Visible light-mediated synthesis of (Spiro)anellated furans

Article abstract of DOI:10.1002/adsc.201300221

Visible light-mediated decarboxylation using N-acyloxyphthalimides as the source for carbon-centered radicals was applied for the synthesis of spirobutenolides. The utility of this approach is demonstrated with the formal synthesis of (S)-(+)-lycoperdic acid. Alternatively, 2,3-anellated furans can be obtained in a one-pot procedure via photocyclization following a regioselective semipinacol rearrangement.

Full text of DOI:10.1002/adsc.201300221

FULL PAPERS
DOI: 10.1002/adsc.201300221
Visible Light-Mediated Synthesis of (Spiro)anellated Furans
Georgiy Kachkovskyi,a,b Christian Faderl,a and Oliver Reisera,*
a
Institut fꢀr Organische Chemie, Universitꢁt Regensburg, Universitꢁtsstrasse 31, 93053 Regensburg, Germany
Fax : (+49)-941-943-4121; phone: (+49)-941-943-4631; e-mail: oliver.reiser@chemie.uni-regensburg.de
Institute of Bioorganic Chemistry and Petrochemistry of National Academy of Science of Ukraine, Murmanska str., 1,
b
02660, Kyiv, Ukraine
Received: March 14, 2013; Revised: May 23, 2013; Published online: August 13, 2013
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201300221.
Abstract: Visible light-mediated decarboxylation tion following a regioselective semipinacol rear-
using N-acyloxyphthalimides as the source for rangement.
carbon-centered radicals was applied for the synthe-
sis of spirobutenolides. The utility of this approach is
demonstrated with the formal synthesis of (S)-(+)-ly- Keywords: butenolides; green chemistry; photoredox
coperdic acid. Alternatively, 2,3-anellated furans can catalysis; semipinacol rearrangement; sensitization;
be obtained in a one-pot procedure via photocycliza- spirocyclization
Introduction
mediate, which led to the here reported development
of visible light-initiated radical spirocyclizations onto
Visible light-mediated photoredox catalysis is an furans starting from simple activated esters of carbox-
active research area of organic chemistry that meets ylic acids (Scheme 1).
the principles of green chemistry.[1] Several recent re-
Visible light-mediated redox transformations usual-
views[2] and publications[3] impressively document the ly require substrates with low activation energies for
potential for organic synthesis in this field. Another electron transfer, being expressed by their redox po-
“green” approach highlights the exploitation of re-
newable resources, and especially simple furans or
amino acids have been recognized as valuable starting
materials.[4] A central theme in our research is fo-
cused on the development of methodology that allows
the stereoselective dearomatization of furans or pyr-
roles to arrive at intermediates of relevance for fine
chemicals or natural products and analogues.[5] In this
study we demonstrate the application of visible light-
mediated photoredox-catalyzed carbon-carbon bond
forming reactions for the functionalization of furans,
leading either to butenolides or anellated furans.
Zard et al. reported the oxidative dearomatization
of furans via intramolecular radical spirocyclization
(Scheme 1),[6] utilizing xanthates as precursors for a-
keto radicals that are thermally generated in the pres-
ence of two equivalents of dilauroyl peroxide. We en-
visaged that a similar process should also be possible
via a visible light-mediated photoredox-catalyzed
pathway, and indeed, a great number of transforma-
tions have been recently developed by generating a-
keto radicals from a-halo ketones or epoxy ketones in
this way.[7] Nevertheless, we were looking for a more
general precursor to generate a reactive radical inter- Scheme 1. Oxidative furan spirocyclizations.
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Visible Light-Mediated Synthesis of (Spiro)anellated Furans
Results and Discussion
We started our investigation by screening catalysts
(Table 1) on the readily available model compound
1a.
Following the protocol developed by Okada et al,
that calls for the combination of catalytic amounts of
[12]
Ru
G
and 1.5 equivalents of 1-benzyl-l,4-di-
hydronicotinamide (BNAH) as sacrificial electron
donor, thus utilizing the reductive quenching cycle
Ru+/Ru2+, irradiation at 455 nm resulted in complete
consumption of 1a. However, rather than the desired
decarboxylated and spirocyclized product 2a only the
Scheme 2. Redox photodecarboxylation of N-(acyloxy)-
phthalimides.[9]
tentials, which results in the preferential formation of formation of a complex mixture characterized by mul-
electron-deficient radicals.[8] However, the generation tiple spots on TLC was observed (entry 1). On the
of alkyl radicals using this strategy was also reported: other hand in the absence of BNAH (thus attempting
Okada et al.[9] and very recently Overman et al.[10] to utilize the oxidative quenching cycle [Ru2+]*/Ru3+)
have demonstrated that O-acyl-N-hydroxyphthali- no reaction at all was observed under otherwise in-
mides could undergo cascade reductive decomposition dentical conditions (entry 2).
ꢀ
accompanied with N O bond cleavage followed by
Since the excited catalyst is assumed to act as a re-
decarboxylation (Scheme 2) The resulting alkyl radi- ductant, we next turned our attention to Cu-
cals could be trapped with various reagents (diselen-
(dap)2Cl[13a] [dap=2,9-bis(para-anisyl)-1,10-phenan-
ACHTUNGTRENNUNG
[9c]
N
or thiols[9d]
)
throline] that is known to follow an oxidative quench-
ing cycle in photoredox processes.[13] The desired
Applying this strategy, we planned to combine product 2a was indeed isolated in 40% yield
furans (furoic acid, furfural, furfuryl amine etc.) with (entry 3), but relatively high catalyst loading
amino acids, overall merging green chemistry princi- (5 mol%) and slow conversion led us seek further im-
ples utilizing renewable resources and visible light- provements. Among two types of iridium-based cata-
mediated photoredox catalysis.[11]
lysts tested, IrACHTUGNTRENNU(G ppy)2ACHTGNUTREN(NGNU dtbbpy)PF6 (dtbbpy=4,4’-di-tert-
butyl-2,2’-dipyridyl)[14] (entry 4) was found to be more
effective, resulting at a loading of 1 mol% in the com-
Table 1. Catalyst screening.[a]
Entry
Catalyst (mol%)
Ru(bpy)3Cl2 (2.5)
LED [nm]
Time [h]
Conversion (yield)[a] [%]
1[b]
2
3
G
455
455
420
455
420
420
455
–
24
24
24
8
15
45
8
8
8
18
100 (0)
0 (0)
Ru(bpy)3Cl2 (2.5)
Cu(dap)2Cl (5)
Ir(ppy)2A(dtbbpy)PF6 (1)
Ir(dF(CF3)ppy)2A(dtbbpy)PF6 (1)
ACHTUNGTRENNUNG
R
80 (40)
100 (61)
70 (–)
100 (43)
0 (0)
0 (0)
0 (0)
20 (0)
4
A
CHTUNGTRENNUNG
5
A
U
6[c]
7
perylene (15)
no catalyst
8[d]
9[e]
10[f]
Ir
Ir(ppy)
Ir(ppy)
G
(dtbbpy)PF6 (1)
455
455
[a]
Conversion was estimated by NMR; isolated yields after chromatography; Pht=phthalimid-2-yl).
In the presence of 1.5 equiv. BNAH.
In THF/water=8/1.
Without irradiation.
In anhydrous acetonitrile.
Without degassing.
[b]
[c]
[d]
[e]
[f]
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Georgiy Kachkovskyi et al.
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Table 2. Scope of the reaction.[a]
Entry
1
2
Time [h][b] Yield of 2 [%][c]
4
Overall yield of 4 without isolation of 2 [%][c]
8
61
35
84 (70)[d]
2
20
31
3
4
43
24
21
N/A
N/A
–
–
0[e]
5
6
18
18
50
N/A
N/A
–
–
55[f] (dr 1/1)
7
18
18
55
43
87
8
9
42[f] (dr 1/1)
18
18
38[f] (dr 3/2)
35[f] (dr 1/1)
53
–
10
N/A
[a]
Reaction conditions: 2 (0.1–2 mmol), Ir
(ppy)
E
[b]
[c]
diation with 1W LED (455 nm); Pht=phthalimid-2-yl. Full conversion time, monitored by TLC. Isolated yield after
[d]
chromatography. 10 mmol scale, 0.5 mol% catalyst. 60% conversion, 2d was not detected; acid 3 was the main prod-
[f]
uct. Combined yield for two diastereomers; ratio was estimated by NMR of crude.
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Visible Light-Mediated Synthesis of (Spiro)anellated Furans
Scheme 3. Vinylogous semipinacol rearrangement of 2.
plete conversion of 1a within 8 h, giving rise to 2a in obtained from one-carbon extended homologue 1g by
61% yield.
an analogous 6-exo-trig cyclization (entry 7).
In contrast, using Ir(dF
[dFACHTUNGTRENNUNG(CF3)ppy=2-(2,4-difluorophenyl)-5-trifluorome-
(CF3)ppy)
E
The competing reaction pathway is the formation
of the carboxylic acid 3, which could be considered to
thylpyridine][15] a much slower conversion was ob- arise from simple hydrolysis. However, since 1 is
served (entry 5). Perylene (entry 6), which shows stable under the reaction conditions in the dark and
a strong absorption at 435 nm (together with only based on the presence of N-phthalimide rather than
3 nm of Stokes shift) and has a promising redox po- N-hydroxyphthalimide in the reaction mixture after
tential in ground state[16] required a loading of irradiation, we assume that 3 is a product of a photo-
15 mol% and prolonged reaction time (45 h) in order process as well (see mechanistic discussion below,
to achieve a comparable yield (43%) of 2a. No con- Scheme 5).
version of 1a in the absence of catalyst, light or water
Besides the competing formation of 3 the moderate
was observed (entries 7–9), moreover, the presence of yields of the spiroanellated 2 were also due to the low
oxygen leads to slow consumption of starting materi- stability of the latter. Turning this to an advantage, 2
al, but no product formation was observed (entry 10). can be readily transformed by a vinylogous semipina-
(dtbbpy)PF6 as the best col[18] rearrangement to 4, in which selectively the
Having identified IrACHTNUTRGNENUG(ppy)2ACHTUNGTRENNUGN
catalyst for the transformation, we examined the acyl group undergoes a 1,2-shift with concurrent ref-
scope of the reaction (Table 2). The substitution pat- ormation of the furan moiety (Scheme 3).[19] The X-
tern on nitrogen in 1 played a crucial role, revealing ray structure analysis of 5a unambiguously proved the
that sterically bulky groups favour the desired spiro- course of this reaction (CCDC 944711, these data can
cyclization to 2, being in agreement with the Thorpe– be obtained free of charge from The Cambridge Crys-
Ingold[17] effect (entries 1–4). Substituents R1 and R2 tallographic Data Centre via www.ccdc.cam.ac.uk/da-
are tolerated well in the b-amino acid part of 1, open- ta_request/cif).
ing the possibility for stereocontrol in the spirocycli-
Starting from 1 the overall process, that is, photo-
zation process (entries 8–10). However, the two dia- cyclization and rearrangement can be carried out in
stereomers of 2h and 2j were formed with little pref- a one-pot process, which gives rise to 4 in improved
erence for one over the other, pointing towards an yields with respect to 2 (Table 2). The efficiency of
early transitions state in the cyclization step. Besides this protocol was demonstrated on a 10-mmol scale
the spiroanellation of pyrrolidinones, the formation of reaction giving a 70% isolated yield of 4a after 48 h
pyrrolidines is also possible (entry 5). Water was gen- of irradiation and only 0.5 mol% loading of the iridi-
erally used as the terminal nucleophile, however, the um catalyst (entry 1).
use of methanol leading to acetal 2f is also possible
Alternatively, the spirolactols 2 could be easily oxi-
(entry 6). Moreover, the 6-spiroanellated 2g could be dized (not shown) following a literature precedent[20]
into butenolides 10, a motif present in various natural
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Georgiy Kachkovskyi et al.
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Table 3. Butenolide synthesis.[a]
Entry
1
Product[b]
Yield [%][c]
dr[d]
52
N/A
2
3
61[e]
2/1
3/2
51[e]
4
5
50[f]
2/1
19[e]
2/1
6
48
N/A
[a]
Reaction conditions: 8 (0.1–1 mmol), IrACHTUNGTRNENG(U ppy)2ACHUTNTGREN(NUGN dtbbpy)PF6 (1 mol%) in CH3CN/H2O=9/1 (0.05M concentration) and 18 h
irradiation with 1W LED (455 nm); Pht=phthalimid-2-yl.
Structure of the major diastereomer is shown.
Isolated yield after chromatography.
Ratio was estimated by NMR of crude.
Combined yield for two separable diastereomers.
Combined yield for two inseparable diastereomers.
[b]
[c]
[d]
[e]
[f]
products.[21] However, using the strategy presented diastereomer formed in 10c (entry 3) (CCDC 944712,
here, a more direct approach to 10 could be devel- these data can be obtained free of charge from The
oped (Table 3). Starting from 5-bromofuroic acid de- Cambridge Crystallographic Data Centre via
rivatives 8 instead of 2, the analogous photocycliza- www.ccdc.cam.ac.uk/data_request/cif.
tion led to the bromolactols 9, which collapse upon
An application of this strategy is demonstrated with
elimination of HBr directly to the desired butenolides the formal synthesis of (S)-(+)-lycoperdic acid (16,
10 with overall improved yields compared to the for- Scheme 4).[22,23] The key intermediate 13, readily avail-
mation of lactols 2. Again, if the b-amino acid chain able in four steps from commercially available 5-bro-
in 8 possesses substituents, 10 is formed as a separable mofurfural (11) and (S)-aspartic acid dimethyl ester
(with the exception of 10d) mixture of two diastereo- (12) was photocyclized under the established condi-
mers, which could be assigned by NOESY NMR tions to give rise to butenolide 14 in 40% yield. Upon
measurements and by an X-ray structure of the minor hydrogenation a 2:1 mixture of the diasteromeric bu-
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Visible Light-Mediated Synthesis of (Spiro)anellated Furans
Scheme 4. Formal synthesis of (S)-(+)-lycoperdic acid.
tyrolactones 15 was obtained, which can be trans- thermore, the observed influence of the N-substituent
formed to (S)-(+)-lycoperdic acid by the route de- in 1 on the yield and reaction time (Table 2, entries 1–
scribed by Yoshifuji et al.[23a]
4) points towards the necessity of a suitable confor-
The process described here requires both irradia- mation being populated to a significant extent for the
tion and a photocatalyst, and Ir(ppy)2A(dtbbpy)PF6 was IET to take place. Noteworthy, a similar sensitization
A
CHTUNGTRENNUNG
identified to be most efficient. Analyzing the reducing of phthalimide from the triplet state of acetone was
power of the latter after excitation, (*Ir3+!Ir4+
see the Supporting Information)[24] reveals that an ox-
=
proposed by Griesbeck for the explanation of
ꢀ0.96 V vs. SCE)[15] with that of 1a (ꢀ1.25 V vs. SCE, a photo-Kolbe type transformation.[28,30]
However, as estimated from the emission spectrum
idative quenching cycle for the title transformation is the triplet energy of 1 is considerably higher (approx.
thermodynamically difficult.[8] Likewise, the potential 3.1 eV)[28]
than
that
of
IrACTHUNGTERNNU(G ppy)2CAHTUNGTRENN(UNG dtbbpy)PF6
2+
of *Ru
G
a fluorescence resonance
range to induce an PET to 1a (Table 1, entry 2). energy transfer (FRET or sensitization) also difficult.
Moreover, due to the absence of a suitable electron Also, the formation of by-product acid 3, which in ab-
donor that could act as a reductant for these catalysts, sence of N-hydroxyphthalimide cannot arise from
a reductive quenching cycle can also be ruled out. simple hydrolysis but must involve a photoreduction
Indeed, Sammis et al.[25] recently showed that N-al- process, is not clear. Finally, it must be pointed out
koxyphthalimides can be cleaved by visible light irra- that Cu
diation of Ru(bpy)3A(PF6)2 in the presence of a sacrifi-
ACHTUNGRTENN(UNG dap)2Cl (cf. Table 1) would have a suitable
N
CHTUNGTRENNUNG
cial electron donor, thus utilizing the reductive
quenching cycle [Ru+/Ru2+ (ꢀ1.35 V (vs. SCE)[8]] (cf.
Table 1, entry 1).[26]
Nevertheless, spectroscopic measurements on Ir-
ACHTUNGTRENNUNG(ppy)2ACHTUNGTRENNUNG(dtbbpy)PF6 revealed that the fluorescence of
the catalyst could be quenched by 1 suggesting an
electron or energy transfer from the excited triplet
state of the catalyst. Since the redox pathway is con-
sidered to be difficult, we propose that Ir
ACHTUNGERTN(NUNG ppy)2
ACHTUNGTRENNUNG
from its excited state to the phthalimide moiety in
1 generating its triplet state 17 with a significantly
higher redox potential.[28] Possessing a suitable confor-
mation (17’’), intramolecular electron transfer (IET)
from a furan moiety could occur[29] to give rise to the
intermediate 18, which after protonation undergoes
ꢀ
N O bond homolytic cleavage, followed by decarbox-
ylation and radical recombination[30] leading to the
spirocation 20 (Scheme 5). The sensitization mecha-
nism is supported via direct excitation of the phthali-
mide moiety by UV irradiation of 1a, giving rise to 2a
in 45% yield (see the Supporting Information). Fur- Scheme 5. Mechanistic discussion.
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Georgiy Kachkovskyi et al.
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(dap)+/Cu
E
lution was degassed using three freeze-pump-thaw cycles
and stirred at room temperature at a distance of approxi-
redox potential [*Cu
SCE][13a] to allow a PET process to 1, and while
indeed the conversion of 1a to 2a was achieved with
this catalyst (Table 1, entry 3), its efficiency was much
mately 2 cm from a blue light emitting diode (LED) (lmax
=
455 nm) for 18 h (TLC monitoring). After the starting mate-
rial had been consumed the solvent was removed under re-
duced pressure. The residue was dissolved in TFA (1 mL/
0.1 mmol). The resulting solution was stirred at room tem-
perature overnight, concentrated, dissolved in DCM,
washed with NaHCO3 solution, dried over Na2SO4 and con-
centrated again. The product was purified on silica to afford
4h; yield: 263 mg (87%); Rf =0.10 (DCM/MeOH=30/1).
1H NMR (400 MHz, CDCl3): d=7.34 (d, J=2.0 Hz, 1H),
6.71 (d, J=2.0 Hz, 1H), 6.04 (br.s, 1H), 3.62 (dd, J=12.0,
5.8 Hz, 1H), 3.28 (dd, J=12.0, 9.0 Hz, 1H), 3.25–3.16 (m,
1H), 1.32 (d, J=6.8 Hz, 3H); 13C NMR (100 MHz, CDCl3):
d=165.6, 163.3, 142.6, 114.1, 107.8, 47.7, 28.9, 15.0; HR-MS
(ESI-MS): m/z=152.0709, calculated for C8H9NO2 [M+H]:
152.0706.
lower than that of IrACHTUNRGTENNUG(ppy)2ACHTUNGTRENN(UGN dtbbpy)PF6.
Conclusions
In conclusion, we have developed visible light-mediat-
ed transformations of activated carboxylic acids cata-
lyzed by IrACHTUNGTRENNUNG(ppy)2ACHTUTGNREN(NGUN dtbbpy)PF6, presumably acting as
a sensitizer. The methodology was applied to the syn-
thesis of spirobutenolides, demonstrating a formal
total synthesis of (S)-(+)-lycoperdic acid. Additional-
ly, an acid-catalyzed vinylogous semipinacol rear-
rangement of the initially formed spirobutenolides
was discovered, allowing the anellation of cyclic lac-
tams onto a furan moiety.
Typical Procedure for the Spirocyclization of 8 to 10;
Synthesis of 7-tert-Butyl-1-oxa-7-azaspiroACHTNUTRGNE[UNG 4.5]dec-3-
ene-2,6-dione (10f)
A 50-mL flask equipped with a magnetic stir bar and
septum was charged with 8f (477 mg, 1.00 mmol) and [Ir-
Experimental Section
ACHUNTGRENN(UG dtb-bpy)ACHTUNGTNER(NUGN ppy)2]PF6·H2O (9.3 mg, 0.01 mmol) in 20 mL of
Typical Procedure for the Photodecarboxylative
Spirocyclization of 1 to 2; Synthesis of 7-tert-Butyl-2-
hydroxy-1-oxa-7-azaspiroACTHNUTRGNEUGN[4.4]non-3-en-6-one (2a)
acetonitrile/water 9/1 mixture (0.05M concentration). The
solution was degassed using three freeze-pump-thaw cycles
and stirred at room temperature at a distance of approxi-
mately 2 cm from a blue light emitting diode (LED) (lmax
=
A 50-mL flask equipped with a magnetic stir bar and
455 nm) for 18 h (TLC monitoring). After the starting mate-
rial had been consumed the solvent was removed under re-
duced pressure. The residue was purified on silica to afford
10f; yield: 107 mg (48%); Rf =0.21 (EtOAc/hexanes=1/1).
1H NMR (300 MHz, CDCl3, anomeric mixture ~9/1): d=
7.34 (d, J=5.7 Hz, 1H), 6.17 (d, J=5.7 Hz, 1H), 3.56–3.47
(m, 1H), 3.46–3.36 (m, 1H), 2.18–2.02 (m, 3H), 2.01–1.87
(m, 1H), 1.42 (s, 9H); 13C NMR (100 MHz, CDCl3): d=
172.4, 164.4, 155.9, 122.0, 87.8, 58.6, 44.4, 31.3, 27.9, 20.5;
HR-MS (ESI-MS): m/z=224.1285, calculated for C12H17NO3
[M+H]: 224.1281.
septum was charged with 1a (484 mg, 1.26 mmol) and [Ir-
ACHTUNGTRENNUNG(dtb-bpy)ACHTUNGTRENNUNG(ppy)2]PF6·H2O (12.0 mg, 0.013 mmol) in 25 mL of
acetonitrile/water 9/1 mixture (0.05M concentration). The
solution was degassed using three freeze-pump-thaw cycles
and stirred at room temperature at a distance of approxi-
mately 2 cm from a blue light emitting diode (LED) (lmax
=
455 nm) for 8 h (TLC monitoring). After the starting materi-
al had been consumed the solvent was removed under re-
duced pressure. The residue was purified on silica to afford
2a; yield: 162 mg (61%); Rf =0.20 (DCM/Et2O=5/1).
1H NMR (300 MHz, CDCl3, anomeric mixture ~9/1): d=
6.26 (d, J=6.7 Hz, 0.1H) and 5.88 (d, J=11.5 Hz, 0.9H),
6.13 (dt, J=5.8, 0.5 Hz, 0.9H) and 6.02 (dd, J=5.8, 1.0 Hz,
0.1H), 5.97 (dd, J=5.8, 1.0 Hz, 0.1H) and 5.93 (dd, J=5.8,
0.5 Hz, 0.9H), 4.36 (d, J=11.5 Hz, 0.9H), 4.19 (br.s, 0.1H),
3.57 (ddd, J=10.0, 7.6, 4.5 Hz, 1H) and 3.55–3.48 (m, 0.1H),
3.42 (dt, J=10.0, 7.6 Hz, 1H) and 3.41–3.32 (m, 0.1H), 2.32–
2.17 (m, 2H), 1.400 (s, 8H) and 1.395 (s, 1H); 13C NMR
(100 MHz, CDCl3, major anomer): d=173.2, 132.2, 131.0,
102.7, 92.4, 55.1, 42.2, 28.3, 27.5; HR-MS: (ESI-MS): m/z=
212.1281, calculated for C11H17NO3 [M+H]: 212.1281. In ad-
dition, 176 mg (95%) of phthalimide (Rf =0.48, DCM/
Et2O=10/1) were isolated.
Acknowledgements
This work was supported by the Alexander von Humboldt
Foundation (fellowship for GK) and the DFG (Graduierten-
kolleg 1626 Photocatalysis). We are grateful to Dr. M. Zabel
and S. Stempfhuber for carrying out X-ray structure analyses.
Helpful comments of the referees during the review process
regarding the mechanistic discussion are gratefully acknowl-
edged.
Typical Procedure for the One-Pot Transformation of
1 to 4; Synthesis of 7-Methyl-6,7-dihydrofuroACTHNUGTRNEUNG[3,2-
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Visible Light-Mediated Synthesis of (Spiro)anellated Furans
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