Preparation of Cyclobutene Acetals and Tricyclic Oxetanes through Photochemical Tandem and Cascade Reactions

Article abstract of DOI:10.1002/anie.201803571

We describe a photochemical reaction using two starting materials, a cyclopent-2-enone and an alkene, which are transformed in a controlled manner via the initial [2+2]-photocycloaddition adducts into cyclobutene aldehydes (conveniently trapped as stable acetals) or unprecedented angular tricyclic 4:4:4 oxetane-containing skeletons. These compounds are formed through tandem or triple cascade photochemical reaction processes, respectively. Small libraries of each compound class were prepared, thus suggesting that this photochemistry approach opens new opportunities for synthesis design and for widening molecular diversity.

Full text of DOI:10.1002/anie.201803571

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Accepted Article
Title: Preparation of cyclobutene acetals and tricyclic oxetanes via
photochemical tandem and cascade reactions
Authors: Julien Buendia, Zong Chang, Hendrik Eijsberg, Régis Guillot,
Angelo Frongia, Francesco Secci, Juan Xie, Sylvie Robin,
Thomas Boddaert, and David J Aitken
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201803571
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Preparation of cyclobutene acetals and tricyclic oxetanes via
photochemical tandem and cascade reactions
Julien Buendia,[a],+ Zong Chang,[a],+ Hendrik Eijsberg,[a],[b] Régis Guillot,[a] Angelo Frongia,[b] Francesco
Secci,[b] Juan Xie,[c] Sylvie Robin,[a],[d] Thomas Boddaert,[a] David J. Aitken*[a]
Abstract: We describe a photochemical reaction paradigm using two
The [2+2]-photocycloaddition of an enone and an alkene
provides a versatile access to functionalized cyclobutanes and, in
particular, the use of cyclopent-2-enone derivatives leads to
bicyclo[3.2.0]heptan-2-one adducts (Figure 1).[11] Some reports
starting materials, a cyclopent-2-enone and an alkene, which are
transformed in
a
controlled manner via the initial [2+2]-
photocycloaddition adducts into cyclobutene aldehydes (conveniently
trapped as stable acetals) or unprecedented angular tricyclic 4:4:4
oxetane-containing skeletons. These compounds are formed via
tandem or triple cascade photochemical reaction processes,
respectively. Small libraries of each compound class have been
prepared, suggesting that this photochemistry paradigm opens new
opportunities for fine synthesis design and for widening molecular
diversity.
describe
the
photochemical
transformation
of
bicyclo[3.2.0]heptan-2-one derivatives, via
a
Norrish-Type-I
cleavage followed by γ-hydrogen (γ-H) transfer, to provide
cyclobutenylpropanals (Figure 1).[12] It appeared logical to us that
it should be possible to combine a cyclopent-2-enone and an
alkene in a tandem photochemical reaction involving a [2+2]-
cycloaddition followed by a Norrish-I/γ-H transfer. Indeed, careful
inspection of the literature revealed a few isolated reports on the
formation of cyclobutenylpropanal derivatives via such
a
Photochemical reactions constitute a powerful toolkit for
modern synthetic chemistry.[1] The selective activation of
molecules through the acquisition of a considerable amount of
energy by the absorption of light can provide access to complex
molecular structures which are difficult to obtain otherwise, via
transformations which have no parallel in ground-state chemical
reactivity. Such processes are attractive in the context of
sustainable development, since the explicit reagent is a simple
photon. Significant recent advances have been collated in the
areas of catalyst-free photochemical synthesis,[2] visible-light
process;[13] however, these compounds were formed in low yields
and considered as side-products (Figure 1). To the best of our
knowledge, no general photochemical procedure allowing the
direct and selective preparation of such structures has been
described to date.
photocatalysis,[3]
enantioselective
catalysis,[4]
photochirogenesis[5] and the use of flow processes,[6] testifying to
the potential of organic photochemistry for selective synthesis.
In the context of tandem or cascade reactions, photochemical
transformations play an active role.[7] A photochemical step can
be conveniently combined with other categories of synthetic
transformations, notably those employing transition-metal
catalysis[8] or featuring radical cyclizations.[9] Much less common
are cascade reactions in which two (or more) discreet
photochemical processes are implicated consecutively.[10]
[a]
Dr J. Buendia, Z. Chang, Dr H. Eijsberg, Dr R. Guillot, Dr S. Robin,
Prof Dr D. J. Aitken
ICMMO, Université Paris-Sud, Université Paris-Saclay, CNRS UMR
8182, Orsay (France)
E-mail: david.aitken@u-psud.fr
Figure 1. Background for this study.
[b]
[c]
[b]
[+]
Dr H. Eijsberg, Dr A. Frongia, Dr F. Secci
Dipartimento di Scienze Chimiche e Geologiche, Università degli
Studi di Cagliari, Cagliari (Italy)
Prof Dr J. Xie
Cyclobutene derivatives are attractive intermediates in
synthesis,[14] and the presence of an aldehyde function further
enhances the value of such compounds. The opportunity for
developing an expedient access to cyclobutenylpropanals from
simple cyclopent-2-enone and alkene substrates seemed to us to
have been overlooked or at least underexploited. We therefore
decided to study this tandem photochemical reaction, with the aim
of developing an efficient and viable synthetic procedure. During
PPSM, ENS Paris-Saclay, Universitꢀ Paris-Saclay, CNRS UMR
8531, Cachan (France)
Dr S. Robin
UFR Sciences Pharmaceutiques et Biologiques, Universitꢀ Paris
Descartes, Paris (France)
These authors contributed equally to this work.
Supporting information for this article is given via a link at the end of
the document.
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the course of this study we discovered a triple photochemical
cascade reaction which leads to unprecedented angular tricyclic
structures (Figure 1).
moderate 40% yield. To resolve the problems of product isolation,
we envisaged trapping the aldehyde product as an acetal during
the photochemical reaction. We screened a variety of acid
catalysts and alcohols (see Table S1 in Supporting Information),
and found that the most efficient procedure was to irradiate the
standard mixture (λ = 300 nm) for 6 h in the presence of 10
equivalents of 1,3-propanediol and 10 mol% p-toluenesulfonic
acid. Under these conditions, the stable acetal 4aa could be
isolated easily in 75% yield (Scheme 1).
We first searched for experimental conditions which would
allow a simple and practical preparation of cyclobutenylpropanals
via a tandem photochemical [2+2]-cycloaddition – Norrish-I/γ-H
transfer process. Cyclopent-2-enone 1a and cyclopentene (in a
10-fold excess) were selected as the model substrates, and an
initial screening was performed by irradiating solutions of these
two compounds in different solvents (acetone, acetonitrile,
cyclohexane, benzene, toluene, methanol, dichloromethane)
using polychromatic light (Hg lamp, Pyrex filter). After 6 h
To confirm the role of the tricyclic ketone 2a as an
intermediate in the tandem process, this compound was
converted into cyclobutene acetal 4aa in 73% yield by irradiation
under the standard conditions (Scheme 1). It was noteworthy that
there was no apparent depletion of the chemical yield when the
more convenient tandem reaction protocol was used instead of
the sequential route.
1
irradiation time, the reaction mixture was examined by H NMR
spectroscopy and in each case (with the exception of acetone, in
which the main product was 2a), the target cyclobutenylpropanal
3a was formed along with a number of other photoproducts.
Retaining acetonitrile as a convenient solvent, we irradiated a
solution of the model substrates using fluorescent lamps (RPR-
3500 Å) with an emission maximum at λ = 350 nm for 14 h, which
gave selectively the [2+2]-cycloaddition adduct 2a in 76% isolated
yield as a single cis-anti-cis diastereoisomer (d.r. > 95:5, as
determined by 1H NMR analysis) (Scheme 1).
We then applied these conditions to prepare a wide range of
cyclobutenylpropanal acetals 4aa-jc (Table 1). Cyclopent-2-
enone 1a and a number of 2- or 4-substituted derivatives 1b-j
were examined in combination with three representative alkenes:
ethylene, tetramethylethylene and cyclopentene. While the yields
were not universally high, this procedure harnessed for the first
time the tandem photochemical process by trapping
cyclobutenylpropanals as stable acetals, making the protocol
easy to apply with a satisfactory substrate scope. Moreover,
synthetically useful functional groups such as a nitrile (4ca), and
a protected (4ga, 4hb, 4hc) or a free (4da, 4dc, 4fa, 4fb) hydroxyl
group were tolerated under the reaction conditions. For substrate
1e, bearing a benzyl chromophore, a longer reaction time was
required (48 h), but compounds 4ea and 4ec were obtained in
satisfactory yields.
To confirm the availability of cyclobutene aldehydes by
deprotection of the corresponding acetals, 4aa was refluxed with
5 mol% p-toluenesulfonic acid in aqueous acetone, which
furnished 3a in 68% yield (85% yield based on recovered starting
material) (Scheme 1). Significantly, this procedure gave 3a as a
much cleaner sample than that obtained via additive-free
irradiation of cyclopent-2-enone 1a and cyclopentene.
As mentioned above, the model reaction of cyclopent-2-
enone 1a with cyclopentene (where acetal trapping was not
applied) led to the formation of a second photoadduct along with
3a. We followed the appearance of this new adduct using 1H NMR
spectroscopy, and found that irradiation (λ = 300 nm) of the model
compounds in acetonitrile for 40 h furnished the new adduct as
the only major compound, with no 3a remaining. The new
compound was isolated and its molecular structure was
determined by extensive NMR spectroscopic analysis. It had the
tetracyclic structure 5aa which featured an unprecedented
angular 4:4:4 fused ring architecture incorporating an oxygen
atom at the apex. The compound was formed as a mixture of two
diastereoisomers (d.r. = 4:1) and the major isomer was found to
have a cis-syn-cis geometry of the linear 4:4:5 ring sequence
(Scheme 1).
Scheme 1. Single, tandem, or triple cascade photoreactions between
cyclopent-2-enone and cyclopentene. The intermediacy of the [2+2]-
cycloaddition adduct in the formation of the cyclobutene aldehyde (and thus its
acetal) as well as the intermediacy of both of these compounds in the formation
of the tricyclic oxetane is supported by the independent transformations.
When the model substrates were irradiated in acetonitrile
solution with higher energy light using fluorescent lamps (RPR-
3000 Å) with an emission maximum at λ = 300 nm, the main
photoproduct observed after 6 h was the target cyclobutene
aldehyde 3a. Unsurprisingly, this compound was rather volatile
and easily degraded, making its isolation difficult. Furthermore we
noticed the formation of another unidentified photoproduct, to
which we will return later. At best, 3a could be isolated in a
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Table 1. Preparation of cyclobutene acetals via the tandem photochemical
process.[a]
The unique architecture of 5aa prompted efforts to prepare
other complex polycyclic oxetanes using the same photochemical
procedure. Although 2-substituted cyclopentenones did not lead
to the formation of the desired products, we were pleased to find
that irradiation (λ = 300 nm) of alkenes and selected 4-substituted
derivatives for 40 h in acetonitrile provided a panel of new
compounds 5aa-kc (Table 2). Despite their inherent ring strain,
these compounds displayed remarkable stability and could be
isolated by column chromatography on silica gel. Moreover, the
presence of a free alcohol group in such structures (5fa, 5fb, 5fc)
was fully tolerated by the reaction conditions.
Table 2. Preparation of angular tricyclic oxetanes via the triple
photochemical cascade process.[a]
[a] Reactions were performed with 1 (1.5 mmol), alkene (10 equiv. or
saturated (for ethylene)), 1,3-propanediol (10 equiv.) and TsOH (10 mol%)
in acetonitrile (50 mL) in a sealed quartz tube at room temperature;
irradiation was performed using a 300 nm light source for a period of 6 h (48
h for 4ea and 4ec); yields are given for isolated products; d.r. was
determined by 1H NMR analysis of the crude reaction mixture.
A plausible mechanism for the formation of 5aa is a triple
photochemical
cascade,
starting
with
the
[2+2]-
[a] Reactions were performed with 1 (1.5 mmol) and alkene (10 equiv. or
saturated (for ethylene)), in acetonitrile (50 mL) in a sealed quartz tube at
room temperature; irradiation was performed using a 300 nm light source for
a period of 40 h; yields are given for isolated products; d.r. was determined
by 1H NMR analysis of the crude reaction mixture.
photocycloaddition to give 2a which evolves via the Norrish-I/γ-H
transfer reaction to give 3a, which is then converted into 5aa via
an intramolecular Paternò-Büchi reaction.[15] This hypothesis was
supported by the fact that a sample of 3a (isolated from its acetal
as described above) could be transformed into 5aa in 55% yield
when irradiated (λ = 300 nm) for 40 h in acetonitrile (Scheme 1).
Likewise, the tricyclic [2+2]-photoadduct 2a provided 5aa in 51%
yield in the same conditions (Scheme 1). It was particularly
gratifying to note that the yield of 5aa from cyclopent-2-enone 1a
and cyclopentene obtained directly via the cascade reaction was
significantly higher than that using any combination of sequential
operations which implicate isolated samples of 2a and/or 3a.
Intramolecular Paternò-Büchi reactions have occasionally been
used to construct 4:4 fused ring systems[16] or to elaborate more
complex polycyclic skeletons containing an oxetane core,[17] but
to our knowledge the formation of the angular 4:4:4 fused ring
system of 5aa is unprecedented, and would be difficult to access
by any other synthetic pathway.
Among these compounds, 5fb alone was a solid product, and
we obtained a single crystal which was suitable for X-ray
diffraction (Figure 2).[18] As expected, the oxetane ring was planar,
but both cyclobutanes were unusually flattened with ring
puckering not exceeding 8°. Striking structural features were the
sterically challenging torsion angles of 9.9° and 9.4° for the vicinal
methyl group pairs and the 134.4° C-C-C bond angle connecting
the three rings.
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Figure 2. Structure of compound 5fb. Left: the structure of the molecule in the
single crystal, as determined by X-ray diffraction. Right: the diagnostic NOESY
correlations observed in solution (20 mM in CDCl3) for the syn relative
configuration of the hydroxyl group with respect to the oxetane ring.
Figure 3. UV absorbance in the n* spectral region for compounds 1a (blue),
2a (green) and 3a (red) (c. 40 mM solutions in acetonitrile).
Acknowledgements
NOESY NMR correlations observed for 5fb in CDCl3 solution
were in complete agreement with the geometry indicated by the
X-ray study, in particular the syn orientation of the hydroxyl group
with regards to the fused oxetane (Figure 2). The same
characteristic correlations were observed for each of the
This work was supported by the French ANR through a Labex
Charm3at grant (ANR-11-LABX-0039) to J.B., the China
Scholarship Council for a pre-doctoral grant to Z.C., and the
French MESR for a pre-doctoral grant to H.E. We are grateful to
our now-retired colleagues Prof P. P. Piras (Cagliari) and Dr J.
Ollivier (Orsay) for their help and support during the early stages
of this project.
compounds
5 illustrated on Table 2, indicating that the
intramolecular Paternò-Büchi step proceeds diastereoselectively,
regardless of the identity of the substituent at the 4-position of the
cyclopent-2-enone substrate. This is presumably the result of a
preferred conformation of the cyclobutene aldehyde precursor
which facilitates the intramolecular attack of the alkene by the
excited state carbonyl oxygen atom in a diastereofacially selective
fashion, although this hypothesis has not been investigated in
detail.
Conflict of Interest
The authors declare no conflict of interest.
Collectively, the above results provide an instructive
demonstration of the controlled reorganization of atom
connectivity starting from a simple “substrate package”, whereby
considerable molecular diversity is accessed from the same pair
of substrates. Depending on the conditions employed, irradiation
of a solution containing a cyclopent-2-enone and an alkene leads
selectively to a bicyclic adduct, a cyclobutene aldehyde or a
tricyclic angular oxetane, through a single, tandem, or triple
photochemical process, respectively. The UV absorption spectra
of 1a, 2a and 3a reveal how the wavelength of the light is
determinant for the reactivity profile: with a λ = 350 nm light source
the [2+2]-cycloaddition can proceed but the bicyclo[3.2.0]heptan-
2-one adduct does not react further, whereas with a λ = 300 nm
light source the tandem or triple cascade reaction takes place
efficiently (Figure 3). The importance of trapping the cyclobutene
aldehyde in situ as an acetal is also underlined; without it, the
triple reaction will ensue.
Keywords: organic photochemistry • cascade reactions •
oxetanes • cyclobutenes • cycloaddition
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Entry for the Table of Contents
COMMUNICATION
Julien Buendia, Zong Chang, Hendrik
Eijsberg, Régis Guillot, Angelo Frongia
Francesco Secci, Juan Xie, Sylvie
Robin, Thomas Boddaert, David J.
Aitken*
Page No. – Page No.
Juggling with atom connectivity. Single, tandem or triple cascade photochemical
transformations of the atom set provided by a cyclopent-2-enone and an alkene can
be carried out in a selective manner, providing high added-value products.
Preparation of cyclobutene acetals
and tricyclic oxetanes via
photochemical tandem and cascade
reactions
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