A concise synthesis of (2S,3S)-BocAHPBA and (R)-BocDMTA, chiral building blocks for peptide-mimetic HIV protease inhibitors

Article abstract of DOI:10.1016/S0957-4166(02)00257-4

Scalable syntheses of (2S,3S)-3-N-tert-butoxycarbonylamino-2-hydroxy-4-phenylbutanoic acid (BocAHPBA) 1 and (R)-3-tert-butoxycarbonyl-5,5-dimethyl-1,3-thiazolidine-4-carboxylic acid (BocDMTA) 2 have been developed. Both 1 and 2 can serve as chiral building blocks in assembling JE-2147 (KNI-764) 3, a potent HIV protease inhibitor. The synthesis of (2S,3S)-BocAHPBA 1 is achieved in 41% overall yield from (S)-2-N,N-dibenzylamino-3-phenylpropanal 4 in five steps where Tamao's reagent [Me₂(i-PrO)SiCH₂MgCl] is employed for a one-carbon homologation, and Zhao's oxidation protocol (TEMPO, NaClO₂, NaClO) is applied to convert a 1,2-glycol moiety into an α-hydroxy acid motif. (R)-BocDMTA 2 is synthesized with 99.4% ee in 24% yield via enantioselective hydrolysis of methyl (±)-5,5-dimethyl-1,3-thiazolidine-4-carboxylate 8b by a Klebsiella oxytoca hydrolase; the unreacted (S)-ester 8b can be recovered and racemized with NaOMe to afford (±)-8b in 46% yield for another round of the enzymatic processing.

Full text of DOI:10.1016/S0957-4166(02)00257-4

TETRAHEDRON:
ASYMMETRY
Pergamon
Tetrahedron: Asymmetry 13 (2002) 1201–1208
A concise synthesis of (2S,3S)-BocAHPBA and (R)-BocDMTA,
chiral building blocks for peptide-mimetic HIV protease
inhibitors
Masaya Ikunaka,* Jun Matsumoto and Yukifumi Nishimoto
Research and Development Center, Nagase & Co., Ltd., 2-2-3, Murotani, Nishi-ku, Kobe 651-2241, Japan
Received 7 May 2002; accepted 23 May 2002
Abstract—Scalable syntheses of (2S,3S)-3-N-tert-butoxycarbonylamino-2-hydroxy-4-phenylbutanoic acid (BocAHPBA) 1 and
(R)-3-tert-butoxycarbonyl-5,5-dimethyl-1,3-thiazolidine-4-carboxylic acid (BocDMTA) 2 have been developed. Both 1 and 2 can
serve as chiral building blocks in assembling JE-2147 (KNI-764) 3, a potent HIV protease inhibitor. The synthesis of
(2S,3S)-BocAHPBA 1 is achieved in 41% overall yield from (S)-2-N,N-dibenzylamino-3-phenylpropanal 4 in five steps where
Tamao’s reagent [Me2(i-PrO)SiCH2MgCl] is employed for a one-carbon homologation, and Zhao’s oxidation protocol (TEMPO,
NaClO2, NaClO) is applied to convert a 1,2-glycol moiety into an a-hydroxy acid motif. (R)-BocDMTA 2 is synthesized with
99.4% ee in 24% yield via enantioselective hydrolysis of methyl ( )-5,5-dimethyl-1,3-thiazolidine-4-carboxylate 8b by a Klebsiella
oxytoca hydrolase; the unreacted (S)-ester 8b can be recovered and racemized with NaOMe to afford ( )-8b in 46% yield for
another round of the enzymatic processing. © 2002 Elsevier Science Ltd. All rights reserved.
peptide analogs that mimic the transition-state for the
protease cleaving the gag and gag/pol gene products. In
the present communication we will discuss chiral pool
synthesis of (2S,3S)-BocAHPBA 1 commencing from
(S)-2-N,N-dibenzylamino-3-phenylpropanal 4 (Scheme
1) and chemoenzymatic synthesis of (R)-BocDMTA 2
featuring kinetic resolution of methyl ( )-5,5-dimethyl-
1,3-thiazolidine-4-carboxylate 8b catalyzed by a ther-
mally stable hydrolase of the Klebsiella oxytoca origin
(Scheme 2).
1. Introduction
(2S,3S)-3-N-tert-Butoxycarbonylamino-2-hydroxy-4-
phenylbutanoic acid (Boc AHPBA) 1 and (R)-3-tert-
butoxycarbonyl-5,5-dimethyl-1,3-thiazolidine-4-carbox-
ylic acid (BocDMTA) 2 are chiral building blocks
which are to be used for assembling potent HIV
protease inhibitors, such as JE-2147 (KNI-764) 3 (Fig.
1);1 these protease inhibitors have evolved from inten-
sive structure–activity relationship studies on truncated
OH
CO2H
S
CO2H
N
NHBoc
Boc
2
1
S
OH
N
NH
O
N
H
HO
O
O
3
Figure 1. Structures of (2S,3S)-BocAHPBA 1, (R)-BocDMTA 2, and JE-2147 3.
* Corresponding author. Tel.: +81 78 992 3163; fax: +81 78 992 1050; e-mail: masaya.ikunaka@nagase.co.jp
0957-4166/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0957-4166(02)00257-4

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M. Ikunaka et al. / Tetrahedron: Asymmetry 13 (2002) 1201–1208
OPri
Si
MgCl
THF
OH
Ph OH
H2O2, NaHCO3
MeOH/THF (1:1)
Ph
OH
CHO
NBn2
Ph
NBn2
Bn2N
Si
OPri
4
6a
5
TEMPO, NaClO2, NaOCl
MeCN, Na2HPO4 aq
1. H2, Pd/C, EtOH
2. Boc2O, MeOH
OH
OH
OH
Ph
Ph
CO2H
NHR
NHBoc
6b R = H
6c R = Boc
1
(41% from 4)
HO
Ph
CO2H
CF3CO2H
CHCl3
NH3+-O2CCF3
1'
Scheme 1. Synthesis of (2S,3S)-BocAHPBA 1.
CO2Me
Boc
S
N
CO2H
HS
Me3SiCHN2
MTBE
NH2
(R)-2'
(±)-7
1. CH2O, EtOH (96%)
2. HCl, MeOH (84%)
(Boc)2O
i-PrOH
Aqueous layer
CO2H
CO2H
S
S
N
NH
Boc
K. oxytoca
hydrolase
(R)-9
CO2R
S
(R)-2
NH
[24% from (±)-8b]
(±)-8a R = H
(±)-8b R = Me
CO2Me
S
NH
MTBE layer
(S)-8b
NaOMe
PhMe
[46% from fresh (±)-8b]
Scheme 2. Synthesis of (R)-BocDMTA 2.
2. Results and discussion
challenge to the community of synthetic chemists, a few
of whom have succeeded in developing enantioselective
approaches: For example, using his unique hetero-
bimetallic asymmetric catalyst, La-Li-(R)-Binol,
Shibasaki achieved excellent diastereoselectivity in
adding nitromethane to (S)-2-N-phthaloylamino-3-
phenypropanal.2 Without resorting to any external chi-
2.1. Chiral pool synthesis of (2S,3S)-BocAHPBA 1
2.1.1. Synthetic plan. Its b-amino-a-hydroxy acid motif
being disposed as erythro configuration, synthesis of
(2S,3S)-BocAHPBA 1 has presented an interesting

M. Ikunaka et al. / Tetrahedron: Asymmetry 13 (2002) 1201–1208
1203
ral controller, Terashima also achieved relatively high
diastereoselectivity in transferring a cyanide group from
acetone cyanohydrin [Me2C(OH)CN] to (S)-2-N,N-
dibenzylamino-3-phenylpropanal 4.3
Having served its role in controlling the stereochemistry
of the reaction with Tamao’s reagent, the N,N-dibenzyl
group of 6a was supposed to be exchanged for an
N-Boc group, such that product isolation should be
effected more easily in the later stage(s) of the synthesis.
Catalytic hydrogenolysis (H2, Pd/C) of the crude 6a
proceeded uneventfully to give the completely depro-
tected amino diol 6b, which, because of its increased
polarity and high crystallinity, could be isolated free
from silicon-derived side products without recourse to
chromatography; 6b was obtained in 56% overall yield
from 4 in three steps, its erythro-(2S,3S) configuration
However, each method has drawbacks from a practical
perspective: (1) The use of the employed nucleophiles is
undesirable,
with
nitromethane2
and
acetone
cyanohydrin3 being highly flammable and toxic, respec-
tively; (2) each reaction takes as long as 72 h to go to
completion;2,3 (3) cryogenic conditions are needed for
both Shibasaki’s nitroaldol reaction2 (−40°C) and
Terashima’s cyanohydrin formation3 (−15°C); (4)
Terashima’s cyano group transfer reaction requires a
halogenated solvent;3 and (5) Shibasaki’s and Terashi-
ma’s recipes both end up requiring harsh, strongly
acidic conditions to build a carboxylic acid functional-
ity from the nitro and cyano groups.
1
being corroborated by the H NMR spectrum that was
superimposed on that recorded by Moyano et al.7b The
purified 6b was then treated with (Boc)2O in methanol
to protect its primary amine function as the carbamate
6c.
2.1.3. Oxidation of 1,2-glycol to a-hydroxy acid. To
oxidize the 1,2-glycol functionality of 6c regioselectively
to the a-hydroxy carboxylic acid in a single operation,
we opted to apply Zhao’s procedures8 with reference to
Wovkulich’s precedent.9 When crude diol 6c was
treated with NaOCl and NaClO2 in the presence of
catalytic 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO),
regioselective oxidation took place and, albeit slug-
gishly, reached completion. Finally, single recrystalliza-
tion furnished (2S,3S)-BocAHPBA 1 in a two-step
overall yield of 74% from 6b and a five-step overall
yield of 41% from 4. Its stereochemical integrity was
confirmed unequivocally by comparing the physico-
chemical data with those recorded in the technical
report10 and by converting it to the trifluoroacetic acid
salt of (2S,3S)-3-amino-2-hydroxy-4-phenylbutanoic
To address these issues, we explored another combina-
tion of nitrogen protection on (S)-2-amino-3-phenyl-
propanal and
a functionalized C(1) nucleophile.
Experimental investigation along with a literature
search has led us to devise a new pragmatic methodol-
ogy capitalizing on Tamao’s silicon reagent, isopro-
poxydimethylsilylmethylmagnesium
chloride,4
in
combination with Reetz’s nitrogen protection tactics
featuring an N,N-dibenzyl group5 (Scheme 1).
2.1.2. Diastereoselective installation of a hydroxymethyl
group.
When
(S)-2-N,N-dibenzylamino-3-phenyl-
propanal 4, prepared in quantity according to Ng’s
procedures,6 was treated with isopropoxydimethylsilyl-
methylmagnesium chloride in THF at 0°C, nucleophilic
addition proceeded smoothly providing b-hydroxysilane
5, which, without purification, was subjected to oxida-
tive cleavage of the SiꢀC bond by basic hydrogen
peroxide4b to give 3-amino-1,2-diol 6a as a single
1
acid 1%, H NMR analysis of which showed the same
spectrum as that recorded by Wong et al. to demon-
strate the absence of the threo isomer.11
2.2. Chemoenzymatic synthesis of (R)-BocDMTA 2
1
stereoisomer. Its H NMR spectrum was the same as
that reported by Izawa et al.7a demonstrating that
Tamao’s reagent, under the influence of the N,N-diben-
zyl group (nonchelation control based on the Felkin–
Anh model5a), successfully established the desired
erythro configuration.
2.2.1. Synthetic plan. Naive retrosynthetic analysis of
(R)-BocDMTA 2 would result in (R)-penicillamine 7 as
a deceptive starting material, because, being an unnatu-
ral form of penicillamine, (R)-7 is available less abun-
dantly; additionally, (R)-penicillamine 7 is notorious as
a causative agent for optic atrophy leading to blind-
ness.12 In contrast, besides having played a key role in
Sheehan’s first total synthesis of penicillin V,13 (S)-peni-
cillamine 7 has been used as a chelating agent to
remove heavy metals, such as lead, copper, and mer-
cury, from the body.12 Hence, to meet such medicinal
need, scalable synthesis of (S)-7 had been established
It is noteworthy that complete erythro-selectivity was
seen in the nucleophilic addition of Tamao’s reagent to
4 at temperatures as high as 0°C. Indeed, 4 was shown
to undergo nucleophilic addition of Grignard reagents
under nonchelation control with high diastereoselectiv-
ity; however, the reported diastereoselectivity was less
than complete, ranging from 92:8 (MeMgI) to 97:3
(PhMgBr) each in favor of the erythro-isomers.5 Thus,
in the present case, it seems probable that the silyl
group [Si(OPri)Me2] in Tamao’s reagent exerted its
steric bulk to augment the intrinsic propensity for 4 to
undergo nonchelation-controlled addition. In addition,
its branching being situated b to the nucleophilic center,
Tamao’s reagent could still complex with the nitrogen
atom of 4 via the magnesium center to increase the
passive volume of the amine functionality such that
nonchelation control is further augmented.5a
where
( )-3-formyl-2,2,5,5-tetramethylthiazolidine-4-
carboxylic acid, which, in turn, was assembled from
( )-7, was resolved via diastereomeric salt formation
into its (S)-isomer and the latter was hydrolyzed to
(S)-7 itself. For the chiral separation process the
following resolving agents were employed: brucine
(a natural alkaloid),13 (+)-(1S,2S)-pseudonorephedrine
(an isomer of phenypropanolamine, used to alleviate
cold symptoms),14a (1S,2S)-threo-2-amino-1-(4-nitro-
phenyl)-1,3-propanediol (an industrial precursor to
chloramphenicol, an antibiotic produced by Strepto-

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M. Ikunaka et al. / Tetrahedron: Asymmetry 13 (2002) 1201–1208
mycesvenezuelae),14b and (S)-lysine (a natural amino
acid).14c However, because each antipode of the resolv-
ing agents is not easily obtainable commercially, neither
approach for the resolution seemed applicable for a
scalable synthesis of (R)-penicillamine 7,14b and hence
(R)-2.15
matic hydrolysis, such as substrate concentrations,
enzyme usage, reaction time, and temperatures, to max-
imize the space-time yield (Table 2). The K. oxytoca
hydrolase being able to function at temperatures as
high as 70°C, we took advantage of its thermal stability
to increase the volume efficiency with ( )-8b. In the
event, the substrate concentration {[( )-8b]} could be
increased to 3.0 M (526 g/L) without compromising the
resolution efficiency (E20=145), when the enzymatic
hydrolysis was run at 60°C in the presence of the K.
oxytoca hydrolase at a concentration of 0.3%; the (R)-
acid 8a was generated with 98% ee after 24 h when 35%
conversion was reached.
Thus, as part of our program to develop scalable
chemoenzymatic processes, we opted to explore the
kinetic resolution of methyl ( )-5,5-dimethyl-1,3-thiazo-
lidine-4-carboxylate 8b with
a hydrolytic enzyme
(Scheme 2), taking into account that the synthesis
should commence with the least expensive commercially
available ( )-713,16,17 and the fact that to the best of our
knowledge, no chemoenzymatic approach had been
used to access either enantiomerically enriched (R)-8a
or (R)-2.
2.2.3. Telescoping the enzymatic hydrolysis to incorpo-
rate carbamate formation. Being an amphoteric a-amino
acid, the hydrolyzed (R)-acid 8a was too difficult to
isolate from the aqueous medium. Thus, it should be
more tactically beneficial to postpone product isolation
until the basic nitrogen function has been blocked as a
carbamate. The telescoped process that was eventually
established is as follows: At a 34% conversion of the
enzymatic hydrolysis {[( )-8b]=3.0 M; [K. oxytoca
hydrolase]=0.3%; 60°C; 26 h} where (R)-acid 8a was
generated with 97.3% ee, the reaction mixture was
basified with aqueous NaOH to pH 11.5, and the
unreacted (S)-ester 8b21 of 50.3% ee was extracted into
methyl tert-butyl ether (MTBE). The aqueous layer
containing the (R)-acid 8a was treated with i-PrOH and
then with (Boc)2O under ice-cooling. On complete car-
bamate formation, the volatile components were evapo-
rated in vacuo, and the aqueous residue was acidified
with aqueous HCl to pH 3.0. Extraction with AcOEt
followed by a single recrystallization [n-heptane–AcOEt
(4:1)] provided (R)-BocDMTA 222 of 99.4% ee in 24%
2.2.2. Enzyme-catalyzed enantioselective hydrolysis.
According to the reported procedures,18 ( )-713,16,17 was
converted to ( )-1,3-thiazolidine-4-carboxylic acid 8a in
96% yield, which, in turn, was esterified under acidic
conditions to give ( )-8b uneventfully in 84% yield
(Scheme 2). We then screened 43 different hydrolase
preparations, such as lipases, proteases, and car-
boxyesterases, looking to identify hydrolytic enzymes
able to digest the (R)-ester 8b selectively. This led to
identification of three hydrolases that could discrimi-
nate between the enantiomers of ( )-8b. As can be seen
from Table 1, it was the thermally stable hydrolase
from K. oxytoca15 that showed superior enantioselectiv-
ity in the hydrolysis of the ( )-ester 8b.
With the promising hydrolytic enzyme from K. oxytoca
in hand, we examined parameters affecting the enzy-
Table 1. Hydrolases able to catalyze the enantioselective hydrolysis of (9)-8ba
Enzyme
Microbial source
[Enzyme] (%)
Conversion (%)d Ee (%) for (S)-8be
Ee (%) for (R)-9e
Ef
Proteaseb (XP-488)
Proteaseb (Bioprase)
Hydrolaseb,c
Aspergillus melleus
Bacillus subtillis
Klebsiella oxytoca
0.5
5
0.1
13.3
37.5
41.6
14.0
41.4
71.1
91.0
69.2
99.7
24
8
\500
a The enzymatic reaction was run at 25°C in an unbuffered aqueous medium with ( )-8b at a concentration of 90 mM and with enzymes each
at the indicated concentration for 24 h.
b Available from Nagase ChemteX Corporation.
c See Ref. 19.
d For the HPLC conditions used to monitor the progress of the reaction, see Section 4.3.3.
e For the HPLC conditions used to determine the enantiomeric purity, see Section 4.3.3.
f See Ref. 20.
Table 2. Enantioselective hydrolysis of (9)-8b with K. oxytoca hydrolasea
[(9)-8b] (M)
[Hydrolase] (%)
Time (h)
Temperature (°C)
Conversion (%)b
Ee (%) for (R)-9c
Ed
0.5
1.0
1.0
3.0
0.1
0.1
0.1
0.3
24
24
24
24
40
40
60
60
43.7
30.6
41.8
34.7
99.1
99.4
96.5
97.7
\500
\500
118
145
a The enzymatic reaction was run in an unbuffered aqueous medium under the conditions detailed in Section 4.3.3.
b For the HPLC conditions used to monitor the progress of the reaction, see Section 4.3.3.
c For the HPLC conditions used to determine the enantiomeric purity, see Section 4.3.3.
d See Ref. 20.

M. Ikunaka et al. / Tetrahedron: Asymmetry 13 (2002) 1201–1208
1205
overall yield from ( )-8b; the enantiomeric purity of
4.2. Synthesis of (2S,3S)-BocAHPBA 1
4.2.1. (2S,3S)-3-N,N-Dibenzylamino-1-isopropoxy-
(R)-2 was determined unambiguously by converting it
23
to the methyl ester (R)-2% with Me3SiCHN2 and ana-
lyzing the latter on a chiral HPLC column.
dimethylsilyl-4-phenylbutan-2-ol 5. Under an atmo-
sphere of N2, a portion (5 mL) of a solution of
(isopropoxydimethylsilyl)methyl chloride (available
from Sigma-Aldrich; 4.6 g, 28 mmol) in dry THF (40
mL) was added to Mg turnings (0.67 g, 28 mg atom).
To the stirred mixture was added 1,2-dibromoethane
(50 mL) at room temperature and an exothermic reac-
tion started in several minutes. The remainder of the
THF solution of (isopropoxydimethylsilyl)methyl chlo-
ride was added dropwise over 30 min so that the
reaction temperature did not exceed 40°C. After the
addition was complete, the grey-silver mixture was
stirred and heated at reflux for 30 min. The mixture was
cooled to 0°C, and a solution of (S)-2-N,N-dibenzyl-
amino-3-phenylpropanal 4 (prepared according to Ng’s
procedures;6 5.5 g, 15 mmol) in dry THF (30 mL) was
added dropwise at the same temperature over 30 min.
The mixture was stirred at 0°C for 30 min, and 10%
aqueous NH4Cl solution (80 mL) was added at the
same temperature. The mixture was extracted with
AcOEt (70 mL). The AcOEt extract was washed with
saturated aqueous NaCl solution (50 mL), dried
(Na2SO4), and concentrated in vacuo to give crude 5 as
a pale yellow oil (7.6 g): TLC [n-hexane/EtOAc (4:1)]
Rf=0.60; IR wmax (KBr) 3492, 2970, 2802, 1740, 1603,
2.2.4. Racemization of the unwanted enantiomer (S)-8b.
To increase the throughput (atom economy24) with the
overall process of the kinetic resolution, the unreacted
(S)-ester 8b, recovered by extraction with MTBE,
should be racemized to replenish the enzyme substrate
( )-8b. When a solution of (S)-8b (50.3% ee) in toluene
was treated with catalytic NaOMe (28% in MeOH) at
room temperature, racemization took place and the
reaction went to completion in 4 h to regenerate ( )-8b
in 46% overall yield from the fresh ( )-8b.
3. Conclusion
The tactical features that have benefited the synthesis of
(2S,3S)-BocAHPBA 1 are twofold: In combination
with N,N-dibenzyl protection of an a-amino aldehyde,
Tamao’s reagent enabled the facile construction of an
erythro-amino alcohol structure at 0°C. Zhao’s oxida-
tion protocol proved to be applicable for regioselective
oxidation of a 1,2-glycol to the a-hydroxy acid
functionality.
1
1495, 1454, 1369, 1252, 1122, 1018, 841, 744 cm−1; H
In the synthesis of (R)-BocDMTA 2 the following
measures have been taken for the kinetic resolution of
( )-8b with the K. oxytoca hydrolase to be industrially
viable: (i) Because of the thermally stable nature of the
hydrolase, the enzymatic hydrolysis was run at 60°C
such that ( )-8b could be processed at concentrations as
high as 3 M. (ii) The enzymatic process was telescoped
into carbamate formation to facilitate the product iso-
lation. (iii) The (S)-ester 8b that survived the enzymatic
hydrolysis and could be recovered easily from the spent
reaction mixture was racemized by the catalytic action
of NaOMe to augment the throughput of the kinetic
resolution.
NMR l (CDCl3) 7.25–7.05 (m, 15H), 4.12–4.05 (m,
1H), 3.98–3.90 (m, 1H), 3.70–3.50 (m, 4H), 3.00–2.70
(m, 4H), 1.08 (dd, J=9.6 Hz, 6.4 Hz, 6H), 0.83–0.78
(m, 2H), 0.11–0.04 (m, 6H). This was employed in the
next step without further purification.
4.2.2.
(2S,3S)-3-N,N-Dibenzylamino-4-phenylbutane-
1,2-diol 6a. To a stirred mixture of crude 5 (7.6 g, 15
mmol), NaHCO3 (1.6 g, 19 mmol), MeOH (40 mL),
and THF (40 mL) was added 30% aqueous H2O2
solution (12 mL) dropwise at room temperature. The
mixture was stirred and heated at 70°C for 2.5 h, and
10% aqueous Na2S2O3 solution (150 mL) was added
dropwise carefully at temperatures between 10 and
25°C. Stirring was continued at the same temperature
range for 30 min, and the mixture was extracted with
AcOEt (150 mL). The AcOEt extract was washed with
saturated aqueous NaCl solution (2×50 mL), dried
(Na2SO4), and concentrated in vacuo to give crude 6a
(6.30 g) as a pale yellow oil: TLC [n-hexane/EtOAc
(1:1)] Rf=0.40 for 6a, 0.80 for 5; IR wmax (KBr) 3396,
3061, 3026, 2928, 1950, 1877, 1809, 1738, 1602, 1495,
1454, 1373, 1244, 1028, 974, 906, 876, 744, 698, 611,
509 cm−1; 1H NMR l (CDCl3) 7.31–7.15 (m, 15H),
3.86–3.80 (m, 1H), 3.77 (d, J=13.6 Hz, 2H), 3.68–3.58
(m, 2H), 3.54 (d, J=13.6 Hz, 2H), 3.15 (dd, J=13.2
Hz, 6.4 Hz, 1H), 3.06–2.92 (m, 2H). This was employed
in the next step without further purification.
4. Experimental
4.1. General
Melting points were measured on an Electrothermal
1A8104 melting point apparatus and are uncorrected.
1H NMR spectra were recorded at 400 MHz on a
Varian UNITY-400 spectrometer in a CDCl3 or
DMSO-d6 solution and in a D2O solution with tetra-
methylsilane and 2-(trimethylsilyl)ethanesulfonic acid,
sodium salt, as an internal standard, respectively. FT-
IR spectra were recorded on a Nicolet Avatar 360
FT-IR spectrometer. Mass spectra were recorded on a
Hitachi M-8000 mass spectrometer (ESI). Elemental
analyses were performed on an Elementar vario EL
analyzer. Optical rotations were measured on a Horiba
SEPA-200 polarimeter. Thin-layer chromatography
(TLC) was performed on Merck Kieselgel 60 plates
(0.25 mm thick, art 1.05714.)
4.2.3. (2S,3S)-3-Amino-4-phenylbutane-1,2-diol 6b. To a
solution of crude 6a (4.7 g) in AcOEt (50 mL) was
added activated charcoal (Darco G-60, 0.5 g) at room
temperature, and the mixture was stirred at the same
temperature for 30 min. The activated charcoal was

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M. Ikunaka et al. / Tetrahedron: Asymmetry 13 (2002) 1201–1208
filtered off, and the filtrate was concentrated in vacuo
to give a slightly yellow oil (4.6 g), which was dissolved
in EtOH (50 mL). 5% Pd/C (K type, N.E. Chemcat
Corporation; 0.92 g; water content, 55.58%) was added,
and the mixture was stirred and heated at 50°C under
an atmosphere of H2 (initial pressure, 4.0 kg/cm2) for
6.5 h. (When hydrogen absorption is too sluggish, the
mixture should be filtered and the Pd catalyst replen-
ished.) When 6a was consumed completely as confirmed
by TLC n-hexane/EtOAc (1:1)], the Pd catalyst was
filtered off, and washed with EtOH (20 mL). The
combined filtrate and washing were concentrated in
vacuo. To the residue was added EtOH (15 mL) fol-
lowed by n-hexane (40 mL), and the precipitated solids
were collected by filtration to give white crystals (1.5 g),
which were suspended in MeOH (100 mL) and filtered.
The filter cake was washed with MeOH (20 mL). The
combined filtrate and washing were concentrated in
vacuo to give 6b as white crystals (1.14 g, 55.6% overall
yield from 4): TLC [n-hexane/EtOAc (1:1)] Rf=0.05;
mp 110–113°C (lit.:7b mp 105–107°C); [h]2D5=−35.2 (c
0.95, MeOH) {lit.:7b [h]2D5=−34.2 (c 1.37, MeOH)}; IR
wmax (KBr) 3305, 3081, 2916, 2860, 2703, 1608, 1493,
1454, 1436, 1377, 1078, 1043, 958, 906, 878, 844, 750,
at the same temperature. The mixture was stirred at
35°C for 24.5 h, and further TEMPO (20 mg, 0.13
mmol) and 0.28% aqueous NaOCl solution (0.50 mL,
1.88×10−2 mmol) were added at the same temperature.
The mixture was stirred at 35°C for 23.5 h, then
allowed to cool to room temperature. To the mixture
was added H2O (10 mL), and the pH of the mixture
was adjusted to 8.0 with 2.0 M aqueous NaOH solution
(2.0 mL). Six percent aqueous sodium sulfite (Na2SO3)
solution (10 mL) was added with ice-cooling below
20°C, and pH of the mixture turned 9.0 after the
addition was complete. The mixture was stirred at
room temperature for 30 min and washed with MTBE
(20 mL). To the aqueous layer was added AcOEt (50
mL). To the mixture was added 2.0 M aqueous HCl
solution (3.0 mL) with stirring to adjust pH of the
aqueous layer to 3. The layers were separated. The
AcOEt layer was washed with H2O (25 mL), and
saturated aqueous NaCl solution (2×30 mL), dried
(MgSO4), and concentrated in vacuo to give crude 1
(0.35 g) as white solids, to which was added AcOEt (2.0
mL) followed by n-hexane (6.0 mL). The suspension
was stirred under reflux for 15 min then cooled to 5°C,
and kept at the same temperature for 30 min. The
precipitated solids were collected by filtration, washed
with n-hexane (10 mL), air-dried at an oven tempera-
ture of 50°C overnight to give purified 1 as white
crystals (0.20 g, 74% overall yield from 6b): TLC
[BuOH/H2O/AcOH (4:1:1)] Rf=0.80; mp 147–148°C
(lit.:10a mp 147–148°C); [h]2D0=+2.69 (c 1.00, MeOH)
{lit.:10b [h]2D0=−2.10 (c 1, MeOH) for the (2R,3R)-iso-
mer}; IR wmax (KBr) 3402, 3353, 2985, 2938, 1695, 1518,
1446, 1390, 1288, 1269, 1250, 1169, 1067, 1051, 1027,
933, 845, 760, 704, 656, 602, 515 cm−1; 1H NMR l
(DMSO-d6) 12.80–12.40 (br, 1H), 7.30–7.10 (m, 5H),
6.72 and 6.20 (each d, J=8.8 Hz, J=8.8 Hz, total 1H),
5.80–5.20 (br, 1H), 4.00 (d, J=4.8 Hz, 1H), 4.00–3.80
(m, 1H), 2.80–2.60 (m, 2H), 1.35–1.20 (m, 9H); MS m/z
294 {[M−H]−}. Anal. found: C, 60.8; H, 7.3; N, 4.7.
Calcd for C15H21NO5: C, 61.00; H, 7.17; N, 4.74%.
1
700, 649, 592, 511 cm−1; H NMR l (CD3OD) 7.30–
7.18 (m, 5H), 3.75–3.60 (m, 2H), 3.58–3.52 (m, 1H),
3.08–2.98 (m, 2H), 2.58–2.42 (m, 1H); MS m/z 182
{[M+H]+}.
4.2.4. (2S,3S)-3-N-tert-Butoxycarbonylamino-4-phenyl-
butane-1,2-diol 6c. To a stirred solution of 6b (0.50 g,
2.76 mmol) in MeOH (15 mL) was added di-tert-butyl
dicarbonate (Boc2O, 0.63 g, 2.88 mmol) dropwise at
room temperature. Stirring was continued at room
temperature for 2 h, and the mixture was concentrated
in vacuo. The residue was extracted with AcOEt (50
mL) and the AcOEt extract was washed with 1.0 M
aqueous KHSO4 solution (2×30 mL), and saturated
aqueous NaCl solution (2×30 mL), dried (MgSO4), and
concentrated in vacuo to give crude 6c as a white solid
(0.90 g, quantitative yield): TLC [n-hexane/EtOAc
(1:1)] Rf=0.40; IR wmax (KBr) 3361, 2981, 2930, 1685,
1605, 1525, 1444, 1365, 1315, 1269, 1250, 1175, 1040,
4.2.6. (2S,3S)-3-Amino-2-hydroxy-4-phenylbutanoic acid
trifluoroacetate 1%. To a mixture of 1 (50 mg, 0.17
mmol) in CHCl3 (2.0 mL) was added trifluoroacetic
acid (TFA, 1.0 mL) at room temperature. Stirring was
continued at room temperature for 4 h, during which
the progress of the reaction was monitored by TLC
[n-BuOH/H2O/AcOH (4:1:1); Rf=0.4 for 1%]. The mix-
ture was concentrated in vacuo. The residue was diluted
with CHCl3 (2.0 mL), and the mixture was concen-
trated in vacuo. This operation was repeated three
times to remove the residual TFA completely; 1% (60
mg) was obtained quantitatively: [h]2D4=−3.2 (c 0.74, N
1
1013, 890, 757, 702, 648 cm−1; H NMR l (DMSO-d6)
7.30–7.10 (m, 5H), 6.56 and 6.10 (each d, J=9.2 Hz,
J=9.2 Hz, 1H), 4.80–4.70 (m, 1H), 4.50–4.40 (m, 1H),
3.62–3.56 (m, 1H), 3.50–3.30 (m, 3H), 2.95 (dd, J=
13.6, 3.2 Hz, 1H), 2.58–2.52 (m, 1H), 1.35–1.20 (m,
9H). This was employed without further purification.
4.2.5. (2S,3S)-3-N-tert-Butoxycarbonylamino-2-hydroxy-
4-phenylbutanoic acid 1. To a stirred mixture of crude
6c (0.30 g, 0.92 mmol), phosphate buffer (Na2HPO4,
pH 6.7, 5.0 mL), and MeCN (5.0 mL) was added
2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO, 40 mg,
0.26 mmol) followed by sodium chlorite (NaClO2, 80%,
0.21 g, 1.86 mmol) at room temperature. The mixture
was warmed to 35°C, and 0.28% aqueous sodium
hypochlorite (NaOCl) solution (0.75 mL, 2.8×10−2
mmol) was added at the same temperature. The mixture
was stirred with warming at 35°C for 6.5 h, and
TEMPO (20 mg, 0.13 mmol) and 0.28% aqueous
NaOCl solution (0.75 mL, 0.94×10−2 mmol) were added
1
HCl) {lit.:11 [h]D24=−2.2 (c 0.67, 1M HCl)}; H NMR l
(D2O) 7.28–7.05 (m, 5H), 4.42 (d, J=3.2 Hz, 1H),
3.94–3.88 (m, 1H), 2.92–2.76 (m, 2H).
4.3. Synthesis of (R)-Boc DMTA 2
4.3.1. ( )-5,5-Dimethyl-1,3-thiazolidine-4-carboxylic acid
8a. To a stirred suspension of ( )-penicillamine 7 (avail-
able from Sigma-Aldrich; 50 g, 0.34 mol) in EtOH (250

M. Ikunaka et al. / Tetrahedron: Asymmetry 13 (2002) 1201–1208
1207
mL) was added 37% aqueous solution of formaldehyde
(50 mL) dropwise. The mixture was stirred at room
temperature for 21 h, during which the progress of the
reaction was monitored by TLC [n-BuOH/AcOH/H2O
(7:2:1); ninhydrin; Rf=0.27 for ( )-7 (yellow), 0.32 for
( )-8a (red)]. The precipitated solids were collected by
filtration, washed with EtOH (25 mL), and air-dried at
an oven temperature of 50°C overnight to give ( )-8a
(52.1 g, 96.4%) as white crystals: mp 195.2–196.2°C
(lit.:18 mp 194–196°C); IR wmax (KBr) 2979, 2511, 1642,
1578, 1459, 1382, 1338, 1269, 1192, 1125, 1003, 902,
When HPLC analysis showed that 34.1% of ( )-8b was
consumed and that (R)-8a was produced with 97.3% ee,
the mixture was basified to pH 11.5 with 48% aqueous
NaOH solution (4.0 mL). The mixture was extracted
with MTBE (3×20 mL). The aqueous layer was stirred
with ice-cooling, and diluted with i-PrOH (20 mL).
Boc2O (5.0 g, 23 mmol) was added, and the mixture
was stirred at room temperature for 4 h. The i-PrOH
was evaporated off in vacuo, and the residue was
ice-cooled and acidified to pH 3.0 with 2.0 M HCl
aqueous solution (8.0 mL). The mixture was extracted
with AcOEt (2×100 mL). The combined AcOEt extracts
were washed with H2O (2×10 mL), dried (Na2SO4), and
concentrated in vacuo. The solid residue was recrystal-
lized from n-heptane (16.0 mL) and AcOEt (4.0 mL).
The precipitated solids were collected by filtration,
washed with n-heptane (5.0 mL), and air-dried at an
oven temperature of 50°C overnight to give 2 as white
crystals (3.70 g, 23.6%): TLC [n-hexane/AcOEt/AcOH
(5:5:0.5); ninhydrin] Rf=0.40 (red); mp 109.4–110.4°C;
[h]D24=−76.4 (c 1.0, EtOH) {lit.:22 [h]2D4=−76.2 (c 1.0,
EtOH)}; IR wmax (KBr) 3001, 2977, 1752, 1736, 1660,
1642, 1474, 1402, 1371, 1307, 1231, 1184, 1165, 1149,
1115, 887, 860, 773, 762, 662 cm−1; 1H NMR l (CDCl3)
1.43 (s, 3H), 1.52 (s, 9H), 1.61 (s, 3H), 4.21 and 4.35
(each s, total 1H), 4.62 (s, 1H), 4.69 (s, 1H), 8.11 (s,
1H); HRMS (70 eV; CI) m/z 261.1030 (M+), calcd for
C11H19NO4S: 261.1035. Anal. found: C, 50.56; H, 7.09;
N, 5.33; S, 12.21. Calcd for C11H19NO4S: C, 50.56; H,
7.33; N, 5.36; S, 12.27%. The enantiomeric purity of
(R)-2 was determined to be 99.4% ee by the following
method: To a stirred solution of (R)-2 (30 mg) in
MTBE (0.3 mL) was added a 10% solution of
Me3SiCHN2 in n-hexane (available from Tokyo Kasei
Kogyo Co.; 0.01 mL). The solution was agitated vigor-
ously, and concentrated in vacuo to give (R)-2% as a
white solid, which was dissolved in i-PrOH (0.2 mL).
The solution was diluted with n-hexane (0.8 mL), and
0.02 mL of the solution was subjected to HPLC analy-
sis [Chiralcel OD (Daicel), 25 cm×4.6 mm f; n-hexane/
i-PrOH (98:2); 1.0 mL/min; 25°C; UV (254 nm);
tR=7.7 min for (R)-2% (99.7%), 13.3 min for the (S)-iso-
mer (0.3%)].
1
785, 686 cm−1; H NMR l (D2O) 1.25 (s, 3H), 1.50 (s,
3H), 3.81 (s, 1H), 4.25 (d, J=10.4 Hz, 1H), 4.32 (d,
J=10.4 Hz, 1H).
4.3.2. Methyl ( )-5,5-dimethyl-1,3-thiazolidine-4-car-
boxylate 8b. A stirred suspension of ( )-8a (28.6 g, 0.18
mol) in MeOH (500 mL) was heated to reflux, and HCl
gas bubbled into the refluxing mixture over a period of
4 h. The resulting clear solution was concentrated in
vacuo. To the residue was added H2O (100 mL), and
the pH of the mixture was adjusted to 11.5 with solid
K2CO3 (19 g). The mixture was extracted with MTBE
(2×100 mL). The combined MTBE extracts were dried
(MgSO4), and concentrated in vacuo to give ( )-8b as
white crystals (26.0 g, 84%): TLC [n-BuOH/AcOH/H2O
(7:2:1); ninhydrin] Rf=0.76 (yellow); mp 39.7–40.7°C
[lit.:21 mp 36°C for the (S)-isomer]; IR wmax (KBr) 3309,
2962, 1736, 1448, 1345, 1289, 1236, 1200, 1179, 1146,
1
1124, 1011, 932, 856, 797 cm−1; H NMR l (CDCl3)
1.23 (s, 3H), 1.67 (s, 3H), 3.10 (s, 1H), 3.52 (s, 1H), 3.78
(s, 3H), 4.23 (d, J=10.0 Hz, 1H), 4.35 (d, J=10.0 Hz,
1H).
4.3.3. (R)-3-tert-Butoxycarbonyl-5,5-dimethyl-1,3-thia-
zolidine-4-carboxylic acid 2. A stirred suspension of
( )-8b (10.5 g, 59.9 mmol) in distilled H2O (20 mL) was
heated to 60°C, and K. oxytoca hydrolase (available
from Nagase ChemteX Corporation;19 60 mg) was
added. The mixture was stirred with heating at 60°C for
26 h, during which the progress of the reaction was
monitored by HPLC using one of two methods.
Method 1: [Sumichiral OA-5000 (Sumika), 25 cm×4.6
mm f; 2 mM solution of CuSO4 in H2O/MeCN
(85:15); 1.0 mL/min; 25°C; UV (254 nm); an aliquot
(0.05–0.1 mL) of the reaction mixture was diluted with
MeOH (0.1 mL) and distilled H2O (0.8 mL), well
agitated, and centrifuged to give a supernatant, 0.01
mL of which was injected; tR=12.1 min for (R)-5,5-
dimethyl-1,3-thiazolidine-4-carboxylic acid 8a, 20.7 min
for (S)-8a18].
4.3.4. Racemization of methyl (S)-5,5-dimethyl-1,3-thia-
zolidine-4-carboxylate 8b. The MTBE extracts from the
spent reaction mixture that were obtained in Section
4.3.3 were washed with H2O (10 mL), dried (Na2SO4),
and concentrated in vacuo to give crude (S)-8b of
50.3% ee [Chiralpak AD (Daicel); Method 2 described
in Section 4.3.3], which was dissolved in PhMe (20 mL).
To the solution was added 28% NaOMe solution in
MeOH (0.7 mL) with stirring at room temperature.
Stirring was continued at room temperature for 4 h,
when the complete racemization was confirmed by the
HPLC analysis [Chiralpak AD (Daicel); using Method
2 described in Section 4.3.3]. The mixture was washed
with H2O (3×10 mL), and concentrated in vacuo. The
solid residue was recrystallized from n-heptane (8.0
mL). The precipitated solids were collected by filtration,
washed with ice-chilled n-heptane (2.0 mL), dried in
vacuo at room temperature to give ( )-8b as white
crystals [4.80 g, 46% overall yield from ( )-8b].
Method 2: [Chiralpak AD (Daicel), 25 cm×4.6 mm f;
n-hexane/i-PrOH/Et2NH (75:25:0.3); 1.0 mL/min;
25°C; UV (254 nm); an aliquot (0.05–0.1 mL) of the
reaction mixture was diluted with saturated aqueous
K2CO3 solution (0.1 mL) and n-hexane (1.0 mL), agi-
tated, and centrifuged to separate the n-hexane layer,
0.01 mL of which was injected; tR=6.0 min for methyl
(S)-5,5-dimethyl-1,3-thiazolidine-4-carboxylate 8b,21 7.0
min for (R)-8b.

1208
M. Ikunaka et al. / Tetrahedron: Asymmetry 13 (2002) 1201–1208
10. (a) Sayo, N.; Yamazaki, N.; Kumobayashi, H. (Takasago
Acknowledgements
International Corporation). Jpn Kokai Tokkyo Koho 96
231,462, 1996; (b) An analytical certificate issued by
Sigma-Aldrich for (2R,3R)-1 (Catalog No. 15058).
11. Yuan, W.; Munoz, B.; Wong, C.-H. J. Med. Chem. 1993,
36, 211.
12. Coppola, G. M.; Schuster, H. F. Asymmetric Synthesis:
Construction of Chiral Molecules Using Amino Acids;
John Wiley & Sons: New York, 1987; p. 2.
Thanks are due to Professor Takeshi Kitahara, The
University of Tokyo, for his technical advice. M.I.
thanks Mr. Masafumi Moriwaki, Director of Research
and Development Center, Nagase & Co., Ltd., for his
encouragement. M.I. also thanks Mr. Fumiki Nomoto,
the 2nd Laboratories of the Research and Development
Center, and Dr. Toru Inoue, the 1st Laboratories of the
Research and Development Center, for their incipient
but dedicated contribution to the project.
13. (a) Nicolaou, K. C.; Sorensen, E. J. Classics in Total
Synthesis: Targets, Strategies, Methods. Penicillin
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(Sheehan, J. C. 1957). VCH: Weinheim, 1996; Chapter 3,
pp. 41–53; (b) Sheehan, J. C.; Henery-Logan, K. R. J.
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