Photochemical Alkene Isomerization for the Synthesis of Polysubstituted Furans and Pyrroles under Neutral Conditions

Article abstract of DOI:10.1002/chem.201903590

A photochemical approach to polysubstituted heterocycles using UV-induced alkene isomerization is described. The method allows for the synthesis of disubstituted furans and pyrroles under mild and neutral conditions and also provides access to a class of trisubstituted furans pertinent to natural-product synthesis. The method has broad functional-group tolerance and many richly decorated heterocycles have been prepared incorporating functional groups that are unstable under Br?nsted and Lewis acidic conditions.

Full text of DOI:10.1002/chem.201903590

A Journal of
Accepted Article
Title: Photochemical Alkene Isomerization for the Synthesis of
Polysubstituted Furans and Pyrroles under Neutral Conditions
Authors: Johannes Walker, Simon Werrel, and Timothy James
Donohoe
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Photochemical Alkene Isomerization for the Synthesis of
Polysubstituted Furans and Pyrroles under Neutral Conditions
Johannes C. L. Walker,Ɨ[a] Simon Werrel,Ɨ[a] and Timothy J. Donohoe*[a]
direct excitation of the enone system without the need for a
photocatalyst. UV/Vis absorption spectra of both model enone 3a
(prepared by cross metathesis of allyl alcohol 1a and enone 2a in
94% yield with Hoveyda Grubbs II catalyst, Table 1) and product
furan 4a10 were measured in a series of solvents and indicated
that neither compound absorbed in the visible region (>400 nm).
Both displayed a π–π* absorption below 275 nm. While enone 3a
displayed an n–π* absorption between 275–375 nm, the
absorbance of furan 4a decayed to essentially zero by 350 nm
giving us an opportunity to avoid product degradation (for UV/Vis
spectra, see Supporting Information).
Abstract: A photochemical approach to polysubstituted heterocycles
using UV induced alkene isomerization is described. The method
allows for the synthesis of disubstituted furans and pyrroles under mild
and neutral conditions and also provides access to a class of
trisubstituted furans pertinent to natural product synthesis. The
method has broad functional group tolerance and many richly
decorated heterocycles have been prepared incorporating functional
groups that are unstable under Brønsted and Lewis acidic conditions.
Recent advances in the application of photochemistry to organic
synthesis have been driven by a renewed appreciation of the
orthogonal reaction pathways accessible by photochemical
excitation.1 These provide opportunities for the synthetic chemist
to uncover novel disconnections, aiding the preparation of
complex molecules in the areas of total synthesis and drug
discovery where high levels of chemoselectivity are a necessity.2
Aromatic heterocycles are one of the most common fragment
3
classes found within both natural product
and active
pharmaceutical agents4,5,6 and in this context it is surprising that
photochemical methods for their de novo synthesis from acyclic
precursors remain underdeveloped.7,8 Routes to these fragments
that harness the power of photochemistry would be of undoubted
value.
In a recent research programme, we have become interested in
the development of catalytic chemistry to form aromatic
heterocyclic compounds 9 and reported an alkene metathesis
route to disubstituted furans (Scheme 1, middle).10 However, the
functional group tolerance was limited due to the acidic conditions
and elevated temperatures that were required for aromatisation.
Building on this work, we wondered whether a photochemical
approach, under pH neutral conditions, would permit the
incorporation of acid sensitive functional groups that would not
previously have been tolerated.
Building on the seminal report of House, 11 Alexakis 12 and
13
Gilmour
independently demonstrated the photochemically
enabled isomerization reaction of α-β unsaturated carbonyls from
their more stable (E) geometry to the (Z) isomer in synthetically
useful yields (Scheme 1, middle).14,15,16 Alexakis and co-workers
accessed a photostationary equilibrium between (E) and (Z)
enone isomers through direct excitation of the alkene with a
mercury lamp, from which the (Z) isomer was isolated in 42%
yield. Gilmour and co-workers successfully effected selective (E)
to (Z) acrylate isomerization using (–)-riboflavin as a triplet
sensitiser. By exploiting the steric clash between the phenyl and
carbonyl substituents of the (Z) isomer they could deconjugate the
chromophore and prevent the reverse isomerization.
Scheme 1. Importance of heterocyclic compounds and strategies for their
synthesis from acyclic precursors.
We therefore elected to use a convenient and commercially
available 365 nm source (Pen-Ray; see ESI for details of the light
source used), hoping that the desired isomerization would take
place without product degradation. We were delighted to find that
irradiation of enone 3a at 365 nm in diethyl ether at 0 ºC under an
air atmosphere led to complete consumption of the starting
material within 5 h and formation of furan 4a in 67% yield (Table
1, Entry 1). Careful argon sparging of the solvent and conducting
the reaction under an argon atmosphere increased the yield to
85% (Entry 2). A brief solvent screen revealed that the reaction
tended to be higher yielding in less polar solvents (Entries 3–5)
although the poor solubility of enone 3a in cyclohexane led to
low conversion (Entry 6). However, a 3:1 mixture of cyclohexane
and chloroform provided furan 4a in near quantitative yield (Entry
Turning to our proposed transformation, we recognised that
isomerization of (E) γ-hydroxy enone intermediate I to (Z)-I should
result in spontaneous condensation to furan II (Scheme 1,
bottom). The removal of (Z)-I from a photostationary equilibrium
should allow for full consumption of (E)-I whilst also permitting
[a]
Dr J. C. L. Walker, Dr S. Werrel, Prof. Dr T. J. Donohoe,
Department of Chemistry, Chemistry Research Laboratory,
University of Oxford, Oxford, OX1 3TA (UK)
E-Mail: timothy.donohoe@chem.ox.ac.uk,
These authors contributed equally.
Ɨ
Supporting information for this article is given via a link at the end of
the document.
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7). Conducting the reaction under otherwise identical conditions
but without irradiation led to no reaction (Entry 8) and irradiation
in the presence of pyridine had no observable deleterious effect,
with full conversion to the furan observed by 1H NMR
spectroscopy (Entry 9). This control reaction suggests that any
decomposition of the chloroform solvent by UV light to HCl and
subsequent Brønsted acid mediated isomerization is not a
contributing reaction pathway.17 Full consumption of enone 3a
was observed when conducting the reaction with a 254 nm source
but furan 4a was isolated in only 68% yield (Entry 10 vs 2). The
remainder of the mass balance was made up of decomposition
side-products, as expected by the absorbance at this wavelength
by the furan product 4a. Pleasingly, the key reaction could also
be scaled up to 3.5 mmol with minimal reduction in yield, although
a longer reaction time was required (Entry 11).
into furan 4h in 89% yield.
Table 1 Optimization of Photochemical Isomerization to Disubstituted Furans
Time
(h)
Change to
Procedurea
Yield 4a
(%)
Entry
Solvent
Solvent not
argon
1
Et2O
5
67
sparged
2
3
4
5
6
7
Et2O
EtOAc
5
2
2
2
2
2
-
-
-
-
-
-
85
50
88
91
22
97
CH2Cl2
PhMe
C6H12
C6H12:CHCl3 (3:1)
No
irradiation
8
9
C6H12:CHCl3 (3:1)
C6H12:CHCl3 (3:1)
2
2
0
1 equiv.
pyridine
Quant.b
10
11
Et2O
2
254 nm
68
95
C6H12:CHCl3 (3:1)
48
3.5 mmol
aReaction conditions for photochemical isomerization: 0.25 mmol enone 3a, hv
(365 nm), 0 °C; solvent was sparged with argon for 10 minutes prior to use.
Reactions were performed in a quartz glass vessel. bBy 1H NMR spectroscopy
with respect to starting material.
With optimized conditions in hand we explored the scope of the
reaction with an emphasis on substrates that would not be
tolerated under Brønsted acidic conditions (Scheme 2).10 All
substrates performed well in the cross metathesis reaction;
sterically demanding species (3f) as well as those containing
strong donor atoms (3c-d)18 produced the corresponding enones
in high yield. The photochemical irradiation sequence
demonstrated excellent functional group tolerance; THP
protected alcohols and cyclic acetal protected diols could be
incorporated to provide furans 4b and 4c in 95% and 98%
respectively. Boc protected amines were also suitable substrates
with furan 4d synthesized in 93% yield and aryl bromide
substituted furan 4e was formed in 75% yield. Furan 4f, bearing a
sulfone group, was formed in an excellent 95% yield and primary
silyl ethers were also tolerated, as in 4g. Enone 3h which, rather
than aromatising, cyclized to give acetal 5h under the previously
reported Brønsted acidic conditions,10 was now cleanly converted
Scheme 2 Scope of disubstituted furan formation. aPerformed on 1.00 mmol
scale. bPerformed on 0.25 mmol scale. c2.5 mol% PPTS, CH2Cl2, rt, 24 h.10
Previously, we had shown that (Z)- gamma N-Ts enones could be
isomerized to pyrroles under strongly acidic conditions at elevated
temperatures.19 We were therefore keen to establish whether our
low temperature UV protocol would also enable access to this
widely used class of substrate (Scheme 3). Synthesis of the
required γ-amino enone intermediates by cross metathesis of allyl
amines 6 with α,β-unsaturated enones 2 required slight heating to
offset the reduced amine reactivity.20 The identity of the amine
protecting group had little effect on metathesis reactivity with Cbz
(7a), Boc (7b) and Ts (7c) groups all well tolerated. Subjection of
enones 7a-c to our standard irradiation conditions then furnished
the corresponding Cbz (8a, 91%), Boc (8b, 88%), and Tosyl (8c,
40%) protected pyrroles. The decomposition of N-tosyl protecting
groups under photochemical conditions has previously been
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reported by Yonemitsu and co-workers.21 Note that increased
HG-II catalyst loadings were required to achieve practical cross
metathesis yields of sterically demanding enones 7d and 7e,
although in both cases much of the unreacted starting material
could also be re-isolated. Cyclic acetal and aryl substituted
pyrroles 8d and 8e could then be formed in 67% and 46% yield
respectively under irradiation.
smooth conversion to cyclic silaketal 11a in 88% yield.
Deprotection of the tether using catalytic quantities of TBAF
provided diol 12a in 99% yield.26 Despite the additional steps this
approach to formal cross metathesis chemistry provides a
strategic advantage for complex molecule synthesis as only a
small excess of one valuable metathesis partner (1.1 equiv.) is
required.
Scheme 4. Alkene metathesis approaches to trisubstituted furans.
Efforts to isomerize and aromatise diol 12a to trisubstituted furan
13a using Brønsted or Lewis acid catalysis were unsuccessful.
However, we were delighted to find that, after reoptimization for
the new substrate, irradiation of diol 12a at 365 nm in EtOAc
afforded furan 13a cleanly in almost quantitative yield (Scheme
5), demonstrating the ability of the photochemical isomerization to
facilitated aromatisation of sensitive substrates under mild
conditions, in addition to providing the broad substrate tolerance
described earlier.
Scheme 3. Scope of disubstituted pyrrole formation. aPerformed on 1.00 mmol
scale. bPerformed on 0.25 mmol scale. c44% 6d was recovered. d39% 6e was
recovered.
After these studies showed that the photochemical
isomerisation/aromatisation protocol was viable, we decided to
investigate application in more substituted examples. Specifically,
we are interested in developing routes to trisubstituted furans
bearing a single carbon unit at the C-3 position. 22 These
fragments are present in a wide range of natural product targets,
including many members of the furanocembranoid family (eg.
lophodiol B, Scheme 1).23 In this case, we envisaged that cross
metathesis between allylic alcohols III and 1,1-disubstituted
enones IV would provide γ-hydroxy enones V which could then
be isomerized (and aromatised) to the desired trisubstituted
furans VI (Scheme 4, top). In preliminary work, the direct cross
metathesis between the requisite allylic alcohol (1a) and 1,1-
disubstituted enone (9a) was unsuccessful. Reaction of 1a and
9a together with Hoveyda-Grubbs Second Generation catalyst
gave no observable CM product formation by 1H NMR
spectroscopy. Enone variants of 9a lacking the hydroxyl group (ie.
with a methyl substituent) or with a TES protected alcohol were
also unreactive in cross metathesis (see Supporting Information
for further details). The combination of a secondary allylic alcohol
and a 1,1-disubstituted deactivated alkene is very challenging for
cross metathesis chemistry. However, we were able to overcome
the poor reactivity by using a temporary silicon-tether–ring-closing
metathesis (TST-RCM) approach, commonly used to access Z-
alkenes (Scheme 4, bottom).24 Synthesis of TST silaketal 10a
was straightforward 25 and ring-closing metathesis resulted in
Scheme 5. Isomerization of diol 12a to trisubstituted furan 13a.
Given the high value of the products in synthetic chemistry
projects, the scope of this approach to trisubstituted furans was
then explored (Scheme 6). Formation of the TST metathesis
precursors was achieved in uniformally high yield. The efficiency
of the subsequent ring-closing metathesis step correlated with the
steric demand of the substrates; increased steric bulk adjacent to
the secondary allylic alcohol resulted in reduced conversion (see
11c and 11d). Interestingly, diastereoselective ring-closing
metathesis was observed when the TST starting material
contained two secondary allylic alcohols, giving the syn silaketal
11e in 45% (maximum 50% yield).27 Removal of the TST was
generally high yielding; only for more hindered silaketal 11e was
a lower yield observed. The final photochemical isomerization
step then furnished the desired furans directly. Furan 13b, with
the C-1 and C-4 substitutents swapped, was formed in 89% yield
and those displaying increased steric hindrance adjacent to the
furan (as in 13c and 13d) could also be formed in good to
excellent yield. The extended conjugation found in furan 13c led
to a lower isolated yield together with unidentified decomposition
side-products. Finally, furan 13e, bearing a secondary alcohol at
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C-3, was formed in 67% yield and furan 13f, bearing a TBS
conditions. Additionally, the photochemical route enabled the
isomerization of otherwise unreactive diols to give trisubstituted
furans. We anticipate that this method will find utility in both drug
discovery and natural product synthesis, where there is a
significant need for mild new routes to highly functionalised
aromatic heterocycles.
protected alcohol, in 46% yield.
Acknowledgements
We are grateful to the People Programme (Marie Curie Actions)
of the European Union’s Seventh Framework Programme
(FP7/2007-2013) under REA grant agreement n° 316955 (S.W.),
Cancer Research UK (CR-UK grant number C38302/A13012),
through an Oxford Cancer Research Center Prize DPhil
Studentship (J.C.L.W.).
Conflict of Interest
The authors declare no conflict of interest
Keywords: Photochemical isomerization, heterocycles,
furans, pyrroles, functional group tolerance
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Scheme 6. Scope of trisubstituted furan formation. aPerformed on 3.00 mmol
scale. bPerformed on 1.00 mmol scale. cPerformed on 0.22–0.50 mmol scale.
f
dPerformed on 0.05–0.15 mmol scale. e1.27 mmol scale 10 mol% HG-II, 100
°C. f39% 10c was recovered. gPerformed on a 0.877 mmol scale; 54% 10d was
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Entry for the Table of Contents
COMMUNICATION
A
photochemical
polysubstituted heterocycles using UV
induced alkene isomerization is
approach
to
J. C. L. Walker, S. Werrel and T. J.
Donohoe*
Page No. – Page No.
described. The method allows for the
synthesis of disubstituted furans and
pyrroles under mild and neutral
conditions and also provides access to a
class of trisubstituted furans pertinent to
natural product synthesis.
Photochemical isomerization and
aromatization to heterocycles
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