Dihedral angle restriction within a peptide-based tertiary alcohol kinetic resolution catalyst

Article abstract of DOI:10.1016/j.tet.2006.01.104

Kinetic resolution of several tertiary alcohols has been evaluated with a peptide-based catalyst that was designed to probe the role of dihedral angle restrictions for certain bonds within the catalyst. In particular, the nucleophilic residue (π-Me)-His h

Full text of DOI:10.1016/j.tet.2006.01.104

Tetrahedron 62 (2006) 5254–5261
Dihedral angle restriction within a peptide-based tertiary
alcohol kinetic resolution catalyst
*
Mary C. Angione and Scott J. Miller
Department of Chemistry, Merkert Chemistry Center, Boston College, Chestnut Hill, MA 02467-3860, USA
Received 15 November 2005; revised 1 January 2006; accepted 2 January 2006
Available online 12 April 2006
Abstract—Kinetic resolution of several tertiary alcohols has been evaluated with a peptide-based catalyst that was designed to probe the role
of dihedral angle restrictions for certain bonds within the catalyst. In particular, the nucleophilic residue (p-Me)–His has been replaced with
the (b-Me)–(p-Me)–His. A synthesis of the key residue is presented, along with characterization data that suggests the substituent exerts a
substantial ground state conformational effect. In addition, kinetic resolution data indicate that the H- to Me-substitution confers enhanced
stereoselectivity in several tertiary alcohol resolutions.
Ó 2006 Elsevier Ltd. All rights reserved.
1. Introduction
of b-substituted amino acids, and their incorporation into
bioactive peptides, has resulted in enhanced biological activ-
ity in a variety of contexts. For example, Hruby improved a
synthetic agonist for pain treatment (1a) by synthesizing
isomers of b-methyl-20,60-dimethyltyrosine (TMT) and then
substituting each analogue for the original exocyclic tyrosine
moiety (Fig. 1).8 The (2S,3R)-TMT analogue 1b produced
a seven-fold increase in binding affinity and a two-fold im-
provement in selectivity whereas the (2S,3S)-TMT analogue
1c did not alter the binding affinity and impaired selectivity.
Biomimetic chemistry can encompass a staggering array of
objectives, reflecting the enormity of the field of biochemis-
try.1 Certainly, the goal of achieving ‘artificial enzymes’ is
one avenue that has drawn inspiration from nature’s cata-
lysts.2 Among the many features of macromolecular cata-
lysts is the ability to control conformation through a wide
range of factors that mirror the complexity of the protein
folding (and nucleic acid folding) problem.
Our laboratory has been studying short sequences of pep-
tides as catalysts for an array of synthetically relevant reac-
tions.3 Since even a short peptide sequence has a wide array
of conformational possibilities available within its low en-
ergy conformational ensembles,4 we have sought strategies
to reduce the complexity of the conformational equilibria.5
While in some cases this appears relevant to stereoselective
catalysts, and in others it does not, we do view the simplifi-
cation of the conformational equilibria as one strategy that
may facilitate the rational design of catalytically active
and selective peptide-based catalysts.
Me
OH
OH
Me
H2N
H2N
O
Me
O
O
S
H
H
N
O
N
HN
Me
N
HN
Me
H
O
N
H
H
O
O
H
O
1a
N
H
S
Me
N
H
O
S
Me
S
O
HO
HO
OH
Me
Me
Me
H2N
OH
Me
H2N
Me
O
Me
O
Among the strategies available for limiting the conforma-
tional dynamics, engineered intramolecular hydrogen bonds,
and the incorporation of secondary structure-promoting se-
quence modules have had a major impact.6 A less often ap-
plied approach in the context of peptide-based catalysts is the
strategy of dihedral angle restriction.7 Perhaps inspired by
the proteinogenic amino acids threonine and isoleucine (in
comparison to serine and leucine, respectively), the synthesis
H
H
O
O
N
N
HN
HN
N
N
H
H
H
H
O
O
1b
1c
Impaired selectivity
Improved Binding
and Selectivity
Figure 1. Synthetic agonist for pain treatment and Hruby’s optimization
through b-substitution within an unnatural tyrosine derivative.
Hruby has shown that the basis of the constraining effect of
b-methylation may be two-fold.9 First, b-methylation of
* Corresponding author. E-mail: scott.miller.1@bc.edu
0040–4020/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2006.01.104

M. C. Angione, S. J. Miller / Tetrahedron 62 (2006) 5254–5261
5255
H
H
H
H
χ1 NH
Me
H
R
H
2
Me
R
OH
HOOC
NH2
HOOC
NH2
HOOC
NH2
χ2
Me
R
Me
O
gauche (-)
trans
gauche (+)
β-(Me)-Phenylalanine
Lower Energy Conformers
Both Highly Populated
H
H
H
H
Me
H
MeO
Me
R
H
Me
NH2
R
NH2
HOOC
NH2
HOOC
NH2
HOOC
OH
R
Me
Me Me
O
gauche (+)
trans
gauche (-)
Lowest Energy
Conformer
β-(Me)-2',6'-dimethyltyrosine
70% Populated
Figure 2. Hruby’s analysis of b-substituted amino acid conformational equilibria.
phenylalanine and tyrosine analogues altered the population
of rotamers around the Ca–Cb bond (c1 angle) such that one
or two of the normally available conformations, gauche (ꢀ),
gauche (+), or trans, were significantly disfavored (Fig. 2).
In the case of the (b-Me)-phenylalanine analogue shown in
Figure 2, b-methylation was the only change with respect
to the naturally occurring (S)-amino acid. The two lowest en-
ergy conformations, gauche (ꢀ) and trans, were highly pop-
ulated when compared with the wild type. In the case of the
(b-Me)-tyrosine analogue shown in Figure 2 the 20 and 60 po-
sitions of the aromatic moiety were also methylated. Only
the lower energy trans conformation was highly populated
in this case. The second possible constraining effect of b-
methylation is inhibited by aryl ring rotation around the
Cb–Cg bond (c2 angle). This category of restricted rotation
will be discussed later in the context of our results.
ally defined secondary structure for catalyst 3 in comparison
to that adopted by catalyst 2.
In order to evaluate the potential generality of this approach,
we sought to explore strategic methyl group insertions into
catalysts for a particularly difficult reaction, the kinetic res-
olution of tertiary alcohols.11,12 Although the enzymatic ki-
netic resolution of secondary alcohols is a well-studied area,
there are far fewer examples of tertiary alcohol resolu-
tion.13,14 One likely reason for this lack of success is due
to the steric demand of the substrate.15 Among the literature
reports of biocatalytic resolution of tertiary alcohols, the
degree of enantiodiscrimination ranges from small to very
high, although cases of the latter are quite limited.16,17
Our own studies of tertiary alcohol kinetic resolution with
nonenzymatic peptide-based catalysts reflected these chal-
lenges as well. We recently reported catalyst 4,18 which pro-
vides the only example for a small-molecular catalyst for
kinetic resolution that directly resolves tertiary alcohols
through nonenzymatic asymmetric acylation.19 Catalyst 4
was identified by fluorescence-based screening and was suc-
cessfully utilized to resolve a highly specific class of acet-
amido-substituted tertiary alcohol substrates (Eq. 2). Six
substrates (5–10, Table 1) were resolved with krel values
In our earlier studies, we applied the b-substitution strategy
to the development of peptide-based catalysts for asymmet-
ric conjugate addition of an azide (Eq. 1).10 In these exper-
iments we found a subtle, but significant improvement in
the enantioselectivity (and overall activities) of the catalysts
that contained an appropriately configured substituent in the
histidine residue. As shown in Eq. 1, comparison of catalysts
2 and 3 showed that the b-methyl catalyst 3 outperformed
catalyst 2 in the enantioselective conjugate addition reac-
tions in each of the six substrate cases we explored. While
the precise basis for the ee and rate enhancements was not
fully determined; NMR studies of the two catalysts revealed
significant differences that supported a more conformation-
ꢁ
ranging from 9 to 22 at 4 C and from 19 to >50 at
ꢀ23 C.20 However, reactions were generally quite sluggish
ꢁ
in comparison with the related secondary alcohol substrates,
and krel values were lower when reactions were conducted at
room temperature. Substrates outside this class of tertiary al-
cohol were not found to undergo significant kinetic resolu-
tion with catalyst 4. We therefore set out to examine the
possible advantages of dihedral angle restriction in catalysts
for these challenging processes.
O
Catalytic Peptide (2.5 mol %)
TMSN3, t-BuCO2H, PhCH3
O
O
O
O
N3
N
R
N
R
(1)
O
O
N
tBu
tBu
N
H
Me
N
O
H
H
N
N
HN
N
N
O
H
H
O
O
Cy
BOCHN
BOCHN
HN
HN
NH
N
O
O
N
O
N
Boc
Me
Me
NH-Phe-OMe
4
Np
Me
Np
2
Ac2O, Et3N
10 mol % 4
N
Bn
N
Bn
3
Me OH
R
Me OH
R
Me OAc
NHAc
+
NHAc
NHAc (2)
R
CH2Cl2/Toluene
2-6 days
45 - 85% ee
(6 substrates)
71 - 92% ee
(6 substrates)

5256
M. C. Angione, S. J. Miller / Tetrahedron 62 (2006) 5254–5261
Table 1. Kinetic resolutions of tertiary alcohols with catalyst 4a
O
O
O
O
Me
N
a
N
N
O
Entry
1
(ꢂ)-Substrate
5, R¼Ph
Temp (ꢁC)
krel (% Conv.)b
O
N
79%
(4:1 dr)
N
N
b,
4
ꢀ23
4
ꢀ23
4
ꢀ23
4
ꢀ23
20 (47)
40 (37)
Ph
Ph
72%
Bn
Bn
11
12
O
Me
O
O
Me
22 (52)
>50 (48)
N
2
3
4
6, R¼p-Tol
N
c
MeO
N
O
d,e
78%
75%
N
N3
N
N3
15 (54)
32 (40)
Ph
Bn
7, R¼p-NO2–Phe
8, R¼b-Nap
Bn
14
13
O
Me
Me
N
O
Me
14 (47)
40 (35)
N
–
f
BF4
MeO
MeO
90%
Me
N
HN
NH
N
Me OH
g
9
BOC
16
BOC
15
Bn
Bn
NHAc
4
ꢀ23
20 (51)
39 (38)
5
Me
N
O
Me
Me
N
O
h
MeO
BOC
HO
BOC
18
80%
(2 steps)
NH
N
NH
N
6
10, R¼Cy
Conditions found in Ref. 18.
Calculated according to the method of Kagan.
4
ꢀ23
9 (51)
19 (35)
17
Scheme 1. (a) CH3MgBr, CuBr$DMS, THF, ꢀ40 ꢁC; (b) KHMDS, THF,
ꢀ50 ꢁC; then trisyl azide, ꢀ78 ꢁC; (c) NaOMe, MeOH/CH2Cl2, ꢀ10 ꢁC;
(d) H2, Pd/C, AcOH/MeOH; (e) Boc2O, CH2Cl2; (f) Me3OBF4, CH2Cl2,
25 ꢁC; (g) H2, 10% Pd/C, MeOH/AcOH; (h) LiOH, MeOH/H2O, 25 ꢁC;
H-ion exchange, NH4OH.
a
b
2. Results and discussion
Evaluation of b-methyl substitution in the context of tertiary
alcohol kinetic resolutions through asymmetric acylation re-
hydroxide on ion exchange resin afforded the desired Boc–
(2S,3S)–(b-Me)–(p-Me)–His–OH 18 (80%, two steps: hy-
drogenolysis/hydrolysis). Final purification of 18 was easily
carried out by preparative reverse phase HPLC to yield the
key amino acid derivative as a fine white powder.
~
quired a synthesis of (b-Me)–(p-Me)–His to be accom-
plished. Our earlier studies in azide conjugate addition
~
relied on the incorporation of (b-Me)–(t-Bn)–His into the
peptides. Our synthetic approach to this residue followed
Hruby’s initial synthetic procedure. In the context of the
p-methylated version as required, we opted to simply con-
vert (p-Me)–His to (b-Me)–(p-Me)–His.
The 1H NMR spectrum of amino acid derivative 18
(300 MHz, D2O) exhibited some evidence for restricted ro-
tation about the Cb–Cg bond. As shown in Figure 3, two
peaks of unequal height for the imidazole C2-proton (HA)
ꢁ
were observed at 25 C; the peaks could be observed to
ꢁ
O
O
O
coalesce reversibly upon heating the sample to 55 C. The
H2N
H2N
Me
H2N
Me
OH
OH
OH
N
existence of signal doubling for the proton corresponding
to HA on the imidazole ring may be consistent with restricted
rotation around the Cb–Cg bond (c2) of 18. The barrier to
interconversion was estimated to be 16 kcal/mol according
to the standard analysis of coalescence temperatures.24 Of
note, the temperature dependent 1H NMR spectra were
also observed for methyl ester derivative 17 when the
1H NMR spectrum was analyzed in D2O. However, when
compound 17 was examined in CD2Cl2, the effect was not
observed. While these observations are not fully understood
at this time, it appears that strong solute–solvent interactions
in protic versus aprotic media influence the dynamics of
compounds like 17 and 18. Of note, these observations of
temperature dependent NMR spectra and their relationship
to dihedral angle restriction are quite reminiscent of the ob-
servations of Hruby in the b-substituted tyrosine series.25
N
N
N
N
N
Bn
Me
Me
-Me)-( -Me)-His
-Bn)-( -Me)-His
-Me)-His
The synthesis began in analogy to the literature precedent for
Boc–(2S,3S)–(b-Me)–(t-Bn)–His–OH (Scheme 1).21,22
Starting from urocanic acid-derived imide 11, diastereose-
lective conjugate addition using CH3MgBr and CuBr$DMS
afforded 12 in a 4:1 mixture of the diastereomers with the
(S)-methyl configuration as the major product. The major di-
astereomer was carried on and subjected to diastereoselec-
tive azidation as described by Evans23 to afford azido
imide 13 with complete diastereocontrol. The chiral auxil-
iary was removed by methanolysis to yield the correspond-
ing methyl ester 14. Reduction of the azide 14 (H2, 10%
Pd/C, AcOH/MeOH) followed by Boc-protection afforded
Boc–(2S,3S)–(b-Me)–(t-Bn)–His–OMe 15.
In the case of (2S,3S)–(b-Me)–(p-Me)–His–OH (19), how-
ever, we also were able to show that the dynamic NMR
behavior was coupled to the Boc-protection state of the
backbone nitrogen. Deprotection under standard conditions
produced free amino acid 19 (Fig. 4). The 1H NMR spectrum
of the protecting group-free residue did not exhibit evidence
of restricted rotation about any torsional angles. Based on
these data, we speculate that either a remote steric effect,
or possibly a hydrogen bonding interaction is associated
with the Boc-group of 18, which is related to the dihedral
In order to prepare the desired p-methyl derivative, a proce-
dure was developed in which the p-nitrogen of 15 was alkyl-
ated using trimethyloxonium tetrafluoroborate to yield
imidazolium salt 16 (90%). Subsequent hydrogenolysis of
the benzyl group using 10% Pd/C under 1 atm of H2 for
24 h afforded 17; hydrolysis of the methyl ester (LiOH,
MeOH/H2O) and isolation by elution with ammonium

M. C. Angione, S. J. Miller / Tetrahedron 62 (2006) 5254–5261
5257
Figure 3. 1H NMR spectra (300 MHz, D2O) of amino acid derivative 18 recorded at 25 ꢁC (top) and 55 ꢁC (bottom).
angle restriction. This possibility is notable in the context of
peptide-based catalysts wherein residues like 19 are incorpo-
rated into peptide sequences where nonbonding interactions
within the catalyst structure may contribute strongly to the
overall structure and function.
20, an analog that contained this perturbation. We began
with a comparison of kinetic resolutions of tertiary alcohol
21 (Eq. 3), and the results are presented in Table 2.
A significant improvement in catalyst performance was
observed with catalyst 20 on comparison with 4. Not only
enantioselectivity (krel) was improved, but also performance
was excellent at the higher temperature, reaction times were
greatly diminished, and improved selectivity was sustained
at lower catalyst loadings. For example, catalyst 4 delivers
These structural effects do indeed translate to improved cat-
alysts for the kinetic resolution of tertiary alcohols. Initially,
we sought to examine the direct comparison of catalyst 4,
lacking the b-methyl group in the His-residue, to catalyst
Figure 4. 1H NMR spectrum (400 MHz, D2O) of acid 19.

5258
M. C. Angione, S. J. Miller / Tetrahedron 62 (2006) 5254–5261
Table 2. Kinetic resolutions of substrate 21 with catalysts 4 versus 20a
Table 3. Kinetic resolutions of tertiary alcohols with catalysts 4 versus 20a
Entry
Catalyst
Temp (ꢁC)
Time (h)
krel (% Conv.)b
Entry (ꢂ)-Substrate Catalyst Time (h) Temp (ꢁC) krel (% Conv.)b
22, R1 = Me
1
2
4 (10 mol %)
20 (10 mol %)
25
25
24
24
14 (29)
>50 (51)
1a
1b
4
20
52
52
4
4
9 (33)
>50 (53)
R2
=
3
4
4 (10 mol %)
20 (10 mol %)
4
4
24
24
21 (26)
>50 (50)
23, R1 = Me
5
6
20 (10 mol %)
20 (2.5 mol %)
25
25
12
12
>50 (46)
49 (27)
2a
2b
4
20
20
20
25
25
10 (39)
24 (60)
R2
=
a
Conditions found in Section 4.
Calculated according to the method of Kagan.
O2N
b
24, R1 = Me
R2
3a
3b
4
20
20
20
25
25
3 (32)
18 (65)
=
a krel of 14 at 29% conversion when the reaction is conducted
ꢁ
at 25 C (Table 2, entry 1), whereas catalyst 20 results in
Ph
a krel of >50 at 51% conversion under identical conditions
(entry 2). Furthermore, while peptide 4 may be induced to
deliver a slight boost in selectivity at lower temperature
4a
4b
25, R1¼CO2Me
R2¼Ph
4
20
18
18
25
25
1.6 (21)
2.8 (43)
ꢁ
(krel¼21, 4 C, entry 3), catalyst 20 performs in a nearly
a
Conditions found in Section 4.
Calculated according to the method of Kagan.
identical fashion within this moderate temperature range
(krel>50, 4 C, entry 4). Because of the apparent increase
b
ꢁ
in activity and selectivity with catalyst 20, we examined
its performance within shorter reaction times and at lower
catalyst loading. Notably, within 12 h, the kinetic resolution
of 21 may be performed such that a krel of >50 at 46% con-
version may be achieved (entry 5). In addition, when the cat-
alyst loading is reduced to 2.5 mol %, the reaction may be
performed such that krel of 49 is observed, but at 27% conver-
sion within 12 h (entry 6).26
Substrate 24 (entry 3a–b) provides another case of aryl group
replacement with an aliphatic chain (Ph, 21 to C2H4Ph, 24).
Once again, catalyst 20 provides a substantial improvement
(krel¼3, catalyst 4; krel¼18, catalyst 20), and once again
overall activity is improved as conversion increases from
32 to 65% within a 20 h window. Finally, to illustrate that
catalyst 20 is by no means a universal solution to the chal-
lenging substrate class of tertiary alcohol resolution, we
note that substrate 25 (entry 4a–b) exhibits low selectivity,
and only a small improvement in krel was observed when cat-
alyst 4 is exchanged for 20 (krel¼1.6, catalyst 4; krel¼2.8,
catalyst 20).
O
O
N
H
N
H
N
N
Me
N
O
Me
Me
N
O
HN
HN
O
O
Cy
Cy
NH
NH
N
N
O
O
Boc
Boc
NH-Phe-OMe
NH-Phe-OMe
3. Conclusions
20
4
In combination with our earlier results in the area of peptide-
catalyzed asymmetric conjugate addition of azide, the re-
sults presented above constitute a second example wherein
the rational incorporation of a b-branch into a catalytically
significant histidine residue results in an improvement in
the overall stereoselectivity and activity exhibited by a pep-
tide-based catalyst. The exact mechanistic basis of the rate
and selectivity enhancements for catalysts remains a topic
of investigation. In combination with precedents in the liter-
ature, and our own observations of solution conformational
behavior, it may be noted that the dihedral angle restriction
is indeed at the heart of the improvements in catalysis we
have observed. If these effects are in fact manifested in con-
formational features, it may be noted that we have leveraged
in some way nature’s evolution of b-branched amino acids
as complements of those that are simply ‘–CH2–’ in the b-
position.
Ac2O, Et3N
2.5 to 10 mol %
Catalyst
Me OH
Me OAc
NHAc
Me OH
NHAc
+
NHAc
(3)
Ph
Ph
Ph
CH2Cl2/Toluene
21
Given the marked improvement of catalyst 20 in comparison
with 4, we sought to examine if these effects would translate
to other substrates (Eq. 4). As shown in Table 3, the perfor-
mance enhancements indeed translated to a range of cases,
although they were not universally achieved. Compound
22 (entry 1a–b), bearing a replacement of the phenyl ring
of 21 with a cyclohexyl group, presents a particularly inter-
esting case. Whereas a sluggish reaction occurs with catalyst
4 to give modest stereoselectivity (krel¼9, 33% conv.), cata-
lyst 20 induces a substantial rate enhancement, with a
dramatic improvement in stereoselectivity (krel>50). Com-
pound 23 (entry 2a–b) also undergoes improved kinetic
resolution with catalyst 20, although the selectivity enhance-
ment is less pronounced (krel¼10, catalyst 4; krel¼24, cata-
lyst 20).
4. Experimental
4.1. General procedures
Ac2O, Et3N
10 mol %
Catalyst
Proton NMR spectra were recorded on Varian 400 or 300
spectrometers. Proton chemical shifts are reported in parts
per million (d) relative to an internal tetramethylsilane
(TMS, 0.00 d) or with the solvent reference relative to
R1
OH
R1
R1
OH
OAc
NHAc
NHAc
+
NHAc
(4)
R2
CH2Cl2/Toluene R2
R2
22-25

M. C. Angione, S. J. Miller / Tetrahedron 62 (2006) 5254–5261
5259
TMS employed as the internal standard (CDCl3, d 7.26 ppm;
D2O, d 4.79 ppm). Data are reported as follows: chemical
shift (multiplicity [singlet (s), doublet (d), triplet (t), quartet
(q), broad doublet (br d), broad singlet (br s), and multiplet
(m)], coupling constants [Hz], integration). Carbon NMR
spectra were recorded on Varian 400 (100 MHz), or 300
(75 MHz) spectrometers with complete proton decoupling.
Carbon chemical shifts are reported in parts per million (d)
relative to TMS with the respective solvent resonance as
the internal standard (CDCl3, d 77.0;ꢁ D2O/methanol,
d 49.15). NMR data were collected at 25 C, unless other-
wise indicated. Infrared spectra were obtained on a Nicolet
210 spectrometer. Diagnostic bands are reported (cmꢀ1).
Analytical thin-layer chromatography (TLC) was performed
using Silica Gel 60 F254 pre-coated plates (0.25 mm thick-
ness). TLC Rf values are reported. Visualization was accom-
plished by irradiation with a UV lamp and/or staining with
cerium ammonium molybdenate (CAM) or KMnO4 solu-
tions. Silica gel chromatography was performed using Silica
Gel 60A (32–63 mm) from Scientific Adsorbants, Inc. Opti-
cal rotations were performed on a Rudolf Research Analyt-
ical IV Automatic polaꢁrimeter at the sodium D line (path
length 50 mm) at 20 C. High resolution mass spectra
were obtained at the Mass Spectrometry Facilities at Boston
College (Chestnut Hill, MA, USA). The method of ionization
is given in parentheses. Analytical GC was performed on
a Hewlett–Packard 6890 employing a flame ionization detec-
tor and the column was specified in the individual experi-
ment. Analytical and preparative reverse phase HPLC were
performed on a Rainin SD-200 chromatograph equipped
with a single wavelength UV detector (214 nm). Analytical
normal-phase, chiral HPLC was performed on a Hewlett–
Packard 1100 or Agilent 1100 Series chromatograph equip-
ped with a diode array detector. (The wavelengths of 214
and 254 nm were used for compound detection.)
(s, 1H), 7.38 (m, 5H), 7.27 (s, 1H), 6.98 (br s, 1H), 5.27 (m,
3H), 4.53 (m, 1H), 3.92 (s, 3H), 3.76 (s, 3H), 3.47 (dd,
J¼3.9, 6.8 Hz, 1H), 1.29 (s, 9H), 1.21 (d, J¼7.1 Hz, 3H);
13C NMR (CDCl3, 100 MHz) d 170.3, 155.5, 137.0, 136.8,
132.8, 129.7, 129.2, 119.4, 80.9, 55.9, 53.8, 53.4, 34.3,
32.8, 28.5, 14.2; IR (film, cmꢀ1) 3376, 3144, 3093, 2974,
1740, 1715, 1161, 1061, 746; TLC Rf 0.24 (ethyl acetate);
[a]D +30.7 (c 2.0, CH2Cl2); exact mass calcd for
[C21H30N3O4]+ requires m/z 388.2236. Found 388.2239.
(ESI+).
4.1.2. Preparation of 17. Compound 16 (115 mg,
0.24 mmol) was dissolved in 5 mL of methanol and 200 mL
of acetic acid. Then the flask was flushed with N2 and after
5 min 10% Pd/C was carefully added. The flask was evacu-
ated and purged with H2 two times before being left to stir un-
der 1 atm of H2. After 24 h the reaction mixture was filtered
through Celite, washed with methanol, and the solvent was
evaporated. Crude product was purified using a reverse phase
HPLC [RP-18 X Terra (Waters) column], eluting with 20%
methanol/H2O at a flow rate of 4 mL/min, to give a yellow
oil. The purified oil was then taken up in CH2Cl2 and washed
twice with saturated NaHCO3. The organic layer was dried,
filtered, and evaporated to give 17. (83 mg, 0.22 mmol,
1
90% yield). H NMR (CDCl3, 400 MHz) d 7.38 (s, 1H),
6.80 (s, 1H), 5.24 (d, J¼8.8 Hz, 1H), 4.56 (dd, J¼9.0,
4.6 Hz, 1H), 3.76 (s, 3H), 3.66 (s, 3H), 3.38 (m, 1H), 1.36
(s, 9H), 1.27 (d, J¼3.7 Hz, 3H); 13C NMR (CDCl3,
100 MHz) d 171.2, 155.0, 138.0, 131.6, 127.1, 80.1, 56.7,
52.6, 32.6, 31.6, 28.4, 15.1; IR (film, cmꢀ1) 3371, 3207,
2974, 1753, 1709, 1507, 1369, 1161; TLC Rf 0.33 (1% meth-
anol/CH2Cl2); [a]D +30.7 (c 2.0, CH2Cl2); exact mass calcd
for [C14H23N3O4+H]+ requires m/z 298.1767. Found
298.1769. (ESI+).
4.1.3. Preparation of 18. Compound 17 (85 mg,
0.22 mmol) was dissolved in 3 mL of methanol and 1 mL
of distilled H2O. Lithium hydroxide monohydrate
(LiOH$H2O) was added (36 mg, 0.86 mmol) and the reac-
tion was stirred for 3 h. The solvent was evaporated, and
the crude product was then taken up in 10% NH4OH and
eluted with the same solvent through a column of Dowex-
500 hydrogen ion exchange resin. The solvent was then
evaporated to give a crude off-white powder. The crude ma-
terial was purified using a reverse phase RP-18 X Terra (Wa-
ters) column, eluting with 5% methanol/H2O at a flow rate of
4 mL/min to give 18 as a white powder (55 mg, 0.19 mmol,
All solvents, triethylamine (Et3N), and diisopropylethyl-
amine (DIPEA) were distilled from appropriate drying
agents prior to use. Acetic anhydride was also distilled over
an appropriate drying agent prior to use and stored in
a Schlenk tube over molecular sieves. Amino acid derivatives
(unless otherwise specified) and coupling reagents (EDC,
HOBt) were used as received from NovaBiochem or
Advanced ChemTech. Trimethyloxonium tetrafluoroborate
(Meerwein salt, Me3O+ BF4ꢀ) was used as received from Al-
drich Chemical Company. Palladium on carbon (10% Pd/C)
was purchased from Strem Chemical Company and used as
received.
1
ꢁ
88%). H NMR (D2O, 55 C, 300 MHz) d 8.88 (br s, 1H),
7.59 (br s, 1H), 4.58 (d, J¼3.60 Hz, 1H), 4.26 (s, 1H),
3.94 (m, 1H), 1.63 (br s, 9H), 1.55 (d, J¼7.20 Hz, 3H);
13C NMR (D2O, 100 MHz) d 175.6, 156.8, 136.7, 134.7,
Racemic substrates 21–23 and 25 were prepared from the
corresponding epoxides.27 Substrate 24 was prepared
according to the method of Simona.28 The following com-
pounds have been also previously prepared in the literature:
4, 11–15,10a 21–23, and 25.18
117.1, 81.0, 56.8, 33.4, 32.3, 27.7, 12.6; IR (film, cmꢀ1
)
3320, 2974, 1690, 1614, 1400, 1161, 1029; TLC Rf 0.13
(15% methanol/CH2Cl2); [a]D +8.1 (c 1.0, methanol); exact
mass calcd for [C13H21N3O4+H]+ requires m/z 284.1610.
Found 284.1605. (ESI+).
4.1.1. Preparation of 16. Boc–(2S,3S)–(b-Me)–(t-Bn)–
His–OMe (15, 690 mg, 1.90 mmol) was dissolved in methy-
lene chloride and then trimethyloxonium tetrafluoroborate
(410 mg, 2.80 mmolꢁ) was added. The reaction mixture was
stirred for 1 h at 25 C and then quenched by washing with
2ꢃ15 mL of distilled water. The organic layer was dried, fil-
tered, and evaporated to give 16 as a white solid (710 mg,
4.1.4. Preparation of 19. Compound 18 was dissolved in
1 mL of trifluoroacetic acid (TFA) and 1 mL of CH2Cl2.
The reaction was stirred for 40 min. The solvent was evapo-
rated, and the crude oil was redissolved in toluene, and sub-
sequently concentrated three times, under conditions of
azeotrope to remove residual water. 1H NMR (D2O,
1
1.5 mmol, 80% yield). H NMR (CDCl3, 300 MHz) d 8.86

5260
M. C. Angione, S. J. Miller / Tetrahedron 62 (2006) 5254–5261
400 MHz) d 8.33 (s, 1H), 7.26 (s, 1H), 3.89 (d, J¼4.8 Hz,
1H), 3.80 (s, 3H), 3.69 (m, 1H), 1.36 (d, J¼7.2 Hz, 3H).
tions with literature reports.18 The absolute configurations of
the major and minor enantiomers of the other substrates
were not determined, but are rather drawn in analogy to
the observed fast and slow reacting enantiomers of substrate
21 and 22 with catalysts 4 and 20.
4.1.5. Data for purified peptide 20. 1H NMR (CDCl3,
400 MHz) d 7.32–7.20 (overlapping of s and m, 5H), 7.16
(br d, J¼6.8 Hz, 2H), 6.89 (s, 1H), 6.65 (br d, J¼8.4 Hz,
1H), 6.44 (m, 1H), 6.29 (br s, 1H), 4.92 (m, 1H), 4.65 (t,
J¼10.2 Hz, 1H), 4.30 (m, 1H), 4.16 (m, 1H), 4.03 (m,
1H), 3.75 (m, 1H), 3.66 (s, 3H), 3.62 (s, 3H), 3.31 (m,
1H), 3.17 (dd, J¼14.0, 5.6 Hz, 1H), 3.08 (dd, J¼14.0,
6.2 Hz, 1H), 2.18–1.64 (m, 11), 1.45 (s, 9H), 1.33 (d,
J¼7.2 Hz, 3H), 1.29–1.09 (m, 10H), 1.00–0.81 (m, 4H);
TLC Rf 0.35 (4% methanol/CH2Cl2); exact mass calcd for
[C43H63N7O8+H]+ requires m/z 806.4816. Found 806.4821
(ESI+).
Acknowledgment
This research is supported by National Institutes of Health
(NIGMS-68649). We also wish to thank Elizabeth Colby
Davie and Steven M. Mennen for critical comments during
the course of this project.
References and notes
4.2. Standard conditions for kinetic resolution
1. Breslow, R. Chem. Biol. 1998, 5, R27–R28.
2. Artificial Enzymes; Breslow, R., Ed.; Wiley: Weinheim, 2005.
3. Miller, S. J. Acc. Chem. Res. 2004, 37, 601–610.
4. Fierman, M. B.; O’Leary, D. J.; Steinmetz, W. E.; Miller, S. J.
J. Am. Chem. Soc. 2004, 126, 6967–6971.
5. For representative reviews, see: (a) Degrado, W. F. Chem. Rev.
2001, 101, 3025–3026 (Thematic Issue); (b) Venkatraman, J.;
Shankaramma, S. C.; Balaram, P. Chem. Rev. 2001, 101,
3131–3152; (c) Baltzer, L.; Nilsson, H.; Nilsson, J. Chem.
Rev. 2001, 101, 3153–3164; (d) Gellman, S. H. Curr. Opin.
Chem. Biol. 1998, 2, 717–725.
A stock solution of racemic alcohol (0.015 M) was made by
dissolving the alcohol in 2:3 CH2Cl2/toluene. The stock so-
lution was distributed in 1 mL aliquots to reaction vessels,
and then 20 mL of a stock solution of peptide catalyst
(0.0015 mmol in CH2Cl2) was introduced. Acetic anhydride
(70.0 mL, 0.74 mmol) and triethylamine (42 mL, 0.30 mmol)
were then introduced. The reaction mixture was allowed to
ꢁ
stir at 25 or 4 C. Then the reaction mixture was quenched
with methanol, and the solvent and amine were removed in
vacuo. The peptide catalyst was removed by running the res-
idue through a 1 cm silica plug in a Pasteur pipette, eluting
with ethyl acetate. Solvent was again removed. The residue
was dissolved in 10% 2-propanol in hexane and assayed
by chiral HPLC analysis or dissolved in ethyl acetate and
assayed by chiral GC analysis as previously described18
(substrates 21–23, and 25) or as described below.
6. For example, see: (a) Haque, T. S.; Little, J. C.; Gellman, S. H.
J. Am. Chem. Soc. 1996, 118, 6975–6985; (b) Karle, I. L.;
Awasthi, S. K.; Balaram, P. Proc. Natl. Acad. Sci. U.S.A.
1996, 93, 8189–8193.
7. Hruby, V. J. Life Sci. 1982, 31, 189–199.
8. Hruby, V. J. Acc. Chem. Res. 2001, 34, 389–397.
9. (a) Kover, K. E.; Jiao, D.; Fang, S.; Hruby, V. J. J. Org. Chem.
1994, 59, 991–998; (b) Qian, X.; Shenderovich, M. D.; Kover,
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Porreca, F.; Hruby, V. J. J. Am. Chem. Soc. 1996, 118, 7280–
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10. (a) Guerin, D. J.; Miller, S. J. J. Am. Chem. Soc. 2002, 124,
2134–2136; (b) Horstmann, T. E.; Guerin, D. J.; Miller, S. J.
Angew. Chem., Int. Ed. 2000, 39, 3635–3638.
11. For reviews of nonenzymatic kinetic resolution, see: (a) Vedejs,
E.; Jure, M. Angew. Chem., Int. Ed. 2005, 44, 3974–4001; (b)
Keith, J. M.; Larrow, J. F.; Jacobsen, E. N. Adv. Synth. Catal.
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12. For reviews of asymmetric acylation, see: (a) Spivey, A. C.;
Maddaford, A.; Redgrave, A. J. Org. Prep. Proced. Int. 2000,
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Berlin, 2004; Chapter 43.
1
4.2.1. Data for substrate 24. H NMR (CDCl3, 400 MHz)
d 7.31–7.25 (m, 2H), 7.19–7.16 (m, 3H), 6.26 (br s, 1H),
3.31 (dd, J¼15.0, 5.9 Hz, 1H), 3.28 (dd, J¼13.2, 5.9 Hz,
1H), 3.15 (s, 1H), 2.70 (m, 2H), 2.00 (s, 3H), 1.84–1.70
(m, 2H), 1.23 (s, 3H); IR (film, cmꢀ1) 3452, 3050, 2987,
1671, 1426, 1269, 730; TLC Rf 0.15 (60% ethyl acetate/
hexane); exact mass calcd for [C13H19NO2+Na]+ requires
m/z 244.1313. Found 244.1319 (ESI+).
4.2.2. Data for acylation product 24–Ac. 1H NMR (CDCl3,
400 MHz) d 7.26–7.16 (m, 5H), 6.33 (br s, 1H), 3.68 (dd,
J¼14.65, 6.59 Hz, 1H), 3.58 (dd, J¼14.65, 5.62 Hz, 1H),
2.74–2.57 (m, 2H), 2.23–2.13 (m, 1H), 2.00–1.93 (m, 1H),
2.02 (s, 3H), 1.46 (s, 3H); IR (film, cmꢀ1) 3295, 2924,
1734, 1652, 1558; TLC Rf 0.44 (60% ethyl acetate/hexane);
exact mass calcd for [C13H21NO3+Na]+ requires m/z
286.1419. Found 286.1418 (ESI+).
13. For examples of the application of peptide-based catalysts to
the kinetic resolution of secondary alcohols, see: (a) Miller,
S. J.; Copeland, G. T.; Papaioannou, N.; Horstmann, T. E.;
Ruel, E. M. J. Am. Chem. Soc. 1998, 120, 1629–1630; (b)
Jarvo, E. R.; Copeland, G. T.; Papaioannou, N.; Bonitatebus,
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2001, 123, 6496–6502; (d) Jarvo, E. R.; Evans, C. A.; Cope-
land, G. T.; Miller, S. J. J. Org. Chem. 2001, 66, 5522–5527.
14. For examples of the application of peptide-based catalysts to
the kinetic resolution of primary alcohols, see: Lewis, C. A.;
4.2.3. Assay of enantiomeric purity for 24 and 24–Ac. En-
antiomers of the starting material 24 and the enantiomers of
the corresponding acylated product (24–Ac) were separated
by chiral HPLC using a chiral OD column (Alltech) by elut-
ing with 2% ethanol/hexane at a flow rate of 0.75 mL/min for
90 min. Retention times: 24: 67 and 86 min, 24–Ac: 43 and
51 min.
The absolute configurations of 21, 21–Ac, 22, and 22–Ac
were determined by correlation of the observed optical rota-

M. C. Angione, S. J. Miller / Tetrahedron 62 (2006) 5254–5261
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