A new charge-tagged proline-based organocatalyst for mechanistic studies using electrospray mass spectrometry

Article abstract of DOI:10.3762/bjoc.10.211

A new 4-hydroxy-L-proline derivative with a charged 1-ethylpyridinium-4- phenoxy substituent has been synthesized with the aim of facilitating mechanistic studies of proline-catalyzed reactions by ESI mass spectrometry. The charged residue ensures a stron

Full text of DOI:10.3762/bjoc.10.211

A new charge-tagged proline-based organocatalyst for
mechanistic studies using electrospray mass spectrometry
J. Alexander Willms, Rita Beel, Martin L. Schmidt, Christian Mundt
and Marianne Engeser*
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2014, 10, 2027–2037.
University of Bonn, Kekulé-Institute of Organic Chemistry and
Biochemistry, Gerhard-Domagk-Str. 1, D-53121 Bonn, Germany
doi:10.3762/bjoc.10.211
Received: 06 June 2014
Accepted: 13 August 2014
Published: 28 August 2014
Email:
Marianne Engeser* - Marianne.Engeser@uni-bonn.de
* Corresponding author
This article is part of the Thematic Series "Chemical templates".
Guest Editor: S. Höger
Keywords:
charge tag; electrospray; mass spectrometry; organocatalysis;
proline; template
© 2014 Willms et al; licensee Beilstein-Institut.
License and terms: see end of document.
Abstract
A new 4-hydroxy-L-proline derivative with a charged 1-ethylpyridinium-4-phenoxy substituent has been synthesized with the aim
of facilitating mechanistic studies of proline-catalyzed reactions by ESI mass spectrometry. The charged residue ensures a strongly
enhanced ESI response compared to neutral unmodified proline. The connection by a rigid linker fixes the position of the charge tag
far away from the catalytic center in order to avoid unwanted interactions. The use of a charged catalyst leads to significantly
enhanced ESI signal abundances for every catalyst-derived species which are the ones of highest interest present in a reacting solu-
tion. The new charged proline catalyst has been tested in the direct asymmetric inverse aldol reaction between aldehydes and
diethyl ketomalonate. Two intermediates in accordance with the List–Houk mechanism for enamine catalysis have been detected
and characterized by gas-phase fragmentation. In addition, their temporal evolution has been followed using a microreactor contin-
uous-flow technique.
Introduction
Electrospray ionization (ESI) mass spectrometry [1] has not beyond that a characterization of transient intermediates by
only developed into a standard characterization method for an tandem mass spectrometry. ESI mass-spectrometric mecha-
extremely broad variety of substances [2], but has also been nistic studies have been reported for a broad range of reaction
recognized as a valuable tool for studying reaction mechanisms types ranging from transition metal-catalyzed polymerization
by transferring species of a reacting solution directly into the [6,9] and coupling reactions [8,10-17] to purely organic
gas phase of a mass spectrometer [3-7]. The technique allows Diels–Alder reactions [18,19] to cite only a few representative
glimpses into the reacting solution as a function of time [8] and examples.
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However, the detection of transient reactive species is often via hydrogen bonding [24,25]. It is still controversial whether
hindered by their very low concentration. A reacting solution of oxazolidinone formation plays a pivotal role in the catalytic
a catalytic transformation typically contains quite a number of cycle or just serves as an rate limiting parasitic off-cycle equi-
different species. Side products, off-cycle resting states, reagent librium [31,33,35,52].
degradation products and impurities of various origins may be
present in much higher concentration than the interesting reac- Thus, we aimed to synthesize a charge-tagged L-proline-based
tive intermediates. Thus, ESI spectra of reacting solutions can organocatalyst for mechanistic studies by ESIMS. Few proline
be frustratingly complicated and the transient species of interest derivatives carrying a covalently fixed charge have been
might be superposed with a large number of more intense back- reported by now [43,53]. They consist of an imidazolium salt
ground signals [20]. In quantification using ESI, the detection attached to hydroxyproline via an ester group at the end of a
limit has been lowered significantly by selected ion monitoring flexible alkyl spacer. Interestingly, such charge tags can cause
in MS/MS mode [2]. Similarly, transient reactive species have an enhancement of the catalytic performance through electro-
been successfully extracted from the chemical noise by colli- steric activation [53], but backfolding can also alter and disturb
sion-induced dissociation (CID) MS/MS [20]. However, it is the catalytic process and induce the formation of side products
not possible to identify unknown or unexpected species by this [43]. In order to fix the charge far away from the catalytic
strategy.
center and thus leave the original catalytic activity of L-proline
preferably undisturbed, we chose a stiff 1-ethylpyridinium unit
As a major drawback of ESI mass spectrometry in general, the as charge-carrier separated from the catalytic center by a rigid
signal intensity does not directly parallel the concentration, but phenyl linker (Figure 1).
the so called ESI response, i.e., the ionization probability during
the ESI process [2,21]. Hence it happens that the reaction inter-
mediates of interest are concealed by easily ionizable other
compounds present in the reacting solution. A convenient ap-
proach to solve this problem is the use of covalently attached
charge tags [6,8,9,12,22,23]. Charge-tagging the catalyst selec-
tively enhances the signal abundances of all catalyst-derived
species in a reacting solution and thus facilitates the identifica-
Figure 1: The new charge-tagged proline-derived catalyst 1.
tion of low-concentrated transient catalytic species. As a
complementary approach, charge-tagged substrates have been
used to easily identify (“fish for”) efficient catalysts [6,9].
We then tested the applicability of 1 for ESIMS mechanistic
studies on the first “inverse” crossed aldol reaction (Scheme 1)
Since the year 2000, enantioselective catalysis based on small published in 2002 by Jørgensen and coworkers [54] in which
organic metal-free molecules has become an enormously the aldehyde acts as the donor in contrast to the “normal”
growing research topic [24-30]. A large variety of organocat- crossed aldol mechanism. It represents an interesting version of
alyzed reactions with high efficiency and selectivity are nowa- a typical proline-catalyzed reaction for which, to the best of our
days known so that organocatalysis complements current knowledge, mechanistic studies have not been reported so far.
catalytic fields such as organometallic or enzymatic catalysis as
an independent subdomain [24-30]. Parallel to the enormous
growth of organocatalytic applications in synthesis, mecha-
nistic studies on organocatalytic reactions [31-38] using ESI
mass spectrometry [20,39-49] have been reported. The
pioneering studies of List and Barbas [50] revealed that the
amino acid L-proline is an effective catalyst for a great variety
of organic reactions, such as the direct asymmetric aldol reac-
tion, one of the most important C–C bond-forming reactions in
organic synthesis [51]. The currently accepted mechanism
suggests a central enamine intermediate which forms a
Zimmerman–Traxler-like transition state with the acceptor sub-
Scheme 1: Inverse aldol reaction with aldehyde donors according to
Jørgensen [54]. We studied the reaction for R = Ph (labelled a through-
out this manuscript) and for R = Et (b).
Results and Discussion
strate [36,37]. The activity and enantioselectivity achieved by Synthesis
proline in many cases is thought to be due to a templating effect Formation of the charge-carrying unit was accomplished
of the OH group directing the aldehyde in a preferred position starting from commercially available 4-bromophenol using a
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strategy reported by Diemer et al. [55]. Protection of the
hydroxy group to yield 3 [56] was followed by Suzuki cross-
coupling with commercial pyridine-4-boronic acid leading to 4.
Subsequent deprotection led to 4-(pyridine-4-yl)phenol (5)
(Scheme 2) [55].
The preparation of the charge-tagged catalyst 1 starting from
doubly-protected hydroxyproline 6 [57] is depicted in
Scheme 3. To introduce a suitable leaving group for the
following step of the synthesis, compound 6 was mesylated to
give the derivative 7 [58] for which crystals suitable for X-ray
analysis have been obtained (Figure 2). An SN2 reaction with 5
[55] led to 8. We abandoned our initial shorter synthetic route
based on a Mitsunobu reaction leading from 6 directly to 8 due
to severe purification difficulties. Compound 8 could be charge-
tagged to 9 using ethyl bromide. Finally, the free catalyst 1 was
obtained by acidic deprotection [59].
Figure 2: Molecular structure of 7 in the solid state.
Scheme 2: Synthesis of 4-(pyridin-4-yl)phenol (5).
Scheme 3: Synthesis of the charge-tagged proline catalyst 1.
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Mechanism of the Jørgensen inversed aldol
reaction
The easiest way of ESI reaction monitoring – mixing the
reagents and measuring ESI spectra after various time intervals
According to the mechanistic model for enamine catalysis from – is restricted to reaction times longer than approximately one
List and Houk [36-38], the aldol reaction from Jørgensen should minute and therefore not appropriate for fast conversions like
proceed via the catalytic cycle shown in Scheme 4. We began aldol reactions. We thus decided to use a more complicated
our experiments with a test whether the charge tag does not experimental setup of two mixing tees connected on-line to the
disturb the catalysis. Indeed, 1 can achieve the formation of mass spectrometer (Figure 3) to detect individual intermediates
aldol products 2a and 2b, respectively, under the reaction of both reactions by ESIMS. These so-called continuous-flow
conditions given in the literature [54]. Performed in simulta- experiments [5,20] allow the sampling of reaction times down
neous parallel reaction batches, 1 provides just about the same to seconds. Solutions of the reagents are mixed in the first
yields as unmodified proline and byproducts were not observed. microreactor and diluted to concentrations suitable for ESIMS
Further, it is important to note that the substrates do not show in the second microreactor. The reaction time between the
any reaction when no catalyst, be it charge-tagged or not, is mixing event and the electrospray is determined by the flow
present in the solution.
rates and capillary lengths. Mass spectra of a solution of diethyl
Scheme 4: Proposed catalytic cycle [36-38] for the aldol reaction with aldehyde donors [54]; CT = charge tag, a: R = Ph, b: R = Et.
Figure 3: Experimental setup for continuous-flow ESIMS experiments using two mixing tee microreactors directly coupled to the ESI needle.
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ketomalonate, butyraldehyde and unmodified L-proline or 1, tion of a similar intermediate for the aldol reaction beween
respectively, are depicted in Figure 4.
acetone and selected benzaldehydes using rather unusual and
presumably extremely soft ESI conditions, and their results
The mass spectra shown in Figure 4 do not exhibit abundant indeed confirm its facile fragmentation [20].
signals for the reactants, even though these are present in the
solution in excess to all other species, a fact that is due to the Interestingly, the signal at m/z 290.12 can be assigned to a
poor ESI response of ketoesters and even more so of aldehydes. further transient species of the reaction – it corresponds to a
In the case of the proline-catalyzed solution (Figure 4a), the protonated adduct of proline with diethyl ketomalonate. This
catalyst is not visible either, because of an instrumental adduct might simply be a non-covalently bound aggregate, i.e.,
discrimination of low masses unfortunately unavoidable with a typical ESI phenomenon [2], but with regard to the rather low
our instrument. Instead, two expected intermediates of the concentrations used here and the observation of the analogous
catalytic cycle indeed are observed in reasonable abundances species in the proton-free charge-tagged case (see below), we
(Figure 4a): The signal at m/z 170.12 corresponds to [IIbuntagged prefer its assignment to a hemiaminal species formed in analogy
+ H]+ and the one at m/z 344.17 is assigned to [IIIbuntagged + to intermediate I by interaction of the proline nitrogen with the
H]+. In contrast, signals for the remaining two intermediates keto group of the ketomalonate (structure depicted in
Ibuntagged and IVbuntagged have not been found, which prob- Figure 4a). In this special case, elimination of water is not
ably is due to their very low concentration in the reaction equi- possible due to the lack of adjacent hydrogen atoms. Its forma-
libria as well as to their facile fragmentation during ESI. Note tion thus represents an off-cycle equilibrium dead-end in the
that the group of Metzger has successfully achieved the detec- course of the intended inverse aldol reaction.
Figure 4: ESI mass spectra of acetonitrile solutions of diethyl ketomalonate and butyraldehyde (a) with unmodified L-proline or (b) with the charge-
tagged catalyst 1 recorded with the continous-flow setup shown in Figure 3.
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In light of the relatively high abundances observed for the reac- Again, we unfortunately have not been successful in finding
tion intermediates catalyzed by uncharged L-proline, the imple- suitable electrospray conditions to detect the fragile intermedi-
mentation of a charge tag might have been considered unnec- ates [Ia]+/[Ib]+ and [IVa]+/[IVb]+. More importantly, however,
cessary. However, the effect of using the charge-tagged cata- additional species that are not present in the unlabeled refer-
lyst 1 is impressive (Figure 4b). The obvious reduction of spec- ence system have not been observed. There are no indications
tral complexity and chemical noise due to the strongly enhanced for an interference of the charge tag with the catalysis, in
ESI response of 1 and all its derivatives underlines the great contrast to the findings with the flexible imidazolium-labeled
benefits of the charge-tagging strategy. In addition to the cata- proline derivatives reported previously [43].
lyst 1 at m/z 313.16, the three transient species discussed above
are found almost exclusively and in very high abundances, i.e., The mere detection of ions at m/z 415.20 (R = Ph) and 367.20
the enamine [IIb]+ at m/z 367.20, the iminium [IIIb]+ at m/z (R = Et), respectively, is not a proof for the presence of reac-
541.26 and the side product [1 + ketomalonate]+ at m/z 487.21. tive enamines [IIa]+/[IIb]+ since the isomeric zwitterionic
Please note that the abundance of the latter varies significantly iminiums [II’a]+/[II’b]+ and oxazolidinones [II’’a]+/[II’’b]+
between different reaction runs, in contrast to the signals of the (Scheme 4) have the same elemental composition. Using one of
other intermediates whose appearances are highly reproducible. the most important mass spectrometric means for structure
Very similar findings are observed when phenylacetaldehyde elucidation, collision-induced dissociation (CID) experiments
instead of butyraldehyde is used. In particular, intermediates have been perfomed. The results for R = Et are shown in
[IIa]+ and [IIIa]+ have been detected in high abundances. Figure 5.
Figure 5: ESI(+) CID MS/MS spectra of mass-selected intermediates a) [IIb]+, b) the butyl ester derivative [II*b]+, c) [IIIb]+.
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All four mass-selected ions [IIa]+/[IIb]+ and [IIIa]+/[IIIb]+
show a very strong propensity to expel CO2 which even
happens during the ESI process via in-source fragmentation
when slightly harsher ionization conditions are used. This frag-
mentation is in perfect accordance with the zwitterionic
iminium structures II’ and III and can also be rationalized for
the oxazolidinone alternative II’’, whereas it requires an addi-
tional hydrogen shift from the enamine structure II. On the
other hand, the comparison with the fragmentation of the struc-
turally related ion [II*b]+ (Figure 5b) is instructive. It
doubtlessly possesses an enamine structure since it can neither
form a zwitterionic iminium ion nor undergo lactonization to
oxazolidinone II’’b because of its tert-butyl blocked carboxylic
acid function. The fragmentation of [II*b]+ exclusively consists
of a loss of [C5,H8,O2] which should correspond to a concerted
or very fast stepwise elimination of isobutene and CO2 leading
to the same product ion at m/z 323.21 as the expulsion of CO2
from m/z 367.20. Interpreting the isobutene loss as a closed-
Figure 6: Normalized relative intensities in ESI spectra recorded for
the inverse aldol reaction of butyraldehyde, diethyl ketomalonate and
charge-tagged catalyst 1 at different time stages using the continous-
flow setup with two microreactors shown in Figure 3.
shell McLafferty-type rearrangement leads to the postulation of qualitative picture of the reaction behaviour and show that the
a (very short-lived undetected) intermediate enamine [IIb]+ use of catalyst 1 is suitable for the examination of L-proline-
which then obviously is able to undergo a facile CO2 elimina- catalyzed reactions via ESIMS. However, we refrain from a
tion. Thus, the fragmentation spectra unfortunately do not allow quantitative kinetic modeling of the data to extract rate
a clear discrimination of the three possible structures II, II’, and constants [8] because we encountered certain limitations of the
II’’.
method. Most importantly, we could not obtain an exact repro-
ducibility of reaction times, probably because of quasi-unavoid-
Marquez and Metzger mass-selected a signal corresponding to able variations in the actual (dead) volumes of the setup inter
the protonated enamine from acetone and untagged L-proline alia due to varying minor capillary blockings. Moreover, we
and observed the elimination of CH2O2 (formic acid) instead of face a slight increase of signal abundances with measuring time
CO2 as main fragmentation component during CID [20]. The (not reaction time) because analytes gradually accumulate in the
protonation during the ESI process presumably occurs at the system the longer their solutions are passed through. The extent
nitrogen atom which enables a direct 1,2-elimination of formic of this effect depends on the height of the flow rate which ne-
acid. In our case, the respective charge-tagged species are cessarily has to be changed when observing a reaction process
detected in their original form without additional proton which with the continuous-flow method. Nevertheless, we would like
explains the differing fragmentation route.
to emphasize that the quality of the experiments surely suffices
to depict the “chronological trend” of the reaction process.
To monitor the temporal evolution of intermediates during the
aldol reaction with the continous-flow setup, series of ESI Conclusion
spectra at different reaction time stages have been recorded by We present the synthesis of the charge-tagged L-proline derived
varying the flow rate of the analyte solutions or by changing the catalyst 1 in which a rigid phenylpyridine linker fixes the
length of the capillary connecting both mixing tees. A resulting charge tag far away from the catalytically active center in order
graph of the normalized relative intensities vs calculated reac- to avoid unwanted interactions. In a comparative continous-
tion time is provided in Figure 6.
flow electrospray mass spectrometric study, the new charged
catalyst 1 and neutral L-proline have been used to investigate
The graph displays a rise of the concentration of IIb during the the proline-catalyzed inverse crossed aldol reaction of alde-
first seconds of the reaction accompanied by a decrease of 1. hydes with diethyl ketomalonate. Two key intermediates of the
Subsequently, an increase of IIIb occurs indicating that IIIb is List–Houk mechanism for enamine catalysis in addition to a
formed out of IIb which is consistent with the mechanism in transient off-cycle species could be observed experimentally.
Scheme 4. The gradual increase of the final aldol product 2b is The use of 1 further allows facile access to a qualitative picture
visible as well; an enlargement of a factor of 100 has been used of the temporal evolution of catalyst-containing intermediates.
in the presentation of Figure 6 due to its much lower ESI We plan to use the new proline catalyst with a non-interfering
response. Overall, these experiments reflect a very reasonable charge-label presented here as a tool to study the templating
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role of the hydroxy group in L-proline-catalyzed reactions in aqueous phase was extracted with CH2Cl2 (2 × 75 mL), the
the gas-phase in the near future.
combined organic extracts were dried with MgSO4 and the
solvents were removed in vacuo. The crude product was puri-
fied by column chromatography on silica gel using ethyl acetate
with 5% triethylamine as eluent (Rf 0.50). Compound 4 was
Experimental
Synthesis
Reactions under inert gas atmosphere were performed under obtained as white solid (1.31 g, 88%). The spectroscopic data
argon using standard Schlenk techniques and oven-dried glass- confirm the reported ones [55].
ware prior to use. Thin-layer chromatography was performed on
aluminum TLC plates silica gel 60F254 from Merck. Detection 4-(Pyridine-4’-yl)phenol (5) [55] and (2S,4R)-tert-butyl
was carried out under UV light (254 and 366 nm). Products N-tert-butyloxycarbonyl-4-hydroxyprolinate (6) [57] were
were purified by column chromatography on silica gel 60 prepared according to literature protocols.
(40–63 µm) from Merck. The 1H and 13C NMR spectra were
recorded on a Bruker Avance 300 spectrometer, at 300.1 and (2S,4R)-tert-Butyl N-tert-butoxycarbonyl-4-oxymethanesul-
75.5 MHz, or a Bruker AM 400, at 400.1 MHz and 100.6 MHz, fonyloxyprolinate (7) was prepared according to a known
at 293 K, respectively. The 1H NMR chemical shifts are procedure [58] which was slightly modified: (2S,4R)-tert-butyl
reported on the δ scale (ppm) relative to residual non-deuter- N-tert-butyloxycarbonyl-4-hydroxyprolinate (6, 3.14 g,
ated solvent as the internal standard. The 13C NMR chemical 10.9 mmol) was dissolved in CH2Cl2 (55 mL) and cooled to
shifts are reported on the δ scale (ppm) relative to deuterated 0 °C. Triethylamine (3.01 mL, 21.5 mmol) was added and the
solvent as the internal standard. Signals were assigned on the mixture was stirred for 0.5 h. Methanesulfonyl chloride
basis of 1H, 13C, HMQC, and HMBC NMR experiments. Most (1.33 mL, 17.1 mmol) was added dropwise over 10 min and the
solvents were dried, distilled, and stored under argon according resulting solution was stirred overnight without further cooling.
to standard procedures. 4-Bromophenol, 4-pyridinylboronic The reaction was quenched by the addition of a saturated solu-
acid, trans-N-(tert-butoxycarbonyl)-4-hydroxy-L-proline, tion of NaHCO3 (70 mL) and the aqueous phase was extracted
L-proline, diethyl ketomalonate, phenylacetaldehyde and with CH2Cl2 (2 × 50 mL). The combined organic extracts were
butyraldehyde were used as received from commercial sources. dried with MgSO4 and the solvents were removed in vacuo. The
crude product was purified by column chromatography on silica
4-Bromophenol methoxymethyl ether (3) [56]: 4-Bromo- gel using cyclohexane/ethyl acetate (2:3) as eluent (Rf 0.61).
phenol (3.00 g, 17.2 mmol) was dried under reduced pressure Compound 7 was obtained as colorless solid (3.55 g, 89%).
and dissolved in dry THF (160 mL) under inert gas atmosphere. Crystals suitable for X-ray analysis have been obtained by slow
NaH (1.32 g, 52.0 mmol) was added and the mixture was stirred diffusion of cyclohexane into a Et2O solution of 7. 1H NMR
at rt for 0.5 h. Methoxymethyl chloride (2.08 mL, 26.0 mmol) (400 MHz, CD3OD) δ 5.26 (s, 1H, CH2CHCH2), 4.30–4.23 (m,
was added dropwise and the resulting suspension was stirred for 1H, CH2CHN), 3.83–3.76 (m, 1H, NCH2CH), 3.70–3.63 (m,
19 h. The reaction was quenched by the addition of a MeOH/ 1H, NCH2CH), 3.14 (s, 3H, S-CH3), 2.67–2.56 (m, 1H,
H2O mixture (1:1, 150 mL) and the aqueous phase was CHCH2CH), 2.28–2.19 (m, 1H, CHCH2CH), 1.50–1.44 (m,
extracted with CH2Cl2 (4 × 50 mL). The combined organic 18H, C(CH3)3) ppm; 1H NMR (400 MHz, CD3OD, 333 K) δ
extracts were dried with MgSO4 and the solvents were removed 5.26 (s, 1H, CH2CHCH2), 4.30–4.26 (m, 1H, CH2CHN),
in vacuo. The crude product was purified by column chroma- 3.83–3.74 (m, 1H, NCH2CH), 3.72–3.63 (m, 1H, NCH2CH),
tography on silica gel using cyclohexane/ethyl acetate (10:1) as 3.12 (s, 3H, S-CH3), 2.67–2.54 (m, 1H, CHCH2CH), 2.30–2.20
eluent (Rf 0.61). Compound 3 was obtained as colorless oil (m, 1H, CHCH2CH), 1.48 (s, 9H, C(CH3)3), 1.46 (s, 9H,
(3.38 g, 90%). The spectroscopic data confirm the reported ones C(CH3)3) ppm; 13C NMR (100 MHz, CD3OD) δ 173.0/172.9
[56].
(C═O), 155.6 (C═O), 83.1 (C(CH3)3), 82.1/81.9 (C(CH3)3),
80.6/79.9 (CH2CHCH2), 59.5/59.4 (CH2CHN), 54.1/53.7
4-[4-(Methoxymethoxy)phenyl]pyridine (4) [55]: 4-Bromo- (NCH2CH), 38.3 (S-CH3), 37.3 (CHCH2CH), 28.6 (C(CH3)3),
phenol methoxymethyl ether (3, 1.5 g, 6.9 mmol), 4-pyridinyl- 28.3/28.2 (C(CH3)3) ppm; 13C NMR (125 MHz, CD3OD, 333
boronic acid (1.02 g, 7.5 mmol), [1,1'-bis(diphenyl- K) δ 172.9 (C═O), 155.6 (C═O), 83.1 (C(CH3)3), 82.1
phosphino)ferrocene]palladium(II) dichloride dichloromethane (C(CH3)3), 80.5/79.8 (CH2CHCH2), 59.5 (CH2CHN), 53.9/53.5
complex (0.22 g, 0.28 mmol) and Na2CO3 (8.78 g, 82.4 mmol) (NCH2CH), 38.5 (S-CH3), 38.3/37.3 (CHCH2CH), 28.7
were suspended in a H2O/1,2-dimethoxyethane mixture (1:3, (C(CH3)3), 28.3 (C(CH3)3) ppm; HRESIMS (m/z): [M + Na]+
75 mL), heated to 100 °C und stirred for 16 h. The resulting calcd for C15H27NNaO7S, 388.1406; found, 388.1398. The
mixture was filtered and the filtrate was mixed with H2O NMR data are consistent with the reported ones measured in
(75 mL) and CH2Cl2 (75 mL) for phase separation. The CDCl3 [58,60].
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Beilstein J. Org. Chem. 2014, 10, 2027–2037.
(2S,4S)-tert-Butyl N-tert-butoxycarbonyl-4-(4-(pyridine-4- (CH2CH3), 53.6/53.2 (NCH2CH), 37.1/36.3 (CHCH2CH), 28.7/
yl)phenoxy)prolinate (8): 4-(Pyridine-4’-yl)phenol (5, 0.63 g, 28.6 (C(CH3)3), 28.3 (C(CH3)3), 16.7 (CH2CH3) ppm;
3.7 mmol) and NaH (0.14 g, 5.5 mmol) were dissolved in dry HRESIMS (m/z): [M]+ calcd for C27H37N2O5, 469.2697;
DMSO (100 mL) under inert gas atmosphere. The suspension found, 469.2692.
was heated to 60 °C and stirred for 1.5 h. (2S,4R)-tert-Butyl
N-tert-butoxycarbonyl-4-methanesulfonyloxyprolinate (7, 4-(4-((3S,5S)-5-Carboxypyrrolidin-3-yloxy)phenyl)-1-ethyl-
1.33 g, 3.7 mmol) was added and the mixture was stirred for 5 d pyridinium bromide (1) was prepared analogous to a known
at 60 °C. The reaction progress was controlled by thin-layer procedure [59] and obtained as beige-brown solid (96%). 1H
chromatography. The reaction was quenched by the addition of NMR (400 MHz, CD3OD) δ 8.92 (d, 3J = 6.4 Hz, 2H, Ho-py),
H2O (100 mL) and the aqueous phase was extracted with 8.37 (d, 3J = 6.4 Hz, 2H, Hm-py), 8.06 (d, 3J = 8.6 Hz, 2H,
CH2Cl2 (2 × 80 mL) and with Et2O (2 × 80 mL). The combined Hm-ph), 7.27/7.20 (d, 3J = 8.6 Hz, 2H, Ho-ph), 5.42 (s, 1H,
organic extracts were dried with Na2SO4 and the solvents were CH2CHCH2), 4.73–4.58 (m, 3H, CH2CH3 and CH2CHN),
removed in vacuo. Remaining DMSO was removed by distilla- 3.82–3.62 (m, 2H, NCH2CH), 2.85–2.73 (m, 1H, CHCH2CH),
tion. The crude product was purified by column chromatog- 2.71–2.63 (m, 1H, CHCH2CH), 1.67 (t, 3J = 7.3 Hz, 3H,
raphy on silica gel using cyclohexane/ethyl acetate (1:3) with CH2CH3) ppm; 13C NMR (100 MHz, CD3OD) δ 171.0 (C═O),
5% triethylamine as eluent (Rf 0.50). Compound 8 was obtained 160.9 (Cph−O), 156.9 (Cp-py−C), 145.4 (Co-py−H), 131.0
as colorless solid (1.14 g, 71%).1H NMR (300 MHz, CD3OD) δ (Cm-ph-H), 128.5 (Cp-ph-C), 125.2 (Cm-py−H), 118.0 (Co-ph−H),
8.55–8.45 (m, 2H, Ho-py), 7.72 (d, 3J = 8.8 Hz, 2H, Hm-ph), 7.67 77.3/76.7 (CH2CHCH2), 59.8 (CH2CHN), 57.4 (CH2CH3), 52.7
(d, 3J = 6.1 Hz, 2H, Hm-py), 7.06/7.00 (d, 3J = 8.8 Hz, 2H, (NCH2CH), 35.7 (CHCH2CH), 16.7 (CH2CH3) ppm;
Ho-ph), 5.06 (s, 1H, CH2CHCH2), 4.41–4.26 (m, 1H, HRESIMS (m/z): [M]+ calcd for C18H21N2O3, 313.1547;
CH2CHN), 3.86–3.72 (m, 1H, NCH2CH), 3.72–3.57 (m, 1H, found, 313.1563.
NCH2CH), 2.67–2.49 (m, 1H, CHCH2CH), 2.47–2.37 (m, 1H,
CHCH2CH), 1.51–1.40 (m, 18H, C(CH3)3) ppm; 13C NMR (75 Diethyl 2-hydroxy-2-(2-oxo-1-phenylethyl)malonate (2a) was
MHz, CD3OD) δ 172.5/172.5 (C═O), 159.7/159.5 (C═O), prepared according to the reported procedure [54] and obtained
156.0 (Cph−O), 150.4 (Co-py−H), 150.1 (Cp-py−C), 131.7/131.4 as orange oil. The synthesis was carried out twice, once using
(Cp-ph−C), 129.6/129.4 (Cm-ph−H), 122.5 (Cm-py−H), 117.3/ L-proline (82%), once using 1 as catalyst (79%).
117.2/117.2 (Co-ph−H), 82.9/82.7/82.6 (C(CH3)3), 81.9/81.6
(C(CH3)3), 77.2/76.2 (CH2CHCH2), 60.1/59.8 (CH2CHN), Diethyl 2-hydroxy-2-(1-oxobutan-2-yl)malonate (2b) was
53.6/53.2 (NCH2CH), 37.1/36.4 (CHCH2CH), 28.7/28.6 prepared according to the reported procedure [54] and obtained
(C(CH3)3), 28.4 (C(CH3)3) ppm; HRESIMS (m/z): [M + H]+ as orange oil (using L-proline: 83%, using 1: 80%).
calcd for C25H33N2O5, 441.2389; found, 441.2390.
Crystal structure determination: X-ray crystallographic
4-(4-((3S,5S)-1,5-Bis(tert-butoxycarbonyl)pyrrolidine-3- analysis of 7 was performed on a Nonius KappaCCD diffrac-
yloxy)phenyl)-1-ethylpyridinium bromide (9): (2S,4S)-tert- tometer using graphite monochromated Mo Kα radiation
Butyl N-tert-butoxycarbonyl-4-(4-(pyridine-4-yl)phenoxy)proli- (λ = 0.71073 Å). Intensities were measured by fine-slicing φ-
nate (8, 0.22 g, 0.5 mmol) was dissolved in bromoethane and ω-scans and corrected for background, polarization and
(13.0 mL, 174.2 mmol), heated to 43 °C and stirred for 5 d. Lorentz effects. A semi-empirical absorption correction was
Bromoethane was removed in vacuo. Compound 9 was applied for the data sets following Blessing’s method [61]. The
obtained as yellowish-brown solid (0.26 g, 95%). 1H NMR (400 structure was solved by direct methods and refined anisotropi-
MHz, CD3OD) δ 8.89 (d, 3J = 6.8 Hz, 2H, Ho-py), 8.35 (d, 3J = cally by the least squares procedure implemented in the ShelX
6.8 Hz, 2H, Hm-py), 8.08/7.99 (m, 2H, Hm-ph), 7.18/7.11 (d, 3J = program system [62]. The hydrogen atoms were included
8.9 Hz, 2H, Ho-ph), 5.19–5.13 (m, 1H, CH2CHCH2), 4.63 (q, 3J isotropically using the riding model on the carbon atoms.
= 7.4 Hz, 2H, CH2CH3), 4.43–4.36 (m, 1H, CH2CHN), Selected data: Crystal dimensions 0.36 × 0.20 × 0.11 mm3,
3.89–3.73 (m, 1H, NCH2CH), 3.71–3.58 (m, 1H, NCH2CH), C15H27NO7S, M = 365.4424, orthorhombic, space group
2.71–2.55 (m, 1H, CHCH2CH), 2.47–2.40 (m, 1H, CHCH2CH), P212121, a = 7.44860(10), b = 8.94580(10), c = 28.7024(4) Å,
1.67 (t, 3J = 7.4 Hz, 3H, CH2CH3), 1.52-1.42 (m, 18H, α = 90°, β = 90°, γ = 90°, V = 1912.55(4) Å3, Z = 4, ρ = 1.269 g
C(CH3)3) ppm; 13C NMR (100 MHz, CD3OD) δ 172.5/172.4 cm−3, μ = 0.203 mm−1, F(000) = 748, 18641 reflections
(C═O), 162.0 (Cph−O), 157.0 (Cp-py−C), 155.9 (C═O), 145.2 (2θmax = 27.99°) measured (4566 unique, Rint = 0.0606,
(Co-py−H), 131.2/131.1 (Cm-ph-H), 127.5 (Cp-ph-C), 125.0 completeness = 99.4%), R (I > 2σ(I)) = 0.0708, wR2 (all data) =
(Cm-py−H), 117.8/117.7 (Co-ph−H), 82.7/82.6 (C(CH3)3), 81.6 0.2054. GOF = 1.052 for 224 parameters and 14 restraints,
(C(CH3)3), 77.6/76.6 (CH2CHCH2), 60.0/59.8 (CH2CHN), 57.2 largest diff. peak and hole 1.557/−0.438 e Å3. CCDC-1016532
2035

Beilstein J. Org. Chem. 2014, 10, 2027–2037.
contains the supplementary data for this structure. These data Gregor Schnakenburg for the X-ray structure determination and
can be obtained free of charge from http://www.ccdc.cam.ac.uk/ Prof. A. C. Filippou for providing X-ray infrastructure.
data_request/cif.
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