From simple furans to complex nitrogen-bearing aromatic polycycles by means of a flexible and general reaction sequence initiated by singlet oxygen

Article abstract of DOI:10.1002/chem.201301571

All-in-one: A new and general one-pot reaction sequence initiated by singlet oxygen that transforms simple furan substrates into complex nitrogen-bearing aromatic polycycles having all the structural features of a number of important natural products (for example, the erythrina alkaloids; see scheme, RB=rose Bengal) is reported. The reaction sequence itself uses mild conditions and has wide functional group tolerance. Copyright

Full text of DOI:10.1002/chem.201301571

COMMUNICATION
DOI: 10.1002/chem.201301571
From Simple Furans to Complex Nitrogen-Bearing Aromatic Polycycles
by Means of a Flexible and General Reaction Sequence Initiated
by Singlet Oxygen
Dimitris Kalaitzakis, Tamsyn Montagnon, Eirini Antonatou, Nuria Bardajꢀ, and
Georgios Vassilikogiannakis*[a]
Dedicated to Professor Michael Orfanopoulos on the occasion of his 65th birthday
Employing one-pot reaction sequences in synthesis has
proven to be a powerful way to improve efficiency and ach-
ieve rapid and impressive increases in molecular complexi-
ty.[1] Herein, we report a new and general reaction sequence
initiated by singlet oxygen that transforms simple furan sub-
strates into complex nitrogen-bearing aromatic polycycles
that have all the structural features of a number of impor-
tant natural products (for example; the erythrina alkaloids,[2]
structure J, Scheme 1). The procedure itself uses mild condi-
tions and has wide functional group tolerance.
At the heart of this new method is the generation of an
N-acyliminium ion (NAI, see I, Scheme 1) in a novel way
(A!I, Scheme 1), and, afterwards, in the same pot, its reac-
tion with a pendent aromatic nucleus (I!J) introduced ear-
lier in the reaction sequence. Investigating the diverse reac-
tions of NAIs has long been an extremely productive field
of research,[3] and, in particular their reaction with aromatic
nucleophiles (a sub-group of reactions falling under the
broader Pictet–Spengler[4] banner, I!J) has afforded a vari-
ety of new ways to access complex target skeletons. Many
methods have been reported for the stepwise assembly of a
NAI precursor and its subsequent reactions with a pendent
aromatic nucleus;[3] recent noteworthy examples employing
a diverse range of strategies to accomplish this overall goal
attest to the continuing attraction of the chemistry.[5–11]
Clearly, however, such an approach becomes all the more
powerful when the whole process is included in a one-pot
reaction sequence. This difficult-to-attain goal has only been
achieved relatively rarely, because it requires that the condi-
tions for assembly of the NAI precursor must be the same
as, or, at least, compatible with, those for NAI generation
and its subsequent reaction. The most common approach
successful in attaining an “all-in-one” reaction sequence is
Scheme 1. Summary of mechanistic rationale for proposed one-pot reac-
tion sequence.
[a] Dr. D. Kalaitzakis, Dr. T. Montagnon, E. Antonatou, N. Bardajꢀ,
Prof. Dr. G. Vassilikogiannakis
inspired by the pioneering work of Meyers,[12] wherein the
acid-catalysed condensation of a 4-keto acid (or 4-keto
ester) with an appropriately substituted amine forms, and,
then reacts with an NAI in situ; recent examples have origi-
nated from the laboratories of Dixon (using chiral polyphos-
phoric acids),[13] Domꢀnguez,[14] Fokas,[15] Jida (using micro-
Department of Chemistry, University of Crete
Vasilika Vouton, 71003, Iraklion, Crete (Greece)
Fax : (+30)2810-545166
E-mail: vasil@chemistry.uoc.gr
Homepage: www.chemistry.uoc.gr/vassilikogiannakis
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201301571.
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wave assistance),[16] Padwa[17]
and Tietze.[18] Modifications to
the dielectrophile substrate (the
keto acid), such as its replace-
ment with a d-chloro-a,b-diketo
ester,[19] or a 2,3-dioxopent-4-
enoate,[20] have also yielded
some nice sequences. In a con-
trasting approach, transition-
metal catalysis has also been
successfully employed; for ex-
ample, Liuꢂs gold-catalysed
indole annulation,[21] or the im-
pressive rhodium-catalysed hy-
droformylation sequences used
by Mann[22] and Chiou.[23]
We began our own investiga-
tion from the premise that the
4-keto acid component used
might be replaced by an alter-
native 1,4-dielectrophile (C,[24]
Scheme 1) arising readily from
the photooxygenation in metha-
nol of a simple and readily ac-
cessible furan using singlet
oxygen[25] (Scheme 1). Inter-
mediate C might be intercepted
by an amine bearing a pendant
aromatic nucleus (C!H).[26] It
is important to note that the
nature and reactivity of the die-
lectrophile C may allow us to
conduct this condensation to
pyrrolidinone H under mild re-
action conditions. Following ad-
dition of an appropriate acid
(Brønsted or Lewis), the N-
Table 1. One-pot reaction sequence results for monosubstituted furans (RB=rose Bengal).
Furan 1
Amine 2
Acid (equiv), t
Product 4
Isolated
yield [%]
1
TFA (0.8 equiv), 6 h
60
2
3
TFA (0.4 equiv), 3 h
62
65
AlCl3 (2.4 equiv), 12 h
4
HCOOH,[a] 4 h
58
5
6
TFA (0.8 equiv), 2 h
63
60
AlCl3 (2.4 equiv), 12 h
7
HCOOH,[a] 2 h
67
acylACHTUNGTRENNUNGiminium ion I might be
formed and encouraged to react
with the proximal aromatic nu-
cleus (6-endo cyclisation) to
yield the final desired aromatic
8
9
TFA (0.8 equiv), 3 h
AlCl3 (3 equiv), 12 h
67
63
nitrogen-bearing
polycycle
(H!J). At this stage, it is im-
portant to note that the entire
process described in this
scheme was envisaged as a one-
pot reaction sequence to be
achieved by the serial addition
of the desired reagents as re-
quired when indicated by thin-
layer chromatography (TLC)
analysis of the reaction mixture.
10
HCOOH,[a] 2 h
70
[a] As solvent.
After validation of the general idea, our initial foray
would focus on deconvoluting optimal conditions for the re-
action of monosubstituted furan substrates (1a–1d, Table 1)
in combination with one of three commercially available
amines ArCH2CH2NH2 bearing aromatic units of differen-
tial nucleophilicity (a simple phenyl ring in 2c, a phenyl ring
activated by two methoxy substituents in 2a and an indole
in 2b). Gratifyingly, the cascade reaction sequence proceed-
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Nitrogen-Bearing Aromatic Polycycles
COMMUNICATION
ed as envisaged, and, despite the formation of a tertiary
carbon centre in the final step, the desired products were
obtained, in each of the three series, in remarkably good
yields especially when considering the degree by which mo-
lecular complexity had been increased (58–70%, Table 1).
An indicator that the proposed mechanistic sequence was
indeed in play came from the isolation of several intermedi-
ates of type 3 (Table 1).[27]
While the method by which the NAI precursor had been
obtained (1!3, Table 1) was new and exceptionally mild,
there exists substantial precedent for the last acid-catalysed
Pictet–Spengler-type cyclisation step of the one-pot reaction
sequence in which a diverse array of different acid catalysts
and conditions have been described, some of which are con-
siderably milder than others (vide supra). After surveying a
number of Brønsted and Lewis acids, we were satisfied that
trifluoroacetic acid (TFA) (in CH2Cl2 at room temperature)
or formic acid (as solvent, at room temperature) were the
mildest and most effective conditions for the 6-endo cyclisa-
tion step that terminates the sequence, at least, when the ar-
omatic nucleus was activated (i.e. in the dimethoxyphenyl,
or indole series; entries 1, 2, 4, 5, 7, 8 and 10 in Table 1).
Entry 10, in which the starting furan substrate 1d has an
alkyl iodide side chain appended to it, serves to show just
how mild and tolerant this one-pot reaction sequence is, as
this delicate functionality remains untouched throughout the
complex reaction sequence.
yields 65–78%). In contrast, however, the greater degree of
substitution in the starting furan substrates now precludes
the cyclisation in the case of the unactivated exemplars (in
which Ar=a simple phenyl ring); the intermediates could
not be coaxed into reacting even using the stronger condi-
tions (AlCl3, entries 3 and 7) that had worked for the earlier
examples. We believe that this is due to the rapid isomerisa-
tion of the initially formed 2-pyrrolidinone 6 to product 7,
which now affords a slightly more stable (compared to
Table 1 substrates) tri/tetrasubstituted and conjugated
double bond; indeed it was compounds of type 7 that were
isolated in both cases (entries 3 and 7). For cyclisation, the
2-pyrrolidinone 6 is required (not isomer 7), and, here the
added stability of isomer 7 shuts down the cyclisation path-
way in the case in which the aromatic partner is also reluc-
tant. As an aside, it is worth noting that products 8ba and
8ba’ contain the entire intact skeleton of the erythrina alka-
loids[2] synthesised in one single step; albeit, not yet correct-
ly substituted for completion of a synthesis.
From a stereochemical perspective some very interesting
results were obtained in this second series of examples
(Table 2). Starting with 2,3-dimethylfuran, the NAI cyclisa-
tion reaction with the dimethoxyphenyl residue was highly
selective and the formation of only one diastereoisomer 8aa
was observed; the one in which the aromatic nucleophile
has approached the face of the NAI that is opposite to the
methyl group appended to the adjacent carbon (Table 2,
entry 1). In contrast, with the more facile indole NAI reac-
Achieving this type of cyclisation when the aromatic nu-
cleus is not activated (i.e. Ar=phenyl) is more difficult, re-
quires harsher conditions and has been reported much more
rarely.[5,28] Furthermore, within this relatively sparse prece-
dent there is little agreement on conditions that might be
applied generally. For example, some substrates react
smoothly with BF3·OEt2,[28a] whilst others show poor[28b] or
no reaction[5c] under the same conditions. We found that
[28c]
AlCl3
was the reagent of choice (in CH2Cl2 at 08C!
room temperature) for all the substrates that we tested, fur-
nishing the desired products in highly respectable yield (60–
65%, entries 3, 6 and 9—Table 1). Interestingly, for entry 9,
if the number equivalents of AlCl3 used was reduced slightly
from 3.0 to 2.4, the product 4cc was found to be contaminat-
ed with significant amounts of the uncyclised regioisomer of
3cc (not shown), in which the double bond is exocyclic, not
endocyclic. This result mirrors the one seen in entry 7, in
which use of TFA (0.8 equiv) instead of HCOOH led to the
rapid formation of the uncyclised intermediate with an exo-
cyclic double bond (the regioisomer to 3ca, see Supporting
Information) and that for entry 10 for which similar obser-
vations were also made.
To expand the scope of this productive sequence, we next
sought to explore the reactions of more highly substituted
furans; the results of this survey are shown in Table 2. For
both 2,3-dimethylfuran (5a) and menthofuran (5b) it was
once again possible, when the conditions were appropriately
tailored, to achieve the proposed complex one-pot reaction
sequence for the two aromatic partners (the dimethoxy-
phenyl 2a and indole nucleophiles 2b, entries 1, 2, 5 and 6,
Scheme 2. Observed further oxidations under controlled conditions; Ar=
dimethoxyphenyl in 7aa and 7ba; Ar=phenyl in 7ac and 7bc.
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G. Vassilikogiannakis et al.
Table 2. One-pot reaction sequence results for di- and trisubstituted furans (RB=rose Bengal).
abstraction from intermediate
7ba (and likewise 7ac and 7bc)
to afford the highly stabilised
tertiary
radical
(at
C1,
Scheme 2), which is subsequent-
ly oxidised to the corresponding
cation[29] from which the double
bond is generated following
elimination of a proton. Sup-
port for this scenario comes in
the form of diene 9, isolated as
the only product of the one-pot
reaction sequence involving
menthofuran and a simple un-
Furan 5
Amine 2 (equiv),
solvent, T, t
Acid (equiv),
solvent, T, t
Product
(7 or 8)
Isolated
yield [%]
G
(0.8 equiv),
MeOH,
RT, 1 h
HCOOH,[a]
reflux, 2 h
1
65
(0.8 equiv),
MeOH,
RT, 1 h
TFA (0.8 equiv),
CH2Cl2, RT, 6 h
2
3
67
58
activated
aromatic
partner
(5b!9, Scheme 2). In a similar
manner, diene 10 was isolated
from the reaction of 2,3-dime-
thylfuran (5a) with phenyle-
thylamine 2c. However, in con-
trast to the menthofuran results,
when 2,3-dimethylfuran was
treated with the dimethoxy-
phenyl aromatic partner the cy-
clised product formed (8aa)
without the extra double bond.
We believe that this is due to
the rapid interconversion be-
tween 6aa and 7aa which pro-
motes cyclisation and prevents
7aa having the lifetime re-
quired to undergo the oxidation
sequence (unlike 7ba, which is
stabilised by the additional
double bond substitution and
may undergo oxidation fol-
lowed by cyclisation). It is very
important that these pathways
do not lead to mixtures and
that the outcome is completely
controllable through alterations
(0.8 equiv),
CH2Cl2,
RT, 1 h
AlCl3 (2.4 equiv),
CH2Cl2, RT, 2 h
(1.2 equiv),
MeOH,
reflux, 4 h
HCOOH,[a]
reflux, 8 h
4
5
70
78
(1.2 equiv),
CHCl3,
reflux, 4 h
HCOOH,[a]
RT, 2 h
(1.2 equiv),
CHCl3,
reflux, 4 h
TFA (1 equiv),
CHCl3, RT, 8 h
6
7
75
65
(1.2 equiv),
CHCl3,
reflux, 4 h
AlCl3 (2.4 equiv),
CHCl3, RT, 12 h
[a] As solvent.
tion, which presumably has an earlier transition state less in-
fluenced by the substitution adjacent to the NAI, no such
selectivity was observed (diastereomeric ratio=60:40,
entry 2). Likewise with menthofuran, the slower NAI cycli-
sation is more selective yielding just two seperable diaster-
eoisomers in a ratio of 7:3 (entry 5), while the faster NAI-
indole cyclisation yields four diastereoisomers (entry 6).[27]
With this second series of examples, we began to see an-
other set of synthetically useful and highly tuneable observa-
tions. In the case of menthofuran with the dimethoxyphenyl
aromatic partner, when the process was carried out accord-
ing to the developed protocol (without purification of inter-
mediates), and, when given a slightly longer reaction time
with HCOOH, another oxidation was seen to take place
giving the unsaturated product 8ba (as a single isomer,
Table 2, entry 4). Presumably, this occurs through hydrogen
to the reaction conditions, especially since introduction of
the second bond in 8ba affords a handle from which further
synthetically useful manipulations could be accomplished.
In summary, we have introduced a remarkably mild and
highly controllable one-pot reaction sequence, beginning
from extremely simple furan substrates, which can be ap-
plied generally and successfully to the construction of ad-
vanced nitrogen-bearing polycycles. The one-pot reaction se-
quence is initiated by the highly selective and green oxidant,
singlet oxygen. Given the number of transformations includ-
ed in this reaction sequence and the impressive increases in
molecular complexity achieved, the yields attained are high
and should make this method a genuinely useful synthetic
strategy.
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Nitrogen-Bearing Aromatic Polycycles
COMMUNICATION
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Acknowledgements
The research leading to these results has received funding from the Euro-
pean Research Council under the European Unionꢂs Seventh Framework
Programme (FP7/2007-2013)/ERC grant agreement no. 277588.
Keywords: N-acyliminium · erythrina alkaloids · Pictet-
Spengler reaction · singlet oxygen · sustainable chemistry
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Received: April 24, 2013
Published online: June 14, 2013
Chem. Eur. J. 2013, 19, 10119 – 10123
ꢁ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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