Peptide-catalyzed conversion of racemic oxazol-5(4 H)-ones into enantiomerically enriched α-amino acid derivatives

Article abstract of DOI:10.1021/jo402828f

We report the development and optimization of a tetrapeptide that catalyzes the methanolytic dynamic kinetic resolution of oxazol-5(4H)-ones (azlactones) with high levels of enantioinduction. Oxazolones possessing benzylic-type substituents were found to perform better than others, providing methyl ester products in 88:12 to 98:2 er. The mechanism of this peptide-catalyzed process was investigated through truncation studies and competition experiments. High-field NOESY analysis was performed to elucidate the solution-phase structure of the peptide, and we present a plausible model for catalysis.

Full text of DOI:10.1021/jo402828f

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Peptide-Catalyzed Conversion of Racemic Oxazol-5(4H)‑ones into
Enantiomerically Enriched α‑Amino Acid Derivatives
Anthony J. Metrano and Scott J. Miller*
Department of Chemistry, Yale University, New Haven, Connecticut 06520-8107, United States
S
* Supporting Information
ABSTRACT: We report the development and optimization
of a tetrapeptide that catalyzes the methanolytic dynamic
kinetic resolution of oxazol-5(4H)-ones (azlactones) with high
levels of enantioinduction. Oxazolones possessing benzylic-
type substituents were found to perform better than others,
providing methyl ester products in 88:12 to 98:2 er. The
mechanism of this peptide-catalyzed process was investigated
through truncation studies and competition experiments.
High-field NOESY analysis was performed to elucidate the
solution-phase structure of the peptide, and we present a plausible model for catalysis.
INTRODUCTION
■
The alcoholytic dynamic kinetic resolution1 (DKR) of oxazol-
5(4H)-ones2 (1) has proven to be an important method for the
preparation of a wide variety of enantiomerically enriched α-
amino acid derivatives (2), including many that are non-
proteinogenic (Scheme 1). This process takes advantage of the
Scheme 1
Figure 1. Several catalyst types that have been employed in the ring-
opening DKR of oxazolones (1).6,7,9−11,12a
of intricate catalytic triads12that have been harnessed to
control this intriguing process. We were drawn to this particular
reaction as an inquiry into whether a purely peptide-based
scaffold might be an effective catalyst for a DKR of this type.
Stimulated by potential analogies between peptides and their
enzymatic counterparts,13 we embarked on an investigation that
has culminated in state-of-the-art levels of selectivity over a
rather broad substrate scope for these intriguing reactions.
We began our study with scaffolds that likely take advantage
of a well-defined, β-hairpin-like secondary structure.14 We,13,15
and others,16 have shown previously that a number of reactions
may be catalyzed with significant selectivity when functional
groups are disposed proximally to one another within such a
framework. Catalyst scaffold 9 (Scheme 2), which contains a β-
dimethylaminoalanine (Dmaa) residue at the N-terminal (i)
anomalously acidic oxazolone α-proton (pKa ≈ 9),3 which
allows for facile epimerization, and therefore rapid equilibration
of enantiomers, through the aromatic oxazole enol intermedi-
ate.4 Design of a chiral catalyst that preferentially reacts with
one enantiomer of 1 over the other establishes a Curtin−
Hammett scenario, wherein the product distribution is
governed by the difference in diastereomeric transition state
energies. This process has been used to explore an array of
asymmetric catalyst types (Figure 1).5 It is noteworthy that
both transition metal complexes6,7 (3 and 4) and low molecular
weight organic scaffolds8−11 (5, 6, and 7), as well as hydrolytic
enzymes12 (e.g., 8), have been demonstrated to be effective
catalysts for the DKR of 1. Of particular interest is the diversity
of reaction mechanismsfrom Lewis acid6 and nucleophilic
catalysis7,8 to bifunctional activation,8−11 through the operation
Received: December 19, 2013
Published: February 12, 2014
© 2014 American Chemical Society
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We next proceeded to optimize the i + 2 position of 9, and in
so doing we ultimately discovered a rather uncommon i + 2
residue that proved to be crucial to catalyst performance.
Replacing Aib with cyclopentyl-substituted Cle provided a
modest increase in conversion and er (entry 4), while both
cyclopropyl-substituted Acpc (entry 5) and the mono-α-
substituted Phe19 (entry 6) were found to be deleterious
relative to Cle. However, catalyst 9f, possessing an indane-
substituted Aic residue at the i + 2 position, provided 2a in 84%
conversion and 93:7 er (entry 7), a significant improvement
over the other residues examined in this position. The reason
for this improvement is not fully understood. However, in order
Scheme 2
position,15a,17 presented the possibility for general base-
mediated catalysis through the action of a side chain tertiary
amine. In cases where the β-hairpin conformation is adopted,
the backbone amides also provide the possibility for nominal
bifunctional catalysis, wherein the amide O or NH may
contribute as hydrogen bond acceptors or donors, respec-
tively.13,15−17 Thus, we evaluated families of catalysts
structurally related to scaffold 9 for their ability to effect the
DKR of oxazolones 1 to provide optically enriched methyl
esters 2.
1
to investigate its possible origins, we compared the H NMR
spectra of peptides 9b, 9c, and Aic-containing 9f (Figure 2).20
The signal corresponding to NHPhe of 9f is shifted downfield by
0.10 ppm relative to that of 9c and by 0.18 ppm relative to that
of 9b. Similarly, the NHAic signal in 9f is downfield-shifted by
0.19 ppm relative to NHCle of 9c and by 0.27 ppm relative to
NHAib of 9c. These rather dramatic downfield shifts suggest
that the Aic-residue of 9f may influence the catalyst
conformation, perhaps through a strengthening of intra-
molecular hydrogen bonds. Additional studies of the solution
conformation of the catalyst are presented below.
RESULTS AND DISCUSSION
■
Optimization of the i + 3 position allowed us to fine-tune the
reactivity and selectivity of the catalyst. Using D-Phe in place of
L-Phe decreased the conversion to 75% and the er to 91:9
(entry 8), while substitution with Gly, having no substitutents,
was even more deleterious (entry 9). Catalyst 9i, with its ethyl-
substituted Abu residue, provided 2a in 94% conversion and
96:4 er (entry 10). Both S-methyl cysteine (entry 11) and O-
benzyl serine (entry 12) in the i + 3 position provided modest
increases in conversion to 97% with no observable effect on the
selectivity. Substitution with Leu provided a nearly 30%
decrease in conversion and a decrease in er to 91:9 (entry
13). While Gly-containing 9h is a competent catalyst, these
results show that an i + 3 side chain in the L-configuration and
of appropriate size is required to achieve the highest levels of
Table 1 summarizes the results of our efforts to optimize a
peptidic catalyst for the methanolytic DKR of 1a (Scheme 2).
In the absence of catalyst, no conversion into methyl ester 2a
was observed under the reaction conditions described in
Scheme 2 (entry 1). We were pleased to discover that catalyst
9a, an archetypal β-hairpin that we had investigated previously
in an unrelated transformation,15a provided 2a with 67:33 er,
albeit with low conversion (entry 2). This promising initial
result spurred a full optimization of scaffold 9. Replacing the C-
terminal methyl ester with a dimethyl amide protecting group
was advantageous, providing 2a in 47% conversion and 76:24 er
(entry 3). This is presumably due to a strengthing of the NHi to
Oi+3 hydrogen bond,18 perhaps providing a more defined
secondary structure.
ad
,
Table 1. Catalyst Optimization
b
c
entry
catalyst
i
i + 1
i + 2
i + 3
conversion (%)
er
1
none
9a
9b
9c
9d
9e
9f
−
−
−
−
0
37
47
55
37
50
84
75
62
94
97
97
70
51
31
95
0
n/a
2
Boc-Dmaa
Boc-Dmaa
Boc-Dmaa
Boc-Dmaa
Boc-Dmaa
Boc-Dmaa
Boc-Dmaa
Boc-Dmaa
Boc-Dmaa
Boc-Dmaa
Boc-Dmaa
Boc-Dmaa
Ac-Dmaa
D-Pro
D-Pro
D-Pro
D-Pro
D-Pro
D-Pro
D-Pro
D-Pro
D-Pro
D-Pro
D-Pro
D-Pro
D-Pro
D-Pro
Hyp(Bn)
D-Pro
Aib
Aib
Cle
Acpc
Phe
Aic
Aic
Aic
Aic
Aic
Aic
Aic
Aic
Aic
Aic
Aic
Phe-OMe
67:33
76:24
79:21
75:25
78:22
93:7
3
Phe-NMe2
4
Phe-NMe2
5
Phe-NMe2
6
Phe-NMe2
7
Phe-NMe2
8
9g
9h
9i
D-Phe-NMe2
Gly-NMe2
91:9
9
89:11
96:4
10
11
12
13
14
15
16
17
Abu-NMe2
9j
Cys(Me)-NMe2
Ser(Bn)-NMe2
Leu-NMe2
95:5
9k
9l
96:4
91:9
9m
9n
9o
9p
Leu-NMe2
87:13
84:16
4:96
Ts-Dmaa
Leu-NMe2
Boc-D-Dmaa
Boc-Leu
D-Ser(Bn)-NMe2
Cys(Me)-NMe2
n/a
a
b
Reported results are the average of two trials. Conversion is defined as the 1H NMR integration of the product alpha signal relative to an internal
c
d
standard in the crude reaction mixture. Enantiomer ratios (S:R) were determined using chiral HPLC. Abbreviations: Boc, tert-butoxycarbonyl;
Dmaa, β-dimethylaminoalanine; Aib, α-aminoisobutyric acid; Cle, cycloleucine (1-aminocyclopentane-1-carboxylic acid); Acpc, 1-amino-
cyclopropane-1-carboxylic acid; Aic, 2-aminoindane-2-carboxylic acid; Abu, 2-aminobutyric acid (ethylglycine); Hyp, trans-4-hydroxyproline; Ac,
acetyl; Ts, para-toluenesulfonyl (tosyl).
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Figure 2. A comparison of the 1H NMR spectra of peptides 9b, 9c, and 9f (500 MHz, 0.02 M in CDCl3). Compared to the other two peptides, the
NHPhe and NHi+2 signals of 9f are significantly downfield-shifted. This behavior is consistent with a stronger intramolecular NHi+3 to Oi hydrogen
bond in the Aic-substituted peptide, which may correlate to a more rigid secondary structure. The NHDmaa is also downfield-shifted in 9f, which is
consistent with a tighter NHDmaa to OPhe hydrogen bond.
a b
,
selectivity and conversion. In an attempt to further rigidify the
secondary structure of 9, catalysts 9m and 9n were synthesized
possessing acetyl and tosyl N-terminal protecting groups,
respectively. We expected these groups to acidify the amide
NH of the i position, providing a stronger NHi to Oi+3
intramolecular hydrogen bond. However, both peptides
performed worse than the analogous 9l (entries 14 and
1521). Finally, O-benzyl-Hyp was substituted for Pro at the i + 1
position with no noticeable effect on reactivity or selectivity
(entry 16). The C-isostere of 9j, with Leu at the i position,
provided no conversion into 2a, suggesting that the tertiary
amine moiety of Dmaa was required for catalysis (entry 17).
Having identified peptide 9k as an effective catalyst for the
DKR of 1a, we proceeded to optimize the reaction conditions.
The selectivity of the 9k-catalyzed DKR of 1a remained fairly
constant over a survey of 10 solvents, only fluctuating between
91:9 and 96:4 (Table 2, entries 1−10). Aromatic solvents
(entries 4−7) provided the best results overall, while
chlorinated solvents (entries 1−3) delivered comparable
selectivities with significantly reduced conversion. Ethereal
solvents (entries 8−10) provided the lowest conversions (10−
44%), but the selectivity remained high (91:9 to 96:4 er). This
was interesting given the ability of these ethereal solvents,
especially THF, to compete for hydrogen bonds. Even
methanol (entry 11), a highly competitive hydrogen-bonding
solvent, furnished 2a with some selectivity (64:36 er).
Table 2. Effect of Solvent on Reactivity and Conversion
c
d
e
entry
solvent
CH2Cl2
conversion (%)
er
1
59
80
94:6
95:5
96:4
95:5
95:5
96:4
95:5
95:5
91:9
96:4
2
Cl(CH2)2Cl
CHCl3
C6H6
3
76
4
89
5
C6F6
98
6
PhCF3
PhCl
>99
94
7
8
THF
40
9
Et2O
10
10
11
t-BuOMe
MeOH
44
>99
64:36
a
Reaction conditions: oxazolone 1a (1 equiv, 0.05 mmol), peptide 9k
(20 mol %, 0.01 mmol), MeOH (5 equiv, 0.25 mmol), solvent (0.50
b
mL, 0.10 M), 4 °C, 24 h. Reported results are the average of two
c
trials. All solvents were dried over 4 Å molecular sieves 48 h before
d
use. Conversion is defined as the 1H NMR integration of the product
alpha signal relative to an internal standard in the crude reaction
mixture. Enantiomer ratios (S:R) were determined using chiral
e
HPLC.
Table 4 describes our exploration of the substrate scope.
Each substrate was analyzed on a small (0.05−0.07 mmol) and
large (0.30−0.58 mmol) scale. The small-scale reaction was
always quenched in the vicinity of 24 h, and the conversion
measured was used to assess whether the larger scale reaction
should be allowed to stir longer. Enantioselectivities were
consistent between the two scales in every case. We were
pleased to discover that 9k performed well in the methanolytic
DKR of 14 different oxazolones 1a−n.22 Of particular note,
oxazolones possessing benzylic substitution at the 4-position
were found to provide the best results in terms of selectivity
(entries 1−16). It has been noted previously that oxazolones of
this type racemize with an accelerated rate over alkyl-
Additional studies evaluating reaction conditions are
presented in Table 3. Fortuitously, the conditions that we
had already chosen for the catalyst optimization study above
(Scheme 2) proved to be the most suitable. Taken together, the
data presented in Tables 2 and 3 suggest the following: (1) the
enantioselectivity of the 9k-catalyzed reaction is not conversion
dependent, as low-conversion reactions also exhibit high ers;
and (2) the catalytic system is robust, in that it can provide high
ers under a broad range of conditions.
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a
oxazolone 1j, derived from aspartic acid, which also provides
high conversion and er (entries 19−20). In general, alkyl-
substituted oxazolones provided 2 in the lowest conversion and
er (entries 17−18 and 23−28), though the leucine-derived
oxazolone (1k) is a notable exception (entries 21−22). The
structure−activity relationship highlighted with oxazolones 1k−
n may provide insight into the reaction mechanism. While γ-
disubstituted 1k performed well, γ-trisubstituted 1m provided
2m with significantly reduced er and conversion (entries 21 vs
25). Similarly, valine-derived 1l performed better than its β-
trisubstituted analogue (1n), though neither is particularly
impressive in terms of conversion. It appears that β-
disubstitution and β/γ-trisubstitution is not well-tolerated by
9k. It is plausible that destabilizing syn-pentane type
interactions in the tetrahedral intermediate of methanolysis,
or even in the transition state leading to that intermediate, may
account for this observation. However, oxazolone 1h provided
2h in upward of 90:10 er, despite its β-disubstitution (entries
15−16). This may be due to the presumably increased acidity
of the α-proton or to the presence of aryl groups for interaction
with 9k.
Table 3. Optimization of Reaction Conditions
c
loading
(mol %)
conc
temp
(°C)
time conversion
b
d
entry
x
(M)
(h)
(%)
er
1
4
3
2
3
3
3
3
3
2
3
5
3
3
20
20
20
20
20
10
10
10
20
20
20
20
20
0.10
0.10
0.10
0.05
0.20
0.05
0.10
0.20
0.10
0.05
0.10
0.20
0.10
4
4
24
24
24
24
24
24
24
24
24
24
24
24
12
96
96
83
44
>99
27
54
82
64
50
16
18
75
96:4
96:4
96:4
96:4
95:5
95:5
94:6
94:6
92:8
92:8
96:4
95:5
96:4
2
3
4
4
4
5
4
6
4
7
4
8
4
9
21
21
−40
−40
4
10
11
12
13
a
Reaction conditions: oxazolone 1a (1 equiv, 0.05 mmol), peptide 9k
(20 mol %, 0.01 mmol), MeOH (x equiv, 0.25 mmol), PhMe (conc),
temp, time. Concentration with respect to 1a. Conversion is defined
as the H NMR integration of the product alpha signal relative to an
internal standard in the crude reaction mixture. Enantiomer ratios
b
c
1
d
(S:R) were determined using chiral HPLC.
The high selectivity exhibited by catalyst 9k across a wide
range of substrate types stimulated a desire to understand its
mode of action. These issues were probed with an analysis of
additional catalyst analogues, including several that were
truncated to explore the influence of peptide miniaturization
(Table 5). Dmaa monomers 10a and 10b provided very low
substituted oxazolones.4b It is possible that this observation
partially accounts for their increased conversion and selectivity,
though interaction with the catalyst likely plays an important
role, as well. Further evidence for this hypothesis is provided by
a
Table 4. Substrate Scope
a
Substrate scope examined using optimized conditions: peptide 9k (20 mol %), MeOH (3 equiv), PhMe (0.10 M wrt 1), 4 °C, unless otherwise
b
c
1
specified. Conversion is defined as the ratio of H NMR alpha signals (product to total alpha) in the crude reaction mixture. Isolated yield after
flash chromatography. Enantiomer ratios were determined by chiral HPLC using an AD-H column, unless otherwise noted. Enantiomer ratios
were determined by chiral HPLC using an OD-H column. 5 equiv of MeOH were used to promote conversion.
d
e
f
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a b
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,
Table 5. Truncation Study
Scheme 3
c
conversion
(%)
d
entry catalyst
i
i + 1
i + 2
er
1
2
3
4
5
6
10a
10b
10c
10d
10e
10f
Boc-Dmaa-
NMe2
−
−
−
−
−
−
9
10
32
24
32
62
46:54
44:56
54:46
56:44
66:34
86:14
Boc-Dmaa-
e
Pyr
Boc-Dmaa D-Pro-
NMe2
Boc-Dmaa D-Pro-
NHMe
Boc-Dmaa D-Pro
Aic-
NMe2
Boc-Dmaa D-Pro
Aic-
NHMe
a
Reaction conditions: oxazolone 1a (1 equiv, 0.05 mmol), peptide 10
(20 mol %, 0.01 mmol), MeOH (5 equiv, 0.25 mmol), PhMe (0.50
b
mL, 0.10 M), 4 °C, 24 h. Reported results are the average of two
c
trials. Conversion is defined as the 1H NMR integration of the
product alpha signal relative to an internal standard in the crude
d
reaction mixture. Enantiomer ratios (S:R) were determined using
e
chiral HPLC. Pyr, 1-pyrrolidinyl.
conversion of 1a to 2a, but the er was nonracemic in favor of
the opposite enantiomer provided by the tetramers examined
previously (entries 1 and 2). Dimers 10c and 10d more than
doubled the conversion, and interestingly provided equal and
opposite enantioinduction compared with 10a and 10b (entries
3 and 4). To this point, there had been no noticeable effect of
the C-terminal protecting group on reactivity and selectivity
(entries 1−4). Trimer 10e, possessing a dimethyl amide
protecting group, provided 2a in the same conversion as dimer
10c with slightly elevated enantioinduction (entry 5). However,
trimer 10f, possessing a C-terminal N-methyl amide protecting
group, nearly doubled the conversion relative to 10e and
provided 2a in 86:14 er (entry 6). These levels of conversion
and enantioselectivity are comparable to those obtained with
certain tetramers (Table 1). This suggests that the ability to
form an NHi+3 to Oi hydrogen bond is important to
enantioinduction and conversion. These observations also
lend credence to our hypothesized β-hairpin structure and
reinforce the notion that secondary structure is important to
enantioselective catalysis in this system.
In order to gain further insight into the mechanism of action,
we next subjected 9k to a competition experiment (Scheme 3).
When triethylamine (20 mol %) was used to catalyze the DKR
of 1a under the same conditions described in Scheme 2,
racemic 2a was produced in 86% conversion. However, when
9k and triethylamine were used together (20 mol % of each),
the reaction proceeded to quantitative conversion and an 83:17
er was observed in the product. These results suggest that 9k is
able to out-compete triethylamine in the DKR of 1a. When
peptide 9p (20 mol %), lacking the pendant tertiary amine base,
was used with an equimolar amount of triethylamine, racemic
2a was formed in 78% conversion, suggesting that the pendant
tertiary amine of 9k is important for organizing the transition
state in the selective reaction.
Figure 3. (a) Representative NOEs observed in a 0.02 M solution of
9k in C6D6. (b) The NOESY data are consistent with a hydrogen-
bonded, β-hairpin structure.
in Figure 3b. Furthermore, in the 1H NMR spectrum of 9k, the
NHSer and NHDmaa signals are shifted significantly downfield at
7.93 and 6.92 ppm, respectively (in C6D6), consistent with
intramolecular hydrogen bonds involving these protons. While
we cannot be certain that the active catalyst adopts such a
conformation, the data that we have accumulated herein show
that the efficacy of this catalytic system improves as NMR
signatures of β-turn-like character become more pronounced.
Figure 4 presents a plausible model for the interaction
between peptide 9k and the substrate that is able to account for
the absolute stereochemistry of the products. The β-turn
structure of 9k in Figure 4b is adapted from a previously
reported X-ray structure of a related peptide (Figure 4a).23
Given the results of previous theoretical and mechanistic
studies on analogous systems,24 it is possible that 9k acts to
bind and stabilize the zwitterionic, tetrahedral intermediate of
methanolysis.25,26 In a preliminary NMR complexation study,
no change in the chemical shifts of 9k or oxazolone 1a were
observed in a 1:1 solution relative to individual spectra acquired
at the same concentration and temperature. This result is
consistent with the hypothesized model, as it suggests that the
catalyst does not dock with the oxazolone itself to any
appreciable degree. Furthermore, it seems as though a
bifunctional catalyst such as 9k would be well-poised to
accelerate the formation of a zwitterionic intermediate through
backbone hydrogen bonding26,27 and general base catalysis.
Our mechanistic inquiry was extended to include an NMR
study of the solution-phase conformation of 9k. Using high-
field 1H−1H-NOESY experiments, the correlations summarized
in Figure 3a were observed. Of particular interest are the NOEs
observed between βSer and βDmaa, NHSer and NHDmaa, NHAic
and αD‑Pro, and between trans-NMeSer and NHDmaa. These
NOEs are consistent with a β-hairpin-type structure,23 as shown
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The organic layer was separated and subsequently washed with 100
mL each of saturated aqueous NaHCO3, water, and brine. The
organics were dried over anhydrous Na2SO4, filtered, and concentrated
in vacuo to provide a clear, yellow oil (2.23 g, > 99% crude yield). The
identity of Boc-Ser(Bn)-NMe2 was confirmed by UPLC−MS. MS
exact mass calculated for [C17H26N2O4 + H]+ requires m/z = 323.19,
found 323.29 (ESI+).
Deprotection 1. Crude Boc-Ser(Bn)-NMe2 was then treated with
10 mL of 4 M HCl in 1,4-dioxane to cleave the Boc group. The
resulting yellow solution was allowed to stir at rt for 1 h, before HCl
and 1,4-dioxane were removed in vacuo. Residual 1,4-dioxane was
removed by coevaporation with CH2Cl2 to provide 1.75 g (99% crude
yield) of H-Ser(Bn)-NMe2·HCl as a white foam, which was dried
thoroughly under reduced pressure before being carried forward to the
next coupling step.
Peptide Coupling 1. H-Ser(Bn)-NMe2·HCl (1.75 g, 6.77 mmol)
was added to a round-bottom flask, along with Boc-Aic-OH (2.15 g,
7.74 mmol), EDC·HCl (1.62 g, 8.46 mmol), and HOBt·H2O (1.30 g,
8.46 mmol). The solid mixture was dissolved in CH2Cl2 (33.8 mL,
0.20 M), and the resulting solution was allowed to stir at rt as i-Pr2NEt
(2.94 mL, 16.9 mmol) was added slowly. The deep yellow solution
was allowed to stir at rt for 42 h, after which the solution was poured
into a separatory funnel and washed with 100 mL of 10% aqueous (w/
v) citric acid. The organic layer was separated and subsequently
washed with 100 mL each of saturated aqueous NaHCO3, water, and
brine. The organics were dried over anhydrous Na2SO4, filtered, and
concentrated in vacuo to provide an off-white foam (3.13 g, 96% crude
yield). The identity of Boc-Aic-Ser(Bn)-NMe2 was confirmed by
UPLC−MS. MS exact mass calculated for [C27H35N3O5 + H]+
requires m/z = 482.26, found 482.39 (ESI+).
Deprotection 2. Deprotection of the crude dipeptide Boc-Aic-
Ser(Bn)-NMe2 was accomplished in the same manner as described in
Deprotection 1 (vide supra) to provide H-Aic-Ser(Bn)-NMe2·HCl
(2.72 g, 6.50 mmol) as a white foam.
Peptide Coupling 2. The coupling of H-Aic-Ser(Bn)-NMe2·HCl
(2.72 g, 6.50 mmol) to Boc-D-Pro-OH (2.15 g, 9.97 mmol) was
performed using the procedure detailed in Peptide Coupling 1, above.
Boc-D-Pro-Aic-Ser(Bn)-NMe2 was isolated as a white foam (3.39 g,
90% crude yield), the identity of which was confirmed by UPLC−MS.
MS exact mass calculated for [C32H42N4O6 + H]+ requires m/z =
579.31, found 579.52 (ESI+).
Figure 4. (a) A previously reported X-ray crystal structure showing a
D-Pro-Aib-containing β-hairpin structure (ref 23). Side chains were
omitted for clarity. (b) Plausible model that accounts for the absolute
stereochemistry of products 2.
Mechanistically, it is plausible that the tertiary amine moiety of
9k directs methanol to 1 and stabilizes charge as it develops in
the tetrahedral intermediate (Figure 4b). The catalyst interacts
preferentially with (S)-1, which may benefit from a
minimization of destabilizing syn-interactions between the
base-delivered methoxy group and the side chain. Furthermore,
the side-chain of (S)-1 points away from the Dmaa-residue,
leaving the site of catalysis sterically unencumbered. Tertiary
amine-catalyzed proton transfer to the oxazole N, followed by
ring-opening, provides (S)-2 as observed.
CONCLUSION
■
In summary, we have developed a simple tetrapeptide that
catalyzes the enantioselective ring-opening DKR of oxazolones
1 with state-of-the-art levels of selectivity. The substrate scope
addressed by the catalyst is also rather broad. Structural analysis
of the catalyst allows for the derivation of a self-consistent
mechanistic model that accounts for the absolute sense of
asymmetric induction. The culmination of these concepts
allows analogies to be drawn between the low molecular weight
peptide and far more complex enzymes, which seem to operate
through multifunctional mechanisms that often exploit non-
covalent interactions to stabilize charge development in
transition states.26 In this sense, the presently reported system
may mimic an important aspect of enzymatic chemistry.28
Deprotection 3. Deprotection of the crude tripeptide Boc-D-Pro-
Aic-Ser(Bn)-NMe2 was accomplished in the same manner as described
in Deprotection 1 (vide supra) to provide H-D-Pro-Aic-Ser(Bn)-
NMe2·HCl (3.02 g, 5.86 mmol) as a pale yellow foam.
Peptide Coupling 3. H-D-Pro-Aic-Ser(Bn)-NMe2·HCl (3.02 g, 5.86
mmol), Boc-Dmaa-OH (1.70 g, 7.33 mmol), and HBTU (2.78 g, 7.33
mmol) were added to a round-bottom flask. The solid mixture was
dissolved in CH2Cl2 (29.3 mL, 0.20 M), and the resulting solution was
allowed to stir at rt as i-Pr2NEt (2.55 mL, 14.7 mmol) was added
slowly. The yellow solution was allowed to stir at rt for 38 h, after
which the solution was poured into a separatory funnel and washed
with about 100 mL of saturated aqueous NaHCO3. The organic layer
was separated and subsequently washed with 100 mL each of water
and brine. The organics were dried over anhydrous Na2SO4, filtered,
and concentrated in vacuo to provide a deep yellow oil. The crude
product was loaded onto a flash purification system for reverse-phase
column chromatography (120 g column, 30−100% MeOH/H2O over
16 column volumes with 3 column volume pre- and postrun
equilibrations, 45 mL min−1 flow, collection λ = 210 nm, monitored
λ = 245 nm, 16 × 150 mm test tubes with 21 mL fractions). Fractions
were pooled, concentrated in vacuo, and dried by coevaporation thrice
with CH2Cl2 to provide Boc-Dmaa-D-Pro-Aic-Ser(Bn)-NMe2 (9k, 2.18
g, 54% yield) as white foam: 1H NMR (500 MHz, CDCl3) δ 7.42 (d, J
= 8.9 Hz, 1H), 7.35−7.20 (m, 5H), 7.19−7.09 (m, 4H), 6.25 (d, J =
6.0 Hz, 1H), 5.26 (td, J = 8.2, 5.8 Hz, 1H), 4.51 (q, J = 12.0 Hz, 2H),
4.40−4.33 (m, 2H), 3.89 (d, J = 16.8, 1H), 3.84−3.80 (m, 1H), 3.72
(AMX, JAX = 6.2 Hz, JMX = 5.9 Hz, JAM = 16.3 Hz, νAM = 56.1 Hz, 2H),
3.52 (m, 2H), 3.33 (d, J = 16.7, Hz, 1H), 3.12 (d, J = 17.8 Hz, 1H),
3.10 (s, 3H), 2.94 (s, 3H), 2.53 (AMX, JAX = 6.7 Hz, JMX = 7.5 Hz, JAM
EXPERIMENTAL SECTION
■
Solution Phase Synthesis of Peptides 9a−p and 10a−f. The
solution phase synthesis of peptides 9a−p was accomplished using the
N-tert-butoxycarbonyl (Boc) protecting group strategy (see Scheme S1
in the Supporting Information).4c,29 Boc-L-β-Dimethylaminoalanine
(Boc-Dmaa-OH) was synthesized according to a literature proce-
dure.30 All other amino acid residues and coupling reagents were
purchased from commercial suppliers. Yields are not optimized. Once
synthesized, peptides were stored at 0 °C to prevent epimerization and
other adverse side-reactivity. Peptide 9k was characterized fully (vide
infra). Peptides 9a−j, 9l−p, and 10a−f were characterized by HRMS
(see Supporting Information).
Representative Synthesis and Characterization of Peptide
9k. Installation of C-Terminal Protecting Group. Boc-Ser(Bn)-OH
(2.00 g, 6.77 mmol), dimethylamine hydrochloride (690 mg, 8.46
mmol), EDC·HCl (1.62 g, 8.46 mmol), and HOBt·H2O (1.30 g, 8.46
mmol) were added to a round-bottom flask. The solid mixture was
dissolved in CH2Cl2 (33.8 mL, 0.20 M), and the resulting solution was
allowed to stir at rt as i-Pr2NEt (2.94 mL, 16.9 mmol) was added
slowly. The pale yellow solution was allowed to stir at rt overnight.
After 14 h, the solution was poured into a separatory funnel and
washed with approximately 100 mL of 10% aqueous (w/v) citric acid.
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= 12.4 Hz, νAM = 130.7 Hz, 2H), 2.26 (s, 6H), 2.22−2.18 (m, 1H),
2.08−1.99 (m, 2H), 1.91−1.81 (m, 2H), 1.42 (s, 9H); 13C NMR (125
MHz, CDCl3) δ 172.5, 171.5, 171.4, 170.4, 155.9, 141.4, 139.3, 138.3,
128.4, 127.6, 127.5, 127.0, 126.7, 124.8, 124.4, 79.6, 77.4, 77.2, 76.9,
73.3, 70.8, 67.2, 60.8, 60.0, 50.8, 48.5, 47.3, 45.8, 45.5, 42.6, 37.6, 36.0,
28.5, 27.8, 25.0; IR (film, cm−1) 3296, 3243, 3213, 3184, 2929, 2860,
2773, 2250, 1684, 1632, 1522, 1453, 1404, 1366, 1307, 1274, 1165,
1098, 1051, 1025; HRMS exact mass calculated for [C37H52N6O7 +
H]+ requires m/z = 693.3976, found 693.3971 (ESI+); [α]2D0 = +38.8
(c = 1.0, CHCl3).
through a plug of Celite to remove Pd/C, washing with MeOH. The
filtrate was concentrated in vacuo, and residual solvent was removed
by coevaporation with CH2Cl2 to provide a flaky, white solid. 1H NMR
analysis confirmed the structure of the crude product. The white solid
product was triturated thrice with Et2O and dried under reduced
pressure, providing clean Boc-Dmaa-OH (1.90 g, 89% yield) to be
1
used in peptide synthesis: H NMR (500 MHz, CDCl3) δ 12.0 (bs,
1H), 5.78 (d, J = 4.7 Hz, 1H), 4.13 (q, J = 6.5 Hz, 1H), 3.26 (m, 2H),
2.85 (s, 6H), 1.38 (s, 9H); 13C NMR (125 MHz, CDCl3) δ 172.6,
155.7, 79.7, 58.6, 50.9, 44.2, 28.5; MS exact mass calculated for
[C10H20N2O4 + H]+ requires m/z = 233.15, found 233.13 (ESI+).
Synthesis of Oxazolones 1. All oxazolone substrates 1 were
previously known compounds. They were synthesized from
commercially available amino acids in a precedented two-step
sequence. The amino acids were first acylated with benzoyl chloride
N-Terminal Modification in Peptides 9m−n. Deprotection 4.
Following Peptide Coupling 3, Boc-Dmaa-D-Pro-Aib-Leu-NMe2 (9l,
100 mg, 0.16 mmol) was dissolved in 3 mL of 4 M HCl in 1,4-dioxane
solution and allowed to stir at ambient temperature for 1 h, after which
HCl and 1,4-dioxane were removed in vacuo. The resulting salt was
carried forward assuming quantitative yield (96 mg, 0.16 mmol).
Acetylation (9m). H-Dmaa-D-Pro-Aic-Leu-NMe2·2HCl (96 mg,
0.16 mmol) was suspended in 0.80 mL of dry CH2Cl2 and treated with
i-Pr2NEt (0.11 mL, 0.64 mmol). The resulting solution was cooled to
0 °C over an ice bath and allowed to stir vigorously as Ac2O (19 μL,
0.20 mmol) was added slowly. The flask was sealed with a septum and
purged with N2. The reaction solution was allowed to warm to rt over
2 h, after which the solution was poured into a separatory funnel,
diluted to 10 mL with additional CH2Cl2, and washed once with 20
mL of saturated aqueous NaHCO3 and once with 20 mL of H2O. The
combined organics were dried over anhydrous Na2SO4, filtered, and
concentrated in vacuo to provide a clear, colorless oil. The crude
product was loaded onto a flash purification system for reverse-phase
column chromatography using the same method described above in
Peptide Coupling 3. The appropriate fractions were pooled,
concentrated in vacuo, and dried by coevaporation with CH2Cl2 to
provide 9m (54 mg, 0.095 mmol, 59% yield) as a white foam.
Tosylation (9n). H-Dmaa-D-Pro-Aic-Leu-NMe2·2HCl (96 mg, 0.16
mmol) was tosylated following the above acetylation procedure using
TsCl (45 mg, 0.20 mmol) in place of Ac2O. Workup and purification
by the same method provided 9n (81 mg, 0.12 mmol, 74% yield) as an
off-white foam.
Synthesis of Boc-L-β-Dimethylaminoalanine (Boc-Dmaa-
OH). Boc-L-β-Aminoalanine.30 Boc-L-Asn-OH (10.0 g, 43.1 mmol)
was added to a 500 mL round-bottom flask equipped with a large stir
bar. The white solid was suspended in 120 mL of a 2:2:1 (v/v/v)
solution of MeCN, EtOAc, and water and allowed to stir at 0 °C over
an ice bath. PhI(OAc)2 (16.7 g, 51.7 mmol) was added slowly to the
cloudy, white suspension at 0 °C. The flask was sealed with a septum,
and the reaction was allowed to warm to rt over 4 h under an
atmosphere of N2. Within a few minutes, the white suspension became
a clear, pale yellow solution. Within an hour, a white solid precipitate
began to form. After 4 h, the white suspension was cooled to 0 °C over
an ice bath for an additional 30 min to promote precipitation. The
using the Schotten-Baumann conditions.7,31 The resulting N-benzoyl
̈
amino acids were cyclodehydrated to oxazolones 1 with acetic
9a,32
anhydride in refluxing CH2Cl2
or with EDC·HCl at 0 °C in
CH2Cl2.33 Please consult the Supporting Information for additional
synthetic details and characterization.
Synthesis of N-Benzoyl Amino Acids 1a′−n′. General
Procedure A.7,31 Amino acid (1.0 equiv) was added to a round-
bottom flask equipped with a magnetic stir bar and was suspended in 2
M aqueous NaOH (0.20 M solution). The resulting mixture was
allowed to stir vigorously at 0 °C over an ice bath. Benzoyl chloride
(1.05 equiv) was then added dropwise with a syringe. The mixture was
allowed to warm to rt over 1−3 h, after which the reaction was again
cooled to 0 °C and quenched by acidification to pH < 2 with 1 M
aqueous HCl. A white/off-white precipitate formed instantly upon
acidification. The mixture was allowed to stand at 0 °C for 15 min
before the precipitate was collected by vacuum filtration and
subsequently recrystallized from hot EtOH/H2O. The resulting
crystalline solid was collected by vacuum filtration, rinsed with H2O,
and dried on high vacuum for multiple hours to provide N-benzoyl
amino acids 1′.
General Procedure B.7,31 The reaction was performed in the same
manner as in General Procedure A, except an extractive workup was
performed. After acidification with 1 M aqueous HCl to pH < 2, the
organics were extracted thrice with Et2O. The combined organics were
dried over anhydrous Na2SO4, filtered, and concentrated in vacuo to
provide a white/off-white solid, which was subsequently recrystallized
from hot EtOH/H2O. The resulting crystalline solid was collected by
vacuum filtration, rinsed with H2O, and dried on high vacuum for
multiple hours to provide N-benzoyl amino acids 1′.
General Procedure C.7,31 Amino acid (1.0 equiv) was added to a
round-bottom flask equipped with a magnetic stir bar and was
suspended in 2 M aqueous Na2CO3 (0.25 M solution). The resulting
mixture was allowed to stir vigorously at 0 °C over an ice bath.
Benzoyl chloride (1.05 equiv) was then added dropwise with a syringe.
The mixture was allowed to warm to rt overnight, after which the
reaction was again cooled to 0 °C and quenched by acidification to pH
< 2 with 1 M aqueous HCl. No precipitate formed. The aqueous
solution was extracted thrice with CH2Cl2. The combined organic
layer was dried over anhydrous Na2SO4, filtered, and concentrated in
vacuo to provide a white solid, which was dissolved in a minimal
amount of MeOH and loaded onto a reverse-phase flash purification
system (120g C18 column, 0 to 60% MeOH/H2O linear gradient, 50
mL min−1 flow rate). The appropriate fractions were pooled and
concentrated in vacuo, and adventitious water was removed by
coevaporation with CH2Cl2, providing N-benzoyl amino acids 1′.
N-Benzoyl-DL-phenylalanine (1a′). Title compound was synthe-
sized from DL-phenylalanine (10.00 g, 60.5 mmol) according to
General Procedure A above. Yield: 40% (fine, white powder); 1H
NMR (500 MHz, CD3OD) δ 7.77−7.59 (m, 2H), 7.56−7.44 (m, 1H),
7.45−7.37 (m, 2H), 7.30−7.20 (m, 4H), 7.19−7.12 (m 1H), 4.78 (dd,
J = 8.2, 5.0 Hz, 1H), 3.24 (AMX, JAX = 5.0 Hz, JMX = 8.2 Hz, JAM = 13.8
Hz, νAM = 105.7 Hz, 2H); 13C NMR (125 MHz, CD3OD) δ 176.4,
169.6, 139.1, 135.7, 132.6, 130.5, 129.5, 129.3, 128.2, 127.5, 56.7, 38.7;
MS exact mass calculated for [C16H15NO3 + H]+ requires m/z =
270.11, found 270.11 (ESI+).
white solid was collected by vacuum filtration through a Buchner
̈
funnel, washing with 200 mL of EtOAc. The product was allowed to
dry on high vacuum for multiple hours, providing Boc-L-β-amino-
1
alanine (5.58 g, 63% yield) as a white powdery solid: H NMR (500
MHz, CD3OD) δ 4.06 (t, J = 6.5 Hz, 1H), 3.15 (ABX, JAX = 6.8 Hz, JBX
= 6.2 Hz, JAB = 12.4 Hz, νAB = 19.2 Hz, 2H), 1.46 (s, 9H); 13C NMR
(125 MHz, CDCl3):δ 174.7, 158.0, 80.9, 53.9, 43.1, 28.7; MS exact
mass calculated for [C8H16N2O4 + H]+ requires m/z = 205.12, found
205.13 (ESI+).
Boc-L-β-Dimethylaminoalanine. Boc-L-β-Aminoalanine (1.88 g,
9.19 mmol) was added to a round-bottom flask containing a magnetic
stir bar. Palladium (10% on activated charcoal, 188 mg, 10 mass%) was
added to the flask, and the solid mixture was carefully treated with
formaldehyde (37% aq, 1.71 mL, 23.0 mmol). The flask was filled with
N2, and methanol (40 mL) was added slowly to the slurry. The
resulting black suspension was allowed to stir vigorously. The flask was
sealed with a septum, evacuated, and filled with H2 from a balloon.
Five rounds of filling and evacuating were performed. After the final
evacuation, the flask was again filled with H2, and the reaction was
allowed to stir 48 h at rt under positive H2 pressure (from a balloon).
After 48 h, the heterogeneous reaction mixture was vacuum filtered
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N-Benzoyl-D-β-(2-furyl)alanine (1b′). Title compound was synthe-
sized from D-β-(2-furyl)alanine (750 mg, 4.83 mmol) according to
General Procedure B above with the following modifications: (1) the
reaction concentration was 0.24 M wrt the amino acid. Yield: 42%
2H), 7.48−7.42 (m, 1H), 7.42−7.37 (m, 4H), 7.37−7.31 (m, 2H),
7.30−7.23 (m, 4H), 7.22−7.11 (m, 2H), 5.53 (d, J = 11.4 Hz, 1H),
4.61 (d, J = 11.3 Hz, 1H); 13C NMR (125 MHz, CD3OD) δ 174.3,
170.2, 142.3, 142.3, 135.4, 132.6, 129.5, 129.5, 129.5, 129.4, 129.4,
128.3, 128.0, 127.8, 57.5, 54.6; MS exact mass calculated for
[C22H19NO3 + H]+ requires m/z = 346.14, found 346.15 (ESI+).
N-Benzoyl-L-homophenylalanine (1i′). Title compound was
synthesized from L-homophenylalanine (896 mg, 5.00 mmol)
according to General Procedure A with the following modifications:
(1) the reaction concentration was 0.50 M wrt the amino acid. Yield:
1
(fluffy white solid); H NMR (500 MHz, CD3OD) δ 7.81−7.69 (m,
2H), 7.56−7.49 (m, 1H), 7.47−7.41 (m, 2H), 7.38 (d, J = 1.8 Hz,
1H), 6.30 (dd, J = 3.0, 1.9 Hz, 1H), 6.17 (d, J = 3.1 Hz, 1H), 4.97−
4.74 (m, 1H), 3.28 (AMX, JAX = 4.8 Hz, JMX = 9.3 Hz, JAM = 15.5 Hz,
νAM = 59.7 Hz, 2H); 13C NMR (125 MHz, CD3OD) δ 174.4, 170.1,
152.6, 143.1, 135.2, 132.8, 129.5, 128.3, 111.3, 108.4, 53.4, 30.7; MS
exact mass calculated for [C14H13NO4 + H]+ requires m/z = 260.09,
found 260.10 (ESI+).
1
59% (white, granular solid); H NMR (500 MHz, CD3OD) δ 7.87−
7.82 (m, 2H), 7.58−7.51 (m, 1H), 7.50−7.44 (m, 2H), 7.36−7.19 (m,
4H), 7.20−7.10 (m, 1H), 4.58 (dd, J = 9.6, 4.7 Hz, 1H), 2.86−2.69
(m, 2H), 2.32−2.23 (m, 1H), 2.21−2.11 (m, 1H); 13C NMR (125
MHz, CD3OD) δ 175.5, 170.5, 142.3, 135.4, 132.8, 129.6, 129.5, 128.5,
127.1, 117.3, 53.9, 34.3, 33.4; MS exact mass calculated for
[C17H17NO3 + H]+ requires m/z = 284.13, found 284.13 (ESI+).
N-Benzoyl-L-aspartic acid β-methyl ester (1j′). Title compound
was synthesized from L-aspartic acid β-methyl ester (1.03 g, 6.97
mmol) was synthesized using General Procedure C. Yield: 16% (white,
N-Benzoyl-L-β-(2-thienyl)alanine (1c′). Title compound was
synthesized from L-β-(2-thienyl)alanine (1.00 g, 65.84 mmol)
according to General Procedure B above with the following
modifications: (1) 1.1 equiv of benzoyl chloride were used; (2) the
reaction concentration was 0.97 M wrt the amino acid; (3) the
reaction was allowed to run overnight (12−14 h). Yield: 62% (flaky
1
white solid); H NMR (500 MHz, CD3OD) δ 7.82−7.75 (m, 2H),
7.56−7.50 (m, 1H), 7.49−7.40 (m, 2H), 7.22 (dd J = 4.6, 1.8 Hz, 1H),
6.96−6.88 (m, 2H), 4.89−4.76 (m, 1H), 3.48 (AMX, JAX = 4.4 Hz, JMX
= 9.1 Hz, JAM = 15.0 Hz, νAM = 79.7 Hz, 2H); 13C NMR (125 MHz,
CD3OD) δ 174.2, 170.0, 140.4, 135.4, 132.8, 129.5, 128.4, 127.8,
127.4, 125.4, 55.7, 32.3; MS exact mass calculated for [C14H13NO3S +
H]+ requires m/z = 276.07, found 276.06 (ESI+).
1
flaky solid); H NMR (500 MHz, CD3OD) δ 7.86−7.79 (m, 2H),
7.61−7.51 (m, 1H), 7.50−7.43 (m, 2H), 4.98 (dd, J = 7.5, 5.5 Hz,
1H), 3.70 (s, 3H), 2.98 (AMX, JAX = 7.0 Hz, JMX = 9.3 Hz, JAM = 20.5
Hz, νAM = 57.8 Hz, 2H); 13C NMR (125 MHz, CD3OD) δ 172.8,
170.0, 135.1, 132.9, 129.6, 128.4, 52.4, 52.4, 36.9; MS exact mass
calculated for [C12H13NO5 + H]+ requires m/z = 252.09, found 252.17
(ESI+).
N-Benzoyl-L-β-(1-naphthyl)alanine (1d′). Title compound was
synthesized from L-β-(1-naphthyl)alanine (1.00 g. 4.65 mmol)
according to General Procedure B above with the following
modifications: (1) 1.1 equiv of benzoyl chloride were used; (2) the
reaction concentration was 0.93 M wrt the amino acid; (3) the
reaction was allowed to run overnight (12−14 h). Yield: 67% (white
solid); 1H NMR (500 MHz, CD3OD) δ 8.23 (dd, J = 8.5, 1.1 Hz, 1H),
7.89−7.84 (m, 1H), 7.76 (d, J = 8.1, 1H), 7.69−7.64 (m, 2H), 7.55
(ddd, J = 8.5, 6.8, 1.4 Hz, 1H), 7.52−7.45 (m, 2H), 7.46−7.29 (m,
4H), 5.01 (dd, J = 9.7, 4.7 Hz, 1H), 3.71 (AMX, JAX = 4.7 Hz, JMX = 9.7
Hz, JAM = 14.3 Hz, νAM = 196.3 Hz, 2H); 13C NMR (125 MHz,
CD3OD) δ 175.3, 170.7, 135.9, 135.7, 135.2, 133.8, 133.1, 130.3,
129.8, 129.1, 129.0, 128.8, 127.7, 127.1, 126.7, 124.9, 55.5, 35.9; MS
exact mass calculated for [C20H17NO3 + H]+ requires m/z = 320.13,
found 320.13 (ESI+).
N-Benzoyl-O-tert-butyl-L-tyrosine (1f′). Title compound was
synthesized from O-tert-butyl-L-tyrosine (712 mg, 3.00 mmol)
according to General Procedure A above with the following
modifications: (1) the reaction concentration was 0.41 M wrt the
amino acid; (2) after 1 h, the aqueous solution was acidified with 1 M
HCl until pH 4; (3) recrystallized from EtOH/H2O at 0 °C overnight.
Yield: 68% (colorless needle-like crystals); 1H NMR (500 MHz,
CD3OD) δ 7.72−7.68 (m, 2H), 7.53−7.48 (m, 1H), 7.44−7.39 (m,
2H), 7.21−7.17 (m, 2H), 6.93−6.87 (m, 2H), 4.88−4.81 (m, 1H),
3.20 (AMX, JAX = 5.1 Hz, JMX = 9.7 Hz, JAM = 13.9 Hz, νAM = 118.8 Hz,
2H), 1.28 (s, 9H); 13C NMR (125 MHz, CD3OD) δ 174.8, 170.2,
155.3, 135.4, 133.9, 132.7, 130.8, 129.5, 128.3, 125.2, 79.5, 55.6, 37.6,
29.1; MS exact mass calculated for [C21H25NO4 + H]+ requires m/z =
342.17 Found 342.17 (ESI+).
N-Benzoyl-DL-leucine (1k′). Title compound was synthesized from
DL-leucine (5.00 g, 38.1 mmol) according to General Procedure B with
the following modifications: (1) 0.95 equiv of benzoyl chloride was
used; (2) the reaction concentration was 1.66 M wrt the amino acid;
(3) EtOAc was used to extract the aqueous layer after acidification.
1
Yield: 61% (white powder); H NMR (400 MHz, CD3OD) δ 7.88−
7.80 (m, 2H), 7.57−7.50 (m, 1H), 7.49−7.41 (m, 2H), 4.67 (dd, J =
10.4, 4.2 Hz, 1H), 1.91−1.63 (m, 3H), 0.99 (dd, J = 8.4, 5.7 Hz, 6H);
13C NMR (125 MHz, CD3OD) δ 176.2, 170.5, 135.4, 132.8, 129.5,
128.5, 52.6, 41.4, 26.3, 23.4, 21.8; MS exact mass calculated for
[C13H17NO3 + H]+ requires m/z = 236.13, found 236.13 (ESI+).
N-Benzoyl-DL-valine (1l′). Title compound was synthesized from
DL-valine (5.00 g, 42.7 mmol) using General Procedure A with the
following modifications: (1) the product was recrystallized from hot
1
H2O instead of EtOH/H2O. Yield: 72% (white, granular solid); H
NMR (400 MHz, CD3OD) δ 7.87−7.81 (m, 2H), 7.58−7.51 (m, 1H),
7.50−7.42 (m, 2H), 4.51 (d, J = 6.3 Hz, 1H), 2.29 (o, J = 6.7 Hz, 1H),
1.05 (d, J = 6.8 Hz, 6H); 13C NMR (125 MHz, CD3OD) δ 174.9,
170.7, 135.5, 132.8, 129.5, 128.5, 59.8, 31.8, 19.7, 18.9; MS exact mass
calculated for [C12H15NO3 + H]+ requires m/z = 222.11, found 222.11
(ESI+).
N-Benzoyl-L-β-tert-butylalanine (1m′). Title compound was
synthesized from L-β-tert-butylalanine (500 mg, 3.44 mmol) according
to General Procedure B with the following modifications: (1) 1.1 equiv
of benzoyl chloride was used; (2) the reaction concentration was 1.0
1
M wrt the amino acid. Yield: 63% (white, flaky solid); H NMR (500
MHz, CD3OD) δ 7.87−7.81 (m, 2H), 7.57−7.48 (m, 1H), 7.49−7.42
N-Benzoyl-L-(4-nitro)phenylalanine (1g′). Title compound was
synthesized from L-(4-nitro)phenylalanine (1.00 g, 4.76 mmol)
according to General Procedure A with the following modifications:
(1) the reaction concentration was 0.24 M wrt the amino acid. Yield:
(m, 2H), 4.73 (dd, J = 9.4, 2.8 Hz, 1H), 1.87 (ABX, JAX = 2.5 Hz, JBX
=
9.8 Hz, JAB = 14.5 Hz, νAB = 30.7 Hz, 2H), 1.02 (s, 9H); 13C NMR
(125 MHz, CD3OD) δ 176.5, 170.1, 135.5, 132.7, 129.5, 128.4, 51.7,
45.7, 31.6, 30.0; MS exact mass calculated for [C14H19NO3 + H]+
requires m/z = 250.14, found 250.14 (ESI+).
1
68% (white, flaky solid); H NMR (400 MHz, CD3OD) δ 8.18−8.12
(m, 2H), 7.75−7.70 (m, 2H), 7.56−7.48 (m, 3H), 7.46−7.39 (m, 2H),
4.95 (dd, J = 9.8, 5.0 Hz, 1H), 3.37 (AMX, JAX = 5.0 Hz, JMX = 9.9 Hz,
JAM = 14.0 Hz, νAM = 93.9 Hz, 2H); 13C NMR (100 MHz, CD3OD) δ
174.1, 170.2, 148.3, 147.0, 135.1, 132.9, 131.4, 129.5, 128.3, 124.4,
55.0, 38.0; MS exact mass calculated for [C16H14N2O5 + H]+ requires
m/z = 315.10, found 315.10 (ESI+).
N-Benzoyl-DL-tert-leucine (1n′). Title compound was synthesized
from DL-tert-leucine (5.00 g, 38.1 mmol) according to General
Procedure B with the following modifications: (1) the reaction
1
concentration was 1.59 M. Yield: 49% (white, flaky solid); H NMR
(500 MHz, CD3OD) δ 7.84−7.79 (m, 2H), 7.57−7.52 (m, 1H), 7.50−
7.42 (m, 2H), 4.56 (s, 1H), 1.01 (s, 9H); 13C NMR (125 MHz,
CD3OD) δ 174.2, 170.5, 135.6, 132.8, 129.6, 128.5, 62.4, 35.3, 27.3;
MS exact mass calculated for [C13H17NO3 + H]+ requires m/z =
236.13, found 236.13 (ESI+).
N-Benzoyl-L-β,β-diphenylalanine (1h′). Title compound was
synthesized from L-β,β-diphenylalanine (1.00 g, 4.14 mmol) according
to General Procedure A with the following modifications: (1) the
reaction concentration was 0.41 M wrt the amino acid. Yield: 60%
Synthesis of Oxazolones 1a−n. General Procedure A.9a,32 N-
Benzoyl amino acid 1′ (1.0 equiv) was added to a round-bottom flask
1
(fluffy, white solid); H NMR (500 MHz, CD3OD) δ 7.55−7.51 (m,
1549
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equipped with a magnetic stir bar and subsequently suspended in dry
CH2Cl2 (0.10 M solution). The suspension was allowed to stir
vigorously at rt as Ac2O (2.0 equiv wrt 1′) was added slowly with a
syringe. A reflux condenser was affixed to the flask, and the suspension
was stirred at reflux overnight under an atmosphere of N2. After 12−
14 h, the suspension had often become a clear, colorless or pale yellow
solution. The solution was allowed to cool to rt before solvent was
removed in vacuo. Acetic acid byproduct was removed by
coevaporation three times each with PhMe and CH2Cl2. The resulting
crude product was loaded onto a silica column in CH2Cl2 and purified
by flash chromatography using a suitable EtOAc/hexanes gradient (as
determined by TLC). The appropriate fractions were pooled,
concentrated under reduced pressure, and dried on high vacuum to
afford oxazolone 1.
(1d′, 600 mg, 1.88 mmol) according to General Procedure A above.
Yield: 76% (yellow oily solid); TLC Rf = 0.44 (20% EtOAc/hexanes,
1
UV); H NMR (500 MHz, CDCl3) δ 8.22−8.14 (m, 2H), 7.93−7.89
(m, 2H), 7.86 (dd, J = 8.3, 1.5 Hz, 1H), 7.79 (d, J = 8.1 Hz, 1H),
7.61−7.48 (m, 4H), 7.47−7.40 (m, 3H), 4.83 (dd, J = 7.9, 5.1 Hz,
1H), 3.70 (AMX, JAX = 5.1 Hz, JMX = 7.9 Hz, JAM = 14.5 Hz, νAM
=
156.5 Hz, 2H); 13C NMR (125 MHz, CDCl3) δ 177.8, 161.8, 134.0,
132.8, 132.0, 132.0, 128.9, 128.8, 128.2, 128.1, 128.0, 126.2, 125.9,
125.8, 125.5, 123.9, 66.3, 34.8; MS exact mass calculated for
[C20H15NO2 + H]+ requires m/z = 302.12, found 302.12 (ESI+).
4-((1H-3-Indolyl)methyl)-2-phenyloxazol-5(4H)-one (1e). Title
compound was synthesized from N-benzoyl-L-tryptophan (1e′, 1.00
g, 3.24 mmol) according to General Procedure A above. Yield: 66%
(off-white solid); TLC Rf = 0.20 (20% EtOAc/hexanes, UV); 1H
NMR (500 MHz, CDCl3) δ 8.19 (s, 1H), 8.05−8.00 (m, 2H), 7.89−
7.81 (m, 1H), 7.68−7.60 (m, 1H), 7.58−7.51 (m, 2H), 7.45−7.37 (m,
1H), 7.30−7.24 (m, 2H), 7.22 (d, J = 2.5 Hz, 1H), 4.89 (dd, J = 6.3,
4.9 Hz, 1H), 3.60 (AMX, JAX = 4.8 Hz, JMX = 6.2 Hz, JAM = 14.8 Hz,
νAM = 61.7 Hz, 2H); 13C NMR (125 MHz, CDCl3) δ 178.2, 161.9,
136.1, 132.7, 128.8, 128.0, 127.5, 126.0, 123.5, 122.2, 119.7, 119.3,
111.1, 109.8, 66.7, 27.4; MS exact mass calculated for [C18H14N2O2 +
H]+ requires m/z = 291.11, found 291.12 (ESI+).
General Procedure B.33 N-Benzoyl amino acid 1′ (1.0 equiv) was
added to a round-bottom flask equipped with a magnetic stir bar and
subsequently suspended in dry CH2Cl2 (0.10 M solution wrt 1′). The
suspension was cooled to 0 °C over an ice bath as EDC·HCl (1.1
equiv wrt 1′) was added portionwise. The flask was sealed, and the
reaction mixture was allowed to warm to rt overnight under an
atmosphere of N2. After 12−14 h, the suspension had often become a
clear, colorless or pale yellow solution. The solution was diluted to
double the initial volume with CH2Cl2 and poured into a separatory
funnel containing an approximately equal volume of H2O. The layers
were separated, and the organic layer was washed sequentially with
saturated aqueous NaHCO3 and H2O (equal volume of each relative
to the organics). The combined organics were dried over anhydrous
Na2SO4, filtered, and concentrated in vacuo to provide a white solid.
The crude product was loaded onto a silica column in CH2Cl2 and
purified by flash chromatography using a suitable EtOAc/hexanes
gradient (as determined by TLC). The appropriate fractions were
pooled, concentrated under reduced pressure, and dried on high
vacuum to afford oxazolone 1.
4-(4-(tert-Butoxy)benzyl)-2-phenyloxazol-5(4H)-one (1f). Title
compound was synthesized from N-benzoyl-O-tert-butyl-L-tyrosine
(1f′, 600 mg, 1.76 mmol) according to General Procedure A above.
Yield: 97% (clear, pale yellow oil); TLC Rf = 0.42 (20% EtOAc/
1
hexanes, UV); H NMR (500 MHz, CDCl3) δ 7.91−7.85 (m, 2H),
7.55−7.48 (m, 1H), 7.45−7.38 (m, 2H), 7.18−7.10 (m, 2H), 6.88−
7.81 (m, 2H), 4.66 (t, J = 5.5 Hz, 1H), 3.25 (AMX, JAX = 4.9 Hz, JMX
=
6.1 Hz, JAM = 14.0 Hz, νAM = 79.8 Hz, 2H); 13C NMR (125 MHz,
CDCl3) δ 177.8, 161.7, 154.6, 132.7, 130.2, 130.0, 128.8, 127.9, 125.9,
124.1, 78.5, 66.7, 36.8, 28.9; MS exact mass calculated for [C20H21NO3
+ H]+ requires m/z = 324.16, found 324.16 (ESI+).
4-Benzyl-2-phenyloxazol-5(4H)-one (1a). Title compound was
synthesized from N-benzoyl-DL-phenylalanine (1a′, 6.567 g, 24.4
mmol) according to General Procedure A above. Yield: 89% (white
4-(4-Nitrobenzyl)-2-phenyloxazol-5(4H)-one (1g). Title com-
pound was synthesized from N-benzoyl-L-(4-nitro)phenylalanine
(1g′, 600 mg, 1.91 mmol) according to General Procedure A above.
Yield: 78% (off-white solid); TLC Rf = 0.24 (20% EtOAc/hexanes,
UV); H NMR (500 MHz, CDCl3) δ 8.20−8.08 (m, 2H), 7.96−7.85
(m 2H), 7.62−7.54 (m, 1H), 7.53−7.42 (m, 4H), 4.73 (dd, J = 6.9, 5.0
1
solid); TLC Rf = 0.48 (20% EtOAc/hexanes, UV); H NMR (500
1
MHz, CDCl3) δ 7.95−7.86 (m, 2H), 7.51−7.47 (m, 1H), 7.45−7.38
(m, 2H), 7.29−7.22 (m, 4H), 7.22−7.15 (m, 1H), 4.66 (dd, J = 6.8,
5.0 Hz, 1H), 3.26 (AMX, JAX = 5.0 Hz, JMX = 6.8 Hz, JAM = 14.0 Hz,
νAM = 92.6 Hz, 2H); 13C NMR (125 MHz, CDCl3) δ 177.5, 161.7,
135.4, 132.7, 129.6, 128.7, 128.4, 127.9, 127.2, 125.8, 66.5, 37.3; MS
exact mass calculated for [C16H13NO2 + H]+ requires m/z = 252.10,
found 252.11 (ESI+).
Hz, 1H), 3.38 (AMX, JAX = 5.0 Hz, JMX = 7.0 Hz, JAM = 14.0 Hz, νAM
=
101.1 Hz, 2H); 13C NMR (125 MHz, CDCl3) δ 177.0, 162.4, 147.5,
143.1, 133.3, 130.7, 129.0, 128.1, 125.5, 123.8, 66.0, 37.0; MS exact
mass calculated for [C16H12N2O4 + H]+ requires m/z = 297.09, found
297.09 (ESI+).
4-(2-Furylmethyl)-2-phenyloxazol-5(4H)-one (1b). Title com-
pound was synthesized from N-benzoyl-D-β-(2-furyl)alanine (1b′,
520 mg, 2.01 mmol) according to General Procedure A above. Yield:
88% (pale yellow solid); TLC Rf = 0.41 (20% EtOAc/hexanes, UV);
1H NMR (500 MHz, CDCl3) δ 7.99−7.93 (m, 2H), 7.60−7.53 (m,
1H), 7.50−7.42 (m, 2H), 7.29 (dd, J = 1.9. 0.8 Hz, 1H), 6.26 (dd, J =
3.2, 1.9 Hz, 1H), 6.17 (dd, J = 3.2, 0.9 Hz, 1H), 4.70 (dd, J = 6.3, 5.5
4-Benzhydryl-2-phenyloxazol-5(4H)-one (1h). Title compound
was synthesized from N-benzoyl-L-diphenylalanine (1h′) according
to General Procedure B above. Yield: undetermined (off-white
powder); TLC Rf = 0.50 (20% EtOAc/hexanes, UV); 1H NMR
(500 MHz, CDCl3) δ 8.02−7.96 (m, 2H), 7.75−7.69 (m, 2H), 7.65−
7.56 (m, 1H), 7.57−7.47 (m, 2H), 7.49−4.71 (m, 2H), 7.40−7.30 (m,
3H), 7.30−7.22 (m, 2H), 7.25−7.16 (m, 1H), 5.24 (d, J = 3.2 Hz,
1H), 4.88 (d, J = 3.2 Hz, 1H); 13C NMR (125 MHz, CDCl3) δ 177.5,
161.8, 141.1, 138.3, 132.8, 129.5, 128.8, 128.7, 128.3, 128.0, 127.4,
127.1, 125.9, 69.5, 52.7; MS exact mass calculated for [C22H17NO2 +
H]+ requires m/z = 328.13, found 328.14 (ESI+).
Hz, 1H), 3.35 (AMX, JAX = 5.4 Hz, JMX = 6.4 Hz, JAM = 15.3 Hz, νAM
=
58.1 Hz, 2H); 13C NMR (125 MHz, CDCl3) δ 177.5, 162.5, 149.8,
142.3, 133.1, 129.0, 128.22, 126.0, 110.7, 108.2, 64.7, 30.3; MS exact
mass calculated for [C14H11NO3 + H]+ requires m/z = 242.08, found
242.08 (ESI+).
4-Phenethyl-2-phenyloxazol-5(4H)-one (1i). Title compound was
synthesized from N-benzoyl-L-homophenyl-alanine (1i′, 800 mg, 2.80
mmol) according to General Procedure A above. Yield: 76% (clear,
2-Phenyl-4-(2-thiophenylmethyl)oxazol-j5(4H)-one (1c). Title
compound was synthesized from N-benzoyl-L-β-(2-thienyl)alanine
(1c′, 600 mg, 2.18 mmol) according to General Procedure A above.
Yield: 61% (pale yellow oil); TLC Rf = 0.43 (20% EtOAc/hexanes,
1
colorless oil); TLC Rf = 0.44 (20% EtOAc/hexanes, UV); H NMR
(400 MHz, CDCl3) δ 8.11−7.93 (m, 2H), 7.69−7.54 (m, 1H), 7.54−
7.44 (m, 2H), 7.40−7.07 (m, 5H), 4.40 (dd, J = 7.6, 5.8 Hz, 1H), 2.88
(t, J = 7.8 Hz, 2H), 2.47−2.33 (m, 1H), 2.24−2.11 (m, 1H); 13C NMR
(125 MHz, CDCl3) δ 178.4, 161.9, 140.3, 132.9, 128.9, 128.7, 128.7,
128.0, 126.5, 126.0, 64.6, 33.3, 31.6; MS exact mass calculated for
[C17H15NO2 + H]+ requires m/z = 266.12, found 266.12 (ESI+).
Methyl 2-(5-oxo-2-phenyl-(4H)oxazolyl)acetate (1j). Title com-
pound was synthesized from N-benzoyl-L-aspartic acid β-methyl ester
(1j′, 278 mg, 1.11 mmol) according to General Procedure A above.
Yield: 63% (clear, colorless oil); TLC Rf = 0.19 (20% EtOAc/hexanes,
UV); 1H NMR (500 MHz, CDCl3) δ 7.99 (d, J = 7.5 Hz, 2H), 7.56 (t,
1
UV); H NMR (400 MHz, CDCl3) δ 8.00−7.92 (m, 2H), 7.62−7.52
(m, 1H), 7.52−7.41 (m, 2H), 7.12 (dd, J = 5.1, 1.3 Hz, 1H), 6.97−
6.90 (m, 1H), 6.90 (dd, J = 5.1, 3.5 Hz, 1H), 4.70 (dd, J = 6.0, 4.7 Hz,
1H), 3.54 (AMX, JAX = 4.7 Hz, JMX = 6.0 Hz, JAM = 15.0 Hz, νAM = 44.2
Hz, 2H); 13C NMR (125 MHz, CDCl3) δ 177.0, 162.3, 136.5, 132.8,
128.8, 128.0, 127.1, 126.8, 125.7, 125.2, 66.2, 31.3; MS exact mass
calculated for [C14H11NO2S + H]+ requires m/z = 258.06, found
258.06 (ESI+).
4-(1-Naphthalenylmethyl)-2-phenyloxazol-5(4H)-one (1d). Title
compound was synthesized from N-benzoyl-L-β-(1-naphthyl)alanine
1550
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J = 7.4 Hz, 1H), 7.47 (t, J = 7.7 Hz, 2H), 4.61 (t, J = 5.0 Hz, 1H), 3.66
equiv) was added to the solution. The PTFE-lined vial cap was
replaced and tightened, and the solution was allowed to stir at 4 °C.
The reaction was quenched after 22−24 h by concentration in vacuo
to remove excess MeOH and solvent. Conversion was determined by
1H NMR as described above. The crude reaction mixture was then
purified by filtration through a pipet silica plug (1 × 7 cm, 50%
EtOAc/hexanes). The fractions containing both starting material and
product were pooled together and concentrated in vacuo to provide a
clear, colorless oil. Analytical HPLC analysis was performed to assess
the er of both the starting material and product.
(s, 3H), 3.08 (AMX, JAX = 4.5 Hz, JMX = 5.6 Hz, JAM = 17.3 Hz, νAM
=
47.0 Hz, 2H); 13C NMR (125 MHz, CDCl3) δ 177.6, 169.8, 163.1,
133.0, 128.9, 128.1, 125.8, 61.8, 52.4, 35.1; MS exact mass calculated
for [C12H11NO4 + H]+ requires m/z = 234.08, found 234.08 (ESI+).
4-Isobutyl-2-phenyloxazol-5(4H)-one (1k). Title compound was
synthesized from N-benzoyl-DL-leucine (1k′, 1.00g, 4.25 mmol)
according to General Procedure B above. Yield: 66% (white solid);
1
TLC Rf = 0.44 (10% EtOAc/hexanes, UV); H NMR (500 MHz,
CDCl3) δ 8.03−7.96 (m, 2H), 7.59−7.52 (m, 1H), 7.50−7.43 (m,
2H), 4.40 (dd, J = 8.9, 5.7 Hz, 1H), 2.14−1.98 (m, 1H), 1.83 (ddd, J =
13.6, 7.8, 5.7 Hz, 1H), 1.67 (ddd, J = 13.7, 9.0, 6.3 Hz, 1H), 1.02 (dd, J
= 11.4, 6.7 Hz, 6H); 13C NMR (125 MHz, CDCl3) δ 179.0, 161.5,
132.7, 128.8, 127.9, 126.1, 64.0, 40.9, 25.3, 22.8, 22.2; MS exact mass
calculated for [C13H15NO2 + H]+ requires m/z = 218.12, found 218.12
(ESI+).
4-Isopropyl-2-phenyloxazol-5(4H)-one (1l). Title compound was
synthesized from N-benzoyl-DL-valine (1l′, 1.00 g, 4.52 mmol)
according to General Procedure B above. Yield: 72% (clear, colorless
oil); TLC Rf = 0.40 (10% EtOAc/hexanes, UV); 1H NMR (500 MHz,
CDCl3) δ 8.04−7.98 (m, 2H), 7.61−7.54 (m, 1H), 7.51−7.45 (m,
2H), 4.29 (dd, J = 4.36, 1.1 Hz, 1H), 2.39 (m, 1H), 1.15 (dd, J = 6.8,
0.9 Hz, 3H), 1.02 (dd, J = 6.8, 0.9 Hz, 3H); 13C NMR (125 MHz,
CDCl3) δ 177.9, 161.8, 132.8, 128.9, 128.0, 126.1, 70.9, 31.4, 18.9,
17.7; MS exact mass calculated for [C12H13NO2 + H]+ requires m/z =
204.10, found 204.11 (ESI+).
Scale-Up (0.50 mmol) Procedure. Peptide 9k (69.3 mg, 0.10 mmol,
20 mol %) was first added to an oven-dried round-bottom flask
equipped with a magnetic stir bar. Oxazolone 1 (0.50 mmol, 1.0 equiv)
was added next, and the solid mixture was dissolved in PhMe (5.0 mL,
0.10 M solution). The resulting solution was allowed to stir at 4 °C
(cold room) as MeOH (61 μL, 1.5 mmol, 3 equiv) was added to the
solution with a syringe. The flask was sealed, and the solution was
allowed to stir at 4 °C for 24−70 h depending on the substrate. The
reaction was quenched by concentration in vacuo, and conversion was
1
assessed by crude H NMR (vide supra). The crude reaction mixture
was then loaded onto a silica column in CH2Cl2 and purified by flash
chromatography using an appropriate EtOAc/hexanes gradient chosen
on the basis of TLC. The fractions containing product were pooled
and concentrated in vacuo. Adventitious solvent was removed by
coevaporation with CH2Cl2 before the product was allowed to dry
1
under a vacuum overnight. H and 13C NMR confirmed the structure
4-Neopentyl-2-phenyloxazol-5(4H)-one (1m). Title compound
was synthesized from N-benzoyl-L-tert-butylalanine (1m′, 250 mg,
1.00 mmol) according to General Procedure B above. Yield: 94%
of the methyl ester product. Analytical HPLC was performed to assess
the er of both the remaining starting material and product.
Racemic Standard Procedure A. Oxazolone 1 (1 equiv, 0.050
mmol) was added to an oven-dried vial equipped with a magnetic stir
bar. The substrate was dissolved in a 0.02 M solution of Et3N in PhMe
(0.50 mL, 0.10 M solution). The resulting solution was allowed to stir
at 4 °C (cold room) as MeOH (10.1 μL, 0.25 mmol) was added to the
solution. The PTFE-lined vial cap was replaced and tightened, and the
solution was allowed to stir at 4 °C for 24 h. The reaction was
quenched by concentration in vacuo to remove excess MeOH, Et3N,
and PhMe. The crude reaction mixture was then purified by filtration
through a pipet silica plug (1 × 7 cm, 50% EtOAc/hexanes, isocratic).
The fractions containing starting material and product were pooled
together and concentrated in vacuo. Analytical HPLC analysis was
performed to assess the er of both the remaining starting material and
product to ensure that both were indeed racemic (vide infra for
substrate-specific HPLC methods). Note: In a number of cases, an
unidentified side product was present in the HPLC trace, eluting just
before the first product peak.34
1
(white solid); TLC Rf = 0.65 (20% EtOAc/hexanes, UV); H NMR
(500 MHz, CDCl3) δ 8.03−7.95 (m, 2H), 7.59−7.52 (m, 1H), 7.50−
7.42 (m, 2H), 4.39 (dd, J = 9.8, 3.3 Hz, 1H), 1.77 (AMX, JAX = 3.3 Hz,
JMX = 9.8 Hz, JAM = 14.1 Hz, νAM = 144.4 Hz, 2H), 1.10 (s, 9H); 13C
NMR (125 MHz, CDCl3) δ 180.0, 160.9, 132.6, 128.8, 127.9, 126.3,
63.6, 45.9, 31.0, 29.8; MS exact mass calculated for [C14H17NO2 + H]+
requires m/z = 232.13, found 232.14 (ESI+).
4-tert-Butyl-2-phenyloxazol-5(4H)-one (1n). Title compound was
synthesized from N-benzoyl-DL-tert-leucine (1n′, 1.00 g, 4.25 mmol)
according to General Procedure B above. Yield: 88% (white solid);
1
TLC Rf = 0.62 (20% EtOAc/hexanes, UV); H NMR (500 MHz,
CDCl3) δ 8.07−7.96 (m, 2H), 7.59−7.52 (m, 1H), 7.50−7.43 (m,
2H), 4.07 (s, 1H), 1.14 (s, 9H); 13C NMR (125 MHz, CDCl3) δ
177.0, 161.3, 132.7, 128.8, 128.0, 126.1, 74.1, 36.0, 26.3; MS exact
mass calculated for [C13H15NO2 + H]+ requires m/z = 218.12, found
218.12 (ESI+).
Racemic Standard Procedure B.35 Racemic N-benzoyl amino acid
1′ (1 equiv) was added to an oven-dried round-bottom flask equipped
with a magnetic stir bar. The white solid was dissolved in a 3:1 (v/v)
PhMe/MeOH solution (0.15 M wrt 1′), and the resulting clear,
colorless solution was allowed to stir at rt. The flask was sealed with a
septum, and a N2 inlet was added. (Trimethylsilyl)diazomethane
(TMSCHN2, 2.0 M in hexanes, 1.1 equiv) was added dropwise to the
stirring solution at rt, causing evolution of a gas. Additional
TMSCHN2 was added until the pale yellow color persisted. The
solution was allowed to stir 20 min at rt before the reaction was
quenched by addition of excess silica. The pale yellow solution became
clear and colorless. The reaction mixture was filtered to remove silica,
and the filtrate was concentrated in vacuo to provide white granular
solid. Analytical HPLC analysis was performed to assess the er of the
product (vide infra for substrate-specific HPLC methods).
General Procedures for the DKR of Oxazolones 1. All vials,
flasks, and magnetic stir bars were oven-dried overnight before use in
DKR experiments. Peptides 9 (and truncated peptides 10) and
reaction conditions were screened using the small-scale procedure
detailed below (0.05 mmol of 1a). 2,3-Dimethylnaphthalene (3.9 mg,
0.025 mmol) was used as an internal standard. Competition
experiments (Scheme 3) were also performed using the small-scale
procedure, with triethylamine (20 mol % wrt 1a) added to the reaction
mixture along with peptide 9k or 9p. The substrate scope was
examined on small scale (0.05 mmol of 1), as well as on a larger scale
(0.50 mmol of 1). The 0.05 mmol scale reactions were quenched after
22−24 h. Conversion was determined by 1H NMR of the crude
reaction mixture and is defined as the ratio of product α-signal
integration (αpdt) to the total α-signal integration (αsm + αpdt). The
0.50 mmol scale reactions were allowed to stir longer than 24 h if the
conversion of the small-scale counterparts (which were started at the
same time) was low at the 24 h mark. Additional MeOH (5 equiv) was
used for less reactive, alkyl-substituted substrates (1k−n). Results were
consistent between the two scales in every case. Isolated yields are only
reported for the scale-up reactions.
N-Benzoyl-L-phenylalanine methyl ester (2a). Title compound
was synthesized from oxazolone 1a (126 mg, 0.50 mmol) according to
the Scale-Up Procedure above. TLC Rf = 0.22 (20% EtOAc/hexanes,
UV). Conversion: 94% (0.050 mmol scale, 24 h); 90% (0.50 mmol
1
scale, 24 h). Yield: 86% isolated (white solid), 97% brsm; H NMR
Small (0.05 mmol) Scale Procedure. Peptide 9k (6.9 mg, 0.01
mmol, 20 mol %) was added to a 4 mL vial, followed by oxazolone 1
(0.05 mmol, 1.0 equiv). The solid mixture was dissolved in PhMe
(0.50 mL, 0.10 M solution). The resulting clear colorless solution was
allowed to stir at 4 °C (cold room) as MeOH (6.1 μL, 0.15 mmol, 3
(400 MHz, CDCl3) δ 7.80−7.69 (m, 2H), 7.56−7.48 (m, 1H), 7.47−
7.38 (m, 2H), 7.35−7.24 (m, 3H), 7.20−7.11 (m, 2H), 6.71 (d, J = 7.6
Hz, 1H), 5.12 (dt, J = 7.7, 5.6 Hz, 1H), 3.78 (s, 3H), 3.29 (ABX, JAX
=
5.7 Hz, JBX = 5.6 Hz, JAB = 13.8 Hz, νAB = 27.2 Hz, 2H); 13C NMR
(125 MHz, CDCl3) δ 172.1, 166.9, 136.0, 134.0, 131.8, 129.4, 128.7,
1551
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128.7, 127.2, 127.1, 53.6, 52.5, 38.0; HRMS exact mass calculated for
[C17H17NO3 + H]+ requires m/z = 284.1287, found 284.1261 (ESI+);
HPLC (Chiralpak AD-H column, 10% i-PrOH/hexanes eluent, 1.0 mL
min−1 flow, 20 °C uniform column temperature, 230 nm) 96:4 er
(0.050 mmol scale, minor tR = 12.6 min, major tR = 15.4 min); 96:4 er
(0.50 mmol scale, minor tR = 12.5 min, major tR = 15.4 min).
N-Benzoyl-L-β-(2-furyl)alanine methyl ester (2b). Title compound
was synthesized from oxazolone 1b (121 mg, 0.50 mmol) according to
the Scale-Up Procedure above. TLC Rf = 0.16 (20% EtOAc/hexanes,
UV). Conversion: 96% (0.050 mmol scale, 23 h); 99% (0.50 mmol
53.6, 52.6, 27.8; HRMS exact mass calculated for [C19H18N2O3 + H]+
requires m/z = 323.1396, found 323.1375 (ESI+); HPLC (Chiralpak
AD-H column, 10% EtOH/hexanes eluent, 0.70 mL min−1 flow, 20 °C
uniform column temperature, 230 nm) 98:2 er (0.050 mmol scale,
minor tR = 49.2 min, major tR = 61.3 min); 97:3 er (0.50 mmol scale,
minor tR = 50.2 min, major tR = 61.5 min).
N-Benzoyl-O-tert-butyl-L-tyrosine methyl ester (2f). Title com-
pound was synthesized from oxazolone 1f (162 mg, 0.50 mmol)
according to the Scale-Up Procedure above. TLC Rf = 0.11 (20%
EtOAc/hexanes, UV). Conversion: 69% (0.050 mmol scale, 24 h);
92% (0.50 mmol scale, 45 h). Yield: 78% isolated (white solid), 85%
1
scale, 26 h). Yield: 78% isolated (white solid); H NMR (500 MHz,
1
CDCl3) δ 7.79−7.68 (m, 2H), 7.54−7.48 (m, 1H), 7.46−7.39 (m,
2H), 7.33 (dd, J = 1.9, 0.8 Hz, 1H), 6.79 (d, J = 7.4 Hz, 1H), 6.29 (dd,
J = 3.2, 1.9 Hz, 1H), 6.10 (dd, J = 3.1, 0.8 Hz, 1H), 5.05 (dt, J = 7.6,
brsm; H NMR (500 MHz, CDCl3) δ 7.75−7.63 (m, 2H), 7.56−7.46
(m, 1H), 7.45−7.36 (m, 2H), 7.10−6.99 (m, 2H), 6.97−6.88 (m, 2H),
6.54 (d, J = 7.7 Hz, 1H), 5.05 (dt, J = 7.6, 5.7 Hz, 1H), 3.74 (s, 3H),
3.21 (ABX, JAX = 6.0 Hz, JBX = 5.5 Hz, JAB = 13.9 Hz, νAB = 23.4 Hz,
2H), 1.32 (s, 9H); 13C NMR (125 MHz, CDCl3) δ 172.2, 166.9,
134.1, 131.9, 130.7, 129.9, 128.8, 127.1, 124.4, 78.6, 53.7, 52.5, 37.4,
29.0; HRMS exact mass calculated for [C21H25NO4 + H]+ requires m/
z = 356.1862, found 356.1821 (ESI+); HPLC (Chiralpak AD-H
column, 10% i-PrOH/hexanes eluent, 1.0 mL min−1 flow, 20 °C
uniform column temperature, 230 nm) 93:7 er (0.050 mmol scale,
minor tR = 10.1 min, major tR = 13.2 min); 93:7 er (0.50 mmol scale,
minor tR = 10.2 min, major tR = 13.5 min).
5.2 Hz, 1H), 3.78 (s, 3H), 3.31 (ABX, JAX = 5.0 Hz, JBX = 4.8 Hz, JAB
=
15.3 Hz, νAB = 17.0 Hz, 2H); 13C NMR (125 MHz, CDCl3) δ 171.8,
167.0, 150.5, 142.4, 134.1, 131.9, 128.7, 127.2, 110.5, 108.2, 52.8, 52.1,
30.8; HRMS exact mass calculated for [C15H15NO4 + H]+ requires m/
z = 274.1079, found 274.1044 (ESI+); HPLC (Chiralpak AD-H
column, 10% i-PrOH/hexanes eluent, 1.0 mL min−1 flow, 20 °C
uniform column temperature, 230 nm) 96:4 er (0.050 mmol scale,
minor tR = 12.5 min, major tR = 15.9 min); 96:4 er (0.50 mmol scale,
minor tR = 12.5 min, major tR = 15.8 min).
N-Benzoyl-L-β-(2-thienyl)alanine methyl ester (2c). Title com-
pound was synthesized from oxazolone 1c (129 mg, 0.50 mmol)
according to the Scale-Up Procedure above. TLC Rf = 0.17 (20%
EtOAc/hexanes, UV). Conversion: 99% (0.050 mmol scale, 22 h);
95% (0.50 mmol scale, 24 h). Yield: 84% isolated (yellow oily solid);
1H NMR (400 MHz, CDCl3) δ 7.82−7.73 (m, 2H), 7.55−7.48 (m,
1H), 7.47−7.39 (m, 2H), 7.17 (dd, J = 5.1, 1.2 Hz, 1H), 6.94 (dd, J =
5.2, 3.4 Hz, 1H), 6.80 (dd, J = 3.4, 1.2 Hz, 1H), 6.79 (bs, 1H), 5.09
N-Benzoyl-L-(4-nitro)phenylalanine methyl ester (2g). Title
compound was synthesized from oxazolone 1g (148 mg, 0.50
mmol) according to the Scale-Up Procedure above. The oxazolone
substrate was difficult to dissolve, so the reaction solution was
prestirred at RT before addition of MeOH at 4 °C. Addition of MeOH
helped to dissolve the reactants. TLC Rf = 0.33 (50% EtOAc/hexanes,
UV). Conversion: 88% (0.050 mmol scale, 22 h); 94% (0.50 mmol
scale, 30 h). Yield: 88% isolated (off-white, flaky solid); 1H NMR (500
MHz, CDCl3) δ 8.15−7.99 (m, 2H), 7.74−7.60 (m, 2H), 7.50−7.41
(m, 1H), 7.40−7.32 (m, 2H), 7.30−7.20 (m, 2H), 6.64 (d, J = 7.2 Hz,
1H), 5.05 (dt, J = 7.1, 5.7 Hz, 1H), 3.72 (s, 3H), 3.31 (AMX, JAX = 6.0
Hz, JBX = 5.5 Hz, JAM = 13.8 Hz, νAM = 68.4 Hz, 2H); 13C NMR (125
MHz, CDCl3) δ 171.6, 167.0, 147.4, 144.0, 133.6, 132.2, 130.4, 128.9,
127.1, 123.8, 53.5, 52.9, 37.9; HRMS exact mass calculated for
[C17H16N2O5 + H]+ requires m/z = 329.1137, found 329.1128 (ESI
+); HPLC (Chiralpak AD-H column, 10% EtOH/hexanes eluent, 0.70
mL min−1 flow, 20 °C uniform column temperature, 230 nm) 88:12 er
(0.050 mmol scale, major tR = 68.6 min, minor tR = 81.7 min); 88:12
er (0.50 mmol scale, major tR = 69.6 min, major tR = 82.7 min).
N-Benzoyl-L-β,β-diphenylalanine methyl ester (2h). Title com-
pound was synthesized from oxazolone 1h (96 mg, 0.30 mmol)
according to the Scale-Up Procedure above. The oxazolone substrate
was difficult to dissolve, so the reaction solution was prestirred with
gentle heating before addition of MeOH at 4 °C. Even so, the solution
was largely heterogeneous (cloudy, pale-yellow solution). TLC Rf =
0.16 (20% EtOAc/hexanes, UV). Conversion: 49% (0.050 mmol scale,
24 h); 26% (0.30 mmol scale, 38 h). Yield: 19% isolated (white, foamy
solid), 75% brsm; 1H NMR (400 MHz, CDCl3) δ 7.55−7.49 (m, 2H),
7.43−7.36 (m, 1H), 7.33−7.11 (m, 12H), 6.37 (d, J = 8.8 Hz, 1H),
5.55 (t, J = 8.6 Hz, 1H), 4.55 (d, J = 8.4 H, 1H), 3.48 (s, 3H); 13C
NMR (125 MHz, CDCl3) δ 172.4, 167.3, 140.1, 139.6, 133.9, 131.9,
129.0, 128.7, 128.7, 128.6, 128.3, 127.5, 127.3, 127.1, 127.1, 55.7, 53.7,
52.3; HRMS exact mass calculated for [C23H21NO3 + H]+ requires m/
z = 360.1600, found 360.1570 (ESI+); HPLC (Chiralcel OD-H
column, 10% i-PrOH/hexanes eluent, 1.00 mL min−1 flow, 20 °C
uniform column temperature, 230 nm) 91:9 er (0.050 mmol scale,
minor tR = 9.2 min, major tR = 18.6 min); 90:10 er (0.30 mmol scale,
minor tR = 9.0 min, major tR = 17.9 min).
(dt, J = 7.5, 4.8 Hz, 1H), 3.79 (s, 3H), 3.52 (ABX, JAX = 5.0 Hz, JBX
=
4.8 Hz, JAB = 14.9 Hz, νAB = 16.0 Hz, 2H); 13C NMR (125 MHz,
CDCl3) δ 171.6, 167.0, 137.3, 133.9, 131.9, 128.7, 127.2, 127.2, 127.0,
125.1, 53.5, 52.7, 32.2; HRMS exact mass calculated for [C15H15NO3S
+ H]+ requires m/z = 290.0851, found 290.0881 (ESI+); HPLC
(Chiralpak AD-H column, 10% i-PrOH/hexanes eluent, 1.0 mL min−1
flow, 20 °C uniform column temperature, 230 nm) 96:4 er (0.050
mmol scale, minor tR = 13.2 min, major tR = 16.6 min); 97:3 er (0.50
mmol scale, minor tR = 13.0 min, major tR = 16.4 min).
N-Benzoyl-L-β-(1-napthyl)alanine methyl ester (2d). Title com-
pound was synthesized from oxazolone 1d (151 mg, 0.50 mmol)
according to the Scale-Up Procedure above. TLC Rf = 0.14 (20%
EtOAc/hexanes, UV). Conversion: 93% (0.050 mmol scale, 23 h);
93% (0.50 mmol scale, 25 h). Yield: 84% isolated (off-white, foamy
solid), 90% brsm; 1H NMR (400 MHz, CDCl3) δ 8.18 (d, J = 8.2 Hz,
1H), 7.86 (dd, J = 7.6, 2.1 Hz, 1H), 7.77 (d, J = 8.1 Hz, 1H), 7.72−
7.63 (m, 2H), 7.58−7.21 (m, 7H), 6.93 (d, J = 7.9 H, 1H), 5.24 (q, J =
6.9 Hz, 1H), 3.71 (ABX, JAX = 6.4 Hz, JBX = 6.5 Hz, JAB = 14.8 Hz, νAB
= 13.0 Hz, 2H), 3.63 (s, 3H); 13C NMR (125 MHz, CDCl3) δ 172.3,
167.1, 133.9, 133.7, 132.4, 132.2, 131.6, 128.8, 128.4, 128.0, 127.4,
127.0, 126.3, 125.7, 125.2, 123.5, 53.6, 52.2, 35.0; HRMS exact mass
calculated for [C21H19NO3 + H]+ requires m/z = 334.1443, found
334.1473 (ESI+); HPLC (Chiralpak AD-H column, 10% i-PrOH/
hexanes eluent, 1.0 mL min−1 flow, 20 °C uniform column
temperature, 230 nm) 94:6 er (0.050 mmol scale, minor tR = 14.2
min, major tR = 18.1 min); 93:7 er (0.50 mmol scale, minor tR = 14.2
min, major tR = 18.1 min).
N-Benzoyl-L-tryptophan methyl ester (2e). Title compound was
synthesized from oxazolone 1e (145 mg, 0.50 mmol) according to the
Scale-Up Procedure above. TLC Rf = 0.05 (20% EtOAc/hexanes, UV).
Conversion: 94% (0.050 mmol scale, 23 h); 99% (0.50 mmol scale, 26
N-Benzoyl-L-homophenylalanine methyl ester (2i). Title com-
pound was synthesized from oxazolone 1i (133 mg, 0.50 mmol)
according to the Scale-Up Procedure above. The reaction solution was
no longer homogeneous after 48 h at 4 °C. The cloudy, white
suspension became a clear solution again upon warming to rt. TLC Rf
= 0.12 (20% EtOAc/hexanes, UV). Conversion: 40% (0.050 mmol
scale, 22 h); 50% (0.50 mmol scale, 48 h). Yield: 47% isolated (white
solid), 76% brsm; 1H NMR (400 MHz, CDCl3) δ 7.77−7.68 (m, 2H),
7.55−7.47 (m, 1H), 7.47−7.39 (m, 2H), 7.33−7.24 (m, 2H), 7.24−
1
h). Yield: 93% isolated (off-white, foamy solid); H NMR (500 MHz,
CDCl3) δ 8.22 (s, 1H), 7.71−7.65 (m, 2H), 7.56 (dt, J = 8.0, 0.9 Hz,
1H), 7.50−7.45 (m, 1H), 7.41−7.33 (m, 3H), 7.19 (ddd, J = 8.2, 7.0,
1.1 Hz, 1H), 7.08 (ddd, J = 8.1, 7.1, 1.0 Hz, 1H), 7.00 (d, J = 2.4 Hz,
1H), 6.68 (d, J = 7.9 Hz, 1H), 5.16 (dt, J = 7.7, 5.2 Hz, 1H), 3.72 (s,
3H), 3.46 (ABX, JAX = 5.2 Hz, JBX = 4.7 Hz, JAB = 14.8 Hz, νAB = 13.0
Hz, 2H); 13C NMR (125 MHz, CDCl3) δ 172.5, 167.1, 136.3, 134.0,
131.8, 128.7, 127.8, 127.2, 122.9, 122.4, 119.9, 118.8, 111.4, 110.2,
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7.17 (m, 3H), 6.71 (d, J = 7.7 Hz, 1H), 4.91 (td, J = 7.4, 5.0 Hz, 1H),
3.77 (s, 3H), 2.75 (m, 1H), 2.35 (m, 1H), 2.17 (m, 1H); 13C NMR
(125 MHz, CDCl3) δ 173.0, 167.1, 140.8, 134.0, 131.9, 128.7, 128.7,
128.5, 127.2, 126.3, 52.7, 52.6, 34.1, 31.9; HRMS exact mass calculated
for [C18H19NO3 + H]+ requires m/z = 298.1443, found 298.1422 (ESI
+); HPLC (Chiralpak AD-H column, 10% i-PrOH/hexanes eluent,
1.00 mL min−1 flow, 20 °C uniform column temperature, 230 nm)
81:19 er (0.050 mmol scale, minor tR = 11.5 min, major tR = 14.1
min); 81:19 er (0.50 mmol scale, minor tR = 11.7 min, major tR = 14.4
min).
minor tR = 6.6 min, major tR = 9.8 min); 80:20 er (0.50 mmol scale,
minor tR = 6.7 min, major tR = 9.9 min).
N-Benzoyl-L-tert-leucine methyl ester (2n). Title compound was
synthesized from oxazolone 1n (109 mg, 0.50 mmol) according to the
Scale-Up Procedure above. TLC Rf = 0.29 (20% EtOAc/hexanes, UV).
Conversion: 5% (0.070 mmol scale, 24 h); 9% (0.50 mmol scale, 62
1
h). Yield: 5% isolated (yellow oil), 92% brsm; H NMR (500 MHz,
CDCl3) δ 7.83−7.77 (m, 2H), 7.55−7.50 (m, 1H), 7.48−7.42 (m,
2H), 6.64 (d, J = 9.3 Hz, 1H), 4.71 (d, 9.3 Hz, 1H), 3.76 (s, 3H), 1.06
(s, 9H); 13C NMR (125 MHz, CDCl3) δ 172.7, 167.5, 134.7, 132.1,
129.0, 127.4, 60.2, 52.3, 35.6, 27.1; HRMS exact mass calculated for
[C14H19NO3 + H]+ requires m/z = 250.1443, found 250.1442 (ESI+);
HPLC (Chiralpak AD-H column, 10% i-PrOH/hexanes eluent, 1.00
mL min−1 flow, 20 °C uniform column temperature, 230 nm) 72:28 er
(0.070 mmol scale, minor tR = 5.9 min, major tR = 7.9 min); 71:29 er
(0.50 mmol scale, minor tR = 5.8 min, major tR = 7.7 min).
N-Benzoyl-dimethyl-L-aspartate (2j). Title compound was synthe-
sized from oxazolone 1j (109 mg, 0.47 mmol) according to the Scale-
Up Procedure above. TLC Rf = 0.19 (40% EtOAc/hexanes, UV).
Conversion: 94% (0.050 mmol scale, 22 h); 99% (0.47 mmol scale, 25
1
h). Yield: 79% isolated (white solid); H NMR (500 MHz, CDCl3) δ
7.83−7.77 (m, 2H), 7.54−7.48 (m, 1H), 7.47−7.40 (m, 2H), 7.24 (d, J
= 8.0 Hz, 1H), 5.06 (dt, J = 8.3, 4.5 Hz, 1H), 3.78 (s, 3H), 3.69 (s,
3H), 3.06 (AMX, JAX = 4.2 Hz, JBX = 4.5 Hz, JAM = 17.3 Hz, νAM = 75.7
Hz, 2H); 13C NMR (125 MHz, CDCl3) δ 171.8, 171.4, 167.0, 133.7,
132.0, 128.8, 127.3, 53.0, 52.2, 49.0, 36.2; HRMS exact mass calculated
for [C13H15NO5 + H]+ requires m/z = 266.1028, found 266.1022 (ESI
+); HPLC (Chiralpak AD-H column, 10% EtOH/hexanes eluent, 0.70
mL min−1 flow, 20 °C uniform column temperature, 230 nm) 90:10 er
(0.050 mmol scale, major tR = 45.4 min, minor tR = 67.8 min); 90:10
er (0.47 mmol scale, major tR = 45.4 min, minor tR = 68.9 min).
N-Benzoyl-L-leucine methyl ester (2k). Title compound was
synthesized from oxazolone 1k (109 mg, 0.50 mmol) according to
the Scale-Up Procedure above. TLC Rf = 0.08 (10% EtOAc/hexanes,
UV). Conversion: 57% (0.050 mmol scale, 24 h); 60% (0.50 mmol
Assignment of Absolute Stereochemistry of 1a. N-Benzoyl-L-
phenylalanine methyl ester. N-Benzoyl-L-phenylalanine (500 mg,
1.86 mmol) was added to an oven-dried round-bottom flask equipped
with a magnetic stir bar. The white solid was dissolved in a 3:1 (v/v)
PhMe/MeOH solution (12.4 mL, 0.15 M wrt Bz-L-Phe-OH), and the
resulting clear, colorless solution was allowed to stir at ambient
temperature. The flask was sealed with a septum, and a N2 inlet was
added. (Trimethylsilyl)diazomethane (TMSCHN2, 1.02 mL, 2.05
mmol) was added dropwise to the stirring solution at rt, causing
evolution of a gas. Additional TMSCHN2 was added until the pale
yellow color persisted. The solution was allowed to stir 20 min at rt
before the reaction was quenched by addition of excess silica. The pale
yellow solution became clear and colorless. The reaction mixture was
filtered to remove silica, and the filtrate was concentrated in vacuo to
provide white granular solid (488 mg, 93% yield). The product was
shown to be spectroscopically consistent with 2a: 1H NMR (500
MHz, CDCl3) δ 7.77−7.71 (m, 2H), 7.55−7.48 (m, 1H), 7.46−7.40
(m, 2H), 7.34−7.24 (m, 3H), 7.19−7.12 (m, 2H), 6.61 (d, J = 7.8 Hz,
1H), 5.11 (dt, J = 7.5, 5.6 Hz, 1H), 3.78 (s, 3H), 3.28 (ABX, JAX = 5.8
Hz, JBX = 5.5 Hz, JAB = 13.9 Hz, νAB = 32.0 Hz, 2H); 13C NMR (125
MHz, CDCl3) δ 172.2, 166.9, 136.0, 134.0, 131.9, 129.5, 128.7, 128.7,
127.3, 127.1, 53.7, 52.5, 38.0; HRMS exact mass calculated for
[C17H17NO3 + H]+ requires m/z = 284.1287, found 284.1273 (ESI+);
HPLC (Chiralpak AD-H column, 10% i-PrOH/hexanes eluent, 1.0 mL
min−1 flow, 20 °C uniform column temperature, 230 nm) 100:0 er (tR
= 15.6 min); [α]2D0 = +90.9 (c = 1.0, CHCl3); literature [α]2D4 = +98.1
(c = 1.0, CHCl3).36
1
scale, 48 h). Yield: 58% isolated (white solid), 95% brsm; H NMR
(500 MHz, CDCl3) δ 7.80−7.74 (m, 2H), 7.50−7.43 (m, 1H), 7.41−
7.35 (m, 2H), 6.73 (d, J = 8.4 Hz, 1H), 4.84 (td, J = 8.6, 4.8 Hz, 1H),
3.73 (s, 3H), 1.78−1.60 (m, 3H), 0.95 (dd, J = 8.8, 5.9 Hz, 6H); 13C
NMR (125 MHz, CDCl3) δ 173.8, 167.2, 134.0, 131.7, 128.6, 127.2,
52.4, 51.2, 41.8, 25.1, 22.9, 22.1; HRMS exact mass calculated for
[C14H19NO3 + H]+ requires m/z = 250.1443, found 250.1428 (ESI+);
HPLC (Chiralpak AD-H column, 10% i-PrOH/hexanes eluent, 1.00
mL min−1 flow, 20 °C uniform column temperature, 230 nm) 91:9 er
(0.050 mmol scale, minor tR = 7.2 min, major tR = 9.5 min); 93:7 er
(0.50 mmol scale, minor tR = 7.2 min, major tR = 9.5 min).
N-Benzoyl-L-valine methyl ester (2l). Title compound was
synthesized from oxazolone 1l (118 mg, 0.58 mmol) according to
the Scale-Up Procedure above. TLC Rf = 0.22 (20% EtOAc/hexanes,
UV). Conversion: 20% (0.050 mmol scale, 24 h); 30% (0.58 mmol
1
scale, 48 h). Yield: 27% isolated (white solid), 86% brsm; H NMR
ASSOCIATED CONTENT
■
(500 MHz, CDCl3) δ 7.86−7.76 (m, 2H), 7.55−7.49 (m, 1H), 7.48−
7.40 (m, 2H), 6.63 (d, J = 8.4 Hz, 1H), 4.79 (dd, J = 8.7, 4.9 Hz, 1H),
3.77 (s, 3H), 2.34−2.22 (m, 1H), 1.00 (dd, J = 11.8, 6.9 Hz, 6H); 13C
NMR (125 MHz, CDCl3) δ 172.8, 167.4, 134.3, 131.8, 128.7, 127.1,
57.6, 52.4, 31.8, 19.1, 18.1; HRMS exact mass calculated for
[C13H17NO3 + H]+ requires m/z = 236.1287, found 236.1295 (ESI
+); HPLC (Chiralpak AD-H column, 10% i-PrOH/hexanes eluent,
1.00 mL min−1 flow, 20 °C uniform column temperature, 230 nm)
80:20 er (0.050 mmol scale, minor tR = 7.1 min, major tR = 9.2 min);
83:17 er (0.58 mmol scale, minor tR = 7.2 min, major tR = 9.4 min).
N-Benzoyl-L-β-tert-butylalanine methyl ester (2m). Title com-
pound was synthesized from oxazolone 1m (116 mg, 0.50 mmol)
according to the Scale-Up Procedure above. TLC Rf = 0.20 (20%
EtOAc/hexanes, UV). Conversion: 15% (0.050 mmol scale, 46 h);
30% (0.58 mmol scale, 69 h). Yield: 33% isolated (white solid), 79%
brsm; 1H NMR (500 MHz, CDCl3) δ 7.81−7.72 (m, 2H), 7.55−7.47
(m, 1H), 7.46−7.39 (m, 2H), 6.45 (d, J = 8.6 Hz, 1H), 4.88 (td, J =
8.8, 3.5 Hz, 1H), 3.75 (s, 3H), 1.75 (AMX, JAX = 3.8 Hz, JBX = 9.0 Hz,
JAM = 14.4 Hz, νAM = 122.4 Hz, 2H), 1.01 (s, 9H); 13C NMR (125
MHz, CDCl3) δ 174.1, 167.0, 134.2, 131.9, 128.7, 127.1, 52.5, 50.5,
46.5, 31.0, 29.8; HRMS exact mass calculated for [C15H21NO3 + H]+
requires m/z = 264.1600, found 264.1572 (ESI+); HPLC (Chiralpak
AD-H column, 10% i-PrOH/hexanes eluent, 1.00 mL min−1 flow, 20
°C uniform column temperature, 230 nm) 79:21 er (0.050 mmol scale,
S
* Supporting Information
HRMS data for peptides 9a−p and 10a−f, H and 13C NMR
1
spectra of compounds 9k and 2a−n, HPLC traces of 2a−n, and
high-field (2D-)NMR characterization of 9k. This material is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: scott.miller@yale.edu.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
A.J.M. gratefully acknowledges support from the National
Science Foundation Graduate Research Fellowship Program.
Early aspects of this study were supported by the National
Science Foundation (CHE-0848224). We would like to thank
Dr. Eric Paulson for assistance with high-field NMR experi-
ments.
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(19) Other chiral, monosubstituted residues were substituted at the i
+ 2 position in other scaffolds, and none were as successful as Phe.
(20) H NMR spectra of peptides 9b, 9c, and 9f were acquired on
the same 500 MHz spectrometer at ambient temperature. Samples
were freshly prepared, each with the same concentration (0.02 M in
CDCl3).
(21) It was later observed that tosylated 9n had epimerized, which
may partially account for its low selectivity.
(22) The absolute configuration of methyl esters 2 was assigned by
analogy to Bz-L-Phe-OMe (see the Experimental Section for details).
(23) For a crystal structure of a β-hairpin peptide, see: Blank, J. T.;
Miller, S. J. Biopolymers 2006, 84, 38−47.
(24) (a) Oh, S. H.; Rho, H. S.; Lee, J. W.; Lee, J. E.; Youk, S. H.;
Chin, J.; Song, C. E. Angew. Chem. 2008, 120, 7990−7993. (b) Li, H.
M.; Liu, X. F.; Wu, F. H.; Tang, L. A.; Deng, L. Proc. Nat. Acad. Sci. U.
S. A. 2010, 107, 20625−20629.
(25) For a discussion on the mechanism of ester hydrolysis, see:
Smith, M. B.; March, J. March’s Advanced Organic Chemistry, 6th ed.;
Wiley: New York, 2007; Chapter 16.
REFERENCES
■
(1) For relevant reviews, see: (a) Faber, K. Chem.Eur. J. 2001, 7,
5004−5010. (b) Keith, J. M.; Larrow, J. F.; Jacobsen, E. N. Adv. Synth.
Catal. 2001, 343, 5−26. (c) Ward, R. S. Tetrahedron: Asymmetry 1995,
6, 1475−1490. (d) Pellissier, H. Adv. Synth. Catal. 2011, 353, 659−
676. (e) Pellissier, H. Tetrahedron 2008, 64, 1563−1601.
(2) Referred to simply as oxazolones from here forward. Also known
as azlactones.
(3) de Jersey, J.; Zerner, B. Biochemistry 1969, 8, 1967−1974.
(4) For discussions on oxazolone racemization, see: (a) Goodman,
M.; Levine, L. J. Am. Chem. Soc. 1964, 86, 2918−2922. (b) Slebioda,
M.; St-Amand, M. A.; Chen, F. M. F.; Benoiton, N. L. Can. J. Chem.
1988, 66, 2540−2544. (c) Montalbetti, C. A. G. N.; Falque, V.
Tetrahedron 2005, 61, 10827−10852.
1
(5) For a relevant review, see: Fisk, J. S.; Mosey, R. A.; Tepe, J. J.
Chem. Soc. Rev. 2007, 36, 1432−1440 and references therein.
(6) (a) Seebach, D.; Jaeschke, G.; Gottwald, K.; Matsuda, K.;
Formisano, R.; Chaplin, D. A.; Breuning, M.; Bringmann, G.
Tetrahedron 1997, 53, 7539−7556. (b) Gottwald, K.; Seebach, D.
Tetrahedron 1999, 55, 723−738.
(26) For discussions on the stabilizing effects of oxyanion holes, see:
(a) Pihko, P. M., Ed.; Hydrogen Bonding in Organic Synthesis; Wiley-
VCH Verlag GmbH & Co. KGaA: Weinheim, Germany, 2009; pp 43−
72. (b) Etzenbach-Effers, K.; Berkessel, A. Top. Curr. Chem. 2010, 291,
1−27. (c) Kamerlin, S. C. L.; Chu, Z. T.; Warshel, A. J. Org. Chem.
(7) Liang, J.; Ruble, J. C.; Fu, G. C. J. Org. Chem. 1998, 63, 3154−
3155.
(8) (a) Yang, X.; Lu, G.; Birman, V. B. Org. Lett. 2010, 12, 892−895.
(b) Xie, L.; Hua, W.; Chan, A. C. S.; Leung, Y.-C. Tetrahedron:
Asymmetry 1999, 10, 4715−4728.
2010, 75, 6391−6401. (d) Simon
́
, L.; Goodman, J. M. J. Org. Chem.
2010, 75, 1831−1840.
(9) (a) Berkessel, A.; Cleemann, F.; Mukherjee, S.; Muller, T. N.;
̈
(27) For a binding model in a similar system, see: Garand, E.;
Kamrath, M. Z.; Jordan, P. A.; Wolk, A. B.; McCoy, A. B.; Miller, S. J.;
Johnson, M. A. Science 2012, 335, 694−698. See also refs 15a and 17.
(28) Knowles, R. R.; Jacobsen, E. N. Proc. Nat. Acad. Sci. U. S. A.
2010, 107, 20678−20685.
Lex, J. Angew. Chem., Int. Ed. 2005, 44, 807−811. (b) Berkessel, A.;
Mukherjee, S.; Cleemann, F.; Muller, T. N.; Lex, J. Chem. Commun.
̈
2005, 1898−1900.
(10) (a) Hang, J.; Li, H.; Deng, L. Org. Lett. 2002, 4, 3321−3324.
(b) Peschiulli, A.; Quigley, C.; Tallon, S.; Gun’ko, Y. K.; Connon, S. J.
J. Org. Chem. 2008, 73, 6409−6412.
(29) El-Faham, A.; Albericio, F. Chem. Rev. 2011, 111, 6557−6602.
(30) Zhang, L.-h.; Kauffman, G. S.; Pesti, J. A.; Yin, J. J. Org. Chem.
1997, 62, 6918−6920. See also ref 17.
(11) (a) Oh, J. S.; Lee, J. W.; Ryu, T. H.; Lee, J. H.; Song, C. E. Org.
Biomol. Chem. 2012, 10, 1052−1055. (b) Lee, J. W.; Ryu, T. H.; Oh, J.
S.; Bae, H. Y.; Jang, H. B.; Song, C. E. Chem. Commun. 2009, 7224−
7226.
(12) For examples, see: (a) Brown, S. A.; Parker, M.-C.; Turner, N. J.
Tetrahedron: Asymmetry 2000, 11, 1687−1690. (b) Turner, N. J.;
Winterman, R.; McCague, R.; Parratt, J. S.; Taylor, S. J. C. Tetrahedron
́
(31) Kurti, L.; Czako, B. Strategic Applications of Named Reactions in
̈
Organic Synthesis; Elsevier: Boston, 2005; pp 398−399 and references
therein.
(32) Goodman, M.; Glaser, C. B. J. Org. Chem. 1970, 35, 1954−1962.
(33) (a) Melhado, A. D.; Luparia, M.; Toste, F. D. J. Am. Chem. Soc.
2007, 129, 12638−12639. (b) Frebault, F.; Luparia, M.; Oliveira, M.
́
T.; Goddard, R.; Maulide, N. Angew. Chem., Int. Ed. 2010, 49, 5672−
5676.
Lett. 1995, 36, 1113−1116. (c) Podea, P. V.; Tosa̧ , M. I.; Paizs, C.;
Irimie, F. D. Tetrahedron: Asymmetry 2008, 19, 500−511. (d) Crich, J.
Z.; Brieva, R.; Marquart, P.; Gu, R.-L.; Flemming, S.; Sih, C. J. J. Org.
Chem. 1993, 58, 3252−3258. (e) Gu, R.-L.; Lee, I.-S.; Sih, C. J.
Tetrahedron Lett. 1992, 33, 1953−1956. (f) Daffe, V.; Fastrez, J. J. Am.
Chem. Soc. 1980, 102, 3601−3605. (g) For the crystal structure of
Candida antarctica Lipase B (8), see: Uppenberg, J.; Hansen, M. T.;
Patkar, S.; Jones, T. A. Structure 1994, 2, 293−308.
(34) The unidentified side-product may be a cyclodimer that has
been observed when oxazolones are treated with tertiary amine bases.
See: Kobayashi, K.; Bryant, L. L.; Tsukamoto, Y.; Saegusa, T.
Macromolecules 1986, 19, 1547−1551.
(35) Kuhnel, E.; Laffan, D. D. P.; Lloyd-Jones, G. C.; del Campo, T.
̈
M.; Shepperson, I. R.; Slaughter, J. L. Angew. Chem., Int. Ed. 2007, 46,
7075−7078.
(13) Miller, S. J. Acc. Chem. Res. 2004, 37, 601−610.
(14) (a) Haque, T. S.; Little, J. C.; Gellman, S. H. J. Am. Chem. Soc.
1996, 118, 6975−6985. (b) Wilmot, C. M.; Thornton, J. M. J. Mol.
Biol. 1988, 203, 221−232.
(36) Ghosh, S. C.; Ngiam, J. S. Y.; Seayad, A. M.; Tuan, D. T.; Chai,
C. L. L.; Chin, A. J. Org. Chem. 2012, 77, 8007−8015.
(15) For examples, see: (a) Barrett, K. T.; Miller, S. J. J. Am. Chem.
Soc. 2013, 135, 2963−2966. (b) Cowen, B. J.; Saunders, L. B.; Miller,
S. J. J. Am. Chem. Soc. 2009, 131, 6105−6107. (c) Fowler, B. S.;
Mikochik, P. J.; Miller, S. J. J. Am. Chem. Soc. 2009, 132, 2870−2871.
(d) Peris, G.; Jakobsche, C. E.; Miller, S. J. J. Am. Chem. Soc. 2007, 129,
8710−8711.
(16) For examples, see: (a) Kastl, R.; Wennemers, H. Angew. Chem.,
Int. Ed. 2013, 52, 7228−7232. (b) Muller, C. E.; Hrinda, R.; Wende, R.
̈
C.; Schreiner, P. R. Chem.Eur. J. 2011, 17, 6309−6314. For relevant
reviews, see: (c) Wennemers, H. Chem. Commun. 2011, 47, 12036−
12041. (d) Colby Davie, E. A.; Mennen, S. M.; Xu, Y.; Miller, S. J.
Chem. Rev. 2007, 107, 5759−5812. (e) Jarvo, E. R.; Miller, S. J.
Tetrahedron 2002, 58, 2481−2495.
(17) For an example of a Dmaa-containing peptide, see: Gustafson,
J.; Lim, D.; Miller, S. J. Science 2010, 328, 1251−1255. See also ref
15a.
(18) Gilli, P.; Pretto, L.; Bertolasi, V.; Gilli, G. Acc. Chem. Res. 2009,
42, 33−44.
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