was added ammonia solution (7 N in methanol, 1 mmol, 1 equiv)
and stirred for 10 min followed by the addition of isocyanide (1 mmol,
1 equiv) and carboxylic acid (1 mmol, 1 equiv). The reaction was allowed
to stir at room temperature in a close screwed vial for 12 h. The
precipitate was filtered and washed with diethyl ether, affording
the corresponding Ugi adduct. Otherwise, the Ugi adduct was purified
by silica gel flash chromatography using either PE/EA or DCM/MeOH
as the eluent.
General Experimental Procedure of [Pd]-Catalyzed Annulation
The Ugi adduct (0.15–0.4 mmol, 1 equiv), Pd(OAc)2 (0.0075–0.02 mmol, 0.05 equiv), dppf (0.015–0.04 mmol,
0.1 equiv), and K2CO3 (0.3–0.8 mmol,
2 equiv) were added to a 5 mL (or 10 mL) reaction tube equipped with
a magnetic stir bar, and 1.5–4 mL of 1,4-dioxane was added.
The mixture was heated to reflux in a metal heating block overnight.
After the reaction was completed, the solvent was removed under reduced
pressure, and the residue was purified by silica gel flash chromatography
using either PE/EA or DCM/MeOH as the eluent.
Scale-up Synthesis of 34b
A 50 mL flask
equipped with a magnetic stirrer bar was charged with 2-formylbenzonitrile
(10 mmol, 1.3 g, 1 equiv) and a calculated amount of 7 N ammonia solution
(in methanol, 10 mmol, 1.4 mL, 1 equiv) in 2,2,2-trifluoroethanol
(10 mL). We sealed the reaction flask immediately and allowed it to
stir for 10 min. Then, 2-chloro-4-fluoro-3-(isocyanomethyl)-1-methylbenzene
(10 mmol, 1.8 g, 1 equiv) and 2-bromobenzoic acid (10 mmol, 2.0 g,
1 equiv) were added to the solution and the reaction mixture was stirred
at room temperature for 12 h. The precipitate was filtrated and washed
by diethyl ether, affording the pure Ugi adduct 34a with
a yield of 51%. Then, the Ugi adduct (5 mmol, 2.6 g, 1 equiv) was
added to Pd(OAc)2 (0.25 mmol, 56 mg, 0.05 equiv), dppf
(0.5 mmol, 277 mg, 0.1 equiv), and K2CO3 (10
mmol, 1.4 g, 2 equiv) in 1,4-dioxane (50 mL) in a 200 mL round flask
with a magnetic stirrer bar. The reaction mixture was heated to reflux
in an oil bath overnight. After the reaction was completed, the solvent
was removed under reduced pressure, and the crude product was purified
by column chromatography (silica gel, petroleum ether/ethyl acetate
= 2:1) to afford the product 34b (0.88 g, 41% yield).
Synthesis of Luotonin Derivative 35b
To
a stirred solution of 2-quinolinecarbonitrile (1 mmol, 154 mg, 1 equiv)
in 2,2,2-trifluoroethanol (1 mL) was added ammonia solution (7 N in
methanol, 1 mmol, 140 μL, 1 equiv) and stirred for 10 min followed
by the addition of benzyl isocyanide (1 mmol, 117 mg, 1 equiv) and
2-bromobenzoic acid (1 mmol, 201 mg, 1 equiv). The reaction was allowed
to stir at room temperature in a close screwed vial for 12 h. The
reaction was concentrated and purified by column chromatography (silica
gel, petroleum ether/ethyl acetate as eluent) to afford the corresponding
Ugi product 35a. Subsequently, the Ugi adduct (0.15 mmol,
75 mg, 1 equiv), Pd(OAc)2 (0.0075 mmol, 1.7 mg, 0.05 equiv),
dppf (0.015 mmol, 8.3 mg, 0.1 equiv), and K2CO3 (0.3 mmol, 41.5 mg, 2 equiv) were added to a 5 mL reaction tube
equipped with a magnetic stir bar, and 1.5 mL of 1,4-dioxane was added.
The mixture was heated to reflux in a metal heating block overnight.
After the reaction was completed, the solvent was removed under reduced
pressure, and the residue was purified by silica gel flash chromatography
using PE/EA as the eluent.
General Experimental Procedure of Cyanamide-Ugi-4CR
To a stirred solution of aldehyde (1 mmol, 1 equiv) in MeOH (1 mL)
was added cyanamide (1 mmol, 1 equiv) and stirred for 30 min followed
by the addition of isocyanide (1 mmol, 1 equiv) and carboxylic acid
(1 mmol, 1 equiv). The reaction was allowed to stir at room temperature
for 12 h. The reaction was concentrated and purified by column chromatography
(silica gel, petroleum ether/ethyl acetate = 2:1) to afford the corresponding
Ugi product.
General Experimental Procedure of Radical Cyclization
The Ugi adduct (0.15 mmol, 1 equiv), tributyltin hydride (0.3 mmol,
2 equiv), and azobisisobutyronitrile (AIBN, 0.15 mmol, 1 equiv) were
added to a 25 mL round flask equipped with a magnetic stir bar, and
7 mL of benzene was added. The mixture was heated to reflux overnight.
After the reaction was completed, NaOH (aq, 1 M, 10 mL) was added
to the reaction mixture and stirred for 30 min. The organic phase
was extracted with ethyl acetate (2 × 20 mL), dried over MgSO4, and concentrated under vacuum. The residue was purified
by silica gel flash chromatography using PE/EA as the eluent.