Synthesis of maculalactone A and derivatives for environmental fate tracking studies

Article abstract of DOI:10.1039/c4ob02042a

Maculalactone A (1) constitutes a promising antifouling agent, inhibiting the formation of biofilms in marine and freshwater systems. In this study, we developed a new route, based on a late-stage formation of the butenolide core, leading to the total synthesis of maculalactone A (three steps, overall yield of 45%) and delivering material on a gram scale. In addition, analogues of the title compound were assayed concerning their biological activity, utilizing Artemia franciscana and Thamnocephalus platyurus. The most active analogue was functionalized with a rhodamine B fluorophore and was utilized in an in vivo staining experiment in Artemia salina. Two different tissues were found to accumulate this maculalactone A derivative. This journal is

Full text of DOI:10.1039/c4ob02042a

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Biomolecular Chemistry
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Synthesis of maculalactone A and derivatives for
environmental fate tracking studies†
Cite this: Org. Biomol. Chem., 2015,
13, 199
Samuel L. Bader, Michael U. Luescher and Karl Gademann*
Maculalactone A (1) constitutes a promising antifouling agent, inhibiting the formation of biofilms in
marine and freshwater systems. In this study, we developed a new route, based on a late-stage formation
of the butenolide core, leading to the total synthesis of maculalactone A (three steps, overall yield of 45%)
and delivering material on a gram scale. In addition, analogues of the title compound were assayed con-
cerning their biological activity, utilizing Artemia franciscana and Thamnocephalus platyurus. The most
active analogue was functionalized with a rhodamine B fluorophore and was utilized in an in vivo staining
experiment in Artemia salina. Two different tissues were found to accumulate this maculalactone A
derivative.
Received 25th September 2014,
Accepted 20th October 2014
DOI: 10.1039/c4ob02042a
www.rsc.org/obc
Introduction
The accumulation of biological material on all surfaces
immersed in biological fluids (referred to as biofouling) is a
serious challenge for marine vessels, fresh water treatment
and industrial plants.1–5 The ban of previously employed tribu-
tyltin coatings by international organizations further aggra-
vated this issue.6 A bio-inspired approach to address this
problem constitutes the use of natural products as antifouling
agents.7,8 In recent years, marine butenolide natural products
attracted interest, as they show good antifouling activity and
quorum-sensing inhibition.9–12 In the context of our own
research, we have developed several natural product based
approaches to prevent biofouling, ranging from novel algicidal
natural products,13–15 passivated metal oxide surfaces with
antifouling properties16–18 to active surfaces, functionalized
with natural products, for controlling or inhibiting bacterial
growth.19,20
The slowly growing cyanobacterium, Kyrtuthrix maculans,
forms pure colonies on rocky shores, which, by the absence of
other organisms in these colonies, suggests that some sort of
chemical defence could be operating. Based on these obser-
vations, the Brown group isolated the maculalactones, a family
of interesting secondary metabolites, from this cyanobacter-
ium (Fig. 1). While maculalactone A (1), B, C and L consist of a
benzylated γ-lactone, maculalactone D, E, F (2), G, H, I, J and K
Fig. 1 Selected maculalactones isolated from Kyrtuthrix maculans:
maculalactone A,21 F and K.22
(3) include an additional phenylpropanoid unit forming a six
membered carbocycle.21–24 Maculalactone A constitutes the
most abundant congener in this series found in K. maculans.
In addition, Brown et al. conducted preliminary antifouling
experiments by submerging structures coated with a macula-
lactone A containing paint in the sea.25 After 12 weeks, a
reduced area coverage with bivalves was observed. Although
this reduction was partially compensated by an increased
growth of algae, it demonstrated the potential of 1 as an anti-
fouling agent. However, little is known about the structural
requirements or the mode of action of maculalactone A (1). In
addition, the isolation of larger amounts of 1 from the slow-
growing producer is unpractical and the three published total
syntheses offer overall yields below 10%.26–28 In this study, we
have developed an efficient preparation of maculalactone A (1),
performed a preliminary structure–activity relationship study
and investigated the environmental fate of this compound in
model crustaceans via labelled analogues.
Department of Chemistry, University of Basel, St. Johanns-Ring 19, 4056 Basel,
Switzerland. E-mail: karl.gademann@unibas.ch
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
c4ob02042a
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oxidation, or benzylated in a 1,2-addition to obtain alcohol 12.
The two parts were combined in a Steglich esterification,
giving diene 13 in an overall yield of 42% and 38%, respect-
Results and discussion
Improved total synthesis of maculalactone A
To access gram-scale amounts of maculalactone A (1) and its ively. However, the subsequent metathesis reaction under opti-
37
structural analogues, an improved synthesis had to be develo- mized conditions and in the presence of Ti(OiPr)4 resulted
ped. The new route should not only result in a higher overall in a low conversion of 11%. Despite this disappointing result,
yield, but should also allow for the modification of the this route allowed for the first time the modification of a
different benzyl groups. We first targeted an optimization of single benzyl moiety.
the published routes to maculalactone A.26–28 A survey of those
Another potentially rewarding strategy constitutes the late
revealed a general problem in the nucleophilic introduction of stage preparation of the olefinic bond through an elimination
benzyl groups to the butenolide core. In a first phase, we tried approach. For this goal, we investigated a procedure originally
to work around such a nucleophilic benzylation. Messorosh reported by Avetisyan et al.39 in 1981 and later utilized in an
et al. recently described the radical benzylation of 2,3-dichloro- industrial setting.40 Key to this strategy is the intramolecular
maleic anhydride (4) in good yield.29 The resulting anhydride, cyclization of 3,6-dioxo-4-oxa-1-hexanoate derivative, 17, with a
6, is a central intermediate in the synthesis shown by Brown subsequent loss of ethanol and CO2, furnishing the desired
et al. and Argade et al.26,27 However, the dichloride, 4, is butenolide (Scheme 3). In order to test this hypothesis, malo-
expensive and difficult to prepare in the laboratory. The corres- nate 14 was saponified to the half-ester, 15, in a single step41
ponding dibromide, 5, was synthesised30 and subjected to the
same conditions, giving 6 in 28% yield (Scheme 1). Overall,
this route led to no improvement when compared to the
known syntheses of 1.
Alternatively,
a shortcut in the route investigated by
Argade27 used an electrophilic substitution at the γ-position of
intermediate 7 (Scheme 1). An enantioselective phase-transfer
catalyst mediated procedure was expected to give optically
enriched maculalactone A. However, the benzyl group was
directed to the α-position giving the achiral lactone 9.
Since an improvement of the previously published routes
did not appear feasible, new strategies were considered. There-
fore, we focused on strategies with a late stage butenolide for-
mation, thereby, avoiding benzylic disconnections. Among the
various known methods to synthesise butenolides, only few
approaches allow substituents in all three positions.32–34
Alkene metathesis has been applied for butenolide synth-
eses by different groups.35–37 Diene 13 was therefore syn-
Scheme 2 Synthesis of maculalactone A by a late stage ring closing
thesised in a convergent fashion from the starting material 10, metathesis.
which was accessed as described by Pihko et al.38 (Scheme 2).
The enal 10 was either converted to acid 11, applying a Pinnick
Scheme 1 Investigated shortcuts in the total syntheses shown by
Brown et al. and Argade et al. The reaction from 4 to 6 was previously
published by Messorosh et al.29 PTC = N-anthrylated quinine chloride,
synthesised as described by Hofstetter et al.31
Scheme 3 Synthesis of maculalactone A by a late stage butenolide for-
mation. Synthesis of 15 from 14, as published by Goel et al.41
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and converted to the corresponding anhydride with pivaloyl
chloride. In the same pot, the anhydride intermediate was
reacted with the known α-hydroxy ketone, 16, which itself is
accessible via a Benzoin-type Umpolung reaction,42 to the
corresponding keto-diester, 17.
The key step, i.e. the late stage cyclization of 17 to buteno-
lide 1, was addressed next. After optimizing the base, solvent
and different additives, the cyclization was accomplished with
caesium carbonate in refluxing THF, giving maculalactone A
(1) in a yield of 76%. In order to drive the reaction to com-
pletion, and to prevent alcoholysis of 17 with the formed
ethanol, the presence of 4 Å molecular sieves was found to be
crucial. Interestingly, the intermolecular version of this reac-
tion led to the formation of product 1, however, only little con-
version could be observed.
We postulate the intramolecular attack of the deprotonated
malonate to the ketone, followed by an intramolecular trans-
esterification (liberation of EtOH) and formation of a β-lactone
as a working hypothesis for this interesting reaction. Sub-
sequent CO2 release would then result in maculalactone A (1).
The mechanism was studied by a series of experiments;
however, direct mass spectrometric detection of the intermedi-
ates was not possible. Interestingly, running the reaction in
sealed tubes did not lead to product formation. Additional
experiments will be necessary to study the mechanism of this
transformation.
Based on this short approach, maculalactone A (1) was syn-
thesised in an overall yield of 45% in three linear steps from
the inexpensive, commercially available starting material, 14.
In addition, this strategy allows for the differentiation of the
benzyl groups via their sequential regioselective introduction.
We took advantage of this approach to access analogues as
described below.
Scheme 4 Synthesis of first generation maculalactone A analogues,
25–27. PMB = para-methoxybenzyl.
Syntheses of maculalactone A derivatives and SAR studies
For further investigations of the mode of action, environ-
mental fate studies and potential applications in antifouling
surface coatings, we were interested in modified maculalac-
tone A derivatives. First, however, the appropriate position for
the modifications had to be identified in a structure–activity
relationship study (SAR). We initially proposed the para-posi-
tions of the respective benzyl groups to have a minimal influ-
ence on the biological activity and, therefore, the three
structural isomers, 25, 26 and 27, each bearing a methoxy
group in the para-position, were synthesised (Scheme 4). Malo-
nate, 18, was synthesised based on a reported procedure43 and
converted to its potassium salt, 19, at reduced temperature to
prevent decarboxylation. The differently substituted α-hydroxy
ketones Bn–CO–CH(OH)–PMB (20) and PMB–CO–CH(OH)–Bn
(21) were prepared from the corresponding aldehydes via for-
mation of the TMS protected cyanohydrins, followed by the
addition of the respective Grignard reagent, as described in
the literature.44,45 The formation of ester, 22–24, proceeded
smoothly, as did the ring closure (yields of 40 to 69% over two
steps).
Fig. 2 Biological evaluation of the first generation maculalactone A
analogues. Grey: A. franciscana, black: T. platyurus. Values are given as
mean of three independent experiments with ten animals; error bars
denote the standard error of the mean (SEM).
We were then interested in profiling the biological activity
of maculalactone A and its analogues. In the work of Brown
et al., the LC50 values of extracted maculalactone A were
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determined in ecotoxicity assays utilizing different stages of centration (10 μM), the ability of the animals to swim in a con-
Tetraclita japonica, Balanus amphitrite, Ibla cumingii and trolled manner is strongly impaired (for a video, see the ESI†).
Artemia salina,25 of which the latter is often used for reference
As analogue 25 showed a similar or higher toxicity as the
toxicity experiments. For comparison, we evaluated the com- parent maculalactone A, it was used for further derivatisation
pounds in the related Artemia franciscana as well as the sensi- studies. The methoxy group was quantitatively cleaved in the
tive freshwater crustacean, Thamnocephalus platyurus, which is presence of boron tribromide to give phenol 28, which was
well-established in our group.46,47
coupled with different residues (Scheme 5). First, etherifica-
The assays show a comparable (same order of magnitude) tion with a triethylene glycol unit48,49 gave the short PEGylated
but slightly lower toxicity of the synthetic maculalactone A (1) lactone 29. After mesitylation of phenol 28, it was coupled
in A. franciscana when compared to the report of Brown et al.25 with an octylborane derivative in a Suzuki reaction, affording
This difference is either caused by racemic 1 or by differences compound 30. Furthermore, phenol 28 was added to the acti-
in the assay set-up, and should not be of concern. Interest- vated PEG azide 31. By a Click reaction, this intermediate was
ingly, compound 25, featuring the PMB group in the α-posi- connected to the rhodamine B derivative 32 50 giving the fluor-
tion, exhibits an increased toxicity in A. franciscana, whereas escently labelled maculalactone A analogue, 33. Additionally,
isomeric lactones 26 and 27 display lower or no toxic effects on compound 34, with a free hydroxyl group instead of the macu-
the test organism (Fig. 2). In addition to the toxicity, the assays lalactone A moiety, was synthesised as a negative control.
exposed a strong effect on the swimming ability of the test However, this compound proved to be unstable (amide hydro-
organisms. While fast movement is observed in both the blank lysis), and, after repeated purification, could only be obtained
and in a 1 μM maculalactone A solution, upon increased con- in very low yield.
Scheme 5 Deprotection and modification of the most active maculalactone A analogue, 25. EtO(CH2CH2O)3Ts was prepared as described in the lit-
erature.48,49 Compound 32 was synthesised as described by Yan et al.50
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Fig. 3 Toxicological tests of modified maculalactone A analogues using
A. franciscana as test organism. Values are given as mean of three inde-
pendent experiments with ten animals; error bars denote the standard
error of the mean (SEM).
Consequently, the PEGylated compound, 29, and the fluor-
escently labelled, rhodamine B derivative, 33, were subjected
to the toxicity assay with A. franciscana (Fig. 3). Surprisingly,
compound 29, with the short PEG residue, did not display any
toxicity in this assay. However, the rhodamine B bearing ana-
logue, 33, exhibited toxicity comparable to the one observed
for 1. Therefore, this fluorescent analogue, 33, was used as a
model to examine the distribution of maculalactone A in
crustaceans.
Fig. 4 Overlay of four microscope pictures of A. salina nauplia, stained
with compound 33 (visual and fluorescence images). Scale bar
100 μm.
=
Visualization of the maculalactone A distribution in
Artemia salina
The media of A. franciscana was found to be incompatible with
the subsequent visualization steps (fixation with formalin).
Therefore, a more robust species of A. salina was employed for
the in vivo experiments with the rhodamine B labelled macula-
lactone A derivative, 33.
A marker concentration of 1 μM was found to be optimal,
as no toxic effects were determined at this level. The two day
old animals were incubated for 24 hours in 1 μM solutions of
the marked maculalactone A, 33, the negative control, 34, rho-
damine B and a blank solution. This was followed by a
washing procedure were the crustaceans were twice transferred
into fresh media and incubated for three hours. Finally, the
larvae were fixed in a formalin solution in PBS buffer. The
stained and fixed organisms were examined by fluorescence
microscopy under identical lighting conditions (see the ESI†
for the micrographs). While rhodamine B led to a general
staining of the entire body, the negative control, 34, did not
show any fluorescence at all. In contrast, the maculalactone A
marker, 33, led to a well defined distribution in the animal
with a selective staining of certain organs (Fig. 4). A highly
stained rod-shaped tissue in the centre of the body is observed,
which can be attributed to the intestine,51 and is sharply ter-
minated towards the tail region.
Fig. 5 The tail region of Artemia salina nauplia stained with compound
33. Visual and fluorescence images; 10 μm slides in the z-axis; scale
bar = 50 μm.
which can be rationalized by a selective staining and little or
no diffusion. A similar, well-defined staining is observed
towards the head region (see the ESI† for details). In summary,
the evidence presented supports the notion that the labelled
maculalactone A derivative is accumulated in the animal in
well-defined tissue. However, as little is known about the
anatomy of A. salina in larvae stage, a more detailed assign-
ment of the involved tissues will require further experiments
based on the preliminary successful use of the fluorescently
labelled probe, 33.
Further support for this observation stems from the exam-
ination using a confocal microscope, which disclosed more
details of this region. Slices through the animal (Fig. 5) reveal
a tube-like structure, which is in agreement with the hypo-
thesis that the intestine, but not its content, is stained by the
marker, 33. Further evidence is presented by the well-defined
localization of the fluorescently labelled natural product, 33,
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HPLC analysis was performed with a Dionex-Chromatography
system using a P680 pump and a PDA-100 detector. Preparative
Conclusions
In conclusion, we have developed a new and efficient route to reversed-phase HPLC was performed using Varian Prep Star
maculalactone A (1) that delivered gram-amounts of material pumps and a Dionex UltiMate 3000 RS detector or a Dionex
in three steps and 45% overall yield. The key step of the syn- P680 pump and a Dionex PDA-100 detector. Melting points
thesis employed a late stage butenolide formation, involving (Mp) were determined using a Büchi B545 apparatus in open
an intramolecular malonate addition with subsequent extru- capillaries and are uncorrected. Absorptions spectra (UV/VIS)
sion of CO2. This route enabled the preparation of maculalac- were recorded on a Shimadzu UV-1650 PC instrument. Emis-
tone analogues, 25, 26 and 27, of which their biological sion spectra were recorded on a Shimadzu RF-5301 PC instru-
activity was compared in a toxicity assay using A. franciscana ment. Additional procedures and characterization data can be
and T. platyurus. Significant activity was observed with com- found in the ESI.†
pound 25, which was further modified. The rhodamine B
labelled derivative, 33 proved to be the most interesting for
tissue distribution and environmental fate studies in A. salina.
Staining of specific tissues of the organisms supported the
hypothesis that selective targets can be reached by maculalac-
tone in the animal. However, further investigations are
required to understand the distribution of maculalactone A (1)
in living organisms, which should be enabled by the probe
reported herein.
Potassium 2-benzyl-3-ethoxy-3-oxopropanoate (15)
A published procedure41 was modified as follows: to a solution
of diethyl benzylmalonate (14, 5.11 g, 20.0 mmol, 1.00 eq.) in
EtOH (20 ml) a freshly filtered solution of potassium hydroxide
(1.39 g, 21.0 mmol, 1.05 eq.) in EtOH (20 ml) was added at
room temperature. The reaction was stirred for six hours at
room temperature. The precipitate was filtered off and the fil-
trate was concentrated to a yellowish foam. The residue was
suspended twice in Et2O and again concentrated. After drying
at high vacuum, the foam was triturated overnight in Et2O
(50 ml). The precipitate was isolated by filtration to give the
title compound, 15, (4.44 g, 17 mmol, 85%) as a white solid.
1H-NMR (400 MHz, D2O): 7.30–7.22 (m, 2H), 7.22–7.15 (m,
3H), 4.06–3.96 (m, 2H), 3.51 (dd, J = 8.8, 7.4 Hz, 1H), 3.05–3.00
(m, 2H), 1.07 (t, J = 7.1 Hz, 3H). 13C-NMR (101 MHz, D2O):
176.0, 173.3, 139.1, 128.7, 128.6, 126.6, 62.0, 57.3, 35.0, 13.2.
IR (neat): 3396, 2980, 1710, 1598, 1496, 1454, 1359, 1321, 1234,
1150, 1095, 1061, 1028, 950, 914, 858, 830, 745, 698. HRMS:
Calculated for C12H13O4 [M–K]−: 221.0819, found: 221.0822.
Mp: 120 °C (decomposition). The analytical data match those
reported in the literature.41
Experimental
All chemicals have been purchased from Acros, Alfa Aesar,
Fluka or Sigma-Aldrich and were used without further purifi-
cation if not otherwise reported. Solvents applied for chemical
transformations were either puriss quality or HPLC grade sol-
vents. Dry solvents were purchased from Sigma-Aldrich. Tech-
nical grade solvents for extractions and chromatography were
distilled before usage. Air or moisture sensitive reactions were
set up in dry glassware under nitrogen atmosphere. All syn-
thetic transformations have been monitored by either thin
layer chromatography (TLC) or 1H-NMR spectroscopy. Yields
refer to purified, dried and spectroscopically pure compounds.
TLC was performed on Merck silica gel 60 F254 plates. The
3-Hydroxy-1,4-diphenylbutan-2-one (16)
developed plates were examined under UV light at 254 nm and A published procedure42 was modified as follows: to a solution
stained with an aqueous KMnO4–Na2CO3 solution. Concen- of sodium acetate (328 mg, 4.00 mmol, 0.200 eq.) and freshly
tration was performed by rotary evaporation at 40 °C under distilled phenyl acetaldehyde (4.77 ml, 40.0 mmol, 2.00 eq.) in
reduced pressure (unless otherwise stated). Flash chromato- EtOH (20 ml), 3-benzyl-5-(2-hydroxyethyl)-4-methylthiazolium
graphy was performed using silica gel 60 (230–400 mesh) from chloride (540 mg, 2.00 mmol, 0.100 eq.) was added. The
Merck or Silicycle with a forced flow eluent at 0.1–0.3 bar mixture was heated to reflux for three hours, cooled down and
1
pressure. All H- and 13C-NMR spectra were recorded using a mixed with ice water (20 ml). After stirring for another hour,
Bruker DPX-NMR 400 MHz (1H) and 100 MHz (13C) spectro- the mixture was saturated with NaCl and extracted three times
meter or a Bruker Avance DRX 500 MHz (1H) and 125 MHz with EtOAc. The org. layers were washed with brine, combined,
(13C) spectrometer at room temperature. Chemical shifts (δ- dried with Na2SO4 and concentrated. The crude product was
values) are reported in ppm, spectra were calibrated related to purified by column chromatography (SiO2, pentane–EtOAc
the solvents’ residual proton chemical shifts, multiplicity is (7 : 1 to 5 : 1)) to give the title compound, 16, (2.9 g, 12 mmol,
reported as follows: s = singlet, d = doublet, t = triplet or m = 61%) as a yellow liquid, which crystallized after several days.
multiplet, and the coupling constant, J, in Hz. IR spectra were 1H-NMR (500 MHz, CDCl3): 7.39–7.26 (m, 6H), 7.25–7.12 (m,
recorded on a Varian 800 FT-IR ATR spectrophotometer. The 4H), 4.51 (ddd, J = 7.4, 5.5, 4.9 Hz, 1H), 3.81 (d, J = 15.9 Hz,
absorptions are reported in cm−1. Mass spectra were recorded 1H), 3.75 (d, J = 15.9 Hz, 1H), 3.22 (d, J = 5.6 Hz, 1H), 3.15 (dd,
either on a high-resolution Bruker maXis 4G spectrometer J = 14.1, 4.7 Hz, 1H), 2.90 (dd, J = 14.1, 7.4 Hz, 1H). 13C-NMR
using electrospray ionization (HRMS) or a Finnigan MAT 95Q (126 MHz, CDCl3): 209.2, 136.5, 133.1, 129.6, 129.5, 128.9,
spectrometer using electronic ionization (EI-MS). Elementary 128.8, 127.5, 127.2, 76.8, 45.9, 40.3. The analytical data match
analysis (EA) was measured on a Perkin-Elmer Analysator 240. those reported in the literature.42,44
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1-Ethyl 3-(3-oxo-1,4-diphenylbutan-2-yl) 2-benzylmalonate (17)
as a mixture of diastereomers
The filtrate was concentrated. The crude product was purified
by recrystallization from water and methanol to give the title
compound, 1, (5.1 g, 14 mmol, 76%) as off-white crystals.
1H-NMR (400 MHz, CDCl3): 7.34–7.23 (m, 6H), 7.22–7.10 (m,
5H), 7.03 (d, J = 6.5 Hz, 2H), 6.92–6.85 (m, 2H), 4.99–4.89 (m,
1H), 3.92 (d, J = 15.6 Hz, 1H), 3.63 (d, J = 15.3 Hz, 1H), 3.57 (d,
J = 15.3 Hz, 1H), 3.48 (d, J = 15.6 Hz, 1H), 3.23 (dd, J = 14.5, 3.9
Hz, 1H), 2.82 (dd, J = 14.6, 6.1 Hz, 1H). 13C-NMR (101 MHz,
CDCl3): 173.7, 161.8, 137.8, 136.1, 135.0, 129.6, 129.2, 128.8,
128.7, 128.3, 127.5, 127.3, 126.5, 81.7, 77.4, 38.1, 33.4, 29.5.
The analytical data match those reported in the literature.26
To a suspension of potassium 2-benzyl-3-ethoxy-3-oxopropano-
ate (15, 19.0 g, 72.8 mmol, 1.40 eq.) in dry THF (250 ml) at
0 °C, DMF (201 μl, 5.20 mmol, 0.050 eq.) was added followed
by the dropwise addition of pivaloyl chloride (11.0 ml,
88.4 mmol, 1.70 eq.) over five minutes. During the addition of
pivaloyl chloride, the reaction mixture turned to a solution
and after another 10 minutes to a gel. This gel was then kept
for one hour at 0 °C and for 2.5 hours at room temperature,
during which the gel turned back into a solution. The resulting
solution was concentrated, dried in high vacuum, again dis-
solved in dry CH2Cl2 (250 ml), concentrated and dried again in
high vacuum without exceeding a temperature of 25 °C. The Acknowledgements
residue was dissolved in dry CH2Cl2 (250 ml) and cooled to
The support by the Swiss National Science Foundation
0 °C. The addition of 4-dimethylaminopyridine (635 mg,
5.20 mmol, 0.100 eq.) to the reaction mixture was followed by
the addition of a solution of 3-hydroxy-1,4-diphenylbutan-2-
one (16, 12.5 g, 52.0 mmol, 1.00 eq.) in dry CH2Cl2 (50 ml)
over 10 minutes. The reaction was allowed to reach room temp-
erature overnight before it was quenched at 0 °C with a
mixture of water and a sat. aq. NaHCO3 solution (1 : 1 v/v). The
(200020_130475) is gratefully acknowledged. We thank Dr
Heinz Nadig for MS, Werner Kirsch for EA, Verena Grundler
for HPLC and Nik Hostettler for emission spectroscopic
measurements.
layers were separated and the aq. layer was extracted three Notes and references
times with CH2Cl2. The org. layers were washed with a sat. aq.
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trated. The crude product was purified by column chromato-
graphy (SiO2, pentane–EtOAc (6 : 1)) to give the title
compound, 17, (23.1 g, 52 mmol, quantitative) as a yellowish
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1
liquid. Rf (SiO2, pentane–Et2O (2 : 1)): 0.7. H-NMR (500 MHz,
CDCl3): 7.32–7.20 (m, 8H), 7.20–7.15 (m, 2H), 7.14–7.09 (m,
2H), 7.06–6.99 (m, 3H), 5.35–5.25 (m, 1H), 4.21–4.04 (m, 2H),
3.76–3.69 (m, 1H), 3.58 (s, 1H), 3.50 (d, J = 16.9 Hz, 0.5H), 3.38
(d, J = 16.9 Hz, 0.5H), 3.26–3.11 (m, 2H), 3.06–2.93 (m, 2H),
1.19–1.11 (m, 3H). 13C-NMR (126 MHz, CDCl3): 204.4, 204.3,
168.5, 168.4, 168.4, 168.3, 137.7, 137.6, 135.7, 135.5, 132.9,
132.9, 129.9, 129.9, 129.6, 129.6, 129.0, 129.0, 128.7, 128.7,
128.7, 127.3, 127.3, 127.2, 127.1, 127.1, 79.6, 79.4, 61.9, 61.9,
53.8, 53.7, 46.5, 46.4, 37.1, 34.7, 34.7, 14.1, 14.1. IR (neat):
3031, 2982, 2936, 1732, 1497, 1454, 1369, 1276, 1229, 1149,
701, 633. HRMS: Calculated for C28H32NO5 [M + NH4]+:
462.2275, found: 462.2274. EI-MS (70 eV) m/z (%): 444 (3,
[M]+), 353 (9), 205 (43), 160 (10), 159 (100), 131 (68), 91 (94).
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