Exploring O-stannyl ketyl and acyl radical cyclizations for the synthesis of γ-lactone-fused benzopyrans and benzofurans

Article abstract of DOI:10.1039/c3ob42090f

The synthesis of a series of γ-lactone-fused benzopyrans and benzofurans, analogues of the pyranonaphthoquinone antibiotics, is reported. Preparation of the heterocycles was achieved by either O-stannyl ketyl or acyl radical cyclization of benzaldehyde pr

Full text of DOI:10.1039/c3ob42090f

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Exploring O-stannyl ketyl and acyl radical
cyclizations for the synthesis of γ-lactone-fused
benzopyrans and benzofurans†
Cite this: Org. Biomol. Chem., 2014,
12, 171
Helen Santoso,a,b Myriam I. Casanac and Christopher D. Donner*a,b
The synthesis of a series of γ-lactone-fused benzopyrans and benzofurans, analogues of the pyrano-
naphthoquinone antibiotics, is reported. Preparation of the heterocycles was achieved by either O-stannyl
ketyl or acyl radical cyclization of benzaldehyde precursors followed by oxidation to give the pyrano- and
furanobenzoquinone systems. The observed diastereoselectivity during O-stannyl ketyl radical cyclization
is influenced by aromatic substitution ortho to the aldehyde, whilst acyl radical cyclization followed by
stereoselective reduction of the resulting pyranones provides a complimentary approach to forming the
required γ-lactone-fused benzopyran systems.
Received 21st October 2013,
Accepted 7th November 2013
DOI: 10.1039/c3ob42090f
www.rsc.org/obc
Introduction
The redox chemistry of quinones is often linked to their bio-
logical relevance, from the central involvement of coenzyme
Q10 (ubiquinone) in the electron transport chain of eukaryotic
organisms, through to the defence mechanism of the bombar-
dier beetle. Not surprisingly, the presence of a quinonoid
system in compounds such as 1–4 (Fig. 1) is crucial to their
bioactivity, with doxorubicin 1 and mitomycin C 2 both being
in clinical use for the treatment of a variety of cancers. The
pyranoquinones kalafungin 3 and frenolicin B 4 are members
of a large family of natural products that consistently show a
broad range of antibiotic activities, many possessing cytotoxic
properties.1 A mechanism of action involving bioreductive
alkylation was proposed by Moore and Czerniak for pyrano-
quinones such as 3 and 4,2 and is supported by more recent
theoretical,3 chemical4 and biochemical studies.5
Fig. 1 Bioreductive alkylating agents.
The tricyclic quinone 5 (Scheme 1) contains the essential
structural features required for the bioactivity of pyrano-
naphthoquinones 3 and 4, based on their proposed bioreduc-
tive alkylation mode of action, and can thus be considered the
pharmacophore for these natural products. The process
involves an initial two-electron reduction of quinone 5 to
aARC Centre of Excellence for Free Radical Chemistry and Biotechnology, Australia.
E-mail: cdonner@unimelb.edu.au; Fax: +61 3 9347 8189; Tel: +61 3 8344 2411
bSchool of Chemistry and Bio21 Molecular Science and Biotechnology Institute,
The University of Melbourne, Victoria 3010, Australia
cECPM, School of Chemistry, Polymers and Materials Science, The University
of Strasbourg, 67000 Strasbourg, France
†Electronic supplementary information (ESI) available: Full experimental details
and copies of 1H and 13C NMR spectra of new compounds. See DOI: 10.1039/
c3ob42090f
Scheme 1 Mechanism of bioreductive alkylation for γ-lactone-fused
pyranoquinones.
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hydroquinone 6, this being followed by opening of the
γ-lactone to form the reactive ortho-quinone methide 7, which
upon reaction with a nucleophile leads to the alkylated
product 8. Some of the biological effects of doxorubicin 1 have
been attributed to a related one-electron bioreductive process
that generates reactive oxygen species,6 whilst mitomycin C 2
acts as a sequence specific cross-linker of DNA via alkylation at
C-1 and C-10.7 In the later case, reduction of the quinone in 2
initiates a cascade involving loss of methanol, opening of
the aziridine ring and loss of carbamate to form the alkylating
species.
Surprisingly, the proposed pharmacophore 5 of the pyrano-
quinones has not itself been prepared previously, although a
number of analogues having substituents on the benzoqui-
none ring8,9 or at C-5 on the pyran ring10–15 are known. Apart
from the alkoxycarbonylative annulation methodology16 and
Lewis acid-catalyzed aryldioxolanylacetate rearrangement,12
preparation of the fused benzopyran-γ-lactone structural
motif contained in 6, which may then serve as a precursor to
quinone 5, usually involves formation of each of the hetero-
cyclic rings in separate steps. Examples include using sequen-
tial Wittig-oxa-Michael reactions followed by lactonization,8
Sharpless dihydroxylation to form the γ-lactone followed
by oxa-Pictet-Spengler reaction10 and 2-alkoxyfuran addition
to 2-acetylbenzoquinone with subsequent acid-catalyzed
rearrangement.13,15
Scheme 2 Reagents and conditions: (a) ethyl propiolate, NMM, CH2Cl2,
1.5 h (10 45%; 11 47%); (b) TEMPO, PhI(OAc)2, CH2Cl2, 3 h (98%);
(c) Bu3SnH, AIBN, benzene, reflux, 5 h (15 13%, 17 46%).
Under these conditions the required γ-lactone-containing
As part of an ongoing program exploring the use of product and the trans-product are formed in equal amounts
O-stannyl ketyl radical cyclizations for the synthesis of a range (14 : 16 1 : 1). When the para-dimethoxy substituted benz-
of structure classes, including prostanoids and dioxaspirono- aldehyde 13 was reacted under similar conditions, however,
nanes,17 we have also applied this approach to the synthesis of the diastereoselectivity of the cyclization changed in favour of
tricyclic systems related to 6 en route to frenolicin B 4.18 This the trans-product 17 (15 : 17 1 : 3.4).18 The conversion of trans-
method enabled both the pyran and cis-fused γ-lactone product 17 to lactone 15 can be effected by exposure to potas-
rings to be formed in a single step, with excellent diastereo- sium carbonate.18
selectivity, from a benzaldehyde precursor. The ability to
To further understand the factors influencing the stereo-
rapidly access heterocycles such as 6 using this method gave selectivity of these O-stannyl ketyl radical cyclizations the iso-
us the incentive to explore the scope of this approach further meric dimethoxybenzaldehydes 22, 28 and 29 were prepared
for the synthesis of analogous systems that may potentially act as outlined in Scheme 3. Thus, reduction of phthalide 18 gave
as bioreductive alkylating agents. Additionally, it was antici- the symmetrical diol 1920 that upon alkylation formed benzyl
pated that during the course of these studies factors affecting alcohol 20, along with dialkylated product 21. Oxidation of
the diastereoselectivity during O-stannyl ketyl radical cycliza- alcohol 20 then gave benzaldehyde 22 in 41% yield over the
tion of benzaldehyde substrates would be assessed to establish three steps. A similar sequence of reactions beginning from
more efficient and predictable outcomes in these multi-bond phthalide 23 initially gave diol 2421 that upon alkylation
forming processes.
formed isomeric benzyl alcohols 25 and 26. Separation of the
isomers was best achieved after their subsequent oxidation to
the corresponding benzaldehydes 28 and 29. With these sub-
strates in hand, the influence of substitution pattern on the
O-stannyl ketyl radical cyclization could be assessed.
Results and discussion
To assess the viability of this approach we firstly prepared the
The ortho-dimethoxybenzaldehydes 22 and 28, in which the
cyclization precursor benzaldehyde 12 in two steps from methoxy substituents are remote to the aldehyde group, both
benzene dimethanol 919 (Scheme 2). When benzaldehyde 12 gave a near 1 : 1 mixture of cis- and trans-products (Table 1,
was treated with tributyltin hydride (0.05 M) and AIBN in entries 3 and 4) that are comparable to the unsubstituted
benzene at reflux a mixture of cis-14 and trans-16 products system 12. The ortho-dimethoxy isomer 29 in which the alde-
were formed along with benzyl alcohol 10 resulting from hyde group has a neighbouring methoxy group gave a prefer-
reduction of the intermediate O-stannyl ketyl radical. Lowering ence for trans-product 35 (entry 5), with the cis-/trans-product
the concentration of tributyltin hydride to 0.02 M gave ratio very similar to that formed from the para-dimethoxy sub-
improved conversion to cyclized products 14 and 16 (Table 1). stituted system 13. With dimethoxybenzaldehydes 13, 22, 28
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Table 1 O-Stannyl ketyl radical cyclization products from benz-
aldehydes 12, 13, 22, 28 and 29
Entry
1
Substrate
Productsa,b
12
2
13
3
4
22
28
5
29
Scheme 3 Reagents and conditions: (a) LiAlH4, THF, 0 °C, 2.5 h (19 80%;
24 93%); (b) ethyl propiolate, NMM, CH2Cl2, 1.5 h (20 56%, 21 24%; 25 + 26
60%, 27 21%); (c) TEMPO, PhI(OAc)2, CH2Cl2, 2–3 h (22 91%; 28 + 29 90%).
a Conditions: Bu3SnH (0.02 M, 1.5 equiv.), AIBN (0.1–0.3 equiv.),
benzene, reflux, 5 h. b Ratio and yield of cyclized products obtained by
integration of 1H NMR spectrum of the mixture after filtration through
10% KF/silica. In each case, 15–25% of the products resulting from
reduction were also observed. c [Bu3SnH] = 0.05 M. d Isolated yield.
and 29 having similar electronic features, the change in stereo-
selectivity may be a result of the steric influence of the
methoxy groups. Thus, when a methoxy group is ortho to the
aldehyde (Table 1, entries 2 and 5) then trans-products are
favoured. Conversely, less hindered aldehyde groups (entries 1,
3 and 4) tend to give similar amounts of cis- and trans-
products. The preference for trans-product formation from
benzaldehydes 13 and 29 may also result from electrostatic
repulsion between the oxygen of the intermediate O-stannyl
ketyl group and adjacent methoxy substituent being mini-
mized in the transition state leading to trans-products 17 and
35. Samarium diiodide has been used to effect reductive cycli-
zation of benzaldehydes,22 however, under these conditions a
strong preference for trans-product formation is observed and
as such has not been attempted in the present study.
Scheme 4 Reagents and conditions: (a) tert-dodecanethiol (0.3 equiv.),
ACCN (0.3 equiv.), PhMe, reflux, 22 h (37 23% [48% recovered 13]).
reduction of ketone 37 (Scheme 4). To prepare the required
ketone we envisaged the direct formation of 37 from benz-
aldehyde 13 using an acyl radical cyclization under conditions
of polarity reversal catalysis.23,24 Firstly using the unsubsti-
tuted benzaldehyde 12, treatment with tert-dodecanethiol and
1,1′-azobis(cyclohexanecarbonitrile) (ACCN) resulted in only
30% conversion to ketone 36 after 18 hours in toluene at
reflux. Similar treatment of the dimethoxybenzaldehyde 13
gave 50% conversion after 22 hours, with ketone 37 being iso-
lated in 23% yield (44% based on recovered 13). Longer reac-
tion times with further addition of initiator and/or catalyst
failed to complete the conversion of benzaldehyde 13 to
ketone 37 without further decomposition taking place. We
With the goal of more efficiently accessing the required
γ-lactone-containing product 15 from benzaldehyde 13,
we considered an approach that involved a diastereoselective
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Scheme 6 Reagents and conditions: (a) Bu3SnH, AIBN, benzene, reflux,
2 h (44 + 46 92%; 45 39%, 47 40%).
Scheme 5 Reagents and conditions: (a) methyl-4-bromobut-2-enoate,
K2CO3, DMF, 16–20 h (40 61%; 41 80%); (b) tert-dodecanethiol (3.0
equiv.), ACCN (1.5 equiv.), PhCl, 100 °C, 20 h (42 68% [26% recovered
40]; 43 64%); (c) (i) CeCl3·7H2O, NaBH4, CH2Cl2, MeOH, −78 °C, 45 min;
(ii) pTsOH·H2O, CHCl3, 2 h (44 69%; 45 84%).
considered that competitive abstraction of hydrogen at the
activated benzylic ether positions in 12 and 13 by the thiyl
radical may be inhibiting efficient intramolecular hydroacyla-
tion to form the expected ketones 36 and 37. In accord with
this, the analogous carbocyclic ketone products are obtained
efficiently using this method24a and we have observed that
increasing steric hindrance by the introduction of further sub-
stitution at the benzylic ether position in 13 gives improved
conversion to the corresponding ketone using the thiyl radical
catalysed process.24b
Scheme 7 Reagents and conditions: (a) Bu3SnH, AIBN, benzene, reflux,
2–3 h (50 + 52 86%; 51 71%, 54 25%); (b) Ac2O, pyridine, DMAP, CH2Cl2,
1.5 h; (c) tert-dodecanethiol (3.0 equiv.), ACCN (1.5 equiv.), PhCl, 100 °C,
18 h (56 47%, 57 11%).
β-alkoxy acrylate systems described earlier (Table 1), the iso-
meric crotonate systems 40 and 41 showed no evidence of
undergoing competitive reduction and shorter reaction times
were required suggesting that the barrier to radical cyclization
may be less significant for 40 and 41.
We then sought to further explore both the O-stannyl ketyl
and acyl radical cyclization approach for the formation of
benzofuran systems having an appended γ-lactone ring
(Scheme 7). Such systems, when converted to benzoquinones,
should potentially be able to act as bioreductive alkylating
agents in a manner similar to that proposed for the benzo-
pyran systems (Scheme 1). The 6,5,5-ring systems in 50/51
resemble the ring system contained in mitomycin C 2 and may
be considered as a ‘hybrid’ of the mitomycin C 2 and pyrano-
naphthoquinone 3 pharmacophores.
We then turned to the isomeric benzaldehyde 40
(Scheme 5), prepared from salicylaldehyde 38,25 to further
investigate the acyl radical cyclization process. When 40 was
heated in the presence of thiol and initiator the ketone
product 4225 was isolated in 68% yield. Similarly, the
dimethoxy-substituted system 41 underwent acyl radical cycli-
zation efficiently to form ketone 43 in 64% yield. Complete
consumption of benzaldehyde 41 was evident after 20 h, sup-
porting the proposition that the presence of readily abstract-
able hydrogen at the benzylic ether position in previous
substrates 12 and 13 significantly hinders the efficiency of
these thiyl radical-catalysed reactions. As anticipated, ketones
42 and 43 could be stereoselectively reduced under Luche
conditions (CeCl3–NaBH4–MeOH) to form the corresponding
lactones 44 and 45 in 69% and 84% yields, respectively. The
presence of trans-alcohol products under these conditions was
not evident.
Direct formation of the γ-lactone-fused benzopyrans 44 and
45 should also be possible using the previously described
O-stannyl ketyl radical cyclization conditions. Thus, upon treat-
ment of benzaldehyde 40 with tributyltin hydride an insepar-
able 1.4 : 1 mixture of lactone 44 and alcohol 46 (Scheme 6)
was obtained in a combined 92% yield, in accord with pre-
vious reports.26 Incorporation of methoxy substituents onto
the aromatic ring in benzaldehyde 41 led to only a slight
change in the diastereoselectivity with 45 and 47 being iso-
lated in 39% and 40% yields, respectively. In contrast to the
Consistent with previous reports benzaldehyde 4827
(Scheme 7) underwent O-stannyl ketyl radical cyclization
efficiently to form an inseparable mixture of benzopyrans 50
and 52 in a combined 86% yield (50 : 52 34 : 66).28 Acetylation
of this mixture assisted separation of acetate 53 from lactone
50 to allow characterization of the products. The corres-
ponding dimethoxy benzaldehyde 49 also cyclized efficiently
to form cis-lactone 51 (71% yield) and trans-alcohol 54 (25%
yield). In contrast to formation of the corresponding benzo-
pyran systems (Table 1, entries 1 and 2) in which the incorpor-
ation of para-methoxy substituents led to the trans-product
being favoured, introduction of methoxy substituents onto
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products, whereas an unsubstituted ortho position (12, 22 and
28) leads to formation of a near equal mixture of cis (lactone)
and trans products. A similar trend, though less pronounced,
is observed for the isomeric crotonate systems 40 and 41.
During formation of benzofurans the opposite outcome is
observed, with the addition of methoxy groups onto the aryl
system favouring the formation of the cis-lactone product 51.
Furthermore, a complimentary approach that uses an acyl
radical cyclization of the same benzaldehyde substrates
followed by diastereoselective reduction of the resulting pyra-
nones has provided a more efficient method for the prepa-
ration of the required cis-lactone products. These approaches
have made available a series of benzopyran- and benzofuran-
fused γ-lactones that have been converted to benzoquinones 5,
58 and 60 and naphthoquinone 59, which are expected to have
potential as bioreductive alkylating agents.
Scheme 8 Reagents and conditions: (a) CAN, MeCN, H2O, 30 min
(93%); (b) PIFA, MeCN, H2O, 1–2 h (58 76%; 60 63%); (c) 1,3-dimethoxy-
1-trimethylsiloxybuta-1,3-diene, CH2Cl2, 1.5 h, then SiO2, air, 1 h (55%).
Acknowledgements
Financial support from the Australian Research Council
through the Centres of Excellence Scheme is gratefully
acknowledged. Professor Carl Schiesser, The University of
Melbourne, is acknowledged for useful discussions.
benzaldehyde 49 results in the cis-lactone 51 being formed in
preference to the trans-alcohol 54 (51 : 54 2.8 : 1).
Benzaldehydes 48 and 49 also underwent acyl radical cycli-
zation, with the unsubstituted system 48 only progressing to
approximately 30% conversion to ketone 55 after 18 h at
100 °C in chlorobenzene. The dimethoxy benzaldehyde 49,
however, was completely consumed after the same reaction
time to give ketone 56 and enol 57 in 47% and 11% yields,
respectively. Interconversion of ketone 56 and enol 57 was not
observed upon standing the purified materials in deuterated
chloroform for two days.
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