sized dienone zerumbone (3)[9] behaved as a “doubled enone”
rather than a cross-conjugated dienone, and gave a bis-
adduct, presumably because the noncoplanarity of the two
strained double bonds prevents cross-conjugation.[22] Ethyl
acrylate (18) gave a positive assay, but the b-phenyl acrylates
phenethylcaffeate (19a, CAPE)[9] from propolis and trans-
caffeoyltyramine (19b)[9] gave no reaction, as did the enoyl
lactone swertiamarin (20). The coumarin umbelliferone (21)
gave a sluggish reaction that required long reaction times (ca.
24 h) or a large excess (8 equiv) of cysteamine to go to
completion.[23] While ambiguous in terms of biological
implications, the data on umbelliferone nevertheless support
an organocatalyzed mechanism for the rapid addition of
cysteamine to conjugated carbonyl compounds. Because of
the constrained s-trans conformation of the unsaturated
system, a concerted proton-transfer process as in A would,
in fact, be impossible for coumarins.
(45.2 Æ 3.4) mm for 4, 6, 7, and 8, respectively), with
11,13-dihydrocostunolide and 11,13-dihydroparthenolide[29]
being inactive. Given the potent anti-inflammatory activity
of parthenolide,[30] it is surprising that TRPA1 activation/
desensitization has so far been overlooked as a possible
mechanistic rationale, in addition to the inhibition of tran-
scription factors like NF-kB,[30] for this action. As predicted by
the NMR spectroscopic assay, the cross-conjugated dienone
santonin was inactive, while its lumi-derivative (12) gave a
positive
TRPA1-activating
response
(EC50 = (36.6 Æ
0.02) mm), as did zerumbone (3; EC50 = (14.8 Æ 4.5) mm),
while umbelliferone (21), despite its slow kinetics of thiol
addition, was a good activator of TRPA1 (EC50 = (6.0 Æ
0.8) mm). The potent activity of curcumin (EC50 = (3.0 Æ
0.5) mm) adds TRPA1 to the almost 100 biological end
points[31] addressed by this dietary diarylheptanoid.[32]
Reversible thia-Michael adducts decompose during
workup, and attempts to isolate them from the reaction
medium fail.[7,20] With the exception of enals, all compounds
that gave Michael adducts in [D6]DMSO were unreactive
with cysteamine in deuterochloroform. Therefore, dilution
with this solvent could be used as a test for reversibility since,
in the presence of an equilibrium, the change of polarity
would reverse the reaction, with reappearance of the olefin
resonance(s) lost during the addition reaction in DMSO.
Adduct stability is a critical determinant for the biological
profile of thiol-trapping agents, with rapid reversibility being
associated with low toxicity.[33] Therefore, the availability of a
rapid method to identify these compounds could be of
interest. In the event, an aliquot (25 mL) of the [D6]DMSO
solution of the in-situ-generated Michael adducts was diluted
Under the conditions of the assay, aldehydes gave thiazo-
line derivatives,[24] and the a,b-unsaturated aldehydes citral
(23) and perillaldehyde (24)[9] gave a mixture of the corre-
sponding 1,4-thiazepines (25 and 26, respectively) and the bis-
adducts 27 and 28.[25] In the conditions of the assay,
1
1:20 with CDCl3, and the H NMR spectrum was recorded.
Within the set of compounds investigated, only the cyste-
amine adducts of umbellulone (2) and zerumbone (3) showed
solvent-induced reversibility. Interestingly, reversal of
Michael addition was only observed for the disubstituted
D9 double bond of zerumbone (see the Supporting Informa-
tion). Remarkably, the Michael adducts of umbellulone and
zerumbone were thermally stable in DMSO, with no degra-
dation being detected upon heating to 508C, a critical
threshold for reversibility with the thiol adduct of bardox-
olone methyl.[7]
The reversibility of the thia-Michael addition of prosta-
glandin dienones has been attributed to cross-conjugation.[20]
While endocyclic cyclohexadienones such as santonin and
prednisone do not behave as Michael acceptors, it is never-
theless interesting that the UV spectrum of umbellulone is
similar to that of cross-conjugated dienones,[34] which suggests
that this compound behaves as the cyclopropylogous version
of a reactive cyclopentadienone (Scheme 1, A’). A very
unusual electronic structure for umbellulone is also suggested
by the dramatic upfield shift (ca. 25 ppm) of the cyclopropane
methylene (C5) upon hydrogenation of the double bond
(d C5 = 38.1 in 2, and 13.7 in its dihydro derivative, see the
Supporting Information). The inactivity of piperitone and
verbenone in the thiol assay shows that the presence of a
cyclopropane ring is critical for the Michael addition. Since a
cyclopropane ring has a strong stabilizing effect on an
adjacent cationic center,[35] it is tempting to speculate that
a,b-unsaturated carboxylic acids were deprotonated, and no
reaction took place. Remarkably, the diterpene pseudolaric
acid B[9] (29) was totally unreactive with cysteamine, despite
the presence of two unsaturated carbonyl groups and one
strained lactone ring.
Having established a quick and straightforward method to
identify Michael acceptors, we validated its extension to
biological systems using the activation of TRPA1, a thiol-
sensitive assay, as an end point.[26] TRPA1 has an intracellular
thiol-rich ankyrin domain, the alkylation of which by Michael
acceptors triggers opening of the channel pore and the
development of an ion current, easily detectable by calcium
imaging.[26] Various classes of thiol traps (sulfinates, isocya-
nates, enals)[27] are known to activate TRPA1,[28] and we
validated the extension of our assay to biological systems with
the exomethylene-g-lactones 4 and 6–8, which gave a clear
activation of TRPA1 in HEK 293 cells transfected with
rTRPA1 (EC50 = (15.75 Æ 0.01), (25.9 Æ 4.9), (63.9 Æ 6.0), and
Angew. Chem. Int. Ed. 2011, 50, 467 –471
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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