Tandem electrocyclic ring opening/radical cyclization: Application to the total synthesis of cribrostatin 6

Article abstract of DOI:10.1016/j.tet.2011.08.064

A concise total synthesis of cribrostatin 6 (1), an antimicrobial and antineoplastic agent, was accomplished using a tandem electrocyclic ring opening/radical cyclization sequence. Specifically, intermediate 4 underwent a 4π-electrocyclic ring opening, radical cyclization, and homolytic aromatic substitution sequence followed by an oxidation to afford the natural product 1 in one pot. Owing to the rapid buildup of complexity in the key step, 1 could be synthesized from commercially available starting materials in only four linear steps. Application of this chemistry to the concise syntheses of analogs of cribrostatin 6 (1) is also reported.

Full text of DOI:10.1016/j.tet.2011.08.064

Tetrahedron 67 (2011) 9765e9770
Contents lists available at SciVerse ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Tandem electrocyclic ring opening/radical cyclization: application to the total
synthesis of cribrostatin 6
,
y
*
*
Daniel Knueppel , Stephen F. Martin
Department of Chemistry and Biochemistry and Texas Institute for Drug and Diagnostic Development, The University of Texas at Austin, 1 University Station A5300, Austin,
TX 78712-0165, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
A concise total synthesis of cribrostatin 6 (1), an antimicrobial and antineoplastic agent, was accom-
plished using a tandem electrocyclic ring opening/radical cyclization sequence. Specifically, intermediate
4 underwent a 4p-electrocyclic ring opening, radical cyclization, and homolytic aromatic substitution
sequence followed by an oxidation to afford the natural product 1 in one pot. Owing to the rapid buildup
of complexity in the key step, 1 could be synthesized from commercially available starting materials in
only four linear steps. Application of this chemistry to the concise syntheses of analogs of cribrostatin 6
(1) is also reported.
Received 5 August 2011
Accepted 23 August 2011
Available online 27 August 2011
This manuscript is dedicated to Gilbert Stork
on the occasion of his 90th birthday and in
honor of his many significant contributions
to the methods and the art of organic
synthesis
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
Cyclobutenediones
Natural products
Quinones
Rearrangement
Total synthesis
1. Introduction
cribrostatin 6 (1) was active against S. pneumoniae with minimum
bactericidal concentration (MBC)/MIC ratios of ꢀ2 for 75% of S.
The spread of multi-drug-resistant bacteria has fueled the need
for new structural classes of antibiotics. Resistant strains of en-
terococci and Staphylococcus aureus, Pseudomonas aeruginosa, and
Streptococcus pneumoniae are emerging with increased frequency.
S. pneumoniae, which is the most common bacterial cause of acute
respiratory infection and otitis media, causes millions of deaths
each year.1 Indeed, pneumonia is the eighth leading cause of death
in the United States.2 A growing resistance of S. pneumoniae against
trimethoprim-sulfamethoxazole, penicillin, macrolide, and tetra-
cycline antibiotics, has spurred the search for novel antibiotic leads.
In 2003 Pettit and co-workers reported the isolation and char-
acterization of the novel heterocycle cribrostatin 6 (1) from the
marine sponge Cribrochalina sp. (Fig. 1).3 The dark blue solid was
found to inhibit the growth of a number of antibiotic-resistant,
Gram-positive bacteria and pathogenic fungi. Most notably,
pneumoniae clinical isolates.1 Cribrostatin 6 also displayed signifi-
cant antineoplastic activity against several human cancer cell lines
(GI50¼0.2e0.4
mg/mL). Hergenrother and co-workers have recently
found that 1 has broad anticancer activity, and they have also made
the important discovery that it induces apoptosis through a re-
active oxygen species (ROS)-mediated mechanism.4 Given the
success of known ROS producers as anticancer agents,5 their find-
ings strongly suggest that cribrostatin 6 (1) has significant potential
as a novel chemotherapeutic agent.
The important biological activity of cribrostatin 6 (1) coupled
with its tricyclic imidazo[5,1-a]isoquinolinedione architecture,
O
O
Me
R1
R2
N
N
Me
R3
EtO
X
O
O
N
Y
* Corresponding authors. E-mail addresses: DIKnueppel@dow.com (D. Knueppel),
Cribrostatin 6
(
1)
2
sfmartin@mail.utexas.edu (S.F. Martin).
y
Present address: Dow AgroSciences LLC, 9330 Zionsville Road, Indianapolis, IN
Fig. 1. Natural product cribrostatin 6 (1) and potential analogs (2).
46268, United States.
0040-4020/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2011.08.064

9766
D. Knueppel, S.F. Martin / Tetrahedron 67 (2011) 9765e9770
which is unprecedented among known natural products, has
attracted the attention of the synthetic community. Nakahara and
co-workers were the first to complete a total synthesis of 1, using
a modified PomeranzeFritsch isoquinoline synthesis as the key
step to give 1 in 18 steps and 0.79% overall yield.6 Kelly and co-
workers subsequently reported an alternative synthesis that
employed two Stille couplings and a one-pot process for oxidative
olefin cleavage, N-Boc deprotection, and hemiaminal formation to
furnish 1 via a longest linear sequence of 13 steps (15 steps total)
and an improved 3.1% yield.7
O
Me
O
Me
radical
4 -electrocyclic
ring opening
EtO
N
cyclization
Me
EtO
OH
OH
N
N
8
Me
N
4
In the context of our general interest in quinone-derived natural
products,8 we were attracted to the challenge of developing a novel
route to cribrostatin 6 (1) that would be both more concise and
more efficient than previous syntheses. Because of the clear bi-
ological potential of 1, we were also interested in designing an
approach that would be readily amenable to the construction of
analogs of 1 having the general structure 2 for further biological
evaluation. We now report a detailed account of our concise total
synthesis of cribrostatin 6 (1) and the facile modification of the
approach to the construction of several cribrostatin 6 analogs.9
Our retrosynthetic analysis of cribrostatin 6 (1) is outlined in
Scheme 1. In particular we envisioned that 1 would be accessed
from 3 via oxidation of the hydroquinone and dehydrogenation of
the hydroisoquinoline ring. The key step of the synthesis drew
upon the prior work of Moore and co-workers, who studied ex-
tensively the conversions of squarate derivatives to quinones,10 and
involved the transformation of the squarate 4 into the hydroqui-
none 3 via a tandem electrocyclic ring opening/radical cyclization.
Intermediate 4 would then be prepared from the known squarate
OH
OH
O
homolytic
Me
EtO
Me
EtO
[O]
aromatic
1
Me
N
N
substitution
Me
N
OH
N
3
9
Scheme 2.
2. Results and discussion
2.1. Synthesis of cribrostatin 6: Optimization of tandem ring
opening/radical cyclization sequence
In accordance with our plan (Scheme 1), efforts toward cri-
brostatin 6 (1) commenced with the synthesis of the requisite
squarate analog 5 following a literature procedure (Scheme 3).11 In
the event, commercially available diethyl squarate (10) was treated
with MeLi to give the intermediate adduct 11, which underwent
elimination upon reaction with TFAA to deliver 5 in 60% yield.
derivative 511 and commercially available alcohol
6 and 2-
methylimidazole (7). By simply modifying the structure of the in-
puts 5 and 7, we envisioned that it would be possible to quickly
access numerous analogs of 1.
OLi
Me
O
O
O
O
O
MeLi
TFAA
60%
Me
O
O
OH
OH
radical cyclization
THF, 78 °C
EtO
EtO
OEt
EtO
OEt
Me
Me
5
11
10
N
N
Me
EtO
Me
EtO
Scheme 3.
N
N
homolytic aromatic
The next task required the preparation of 4. Toward this end,
commercially available alcohol 6 was converted to tosylate 12 in
95% yield using a slightly modified version of a known procedure
(Scheme 4).14 Initial experiments to prepare the alkyne 13 fo-
cused on the reaction of tosylate 12 with 2-methylimidazole (7)
in the presence of bases, such as t-BuOK, Cs2CO3, and NaH.
However, none of these reactions produced significant quantities
of the desired 13. After some experimentation, we discovered
that heating the tosylate 12 with an excess of 7 in acetonitrile at
70 ꢁC gave the requisite alkyne 13 in 92% yield. Deprotonation of
the acetylenic proton of 13 with n-BuLi and regioselective
3
1
substitution
Me
O
1,2-addition
Me
O
H
N
+
+
EtO
Me
OH
N
HO
N
EtO
O
alkylation
Me
5
6
7
N
4
Scheme 1.
As noted previously, our approach to 1 was influenced by the
seminal work of Moore and co-workers. Namely, there was ample
precedent for the thermal ring opening of squarate derivatives
n-BuLi, THF, −78 → 0 °C
7
OH
OTs
then
CH3CN, 70 °C
92%
p-TsCl, THF, 0 °C → rt
6
12
closely related to 4 via a 4p-electrocyclic ring opening reaction to
95%
furnish ketene intermediates, such as 8 (Scheme 2). Radical cycli-
zation of 8 would then deliver the diradical 9,12 which we antici-
pated would undergo a homolytic aromatic substitution onto the
pendant imidazole to afford hydroquinone 3, which possesses the
tricyclic core of cribrostatin 6 (1). Although cyclizations of phenyl
radicals related to 9 onto pendant alkenyl and alkynyl groups had
been documented,12 there were only a few examples of cyclizations
onto aryl rings, none of which involved azoles.13 Hence, the suc-
cessful transformation of 9 to deliver 3 would significantly expand
the utility of such cyclizations. Oxidation and dehydrogenation of 3,
ideally in the same pot, would then deliver cribrostatin 6 (1).
Me
O
n-BuLi, THF, –78 °C
N
EtO
then
OH
Me
N
5, –78 → 0 °C
N
Me
62%
13
N
4
Scheme 4.

D. Knueppel, S.F. Martin / Tetrahedron 67 (2011) 9765e9770
9767
addition of the anion thus formed to the squarate derivative 5 in
accordance with literature precedent10,11 afforded the key in-
termediate 4 in 62% yield.
our preliminary experiments, we discovered that 3 like 14 was
also labile toward facile aerial oxidation. We thus stirred the so-
lution of 3 open to air overnight at room temperature. We were
somewhat surprised to discover that a mixture (2:1) of quinone 16
and cribrostatin 6 (1) was produced in 24% combined yield. This
result not only confirmed that our plan for preparing cribrostatin 6
(1) was viable, but it also suggested that 1 might be obtained di-
rectly from 4 in one pot. However, extended stirring of the mixture
of 16 and 1 in an open flask gave at best a mixture of 1 and 16 in
a 2:1 ratio.
With the squarate derivative 4 in hand, the stage was set to
explore its pivotal transformation into 3. In the event, 4 was
heated in refluxing PhCl (0.02 M) for 30 min to give hydroqui-
none 14 (Scheme 5). Based upon examination of the 1H NMR
spectrum of the crude reaction mixture, 14 was the only identi-
fiable product of the reaction; none of the desired 3 was detec-
ted. However, it was not possible to isolate 14 in pure form
because it underwent facile aerial oxidation to give quinone 15.
Although obtaining the quinone 15 was not the desired outcome,
we were able to convert 4 cleanly into 15 by the simple expe-
dient of heating 4 in refluxing PhCl for 30 min, followed by
stirring the resultant solution at room temperature open to air
overnight. Quinone 15 was thus isolated as the sole product of
the reaction in 25% yield (unoptimized).
Me
Me
O
N
OH
OH
Me
N
Δ, 30 min
EtO
OH
PhCl (0.02 M)
N
EtO
Me
N
14
4
Me
O
OH
N
Scheme 6.
Me
EtO
Me
EtO
N
aerial oxidation, rt
25%
In as much as dehydrogenation of 16 to naturally-occurring 1
was only partially successful using air, we screened additional ox-
idants. Gratifyingly, after screening a number of potential oxidants
(e.g., CAN, DDQ, MnO2, Ag2O), we found that heating the crude
mixture of 16 and 1 with Pd/C in anisole gave exclusively cri-
brostatin 6 (1) in 17% overall yield from 4.
N
Me
OH
3
N
O
15
Scheme 5.
At this juncture we wanted to further streamline the conversion
of 4 to cribrostatin 6 (1). In the event, a solution of 4 in CH3CN
(1.0 mM) was first heated until starting material had been con-
sumed (TLC) (Scheme 7). The majority of the solvent was evapo-
rated under reduced pressure, Pd/C was added, and the reaction
mixture was heated at 80 ꢁC for 4 h to give cribrostatin 6 (1) in one
pot and 26% overall yield. The synthetic 1 thus obtained gave 1H
and 13C NMR spectra consistent with those reported in the litera-
ture.3,6,7 This total synthesis of cribrostatin 6 is remarkably concise
as it requires only four steps in the longest linear sequence and only
five steps from commercially available starting materials, and it
proceeds in 14.1% overall yield. Notably, no protecting groups are
used.
This result suggests that the diradical intermediate 9 (cf.
Scheme 2) did indeed form, but rather than undergoing the de-
sired cyclization via homolytic aromatic substitution onto the
pendant imidazole, two intermolecular hydrogen atom transfers
ensued to deliver hydroquinone 14. Because Moore and Xiong
had also isolated hydroquinone products,13a we were not com-
pletely surprised by this finding. Mechanistic considerations
clearly suggested that we should conduct the reaction under
more dilute conditions as the desired cyclization was unim-
olecular, whereas the hydrogen atom transfer was inter-
molecular.12b Accordingly, we conducted a number of exploratory
experiments on a small scale to identify the conditions (solvent,
temperature, time, and concentration) that would provide the
cyclized material 3. Indeed, we heated a more dilute solution of 4
in PhCl (1.0 mM) under reflux, and the 1H NMR spectrum of the
crude reaction mixture revealed that we had formed a mixture
(ca. 1:3) that contained both 14 and the desired hydroquinone 3.
Further experimentation demonstrated that the solvent also
played an important role in this tandem ring opening and radical
cyclization sequence. For example, use of dichloroethane, dime-
thylformamide, and anisole as solvent (1.0 mM) gave similar
mixtures in which 3 was formed in increased amounts relative to
14. Eventually we discovered that CH3CN gave the best results as
heating a solution of 4 in refluxing CH3CN (1.0 mM) gave a mix-
ture containing 3 and 14 in an approximate ratio of 8:1.
Me
O
O
O
Δ, 35 min
Me
CH3CN (1.0 mM)
EtO
OH
N
then
Me
EtO
N
Pd/C, CH3CN, 80 °C
N
Me
N
26%
4
1
Scheme 7.
Having identified the optimal conditions for the ring opening/
radical cyclization sequence, we then conducted a preparative
experiment in which a solution of 4 in CH3CN (1.0 mM) was heated
to give hydroquinone 3 as the major product (Scheme 6). During
2.2. Synthesis of cribrostatin 6 analogs
Owing to the extraordinarily concise nature of our approach to
cribrostatin 6 (1), the synthesis of its analogs became a practical

9768
D. Knueppel, S.F. Martin / Tetrahedron 67 (2011) 9765e9770
matter. As noted previously, we had envisioned that changing the
starting materials would give rapid access to a variety of cri-
brostatin 6 analogs (2) (Scheme 8). More specifically, we envisioned
that the N-alkylated azole 17 could be readily modified by varying
X, Y, and R3. It would also be a simple matter to alter R1 and R2 on
the squarate input 18, because a large number of squarate de-
rivatives are known and can be readily accessed.11 Combining these
inputs would then furnish a number of different intermediates 19
that should deliver cribrostatin 6 analogs 2 upon ring opening,
radical cyclization, and oxidation.
Scheme 10.
Ready access to cribrostatin 6 (1) itself creates the possibility
that other analogs might be prepared from the natural product
itself. In order to explore the feasibility of conducting such trans-
formations, 1 was treated with methylamine to afford 25 in 66%
yield (Scheme 11). This important discovery now opens the door for
synthesizing a wide range of cribrostatin 6 analogs via analogous
addition/elimination sequences using alcohols, thiols, and other
amines.
O
O
Scheme 8.
Me
Me
MeNH2, EtOH, rt
66%
Toward the objective of synthesizing cribrostatin 6 analogs, we
first explored a route to the diethoxy analog 21 (Scheme 9). Thus,
commercially available diethyl squarate (10) was coupled with the
anion of alkyne 13 to afford cyclobutenol 20. When 20 was heated
under reflux in CH3CN for 24 h, only unreacted starting material
was recovered. On the other hand, heating a solution of 20 in ani-
sole (1.0 mM), followed by evaporating most of the solvent, and
heating the resulting solution with Pd/C afforded cribrostatin 6
analog 21 in 18% overall yield from 20.
N
N
Me
Me
EtO
MeHN
O
O
N
N
1
25
Scheme 11.
3. Summary
We have completed a remarkably concise synthesis of cri-
brostatin 6 (1) and several analogs by a novel approach that in-
volves the rapid buildup of molecular complexity. The synthesis of
1 required a mere four steps in the longest linear sequence (five
total steps from commercially available starting materials) and
proceeded in 14.1% overall yield. The approach features the as-
sembly of the natural product by a convenient, one-pot operation
that proceeds via a tandem 4p-electrocyclic ring opening of
a squarate derivative and a radical cyclization/homolytic aromatic
substitution, followed by oxidation. The use of an azole as a reaction
partner in the cyclization of phenyl radicals generated from squa-
rates also represents a significant expansion of a related process
discovered by Moore and co-workers. Further applications of this
chemistry to the synthesis of additional cribrostatin 6 analogs as
well as evaluation of the biological potency of these novel com-
pounds is currently underway and will be reported in due course.
Scheme 9.
4. Experimental
4.1. General
Several additional analogs of cribrostatin 6 were then prepared
using the known squarate derivatives 22a and 22b as inputs
(Scheme 10).11 The standard coupling protocol with 13 was
employed to afford key intermediates 23a,b. Thermolysis of these
cyclobutenols followed by oxidation in one pot provided the ex-
pected cribrostatin 6 analogs 24a,b in acceptable overall yields
(unoptimized).
Solvents and reagents were reagent grade and used without
purification unless otherwise specified. THF was passed through
two columns of neutral alumina. CH3CN and DMF were passed
through two columns of molecular sieves. Reactions involving air-
or moisture-sensitive reagents or intermediates were performed

D. Knueppel, S.F. Martin / Tetrahedron 67 (2011) 9765e9770
9769
under an inert atmosphere of argon or nitrogen in glassware that
had been oven dried. Reaction temperatures refer to bath tem-
peratures. Melting points are uncorrected. Infrared (IR) spectra
were recorded neat on sodium chloride plates and are reported in
wave numbers (cmꢂ1). 1H and 13C NMR spectra were obtained as
solutions in CDCl3 unless otherwise noted, and chemical shifts
are reported in parts per million (ppm) in reference to CDCl3
(7.24 ppm and 77.0 ppm, respectively). Coupling constants are
reported in hertz (Hz). Spectral splitting patterns are designated
as follows: s, singlet; d, doublet; t, triplet; q, quartet; app,
apparent; br, broad; m, multiplet; comp, overlapping multiplets
of magnetically nonequivalent protons. All products that were
used without further purification were >95% pure by 1H NMR
spectroscopy.
(neat) 2986, 1763, 1620, 1327 cmꢂ1; mass spectrum (ESI, CI) m/z
275.1390 [C15H18O3N2 (Mþ1) requires 275.1393], 275 (base).
4.2.4. 2-Ethoxy-3-methyl-5-(2-(2-methyl-1H-imidazol-1-yl)ethyl)-
cyclohexa-2,5-diene-1,4-dione (15). A solution of
4
(40 mg,
0.15 mmol) in chlorobenzene (6.2 mL) was heated for 30 min at
130 ꢁC in a preheated oil bath. After cooling to room temperature,
the reaction was stirred open to air for 16 h. The solvent was re-
moved under reduced pressure, and the residue was purified by
flash column chromatography eluting with CHCl3/MeOH (10:1) to
give 10 mg (25%) of 15 as a yellow oil; 1H NMR (400 MHz, CDCl3)
d
6.87 (d, J¼1.2 Hz, 1H), 6.75 (d, J¼1.2 Hz, 1H), 6.27 (s, 1H), 4.29 (q,
J¼7.0 Hz, 2H), 4.00 (t, J¼7.3 Hz, 2H), 2.78 (t, J¼7.3 Hz, 2H), 2.34 (s,
3H), 1.94 (s, 3H), 1.33 (t, J¼7.0 Hz, 3H); mass spectrum (ESI) m/z
275.1395 [C15H18N2O3 (Mþ1) requires 275.1390].
4.2. Experimental procedures
4.2.5. Cribrostatin 6 (1). A solution of 4 (96 mg, 0.35 mmol) in
CH3CN (350 mL) was heated under reflux for 35 min in a preheated
oilbath (130 ꢁC). Aftercoolingtoroomtemperature, the solutionwas
concentrated to approximately 5 mL by evaporation under reduced
pressure. Pd/C (15 mg, 10 wt % loading) was added, and the reaction
was heated for 4 h at 80 ꢁC. After cooling to room temperature, the
solvent was removed under reduced pressure, and the residue was
purified by flash column chromatography eluting with CHCl3/MeOH
(10:1) to give 25 mg (26%) of 1 as a blue solid: mp 165e167 ꢁC (lit.3
4.2.1. But-3-ynyl-4-methylbenzenesulfonate (12). A solution of n-
BuLi (2.80 mL, 2.60 M, 7.27 mmol) in hexanes was added dropwise
to a solution of 6 (0.50 mL, 6.61 mmol) in THF (7.5 mL) at ꢂ78 ꢁC.
After 5 min at ꢂ78 ꢁC, stirring was continued for 30 min at 0 ꢁC. p-
TsCl (1.51 g, 7.93 mmol) in THF (8 mL) was added, and the reaction
was stirred for 30 min at 0 ꢁC. H2O (2 mL) was added, and the
mixture was extracted with Et2O (3ꢃ10 mL). The combined organic
phases were dried (Na2SO4), and the solvent was removed under
reduced pressure to give 1.41 g (95%) of 12 as a pale yellow oil; 1H
169e171 ꢁC, lit.7 165e167 ꢁC); 1H NMR (600 MHz, CDCl3)
d 8.23 (d,
NMR (400 MHz, CDCl3)
d
7.78 (d, J¼8.0 Hz, 2H), 7.33 (d, J¼8.0 Hz,
J¼0.9 Hz, 1H), 7.83 (dd, J¼7.3, 0.9 Hz, 1H), 7.17 (d, J¼7.3 Hz, 1H), 4.38
2H), 4.08 (t, J¼7.1 Hz, 2H), 2.53 (dt, J¼7.1, 2.6 Hz, 2H), 2.43 (s, 3H),
(q, J¼7.0 Hz, 2H), 2.67 (s, 3H), 2.04 (s, 3H), 1.40 (t, J¼7.0 Hz, 3H); 13C
1.95 (t, J¼2.6 Hz, 1H); 13C NMR (100 MHz, CDCl3)
d
145.0, 132.7,
NMR (151 MHz, CDCl3) d 184.9, 180.7,156.2, 137.7,130.1,125.9, 125.0,
129.9, 127.9, 78.3, 70.7, 67.4, 21.6, 19.4; IR (neat) 3290, 2962,
2919, 1598, 1359, 1190, 1176, 980, 904, 815 cmꢂ1; mass spectrum
(ESI) m/z 247.0399 [C11H12O3S (Mþ23) requires 247.0400], 471
(base), 247, 225.
124.7, 123.9, 123.5, 107.6, 69.6, 16.0, 12.6, 9.2; IR (neat) 2923, 1662,
1626, 1611, 1527, 1172 cmꢂ1; mass spectrum (ESI) m/z 271.1077
[C15H14N2O3 (Mþ1) requires 271.1078], 271 (base), 243, 215.
4.2.6. 2,3-Diethoxy-4-hydroxy-4-(4-(2-methyl-1H-imidazol-1-yl)
but-1-ynyl)cyclobut-2-enone (20). A solution of n-BuLi (1.27 mL,
2.29 M, 2.91 mmol) in hexanes was added dropwise to a solution of 13
(340 mg, 2.53 mmol) in THF (12 mL) at ꢂ78 ꢁC. After 35 min at ꢂ78 ꢁC,
a solution of 3,4-diethoxycyclobut-3-ene-1,2-dione (10) (862 mg,
5.07 mmol) in THF (6 mL) at 0 ꢁC was added dropwise via cannula.
After 10 min at ꢂ78 ꢁC, stirring was continued for 1.5 h at 0 ꢁC. Satu-
rated aqueous NH4Cl (5 mL) and brine (5 mL) were added, and the
mixture was extracted with EtOAc (3ꢃ30 mL). The combined organic
phases were dried (Na2SO4), and the solvent was removed under re-
duced pressure. The residue was purified by flash column chroma-
tography eluting with CH2Cl2/MeOH (10:1/5:1) to give 379 mg (49%)
4.2.2. 1-(But-3-ynyl)-2-methyl-1H-imidazole (13). 2-Methylimidazole
(7) (4.456 g, 54.27 mmol) was added to a solution of 12 (2.434 g,
10.85 mmol) in CH3CN (15 mL). The reaction was heated for 18 h at
70 ꢁC, whereupon it was cooled to room temperature, and the solvent
was removed under reduced pressure. The residue was purified by
flash column chromatography eluting with CHCl3/MeOH (10:1) to
give an oil that was further purified by short path distillation under
reduced pressure (2 Torr) at 150 ꢁC to give 1.346 g (92%) of 13 as
a clear oil; 1H NMR (400 MHz, CDCl3)
d
6.85 (d, J¼1.2 Hz, 1H), 6.82 (d,
J¼1.2 Hz, 1H), 3.95 (t, J¼7.0 Hz, 2H), 2.53 (dt, J¼7.0 Hz, 2H), 2.35 (s,
3H), 1.99 (t, J¼2.6 Hz, 1H); 13C NMR (100 MHz, CDCl3)
d 144.3,
127.1, 118.9, 79.6, 71.1, 44.3, 20.9, 12.8; IR (neat) 3287, 2923, 1501,
1425, 1281 cmꢂ1; mass spectrum (CI) m/z 135.0926 [C8H10N2 (Mþ1)
requires 135.0922], 135 (base).
of 20 as a yellow oil; 1H NMR (400 MHz, CDCl3)
d
6.83 (d, J¼1.4 Hz, 1H),
6.79 (d, J¼1.4 Hz,1H), 4.53e4.38 (comp, 2H), 4.25 (q, J¼7.1 Hz, 2H), 3.96
(t, J¼6.6 Hz, 2H), 2.63 (t, J¼6.6 Hz, 2H), 2.35 (s, 3H), 1.40 (t, J¼7.0 Hz,
3H), 1.27 (t, J¼7.1 Hz, 3H); 13C NMR (100 MHz, CDCl3)
d 181.3, 165.1,
4.2.3. 3-Ethoxy-4-hydroxy-2-methyl-4-(4-(2-methyl-1H-imidazol-1-
yl)but-1-ynyl)cyclobut-2-enone (4). A solution of n-BuLi (0.53 mL,
2.44 M, 1.30 mmol) in hexanes was added dropwise to a solution of
13 (145 mg, 1.08 mmol) in THF (5.4 mL) at ꢂ78 ꢁC. After 35 min at
ꢂ78 ꢁC, a solution of 3-ethoxy-4-methylcyclobutene-1,2-dione (5)
(242 mg, 1.73 mmol) in THF (5 mL) at ꢂ78 ꢁC was added dropwise
via cannula. After 10 min at ꢂ78 ꢁC, stirring was continued for 1.5 h
at 0 ꢁC. Saturated aqueous NH4Cl (5 mL) and brine (5 mL) were
added, and the mixture was extracted with EtOAc (3ꢃ20 mL). The
combined organic phases were dried (Na2SO4), and the solvent was
removed under reduced pressure. The residue was purified by flash
column chromatography eluting with CHCl3/MeOH (10:1) to give
144.4,134.2,126.5,119.1, 84.2, 78.6, 78.1, 69.4, 67.0, 44.4, 21.5,15.5,15.2,
12.6; IR (neat) 3119, 2982,1776,1634,1324,1045 cmꢂ1; mass spectrum
(ESI) m/z 305.1496 [C16H20O4N2 (Mþ1) requires 305.1499], 305 (base).
4.2.7. 8,9-Diethoxy-3-methylimidazo[5,1-a]isoquinoline-7,10-dione
(21). A solution of 20 (89 mg, 0.29 mmol) in degassed anisole
(290 mL) was heated at 120 ꢁC for 2.5 h in a preheated oil bath.
After cooling to room temperature, the solution was concentrated
to approximately 5 mL by evaporation under reduced pressure. Pd/
C (12 mg, 10 wt % loading) was added, and the reaction was heated
for 15 h at 90 ꢁC. After cooling to room temperature, the solvent was
removed under reduced pressure, and the residue was purified by
flash column chromatography eluting with CHCl3/MeOH (10:1) to
give 16 mg (18%) of 21 as an aquamarine solid: mp 125e127 ꢁC; 1H
185 mg (62%) of 4 as an amber oil; 1H NMR (400 MHz, CDCl3)
d 6.85
(d, J¼1.4 Hz, 1H), 6.84 (d, J¼1.4 Hz, 1H), 4.56e4.38 (comp, 2H), 3.99
(t, J¼6.7 Hz, 2H), 2.67 (t, J¼6.7 Hz, 2H), 2.39 (s, 3H), 1.65 (s, 3H), 1.43
NMR (400 MHz, CDCl3)
d
8.30 (s, 1H), 7.85 (d, J¼7.4 Hz, 1H), 7.18 (d,
(t, J¼7.2 Hz, 3H); 13C NMR (100 MHz, CDCl3)
d
187.9, 181.1, 144.4,
J¼7.4 Hz, 1H), 4.35 (q, J¼7.1 Hz, 2H), 4.34 (q, J¼7.1 Hz, 2H), 2.69 (s,
126.6, 123.9, 119.1, 85.0, 82.7, 78.6, 69.0, 44.3, 21.6, 15.1, 12.7, 6.4; IR
3H), 1.42 (t, J¼7.1 Hz, 3H), 1.41 (t, J¼7.1 Hz, 3H); 13C NMR (151 MHz,

9770
D. Knueppel, S.F. Martin / Tetrahedron 67 (2011) 9765e9770
CDCl3)
d
181.7, 181.3, 145.9, 145.6, 137.9, 126.6, 124.5, 124.0, 123.5,
15 h at 80 ꢁC. After cooling to room temperature, the solvent was
removed under reduced pressure, and the residue was purified by
flash column chromatography eluting with (CHCl3/MeOH 10:1) to
give 15 mg (14%) of 24b as a purple solid: mp >300 ꢁC; 1H NMR
123.3, 107.5, 69.91, 69.88, 15.6 (2C), 12.7; IR (neat) 2925, 1666, 1621,
1601, 1531, 1271, 1176 cmꢂ1; mass spectrum (CI) m/z 301.1182
[C16H16N2O4 (Mþ1) requires 301.1183].
(400 MHz, CDCl3)
d
8.25 (s, 1H), 7.82 (d, J¼7.4 Hz, 1H), 7.16 (d,
4.2.8. 4-Hydroxy-3-methoxy-2-methyl-4-(4-(2-methyl-1H-imidazol-
1-yl)but-1-ynyl)cyclobut-2-enone (23a). A solution of n-BuLi
(1.30 mL, 2.61 M, 3.39 mmol) in hexanes was added dropwise to
a solution of 13 (396 mg, 2.95 mmol) in THF (10 mL) at ꢂ78 ꢁC. After
35 min at ꢂ78 ꢁC, a solution of 3-methoxy-4-methylcyclobut-3-ene-
1,2-dione (22a) (595 mg, 4.72 mmol) inTHF (5 mL) at 0 ꢁC was added
dropwise via cannula. After 10 min at ꢂ78 ꢁC, stirring was continued
for 1.5 h at 0 ꢁC. Saturated aqueous NH4Cl (5 mL) and brine (5 mL)
were added, and the mixture was extracted with EtOAc (3ꢃ20 mL).
The combined organic phases were dried (Na2SO4), and the solvent
was removed under reduced pressure. The residue was purified by
flash column chromatography eluting with CHCl3/MeOH
(10:1/5:1) to give 241 mg (38%) of 23a as an orange oil; 1H NMR
J¼7.4 Hz, 1H), 2.67 (d, 3H), 2.12 (s, 3H), 2.11 (s, 3H); 13C NMR
(100 MHz, CDCl3, one ArC not observed) d 184.5, 183.9, 142.3, 141.3,
137.8, 126.4, 124.8, 124.5, 124.1, 107.6, 12.64, 12.60, 12.5; IR (neat)
2922, 1649, 1609, 1298, 1284 cmꢂ1; mass spectrum (ESI) m/z
241.0975 [C14H12N2O2 (Mþ1) requires 241.0972], 241 (base).
4.2.12. 9-(Dimethylamino)-3,8-dimethylimidazo[5,1-a]isoquinoline-
7,10-dione (25). A solution of MeNH2 (28 mL, 8.03 M, 0.23 mmol) in
EtOH was added to a solution of 1 (12 mg) in EtOH (2.5 mL). After 18 h
at room temperature, the solvent was removed under reduced
pressure and the residue was purified by flash column chromatog-
raphy eluting with CHCl3/MeOH (10:1) to give 8 mg (66%) of 25 as
a green solid: mp 208e210 ꢁC; 1H NMR (400 MHz, CDCl3, mixture of
(400 MHz, CDCl3)
(s, 3H), 3.93 (t, J¼6.7 Hz, 2H), 2.61 (t, J¼6.7 Hz, 2H), 2.30 (s, 3H), 1.59
(s, 3H); 13C NMR (100 MHz, CDCl3)
188.0, 181.7, 144.4, 126.4, 124.1,
119.0, 84.9, 82.5, 78.6, 59.5, 44.3, 21.5, 12.5, 6.3; IR (neat) 3115, 2956,
1765, 1625, 1339 cmꢂ1
mass spectrum (ESI) m/z 261.1237
d
6.79 (d, J¼1.4 Hz, 1H), 6.74 (d, J¼1.4 Hz, 1H), 4.11
rotamers)
5.79 (d, J¼4.0 Hz,1H), 3.24 (d, J¼4.0 Hz,1.5H), 3.23 (d, J¼4.0 Hz,1.5H),
2.68 (s, 3H), 2.22 (s, 3H); 13C NMR (100 MHz, CDCl3)
182.2, 181.5,
145.9, 137.2, 127.6, 125.5, 124.2, 123.7, 121.6, 108.8, 108.4, 32.8, 12.6,
10.5; IR (neat) 3583, 3325, 2922, 1620, 1515 cmꢂ1; mass spectrum
(ESI) m/z 256.1087 [C14H13N3O2 (Mþ1)requires 256.1081], 256 (base).
d
8.10 (s, 1H), 7.87 (d, J¼7.5 Hz, 1H), 7.27 (d, J¼7.5 Hz, 1H),
d
d
;
[C14H16N2O2 (Mþ1) requires 261.1234], 261 (base).
4.2.9. 4-Hydroxy-2,3-dimethyl-4-(4-(2-methyl-1H-imidazol-1-yl)
but-1-ynyl)cyclobut-2-enone (23b). A solution of n-BuLi (1.54 mL,
2.59 M,4.00 mmol)inhexaneswasaddeddropwisetoasolutionof13
(447 mg, 3.33 mmol) inTHF (17 mL)atꢂ78ꢁC. After35 minatꢂ78ꢁC,
a solution of 3,4-dimethylcyclobut-3-ene-1,2-dione (22b) (550 mg,
5.00 mmol) inTHF (5 mL) at ꢂ78 ꢁC was added dropwise via cannula.
After 10 min at ꢂ78 ꢁC, stirring was continued for 1.5 h at 0 ꢁC. Sat-
urated aqueous NH4Cl (5 mL) and brine (5 mL) were added, and the
mixture was extracted with EtOAc (3ꢃ30 mL). The combined organic
phases were dried (Na2SO4), and the solvent was removed under
reduced pressure. The residue was purified by flash column chro-
matography eluting with CHCl3/MeOH (5:1) to give 322 mg (40%) of
Acknowledgements
We thank the National Institutes of General Medical Sciences
(GM31077) and The Robert A. Welch Foundation (F-652) for sup-
port of this research. D.K. also thanks the National Science Foun-
dation for a Graduate Research Fellowship. Ms. Genevieve Pease
(The University of Texas) is gratefully acknowledged for helping
with the preparation of starting materials.
References and notes
23b as a yellow oil; 1H NMR (400 MHz, CDCl3)
6.66 (d, J¼1.4 Hz,1H), 3.86(t, J¼6.7 Hz, 2H), 2.53(t, J¼6.7 Hz, 2H), 2.23
(s, 3H), 2.03 (s, 3H), 1.60 (s, 3H); 13C NMR (100 MHz, CDCl3)
190.1,
d
6.74 (d, J¼1.4 Hz,1H),
1. Pettit, R. K.; Bridget, R. F.; Knight, J. C.; Weber, C. A.; Pettit, G. R.; Cage, G. D.; Pon,
S. J. Med. Microbiol. 2004, 53, 61e65 and references therein.
2. Kung, H.-C.; Loyert, D. L.; Xu, J.; Murphy, S. L. Natl. Vital Stat. Rep. 2008, 56, 1e121.
3. Pettit, G. R.; Collins, J. C.; Knight, J. C.; Herald, D. L.; Nieman, R. A.; Williams, M.
D.; Pettit, R. K. J. Nat. Prod. 2003, 66, 544e547.
d
178.1, 149.0, 144.1, 126.0, 118.9, 85.4, 84.0, 79.3, 44.2, 21.3, 12.3, 10.3,
7.5; IR (neat) 3520, 2922,1760,1644,1426 cmꢂ1; mass spectrum (ESI)
m/z 245.1290 [C14H16N2O2 (Mþ1) requires 245.1285].
4. Hoyt, M. T.; Palchaudhuri, R.; Hergenrother, P. J. Invest. New Drugs 2011, 29,
562e573.
5. (a) McFadyen, M. C. E.; Melvin, W. T.; Murray, G. I. Mol. Cancer Ther. 2004, 3,
363e371; (b) Shawa, A. T.; Winslowa, M. M.; Magendantza, M.; Ouyang, C.;
Dowdle, J.; Subramaniand, A.; Lewis, T. A.; Maglathind, R. L.; Tollidayd, N.; Jacks,
T. Proc. Natl. Acad. Sci. U.S.A. 2011, 108, 8773e8778.
6. (a) Nakahara, S.; Kubo, A. Heterocycles 2004, 63, 2355e2362; (b) Nakahara, S.;
Kubo, A.; Mikami, Y.; Ito, J. Heterocycles 2006, 68, 515e520.
7. Markey, M. D.; Kelly, T. R. J. Org. Chem. 2008, 73, 7441e7443.
8. For examples, see: (a) Apsel, B.; Bender, J. A.; Escobar, M.; Kaelin, D. E., Jr.;
Lopez, O. D.; Martin, S. F. Tetrahedron Lett. 2003, 44, 1075e1077; (b) Chen, C.-L.;
Martin, S. F. J. Am. Chem. Soc. 2006, 128, 13696e13697; (c) Sparks, S.; Chen, C.;
Martin, S. Tetrahedron 2007, 63, 8619e8635; (d) O’Keefe, B. M.; Mans, D. M.;
Kaelin, D. E., Jr.; Martin, S. F. J. Am. Chem. Soc. 2010, 132, 15528e15530; (e)
O’Keefe, B. M.; Mans, D. M.; Kaelin, D. E., Jr.; Martin, S. F. Tetrahedron 2011, 67,
6524e6538; (f) Nichols, A. L.; Zhang, P.; Martin, S. F. Org. Lett. 2011, 13,
4696e4699.
4.2.10. 9-Methoxy-3,8-dimethylimidazo[5,1-a]isoquinoline-7,10-
dione (24a). A solution of 23a (240 mg, 0.92 mmol) in CH3CN
(400 mL) was heated under reflux for 35 min in a preheated oil bath
(130 ꢁC). After cooling to room temperature, the solution was con-
centrated to approximately 5 mL by evaporation under reduced pres-
sure. Pd/C (31 mg, 10 wt % loading) was added, and the reaction was
heated for 20 h at 80 ꢁC. After cooling to room temperature, the solvent
was removed under reduced pressure, and the residue was purified by
flash column chromatography eluting with CHCl3/MeOH (20:1) to give
60 mg (25%) of 24a as a green-blue solid: mp 149e151 ꢁC; 1H NMR
9. For a preliminary account of some of this work, see: Knueppel, D.; Martin, S. F.
Angew. Chem., Int. Ed. 2009, 48, 2569e2571.
10. For reviews of ring expansions of cyclobutenones, see: (a) Moore, H. W.; Yerxa,
B. R. Adv. Strain Org. Chem. 1995, 4, 81e162; (b) Moore, H. W.; Yerxa, B. R.
Chemtracts 1992, 5, 273e313.
(400 MHz, CDCl3)
d
8.22 (s,1H), 7.82 (d, J¼7.2 Hz, 1H), 7.15 (d, J¼7.2 Hz,
1H), 4.09 (s, 3H), 2.67 (s, 3H), 2.02 (s, 3H); 13C NMR (100 MHz, CDCl3)
d
184.8, 180.5, 156.6, 137.7, 129.4, 125.9, 124.8, 124.7, 123.8, 123.5, 107.6,
61.1, 12.6, 9.0; IR (neat) 2925, 1666, 1628, 1291, 1156 cmꢂ1; mass spec-
11. Reed, M. W.; Pollart, D. J.; Perri, S. T.; Foland, L. D.; Moore, H. W. J. Org. Chem.
trum (ESI) m/z 257.0924 [C14H12N2O3 (Mþ1) requires 257.0921].
1988, 53, 2477e2482.
12. (a) Xu, S. L.; Taing, M.; Moore, H. W. J. Org. Chem. 1991, 56, 6104e6109; (b) Xia,
H.; Moore, H. W. J. Org. Chem. 1992, 57, 3765e3766; (c) Xiong, Y.; Xia, H.; Moore,
H. W. J. Org. Chem. 1995, 60, 6460e6467; (d) Wipf, P.; Hopkins, C. R. J. Org. Chem.
1999, 64, 6881e6887.
13. (a) Xiong, Y.; Moore, H. W. J. Org. Chem. 1996, 61, 9168e9177; (b) Hergueta, A.
R.; Moore, H. W. J. Org. Chem. 1999, 64, 5979e5983.
14. (a) Negishi, E.; Boardman, L. D.; Sawada, H.; Bagheri, V.; Stoll, A. T.; Tour, J. M.;
Rand, C. L. J. Am. Chem. Soc. 1988, 110, 5383e5396; (b) Falck, J. R.; He, A.; Fukui,
H.; Tsutsui, H.; Radha, A. Angew. Chem., Int. Ed. 2007, 46, 4527e4529.
4.2.11. 3,8,9-Trimethylimidazo[5,1-a]isoquinoline-7,10-dione
(24b). A solution of 23b (108 mg, 0.44 mmol) in CH3CN (400 mL)
was heated under reflux for 35 min in a preheated oil bath (130 ꢁC).
After cooling to room temperature, the solution was concentrated to
approximately 5 mL by evaporation under reduced pressure. Pd/C
(10 mg,10 wt % loading) was added, and the reaction was heated for