Photocycloadditions of thiomaleimides

Article abstract of DOI:10.1039/c2cc31673k

Thiomaleimides, generated by the addition of bromomaleimides to thiols including cysteine, undergo highly efficient [2+2] photocycloadditions.

Full text of DOI:10.1039/c2cc31673k

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COMMUNICATION
[2 + 2] Photocycloadditions of thiomaleimidesw
Lauren M. Tedaldi, Abil E. Aliev and James R. Baker*
Received 6th March 2012, Accepted 15th March 2012
DOI: 10.1039/c2cc31673k
Thiomaleimides, generated by the addition of bromomaleimides
to thiols including cysteine, undergo highly efficient [2 + 2]
photocycloadditions.
Table 1 Photochemical dimerisation of thiomaleimides
The maleimide motif has found widespread use in a range of
fields due its diverse reactivity profile. It is a highly reactive
conjugate acceptor and as such can be employed effectively in
selective reactions with thiols, such as cysteine residues in
proteins. As a result it is one of the most widely used functional
groups in bioconjugation.1 Maleimides are also photoactive,
undergoing reactions including [2 + 2] photocycloadditions,2
which have been exploited in applications ranging from total
synthesis3 to polymer cross-linking.4
Entry
R1
R2
Starting Material Dimer Product
1
2
n-Hex
n-Hex
H
Me
3
5
4
6
3
H
H
7
9
8a
We envisaged a situation in which these two modes of
reactivity could be combined. The product of the addition of
a thiol to a maleimide is the saturated thiosuccinimide 1
(Scheme 1), which can no longer undergo the [2 + 2] photo-
cycloadditions. In contrast when a thiol is added to a bromo-
maleimide an addition-elimination reaction takes place and
the product is the unsaturated thiomaleimide 2. We have
shown this reaction to have broad applications in bioconjuga-
tion via the highly efficient, selective and reversible modification
of single cysteine residues and disulfide bonds in proteins.5 We
postulated that such thiomaleimides may also retain the photo-
activity of the maleimides, and we report here on our results.
Irradiation of hexylthiomaleimide 3 with a 125 W medium
pressure mercury lamp in pyrex glassware afforded after just
five minutes a single dimeric product 4 in quantitative yield
(Table 1). This is an extremely rapid reaction in comparison to
maleimide itself which undergoes dimerization in one hour under
the same conditions (see ESI). The reaction proceeds in acetonitrile
or water/acetonitrile (95 : 5) down to at least 72 mM (see ESI),
4
10a
a
Obtained as a mixture of diastereomers.
indicating that it could be applied in conditions suitable for
bioconjugation. The dimerisation also proceeds quantitatively
when the nitrogen is methylated (entry 2). By analysing NMR
J-couplings and nuclear Overhauser effects (NOEs) from the
irradiation of a 1 : 1 mixture of the N–H and N–Me substrates
3 and 5 we deduced that the dimers formed are the exo head-
to-head products (see ESI). Head-to-head dimers were expected
according to FMO theory and in line with literature precedent,
whilst the exo- orientation can be explained by avoidance of the
worst of the steric clashes.
To test the hypothesis that this strategy could be used on
substrates containing cysteine, we irradiated a solution of
N-Boc-Cys(Mal)-OMe 75a resulting in the formation of dimeric
products 8 in just 5 min. The dimerisation of N-AcCys(Mal)NHBn
9 was also carried out, as analysis of the conveniently visible
13C satellite peaks provided further evidence for the exo head-
to-head outcome in these reactions (See ESI). These dimers
can be considered as photochemically formed disulfide mimics,
in which the two cysteine residues are bridged by a two carbon
linker, and which are stable to reductive conditions. Indeed
treatment of 4 with excess reducing agent, b-mercaptoethanol,
showed no reaction (see ESI).
Scheme 1 Addition of a thiol to a maleimide vs. a bromomaleimide.
We investigated suitable olefinic partners for the [2 + 2]
photocycloaddition as alternatives to this dimerisation. We
were pleased to find that with a variety of olefins the expected
[2 + 2] photocycloaddition takes place, again in just 5 min
(Table 2). Excess olefin is always necessary to stop the competing
Department of Chemistry, University College London, 20 Gordon St,
London, UK. E-mail: j.r.baker@ucl.ac.uk; Tel: (+44) 2076792653
w Electronic supplementary information (ESI) available: Full experi-
mental details. See DOI: 10.1039/c2cc31673k
c
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Chem. Commun., 2012, 48, 4725–4727 4725

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Table 2 Thiomaleimide-alkene photocycloadditions
Entry
1
R
Equiv
300
Product
Yield
41%
n-Bu
11
2
300
12
49%
Scheme 3 Further photocyloadditions of thiomaleimides.
Table 3 Styrene photocycloadditions with thiomaleimides
3
10
10
13
14
48%
4
Ph
70%a
a
30% insertion product 17 was also isolated.
Entry
R1
R2
X
Yield
1
2
3
n-Hex
n-Hex
n-Hex
Me
CH2Cy
Ph
H
H
H
70%a
64%b
80%b,c
4
5
6
H
H
H
H
63%d
70%b,e
60%b,e
n-Hex
n-HexS
Scheme 2 Insertion of styrene in to the C–S bond.
a
b
Combined yield of two separable diastereomers. Combined yield of
thiomaleimide dimerisation reaction. The products isolated
were all exo ‘head-to-head’.
c
two inseparable diastereomers. 1% of the C–S insertion product
d
also isolated. Combined yield of four inseparable diastereomers.
By far the best partner (entry 4) was styrene; only requiring
10 equivalents and forming the cyclobutane 14 in 70% yield.
An intriguing isomeric product 17 was also isolated in 30%
yield, derived from an insertion of the styrene in to the C–S
bond (Scheme 2). The mechanism for this unprecedented side-
reaction is not known, but one possibility is best visualized by
drawing a resonance form of the excited state, 15, which
represents the double bond character present in the C–S bond.
A [2 + 2] photocycloaddition would thus afford zwitterionic
intermediate 16 which then undergoes a fragmentation to
afford the product 17. Alternative mechanisms such as via
electron transfer or via homolysis of the C–S bond are also
possible. It is intriguing to note that maleimides are also
known to undergo alkene insertions into the imide, as widely
demonstrated by Booker-Milburn et al.,6 and thus demon-
strate an incredibly varied photochemical reactivity profile.
Importantly for potential applications in bioconjugation the
two products 14 and 17 represent an overall 100% attachment
of the styrene to the maleimide.
e
Irradiation time 20 min.
With styrene demonstrated as a suitable olefenic partner for
the intermolecular [2+ 2] photocycloaddition, we also explored
the effect of varying the thiomaleimide (Table 3). Initially the
effect of maleimide N-substitution on the outcome of the
reaction was considered (entries 1–3). Alkyl and aryl substi-
tuents were tolerated. Notably alkyl substituents led to com-
plete elimination of the C–S insertion by-product, whilst in
the aryl case it was present in just 1% yield. The cysteine
thiomaleimide 7 also underwent effective [2 + 2] photocyclo-
addition with styrene (entry 4) to afford four diastereomeric
cyclobutanes as a mixture. Again no C–S bond insertion
product was isolated in this case. We then switched our attention
to dithiomaleimides, which upon irradiation with styrene
afforded dithio-cyclobutanes (entries 5 and 6), following a
slightly extended irradiation time of 20 min. Interestingly
irradiation of these dithiomaleimides independently led to no
observable dimerisation even after 8 h, possibly due to increased
steric effects of the tetrasubstituted double bonds.
The broad potential scope for this [2 + 2] photocycloaddition
is further exemplified by the reactions of thiomaleimide 3 with
cyclopentene to form tricyclic product 18, with hex-3-yne to
form cyclobutene 19, and the intramolecular cycloaddition of
thiomaleimide 20 to afford 21 (Scheme 3).
Low wavelength, high energy UV exposure can cause
undesirable side-reactions in proteins and other biomolecules.7
To minimize such photochemical degradation UV wavelengths
c
4726 Chem. Commun., 2012, 48, 4725–4727
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We also thank Dr Lisa Harris for assistance with the mass
spectrometry analyses, and Prof Steve Caddick, Prof Kevin
Booker-Milburn and Dr Hugh Britton for helpful discussions.
Notes and references
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protein modification, Boca Raton, Florida, 2005; (c) I. S. Carrico,
Chem. Soc. Rev., 2008, 37, 1423–1431.
2 (a) K. I. Booker-Milburn, J. K. Cowell, F. D. Jimenez, A. Sharpe
and A. J. White, Tetrahedron, 1999, 55, 5875–5888; (b) P. Boule
and J. Lemaire, J. Chim. Phys. Phys.-Chim. Biol., 1980, 77,
161–165; (c) C. Roscini, K. L. Cubbage, M. Berry, A. J. Orr-Ewing
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3 B. A. Pearlman, J. Am. Chem. Soc., 1979, 101(21), 6398–6404.
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(b) N. Schmeling, K. Hunger, G. Engler, B. Breiten, P. Rolling,
¨
Fig. 1 Overlayed UV absorbance spectra of maleimides at 1mM in
acetonitrile.
above 350 nm are sought.8 Thiomaleimides may prove more
useful than simple maleimides for photochemical applications
in such areas as successive substitutions of the maleimide cause a
bathochromic shift, as seen in Fig. 1. Maleimide itself absorbs at a
A. Mixa, C. Staudt and K. Kleinermanns, Polym. Int., 2009, 58,
720–727.
5 (a) L. M. Tedaldi, M. E. B. Smith, R. Nathani and J. R. Baker,
Chem. Commun., 2009, 6583–6585; (b) M. E. B. Smith,
F. F. Schumacher, C. P. Ryan, L. M. Tedaldi, D. Papaioannou,
G. Waksman, S. Caddick and J. R. Baker, J. Am. Chem. Soc., 2010,
132, 1960–1965; (c) F. F. Schumacher, M. Nobles, C. P. Ryan,
M. E. B. Smith, A. Tinker, S. Caddick and J. R. Baker, Bioconjugate
Chem., 2011, 22, 132–136; (d) C. P. Ryan, M. E. B. Smith,
F. F. Schumacher, D. Grohmann, D. Papaioannou, G. Waksman,
F. Werner, J. R. Baker and S. Caddick, Chem. Commun., 2011, 47,
5452–5454; (e) P. Moody, M. E. Smith, C. P. Ryan, V. Chudasama,
J. R. Baker, J. Molloy and S. Caddick, ChemBioChem, 2012, 47,
39–41.
6 (a) K. I. Booker-Milburn, C. E. Anson, C. Clissold, N. J. Costin,
R. F. Dainty, M. Murray, D. Patel and A. Sharpe, Eur. J. Org.
Chem., 2001, 1473–1482; (b) D. M. E. Davies, C. Murray, M. Berry,
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1449–1457.
l
max of around 273 nm, whereas the thiomaleimide absorbs at a
lmax of 339 nm and the dithiomaleimide at lmax of 393 nm. An
increase in extinction coefficients of the thiomaleimides compared
to maleimide is also apparent from this figure, which in part
explains the increased efficiency of their photocycloadditions.
In conclusion, thiomaleimides readily generated by the
addition of thiols to bromomaleimides undergo highly efficient
[2 + 2] photocycloadditions. This can be utilized to form
dimers, or in the presence of suitable olefins such as styrene
to afford the corresponding cyclobutanes. The bathocromic
shift observed for thiomaleimides, along with the efficiency of
the photochemical reactions, suggests these compounds will be
more effectively employed in [2 + 2] photocycloadditions than
simple maleimides. We envisage broad potential applications
including photochemical bioconjugation, cross-linking studies,
and surface modification.
7 D. B. Volkin, H. Mach and C. Russell Middaugh, Method. Molec.
Biol., 1995, 40, 35–63.
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4, 473–480; (c) Y. Z. Wang, W. J. Hu, W. J. Song, R. K. V. Lint and
Q. Lin, Org. Lett., 2008, 10, 3725–3728.
The authors are grateful to RCUK, EPSRC, BBSRC, the
Wellcome Trust and UCLB for support of our programme.
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 4725–4727 4727