Biomimetic total synthesis of gambogin and rate acceleration of pericyclic reactions in aqueous media

Article abstract of DOI:10.1002/anie.200462211

A significant rate acceleration of a Claisen rearrangement and of a Claisen/Diels-Alder reaction cascade sequence was observed in the presence of water. This reaction sequence was used in a total synthesis of gambogin (1) from a tricyclic dialkene 3 via 2

Full text of DOI:10.1002/anie.200462211

Communications
Our retrosynthetic plan for the molecule of gambogin (1),
whose resemblance to forbesione and lateriflorone[5] influ-
enced our thinking, is shown in Scheme 1. Thus, applying a
retro-Claisen rearrangement[6] on ring A (benzopyran ring,
mixture of diastereomers) unraveled the acetylenic benze-
noid compound 2, which could be derived from 3a through O-
Natural Products Synthesis
Biomimetic Total Synthesis of Gambogin
and Rate Acceleration of Pericyclic
Reactions in Aqueous Media**
K. C. Nicolaou,* Hao Xu, and Markus Wartmann
Scheme 1. Retrosynthetic analysis of gambogin (1): a) Claisen rearrangement;
b) O-alkylation; c) Diels–Alder reaction.
Gambogin (1, Scheme 1) has an unusual molecular
architecture and exhibits cytotoxic properties against
the Hela and HEL cell lines (MIC: 6.25 and
3.13 mgmLꢀ1, respectively).[1] Isolated from the gamboge
resin of Garcina hamburyi in 1996, this naturally occurring
substance provides an intriguing synthetic challenge and an
opportunity for the development of new synthetic technology
and biological tools. Herein we report a biomimetic[2] total
synthesis of gambogin[3] and the observation of dramatic rate
accelerations of the Claisen rearrangement and the Claisen/
Diels–Alder cascade reaction[4] in protic solvents, most
notably in water.
alkylation and, yet again, a Claisen rearrangement. Finally,
the
cagelike
trioxatetracyclo[7.4.1.02,7.02,11]tetradecane
system[7] was dismantled as previously[5] to the BCD tricycle
4, whose construction from readily available starting materi-
als was reasonably anticipated. Most significantly, the syn-
thetic plan that emerged from this analysis offered us the
opportunity to investigate the possibility of accelerating
pericyclic reactions such as the Claisen and Diels–Alder
reactions beyond their previously defined boundaries.
The required intermediate 4 was synthesized as summar-
ized in Scheme 2. Phloroglucinol (5) was protected with three
MOM groups, and the resulting compound was brominated
with NBS to afford intermediate 6 in 62% overall yield.
Lithiation of 6, followed by trapping of the generated anion
with fully functionalized benzaldehyde 7[5b,c] in diethyl ether
generated bis-aromatic system 8a in 85% yield. However,
when THF was used as the solvent in large-scale operations
for solubility considerations, a spontaneous silyl group
migration[8] took place, affording 8b cleanly and in 85%
yield. Desilylation of 8a or 8b with TBAF and subsequent
oxidation with MnO2 furnished ketone 9 in 68% overall yield.
Ketone 9 was heated at reflux in a solution of KOH (EtOH/
H2O) which induced an intramolecular conjugate addition/
elimination[9] to afford tricyclic system 10. Subsequent hydro-
genolysis of the two benzyl groups of 10 in THF led to the
highly oxygenated xanthone 11 in 76% overall yield. Lacto-
lization of the catechol system of 11 through our previously
reported method[5b,c] (KOtBu, [18]crown-6) failed to produce
the desired lactols (12a/12b) in high yield. The procedure was
[*] Prof. Dr. K. C. Nicolaou, H. Xu
Department of Chemistry and
The Skaggs Institute for Chemical Biology
The Scripps Research Institute
10550 North Torrey Pines Road, La Jolla, CA 92037 (USA)
Fax: (+1)858-784-2469
E-mail: kcn@scripps.edu
and
Department of Chemistry and Biochemistry
University of California, San Diego
9500 Gilman Drive, La Jolla, CA 92093 (USA)
Dr. M. Wartmann
Novartis Institutes for BioMedical Research
Basel (Switzerland)
[**] We thank Dr. D. H. Huang and Dr. G. Siuzdak for NMR spectro-
scopic and mass spectrometric assistance, respectively. We thank
Robert Reuter (Novartis) for the biological evaluations of gambogin
and its analogues. Financial support for this work was provided by
grants from the National Institutes of Health (USA) and the Skaggs
Institute for Chemical Biology, and a fellowship from the Skaggs
Institute for Research (to H.X.).
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DOI: 10.1002/anie.200462211
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modified so as to include NaHMDS and [15]crown-5
instead. The disodium salt of 11, generated by
addition of NaHMDS, was first suspended and then
completely dissolved in acetonitrile after addition of
[15]crown-5 and sonication. Treatment of this so-
lution with freshly prepared bromoisobutyralde-
hyde[10] afforded a mixture of regioisomeric TMS-
protected lactols (ꢁ 1:1) at 08C in only 20 min. The
unexpected silylation of the lactol moiety might be
rationalized by envisioning an intermolecular lactol
anion attack of the hexamethyldisilazane generated
in situ owing to the enhanced nucleophilicity of the
former. Desilylation (TBAF) followed by reaction
of the resulting lactols (12a/12b) with methylene
phosphorane provided regioisomeric olefins 13a
and 13b (ꢁ 1:1, 57% over three steps). The original
KOtBu/[18]crown-6 protocol proved successful in
the second alkylation. Thus, a Wittig olefination
sequence converted 13a/13b, via aldehydes 14a/
14b, into the desired diolefin 4 in 74% overall yield.
Upon heating in DMF at 1208C, the xanthone
derivative 4 smoothly underwent the expected
Claisen/Diels–Alder cascade sequence through the
presumed intermediates 4a and 4b (Scheme 3) to
furnish 3a and 3b in 69% and 23% yield, respec-
tively. The structural assignments of these com-
pounds were supported by NOE studies. Removal of
the MOM groups from the desired isomer 3a was
smoothly effected through the action of HCl (1.0m
in CH2Cl2/Et2O (1:1)), which generated catechol 15
in 85% yield. As one of the phenolic groups in 15
was deactivated owing to internal hydrogen bond-
ing, we were able to effect selective monopropargy-
ꢂ
lation of the other with CH CC(Me)2OCOCF3 in
the presence of DBU and CuCl[11] in 60% yield
(85% based on 70% conversion). Further selective
reduction and acetylation furnished olefin 17, whose
Claisen rearrangement proceeded regioselectively
and in 69% overall yield from 16, to afford the
prenylated aromatic system 18 as a single isomer. A
ꢂ
second propargylation, this time with CH CC(Me)-
[12]
=
[CH2CH2CH CC(Me)2]OCOCF3, was applied to
convert 18 into 2 cleanly, albeit in relatively low
conversion as a result of the steric hindrance
encountered in the transition state (26% yield;
88% based on 30% conversion). Finally, heating of
propargyl ether 2 in [D7]DMF at 1408C led directly
to gambogin (1) as a yellow foam in 65% yield.[13]
The high temperature employed in the last opera-
tion was apparently sufficient encouragement for
the acetate group to depart. Three gambogin
analogues (20a–c) were synthesized through the
same strategy (Scheme 4).
The conversion of aryl ether 17 into the preny-
lated aromatic system 18 provided a good oppor-
tunity to investigate solvent effects on the rate of the
Claisen rearrangement. Previous work in this
field,[14] including ours,[6a] was suggestive of the
requirement of heating up to 1808C in aprotic
Scheme 2. Synthesis of advanced intermediate 4: a) NaH (4.0 equiv), MOMCl
(3.5 equiv), DMF, 0!258C, 2 h, 65%; b) NBS (1.05 equiv), CH2Cl2, 258C, 1 h,
95%; c) nBuLi (1.08 equiv), THF or diethyl ether, ꢀ788C, 2 h; then 7 (1.0 equiv),
ꢀ78!08C, 0.5 h, 85%; d) TBAF (1.1 equiv), THF, 08C, 15 min, 90%; e) MnO2
(10.0 equiv), CH2Cl2, 258C, 12 h, 75%; f) NaOH (5.0 equiv), EtOH, 908C, 24 h,
80%; g) Pd/C (10 wt%), H2 (1 atm), THF, 258C, 2 h, 95%; h) NaHMDS
(2.2 equiv), THF, 08C, 30 min; then concentrated and suspended in MeCN; then
[15]crown-5 (2.2 equiv), 15 min, bromoisobutyraldehyde (4.0 equiv), 0!258C,
30 min; i) TBAF (1.2 equiv), THF, 08C, 15 min, 71% over two steps;
j) CH3P+Ph3Brꢀ (3.5 equiv), NaHMDS (3.5 equiv), THF, 08C, 1 h; then 12 was
added to the generated ylide, 0!258C, 0.5 h, 80%; k) KOtBu (1.2 equiv), THF,
08C, 30 min; then concentrated and suspended in MeCN; then [18]crown-6
(1.2 equiv), 15 min, bromoisobutyraldehyde (4.0 equiv),
0!258C, 2 h, 85%; l) CH3P+Ph3Brꢀ (1.5 equiv), NaHMDS (1.5 equiv), THF, 08C,
1 h; then 14 was added to the generated ylide, 0!258C, 0.5 h, 87%. MOM=
methoxymethyl, DMF=N,N-dimethylformamide, NBS=N-bromosuccinimide,
TBAF=tetra-n-butylammonium fluoride, HMDS=hexamethyldisilazane.
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solvents such as DMF or N,N-diethylaniline, although
temperatures as low as 608C were successfully employed
by Grieco and co-workers[14f] for a Claisen rearrange-
ment of an aliphatic system. The theoretical studies of
Jorgensen and Severance,[15] in particular, were encour-
aging to us, for they pointed to a favorable hydrogen-
bonding effect in protic solvents on the rate of pericyclic
reactions such as the one under investigation. Table 1
summarizes the results of this study in which various
solvents and temperatures were employed. As expected,
in [D6]benzene, there was no detectable reaction of 17
after 4 h at 1008C, nor did the substrate react in ethanol
or trifluoroethanol at 258C after the same period of
time. In [D7]DMF, however, we began to observe
conversion of 17 into 18 through the Claisen rearrange-
ment at 508C (4.5 h, 25% conversion), whereas at
1008C, the reaction was complete within 0.5 h. The same
accelerating effect was noticed in methanol at 508C
(4.5 h, 50% conversion) and 1008C (0.5 h, 100% con-
version). The addition of water to the latter solvent
(MeOH/H2O = 2:1) resulted in an even faster reaction
(508C: 3.5 h, 100% conversion or 1008C: 0.25 h, 100%
conversion), whereas an increase in the water content of
the solvent mixture to 1:1 provided similar rate accel-
eration (508C: 2.5 h, 100% conversion or 1008C: 0.25 h,
100% conversion). Further addition of water was
detrimental owing to precipitation of the substrate.
The highest rate acceleration, however, was observed in
trifluoroethanol/H2O (1:1) (258C: 75 h, 100% conver-
sion or 408C: 65 h, 100% conversion) and ethanol/H2O
(1:1) (258C: 72 h, 100% conversion or 408C: 60 h,
100% conversion). The ability to carry out this type of
Claisen rearrangement at ambient temperature is
unprecedented and is expected to expand the scope of
this process to include otherwise fragile substrates and/
or products.
Encouraged by these results, we proceeded to
explore the effect of the same solvents on the Claisen/
Diels–Alder cascade sequence involved in the conver-
sion of 4 into 3a (and 3b, see Scheme 3). Table 2 exhibits
the findings of this study. Thus, in [D6]benzene, there
was no reaction at 1008C after 4 h, and no detectable
reactions occurred in ethanol, trifluoroethanol or
[D7]DMF at 658C after 4 h. In [D7]DMF at 1008C,
Scheme 3. Total synthesis of gambogin (1): a) DMF, 1208C, 1 h,
3a (69%) and 3b (23%); b) HCl (1.0m) in Et2O/CH2Cl2 (1:1),
258C, 24 h, 85%; c) propargyl alcohol (1.4 equiv), DBU
(1.4 equiv), TFAA (1.4 equiv), MeCN, 08C, 30 min; DBU
(1.5 equiv), CuCl (0.1% equiv), 15 min; then TFA propargyl
ester, 08C, 5 h, 60% (85% based on 70% conversion); d) Lin-
dlar catalyst (10 wt%), H2 (1 atm), EtOAc (contaminated with
0.02% quinoline), 258C, 1 h, 95%; e) Ac2O (10.0 equiv), py
(10.0 equiv), 4-DMAP (1.0 equiv), CH2Cl2, 258C, 8 h, 85%;
f) DMF, 1208C, 0.5 h, 85%; g) propargyl alcohol (1.4 equiv),
DBU (1.4 equiv), TFAA (1.4 equiv), MeCN, 08C, 30 min; DBU
(1.5 equiv), CuCl (0.1% equiv), 15 min; then TFA propargyl
ester, 08C, 5 h, 26% (88% based on 30% conversion);
h) [D7]DMF, 1408C, 2 h, 65%. DBU=1,8-diazabicyclo[5, 4,
0]undec-7-ene, TFAA=trifluoracetic anhydride, py=pyridine, 4-
DMAP=4-dimethylaminopyridine.
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Table 2: Solvent effects on the rate of the Claisen/Diels–Alder cascade of
4 into 3a.[a]
Entry
Solvent
T [8C]
t [h]
Conversion [%]
1
2
[D6]Benzene
EtOH
100
65
4
4
0
0
3
4
5
6
7
8
9
10
11
12
13
14
15
TFE
[D7]DMF
[D7]DMF
MeOH
65
65
100
65
100
65
100
65
100
65
65
65
4
4
2.0
4
1.0
4.5
1.0
4.0
0.5
4.0
4.0
3.5
0.5
0
0
75
0
100
100
100
100
100
100
100
100
100
Scheme 4. Synthesis of analogues 20a–c of gambogin: a) [D7]DMF, 1408C,
2 h, 65%.
Table 1: Solvent effects on the rate of the Claisen rearrangement of 17
into 18.[a]
MeOH
MeOH/H2O (2:1)
MeOH/H2O (2:1)
MeOH/H2O (1:1)
MeOH/H2O (1:1)
TFE/H2O (1:1)
EtOH/H2O (1:1)
MeOH/H2O (1:2)
MeOH/H2O (1:2)
100
[a] The reactions were carried out on a 0.1–1.0-mmol scale and
monitored by disappearance of starting material by TLC and H NMR
Entry
Solvent
T [8C]
t [h]
Conversion [%]
1
spectroscopy. [b] The ratio of the two products 3a:3b (see Scheme 3)
stayed constant (ꢁ3:1) during the course of the reactions.
1
2
3
[D6]benzene
EtOH
100
25
25
4
4
4
0
0
0
TFE[b]
4
5
6
7
8
9
10
11
12
13
14
15
[D7]DMF
[D7]DMF
MeOH
MeOH
MeOH/H2O (2:1)
MeOH/H2O (2:1)
MeOH/H2O (1:1)
MeOH/H2O (1:1)
TFE/H2O (1:1)
TFE/H2O (1:1)
EtOH/H2O (1:1)
EtOH/H2O (1:1)
50
100
50
100
50
100
50
100
25
40
25
40
4.5
0.5
4.5
0.5
3.5
0.25
2.5
0.25
75
25
100
50
Severance[15] based on Monte Carlo calculations, postulates
one hydrogen bond from the solvent to one of the lone pairs
of electrons of the ethereal oxygen atom in the ground state 4
and two hydrogen bonds from two solvent molecules to both
lone pairs of electrons of the oxygen atom in the transition
state 4-TS, by virtue of the distortion of conjugation as the
reaction begins to occur, a condition that results in catalysis.
This bifurcated-hydrogen-bond hypothesis has found corrob-
oration in the X-ray crystal structures of chorismate mutase
isolated from Bacillus subtilis (BsCM)[17] and Escherichia coli
(EcCM),[18] the enzyme that catalyzes the Claisen rearrange-
ment[19] of the chorismate anion to the prephenate anion.
Thus, whereas the polar aprotic solvent DMF can only
accelerate the nonsynchromic, semi-zwitterionic[20] Claisen
rearrangement by stabilizing its polar transition state, the
protic solvents, because of their hydrogen-bond-donor abil-
ities[21] (most notably water), are able to provide additional
stabilization of the transition state and thus promote the
dramatic acceleration effects observed.[22] It was interesting to
note that a further increase in the water content in these
alcoholic solvents resulted in the precipitation of the substrate
and shut-down of the reaction, presumably as a result of the
insufficient solvation that accompanied the observed precip-
itation of the reacting substrate. The concurrent acceleration
of the Diels–Alder component of the cascade is due, as
explained by Breslow and co-workers,[16,23] to the hydrophobic
effect[24] and the unique internal pressure of water rather than
simple polarity or hydrogen-bonding phenomena. It was
interesting to note that at no temperature in any solvent
system could we detect any of the postulated intermediate
dienone 22, leading us to conclude that the Diels–Alder[25]
100
100
100
100
100
100
100
100
100
65
72
60
[a] The reactions were carried out on a 0.1–1.0-mmol scale and
monitored by disappearance of starting material by TLC and H NMR
spectroscopy. [b] TFE=trifluoroethanol.
1
however, the reaction started to proceed (2 h, 75% conver-
sion). In methanol, although there was no reaction at 658C
after 4 h, the sequence was complete after only 1 h at 1008C.
An increase in the amount of water in the methanolic solvent
(MeOH/H2O 2:1!1:1!1:2) caused a rather dramatic effect
in rate acceleration (100% conversion at 1008C after 0.5–1 h,
Table 2, entries 9, 11, and 15, or 100% conversion at 658C
after 3.5–4.5 h, Table 2, entries 8, 10, and 14). This remarkable
effect was also observed more or less unchanged in ethanol/
water (1:1) and trifluoroethanol/water (1:1) mixtures
(Table 2, entries 12 and 13).
The explanation for the observed rate acceleration of both
the Claisen rearrangement and the Diels–Alder reaction
constituting the cascade sequence in the conversion of 4 into
3a can be found in previous theoretical as well as exper-
imental work.[14,16] For the Claisen rearrangement, the
model[14g] shown in Figure 1, as developed by Jorgensen and
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Keywords: Claisen rearrangement · Diels–Alder
.
reaction · natural products · rate acceleration · total
synthesis
[1] J. Asano, K. Chiba, M. Tada, T. Yoshii, Phytochemistry
1996, 41, 815 – 820.
[2] For a number of biologically active, phloroglucinol-
derived natural products, see: a) M. Tada, K. Chiba, H.
Yamada, H. Maruyama, Phytochemistry 1991, 30, 2559 –
2562; b) M. Nagai, M. Tada, Chem. Lett. 1987, 1337 –
1340.
[3] An elegant total synthesis of gambogin was recently
reported: E. J. Tisdale, I. Slobodov, E. A. Theodorakis,
Proc. Natl. Acad. Sci. USA 2004, 101, 12030 – 12035; for
other related publications from this group, see: a) E. J.
Tisdale, C. Chowdhury, B. G. Vong, H. Li, E. A.
Theodorakis, Org. Lett. 2002, 4, 909 – 912; b) E. J.
Tisdale, B. G. Vong, H. Li, S. H. Kim, C. Chowdhury,
E. A. Theodorakis, Tetrahedron 2003, 59, 6873 – 6887;
c) E. J. Tisdale, I. Slobodov, E. A. Theodorakis, Org.
Biomol. Chem. 2003, 1, 4418 – 4422.
[4] For recent reviews on biomimetic and cascade-type
strategies in total synthesis, see: a) K. C. Nicolaou, S. A.
Snyder, T. Montagnon, G. E. Vassilikogiannakis, Angew.
Chem. 2002, 114, 1742 – 1773; Angew. Chem. Int. Ed.
2002, 41, 1668 – 1698; b) K. C. Nicolaou, T. Montagnon,
S. A. Snyder, Chem. Commun. 2003, 551 – 564.
Figure 1. Molecular-orbital rationale for the acceleration of the Claisen/Diels–Alder
cascade reaction (4!3a) by protic solvents.
[5] For previous total syntheses of natural products with
trioxatetracyclo [7.4.1.02,7.02,11]tetradecane-type struc-
tures from this group, see: a) K. C. Nicolaou, J. Li, Angew.
Chem. 2001, 113, 4394 – 4398; Angew. Chem. Int. Ed. 2001, 40,
4264 – 4268; b) K. C. Nicolaou, P. K. Sasmal, H. Xu, K. Namoto,
A. Ritzꢀn, Angew. Chem. 2003, 115, 4357 – 4361; Angew. Chem.
Int. Ed. 2003, 42, 4225 – 4229; c) K. C. Nicolaou, P. K. Sasmal, H.
Xu, J. Am. Chem. Soc. 2004, 126, 5493 – 5501.
intramolecular collapse was the faster of the two pericyclic
reactions involved in this cascade. To the best of our
knowledge, this is the first example of a Claisen/Diels–
Alder cascade sequence accelerated by water.
The synthesized compounds 20a–c, except for 20a, which
are diastereomeric mixtures, like gambogin 1 itself, were
tested for their cytotoxicity against human epidermoid cancer
cell line KB-31 and its taxol-resistant mutant cell line KB-
8511. The results are shown in Table 3. Both the methyl- and
isopentyl-substituted analogues 20a and 20c exhibited
[6] For recent examples of Claisen rearrangements in organic
synthesis, see: a) K. C. Nicolaou, J. A. Pfefferkorn, A. J.
Roecker, G.-Q. Cao, S. Barluenga, H. J. Mitchell, J. Am. Chem.
Soc. 2000, 122, 9939 – 9953, and references therein; b) S.
Yamaguchi, M. Ishibashi, K. Akasaka, H. Yokoyama, M.
Miyazawa, Y. Hirai, Tetrahedron Lett. 2001, 42, 1091 – 1093;
c) I. Iwai, J. Ide, Chem. Pharm. Bull. 1963, 11, 1042 – 1049; d) J.
Hlubucek, E. Ritchie, W. C. Taylor, Tetrahedron Lett. 1969, 10,
1369 – 1370; e) W. K. Anderson, E. J. LaVoie, P. G. Whitkop, J.
Org. Chem. 1974, 39, 881 – 884.
Table 3: Cytotoxicity of gambogin (1) and its analogues (20a–c).[a]
Entry
Compound
IC50 [mM]
KB-31
KB-8511
[7] For the Claisen-rearrangement-based hypothesis regarding the
biosynthesis of Garcina family natural products, see: a) A. J.
Quillinan, F. Scheinmann, J. Chem. Soc. Perkin Trans. 1 1972,
1382 – 1387; b) A. J. Quillinan, F. Scheinmann, J. Chem. Soc.
1971, 966 – 967.
[8] J. Mulzer, B. Schꢁllhorn, Angew. Chem. 1990, 102, 433 – 434;
Angew. Chem. Int. Ed. Engl. 1990, 29, 431 – 432.
[9] S. Horne, R. Rodrigo, J. Org. Chem. 1990, 55, 4520 – 4522.
[10] A. L. J. Bechwith, C. B. Thomas, J. Chem. Soc. Perkin Trans. 2
1973, 861 – 872.
1
2
3
4
1
9.35
8.41
>10
6.01
>10
>10
>10
>10
20a
20b
20c
[a] The antiproliferative effects of the tested compounds were assessed
in human epidermoid cancer cell lines: the parent cell line (KB-31) and
the taxol-resistant (due to Pgp-overexpression) cell line (KB-8511).
slightly higher potencies than the natural compound 1 against
KB-31, while the ethyl derivative 20b was less active. All
compounds, including gambogin itself, failed to exhibit
significant cytotoxicity against the taxol-resistant mutant
cell line KB-8511 at concentrations below 10 mm. The
chemistry described herein may provide entries to biologi-
cally active molecules of the gambogin type and stimulate
further studies in the reaction process development.
[11] J. D. Godfrey, Jr., R. H. Mueller, T. C. Sedergran, N. Soundar-
arajan, V. J. Colandrea, Tetrahedron Lett. 1994, 35, 6405 – 6408.
[12] P. H. Kahn, J. Cossy, Tetrahedron Lett. 1999, 40, 8113 – 8114.
[13] We thank Professor M. Tada (Tokyo University of Agriculture
and Techonology, Japan) for sending us the 1H NMR and
13C NMR spectra of natural gambogin for comparison.
[14] For Claisen rearrangements accelerated in protic solvents, see:
a) J. F. Kincaid, D. S. Tarbell, J. Am. Chem. Soc. 1939, 61, 3085 –
3089; b) W. N. White, D. Gwynn, R. Schlitt, C. Girard, W. Fife, J.
Am. Chem. Soc. 1958, 80, 3271 – 3277; c) H. L. Goering, R. R.
Jacobson, J. Am. Chem. Soc. 1958, 80, 3277 – 3285; d) R. M.
Received: October 5, 2004
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[26] We thank Professor K. B. Sharpless and Professor M. G. Finn for
helpful discussions on the rate-acceleration phenomena in water.
Angew. Chem. Int. Ed. 2005, 44, 756 –761
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