Dual Lewis Acid/Photoredox-Catalyzed Addition of Ketyl Radicals to Vinylogous Carbonates in the Synthesis of 2,6-Dioxabicyclo[3.3.0]octan-3-ones

Article abstract of DOI:10.1021/acs.orglett.8b02442

A combined Lewis acid/photoredox catalyst system enabled the intramolecular umpolung addition of ketyl radicals to vinylogous carbonates in the synthesis of 2,6-dioxabicyclo[3.3.0]octan-3-ones. This reaction proceeded on a variety of aromatic ketones to provide THF rings in good yield (up to 95%). Although diastereoselectivity was found to be modest (1.4-5:1) for the C-C bond forming reaction, the minor diastereomers were converted to 2,6-dioxabicyclo[3.3.0]octan-3-ones by an efficient Lewis acid-mediated epimerization cascade in up to 90% yield.

Full text of DOI:10.1021/acs.orglett.8b02442

Letter
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/OrgLett
Dual Lewis Acid/Photoredox-Catalyzed Addition of Ketyl Radicals to
Vinylogous Carbonates in the Synthesis of 2,6-
Dioxabicyclo[3.3.0]octan-3-ones
Nicholas J. Foy, Katherine C. Forbes, Anne Marie Crooke, Maxwell D. Gruber, and Jeffrey S. Cannon*
Department of Chemistry, Occidental College, Los Angeles, California 90041, United States
*
S Supporting Information
ABSTRACT: A combined Lewis acid/photoredox catalyst
system enabled the intramolecular umpolung addition of ketyl
radicals to vinylogous carbonates in the synthesis of 2,6-
dioxabicyclo[3.3.0]octan-3-ones. This reaction proceeded on
a variety of aromatic ketones to provide THF rings in good
yield (up to 95%). Although diastereoselectivity was found to
be modest (1.4−5:1) for the C-C bond forming reaction, the minor diastereomers were converted to 2,6-dioxabicyclo[3.3.0]-
octan-3-ones by an efficient Lewis acid-mediated epimerization cascade in up to 90% yield.
etrahydrofuran fused γ-lactones, specifically 2,6-
dioxabicyclo[3.3.0]octan-3-ones (1), are a prevailing
found as key components of other natural products such as the
2 3
T
pallambins and the rubriflordilactones. Of particular interest is
the presence of this moiety in both polyketide and terpenoid
natural products. The prevalence and congested complexity of
this core structure call for efficient methods for its construction.
The most common method for the synthesis of these bicyclic
structures is by conjugate addition of a pendant alcohol to an
structural feature in a number of biologically active natural
products (Figure 1). For instance, the plakortones are a large
family of natural products with applications in cardiac
arrhythmia, and it is hypothesized that the oxabicylic “head” is
1
central to its function. More highly substituted forms have been
4
−11
already constructed unsaturated γ-lactone.
Other assemblies
of this moiety include palladium-catalyzed carbonylative
1
2−16
17
oxidation,
iodolactonization of a dihydrofuran, and by
18,19
ionic rearrangement cascades.
These methods focus
primarily on the formation of C-O bonds in the core structure;
methods that center around C-C bond formation remain
lacking.
Although these methods provide access to the key oxabicyclic
structure, they require at least one stereocenter of the ring fusion
to be established prior to construction of the ring system. We
anticipated that the simultaneous construction of the ring-fusion
C-C and the lactone C-O bonds of the bicyclo[3.3.0]octane
would greatly simplify their synthesis by generating both of these
stereocenters in a single step (Figure 1). Stereoselectivity would
then be under reagent control, and it would allow for
deconstruction to a highly simplified linear precursor that is
readily available from β-hydroxy ketones. We hoped to employ
the intramolecular umpolung addition of a ketyl radical from a
linear vinylogous carbonate (2) to achieve the direct synthesis of
the ring-fusion C-C bond. Ketyls are powerful nucleophilic
radical intermediates that have been used widely in C-C bond
20,21
forming processes.
However, the challenging single electron
reduction of ketones has made ketyl radical chemistry difficult to
render catalytic. To date, addition of ketyl radicals to vinylogous
2
2−24
carbonates has been limited to tin hydride,
or samarium-
25−27
(
II) mediated processes.
These reagents can be toxic or
Figure 1. Example of bioactive natural products bearing 2,6-
dioxabicyclo[3.3.0]octan-3-one cores.
Received: July 31, 2018
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b02442
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
highly sensitive to adventitious water and O , further reducing
their practical use. Although samarium(II) is a convenient
coordination to a Lewis acid (4) or by protonation by a Brønsted
acid. The resulting ketyl radical (5) would add to the pendant
vinylogous carbonate to provide THF 6 followed by H atom
transfer from a terminal reductant such as Hantzsch ester
(HEH) to provide 3-hydroxytetrahydrofurans such as 7. Syn-
THF 7 could cyclize to the corresponding 2,6-
dioxabicyclo[3.3.0]octan-3-one.
2
reagent for this transformation, reduction of samarium(III) to
28−31
the active samarium(II) is rare in catalysis.
We aimed to
effect this transformation with mild, catalytic conditions that
would avoid toxic intermediates or stoichiometric transition
metals.
Photoredox catalysis has enabled a growing and powerful
family of reactions that harvest visible light to achieve often-
Vinylogous carbonate 3a was chosen for the optimization of
THF synthesis (Table 1). Initial studies began by investigating
32−34
challenging electron transfer reactions.
The combination of
+
2
a
photoredox catalysts such as Ru(bpy)3 and Ir(ppy) with acids
Table 1. Discovery and Optimization of THF Synthesis
3
has enabled the mild reduction of carbonyl derivatives to the
3
5−37
corresponding ketyl. Recent reports by Knowles,
Ruep-
3
8−40
41−43
ing,
and others
have shown that activated carbonyl
compounds can accept an electron from the reduced form of the
photocatalyst. These catalyst systems provide an exciting
opportunity for the reductive umpolung reactivity of ketones
and aldehydes in C-C bond forming reactions. In these systems,
a Brønsted or Lewis acidic cocatalyst lowers the effective
reduction potential of the carbonyl, enabling electron transfer
b
c
entry
photocat
acid (mol %)
solvent
yield (%) dr 8a:9a
d
1
Ir(ppy)3
Ir(ppy)3
p-TSA (10)
p-TSA (10)
p-TSA (10)
p-TSA (20)
p-TSA (20)
THF
THF
THF
THF
MeCN
MeCN
MeCN
DCM
DMF
MeCN
MeCN
MeCN
28
22
21
41
44
51
75
49
59
2.2:1
1.4:1
1.4:1
1.7:1
1.7:1
2.1:1
2.4:1
2.1:1
2.4:1
e
2
e
3
Ru(bpy)3
Ru(bpy)3
Ru(bpy)3
Ru(bpy)3
Ru(bpy)3
Ru(bpy)3
Ru(bpy)3
Ru(bpy)3
Ru(bpy)3
4
5
6
7
8
9
4
4,45
from photoexcited transition metal complexes.
This
approach could serve as a general catalytic strategy for the
reductive umpolung reactions of carbonyls in place of systems
requiring full equivalents of transition metal.
Inspired by these recent studies, we were interested in their
application to the cyclization of γ-ketovinylogous carbonates
SnCl (20)
2
La(OTf) (40)
3
La(OTf) (40)
3
La(OTf) (40)
3
g
10
11
12
trace
trace
21
nd
nd
nd
(
2). We anticipated some additional challenges for the proposed
f
g
La(OTf) (40)
3
substrates relative to previously reported reactions. The
sensitivity of the vinylogous carbonate to hydrolysis under
acidic conditions were of primary concern. Additionally,
vinylogous carbonates are much more sluggish electrophiles
than α,β-unsaturated esters. In light of these two considerations,
a full re-evaluation of the reaction conditions was warranted.
The proposed catalytic cycle for the synthesis of furan-fused
lactones 1 involves cooperation between an acid and a transition
metal photocatalyst (Scheme 1). In this cycle, photoexcited
g
La(OTf) (40)
3
a
0
.2 mmol 3a, 2 mol % photocatalyst, 3 equiv HEH, [3a] = 0.05 M.
b
c
Combined yield of 8a and 9a after chromatography. Determined by
d
relative yields of isolated material after chromatography. 2,3-
Dihydro-2-phenylbenzothiazole (1.5 equiv) used in place of HEH.
1.5 equiv of HEH. Conducted in the absence of light. Not
e
f
g
determined.
proton-coupled electron transfer (PCET) to β-keto vinylogous
carbonate 3a. Under the conditions reported by Knowles,
cyclization to 8/9a was observed in low yield (entry 1). It was
+
2
Ru(bpy)3 would be reduced by a terminal reductant to provide
the super-reductant Ru(bpy) . This complex would then
4
+
3
found that 2,3-dihydro-2-phenylbenzothiazole was too active as
a terminal reductant, and the predominant side product was the
direct reduction of the ketone to the alcohol. Changing to
Hantzsch ester (HEH) was found to reduce this undesired
reactivity (entry 2). Although changing reductants did not
significantly impact the yield, the mass balance in this case was
transfer an electron to a ketone that has been activated by
Scheme 1. Proposed Dual-Catalytic Cycle
+
2
now found to be predominantly unreacted 3a. Ru(bpy)3
proved to be the optimal photoredox catalyst for this process
entry 3). Increasing the amount of Hantzsch ester and acid
(
catalyst dramatically improved the yield (entry 4). After a survey
of protic acids, it was found that Lewis acids provided superior
conversion, with SnCl and La(OTf) having the best perform-
2
3
ance (entries 5−7). Acetonitrile was determined to be the
optimal solvent (entries 8 and 9). Ultimately, 2 mol % of
photocatalyst, combined with 40 mol % of Lewis acidic
La(OTf) in acetonitrile with 3 equiv Hantzsch ester irradiated
3
with blue LED light proved to be the optimal conditions (entry
7). Control experiments proved that the combined action of the
photocatalyst, Lewis acid, and light to be necessary for sufficient
reactivity (entries 10−12). Interestingly, in the absence of
photocatalyst, a small amount of cyclization was observed (entry
1
2). It is thought that the photoexcited state of Hantzsch ester
(
ca. −2.0 V) could directly reduce activated complex 4, albeit at
+2
3
46
a reduced rate than photoexcited Ru(bpy)
. Although it was
anticipated that Lewis acids might preferentially template a
B
DOI: 10.1021/acs.orglett.8b02442
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
single diastereomeric transition state, no experimental con-
ditions were found that significantly improved the diaster-
eoselectivity of the C-C bond formation.
electron-donating and electron-withdrawing aromatic substitu-
ents (entries 2−4). α-Substituted ketones proved to be excellent
substrates for this reaction (entries 5−8), providing the highest
yields of cyclized products 8 and 9. Interestingly, these
substituents also had an effect on the diastereoselectivity of
the reaction. Larger substituents increasingly favored the anti-
diastereomer, with benzyl substrate 3g completely inverting the
selectivity. Furthermore, the diastereoselectivity relative to the
4-position was typically high, with only small amounts of other
In general, we found vinylogous carbonates 3 to be sluggish to
react compared to other α,β-unsaturated carbonyl electrophiles.
These substrates therefore required up to 48 h to achieve full
consumption of starting material. However, the vinylogous
carbonates were surprisingly stable to the reaction conditions,
with no byproducts of carbonate hydrolysis observed, and
predominantly starting material (3) was recovered from slow
reactions. Additives that have been found to be beneficial for
combined photoredox/Lewis acid catalysts, such as 2,2′-
1
diastereomers visible by H NMR. The alkene and aromatic
functionality of 3f and 3g were also well-tolerated (entries 7 and
8), and no products of radical addition to these functional
groups were observed. Tetralone derivative 3h provided
tetracyclic products 8h and 9h in good yield. Salicylate-derived
vinylogous carbonate was also cyclized, however the product
readily aromatized under the acidic conditions to provide
benzofuran 10 (entry 9). It is also important to note the general
stability of the resulting 3° benzylic alcohol products to the
reaction conditions, which underscores their mild nature.
Overall, substituted tetrahydrofurans could be obtained in
good yields from a variety of linear vinylogous carbonates.
Although the diastereoselectivities of the C-C bond formation
remained modest, a strategy for converting the minor
diastereomer to the desired lactone was developed (Scheme
4
1
bipyridyl, had little effect on reactivity in our case. Although
it is possible that the enhanced reactivity of Lewis acids when
compared to Brønsted acids is due to additional Lewis acid
activation of the vinylogous carbonate, we have no additional
evidence to support this hypothesis.
A number of vinylogous carbonates were investigated in this
THF synthesis (Table 2). The reaction could be scaled to 1
mmol of 3a without a significant impact on overall yield (entry
1
). Aromatic substitution did not dramatically impact the yield
or selectivity of cyclization, and the reaction was tolerant of
a
Table 2. Scope of Ketyl Cyclization
2
). It was initially proposed that epimerization could proceed
Scheme 2. Epimerization of Tetrahydrofurans 9 to Lactones 8
through β-elimination of the THF oxygen atom, attempts to
epimerize 9a under basic conditions were met with only
47
moderate success. It was eventually found that Lewis acids
were capable of mediating the desired transformation, with
SnCl performing the best. Under the optimized conditions, 9a
2
was converted to 2,6-dioxabicyclo[3.3.0]octan-3-one 8a in the
presence of SnCl and 5 Å molecular sieves in high yield. The
2
Lewis acid likely ionizes the alcohol to generate a benzylic
cation, ablating the C3 stereocenter, or the stereocenter is
inverted by anchimeric assistance from the neighboring ester
(i.e., 11). Lactone 8 would then be generated either by
lactonization of an epimerized alcohol or by loss of ethanol from
11. Sequestration of ethanol by molecular sieves then drives the
equilibrium to 2,6-dioxabicyclo[3.3.0]octan-3-one 8.
a
0
.20 mmol 3, 2 mol % Ru(bpy) (PF ) , 40 mol % La(OTf) , 3 equiv
3 6 2 3
b
HEH, [3] = 0.05 M, 48 h. Determined by relative yields of isolated
material after chromatography. 1 mmol 3a. Configuration of major
4
9
c
d
-substituent diastereomer determined by 1D NOE analysis of 8g and
g, others assigned by analogy. Ratio of 8:4-epi-8:9:4-epi-9
e
C
DOI: 10.1021/acs.orglett.8b02442
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
This epimerization approach proved to be general for the
conversion of the anti cyclization diastereomer (9) to the
corresponding 2,6-dioxabicyclo[3.3.0]octan-3-one in good
yield. Furthermore, diastereomeric mixtures of 9 could be
converted to a single isomer of 8 using these conditions.
Although electronic substitution of the aromatic ring affected
the rate, the yield did not suffer. Overall, this two-step process
renders this synthesis of 2,6-dioxabicyclo[3.3.0]octan-3-ones
from vinylogous carbonates 3 to be highly diastereoselective.
In conclusion, a new dual Lewis acid/photoredox catalyst
system has been developed for the umpolung reactivity of
ketones. This method has been specifically applied to the
intramolecular addition to vinylogous carbonates to provide
polysubstituted tetrahydrofurans in good yield. Although the
diastereoselectivity of the C-C bond formation was modest, a
strategy to convert the minor anti diastereomer to the bicyclic
lactone product was implemented. Overall, this two-step
procedure provides ready access to 2,6-dioxabicyclo[3.3.0]-
octan-3-ones from a linear precursor where the relative
stereochemistry of the product can be controlled.
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ASSOCIATED CONTENT
■
*
S
Supporting Information
(
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.8b02442.
(12) Semmelhack, M. F.; Hooley, R. J.; Kraml, C. M. Synthesis of
1
Experimental details, NOE studies of 8g and 9g, H and
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C NMR spectra of new compounds (PDF)
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14) Hayes, P. Y.; Chow, S.; Rahm, F.; Bernhardt, P. V.; De Voss, J. J.;
AUTHOR INFORMATION
■
Kitching, W. Synthesis of the Sponge-Derived Plakortone Series of
Corresponding Author
E-mail: jcannon@oxy.edu
Bioactive Compounds. J. Org. Chem. 2010, 75, 6489.
(
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*
ORCID
4
Jeffrey S. Cannon: 0000-0002-3407-2235
Author Contributions
(16) Ebner, C.; Carreira, E. M. Pentafulvene for the Synthesis of
Complex Natural Products: Total Syntheses of (±)-Pallambins A and
B. Angew. Chem., Int. Ed. 2015, 54, 11227.
The manuscript was written through contributions of all
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ACKNOWLEDGMENTS
■
This work was financially supported by the Donors of the
Petroleum Research Fund (57208-UNI1), which is adminis-
tered by the American Chemical Society; fellowships from the
Kenneth T. and Eileen L. Norris Foundation (AMC); the ACS
Division of Organic Chemistry (KCF); and the Occidental
College Undergraduate Research Center. NMR data was
acquired on instruments purchased with a grant from the NSF
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DOI: 10.1021/acs.orglett.8b02442
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