Photocycloaddition of 3-alkenyloxy-2-cycloalkenones: Enantioselective lewis acid catalysis and ring expansion

Article abstract of DOI:10.1002/anie.201407832

By application of substoichiometric amounts (50 mol %) of a chiral Lewis acid, the intramolecular [2+2] photocycloaddition of the title compounds was achieved with high enantioselectivity (up to 94 % ee). Upon cleavage of the cyclobutane ring the resulting tricyclic products underwent ring-expansion reactions under acidic conditions and formed anellated seven- or eight-membered-ring systems without racemization. The ring expansion could be combined with a diastereoselective reduction (triethylsilane) or allylation (allyltrimethylsilane) upon BF₃ catalysis (48-87 % yield). Ring-a-ring o'roses: An enantioselective, Lewis acid catalyzed [2+2] photocycloaddition (9 examples, 69-94 % yield, up to 94 % ee) leads to tricyclic products, in which the marked bond of the cyclobutane ring can be cleaved to form medium-sized rings as shown in the example above.

Full text of DOI:10.1002/anie.201407832

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Chemie
DOI: 10.1002/anie.201407832
Photochemistry
Hot Paper
[2+2] Photocycloaddition of 3-Alkenyloxy-2-cycloalkenones:
Enantioselective Lewis Acid Catalysis and Ring Expansion**
Richard Brimioulle and Thorsten Bach*
Abstract: By application of substoichiometric amounts
(50 mol%) of a chiral Lewis acid, the intramolecular [2+2]
photocycloaddition of the title compounds was achieved with
high enantioselectivity (up to 94% ee). Upon cleavage of the
cyclobutane ring the resulting tricyclic products underwent
ring-expansion reactions under acidic conditions and formed
anellated seven- or eight-membered-ring systems without
racemization. The ring expansion could be combined with
a diastereoselective reduction (triethylsilane) or allylation
(allyltrimethylsilane) upon BF3 catalysis (48–87% yield).
Lewis acid catalyzed photocycloaddition of enones, which was
first described for dihydropyridones,[10d] is generally applica-
ble, and we commenced this study with 3-but-3-enyloxy-2-
cyclohexenone (1a). The intramolecular [2+2] photocycload-
dition of this compound has been reported[12] and leads to the
racemic photocycloaddition product rac-2a. Its enantiomers
can be readily separated by GC analysis on a chiral stationary
phase. The Lewis acid 3a,[10b,d] which we used for previous
enantioselective [2+2] photocycloaddition reactions, gave
only moderate enantioselectivities (Table 1, entry 1). An
extended optimization of the aryl substituent of the oxaza-
T
he ring strain of cyclopropanes and cyclobutanes can be
exploited to conduct ring-expansion reactions in di- and
tricyclic ring systems.[1] In particular, by cleavage of a carbon–
carbon bond, seven- and eight-membered carbocycles can be
generated, which are difficult to obtain by conventional
methods, and which are recurring elements in numerous
natural products.[2] In the context of [2+2] photocycloaddition
reactions, ring expansions have been performed by a radical
fragmentation[3,4] or preferably by a retro-aldol reaction
(de Mayo reaction)[5] or a retro-Mannich reaction.[6] Any
stereogenic center that is located at the cleaved bond is—at
least temporarily—lost, while the configuration of other
stereogenic centers in the molecular framework is retained.
Against this backdrop, the enantioselective photochemical
generation of cyclobutanes poses a significant challenge,
which in the past was very often solved by auxiliary-controlled
methods[7] or by the application of stoichiometric chiral
templates.[8] In recent times, there have been initial hints that
catalytic enantioselective [2+2] photocycloaddition variants[9]
can also be used for this purpose.[10,11] Herein we demonstrate
that chiral Lewis acids can be employed to perform enantio-
selective [2+2] photocycloaddition reactions of 2-cycloalke-
nones. In addition, the cyclobutanes generated in this reaction
were found to undergo ring-opening and ring-expansion
reactions, which have so far received little attention.
Table 1: Examples from the optimization of reaction conditions for the
enantioselective intramolecular [2+2] photocycloaddition of 2-cyclo-
hexenone 1a in the presence of chiral Lewis acids 3.
Entry[a]
Cat.
Loading[b]
[mol%]
l[c]
Power[d]
[W]
Conv.[e]
[%]
ee[f]
[%]
[nm]
1
2
3
4
5
6
7
3a
3b
3c
–
3c
3c
3c
3c
3c
50
50
50
–
50
40
30
50
50
300
300
300
300
300
300
300
366
300
36
36
36
36
84
36
36
128
36
80
89
67
79
98
42
25
52
99
46
75
80
–
56
80
74
80
80
8[g]
9[g]
In the beginning of our investigations, the goal was to
demonstrate that the previously discovered enantioselective
[a] All reactions were performed in CH2Cl2 as the solvent with a substrate
concentration of 20 mm and at a temperature of ꢀ708C (Duran glass).
[b] Amount of the chiral Lewis acid used for the reaction. [c] Emission
maximum[13] of the fluorescence lamps. [d] Overall power of the
fluorescence lamps. [e] The conversion was determined by GC analysis.
[f] The enantiomeric excess (ee) was calculated from the ratio of
enantiomers as determined by GC analysis on a chiral stationary phase.
[g] The solution was irradiated for 48 h.
[*] M. Sc. R. Brimioulle, Prof. Dr. T. Bach
Lehrstuhl fꢀr Organische Chemie I und Catalysis Research Center
(CRC), Technische Universitꢁt Mꢀnchen
Lichtenbergstrasse 4, 85747 Garching (Germany)
E-mail: thorsten.bach@ch.tum.de
Homepage: http://www.oc1.ch.tum.de/home_en/
[**] This project was supported by the Deutsche Forschungsgemein-
schaft (DFG, GRK 1626) and the Fonds der Chemischen Industrie
(PhD scholarship to R.B.). We thank Olaf Ackermann, Florian Mayr,
and Marcus Wegmann for help with the HPLC analysis.
borolidine skeleton 3 (see Supporting Information) delivered
significantly better results with Lewis acids 3b (entry 2) and
3c (entry 3).
Fluorescence lamps with an emission maximum at l =
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201407832.
300 nm[13] were used in these experiments as the irradiation
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Table 2: Yields and enantioselectivities for the intramolecular [2+2]
photocycloaddition of the 2-cycloalkenones 1.
Entry Substrate[a]
Power[b] t[c] Product
Yield[d] ee[e]
Figure 1. UV/Vis spectrum of compound 1a in CH2Cl2 as solvent
(c=0.5 mm) without Lewis acid (black line) and after addition of
10 equivalents of EtAlCl2 (gray line).
[W]
[h]
[%]
[%]
1
36
48
90
80
source. The choice of wavelength was based on the observa-
tion that cyclohexenone 1a shows an absorption maximum in
its UV/Vis spectrum at 245 nm (e = 21790mꢀ1 cmꢀ1) which
was shifted to 281 nm (e = 27270mꢀ1 cmꢀ1) upon complex-
ation by a Lewis acid (Figure 1). The effect of the Lewis acid
is based on the fact that the complex shows a significant
absorption in a wavelength range, in which the uncomplexed
substrate only shows very weak absorption.[10d] However, in
the absence of any Lewis acid this weak absorption—in this
case the np* band at l = 299 nm (e = 90mꢀ1 cmꢀ1)—is suffi-
cient to induce a relatively fast [2+2] photocycloaddition
reaction (Table 1, entry 4).
The photon flux was regulated by the power (number of
lamps) of the irradiation source. The photoreaction was
complete after shorter irradiation times at higher photon flux
(entry 5), but the enantioselectivity decreased due to the
racemic background reaction (vide supra).[10d] To our delight,
catalyst 3c still delivered high enantioselectivities (entry 6) at
a catalyst loading of 40 mol%, however at lower conversion.
With 30 mol% catalyst, both the conversion and the enantio-
selectivity decreased (entry 7). Fluorescence lamps with
a red-shifted emission[13] led to a similar result as the
300 nm lamps concerning the enantioselectivity. However,
even at a higher irradiation power the conversion remained
low (entry 8). The reaction 1a!2a could be completed at low
photon flux after longer irradiation times without loss of
enantioselectivity (entry 9).
Under the reaction conditions of entry 9 in Table 1,
photocycloaddition product 2a could be isolated in 90%
yield and with 80% ee (Table 2, entry 1). Further cycloalke-
nones 1 reacted equally efficiently under optimized reaction
conditions[14] and in several cases (entries 2, 5–9) with
significantly higher enantioselectivities. Only sterically
demanding (entry 3) and electron-deficient olefins (entry 4)
led to slightly lower selectivities. In analogy to the bath-
ochromic shift upon addition of EtAlCl2, we assume that the
substrates are complexed by the chiral Lewis acid, resulting in
the same photophysical effect: The high absorption coeffi-
cient of the Lewis acid/substrate complex at l ꢁ 300 nm
suppresses the background reaction of the substrate, and the
[2+2] photocycloaddition reaction occurs exclusively in the
presence of the chiral Lewis acid. The absolute configuration
of the photoproducts was assigned based on the face differ-
entation suggested for chiral oxazaborolidines, which has
been previously verified not only in thermal[15] but also in
photochemical reactions.[10b,d]
2
3
4
5
6
7
8
9
84
84
54
84
48
72
84
84
24
48
48
48
24
48
48
48
93[f]
93[g]
91
82
75
76
86
86
87
82
94
81
94[h]
69
86[i]
93
[a] All reactions were performed with 0.1 mmol of starting material in
a Rayonet reactor with fluorescence lamps (6 W power output each;
emission maximum at l=300 nm)[13] as the light source. [b] Overall
power of the fluorescence lamps used. [c] Irradiation time. [d] Yield of
isolated product. [e] The enantiomeric excess (ee) was calculated from
the ratio of enantiomers as determined by GC analysis on a chiral
stationary phase. [f] The reaction was additionally performed on a larger
scale (0.6 mmol) (81% yield, 85% ee). [g] Based on conversion (55%).
[h] Based on conversion (72%). [i] Based on conversion (29%).
After completion of the photoreaction, a base (NEt3) was
routinely added to the reaction mixture at ꢀ708C, since ring-
opening products seemed to be formed when the solution was
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warmed to room temperature in the presence of the chiral
Lewis acid 3c. In fact, it has been reported for the reaction
1 f!2 f that such a ring-opening reaction can occur, which
leads to an achiral compound after elimination of a pro-
ton.[14b,16] This elimination cannot take place in product 2b
(Table 2, entry 2), which bears a methyl group instead of
a proton in position C3a of the hexahydro-2H-benzo-
[1,4]cyclobuta[1,2-b]furan framework. Consequently, upon
treatment of compound 2b with a Brønsted acid (1n HCl in
CH2Cl2), the chiral tricyclic compound 4b was isolated in very
good yield (Figure 2). The product is the acetal of the
Scheme 1. Diastereoselective BF3-catalyzed ring opening of the photo-
cycloaddition products 2b und 2h to deliver cyclooctanones 5b and
6b and cycloheptanones 5h and 6h.
was obtained in 76% yield (d.r. = 87/13) and product 6h was
even obtained in diastereomerically pure form (d.r. > 95/5).
In summary, for the first time representatives of the most
important substrate class for [2+2] photocycloaddition reac-
tions (2-cycloalkenones) were reacted in an enantioselective,
intramolecular variant of the reaction. Thus, this study
confirms that enantioselective reaction control by the selec-
tive excitation of a Lewis acid/substrate complex is generally
applicable. So far, this principle has only been applied to 5,6-
dihydro-4-pyridones as substrates.[10d] Furthermore, the prod-
ucts obtained in the present study open up new pathways to
interesting molecular frameworks. In this context, the octa-
hydrocyclohepta[b]furan framework (compounds 5h, 6h)
seems to be of particular relevance.[20]
Figure 2. Products 4 and yields of the acid-catalyzed ring opening (HCl
in CH2Cl2) of cyclobutanes 2 formed in the intramolecular [2+2]
photocycloaddition reaction.
corresponding 1,5-cyclooctadione, which in turn is the prod-
uct of a cyclobutane ring expansion (vide infra). The ring-
opening reaction is generally applicable and in the same
fashion products 4c, 4d, 4e, and 4g were obtained.
The assignment of the relative configuration was not
trivial for compounds 4, since the signals in the 1H NMR
spectra overlapped significantly. Although cyclobutane 2d
was—presumably due to the electron-withdrawing fluoro
substituents—more difficult to hydrolyze than the other
cyclobutanes and delivered only relatively low yields, product
4d turned out to be useful because distinct NOE assignments
could be made in this case (see the Supporting Information).
The formation of the tricyclic compounds 4 occurs diastereo-
selectively with retention of the stereogenic center at position
C3a formed in the course of the [2+2] photocycloaddition
reaction. For compounds 4d (76% ee) and 4e (86% ee), it
was explicitly proven that the enantiomeric excess remained
unchanged relative to that of the starting materials 2d and 2e.
The ring expansion can be explained by the intermediacy
of an onium ion. It is formed from the [2+2] photocycloaddi-
tion products by activation of the carbonyl group with a Lewis
or Brønsted acid and fragmentation of the central carbon–
carbon bond (Scheme 1).[17] Since we hoped that this inter-
mediate could be trapped in a subsequent intermolecular
reaction,[18] products 2b and 2h were treated with a Lewis acid
and a silyl nucleophile (NuSiR3). In the case of the eight-
membered ring, cyclooctanone 5b was obtained with Et3SiH
as hydride source, whereas over-reduction occurred in the
case of compound 2h under the same reaction conditions.
However, the intermediary alcohol 5h’ was readily oxidized
(IBX = 2-iodoxybenzoic acid), affording the desired cyclo-
heptanone 5h. The reaction proceeded with perfect diaste-
reoselectivity, which is expected due to better shielding at the
concave bottom face of the carbocycle.[19] In the same way,
allyltrimethylsilane could be used to perform a diastereose-
lective carbon–carbon bond-formation reaction. Product 6b
Received: August 1, 2014
Published online: && &&, &&&&
Keywords: cycloaddition · enantioselectivity · Lewis acids ·
.
photochemistry · ring expansion
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4
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Photochemistry
R. Brimioulle, T. Bach*
&&&& — &&&&
[2+2] Photocycloaddition of 3-Alkenyloxy-
2-cycloalkenones: Enantioselective Lewis
Acid Catalysis and Ring Expansion
Ring-a-ring o’roses: An enantioselective,
Lewis acid catalyzed [2+2] photocy-
cloaddition (9 examples, 69–94% yield,
up to 94% ee) leads to tricyclic products,
in which the marked bond of the cyclo-
butane ring can be cleaved to form
medium-sized rings as shown in the
example above.
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