Silicon-containing dipeptidic aspartame and neotame analogues

Article abstract of DOI:10.1021/om300442e

A series of seven silicon-containing dipeptides ((S,R)-5, (S,S)-5, (S,R)-6, (S,R)-7, (S,S)-7, (S,R)-8, and (S,S)-8) was prepared in multistep syntheses, starting from the enantiopure silicon-containing α-methylated α-amino acids (R)- or (S)-α-[(trimethylsilyl)methyl]alanine ((R)-4 or (S)-4). For the preparation of (R)-4 and (S)-4, a robust synthesis on a multigram scale was developed, using an enzyme-catalyzed stereoselective ester hydrolysis as the key step. Coupling of (R)-4 and (S)-4 with different S-configured aspartate residues yielded a series of dipeptides that can be best described as silicon analogues of the artificial sweeteners aspartame and neotame. The identity of these dipeptides was established by elemental analyses and NMR spectroscopic studies (¹H, ¹³C, ¹⁵N, ²⁹Si). Some of the title compounds and some of their precursors were structurally characterized by single-crystal X-ray diffraction. The silicon-containing dipeptides were shown to display attractive ADME properties, including good solubility and plasma protein binding capabilities, no CYP inhibition, and no metabolic stability liabilities. Thus, the silicon-containing α-amino acids (R)-4 and (S)-4 proved to be promising building blocks for the design of biologically active peptides. Attempts to characterize the title compounds for their T1R2/R3 activating properties by testing their ability to induce GLP-1 secretion from the intestinal endocrine cell line STC-1 failed for unknown reasons. Sensory evaluation of the silicon-containing dipeptides demonstrated the aspartame analogue (S,R)-5 and the neotame analogue (S,R)-8 to be very potent artificial sweeteners, whereas the corresponding diastereomers tasted bitter. The silicon compounds (S,R)-5 and (S,R)-8 are approximately 50 and 600 times, respectively, as sweet as sucrose on a weight basis.

Full text of DOI:10.1021/om300442e

Article
pubs.acs.org/Organometallics
Silicon-Containing Dipeptidic Aspartame and Neotame Analogues
Steffen Dorrich,† Steffen Falgner,† Sarah Schweeberg,† Christian Burschka,† Peter Brodin,‡
̈
Britt Marie Wissing,‡ Babro Basta,‡ Peter Schell,‡ Udo Bauer,‡ and Reinhold Tacke*,†
†Institut fur Anorganische Chemie, Universitat Wurzburg, Am Hubland, D-97074 Wurzburg, Germany
̈
̈
̈
̈
‡AstraZeneca, R&D Molndal, Pepparedsleden 1, S-43183 Molndal, Sweden
̈
̈
S
* Supporting Information
ABSTRACT: A series of seven silicon-containing dipeptides ((S,R)-5,
(S,S)-5, (S,R)-6, (S,R)-7, (S,S)-7, (S,R)-8, and (S,S)-8) was prepared in
multistep syntheses, starting from the enantiopure silicon-containing
α-methylated α-amino acids (R)- or (S)-α-[(trimethylsilyl)methyl]-
alanine ((R)-4 or (S)-4). For the preparation of (R)-4 and (S)-4, a
robust synthesis on a multigram scale was developed, using an enzyme-
catalyzed stereoselective ester hydrolysis as the key step. Coupling of (R)-4 and (S)-4 with different S-configured aspartate
residues yielded a series of dipeptides that can be best described as silicon analogues of the artificial sweeteners aspartame and
neotame. The identity of these dipeptides was established by elemental analyses and NMR spectroscopic studies (1H, 13C, 15N,
29Si). Some of the title compounds and some of their precursors were structurally characterized by single-crystal X-ray diffraction.
The silicon-containing dipeptides were shown to display attractive ADME properties, including good solubility and plasma
protein binding capabilities, no CYP inhibition, and no metabolic stability liabilities. Thus, the silicon-containing α-amino acids
(R)-4 and (S)-4 proved to be promising building blocks for the design of biologically active peptides. Attempts to characterize
the title compounds for their T1R2/R3 activating properties by testing their ability to induce GLP-1 secretion from the intestinal
endocrine cell line STC-1 failed for unknown reasons. Sensory evaluation of the silicon-containing dipeptides demonstrated the
aspartame analogue (S,R)-5 and the neotame analogue (S,R)-8 to be very potent artificial sweeteners, whereas the corresponding
diastereomers tasted bitter. The silicon compounds (S,R)-5 and (S,R)-8 are approximately 50 and 600 times, respectively, as
sweet as sucrose on a weight basis.
peptides, which can serve as valuable tools in the search for both
the preferred bioactive conformation and the active site of the
receptor.6 This approach has also been used for the development
of the theory of sweet taste.7 With these points in mind, the
incorporation of the silicon-containing α-methylated α-amino
acids (R)-4 and (S)-4 into the dipeptides 5−8 offered great
scope to further our understanding of taste mechanisms.
The design of the target dipeptides presented herein takes
into account that the absolute configuration of the aspartate
residue (S configuration) is important for the sweet taste of
dipeptidic sweeteners.4d Thus, the aspartame analogues 5 and 6
and the neotame analogues 7 and 8 synthesized in this study
contain S-configured aspartate residues. We report here on the
synthesis of the silicon-based dipeptides (S,R)-5, (S,S)-5, (S,R)-
6, (S,R)-7, (S,S)-7, (S,R)-8, and (S,S)-8 and the evaluation of
their ADME profiles. In addition, we report on our (failed)
attempts to evaluate the sweet-tasting properties of these dipep-
tides by testing their ability to induce GLP-1 secretion from the
mouse intestinal endocrine cell line STC-1. Furthermore, the
results of sensory evaluations of compounds (S,R)-5, (S,S)-5,
(S,R)-7, (S,S)-7, (S,R)-8, and (S,S)-8 are described. The studies
INTRODUCTION
■
In light of our systematic investigations on silicon-containing α-
amino acids1 and their application in the synthesis of silicon-
containing peptidic drugs,1b,c and inspired by Weidmann’s
finding that phenylalanine (1) and β-(trimethylsilyl)alanine (2)
can behave as bioisosteric building blocks in biologically active
peptides,2 we have been interested in the design of silicon-
based dipeptidic sweeteners (Scheme 1) (for recent reviews
dealing with silicon-containing α-amino acids and peptides, see
ref 3). Previous work, in which α-methyl-phenylalanine (3) was
used for the development of artificial sweeteners,4 inspired us
to incorporate the α-methylated α-amino acids (R)- and (S)-α-
[(trimethylsilyl)methyl]alanine ((R)-4 and (S)-4)1h,5 into
several dipeptides (5−8). Keeping in mind that the commercial
artificial dipeptidic sweeteners aspartame ((S,S)-9) and neo-
tame ((S,S)-10) and the related sweetener (S,S)-11 (Scheme 1)
are built up from (S)-aspartic acid and (S)-phenylalanine
(or (S)-α-methyl-phenylalanine), the analogous silicon-containing
dipeptides (S,R)-5, (S,R)-6, (S,R)-7, and (S,R)-8 were regarded as
potential sweeteners.5
Linear biologically active peptides that are built up from
natural α-amino acids show a high conformational flexibility
and therefore adopt a multitude of different conformations in
solution, although only a few of these represent a bioactive
conformation. However, incorporation of nonproteinogenic
α-methylated α-amino acids leads to conformationally restricted
Special Issue: Organometallics in Biology and Medicine
Received: May 22, 2012
Published: July 25, 2012
© 2012 American Chemical Society
5903
dx.doi.org/10.1021/om300442e | Organometallics 2012, 31, 5903−5917

Organometallics
Article
Scheme 1. Structural Formulas of the α-Amino Acids 1−4,
the Target Dipeptides 5−8, and the Artificial Sweeteners
Aspartame (9), Neotame (10), and 11
Scheme 2. Modified Syntheses of the Enantiomerically
Enriched Intermediates (R)-13 and (S)-14 and the
Enantiopure Silicon-Containing α-Amino Acids (R)-4 and
(S)-4
reported in this paper represent an extension of our ongoing
investigations on silicon-containing drugs8,9 and odorants.10,11
RESULTS AND DISCUSSION
Scheme 3. Syntheses of the Silicon-Based Aspartame
Analogues (S,R)-5·TFA and (S,R)-6·TFA
■
Syntheses. The silicon-based aspartame and neotame
analogues (S,R)-5, (S,S)-5, (S,R)-6, (S,R)-7, (S,S)-7, (S,R)-8,
and (S,S)-8 were synthesized, starting from the silicon-
containing α-amino acids (R)-4 and (S)-4. The enantioselective
syntheses of (R)-4 and (S)-4 have already been reported
elsewhere;1h however, the scale-up of these syntheses required
several modifications (Scheme 2).
The enzyme-catalyzed preparation of the key intermediate in
the synthesis of (R)-4 and (S)-4, (R)-3-ethoxy-2-methyl-3-oxo-
2-[(trimethylsilyl)methyl]propanoic acid ((R)-13), was per-
formed on the 100 g scale to give (R)-13 in 97% yield. For
reasons not yet fully understood, the enantiomeric excess of
(R)-13 was only 75% ee (compared to 85% ee reported in ref
1h); however, this problem could be overcome by appropriate
modifications to the subsequent transformations (R)-13 →
(R)-4 and (R)-13 → (S)-14 → (S)-4 (Scheme 2). The
enantiomeric excess of (R)-4 was increased stepwise from 75%
ee to 90% ee and finally to >99% ee through 2-fold
crystallization of (R)-4 from methanol/water (16% yield). As
the synthesis of (S)-14 and its subsequent crystallization from
n-hexane led to an increase of the enantiomeric excess from
75% to 95% ee (30% yield), a much higher yield (56% (relative
to (S)-14) or 17% (relative to (R)-13)) was obtained in the
subsequent transformations of (S)-14 into (S)-4 (compared to
(R)-13 → (R)-4; for details, see the Experimental Section).
Both silicon-containing α-amino acids were obtained with an
enantiomeric excess of >99% ee.
the hydrochlorides of the corresponding methyl (or ethyl)
esters (R)-15, (S)-15, and (R)-16 (yields: (R)-15·HCl, 99%;
(S)-15·HCl, 99%; (R)-16·HCl, 99%). Coupling of (R)-15·HCl,
(S)-15·HCl, and (R)-16·HCl with N-tert-butoxycarbonyl-4-tert-
butoxy-(S)-aspartic acid ((S)-17) using N-methylmorpholine as
the auxiliary base and O-(6-chlorobenzotriazol-1-yl)-N,N,N′,N′-
tetramethyluronium hexafluorophosphate (HCTU, 18) as the
coupling reagent yielded the protected dipeptidic intermediates
(S,R)-19 (86% yield), (S,S)-19 (98%), and (S,R)-20 (98%), res-
pectively. Removal of the tert-butoxycarbonyl (Boc) and tert-butyl
protecting groups by treatment with an excess of trifluoroacetic
The silicon-based aspartame analogues (S,R)-5, (S,S)-5, and
(S,R)-6 were synthesized according to Schemes 3 and 4 and
were isolated as the corresponding ammonium trifluoroacetates
(S,R)-5·TFA, (S,S)-5·TFA, and (S,R)-6·TFA (TFA = trifluoro-
acetic acid). In the first step, compounds (R)-4 and (S)-4 were
treated with thionyl chloride in methanol (or ethanol) to give
5904
dx.doi.org/10.1021/om300442e | Organometallics 2012, 31, 5903−5917

Organometallics
Article
the tert-butoxycarbonyl (Boc) protecting group was accom-
plished by using a mixture of chlorotrimethylsilane and
phenol,12 without affecting the benzyl ester group, to furnish
the N-deprotected dipeptides (S,R)-23 (70% yield) and (S,S)-
23 (90%). Reaction of (S,R)-23 and (S,S)-23 with 2,2-
dimethylpropanal (24), in the presence of magnesium sulfate,
yielded the N-2,2-dimethylpropylidene-substituted derivatives
(S,R)-25 and (S,S)-25, respectively. The corresponding N-3,3-
butylidene analogues (S,R)-26 and (S,S)-26 were synthesized
analogously by treating (S,R)-23 or (S,S)-23 with 3,3-
dimethylbutanal (27). Compounds (S,R)-25, (S,S)-25, (S,R)-
26, and (S,S)-26 were isolated as crude products, followed by
removal of the benzyl (Bn) protecting group and hydro-
genation of the imine moiety in a single step by reaction with
hydrogen in the presence of palladium/carbon as the catalyst to
give the target compounds (S,R)-7 (95% yield), (S,S)-7 (89%),
(S,R)-8 (92%), and (S,S)-8 (93%) (the yields are reported
relative to the respective intermediates (S,R)-23 and (S,S)-23).
The identities of the silicon-based aspartame and neotame
analogues (S,R)-5·TFA, (S,S)-5·TFA, (S,R)-6·TFA, (S,R)-7,
(S,S)-7, (S,R)-8, and (S,S)-8 and the intermediates (R)-15·HCl,
(S)-15·HCl, (R)-16·HCl, (S,R)-19, (S,S)-19, (S,R)-20, (S,R)-
22, (S,S)-22, (S,R)-23, and (S,S)-23 were established by NMR
spectroscopic studies (1H, 13C, 15N, 19F, 29Si), mass
spectrometric investigations, and elemental analyses (C, H, N)
or high-resolution mass spectrometry. In addition, compounds
(S,R)-5·2H2O, (S,R)-5·TFA, (S,R)-7·H2O, (S,R)-8·2.5H2O,
(R)-15·HCl, (S,R)-19·0.22H2O, (S,R)-20, (S,R)-22, and
(S,S)-22 were characterized by crystal structure analyses.
Determination of the Enantiomeric and Diastereo-
meric Excesses. The enantiomeric excess of (R)-4 and (S)-4
Scheme 4. Synthesis of the Silicon-Based Aspartame
Analogue (S,S)-5·TFA
acid afforded the target compounds (S,R)-5·TFA (54% yield),
(S,S)-5·TFA (89%), and (S,R)-6·TFA (50%).
The silicon-based neotame analogues (S,R)-7, (S,S)-7, (S,R)-
8, and (S,S)-8 were synthesized according to Schemes 5 and 6,
starting from (R)-15·HCl or (S)-15·HCl. Coupling of (R)-
15·HCl and (S)-15·HCl with N-tert-butoxycarbonyl-4-benzyl-
oxy-(S)-aspartic acid ((S)-21) using N-methylmorpholine as
the auxiliary base and 18 as the coupling reagent yielded the
protected dipeptidic intermediates (S,R)-22 (97% yield) and
(S,S)-22 (98%), respectively. Subsequent selective removal of
Scheme 5. Syntheses of the Silicon-Based Neotame Analogues (S,R)-7 and (S,R)-8
5905
dx.doi.org/10.1021/om300442e | Organometallics 2012, 31, 5903−5917

Organometallics
Article
Scheme 6. Syntheses of the Silicon-Based Neotame Analogues (S,S)-7 and (S,S)-8
was determined, after Mosher-type derivatization (Scheme 7),13
by GC-MS studies and NMR spectroscopic measurements. For
excess of the silicon-containing α-amino acids (R)-4 and (S)-4
was then accomplished by their derivatization according to
Scheme 7 and subsequent GC-MS studies of the analytical
samples obtained, using the 1/1 mixture of (R,R)-29 and (R,S)-
29 as the reference sample (for further details, see the
Experimental Section and the Supporting Information).
According to these studies, compounds (R)-4 and (S)-4 were
obtained with an enantiomeric excess of >99% ee. As the
purchased aspartic acid was enantiomerically pure and all steps
in the synthesis of the target dipeptides should not affect the
absolute configuration of the stereogenic centers, an enantio-
meric excess of >99% ee and a diastereomeric excess of >99%
de of the target compounds and their intermediates can be
assumed. This assumption is further supported by the NMR
spectra of the diastereomeric compounds, which did not
indicate any epimerization at the stereogenic centers.
Scheme 7. Syntheses of the Diastereomers (R,R)-29 and
(R,S)-29, Starting from rac-4 (→(R,R)-29/(R,S)-29, 1/1
Mixture), (R)-4 (→(R,R)-29), and (S)-4 (→(R,S)-29)
Crystal Structure Analyses. Compounds (S,R)-5·2H2O,
(S,R)-5·TFA, (S,R)-7·H2O, (S,R)-8·2.5H2O, (R)-15·HCl,
(S,R)-19·0.22H2O, (S,R)-20, (S,R)-22, and (S,S)-22 were
structurally characterized by single-crystal X-ray diffraction.
The crystal data and the experimental parameters used for the
crystal structure analyses are given in the Supporting
Information. The molecular (zwitterionic) structures of (S,R)-
5, (S,R)-7, and (S,R)-8 in the crystal of the respective hydrates
and the structure of the cation of (S,R)-5·TFA are depicted in
Figures 1−4.
All the bond lengths and angles of the compounds studied
are in the expected ranges and do not need any further discus-
sion, except for the Si−C−C angles (119.75(7)−123.81(14)°),
which differ significantly from the ideal tetrahedral angle. The
same phenomenon has also been observed for the crystal
this purpose, rac-15·HCl (obtained from rac-4 analogously to
the synthesis of (R)-15·HCl and (S)-15·HCl) was treated with
(S)-3,3,3-trifluoro-2-methoxy-2-phenylpropanonyl chloride
((S)-28) to afford a 1:1 mixture of the two reference com-
pounds (R,R)-29 and (R,S)-29 (yield 84%). Their identities
were established by elemental analyses (C, H, N), NMR spectro-
scopic studies (1H, 13C, 15N, 19F, 29Si), and mass spectrometric
investigations (GC/EI-MS). The determination of the enantiomeric
5906
dx.doi.org/10.1021/om300442e | Organometallics 2012, 31, 5903−5917

Organometallics
Article
Figure 4. Molecular (zwitterionic) structure of (S,R)-8 in the crystal of
(S,R)-8·2.5H2O.
Figure 1. Comparison of the molecular (zwitterionic) structure of
(S,R)-5 in the crystal of (S,R)-5·2H2O (left) with that of (S,S)-1117 in
the crystal of (S,S)-11·2H2O (right).
the Si−C−C bond angle appears to be an intrinsic structural
feature of silicon-containing amino acids of this particular
formula type.
As can be seen from Figures 1, 2, and 4, the dipeptides (S,R)-
5, (S,R)-7, and (S,R)-8 adopt a zwitterionic structure in the
crystal. In these structures, the charged ammonium and
carboxylate groups of the zwitterions are close together and
form a “zwitterionic ring” in the aspartate residue, with
intramolecular N−H···O hydrogen bonds.14 The N···O
distances observed for (S,R)-5 (2.938(4) Å), (S,R)-7
(2.5674(11) Å), and (S,R)-8 (2.6619(8) Å) are similar to
those found in the crystal structures of aspartame ((S,S)-9;
2.987 Å),15 its α,α-dialkylated analogue (S,S)-11 (3.056 Å),4b
and neotame ((S,S)-10; 3.187 Å).16 Also, the C terminus of the
silicon-containing dipeptides shows structural similarities with
their corresponding carbon analogues (SiMe3/Ph exchange) in
the solid state. This is shown for the structures of (S,R)-5 and
(S,S)-11 in Figure 1.17
Biological Studies. The silicon-containing aspartame and
neotame analogues were profiled for their ADME properties
using neotame as a reference compound. All compounds were
investigated for their solubility in HBSS buffer (pH 7.4) and
their octananol/water (pH 7.4) distribution coefficient (log D
value). Furthermore, they were studied for their human plasma
protein binding (hPPB) capability and their cytochrome P450
(CYP) inhibition using the two isoforms CYP3A4 and
CYP2D6. To determine the rate of decomposition in vitro,
the intrinsic clearance (CLint) and the half-life (t1/2) of the test
compounds were measured in human liver microsomes. As
shown in Table 1, in reference to neotame no significant impact
on the ADME properties of the test compounds was observed.
Overall, the silicon-containing dipepetidic aspartame and
neotame analogues studied showed an attractive property
profile, including good solubility and plasma protein binding
capabilities, no CYP inhibition, and no metabolic stability
liabilities. Thus, the silicon-containing α-amino acids (R)-4 and
(S)-4 proved to be promising building blocks for the design and
development of biologically active peptides.
Figure 2. Molecular structure of the cation in the crystal of (S,R)-
5·TFA.
Figure 3. Molecular (zwitterionic) structure of (S,R)-7 in the crystal of
(S,R)-7·H2O.
To get some preliminary information about their T1R2/R3
activating and thereby their sweet-tasting properties, the title
compounds as well as aspartame and neotame were tested for
their ability to induce GLP-1 secretion from the mouse intes-
tinal endocrine cell line STC-1.18 This cell line can be activated19,20
structures of rac-β-(trimethylsilyl)alanine (2)1g and (R)-
α-[(trimethylsilyl)methyl]alanine (4).1h Thus, the extension of
5907
dx.doi.org/10.1021/om300442e | Organometallics 2012, 31, 5903−5917

Organometallics
Article
Table 1. ADME Properties of (S,R)-5, (S,S)-5, (S,R)-6, (S,R)-7, (S,S)-7, (S,R)-8, (S,S)-8, and Neotame
IC50 (μM)
compd
solubility (μM)
log D
hPPB (% free)
CYP3A4
CYP2D6
CLint (μL min−1 mg−1
)
t1/2 (min)
a
(S,R)-5
36
61
77
96
97
89
89
0.3
0.3
0.8
2.4
2.3
2.8
2.7
1.8
nd
nd
72
20
16
31
27
17
>20
>20
>20
>20
>20
>20
>20
>20
>20
>20
>20
>20
>20
>20
>20
>20
<5
<5
<5
<5
<5
<5
<5
<5
>139
>139
>139
>139
>139
>139
>139
>139
b
(S,S)-5
(S,R)-6
(S,R)-7
(S,S)-7
(S,R)-8
(S,S)-8
neotame
91
a
b
Studied as (S,R)-5·TFA. Studied as (S,S)-5·TFA.
and release GLP-1 after treatment with sweet- and bitter-tasting
compounds. The artificial sweetener sucralose and phenyl-
thiocarbamide (PTC), a bitter taste receptor (T2R) agonist,
were used as positive controls and resulted in GLP-1 release as
measured by ELISA. However, treatment with the other test
compounds, including neotame and aspartame, at concen-
trations up to 2 mM did not result in any significant increase in
GLP-1 production.
A possible reason for this is species variation in the T1R2/R3
sweet taste receptors. STC-1 is a cell line of murine origin, and
it is known that neither aspartame nor neotame are active at the
mouse sweet taste receptor.21 Further attempts using a human
entero endocrine cell line such as NCI-H716, which is known
to secrete GLP-1, failed to show GLP-1 secretion properties
upon compound exposure for unknown reasons.22
Sensory Evaluation. The silicon-containing dipeptides
(S,R)-5, (S,S)-5, (S,R)-8, and (S,S)-8 were taste-checked by
two volunteers (for details, see the Experimental Section).
Compounds (S,R)-5 (aspartame analogue) and (S,R)-8 (neo-
tame analogue) were found to be approximately 50 and 600
times, respectively, as sweet as sucrose on a weight basis,
whereas the corresponding diastereomers (S,S)-5 and (S,S)-8
tasted bitter (Table 2). This is in good agreement with the
and (S,R)-8 showed lower sweetness intensities than their
respective analogues (S,S)-9 (aspartame), (S,S)-10 (neotame),
and (S,S)-11, they are very potent artificial sweeteners.
CONCLUSION
■
In this study, we have succeeded in scaling up the synthesis of
the enantiopure silicon-containing α-methylated α-amino acids
(R)- and (S)-α-[(trimethyl)silyl)methyl]alanine ((R)-4 and
(S)-4). Coupling of (R)-4 and (S)-4 with different S-configured
aspartate residues yielded a series of silicon-containing
dipeptidic aspartame and neotame analogues. These dipeptides
showed attractive ADME property profiles, including good
solubility and plasma protein binding capabilities, no CYP
inhibition, and no metabolic stability liabilities. Thus, the
silicon-containing α-amino acids proved to be promising
building blocks for the design of biologically active peptides.
Attempts to characterize the title compounds for their T1R2/
R3 activating properties by testing their ability to induce GLP-1
secretion from the mouse intestinal endocrine cell line STC-1
failed for unknown reasons (no significant increase in GLP-1
secretion up to a concentration of 2 mM). Sensory evaluation
of the silicon-containing dipeptides (S,R)-5, (S,S)-5, (S,R)-8,
and (S,S)-8 demonstrated the aspartame analogue (S,R)-5 and
the neotame analogue (S,R)-8 to be very potent artificial
sweeteners, whereas the corresponding diastereomers (S,S)-5
and (S,S)-8 tasted bitter. The silicon compounds (S,R)-5 and
(S,R)-8 are approximately 50 and 600 times, respectively, as
sweet as sucrose on a weight basis.
Table 2. Taste and Sweetness Intensities of (S,R)-5, (S,S)-5,
(S,R)-8, (S,S)-8, (S,S)-9, (S,R)-9, (S,S)-10, (S,R)-10, (S,S)-
11, and (S,R)-11
a
compd
taste
sweet (50)
b
(S,R)-5
c
EXPERIMENTAL SECTION
(S,S)-5
(S,R)-8
(S,S)-8
(S,S)-9
(S,R)-9
(S,S)-10
(S,R)-10
(S,S)-11
(S,R)-11
bitter
■
sweet (600)
bitter
Chemistry: General. All syntheses in organic solvents were carried
out under dry argon. The organic solvents used were dried and puri-
fied according to standard procedures and stored under dry argon.
Starting materials and reagents were purchased from Sigma-Aldrich
(Schnelldorf, Germany), ABCR (Karlsruhe, Germany), or Acros (Geel,
Belgium) and were used without further purification. The enzyme
(porcine liver esterase, 165 U mg−1, EC 3.1.1.1) was purchased from
Fluka (Buchs, Switzerland). Reversed-phase medium-pressure liquid
chromatography (RP-MPLC) was performed as follows: pressure, 16
bar; column, 50 × 2.5 cm; RP-18 silica gel, YMC ODS-A, 15 μm;
detector, Knauer Variable Wavelength Monitor. Bulb-to-bulb dis-
d
sweet (100−200)
d
bitter
e
sweet (8000−10000)
f
not sweet
g
sweet (200)
g
bitter
a
b
Sweetness intensities (in parentheses) refer to sucrose as 1. Studied
c
d
e
as (S,R)-5·TFA. Studied as (S,S)-5·TFA. Reference 23. Reference
24. Reference 24b. Reference 4b.
f
g
tillations were accomplished by using a Buchi GKR-50 Glass Oven
̈
apparatus. Lyophilization was carried out with a Christ alpha 1-2
apparatus. Enzymatic reactions were performed by using a Schott
TitroLine alpha titrator with pH-stat function. Melting points were
known impact of the absolute configuration on the taste
properties of the related dipeptides (S,S)-9, (S,R)-9, (S,S)-10,
(S,R)-10, (S,S)-11, and (S,R)-11 (Table 2). Thus, the amino
acids (R)-4 and (S)-phenylalanine as well as (S)-4 and (R)-
phenylalanine behave as bioisosteric building blocks in these
dipeptides. Although the silicon-containing dipeptides (S,R)-5
determined with a Buchi Melting Point B-540 apparatus using samples
̈
in open glass capillaries or by differential scanning calorimetry using
a Mettler Toledo DSC 823e apparatus. The 1H, 13C, 15N, 19F, and 29Si
NMR spectra were recorded at 23 °C, unless otherwise stated, on a
Bruker Avance 400 (1H, 400.1 MHz; 19F, 376.5 MHz) or a Bruker
5908
dx.doi.org/10.1021/om300442e | Organometallics 2012, 31, 5903−5917

Organometallics
Article
Avance 500 NMR spectrometer (1H, 500.1 MHz; 13C, 125.8 MHz;
15N, 50.7 MHz; 29Si, 99.4 MHz) using [D6]DMSO, C6D6, CD3OH,
CD3OD, or CD2Cl2 as the solvent. Chemical shifts (ppm) were
determined relative to internal [D5]DMSO (1H, δ 2.49 ppm;
[D6]DMSO), internal C6HD5 (1H, δ 7.28 ppm; C6D6), internal
CHD2OH (1H, δ 3.30 ppm; CD3OH), internal CHD2OD (1H, δ 3.30
ppm; CD3OD), internal CHDCl2 (1H, δ 5.32 ppm; CD2Cl2), internal
[D6]DMSO (13C, δ 39.5 ppm; [D6]DMSO), internal C6D6 (13C, δ
128.0 ppm; C6D6), internal CD3OH (13C, δ 49.0 ppm; CD3OH),
internal CD3OD (13C, δ 49.0 ppm; CD3OD), internal CD2Cl2 (13C, δ
53.8 ppm; CD2Cl2), external H2NC(O)H (90% in [D6]DMSO) (15N,
δ −268.0 ppm; [D6]DMSO, C6D6, CD3OH, CD3OD, CD2Cl2),
external CFCl3 (19F, δ 0 ppm; [D6]DMSO, C6D6, CD3OD), or
external TMS (29Si, δ 0 ppm; [D6]DMSO, C6D6, CD3OH, CD3OD,
CD2Cl2). Analysis and assignment of the 1H and 13C NMR
was dissolved in hydrochloric acid (6 M, 700 mL) at 20 °C, and the
resulting mixture was heated under reflux for 1 h. The solvent was
removed under reduced pressure, the solid residue was dissolved in
water (40 mL), and the resulting solution was freeze-dried to furnish a
colorless powder ((R)-4·HCl; 5.08 g; crude product, not further
purified), which was dissolved in a mixture of methanol (95 mL) and
an aqueous sodium hydroxide solution (1 M, 22.5 mL; 22.5 mmol of
NaOH).26 The solution was kept undisturbed at 20 °C for 4 days, and
the resulting precipitate was separated by suction filtration and washed
with diethyl ether (2 × 5 mL) to furnish (R)-4 (2.93 g, 16.7 mmol;
16% yield; enantiomeric excess, >99% ee) as a colorless crystalline
solid. The NMR data of the product (solvent, D2O) were identical
with those reported in ref 1h.
Synthesis of (S)-2-Amino-2-methyl-3-(trimethylsilyl)-
propanoic Acid ((S)-α-[(Trimethylsilyl)methyl]alanine, (S)-4).
This compound was synthesized according to ref 1h. A solution of
thionyl chloride (2.74 g, 23.0 mmol) in dichloromethane (25 mL) was
added dropwise at 0 °C within 15 min to a stirred solution of (S)-14
(5.00 g, 19.2 mmol; enantiomeric excess, 95% ee) in a mixture of
dichloromethane (25 mL) and N,N-dimethylformamide (50 μL). The
resulting mixture was warmed to 20 °C and was then stirred at this
temperature for 15 h. The solvent and the excess thionyl chloride were
removed under reduced pressure (20 °C, 0.02 mbar), the residue was
dissolved in acetone (20 mL), and the resulting solution was added
dropwise at 0 °C within 10 min to a stirred suspension of sodium azide
(1.37 g, 21.1 mmol) in acetone (30 mL). The mixture was warmed to
20 °C and was then stirred at this temperature for 3 h. The resulting
precipitate was separated by suction filtration, washed with acetone
(3 × 40 mL), and discarded. The filtrate (including the wash solutions)
was heated under reflux for 16 h, the mixture was cooled to 20 °C, and
the solvent was removed under reduced pressure. Subsequently,
hydrochloric acid (6 M, 300 mL) was added to the residue in a single
portion at 20 °C, and the resulting mixture was heated under reflux for
16 h. The solvent was removed under reduced pressure, the solid
residue was dissolved in water (400 mL), and the resulting aqueous
solution was extracted continuously with diethyl ether (500 mL) over
a period of 1 day using a perforator, and the extract was discarded. The
remaining aqueous phase was freeze-dried to furnish a colorless
powder ((S)-4·HCl; 3.30 g; crude product, not further purified),
which was dissolved in a mixture of methanol (36 mL) and an aqueous
sodium hydroxide solution (1 M, 14.6 mL; 14.6 mmol of NaOH).26
The solution was kept undisturbed at 10 °C for 1 day, and the
resulting precipitate was separated by suction filtration and washed
with diethyl ether (2 × 5 mL) to furnish (S)-4 (1.89 g, 10.8 mmol;
56% yield; enantiomeric excess, >99% ee) as a colorless crystalline
solid. The NMR data of the product (solvent, D2O) were identical
with those reported in ref 1h.
1
spectroscopic data was supported by H,1H gradient selected COSY
along with 13C,1H gradient selected HMQC and HMBC experiments.
Assignment of the 13C NMR spectroscopic data was additionally
supported by DEPT 135 experiments. The 15N NMR spectra were
obtained by using inverse correlation 15N,1H HMQC and HMBC
experiments. The spin systems were analyzed by using the WIN-
DAISY software package (version 4.05, Bruker).25 Coupling constants
are given as their absolute values. The assignment of the NMR spec-
troscopic data does not take into account H/D exchange processes
when using CD3OD as the solvent. In addition, the assignment of the
NMR spectroscopic data does not take into account that compounds
(S,R)-7, (S,S)-7, (S,R)-8, and (S,S)-8 can adopt a zwitterionic structure
in solution. EI-MS spectra (70 eV) and CI-MS spectra (reactant gas,
methane) were recorded on a Varian 320-MS SQ mass spectrometer.
HRMS spectra were recorded on a Bruker MicrOTOF mass spectro-
meter using solutions in methanol (ESI). Elemental analyses were
performed by using a VarioMicro apparatus (Elementar Analysensys-
teme GmbH) or a EURO EA Elemental Analyzer (EuroVector).
Synthesis of rac-2-Amino-2-methyl-3-(trimethylsilyl)-
propanoic Acid (rac-α-[(Trimethylsilyl)methyl]alanine, rac-4).
This compound was synthesized according to ref 1h.
Synthesis of (R)-2-Amino-2-methyl-3-(trimethylsilyl)-
propanoic Acid ((R)-α-[(Trimethylsilyl)methyl]alanine, (R)-4).
This compound was synthesized according to ref 1h. A solution of
thionyl chloride (14.6 g, 123 mmol) in dichloromethane (50 mL) was
added dropwise at 0 °C within 15 min to a stirred solution of (R)-13
(23.7 g, 102 mmol; enantiomeric excess, 75% ee) in a mixture of
dichloromethane (150 mL) and N,N-dimethylformamide (200 μL).
The resulting mixture was warmed to 20 °C and was then stirred at
this temperature for 18 h. The solvent and the excess thionyl chloride
were removed under reduced pressure (20 °C, 0.02 mbar), the residue
was dissolved in acetone (200 mL), and the resulting solution was
added dropwise at 0 °C within 10 min to a stirred suspension of
sodium azide (7.28 g, 112 mmol) in acetone (150 mL). The mixture
was warmed to 20 °C and was then stirred at this temperature for 3 h.
The resulting precipitate was separated by suction filtration, washed
with acetone (3 × 40 mL), and discarded. The filtrate (including the
wash solutions) was heated under reflux for 16 h, the mixture was
cooled to 20 °C, and the solvent was removed under reduced pressure.
Subsequently, hydrochloric acid (6 M, 700 mL) was added to the
residue in a single portion at 20 °C, and the resulting mixture was
heated under reflux for 6 h. The solvent was removed under reduced
pressure, the solid residue was dissolved in water (500 mL), and the
resulting aqueous solution was extracted with diethyl ether (3 × 100 mL)
and then freed of water under reduced pressure. The solid residue was
dissolved in water (250 mL), and the resulting solution was freeze-
dried to furnish a colorless powder ((R)-4·HCl; 17.3 g; crude product,
not further purified), which was dissolved in a mixture of methanol
(324 mL) and an aqueous sodium hydroxide solution (1 M, 76.6 mL;
76.6 mmol of NaOH).26 The solution was kept undisturbed at 20 °C
for 7 days, and the resulting precipitate was separated by suction
filtration and washed with diethyl ether (3 × 20 mL) to furnish (R)-4
(3.96 g, 22.6 mmol; 22% yield; enantiomeric excess, 90% ee) as a
colorless crystalline solid. To increase the enantiomeric excess,
compound (R)-4 (3.96 g, 22.6 mmol; enantiomeric excess, 90% ee)
Synthesis of (S)-Aspart-1-yl-(R)-2-[(trimethylsilyl)methyl]-
alanine Methyl Ester Hydrotrifluoroacetate ((S,R)-5·TFA).
Trifluoroacetic acid (4.45 g, 39.0 mmol) was added in a single
portion at 20 °C to (S,R)-19 (242 mg, 525 μmol), and the resulting
solution was stirred at 20 °C for 20 min. Subsequently, the volatile
components were removed under reduced pressure, and water
(150 mL) was added to the residue. The aqueous phase was extracted
continuously with diethyl ether (400 mL) over a period of 8 h using a
perforator, and the extract was discarded. The remaining aqueous
solution was freed of water under reduced pressure, the solid residue
was crystallized from water (500 μL; cooling of a boiling solution to
20 °C and crystallization over a period of 2 days), and the resulting
crystals were washed with n-heptane (−20 °C, 500 μL). Recrystalliza-
tion from water/methanol (10/1 v/v, 500 μL; cooling of a boiling
solution to 20 °C and crystallization over a period of 2 days) and
washing of the resulting crystals with n-heptane (−20 °C, 500 μL)
furnished (S,R)-5·TFA (119 mg, 284 μmol; 54% yield) as a colorless
crystalline solid. Mp: 219 °C dec. 1H NMR (400.1 MHz,
[D6]DMSO): δ 0.00 (s, 9 H; Si(CH3)3), 1.18 (δA) and 1.20 (δB)
(AB system, 2J(A,B) = 13.7 Hz, 2 H; SiCHAHB), 1.44 (s, 3 H; CCH3),
2
2.41 (δA), 2.61 (δB), and 3.79 (δX) (ABX system, J(A,B) = 17.0 Hz,
3
3J(A,X) = 9.1 Hz, J(B,X) = 4.1 Hz, 3 H; CHAHBCHX), 3.55 (s, 3 H;
OCH3), 8.6 ppm (br s, 1 H; C(O)NH), NH3 and C(O)OH not
5909
dx.doi.org/10.1021/om300442e | Organometallics 2012, 31, 5903−5917

Organometallics
Article
detected. 13C NMR (125.8 MHz, CD3OH): δ −0.4 (Si(CH3)3), 25.1
(CCH3), 29.2 (SiCH2), 36.3 (CH2CH), 51.1 (CH2CH), 52.7 (OCH3),
60.0 (CCH3), 168.3 (C(O)NH), 173.1 (C(O)OH), 175.5 ppm
(C(O)OCH3), CF3 and CF3C not detected. 15N NMR (50.7 MHz,
CD3OH): δ −346.8 (NH3), −245.7 ppm (C(O)NH). 19F NMR
(376.5 MHz, [D6]DMSO): δ −73.4 ppm. 29Si NMR (99.4 MHz,
CD3OH): δ −1.0 ppm. Anal. Calcd for C14H25F3N2O7Si: C, 40.19; H,
6.02; N, 6.69. Found: C, 40.17; H, 6.28; N, 6.79.
diethyl ether (400 mL) over a period of 8 h using a perforator, and the
extract was discarded. The remaining aqueous solution was freed of
water under reduced pressure, and the solid residue was dissolved in
methanol/water (80/20 v/v, 500 μL) and was further purified by RP-
MPLC (eluent, methanol/water (80/20 v/v); flow rate, 15 mL min−1;
detector wavelength, 210 nm). The resulting product was dissolved in
water (8 mL), and the solution was freeze-dried to furnish (S,R)-
6·TFA (46.0 mg, 106 μmol; 50% yield) as a colorless solid. Mp: 235
1
Synthesis of (S)-Aspart-1-yl-(S)-2-[(trimethylsilyl)methyl]-
alanine Methyl Ester Hydrotrifluoroacetate ((S,S)-5·TFA).
Trifluoroacetic acid (4.45 g, 39.0 mmol) was added in a single
portion at 20 °C to (S,S)-19 (250 mg, 543 μmol), and the resulting
solution was stirred at 20 °C for 20 min. Subsequently, the volatile
components were removed under reduced pressure, and water (40
mL) was added to the residue. The aqueous phase was extracted with
diethyl ether (3 × 15 mL), the extracts were discarded, and the
remaining aqueous solution was freed of water under reduced
pressure. The solid residue was dissolved in methanol/water (7/3 v/
v, 1 mL) and was further purified by RP-MPLC (eluent, methanol/
water (7/3 v/v); flow rate, 15 mL min−1; detector wavelength, 210
nm). The resulting product was dissolved in water (12 mL), and the
solution was freeze-dried to furnish (S,S)-5·TFA (202 mg, 483 mmol;
°C dec. H NMR (500.1 MHz, CD3OD): δ 0.05 (s, 9 H; Si(CH3)3),
1.22 (δA) and 1.29 (δB) (AB system, 2J(A,B) = 14.5 Hz, 2 H;
SiCHACHB), 1.26 (δX), 4.10 (δA), and 4.13 (δB) (ABX3 system,
2J(A,B) = 10.8 Hz, 3J(A,X) = 7.1 Hz, 3J(B,X) = 7.2 Hz, 5 H;
OCHAHBC(HX)3), 1.55 (s, 3 H; CCH3), 2.64 (δA), 2.83 (δB), and 4.05
ppm (δX) (ABX system, 2J(A,B) = 17.6 Hz, 3J(A,X) = 8.8 Hz, 3J(B,X)
= 4.8 Hz, 3 H; CHAHBCHX), NH3, C(O)NH, and C(O)OH not
detected. 13C NMR (125.8 MHz, CD3OD): δ −0.2 (Si(CH3)3), 14.4
(OCH2CH3), 25.2 (CCH3), 29.3 (SiCH2), 37.4 (CH2CH), 51.6
(CH2CH), 59.9 (CCH3), 62.5 (OCH2CH3), 168.6 (C(O)NH), 174.6
(C(O)OH), 175.3 ppm (C(O)OCH2CH3), CF3C and CF3C not
detected. 15N NMR (50.7 MHz, CD3OD): δ −348.3 (NH3), −246.7
ppm (C(O)NH). 19F NMR (CD3OD, 376.5 MHz): δ −76.9 ppm. 29Si
NMR (99.4 MHz, CD3OD): δ −0.9 ppm. Anal. Calcd for
C15H27F3N2O7Si: C, 41.66; H, 6.29; N, 6.48. Found: C, 41.35; H,
6.32; N, 6.19.
1
89% yield) as a colorless solid. Mp: 250 °C dec. H NMR (500.1
MHz, [D6]DMSO): δ 0.01 (s, 9 H; Si(CH3)3), 1.20 (δA) and 1.28 (δB)
(AB system, 2J(A,B) = 14.5 Hz, 2 H; SiCHAHB), 1.39 (s, 3 H; CCH3),
Synthesis of N-(2,2-Dimethylpropyl)-(S)-aspart-1-yl-(R)-2-
[(trimethylsilyl)methyl]alanine Methyl Ester ((S,R)-7). Com-
pound 24 (127 mg, 1.47 mmol) and anhydrous magnesium sulfate
(354 mg, 2.94 mmol) were added at 20 °C to a stirred solution of
(S,R)-23 (580 mg, 1.47 mmol) in dichloromethane (25 mL), and the
resulting mixture was stirred at this temperature for 24 h. The
magnesium sulfate (including its hydrate) was filtered off and
discarded, and the solvent of the filtrate was removed under reduced
pressure to furnish N-(2,2-dimethylpropylidene)-4-benzyloxy-(S)-
aspart-1-yl-(R)-2-[(trimethylsilyl)methyl]alanine methyl ester ((S,R)-
25; 596 mg; crude product, not further purified) as a colorless oily
2
2.64 (δA), 2.74 (δB), and 3.91 (δX) (ABX system, J(A,B) = 17.5 Hz,
3
3J(A,X) = 8.5 Hz, J(B,X) = 3.8 Hz, 3 H; CHAHBCHX), 3.56 (s, 3 H;
OCH3), 8.6 ppm (br s, 1 H; C(O)NH), NH3 and C(O)OH not
detected. 13C NMR (125.8 MHz, CD3OH): δ −0.1 (Si(CH3)3), 25.1
(CCH3), 28.5 (SiCH2), 37.1 (CH2CH), 51.3 (CH2CH), 52.9 (OCH3),
60.0 (CCH3), 168.5 (C(O)NH), 174.1 (C(O)OH), 176.1 ppm
(C(O)OCH3), CF3 and CF3C not detected. 15N NMR (50.7 MHz,
CD3OH): δ −348.1 (NH3), −247.5 ppm (C(O)NH). 19F NMR
(376.5 MHz, [D6]DMSO): δ −73.5 ppm. 29Si NMR (99.4 MHz,
CD3OH): δ −0.9 ppm. Anal. Calcd for C14H25F3N2O7Si: C, 40.19; H,
6.02; N, 6.69. Found: C, 39.97; H, 6.36; N, 6.75.
+
liquid (EI-MS: m/z (%) 447 (1) [M+ − CH3], 91 (100) [C7H7 ]). At
Synthesis of (S)-Aspart-1-yl-(R)-2-[(trimethylsilyl)methyl]-
alanine Methyl Ester Hydrochloride ((S,R)-5·HCl). A 2 M solution
of hydrogen chloride in diethyl ether (50 mL, 100 mmol of HCl) was
added in a single portion at 20 °C to (S,R)-19 (502 mg, 1.09 mmol),
and the resulting solution was stirred at 20 °C for 41 h. Subsequently,
the volatile components were removed under reduced pressure, and
water (60 mL) was added to the residue. The aqueous phase was
extracted with diethyl ether (3 × 25 mL), and the extracts were
discarded. The remaining aqueous solution was freed of water under
reduced pressure, and the solid residue was dissolved in methanol/
water (3/2 v/v, 700 μL) and was further purified by RP-MPLC
(eluent, methanol/water (3/2 v/v); flow rate, 14 mL min−1; detector
wavelength, 210 nm). The resulting product was dissolved in water
(15 mL), and the solution was freeze-dried to furnish (S,R)-5·HCl
(308 mg, 904 μmol; 83% yield) as a colorless solid. Mp: 234 °C dec.
1H NMR (500.1 MHz, CD3OH): δ 0.04 (s, 9 H; Si(CH3)3), 1.22 (δA)
20 °C, palladium/carbon (200 mg, 10 wt %, 188 μmol of Pd) was
added to a stirred solution of (S,R)-25 (596 mg; crude product) in
methanol (25 mL), and the resulting suspension was stirred at this
temperature under an atmosphere of hydrogen for 20 min. The
insoluble components were filtered off, washed with methanol (2 ×
10 mL), and discarded. The filtrate and the wash solutions were
combined, and the solvent was removed under reduced pressure. The
residue was dissolved in methanol (1.5 mL) and further purified by
RP-MPLC (eluent, methanol/water (7/3 v/v); flow rate, 15 mL
min−1; detector wavelength, 210 nm) to furnish (S,R)-7 (521 mg,
1.39 mmol; 95% yield) as a colorless solid. Mp: 140 °C dec. 1H NMR
(500.1 MHz, [D6]DMSO): δ −0.00 (s, 9 H; Si(CH3)3), 0.87 (s, 9 H;
C(CH3)3), 1.227 (δA) and 1.230 (δB) (AB system, 2J(A,B) = 3.0 Hz, 2
H; SiCHAHB), 1.42 (s, 3 H; CCH3), 2.22 (δA) and 2.28 (δB) (AB
system, 2J(A,B) = 11.2 Hz, 2 H; CHAHBNH), 2.30 (δA), 2.40 (δB), and
3.35 (δX) (ABX system, 2J(A,B) = 15.9 Hz, 3J(A,X) = 10.1 Hz, 3J(B,X)
= 4.3 Hz, 3 H; CHAHBCHX), 3.54 (s, 3 H; OCH3), 8.2 ppm (br s, 1 H;
C(O)NH), CH2NH, and C(O)OH not detected. 13C NMR (125.8
MHz, CD3OD): δ −0.1 (Si(CH3)3), 25.6 (CCH3), 27.4 (C(CH3)3),
28.5 (SiCH2), 31.5 (C(CH3)3), 36.1 (CH2CH), 52.9 (OCH3), 59.4
(CH2NH), 60.2 (CCH3), 60.7 (CH2CH), 168.0 (C(O)NH), 175.7
(C(O)OCH3), 176.6 ppm (C(O)OH). 15N NMR (50.7 MHz,
CD3OD): δ −340.5 (CH2NH), −244.4 ppm (C(O)NH). 29Si NMR
(99.4 MHz, CD3OD): δ −1.0 ppm. EI-MS: m/z (%) 374 (1) [M+],
172 (100). Anal. Calcd for C17H34N2O5Si: C, 54.51; H, 9.15; N, 7.48.
Found: C, 54.14; H, 8.87; N, 7.47.
and 1.29 (δB) (AB system, 2J(A,B) = 14.5 Hz, 2 H; SiCHAHB), 1.54 (s,
3 H; CCH3), 2.50 (δA), 2.70 (δB), and 3.98 (δX) (ABX system, 2J(A,B)
3
3
= 17.1 Hz, J(A,X) = 5.3 Hz, J(B,X) = 9.0 Hz, 3 H; CHAHBCHX),
3.65 (s, 3 H; OCH3), 8.6 ppm (br s, 1 H; C(O)NH), NH3 and
C(O)OH not detected. 13C NMR (125.8 MHz, CD3OH): δ −0.4
(Si(CH3)3), 25.2 (CCH3), 27.7 (SiCH2), 38.3 (CH2CH), 52.3
(CH2CH), 52.6 (OCH3), 59.8 (CCH3), 169.3 (C(O)NH), 175.7
(C(O)OCH3), 176.1 ppm (C(O)OH). 15N NMR (50.7 MHz,
CD3OH): δ −347.5 (NH3), −246.0 ppm (C(O)NH). 29Si NMR
(99.4 MHz, CD3OH): δ −1.0 ppm. Anal. Calcd for C12H25ClN2O5Si:
C, 42.28; H, 7.39; N, 8.22. Found: C, 42.31; H, 7.49; N, 7.99.
Synthesis of (S)-Aspart-1-yl-(R)-2-[(trimethylsilyl)methyl]-
alanine Ethyl Ester Hydrotrifluoroacetate ((S,R)-6·TFA). Tri-
fluoroacetic acid (4.45 g, 39.0 mmol) was added in a single portion at
20 °C to (S,R)-20 (100 mg, 211 μmol), and the resulting solution was
stirred at 20 °C for 20 min. Subsequently, the volatile components
were removed under reduced pressure, and water (150 mL) was added
to the residue. The aqueous phase was extracted continuously with
Synthesis of N-(2,2-Dimethylpropyl)-(S)-aspart-1-yl-(S)-2-
[(trimethylsilyl)methyl]alanine Methyl Ester ((S,S)-7). Com-
pound 24 (49.1 mg, 570 μmol) and anhydrous magnesium sulfate
(137 mg, 1.14 mmol) were added at 20 °C to a stirred solution of
(S,S)-23 (225 mg, 570 μmol) in dichloromethane (25 mL), and the
resulting mixture was stirred at this temperature for 24 h. The
magnesium sulfate (including its hydrate) was filtered off and
discarded, and the solvent of the filtrate was removed under reduced
5910
dx.doi.org/10.1021/om300442e | Organometallics 2012, 31, 5903−5917

Organometallics
Article
pressure to furnish N-(2,2-dimethylpropylidene)-4-benzyloxy-(S)-
aspart-1-yl-(S)-2-[(trimethylsilyl)methyl]alanine methyl ester ((S,S)-
25; 200 mg; crude product, not further purified) as a colorless oily
Synthesis of N-(3,3-Dimethylbutyl)-(S)-aspart-1-yl-(S)-2-
[(trimethylsilyl)methyl]alanine Methyl Ester ((S,S)-8). Com-
pound 27 (45.2 mg, 451 μmol) and anhydrous magnesium sulfate
(109 mg, 906 μmol) were added at 20 °C to a stirred solution of (S,S)-
23 (178 mg, 451 μmol) in dichloromethane (10 mL), and the
resulting mixture was stirred at this temperature for 16 h. The
magnesium sulfate (including its hydrate) was filtered off and
discarded, and the solvent of the filtrate was removed under reduced
pressure to furnish N-(3,3-dimethylbutylidene)-4-benzyloxy-(S)-as-
part-1-yl-(S)-2-[(trimethylsilyl)methyl]alanine methyl ester ((S,S)-
26; 199 mg; crude product, not further purified) as a colorless oily
+
liquid (EI-MS: m/z (%) 447 (2) [M+ − CH3], 91 (100) [C7H7 ]). At
20 °C, palladium/carbon (150 mg, 10 wt %, 141 μmol of Pd) was
added to a stirred solution of (S,S)-25 (200 mg; crude product) in
methanol (20 mL), and the resulting suspension was stirred at this
temperature under an atmosphere of hydrogen for 20 min. The
insoluble components were filtered off, washed with methanol (2 ×
10 mL), and discarded. The filtrate and the wash solutions were
combined, and the solvent was removed under reduced pressure.
The residue was dissolved in methanol (1 mL) and further purified by
RP-MPLC (eluent, methanol/water (4/1 v/v); flow rate, 15 mL
min−1; detector wavelength, 210 nm) to furnish (S,S)-7 (191 mg,
+
liquid (EI-MS: m/z (%) 461 (2) [M+ − CH3], 91 (100) [C7H7 ]). At
20 °C, palladium/carbon (75 mg, 10 wt %, 70.5 μmol of Pd) was
added to a stirred solution of (S,S)-26 (199 mg; crude product) in
methanol (10 mL), and the resulting suspension was stirred at this
temperature under an atmosphere of hydrogen for 20 min. The
insoluble components were filtered off, washed with methanol (2 × 10
mL), and discarded. The filtrate and the wash solutions were
combined, and the solvent was removed under reduced pressure.
The residue was dissolved in methanol (1 mL) and further purified by
RP-MPLC (eluent, methanol/water (7/3 v/v); flow rate, 13 mL
min−1; detector wavelength, 210 nm) to furnish (S,S)-8 (163 mg, 419
1
510 μmol; 89% yield) as a colorless solid. Mp: 121 °C dec. H NMR
(500.1 MHz, [D6]DMSO): δ −0.01 (s, 9 H; Si(CH3)3), 0.87 (s, 9 H;
2
C(CH3)3), 1.23 (δA) and 1.28 (δB) (AB system, J(A,B) = 14.5 Hz,
2 H; SiCHAHB), 1.40 (s, 3 H; CCH3), 2.22 (δA) and 2.27 (δB) (AB
system, 2J(A,B) = 10.9 Hz, 2 H; CHAHBNH), 2.30 (δA), 2.39 (δB), and
3.33 (δX) (ABX system, 2J(A,B) = 15.9 Hz, 3J(A,X) = 9.5 Hz, 3J(B,X) =
4.6 Hz, 3 H; CHAHBCHX), 3.55 (s, 3 H; OCH3), 8.3 ppm (br s, 1 H;
C(O)NH), CH2NH and C(O)OH not detected. 13C NMR (125.8
MHz, CD3OD): δ −0.1 (Si(CH3)3), 25.1 (CCH3), 27.4 (C(CH3)3),
28.8 (SiCH2), 31.6 (C(CH3)3), 36.3 (CH2CH), 52.9 (OCH3), 59.6
(CH2NH), 60.0 (CCH3), 60.7 (CH2CH), 168.3 (C(O)NH), 175.8
(C(O)OCH3), 176.7 ppm (C(O)OH). 15N NMR (50.7 MHz,
CD3OD): δ −340.5 (CH2NH), −244.4 ppm (C(O)NH). 29Si NMR
(99.4 MHz, CD3OD): δ −0.9 ppm. EI-MS: m/z (%) 374 (1) [M+],
172 (100). Anal. Calcd for C17H34N2O5Si: C, 54.51; H, 9.15; N, 7.48.
Found: C, 54.25; H, 8.93; N, 7.45.
1
μmol; 93% yield) as a colorless solid. Mp: 149 °C dec. H NMR
(500.1 MHz, CD3OD): δ 0.06 (s, 9 H; Si(CH3)3), 0.97 (s, 9 H;
2
C(CH3)3), 1.28 (δA) and 1.32 (δB) (AB system, J(A,B) = 14.6 Hz, 2
H; SiCHAHB), 1.51 (s, 3 H; CCH3), 1.60 (δA), 1.64 (δB), 2.95 (δC),
2
3
and 2.98 (δD) (ABCD system, J(A,B) = 12.9 Hz, J(A,C) = 12.5 Hz,
3J(A,D) = 4.6 Hz, 3J(B,C) = 4.9 Hz, 3J(B,D) = 12.6 Hz, 2J(C,D) = 11.8
Hz, 4 H; CHAHBCHCHDNH), 2.52 (δA), 2.62 (δB), and 3.98 (δX)
(ABX system, 2J(A,B) = 17.0 Hz, 3J(A,X) = 10.1 Hz, 3J(B,X) = 4.5 Hz,
3 H; CHAHBCHX), 3.67 ppm (s, 3 H; OCH3), C(O)NH, CH2NH,
and C(O)OH not detected. 13C NMR (125.8 MHz, CD3OD): δ −0.1
(Si(CH3)3), 25.0 (CCH3), 28.9 (SiCH2), 29.5 (C(CH3)3), 30.6
(C(CH3)3), 37.5 (CH2CH), 40.8 (CH2CH2NH), 44.6 (CH2CH2NH),
52.9 (OCH3), 59.5 (CH2CH), 60.0 (CCH3), 168.3 (C(O)NH), 175.7
(C(O)OCH3), 175.9 ppm (C(O)OH). 15N NMR (50.7 MHz,
CD3OD): δ −333.8 (CH2NH), −243.6 ppm (C(O)NH). 29Si NMR
(99.4 MHz, CD3OD): δ −0.9 ppm. EI-MS: m/z (%) 388 (1) [M+],
186 (100). Anal. Calcd for C18H36N2O5Si: C, 55.64; H, 9.34; N, 7.21.
Found: C, 55.51; H, 9.46; N, 7.14.
Synthesis of N-(3,3-Dimethylbutyl)-(S)-aspart-1-yl-(R)-2-
[(trimethylsilyl)methyl]alanine Methyl Ester ((S,R)-8). Com-
pound 27 (53.3 mg, 532 μmol) and anhydrous magnesium sulfate
(129 mg, 1.07 mmol) were added at 20 °C to a stirred solution of
(S,R)-23 (210 mg, 532 μmol) in dichloromethane (10 mL), and the
resulting mixture was stirred at this temperature for 2.5 h. The
magnesium sulfate (including its hydrate) was filtered off and
discarded, and the solvent of the filtrate was removed under reduced
pressure to furnish N-(3,3-dimethylbutylidene)-4-benzyloxy-(S)-as-
part-1-yl-(R)-2-[(trimethylsilyl)methyl]alanine methyl ester ((S,R)-
26; 285 mg; crude product, not further purified) as a colorless oily
Synthesis of Diethyl Methyl[(trimethylsilyl)methyl]malonate
(12). This compound was synthesized according to ref 1h.
+
liquid (EI-MS: m/z (%) 461 (2) [M+ − CH3], 91 (100) [C7H7 ]). At
20 °C, palladium/carbon (110 mg, 10 wt %, 103 μmol of Pd) was
added to a stirred solution of (S,R)-26 (285 mg; crude product) in
methanol (10 mL), and the resulting suspension was stirred at this
temperature under an atmosphere of hydrogen for 20 min. The
insoluble components were filtered off, washed with methanol (2 × 10
mL), and discarded. The filtrate and the wash solutions were
combined, and the solvent was removed under reduced pressure.
The residue was dissolved in methanol (1 mL) and further purified by
RP-MPLC (eluent, methanol/water (7/3 v/v); flow rate, 13 mL
min−1; detector wavelength, 210 nm) to furnish (S,R)-8 (191 mg, 492
Synthesis of (R)-3-Ethoxy-2-methyl-3-oxo-2-[(trimethylsilyl)-
methyl]propanoic Acid ((R)-13; Enantiomeric Excess, 75% ee).
This compound was synthesized according to ref 1h. Porcine liver
esterase (60 mg, 165 U mg−1, EC 3.1.1.1) was added in a single
portion at 38 °C to a stirred emulsion of 12 (100 g, 384 mmol) in
water (3 L). The mixture was stirred at 38 °C for 94 h while the pH
value was adjusted to 7.60 with a 1 M aqueous sodium hydroxide
solution by using a pH-stat titrator. The enzyme was separated by
suction filtration (Celite), washed with water (3 × 150 mL), and
discarded. The aqueous phase was extracted continuously with diethyl
ether (1.5 L) over a period of 4 days using a perforator, and the extract
was discarded. The remaining aqueous solution was acidified with
concentrated hydrochloric acid (40 mL) and extracted continuously
with diethyl ether (1.5 L) over a period of 18 h using a perforator. The
organic phase was dried over anhydrous sodium sulfate, the solvent
was removed under reduced pressure, and the residue was purified by
bulb-to-bulb distillation (110 °C, 0.02 mbar) to furnish (R)-13 (86.9 g,
374 mmol; 97% yield; enantiomeric excess, 75% ee) as a colorless
liquid. The NMR data of the product (solvent, [D6]DMSO) were
identical with those reported in ref 1h.
1
μmol; 92% yield) as a colorless solid. Mp: 114 °C dec. H NMR
(500.1 MHz, CD3OD): δ −0.05 (s, 9 H; Si(CH3)3), 0.95 (s, 9 H;
2
C(CH3)3), 1.31 (δA) and 1.34 (δB) (AB system, J(A,B) = 14.5 Hz, 2
H; SiCHAHB), 1.54 (s, 3 H; CCH3), 1.59 (δA), 1.61 (δB), 2.95 (δC),
2
3
and 2.96 (δD) (ABCD system, J(A,B) = 13.0 Hz, J(A,C) = 12.2 Hz,
3J(A,D) = 4.9 Hz, 3J(B,C) = 4.9 Hz, 3J(B,D) = 12.4 Hz, 2J(C,D) = 12.1
Hz, 4 H; CHAHBCHCHDNH), 2.52 (δA), 2.66 (δB), and 3.96 (δX)
(ABX system, 2J(A,B) = 17.0 Hz, 3J(A,X) = 9.9 Hz, 3J(B,X) = 4.5 Hz, 3
H; CHAHBCHX), 3.67 ppm (s, 3 H; OCH3), C(O)NH, CH2NH, and
C(O)OH not detected. 13C NMR (125.8 MHz, CD3OD): δ −0.1
(Si(CH3)3), 25.6 (CCH3), 28.6 (SiCH2), 29.5 (C(CH3)3), 30.6
(C(CH3)3), 37.6 (CH2CH), 41.0 (CH2CH2NH), 44.6 (CH2CH2NH),
52.9 (OCH3), 59.5 (CH2CH), 60.1 (CCH3), 168.7 (C(O)NH), 175.7
(C(O)OCH3), 176.2 ppm (C(O)OH). 15N NMR (50.7 MHz,
CD3OD): δ −334.3 (CH2NH), −244.4 ppm (C(O)NH). 29Si NMR
(99.4 MHz, CD3OD): δ −1.0 ppm. EI-MS: m/z (%) 388 (1) [M+],
186 (100). Anal. Calcd for C18H36N2O5Si: C, 55.64; H, 9.34; N, 7.21.
Found: C, 55.46; H, 9.27; N, 7.07.
Synthesis of [(S)-3-tert-Butoxy-2-methyl-3-oxo-2-
[(trimethylsilyl)methyl]propanoic Acid ((S)-14; Enantiomeric
Excess, 95% ee). This compound was synthesized according to ref
1h. At −20 °C, 2-methylpropene (83.0 g, 1.48 mol) was passed
through a stirred mixture of (R)-13 (40.0 g, 172 mmol; enantiomeric
excess, 75% ee), dichloromethane (400 mL), and concentrated sulfuric
acid (4.00 mL) over a period of 1 h. The mixture was warmed to
20 °C and was then stirred at this temperature for 20 h. Subsequently,
5911
dx.doi.org/10.1021/om300442e | Organometallics 2012, 31, 5903−5917

Organometallics
Article
ethanol (400 mL) and potassium hydroxide (55.0 g, 980 mmol) were
added one after another in single portions, and the resulting mixture
was heated under reflux for 54 h and then cooled to 20 °C. The
solvent was removed under reduced pressure, water (3 L) was added,
the resulting mixture was extracted continuously with diethyl ether
(1.5 L) over a period of 2 days using a perforator, and the extract was
discarded. The remaining aqueous phase was acidified with
concentrated hydrochloric acid (80 mL) and then extracted
continuously with diethyl ether (1.5 L) over a period of 45 h using
a perforator. The organic phase was dried over anhydrous sodium
sulfate, the solvent was removed under reduced pressure, and the
residue was dried in vacuo (20 °C, 0.02 mbar, 2 h) and then
crystallized from n-hexane (200 mL) at −20 °C over a period of 1 day
to furnish (S)-14 (30.5 g, 117 mmol; 68% yield; enantiomeric excess,
75% ee) as a colorless crystalline solid. Further product fractions of
(S)-14 (in total 11.2 g, 43.0 mmol; 25% yield; enantiomeric excess,
75% ee) were obtained by applying the crystallization procedure
described above to the mother liquor (crystallization from 50 mL, 12
mL, and 2 mL n-hexane, respectively) to give (S)-14 in a total yield of
93% as a crystalline solid (enantiomeric excess, 75% ee).
The solvent and excess thionyl chloride were removed under reduced
pressure, and the resulting residue was washed with diethyl ether (2 × 2
mL) and then dissolved in water (3 mL). The resulting solution was
freeze-dried to furnish (S)-15·HCl (128 mg, 567 μmol; 99% yield) as a
colorless solid. Mp: 258 °C dec. The NMR data of the product (solvent,
[D6]DMSO) were identical with those obtained for rac-15·HCl. EI-MS:27
m/z (%) 174 (6) [M+ − CH3], 73 (100) [Si(CH3)3+]. HRMS (ESI):
m/z calcd for [M − HCl + Na]+, 212.1077; found, 212.1077.
Synthesis of (R)-2-Amino-2-methyl-3-(trimethylsilyl)-
propionic Acid Ethyl Ester Hydrochloride ((R)-α-
[(Trimethylsilyl)methyl]alanine Ethyl Ester Hydrochloride (R)-
16·HCl). Thionyl chloride (6.81 g, 57.2 mmol) was added at −20 °C
to a stirred suspension of (R)-4 (1.00 g, 5.70 mmol) in ethanol (100
mL). After the mixture had been warmed to 20 °C, it was stirred at this
temperature for 5 min and then heated under reflux for 16 h. The
solvent and excess thionyl chloride were removed under reduced
pressure, and the resulting residue was washed with diethyl ether (3 ×
30 mL) and then dissolved in water (50 mL). The resulting solution
was freeze-dried to furnish (R)-16·HCl (1.35 g, 5.63 mmol; 99% yield)
as a colorless solid. Mp: 263 °C dec. 1H NMR (500.1 MHz,
[D6]DMSO): δ 0.02 (s, 9 H; Si(CH3)3), 1.25 (δX), 4.13 (δA), and 4.20
Enhancement of the enantiomeric excess was accomplished by
recrystallizing (S)-14 (19.7 g, 75.7 mmol; enantiomeric excess, 75%
ee) from n-hexane (129 mL) at −20 °C over a period of 1 day to
furnish (S)-14 (5.86 g, 22.5 mmol; 30% yield (relative to 19.7 g of
(S)-14); enantiomeric excess, 95% ee) as a colorless crystalline solid.
The NMR data of the product (solvent, C6D6) were identical with
those reported in ref 1h.
2
3
(δB) (ABX3 system, J(A,B) = 10.9 Hz, J(A,X) = 3J(B,X) = 7.1 Hz, 5
2
H; OCHAHBC(HX)3), 1.24 (δA) and 1.26 (δB) (AB system, J(A,B) =
13.1 Hz, 2 H; SiCHAHB), 1.49 (s, 3 H; CCH3), 8.5 ppm (br s, 3 H;
NH3). 13C NMR (125.8 MHz, [D6]DMSO): δ −0.5 (Si(CH3)3), 13.8
(OCH2CH3), 23.6 (CCH3), 27.3 (SiCH2), 58.4 (CCH3), 62.0
(OCH2CH3), 171.4 ppm (C(O)OC2H5). 15N NMR (50.7 MHz,
[D6]DMSO): δ −319.5 ppm. 29Si NMR (99.4 MHz, [D6]DMSO): δ
−1.0 ppm. EI-MS:27 m/z (%) 188 (5) [M+ − CH3], 130 (100) [M+ −
Si(CH3)3]. HRMS (ESI): m/z calcd for [M − HCl + Na]+, 226.1234;
found, 226.1233.
N-tert-Butoxycarbonyl-4-tert-butoxy-(S)-aspartic Acid ((S)-
17). This compound was commercially available (Sigma-Aldrich).
O-(6-Chlorobenzotriazol-1-yl)-N,N,N′,N′-tetramethyluro-
nium Hexafluorophosphate (18). This compound was commer-
cially available (Sigma-Aldrich).
Synthesis of rac-2-Amino-2-methyl-3-(trimethylsilyl)-
propionic Acid Methyl Ester Hydrochloride (rac-α-
[(Trimethylsilyl)methyl]alanine Methyl Ester Hydrochloride,
rac-15·HCl). Thionyl chloride (6.79 g, 57.1 mmol) was added at
−20 °C to a stirred suspension of rac-4 (1.00 g, 5.70 mmol) in
methanol (50 mL). After the mixture had been warmed to 20 °C, it
was stirred at this temperature for 5 min and then heated under reflux
for 2 h. The solvent and excess thionyl chloride were removed under
reduced pressure, and the resulting residue was washed with diethyl
ether (2 × 10 mL) and then dissolved in water (150 mL). The
resulting solution was freeze-dried to furnish rac-15·HCl (1.28 g, 5.67
Synthesis of N-tert-Butoxycarbonyl-4-tert-butoxy-(S)-aspart-
1-yl-(R)-2-[(trimethylsilyl)methyl]alanine Methyl Ester ((S,R)-
19). (S)-17 (987 mg, 3.41 mmol), 18 (1.54 g, 3.72 mmol), and N-
methylmorpholine (941 mg, 9.30 mmol) were added one after another
in single portions at 20 °C to a stirred suspension of (R)-15·HCl
(700 mg, 3.10 mmol) in dichloromethane (30 mL). The resulting mixture
was stirred at this temperature for 30 min and then extracted with
water (2 × 20 mL). The combined aqueous solutions were extracted
with dichloromethane (3 × 20 mL) and discarded, the solvent of the
combined organic solutions was removed under reduced pressure, and
the solid residue was crystallized from methanol/water (58/42 v/v,
76 mL; slow cooling of a boiling solution to 20 °C and crystallization
over a period of 1 day). The precipitate was washed with methanol
(−20 °C, 10 mL) and dried under vacuum (0.05 mbar, 20 °C, 4 h) to
furnish (S,R)-19 (1.23 g, 2.67 mmol; 86% yield) as a colorless
crystalline solid. Mp: 239 °C. 1H NMR (500.1 MHz, C6D6): δ 0.14 (s,
1
mmol; 99% yield) as a colorless solid. Mp: 253 °C dec. H NMR
(500.1 MHz, [D6]DMSO, 27 °C): δ 0.01 (s, 9 H; Si(CH3)3), 1.26
2
(δA) and 1.30 (δB) (AB system, J(A,B) = 14.4 Hz, 2 H; SiCHAHB),
1.51 (s, 3 H; CCH3), 3.72 (s, 3 H; OCH3), 8.7 ppm (br s, 3 H; NH3).
13C NMR (125.8 MHz), [D6]DMSO, 27 °C): δ −0.7 (Si(CH3)3), 23.3
(CCH3), 27.3 (SiCH2), 52.8 (OCH3), 58.4 (CCH3), 171.6 ppm
(C(O)OCH3). 15N NMR (50.7 MHz, [D6]DMSO, 27 °C): δ −319.5
ppm. 29Si NMR (99.4 MHz, [D6]DMSO, 27 °C): δ −1.0 ppm. EI-
MS:27 m/z (%) 174 (4) [M+ − CH3], 73 (100) [Si(CH3)3 ]. HRMS
+
(ESI): m/z calcd for [M − HCl + Na]+, 212.1077; found, 212.1077.
Synthesis of (R)-2-Amino-2-methyl-3-(trimethylsilyl)-
propionic Acid Methyl Ester Hydrochloride ((R)-α-
[(Trimethylsilyl)methyl]alanine Methyl Ester Hydrochloride,
(R)-15·HCl). Thionyl chloride (508 mg, 4.27 mmol) was added at
−20 °C to a stirred suspension of (R)-4 (75.0 mg, 428 μmol) in
methanol (3 mL). After the mixture had been warmed to 20 °C, it was
stirred at this temperature for 5 min and then heated under reflux for 2
h. The solvent and excess thionyl chloride were removed under
reduced pressure, and the resulting residue was washed with diethyl
ether (2 × 3 mL) and then dissolved in water (3 mL). The resulting
solution was freeze-dried to furnish (R)-15·HCl (96.0 mg, 425 μmol;
99% yield) as a colorless solid. Mp: 271 °C dec. The NMR data of
the product (solvent, [D6]DMSO) were identical with those obtained
for rac-15·HCl. EI-MS:27 m/z (%) 174 (5) [M+ − CH3], 73 (100)
2
9 H; Si(CH3)3), 1.26 (δA) and 1.73 (δB) (AB system, J(A,B) = 14.6
Hz, 2 H; SiCHAHB), 1.43 (s, 9 H; C(CH3)3; Boc or tBu), 1.54 (s, 9 H;
C(CH3)3; Boc or tBu), 1.83 (s, 3 H; CCH3), 2.58 (δA), 2.92 (δB), 4.80
2
3
(δM), and 6.0 (br, δx) (ABMX system, J(A,B) = 16.8 Hz, J(A,M) =
7.1 Hz, 3J(B,M) = 4.5 Hz, 3J(M,X) = 8.8 Hz, 4 H; CHAHBCHMNHX),
3.47 (s, 3 H; OCH3), 7.4 ppm (br s, 1 H; C(O)NH). 13C NMR (125.8
MHz, C6D6): δ −0.4 (Si(CH3)3), 25.8 (CCH3), 27.7 (SiCH2), 27.9
(C(CH3)3; Boc or tBu), 28.3 (C(CH3)3; Boc or tBu), 37.2 (CH2CH),
51.4 (CH2CH), 51.9 (OCH3), 59.1 (CCH3), 79.7 (C(CH3)3; Boc or tBu),
81.0 (C(CH3)3; Boc or tBu), 155.8 (OC(O)NH), 170.2 (CC(O)NH),
171.7 (C(O)OC(CH3)3), 174.9 ppm (C(O)OCH3). 15N NMR (50.7
MHz, C6D6): δ −293.8 (OC(O)NH), −252.2 ppm (CC(O)NH). 29Si
NMR (99.4 MHz, C6D6): δ −1.1 ppm. CI-MS: m/z (%) 489 (1) [M +
C2H5]+, 461 (5) [M + H]+, 349 (100). Anal. Calcd for C21H40N2O7Si: C,
54.76; H, 8.75; N, 6.08. Found: C, 54.38; H, 8.69; N, 6.13.
+
[Si(CH3)3 ]. HRMS (ESI): m/z calcd for [M − HCl + Na]+,
212.1077; found, 212.1078.
Synthesis of (S)-2-Amino-2-methyl-3-(trimethylsilyl)-
propionic Acid Methyl Ester Hydrochloride ((S)-α-
[(Trimethylsilyl)methyl]alanine Methyl Ester Hydrochloride,
(S)-15·HCl). Thionyl chloride (678 mg, 5.70 mmol) was added at
−20 °C to a stirred suspension of (S)-4 (100 mg, 570 μmol) in
methanol (3 mL). After the mixture had been warmed to 20 °C, it was
stirred at this temperature for 5 min and then heated under reflux for 2 h.
Synthesis of N-tert-Butoxycarbonyl-4-tert-butoxy-(S)-aspart-
1-yl-(S)-2-[(trimethylsilyl)methyl]alanine Methyl Ester ((S,S)-
19). (S)-17 (894 mg, 3.09 mmol), 18 (1.39 g, 3.36 mmol), and
N-methylmorpholine (852 mg, 8.42 mmol) were added one after
5912
dx.doi.org/10.1021/om300442e | Organometallics 2012, 31, 5903−5917

Organometallics
Article
another in single portions at 20 °C to a stirred suspension of (S)-
15·HCl (634 mg, 2.81 mmol) in dichloromethane (30 mL). The
resulting mixture was stirred at this temperature for 30 min and then
extracted with water (2 × 20 mL). The combined aqueous solutions
were extracted with dichloromethane (3 × 20 mL) and discarded, the
solvent of the combined organic solutions was removed under reduced
pressure, and the remaining residue was dissolved in methanol/water
(4/1 v/v, 4 mL) and then further purified by RP-MPLC (eluent,
methanol/water (4/1 v/v); flow rate, 9 mL min−1; detector
wavelength, 210 nm) to furnish (S,S)-19 (1.27 g, 2.76 mmol; 98%
yield) as a colorless solid. Mp: 244 °C. 1H NMR (500.1 MHz, C6D6):
δ 0.16 (s, 9 H; Si(CH3)3), 1.35 (δA) and 1.98 (δB) (AB system, 2J(A,B)
= 14.7 Hz, 2 H; SiCHAHB), 1.42 (s, 9 H; C(CH3)3; Boc or tBu), 1.56
(s, 9 H; C(CH3)3; Boc or tBu), 1.85 (s, 3 H; CCH3), 2.54 (δA), 2.98
extracted with dichloromethane (3 × 20 mL) and discarded, the
solvent of the combined organic solutions was removed under reduced
pressure, and the remaining residue was dissolved in methanol (1.6
mL) and then further purified by RP-MPLC (eluent, methanol/water
(4/1 v/v); flow rate, 19 mL min−1; detector wavelength, 225 nm) to
furnish (S,R)-22 (3.73 g, 7.54 mmol; 97% yield) as a colorless solid.
Mp: 250 °C. 1H NMR (500.1 MHz, C6D6): δ 0.14 (s, 9 H; Si(CH3)3),
1.26 (δA) and 1.72 (δB) (AB system, 2J(A,B) = 14.6 Hz, 2 H;
SiCHAHB), 1.54 (s, 9 H; C(CH3)3), 1.81 (s, 3 H; CCH3), 2.61 (δA),
2
2.93 (δB), 4.80 (δM), and 6.0 (br, δx) (ABMX system, J(A,B) = 16.9
3
3
3
Hz, J(A,M) = 7.0 Hz, J(B,M) = 4.5 Hz, J(M,X) = 8.8 Hz, 4 H;
CHAHBCHMNHX), 3.46 (s, 3 H; OCH3), 4.99 (δA) and 5.06 (δB) (AB
system, 2J(A,B) = 12.4 Hz, 2 H; OCHAHB), 7.13−7.17 (m, 1 H; p-CH,
C6H5), 7.18−7.22 (m, 2 H; m-CH, C6H5), 7.23−7.26 (m, 2 H; o-CH,
C6H5), 7.4 ppm (br s, 1 H; C(O)NH). 13C NMR (125.8 MHz, C6D6):
δ −0.4 (Si(CH3)3), 25.9 (CCH3), 27.6 (SiCH2), 28.3 (C(CH3)3), 35.9
(CH2CH), 51.3 (CH2CH), 52.0 (OCH3), 59.1 (CCH3), 66.6 (OCH2),
79.9 (C(CH3)3), 128.2 (p-C, C6H5), 128.4 (o-C, C6H5), 128.6 (m-C,
C6H5), 136.2 (i-C, C6H5), 155.8 (OC(O)NH), 169.9 (CC(O)NH),
172.0 (C(O)OCH2C6H5), 174.8 ppm (C(O)OCH3). 15N NMR (50.7
MHz, C6D6): δ −293.9 (OC(O)NH), −252.7 ppm (CC(O)NH). 29Si
NMR (99.4 MHz, C6D6): δ −1.1 ppm. EI-MS: m/z (%) 479 (10)
2
(δB), 4.76 (δM), and 6.0 (br, δx) (ABMX system, J(A,B) = 16.7 Hz,
3J(A,M) = 5.9 Hz, 3J(B,M) = 4.6 Hz, 3J(M,X) = 8.8 Hz, 4 H;
CHAHBCHMNHX), 3.46 (s, 3 H; OCH3), 7.6 ppm (br s, 1 H;
C(O)NH). 13C NMR (125.8 MHz, C6D6): δ −0.5 (Si(CH3)3), 26.2
(CCH3), 26.9 (SiCH2), 27.9 (C(CH3)3; Boc or tBu), 28.3 (C(CH3)3;
Boc or tBu), 36.7 (CH2CH), 51.7 (CH2CH), 52.0 (OCH3), 59.1
(CCH3), 79.8 (C(CH3)3; Boc or tBu), 80.9 (C(CH3)3; Boc or tBu),
155.8 (OC(O)NH), 170.1 (CC(O)NH), 171.7 (C(O)OC(CH3)3),
175.3 ppm (C(O)OCH3). 15N NMR (50.7 MHz, C6D6): δ −293.6
(OC(O)NH), −255.5 ppm (CC(O)NH). 29Si NMR (99.4 MHz,
C6D6): δ −1.1 ppm. EI-MS: m/z (%) 445 (3) [M+ − CH3], 57 (100)
+
[M+ − CH3], 91 (100) [C7H7 ]. Anal. Calcd for C24H38N2O7Si: C,
58.28; H, 7.74; N, 5.66. Found: C, 58.14; H, 7.70; N, 5.70.
Synthesis of N-tert-Butoxycarbonyl-4-benzyloxy-(S)-aspart-
1-yl-(S)-2-[(trimethylsilyl)methyl]alanine Methyl Ester ((S,S)-
22). (S)-21 (1.13 g, 3.49 mmol), 18 (1.58 g, 3.82 mmol), and N-
methylmorpholine (970 mg, 9.59 mmol) were added one after another
in single portions at 20 °C to a stirred suspension of (S)-15·HCl
(720 mg, 3.19 mmol) in dichloromethane (40 mL). The resulting
mixture was stirred at this temperature for 30 min and then extracted
with water (2 × 20 mL). The combined aqueous solutions were
extracted with dichloromethane (3 × 20 mL) and discarded, the
solvent of the combined organic solutions was removed under reduced
pressure, and the remaining residue was dissolved in methanol (1 mL)
and then further purified by RP-MPLC (eluent, methanol/water (75/
25 v/v); flow rate, 18 mL min−1; detector wavelength, 225 nm) to
furnish (S,S)-22 (1.55 g, 3.13 mmol; 98% yield) as a colorless oily
+
[C(CH3)3 ]. Anal. Calcd for C21H40N2O7Si: C, 54.76; H, 8.75; N,
6.08. Found: C, 54.76; H, 8.40; N, 5.93.
Synthesis of N-tert-Butoxycarbonyl-4-tert-butoxy-(S)-aspart-
1-yl-(R)-2-[(trimethylsilyl)methyl]alanine Ethyl Ester ((S,R)-20).
(S)-17 (1.81 g, 6.26 mmol), 18 (2.83 g, 6.84 mmol), and N-
methylmorpholine (1.74 g, 17.2 mmol) were added one after another
in single portions at 20 °C to a stirred suspension of (R)-16·HCl
(1.35 g, 5.63 mmol) in dichloromethane (150 mL). The resulting
mixture was stirred at this temperature for 11 h and then extracted
with water (3 × 50 mL). The combined aqueous solutions were
extracted with dichloromethane (3 × 50 mL) and discarded, the
solvent of the combined organic solutions was removed under reduced
pressure, and the solid residue was then crystallized from methanol/
water (73/27 v/v, 190 mL; slow cooling of a boiling solution to 20 °C
and crystallization over a period of 1 day) to furnish (S,R)-20 (2.62 g,
5.52 mmol; 98% yield) as a colorless crystalline solid. Mp: 242 °C dec.
1H NMR (500.1 MHz, CD2Cl2): δ 0.03 (s, 9 H; Si(CH3)3), 1.21 (δA)
1
liquid. H NMR (500.1 MHz, C62D6): δ 0.16 (s, 9 H; Si(CH3)3), 1.30
(δA) and 1.96 (δB) (AB system, J(A,B) = 14.7 Hz, 2 H; SiCHAHB),
1.56 (s, 9 H; C(CH3)3), 1.84 (s, 3 H; CCH3), 2.52 (δA), 3.02 (δB),
4.78 (δM), and 5.9 (br, δx) (ABMX system, 2J(A,B) = 17.1 Hz, 3J(A,M)
= 6.0 Hz, 3J(B,M) = 4.7 Hz, 3J(M,X) = 9.3 Hz, 4 H;
CHAHBCHMNHX), 3.44 (s, 3 H; OCH3), 4.99 (δA), and 5.01 (δB)
2
and 1.46 (δB) (AB system, J(A,B) = 14.6 Hz, 2 H; SiCHAHB), 1.24
(δX), 4.10 (δA), and 4.12 (δB) (ABX3 system, 2J(A,B) = 10.9 Hz,
3J(A,X) = 7.1 Hz, 3J(B,X) = 7.2 Hz, 5 H; OCHAHBC(HX)3), 1.437 (s,
9 H; C(CH3)3; Boc or tBu), 1.440 (s, 9 H; C(CH3)3; Boc or tBu), 1.53
(s, 3 H; CCH3), 2.55 (δA), 2.73 (δB), 4.37 (δM), and 5.6 (br, δX)
2
(AB system, J(A,B) = 12.3 Hz, 2 H; OCHAHB), 7.13−7.17 (m, 1 H;
p-CH, C6H5), 7.18−7.21 (m, 2 H; m-CH, C6H5), 7.22−7.26 (m, 2 H;
o-CH, C6H5), 7.6 ppm (br s, 1 H; C(O)NH). 13C NMR (125.8 MHz,
C6D6): δ −0.5 (Si(CH3)3), 26.2 (CCH3), 27.0 (SiCH2), 28.3
(C(CH3)3), 35.4 (CH2CH), 51.5 (CH2CH), 52.0 (OCH3), 59.1
(CCH3), 66.5 (OCH2), 80.0 (C(CH3)3), 128.2 (p-C, C6H5), 128.4 (o-
C, C6H5), 128.6 (m-C, C6H5), 136.2 (i-C, C6H5), 155.8 (OC(O)NH),
170.0 (CC(O)NH), 171.9 (C(O)OCH2C6H5), 175.3 ppm (C(O)-
OCH3). 15N NMR (50.7 MHz, C6D6): δ −293.7 (OC(O)NH),
−255.3 ppm (CC(O)NH). 29Si NMR (99.4 MHz, C6D6): δ −1.1
2
3
3
(ABMX system, J(A,B) = 16.9 Hz, J(A,M) = 8.0 Hz, J(B,M) = 5.1
Hz, 3J(M,X) = 9.9 Hz, 4 H; OCHAHBCHMNHX), 7.0 ppm (br s, 1 H;
C(O)NH). 13C NMR (125.8 MHz, CD2Cl2): δ 0.0 (Si(CH3)3), 14.5
(OCH2CH3), 25.8 (CCH3), 28.1 (SiCH2), 28.4 (C(CH3)3; Boc or
tBu), 28.7 (C(CH3)3; Boc or tBu), 38.2 (CH2CH), 51.4 (CH2CH),
59.3 (CCH3), 61.9 (OCH2CH3), 80.5 (C(CH3)3; Boc or tBu), 82.1
(C(CH3)3; Boc or tBu), 155.9 (OC(O)NH), 170.4 (CC(O)NH),
171.9 (C(O)OC(CH3)3), 175.0 ppm (C(O)OCH2CH3). 15N NMR
(50.7 MHz, CD2Cl2): δ −294.3 (OC(O)NH), −251.6 ppm (CC(O)-
NH). 29Si NMR (99.4 MHz, CD2Cl2): δ 0.8 ppm. EI-MS: m/z (%)
+
ppm. EI-MS: m/z (%): 479 (10) [M+ − CH3], 91 (100) [C7H7 ].
Anal. Calcd for C24H38N2O7Si: C, 58.28; H, 7.74; N, 5.66. Found: C,
58.00; H, 7.78; N, 5.61.
+
459 (4) [M+ − CH3], 57 (100) [C(CH3)3 ]. Anal. Calcd for
Synthesis of 4-Benzyloxy-(S)-aspart-1-yl-(R)-2-
[(trimethylsilyl)methyl]alanine Methyl Ester ((S,R)-23). A
mixture of chlorotrimethylsilane (543 mg, 5.00 mmol), phenol (1.41
g, 15.0 mmol), and dichloromethane (5 mL) was added at 20 °C in a
single portion to (S,R)-22 (534 mg, 1.08 mmol), and the resulting
mixture was stirred at this temperature for 20 min. Subsequently,
dichloromethane (30 mL) and an aqueous sodium hydroxide solution
(1 M, 10 mL) were added. The organic phase was separated and
washed with an aqueous sodium hydroxide solution (1 M, 3 × 10 mL).
The combined aqueous layers were extracted with dichloromethane
(3 × 20 mL), and the combined organic extracts were washed
sequentially with water (10 mL) and a saturated aqueous sodium
chloride solution (2 × 10 mL). The organic solution was dried over
C22H42N2O7Si: C, 55.67; H, 8.92; N, 5.90. Found: C, 55.65; H, 8.79;
N, 6.13.
N-tert-Butoxycarbonyl-4-benzyloxy-(S)-aspartic Acid ((S)-
21). This compound was commercially available (Sigma-Aldrich).
Synthesis of N-tert-Butoxycarbonyl-4-benzyloxy-(S)-aspart-
1-yl-(R)-2-[(trimethylsilyl)methyl]alanine Methyl Ester ((S,R)-
22). (S)-21 (2.77 g, 8.57 mmol), 18 (3.87 g, 9.35 mmol), and N-
methylmorpholine (2.37 g, 23.4 mmol) were added one after another
in single portions at 20 °C to a stirred suspension of (R)-15·HCl
(1.76 g, 7.79 mmol) in dichloromethane (60 mL). The resulting
mixture was stirred at this temperature for 30 min and then extracted
with water (2 × 30 mL). The combined aqueous solutions were
5913
dx.doi.org/10.1021/om300442e | Organometallics 2012, 31, 5903−5917

Organometallics
Article
anhydrous sodium sulfate, and the solvent was removed under reduced
pressure. The remaining residue was dissolved in methanol (1.2 mL)
and then further purified by RP-MPLC (eluent, methanol/water (7/3
v/v); flow rate, 16 mL min−1; detector wavelength, 210 nm) to furnish
(S)-3,3,3-Trifluoro-2-methoxy-2-phenylpropanonyl Chloride
((S)-28). This compound was commercially available (Sigma-Aldrich;
enantiomeric excess, >99% ee).
Synthesis of N-[(R)-3,3,3-Trifluoro-2-methoxy-2-phenylpro-
panonyl]-(R)-2-[(trimethylsilyl)methyl]alanine Methyl Ester
((R,R)-29) and N-[(R)-3,3,3-trifluoro-2-methoxy-2-phenylpropa-
nonyl]-(S)-2-[(trimethylsilyl)methyl]alanine Methyl Ester ((R,S)-
29). Propylene oxide (124 mg, 2.13 mmol) and (S)-28 (134 mg, 530
μmol) were added one after another in single portions to a stirred
suspension of rac-15·HCl (120 mg, 531 μmol) in tetrahydrofuran
(5 mL) at 20 °C, and the resulting mixture was heated under reflux for
2 h. Subsequently, the volatile components were removed under
reduced pressure, and the residue was dissolved in diethyl ether (30
mL). The resulting solution was washed successively with a saturated
aqueous sodium carbonate solution (3 × 15 mL), hydrochloric acid
(2 M, 3 × 15 mL), and a saturated aqueous sodium chloride solution
(2 × 10 mL). The organic phase was dried over anhydrous sodium
sulfate, the solvent was removed under reduced pressure, and the
residue was dissolved in methanol (800 μL) and then further purified by
RP-MPLC (eluent, methanol/water (4/1 v/v); flow rate, 19 mL min−1;
detector wavelength, 210 nm) to furnish a 1/1 mixture of the two
diastereomers (R,R)-29 and (R,S)-29 (181 mg, 446 μmol; 84% yield) as
a colorless liquid. 1H NMR (500.1 MHz, C6D6; data for two
diastereomers (molar ratio, 1/1); the resonance signals of (R,S)-29
are marked with asterisks):28 δ −0.01*/0.07 (s, 9 H; Si(CH3)3), 0.92*/
1.02 (δA) and 1.35/1.38* (δB) (AB system, 2J(A,B) = 14.6*/14.6 Hz; 2
1
(S,R)-23 (297 mg, 753 μmol; 70% yield) as a colorless liquid. H
NMR (400.1 MHz, CD2Cl2): δ 0.02 (s, 9 H; Si(CH3)3), 1.24 (δA) and
2
1.48 (δB) (AB system, J(A,B) = 14.6 Hz, 2 H; SiCHAHB), 1.54 (s, 3
H; CCH3), 1.8 (br s, 2 H; NH2), 2.59 (δA), 2.94 (δB), and 3.63 (δX)
(ABX system, 2J(A,B) = 16.8 Hz, 3J(A,X) = 8.4 Hz, 3J(B,X) = 3.9 Hz, 3
H; CHAHBCHX), 3.66 (s, 3 H; OCH3), 5.13 (δA) and 5.14 (δB) (AB
system, 2J(A,B) = 12.4 Hz, 2 H; OCHAHB), 7.29−7.39 (m, 5 H;
C6H5), 7.9 ppm (br s, 1 H; C(O)NH). 13C NMR (125.8 MHz,
CD2Cl2): δ −0.3 (Si(CH3)3), 25.7 (CCH3), 27.8 (SiCH2), 39.7
(CH2CH), 52.4 (CH2CH), 52.5 (OCH3), 58.5 (CCH3), 66.8 (OCH2),
128.5 (o-C, C6H5), 128.6 (p-C, C6H5), 128.9 (m-C, C6H5), 136.3 (i-C,
C6H5), 172.1 (C(O)OCH2C6H5), 172.3 (CC(O)NH), 175.4 ppm
(C(O)OCH3). 15N NMR (50.7 MHz, CD2Cl2): δ −352.6 (NH2),
−253.0 ppm (C(O)NH). 29Si NMR (99.4 MHz, CD2Cl2): δ −0.8
+
ppm. EI-MS: m/z (%) 379 (21) [M+ − CH3], 91 (100) [C7H7 ].
HRMS (ESI): m/z calcd for [M + Na]+, 417.1816; found, 417.1815.
Synthesis of 4-Benzyloxy-(S)-aspart-1-yl-(S)-2-
[(trimethylsilyl)methyl]alanine Methyl Ester ((S,S)-23). A
mixture of chlorotrimethylsilane (737 mg, 6.78 mmol), phenol (1.98
g, 21.0 mmol), and dichloromethane (10 mL) was added at 20 °C in a
single portion to (S,S)-22 (747 mg, 1.51 mmol), and the resulting
mixture was stirred at this temperature for 20 min. Subsequently,
dichloromethane (60 mL) and an aqueous sodium hydroxide solution
(1 M, 60 mL) were added. The organic phase was separated and
washed with an aqueous sodium hydroxide solution (1 M, 3 × 20 mL).
The combined aqueous layers were extracted with dichloromethane
(3 × 30 mL), and the combined organic extracts were washed
sequentially with water (10 mL) and a saturated sodium chloride
solution (2 × 10 mL). The organic solution was dried over anhydrous
sodium sulfate, and the solvent was removed under reduced pressure.
The remaining residue was dissolved in methanol (1 mL) and then
further purified by RP-MPLC (eluent, methanol/water (7/3 v/v); flow
rate, 15 mL min−1; detector wavelength, 220 nm) to furnish (S,R)-23
(537 mg, 1.36 mmol; 90% yield) as a colorless liquid. 1H NMR (500.1
MHz, CD2Cl2): δ 0.01 (s, 9 H; Si(CH3)3), 1.22 (δA) and 1.59 (δB)
(AB system, 2J(A,B) = 14.7 Hz, 2 H; SiCHAHB), 1.56 (s, 3 H; CCH3),
1.8 (br s, 2 H; NH2), 2.62 (δA), 2.92 (δB), and 3.61 (δX) (ABX system,
2J(A,B) = 16.7 Hz, 3J(A,X) = 8.4 Hz, 3J(B,X) = 3.8 Hz, 3 H;
5
H; SiCHAHB), 1.78*/1.80 (s, 3 H; CCH3), 3.27/3.49* (q, J(H,F) =
1.5/1.8 Hz*, 3 H; COCH3), 3.45/3.47* (s, 3 H; C(O)OCH3), 7.1*/7.4
(br s, 1 H; NH), 7.14−7.19 (m, 1 H; p-CH, C6H5), 7.20−7.26 (m, 2 H;
m-CH, C6H5), 7.86−7.90 ppm (m, 2 H; o-CH, C6H5). 13C NMR (125.8
MHz, C6D6; data for two diastereomers (molar ratio, 1/1); the
resonance signals of (R,S)-29 are marked with asterisks):28 δ −0.6*/−
0.5 (Si(CH3)3), 24.8*/25.1 (CCH3), 28.0*/28.4 (SiCH2), 51.9/52.0*
(C(O)OCH3), 54.7/55.1* (q, 4J(C,F) = 1.9/1.9* Hz; COCH3),
58.88*/58.91 (CCH3), 84.3*/84.4 (q, 2J(C,F) = 26.0/26.0* Hz;
1
CCF3), 124.5*/124.7 (q, J(C,F) = 290.0/290.0* Hz; CCF3), 127.9*/
128.5 (q, 4J(C,F) = 1.5/1.5* Hz; o-CH, C6H5), 128.57*/128.58 (m-CH,
C6H5), 129.47*/129.50 (p-CH, C6H5), 133.1/133.7* (i-C, C6H5),
165.0/165.2* (C(O)NH), 174.1/174.3* ppm (C(O)OCH3). 15N NMR
(50.7 MHz, C6D6; data for two diastereomers (molar ratio, 1/1); the
resonance signal of (R,S)-29 is marked with an asterisk):28 δ −252.5*/−
252.1 ppm; 19F NMR (376.5 MHz, C6D6; data for two diastereomers
(molar ratio, (1/1); the resonance signal of (R,S)-29 is marked with an
asterisk):28 δ −68.27*/−68.32 ppm. 29Si NMR (99.4 MHz, C6D6; data
for two diastereomers (molar ratio, 1/1); the resonance signal of (R,S)-
29 is marked with an asterisk):28 δ −1.23*/−1.20 ppm. GC/EI-MS
(data for two diastereomers (molar ratio, 1:1); the signal of (R,S)-29
is marked with an asterisk):29 tR = 10.6*/10.7 min; m/z (%) 390 (40)
[M+ − CH3], 346 (100) [M+ − C(O)OCH3]. Anal. Calcd for
C18H26F3NO4Si: C, 53.32; H, 6.46; N, 3.45. Found: C, 53.25; H, 6.62;
N, 3.54.
CHAHBCHX), 3.67 (s, 3 H; OCH3), 5.127 (δA) and 5.134 (δB) (AB
system, 2J(A,B) = 14.4 Hz, 2 H; OCHAHB), 7.30−7.39 (m, 5 H;
C6H5), 8.0 ppm (br s, 1 H; C(O)NH). 13C NMR (125.8 MHz, C6D6):
δ −0.5 (Si(CH3)3), 26.4 (CCH3), 27.0 (SiCH2), 39.2 (CH2CH), 51.9
(OCH3), 52.6 (CH2CH), 58.6 (CCH3), 66.2 (OCH2), 128.2 (p-C,
C6H5), 128.5 (o-C, C6H5), 128.7 (m-C, C6H5), 136.5 (i-C, C6H5),
171.7 (C(O)OCH2C6H5), 172.0 (CC(O)NH), 175.4 ppm (C(O)-
OCH3). 15N NMR (50.7 MHz, CD2Cl2): δ −352.6 (NH2), −253.9
ppm (C(O)NH). 29Si NMR (99.4 MHz, C6D6): δ −1.1 ppm. EI-MS:
Determination of the Enantiomeric Excess of (R)-4 and (S)-4
by NMR Spectroscopy. The enantiomeric excesses of (R)-4 and (S)-
+
m/z (%): 379 (1) [M+ − CH3], 91 (100) [C7H7 ]. HRMS (ESI): m/z
1
4 were determined by H, 13C, 15N, 19F, and 29Si NMR experiments
calcd for [M + Na]+, 417.1816; found, 417.1814.
2,2-Dimethylpropanal (24). This compound was commercially
available (Sigma-Aldrich).
after Mosher-type derivatization13 (rac-4 → rac-15·HCl → (R,R)-29/
(R,S)-29; (R)-4 → (R)-15·HCl → (R,R)-29; (S)-4 → (S)-15·HCl →
(R,S)-29) (see section above). The NMR spectra were recorded at
23 °C on a Bruker Avance 400 (19F, 376.5 MHz) or Bruker Avance
500 NMR spectrometer (1H, 500.1 MHz; 13C, 125.8 MHz; 15N, 50.7
MHz; 29Si, 99.4 MHz). The composition of the samples used for the
NMR experiments was as follows: sample substance (29), 18 mg;
C6D6, 750 μL.
Synthesis of N-(2,2-Dimethylpropylidene)-4-benzyloxy-(S)-
aspart-1-yl-(R)-2-[(trimethylsilyl)methyl]alanine Methyl Ester
((S,R)-25). See synthesis of (S,R)-7.
Synthesis of N-(2,2-Dimethylpropylidene)-4-benzyloxy-(S)-
aspart-1-yl-(S)-2-[(trimethylsilyl)methyl]alanine Methyl Ester
((S,S)-25). See synthesis of (S,S)-7.
Determination of the Enantiomeric Excess of (R)-4 and (S)-4
by Gas Chromatography. The enantiomeric excesses of (R)-4 and
(S)-4 were determined by capillary gas chromatography after Mosher-
type derivatization13 (see section below). The experimental conditions
were as follows: gas chromatograph, Varian 450-GC; column, Varian
FactorFour VF-5 ms capillary column (length, 30 m; internal diameter,
0.25 mm; film thickness, 0.25 μm); carrier gas, helium; constant
column flow, 1 mL min−1; temperature program, 80 °C (2 min) to 280
Synthesis of N-(3,3-Dimethylbutylidene)-4-benzyloxy-(S)-
aspart-1-yl-(R)-2-[(trimethylsilyl)methyl]alanine Methyl Ester
((S,R)-26). See synthesis of (S,R)-8.
Synthesis of N-(3,3-Dimethylbutylidene)-4-benzyloxy-(S)-
aspart-1-yl-(S)-2-[(trimethylsilyl)methyl]alanine Methyl Ester
((S,S)-26). See synthesis of (S,S)-8.
3,3-Dimethylbutanal (27). This compound was commercially
available (Sigma-Aldrich).
5914
dx.doi.org/10.1021/om300442e | Organometallics 2012, 31, 5903−5917

Organometallics
Article
°C (13 min) with 20 °C min−1; injector temperature, 220 °C; split,
1/100; detector, Varian 320-MS SQ (EI-MS, 70 eV); reconstructed
ion current (RIC), mass range m/z 200−410. The retention times of
the derivatives of (R)-4 ((R,R)-29) and (S)-4 ((R,S)-29) were as
follows: (R,R)-29, 10.7 min; (R,S)-29, 10.6 min.
The software used for evaluation of the data was QuanLynx (Waters).
As standards for the concentration estimations, samples with the same
degree of dilution were prepared, but with acetonitrile instead of the
buffer.
(c). log D Values. The log D values were obtained by determining
the capacity factor k′. Thus, an Agilent 1100 HPLC system was used to
inject samples onto a reversed-phase LC column (Waters XTerra C18,
3.5 μm, 100 × 2.1 mm). The outlet from the LC column was
connected to a Micromass LC-TOF mass spectrometer (Waters)
equipped with an electrospray interface. The pumps were programmed
to deliver the following gradient at a flow rate of 300 μL min−1
(mobile phase A, 5% acetonitrile/95% aqueous ammonium acetate
solution (10 mM, pH 7.4); mobile phase B, 95% acetonitrile/5%
aqueous ammonium acetate solution (10 mM, pH 7.4)): 100% A
(2 min), linear gradient from 100% A to 100% B (2−17 min), 100% B
Derivatization of (R)-4 and (S)-4 for the Determination of
the Enantiomeric Excess by Gas Chromatography. A 1.25 M
solution of hydrogen chloride in methanol (4 mL, 5.00 mmol of HCl)
was added at 20 °C to 4 (1.00 mg, 5.70 μmol), and the resulting
solution was heated under reflux for 15 min. The solvent and excess
hydrogen chloride were removed under reduced pressure to furnish
15·HCl (crude product, not further purified). 15·HCl was dissolved in
tetrahydrofuran (3 mL), followed by sequential addition of propylene
oxide (1.32 mg, 22.7 μmol) and (S)-28 (1.44 mg, 5.70 μmol) at 20 °C.
The resulting mixture was heated under reflux for 30 min, and the
volatile components were removed under reduced pressure. The
residue was dissolved in diethyl ether (700 μL), the resulting solution
was extracted with water (300 μL), and a 2 μL sample of the organic
phase was injected into the gas chromatograph.
́
(3 min). Standardized k′ data were obtained by calibrating against a set
́
of compounds with precisely determined k′ values (warfarin,
testosterone, metoprolol, propranolol, felodipine). The log D values
in octanol/water are also known for these compounds, which allowed
mapping of the results onto a log D scale.
Crystal Structure Analyses. A 1 M solution of sodium hydroxide
(48 μL, 48 μmol of NaOH) was added at 20 °C to a solution of (S,R)-
5·HCl (16.4 mg, 48 μmol) in methanol (500 μL), and the resulting
mixture was kept undisturbed at 10 °C for 35 days to give suitable
single crystals of (S,R)-5·2H2O. Suitable single crystals of (S,R)-22
were obtained by undisturbed storage of the oily product obtained by
the synthesis of (S,R)-22 as described above. Suitable single crystals of
the other compounds were obtained by crystallization from water at
20 °C ((S,R)-5·TFA)], from methanol/water (83/17 v/v) at 10 °C ((S,R)-
7·H2O), from methanol/water (88/12 v/v) at 10 °C ((S,R)-8·2.5H2O),
from dichloromethane/n-hexane (1/1 v/v) at 10 °C ((R)-15·HCl),
from methanol/water (58/42 v/v) at 20 °C ((S,R)-19·0.22H2O),
from methanol/water (68/32 v/v) at 20 °C ((S,R)-20), and from n-
hexane at 20 °C (cooling of a boiling solution to 20 °C; (S,S)-22).
The crystals were mounted in inert oil (perfluoropolyalkyl ether,
ABCR) on a glass fiber and then transferred to the cold nitrogen gas
stream of the diffractometer ((S,R)-5·2H2O, (R)-15·HCl, and (S,R)-
22, Stoe IPDS, graphite-monochromated Mo Kα radiation
(λ = 0.710 73 Å); (S,R)-5·TFA, (S,R)-7·H2O, (S,R)-8·2.5H2O,
(S,R)-19·0.22H2O, (S,R)-20, and (S,S)-22, Bruker Nonius KAPPA
APEX II, Montel mirror, MoKα radiation (λ = 0.710 73 Å)). The
structures were solved by direct methods.30 All non-hydrogen atoms
were refined anisotropically.30 A riding model was employed in the
refinement of the CH hydrogen atoms. CCDC-877521 ((S,R)-5·2H2O),
CCDC-877522 ((S,R)-5·TFA), CCDC-877523 ((S,R)-7·H2O), CCDC-
877524 ((S,R)-8·2.5H2O), CCDC-877525 ((R)-15·HCl), CCDC-
877526 ((S,R)-19·0.22H2O), CCDC-877527 ((S,R)-20), CCDC-877528
((S,R)-22), and CCDC-877529 ((S,S)-22) contains supplementary
crystallographic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via www.
ccdc.cam.ac.uk/data_request/cif.
Determination of Physicochemical Properties. (a). Materials.
Hank's balanced saline solution (HBSS) and 2-[4-(2-hydroxyethyl)-
piperazino]ethanesulfonic acid (HEPES) were purchased from
GIBCO (Carlsbad, CA). The solvents used for the experiments
were of analytical grade, and the water used was obtained from a water
purification system (Elgastat Maxima, ELGA, Lane End, U.K.). The
HBSS buffer (pH 7.4) was prepared as follows: an aqueous solution of
HEPES (12.5 mL, 1 M) was added to HBSS (500 mL), and the
pH was titrated to 7.4 with an aqueous sodium hydroxide solution
(10 mM).
(b). Solubility in HBSS Buffer (pH 7.4). The liquid handling for the
solubility assay was automated in a 96-well format using a Multiprobe
II HT EX robot (Packard, Meriden, CT). The test compounds (10 μL,
10 mM in DMSO) were diluted in HBSS buffer (990 μL, pH 7.4) and
shaken at room temperature on a flatbed shaker. After 16 h, samples of
400 μL were filtered through a 8 × 12 Whatman GF/B well filter
(assisted by vacuum). The filtered samples were analyzed on an
Agilent 1100 HPLC system (Waldbronn, Germany) with a diode array
detector and coupled to a Micromass LC-TOF mass spectrometer
(Waters, Wythenshawe, U.K.) equipped with an electrospray interface.
Determination of hPPB Values. (a). Materials. The potassium
phosphate buffer (122 μM, pH 7.4) was of analytical grade. The water
used in the experiments was obtained from a water purification system
(Elgastat Maxima).
(b). Assays. Equilibrium dialysis was carried out using a laboratory-
made dialysis plate, made of polyether ether ketone (PEEK),
consisting of two symmetric halves, and a semipermeable membrane
with a molecular cutoff of 6−8 KDa (Spectra/Por, Spectrum
Laboratories Inc., CA). The membrane was first soaked in water for
30 min, before being placed between the dialysis plate halves. A
phosphate buffer (122 μM, 190 mL, pH 7.4) was added into one side
of the chamber, and an equivalent volume of plasma with spiked test
compounds (10 mM of each compound) was added to the other side
of the membrane. The test compounds were dialyzed for 18 h at 37
1 °C in an air incubator on an orbital shaker. A 50 mL volume from
the buffer side (representing the unbound concentration) and an
equivalent sample volume from the plasma side (representing the
total concentration) were transferred from the dialysis cells to a
96-deep-well plate for LC-MS analysis.
Determination of CYP Inhibition. (a). Materials. Nicotinamide
adenine dinucleotide phosphate (NADPH, reduced form, tetrasodium
salt, 98%) and Tris base were purchased from Sigma-Aldrich (St.
Louis, MO). The potassium phosphate buffer (pH 7.4) and
acetonitrile were of analytical grade. The water used in the
experiments was obtained from a water purification system (Elgastat
Maxima). The recombinant human enzymes used were prepared in
house,31 except for CYP2D6, which was purchased from CYPEX
(Dundee, U.K.). The following coumarin substrates, biotransformed
into fluorescent metabolites, were used as probes for each individual
CYP: CYP3A4, 7-benzyloxy-4-(trifluoromethyl)coumarin (BFC;
Gentest, Woburn, MA); CYP2D6, 7-methoxy-4-(aminomethyl)-
coumarin (MAMC; Gentest). A fluorescence plate reader (SpectraMax
GeminiXS, Molecular Devices, Sunnyvale, CA) was used to measure
the levels of metabolites formed.
(b). Assays. A fluorescence-based method according to ref 32 in 96-
well format was used to determine the inhibition of two different CYPs
(3A4 and 2D6). Dilution series of the test compounds were prepared
at eight different concentrations (for CYP3A4, 50.0, 16.7, 5.56, 1.85,
0.617, 0.206, 0.0686, and 0.0229 μM; for CYP2D6, 20.0, 6.67, 2.22,
0.741, 0.247, 0.0823, 0.0274, and 0.00914 μM). For each CYP, a
mixture of the enzyme, the corresponding coumarin substrate,
potassium phosphate buffer (pH 7.4), and water (concentrations
and volumes were CYP dependent) were added to each well in a black
96-well plate. The test compounds at different concentrations were
added. After 10 min of preincubation, the cofactor NADPH was added
to initiate the reaction. After 20−50 min (CYP and substrate
dependent), the reaction was terminated by addition of 0.1 M Tris
base in acetonitrile/water (4/1 v/v). The plates were transferred to the
fluorescence plate reader, where the wavelengths were set individually
for the different coumarin substrates and their respective fluorescent
5915
dx.doi.org/10.1021/om300442e | Organometallics 2012, 31, 5903−5917

Organometallics
Article
metabolites (CYP3A4, 7-hydroxy-4-(trifluoromethyl)coumarin
(HFC); CYP2D6, 7-hydroxy-4-(aminomethyl)coumarin (HAMC)).
The responses were exported to Excel, with which the IC50 curves
(XLfit) were plotted (percent inhibition versus concentration) and
IC50 values calculated for each test compound and enzyme.
Determination of Intrinsic Clearance and Half-Lives in Liver
Microsomes. (a). Materials. Human liver microsomes were prepared
according to ref 33 and stored as granulates at −70 °C. Nicotinamide
adenine dinucleotide phosphate (NADPH, reduced form, tetrasodium
salt, 98%) was purchased from Sigma-Aldrich. The potassium
phosphate buffer (93 mM, pH 7.4) and acetonitrile were of analytical
grade. The water used in the experiments was obtained from a water
purification system (Elgastat Maxima).
and (S,S)-23, and tables and CIF files giving the crystallo-
graphic data for (S,R)-5·2H2O, (S,R)-5·TFA, (S,R)-7·H2O,
(S,R)-8·2.5H2O, (R)-15·HCl, (S,R)-19·0.22H2O, (S,R)-20,
(S,R)-22, and (S,S)-22. This material is available free of charge
via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: r.tacke@uni-wuerzburg.de.
Notes
The authors declare no competing financial interest.
(b). Assays. The metabolic stability in human liver microsomes was
tested by a substrate depletion method.34 The liquid handling was
automated in a 96-well format using a Tecan Genesis Workstation 200
REFERENCES
■
(1) (a) Handmann, V. I.; Merget, M.; Tacke, R. Z. Naturforsch. 2000,
55b, 133. (b) Tacke, R.; Merget, M.; Bertermann, R.; Bernd, M.;
Beckers, T.; Reissmann, T. Organometallics 2000, 19, 3486. (c) Merget,
equipped with a Tecan GenMate 96 pipetting robot (Mannerdorf,
̈
Switzerland). A mixture of microsomes (0.5 mL−1), potassium
phosphate buffer (93 mM, pH 7.4), and the test substrate (1 μM)
was prepared in the 96-well plates and incubated at 37 °C. The reac-
tion was initiated by addition of the cofactor NADPH (1 mM). Aliquots
were withdrawn from the incubation mixture at 0, 3, 7, 15, and 30 min,
and the reaction was terminated by precipitation of the proteins with
3.5 parts of cold acetonitrile. After centrifugation (20 min, 2900g), the
supernatant was diluted with an equal amount of water and then
analyzed by LC-MS. The LC system consisted of an Agilent 1100 pump
and a CTC HTS PAL injector (Zwingen, Switzerland). A Waters
Quattro Ultima triple-quadrupole mass spectrometer (Wythenshawe,
U.K.), equipped with an electrospray ion source, was used for
quantification by selected reaction monitoring. The peaks in the chro-
matograms were integrated by the QuanLynx software, and the peak
areas were exported to Excel, with which log [area] versus time (min)
was plotted (XLfit). The rate of compound disappearance over time was
calculated from the slope of the line (rate constant k). Both clearance
values (CLint) and half-lives (t1/2) were calculated from k.
M.; Gunther, K.; Bernd, M.; Gunther, E.; Tacke, R. J. Organomet.
̈
̈
Chem. 2001, 628, 183. (d) Tacke, R.; Handmann, V. I. Organometallics
2002, 21, 2619. (e) Tacke, R.; Handmann V. I. (inventors), PCT Int.
Pat. Appl. WO 03/082880 A1, October 9, 2003. (f) Tacke, R.; Schmid,
T.; Merget, M. Organometallics 2005, 24, 1780. (g) Falgner, S.;
Schmidt, D.; Bertermann, R.; Burschka, C.; Tacke, R. Organometallics
2009, 28, 2927. (h) Falgner, S.; Burschka, C.; Wagner, S.; Bohm, A.;
̈
Daiss, J. O.; Tacke, R. Organometallics 2009, 28, 6059. (i) Falgner, S.;
Buchner, G.; Tacke, R. J. Organomet. Chem. 2010, 695, 2614.
(2) Weidmann, B. Chimia 1992, 46, 312.
(3) (a) Mortensen, M.; Husmann, R.; Veri, E.; Bolm, C. Chem. Soc.
Rev. 2009, 38, 1002. (b) Qi, Y.; Sieburth, S. McN. In Amino Acids,
Peptides and Proteins in Organic Chemistry: Modified Amino Acids,
Organocatalysis and Enzyme; Hughes, A. B., Ed.; Wiley-VCH:
Weinheim, Germany, 2009; Vol. 2, pp 261−280.
(4) (a) Kamphuis, J.; Lelj, F.; Tancredi, T.; Tonioli, C.; Temussi, P.
A. Quant. Struct.-Act. Relat. 1992, 11, 486. (b) Polinelli, S.; Broxterman,
Q. B.; Schoemaker, H. E.; Boesten, W. H. J.; Crisma, M.; Valle, G.;
Tonioli, C.; Kamphuis, J. Bioorg. Med. Chem. Lett. 1992, 2, 453.
(c) Hooper, N. M.; Hesp, R. J.; Tieku, S.; Boesten, W. H. J.; Toniolo,
C.; Kamphuis, J. J. Agric. Food Chem. 1994, 42, 1397. (d) Benedetti, E.;
Gavuzzo, E.; Santini, A.; Kent, D. R.; Zhu, Y.-F.; Zhu, Q.; Mahr, C.;
Goodman, M. J. Pept. Sci. 1995, 1, 349. (e) Mossel, E.; Formaggio, F.;
Crisma, M.; Toniolo, C.; Broxterman, Q. B.; Boesten, W. H. J.;
Kamphuis, J.; Quaedflieg, P. J. L. M.; Temussi, P. Tetrahedron:
Asymmetry 1997, 8, 1305. (f) Mossel, E.; Formaggio, F.; Valle, G.;
Crisma, M.; Toniolo, C.; Doi, M.; Ishida, T.; Broxterman, Q. B.;
Kamphuis, J. Lett. Pept. Sci. 1998, 5, 223. (g) Goodman, M.; Del Valle,
J. R.; Amino, Y.; Benedetti, E. Pure Appl. Chem. 2002, 74, 1109.
(5) It is worth noting that (R)-4 and (S)-4 are bioisosteric with L-
phenylalanine ((S)-phenylalanine) and D-phenylalanine ((R)-phenyl-
alanine), respectively. Due to changed atom priorities according to the
Cahn−Ingold−Prelog rules, the configuration symbols (R, S) of the
respective bioisosters are different.
Determination of T1R2/R3 Activating Properties. (a). Materi-
als. STC-1 cells (Origin CSH; obtained from Dr. Brian Matlock,
AstraZeneca, Wilmington, DE) were grown in DMEM, glutamax, 4.5 g
L−1 D-glucose, pyruvate (GIBCO) + pen/strep (GIBCO) + 2.5% v/v
FCS (HyClone, Logan, UT) + 10% v/v horse serum (GIBCO).18 The
potassium phosphate buffer (pH 7.4) was of analytical grade. The
water used in the experiments was obtained from a water purification
system (Elgastat Maxima).
(b). Assays. The cells were plated in 96-well poly-D-lysine plates
(Greiner Bio-One, Kremsmunster, Austria) and grown to 90%
̈
confluence. The medium was removed and replaced with assay buffer
(HEPES solution with Ca2+, Mg2+, and 0.1% BSA) with test
compounds at final concentrations of 0.5−2 mM, and it was incubated
for 2 h at 37 °C. Phenylthiocarbamide (PTC) was used at 6 mM,
sucralose at 25 mM, and aspartame and neotame at 2 mM final
concentrations. Supernatants were removed, centrifuged to remove cell
debris, and frozen at −80 °C prior to testing in the Linco GLP-1
(Active) ELISA kit (Millipore, Billerica, MA) according to the Linco
protocol. Plates were read on a Spectramax Gemini XS fluorescence
plate reader with an excitation/emission wavelength of 355 nm/460 nm.
Sensory Evaluation. (S,R)-5·TFA, (S,S)-5·TFA, (S,R)-8, and
(S,S)-8 were taste-checked by two volunteers in blind tests using dilute
aqueous solutions of each test compound. Different concentrations
(between 0.1 and 0.001% (w/v)) of each dipeptide were evaluated at
3−5 min intervals. About 1.5 mL of each solution was applied via
pipettes and allowed to flow over the voluenteer’s tongue for several
seconds. Sweetness intensities were determined relative to aqueous
sucrose solutions of 0.5, 2.0, 4.0, and 8.0% (w/v) as the reference.
(6) Recent reviews dealing with α,α-dialkylated α-amino acids:
(a) Venkatraman, J.; Shankaramma, S. C.; Balaram, P. Chem. Rev.
2001, 101, 3131. (b) Cativiela, C.; Díaz-de-Villegas, M. D.
Tetrahedron: Asymmetry 2007, 18, 569. (c) Tanaka, M. Chem.
Pharm. Bull. 2007, 55, 349. (d) Vogt, H.; Brase, S. Org. Biomol.
̈
Chem. 2007, 5, 406.
(7) Reviews dealing with artificial sweeteners and the theory of sweet
taste: (a) Yamazaki, T.; Benedetti, E.; Kent, D.; Goodman, M. Angew.
Chem. 1994, 106, 1502; Angew. Chem., Int. Ed. Engl. 1994, 33, 1437.
(b) Temussi, P. J. Mol. Recognit. 2006, 19, 188. (c) Meyers, B.; Brewer,
M. J. Food Sci. 2008, 73, R81.
ASSOCIATED CONTENT
(8) Reviews dealing with silicon-based drugs: (a) Bains, W.; Tacke, R.
Curr. Opin. Drug Discovery Dev. 2003, 6, 526. (b) Showell, G. A.; Mills,
J. S. Drug Discovery Today 2003, 8, 551. (c) Mills, J. S.; Showell, G. A.
Expert Opin. Investig. Drugs 2004, 13, 1149. (d) Pooni, P. K.; Showell,
G. A. Mini-Rev. Med. Chem. 2006, 6, 1169. (e) Sieburth, S. McN.;
Chen, C.-A. Eur. J. Org. Chem. 2006, 311. (f) Gately, S.; West, R. Drug
■
S
* Supporting Information
Figures showing the determination of the enantiomeric excess
of (R)-4 and (S)-4, figures giving the 1H and 13C NMR spectra
of rac-15·HCl, (R)-15·HCl, (S)-15·HCl, (R)-16·HCl, (S,R)-23,
5916
dx.doi.org/10.1021/om300442e | Organometallics 2012, 31, 5903−5917

Organometallics
Article
Dev. Res. 2007, 68, 156. (g) Franz, A. K. Curr. Opin. Drug Discovery
Dev. 2007, 10, 654. (h) Meanwell, N. A. J. Med. Chem. 2011, 54, 2529.
(9) Recent publications dealing with silicon-based drugs: (a) Johans-
son, T.; Weidolf, L.; Popp, F.; Tacke, R.; Jurva, U. Drug Metab. Dispos.
2010, 38, 73. (b) Tacke, R.; Nguyen, B.; Burschka, C.; Lippert, W. P.;
Hamacher, A.; Urban, C.; Kassack, M. U. Organometallics 2010, 29,
(R)-4 or (S)-4 ((R)-4 → (R)-15·HCl → (R,R)-29; (S)-4 → (S)-
15·HCl → (R,S)-29).
(29) Assignment of the respective signal was accomplished by
comparison with the gas chromatographic data of the single
diastereomers (R,R)-29 and (R,S)-29, which were synthesized from
(R)-4 or (S)-4 ((R)-4 → (R)-15·HCl → (R,R)-29; (S)-4 → (S)-
15·HCl → (R,S)-29).
1652. (c) Bauer, J. B.; Lippert, W. P.; Dorrich, S.; Tebbe, D.; Burschka,
̈
(30) Sheldrick, G. M. Acta Crystallogr., Sect. A 2008, 64, 112.
C.; Christie, V. B.; Tams, D. M.; Henderson, A. P.; Murray, B. A.;
(31) Masimirembwa, C. M.; Otter, C.; Berg, M.; Jonsson, M.;
̈
Marder, T. B.; Przyborski, S. A.; Tacke, R. ChemMedChem 2011, 6,
Leidvik, B.; Jonsson, E.; Johansson, T.; Backman, A.; Edlund, A.;
̈
1509. (d) Barnes, M. J.; Burschka, C.; Buttner, M. W.; Conroy, R.;
̈
Andersson, T. B. Drug Metab. Dispos. 1999, 27, 1117.
(32) Crespi, C. L.; Miller, V. P.; Penman, B. W. Anal. Biochem. 1997,
248, 188.
Daiss, J. O.; Gray, I. C.; Hendrick, A. G.; Tam, L. H.; Kuehn, D.;
Miller, D. J.; Mills, J. S.; Mitchell, P.; Montana, J. G.; Muniandy, P. A.;
Rapley, H.; Showell, G. A.; Tebbe, D.; Tacke, R.; Warneck, J. B. H.;
Zhu, B. ChemMedChem 2011, 6, 2070. (e) Tacke, R.; Bertermann, R.;
(33) Ernster, L.; Siekevitz, P.; Palade, G. E. J. Cell Biol. 1962, 15, 541.
́
(34) Baranczewski, P.; Stanczak, A.; Sundberg, K.; Svensson, R.;
Wallin, Å.; Jansson, J.; Garberg, P.; Postlind, H. Pharmacol. Rep. 2006,
58, 453.
Burschka, C.; Dorrich, S.; Fischer, M.; Muller, B.; Meyerhans, G.;
̈
̈
Schepmann, D.; Wunsch, B.; Arnason, I.; Bjornsson, R. ChemMed-
̈
Chem 2012, 7, 523.
(10) Review dealing with silicon-based odorants: Tacke, R.; Metz, S.
Chem. Biodiversity 2008, 5, 920.
(11) Recent publications dealing with silicon-based odorants:
(a) Natscher, J. B.; Laskowski, N.; Kraft, P.; Tacke, R. ChemBioChem
̈
2010, 11, 315. (b) Sunderkotter, A.; Lorenzen, S.; Tacke, R.; Kraft, P.
̈
Chem. Eur. J. 2010, 16, 7404. (c) Geyer, M.; Bauer, J.; Burschka, C.;
Kraft, P.; Tacke, R. Eur. J. Inorg. Chem. 2011, 2769.
(12) Kaiser, E., Sr.; Picart, F.; Kubiak, T.; Tam, J. P.; Merrifield, R. B.
J. Org. Chem. 1993, 58, 5167.
(13) (a) Dale, J. A.; Dull, D. L.; Mosher, H. S. J. Org. Chem. 1969, 34,
2543. (b) Bringmann, G.; God, R.; Schaffer, M. Phytochemistry 1996,
̈
43, 1393.
(14) The hydrogen-bonding systems were analyzed by using the
program system PLATON: Spek, A. L. PLATON; University of
Utrecht, Utrecht, The Netherlands, 2008.
(15) Hatada, M.; Jancarik, J.; Graves, B.; Kim, S.-H. J. Am. Chem. Soc.
1985, 107, 4279.
(16) Wink, D. J.; Schroeder, S. A.; Prakash, I.; Lam, K.-C.; Rheingold,
A. L. Acta Crystallogr., Sect. C 1999, C55, 1365.
(17) The molecular structure of (S,S)-11 in Figure 1 was generated
from data given in ref 4b.
(18) Rindi, G.; Grant, S. G. N.; Yiangou, Y.; Ghatei, M. A.; Bloom, S.
R.; Bautch, V. L.; Solcia, E.; Polak, J. M. Am. J. Pathol. 1990, 136, 1349.
(19) Wu, S. V.; Rozengurt, N.; Yang, M.; Young, S. H.; Sinnett-
Smith, J.; Rozengurt, E. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 2392.
(20) Saitoh, O.; Hirano, A.; Nishimura, Y. NeuroReport 2007, 18,
1991.
(21) Li, X.; Staszewski, L.; Xu, H.; Durick, K.; Zoller, M.; Adler, E.
Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 4692.
(22) Reimer, R. A.; Darimont, C.; Gremlich, S.; Nicolas-Met
́
ral, V.;
Ruegg, U. T.; Mace, K. Endocrinology 2001, 142, 4522.
́
̈
(23) (a) Mazur, R. H.; Schlatter, J. M.; Goldkamp, A. H. J. Am. Chem.
Soc. 1969, 91, 2684. (b) Suami, T.; Hough, L. Food Chem. 1993, 46,
235.
(24) (a) Nofre, C.; Tinti, J.-M. (inventors) U.S. Pat. Appl. 5 480 668,
January 2, 1996. (b) Prakash, I.; Bishay, I.; Schroeder, S. Synth.
Commun. 1999, 29, 4461.
(25) (a) Program WIN-DAISY 4.05; Bruker-Franzen, Bremen,
Germany, 1998. (b) Weber, U.; Germanus, A.; Thiele, H. Fresenius J.
Anal. Chem. 1997, 359, 46.
(26) The molar ratio (R)-4·HCl/NaOH or (S)-4·HCl/NaOH takes
into account the amount of the crude product (R)-4·HCl or (S)-4·HCl
obtained in the former step.
(27) The ammonium salt (15·HCl or 16·HCl) was dissolved in a
saturated aqueous sodium carbonate solution (700 μL), the resulting
free base (15 or 16) was extracted with diethyl ether (700 μL), and the
resulting organic solution was then injected into the mass
spectrometer.
(28) Assignment of the respective resonance signals was accom-
plished by comparison with the NMR spectroscopic data of the single
diastereomers (R,R)-29 and (R,S)-29, which were synthesized from
5917
dx.doi.org/10.1021/om300442e | Organometallics 2012, 31, 5903−5917