Stereoselective synthesis of N-benzyl (2S,3S,4S)-3-hydroxy-4-methylproline

Article abstract of DOI:10.1016/j.tetasy.2016.02.008

The synthesis of (2S,3S,4S)-N-benzyl-3-hydroxy 4-methyl proline and its stereoisomer (2R,3R,4S)-N-benzyl-3-hydroxy 4-methyl proline has been achieved starting with the conjugate addition of methyl methacrolate and benzylamine. The other key reactions performed in our strategy include enzymatic separation of an α-methyl β-amino alcohol, Sharpless asymmetric dihydroxylation of an α,β-unsaturated δ-amino ester, and intramolecular cyclization of a sulfonate to a pyrrolidine ring system. Two stereoisomers were synthesized from the common α,β-unsaturated δ-amino ester intermediate. This protocol is simple, straightforward, efficient, highly stereoselective, and economically viable.

Full text of DOI:10.1016/j.tetasy.2016.02.008

Tetrahedron: Asymmetry 27 (2016) 261–267
Contents lists available at ScienceDirect
Tetrahedron: Asymmetry
journal homepage: www.elsevier.com/locate/tetasy
Stereoselective synthesis of N-benzyl (2S,3S,4S)-3-hydroxy-4-
methylproline
Mahanthaswamy Hiremath a,b, Yalechakanahalli C. Sunil Kumar a, Muddegowda Umashankara a,
Nagarakere C. Sandhya a, Siddapa Chandrappa a, Chottanahalli S. Pavan Kumar a,
a,
Kempegowda Mantelingu a, , Kanchugarakoppalu S. Rangappa
⇑
⇑
a Department of Studies in Chemistry, University of Mysore, Manasagangotri, Mysuru 570006, Karnataka, India
b Syngene International Ltd, Bengaluru, Karnataka, India
a r t i c l e i n f o
a b s t r a c t
Article history:
The synthesis of (2S,3S,4S)-N-benzyl-3-hydroxy 4-methyl proline and its stereoisomer (2R,3R,4S)-N-ben-
zyl-3-hydroxy 4-methyl proline has been achieved starting with the conjugate addition of methyl
methacrolate and benzylamine. The other key reactions performed in our strategy include enzymatic
Received 4 February 2016
Accepted 16 February 2016
Available online 2 March 2016
separation of an
d-amino ester, and intramolecular cyclization of a sulfonate to a pyrrolidine ring system. Two stereoiso-
mers were synthesized from the common ,b-unsaturated d-amino ester intermediate. This protocol is
simple, straightforward, efficient, highly stereoselective, and economically viable.
Ó 2016 Elsevier Ltd. All rights reserved.
a-methyl b-amino alcohol, Sharpless asymmetric dihydroxylation of an a,b-unsaturated
a
1. Introduction
being investigated in phase II/III clinical studies against Candida
and Aspergillums species.17
Functionalized pyrrolidines are present as major structural
components in many bioactive natural products and pharmaceuti-
cally important substances.1 (2S,3S,4S)-3-Hydroxy-4-methylpro-
line, commonly known as HMP 1, is one example. This
non-proteogenic amino acid was first isolated from Mediterranean
sponge and the seeds of delonix regia2 and is a common component
of several antimicrobial cyclic lipopeptides, such as mucrorin-D,3
tetomycin4 echinocandins B, C, D (Fig. 1),5 pneumocandins,6–8
sporiofungin A, mulundocandin and deoxymulundocandin.9 These
Structurally, 3-hydroxy-4-methylproline has three contiguous
stereogenic carbon atoms at C2, C3, and C4 positions, and so four
diastereomeric racemic forms are possible. (2S,3S,4S)-HMP 1 is a
common structural moiety present in natural peptides. Other iso-
mers such as epi-HMP 2, ent-HMP 3, and allo-HMP 4 are of syn-
thetic origin and their biological importance is not yet known.
Different strategies have been reported for the synthesis of the
naturally occurring (2S,3S,4S) isomer,18–32 although none of them
are straightforward for the synthesis of all four diastereomers of
HMP. Since HMP is present as an important component of most
of the cyclic peptides, it is likely that this component itself would
manifest the activity and could potentiate activities in analogues.
Thus, the development of an efficient enantioselective synthesis
of these unusual pyrrolidine amino acids could be useful for the
study of structural analogues of natural peptides. Herein we report
the synthesis of (2S,3S,4S)-N-benzyl-3-hydroxy 4-methyl proline
and (2R,3R,4S)-N-benzyl-3-hydroxy 4-methyl proline from a com-
mon achiral synthetic precursor.
peptides exhibit inhibitory activities against 1,3-b-D-glucan syn-
thase and fungicidal activities, especially against Candida albicans
and Pneumocystis carinii.10 They have also been found to exhibit
therapeutic activities, such as anticancer, antifungal, antibiotic,
antiviral and as immune stimulating agents.11
It has been demonstrated that the biological activities of these
peptides is significantly diminished by the replacement of HMP
with 3-hydroxyproline, 4-hydroxyproline12 or 4-methylproline.13
Similarly replacement with 4-aminoproline dramatically reduces
the cellular accumulation and antifungal activity.14 HMP is also a
structural component in many pharmaceutically interesting ana-
logues such as LY30336615 and FK-463.16 The latter is currently
2. Results and discussion
Our retrosynthetic analysis of HMP suggested that 3-N-benzyl-
N-(Boc)-2-methyl-1-propanol 8 could serve as a useful common
intermediate for the synthesis of our target compounds (Scheme 1).
We anticipated that the configurations at C-3 and C-4 could be
⇑
Corresponding authors. Tel.: +91 0821 2419497.
E-mail addresses: kmlingu@gmail.com (K. Mantelingu), rangappaks@gmail.com
(K.S. Rangappa).
http://dx.doi.org/10.1016/j.tetasy.2016.02.008
0957-4166/Ó 2016 Elsevier Ltd. All rights reserved.

262
M. Hiremath et al. / Tetrahedron: Asymmetry 27 (2016) 261–267
OH
O
R1
H
R3
H
N
N
R1
O
N
O
HO
OH
Me
O
NH
O
OH
OH
HN
N
H
O
N
O
R2
(2S,3S,4S)-HMP 1
O
OH
OH
LY303366 R1 = R2 = OH
Echinocandin B R1 = R2 = OH, R3 = C17H31
Echinocandin C R1 = OH, R2 = H, R3 = C17H31
Echinocandin D R1 = R2 = H, R3 = C17H31
R3
=
OC5H11
Figure 1. Unnatural amino acid and bioactive cyclic lipopeptides.
O
N
O
O
O
S
OH
OEt
OH
O
O
OEt
N
N
H
Boc
Boc
Bn
13
Bn
1
11
Boc
N
OEt
Bn
CHO
OEt
Boc
O
+
5
10
N
+
Bn
7
HO
Ph
Ph
P
Ph
O
NH2
Scheme 1. Retrosynthetic analysis.
sequentially introduced by Wittig reaction followed by Sharpless
asymmetric dihydroxylation and cyclization to obtain a pyrrolidine
ring structure.
(Scheme 2). The enzymatic acetylation of the (S)-isomer gradually
increased with time and remained unchanged after 16 h (moni-
tored by thin layer chromatography). The reaction mixture was fil-
tered and both compounds were separated by column
chromatography. Free alcohol compound 8 was obtained in 43%
yield and acetylated compound 9 was obtained in 47% yield.
2.1. Synthesis of (2S,3S,4S)-N-benzyl-3-hydroxy-4-methylproline
Our first target was to synthesize (R)-2-methyl-b-anino alcohol
and (S)-2-methyl-b-anino alcohol. We successfully synthesized
achiral 2-methyl-b-aminoester 6 from methyl methacrylate 5 and
benzylamine by conjugate addition reaction. In the presence of bis-
muth nitrate catalyst, the conjugate addition reaction occurred
smoothly and N-benyl-2-methyl methylpropinate was exclusively
obtained in 87% yield. The methyl ester was reduced to an alcohol
using LAH and t-butyl carbamate protection was introduced to the
amino group. These two reactions were carried out in a single step.
The resulting racemic 2-methyl b-amino alcohol 7 was obtained in
90% yield over two steps. Next we attempted the separation of
enantiomers by enzymatic hydrolysis of acetylated 2-methyl
b-amino alcohol with different enzymes under different buffer
conditions. Our efforts were unsuccessful. We then dissolved the
racemic b-amino alcohol 7 in vinyl acetate and stirred the solution
in the presence of 0.5 mol % enzyme Pseudomonas capacia at room
temperature. The progress of the reaction was monitored by thin
layer chromatography. Under these conditions, the (S)-isomer
was acetylated and converted into the corresponding acetyl deriva-
The optical rotation of compound 8 is found to be [
a] +16.6
D
(C = 0.3 MeOH) and the compound 9 is [ +6.6 (C = 0.3 MeOH).
a
]
D
The absolute configuration at C2 position of b-Amino alcohol 8
was assigned by comparing the specific rotation of the free alcohol
derivatives of this compound with the reported values33 and it was
found that compound 8 had an (R)-configuration. Next we hydro-
lyzed the acetylated compound 9 into the corresponding free alco-
hol 10 by K2CO3 in a methanol–water solvent system. The specific
rotation of compound 10 was found to be [
a
]
D = À16.3 (c 0.3
MeOH). The enantiomeric purity (ee) of both compounds 8 and
10 was determined by chiral HPLC. Compound 9 was obtained with
90.33% ee and compound 10 was obtained with 97.34% ee
(Scheme 3).
Our next target was to synthesize naturally occurring HPM 1
pyrrolidine ring structure; for this we carried out the following
sequential reactions with compound 10, which has an (S)-methyl
side chain of the natural compound. Initially we focused on
assembling the required carbon framework. First we oxidized the
primary alcohol 10 to aldehyde 11 using IBX. The resulting
tive
9
while the (R)-isomer was isolated as free alcohol
8
aldehyde was immediately converted into a,b-unsaturated ethyl

M. Hiremath et al. / Tetrahedron: Asymmetry 27 (2016) 261–267
263
NH2
OH
i) THF, LAH
OMe
OMe
MeO
ii) (Boc)2O, NaOH
N
Bi(NO3)3
NH
O
O
Boc
Bn
Bn
6
87%
7 90%
5
vinyl acetate,
Pseudomomas Capacia
OAc
OH
+
N
N
Boc
Bn
Boc
Bn
9 47%
8 43%
K2CO3, MeOH:H2O
98%
OH
N
Boc
Bn
10
Scheme 2. Enzymatic separation of racemic b-amino alcohol.
O
CHO
OEt
benzene, Ph3P=CH-COOEt
IBX, ethyl acetate
reflux, 3h,
10
Boc
Boc
11
reflux, 12h, 89%
N
N
Bn
Bn
12
O
OsO4 (cat), CH3SONH2,
K2CO3, K3Fe(CN)6,
(DHQ)2AQN (cat),
t-butanol : H2O
O
O
OH
O
S
OEt
SOCl2, CH2Cl2
92%
OEt
O
OH
Boc
N
80%, 96%(ee)
N
Boc
14
Bn
Bn
13
OH
OH
i) 2M LiOH,
MeOH:H2O
BF3.O(Et)2,
DCM, 0 oC
COOH
COOEt
83%
N
63%
N
Bn
16
Scheme 3. Synthesis of (2S,3S,4S)-1-benzyl-3-hydroxy-4-methylpyrrolidine-2-carboxylic acid.
Bn
15
ester 12 using a Wittig reaction. In this step, the required carbon
skeleton for cyclization to pyrrolidine ring was achieved. Our next
pyrrolidine derivative 15 was isolated in 63% yield in three steps.
We repeated this reaction to confirm its viability and we found that
the reaction was completed within 5 h and 15 was isolated each
time. We also carried out the deprotection of the Boc group using
1:1 HCl in ether, 50% TFA in dichloromethane and 98% formic acid.
Under all these conditions, we were unable to isolate the cyclized
product 15 and instead only the Boc deprotected compound was
obtained. The cyclization occurred in an SN2 fashion via inversion,
of configuration which introduced the required (2S) stereocenter at
the C-2 position. The ethyl ester was hydrolyzed with 2 M NaOH
solution in a methanol water mixture and gave the N-benzyl pro-
tected molecule 16 in 83% yield. Deprotection of the N-benzyl
group would furnish target molecule 1. For the sake of smooth
extraction from an aqueous solution and solubility in CDCl3
solvent for NMR studies, we retained the N-benzyl protection
group in compound 15. The selective deprotection of the N-benzyl
group has been observed in several literatures; this deprotection
does not affect or alter the stereocenter at the C-2 position, and
task was to introduce a cis-dihydroxy group to
a,b-unsaturated
ester 12; this we achieved via a Sharpless ADH reaction with AD-
mix–b (1 mol % of (DHQ)2AQN.34 This gave (2R,3S)-syn-diol 13 with
96% ee. The (2R,3S)-syn-diol insertion allows for the introduction of
a (2S)-hydroxyl group and provides an electrophilic center for
cyclization via an SN2 type of displacement to give a pyrrolidine
ring. The syn-diol in compound 13 was activated with thionylchlo-
ride to give cyclic sulfate derivative 14. Our initial plan was to oxi-
dize the cyclic sulfate derivative 14 into cyclic sulfonate using
RuCl3 oxidation,35 followed by cyclization via deprotection of the
N-benzyl group. The oxidation of compound 14 using RuCl3 gave
a poor yield. We envisaged that this may be due to the deactivation
of the Ru catalyst by t-butyl carbamate group. Hence we attempted
the deprotection of Boc group with BF3 etherate.21 We observed
that cleavage of the Boc group followed by cyclization and
simultaneous hydrolysis of the sulfate group occurred and the

264
M. Hiremath et al. / Tetrahedron: Asymmetry 27 (2016) 261–267
so we envisaged that the target compound 16 could be easily
3. Conclusion
achieved.
In conclusion, we have reported a versatile synthetic approach
for the synthesis of N-benyl-(2S,3S,4S)-3-hydroxy-4-methylproline
in 15 linear steps and 38% overall yield starting from achiral
methylmethcrolate and benzylamine. Our reaction conditions do
not involve the use of harsh and/or high cost reagents and could
be used to access other members of this class of compound.
2.2. Synthesis of (2R,3R,4S)-ethyl 1-benzyl-3-hydroxy-4-
methylpyrrolidine-2-carboxylate
Next we extended the above synthetic strategy to synthesize a
(2R,3R,4S)-HMP derivative. We achieved this by introducing the
(2S,3R)-syn-diol to compound 12 followed by cyclization as shown
in Scheme 4. The (2S,3R)-syn-diol was selectively introduced using
(DHQD)2PHAL catalyst in a Sharpless ADH reaction. The (2S,3R)-
syn-diol 17 was isolated in 83% yield and with 98% ee. This diol
was converted into cyclic sulfite derivative 18 in 92% yield by
treatment with thionyl chloride. In BF3 etherate solution, com-
pound 17 underwent cyclization to give pyrrolidine compound 19.
4. Experimental
4.1. General
Experiments that required an inert atmosphere were carried
out under argon. THF and Et2O solvent were freshly distilled over
sodium/benzophenone ketyl and transferred via syringe. Dichloro-
methane solvent was freshly distilled from CaH2. Triethylamine
was freshly distilled over KOH. Commercially available reagents
were used as received. Unless noted otherwise, work-up indicates
pouring the reaction mixture into brine, followed by extraction
with solvent. Washing with brine, drying over anhydrous Na2SO4,
and evaporation of the solvent under reduced pressure followed
by chromatography on silica gel (60–120 mesh) with indicated elu-
ent furnished the corresponding products. 1H and 13C NMR chem-
ical shifts are reported in ppm downfield from tetramethylsilane
and coupling constants (J) are reported in Hertz (Hz). IR data were
measured with an oily film on NaCl plates and are given only for
molecules with relevant functional groups (OH, C@O). Specific
rotations are given in 10À1 deg cm2 gÀ1 and were measured at
25 °C. The following abbreviations were used to report the NMR
chemical shift multiplicity s, singlet; d, doublet; dd, double dou-
blet; t, triplet; q, quartet; m, multiplet; br, broad. Elemental anal-
yses are expressed as a percentage with the abbreviation calcd
designating calculated.
OH
O
OsO4 (cat), CH3SONH2
K2CO3, K3Fe(CN)6
(DHQ)2AQN (cat)
t-butanol, H2O
OEt
SOCl2, CH2Cl2
92%
OH
12
N
83%, 98%ee
Boc
Bn
17
O
OH
O
O
S
BF3.O(Et)2, DCM
0 o
C
COOEt
OEt
O
N
62%
N
Boc
Bn
Bn
19
18
Scheme 4. Synthesis of (2R,3R,4S)-ethyl 1-benzyl-3-hydroxy-4-methylpyrrolidine-
2-carboxylate.
2.3. NOE studies
The structure of cyclic compounds 15 and 19 was confirmed by
NOESY studies. The various NOE interactions observed for these
compounds are shown in Figure 2. Compound 15 showed vicinal
coupling constants for J3,4 = 3.8 Hz and J2,3 = 8.4 Hz. Strong NOE
interactions between H-3–H-4, H-4–H-5, and H5–H3 were also
observed. Similarly vicinal coupling constants for J3,4 = 9.1 Hz and
J2,3 = 8.7 Hz were observed for compounds 19. The strong NOE
interaction between H-2 and H-4 indicated a cis-relationship
between H-2 and H-4, whereas no NOE interaction between either
H-4 and H-3 or H-3 and H-2 was observed. This suggested that H-4
and H-3 had a trans-relationship (Fig. 3).
4.2. Methyl 3-(benzylamino)-2-methylpropanoate 6
A mixture of methylmethacrylate (20 g, 0.199 mol) and bismuth
nitrate was (5.6 g, 0.01 mol) was stirred at 0 °C for 15 min. To this
mixture benzylamine (21.5 g, 0.199 mol) was added dropwise after
which the reaction mixture was slowly warmed to room tempera-
ture and stirring was continued for 10 h. The reaction mixture was
filtered off and diluted with dichloromethane, washed with satu-
rated brine solution, and dried over anhydrous Na2SO4. The solvent
was evaporated under reduced pressure and the residue was puri-
fied by column chromatography (silica 60–120 mesh) hexane/ethyl
acetate (7:3). Yield: 36.26 g, (87.53%); IR (NaCl): 1454.7, 1194.1,
OH
OH
Me
OH
Me
Me
1171.6,1120.5 cmÀ1 1H NMR (CDCl3 400 MHz) d 7.30 (m, 4H),
;
OH
OH
OH
7.27 (m, 1H), 3.80 (s, 2H), 3.69 (s, 3H), 2.87 (m, 1H), 2.16 (s, 1H)
2.71 (m, 2H), 1.78 (br, 1H), 1.18 (d, J = 6.92 Hz, 3H); 13C NMR
(CDCl3 100 MHz) d 171.57, 135.44, 123.63, 123.29, 122.20, 48.98,
47.35, 46.91, 35.27, 10.56; MS (ESI): Calcd for (C12H17NO2) m/z
207.27; found: 208.1 (M+H+); Anal. Calcd for (C12H17NO2): C,
69.54; H, 8.27; N, 6.76; O, 15.44; found: C, 69.52; H, 8.24; N,
6.77; O, 15.41.
N
H
N
H
N
H
O
O
O
(2R,3R,4R)-ent-HMP 3
(2S,3R,4S)-allo-HMP 4
(2R,3S,4S)-epi-HMP 2
Figure 2. Unusual pyrrolide amino acids.
4.3. tert-Butyl benzyl(3-hydroxy-2-methylpropyl)carbamate 7
HO
H2
H2
OH
H3
OEt
O
H1
H1
H3
H5
To a stirred solution of lithium aluminum hydride (4.73 g,
0.12 mol) in 300 mL of anhydrous THF at À30 °C, was added solu-
tion of compound 6 (30 g, 0.14 mol) in 20 mL of anhydrous THF
using dropping funnel under argon. The reaction mixture was
slowly warmed to room temperature and stirred for 2 h. The reac-
tion mixture was then cooled to 0 °C and quenched with aqueous
NaOH. To this, 32 mL (0.14 mol) of Boc anhydride was added
OEt
O
H4
N
N
H6
H5
Bn
Bn
15
19
Figure 3. NOE interactions observed for compounds 15 and 19.

M. Hiremath et al. / Tetrahedron: Asymmetry 27 (2016) 261–267
265
dropwise and the reaction mixture was stirred for 4 h. Next, THF
was removed under reduced pressure. The aqueous layer was
added to 100 mL of ethyl acetate and with vigorous stirring the
reaction mixture was acidified to pH 2.0 with saturated KHSO4
solution. The organic layer was separated and the aqueous layer
was further extracted with ethyl acetate. The combined organic
layer was washed with Na2CO3 solution and dried over anhydrous
Na2SO4. The crude product obtained after concentration of ethyl
acetate under reduced pressure was purified by column chro-
matography using silica gel (60–120 mesh) hexane/ethyl acetate
(7:3). Yield: 36.4 g, (90%); IR (NaCl): 2907.7, 2928.6, 1666.1,
hexane/ethyl acetate (8.5:1.5) Yield: 10.2 g (98%); IR (NaCl):
3427.9, 2871.0, 2361.2 cmÀ1 1H NMR (CDCl3, 300 MHz) d 7.31
;
(m, 5H), 4.58 (d, J = 15.63 Hz, 1H), 4.29 (d, J = 15.87 Hz, 1H), 3.72
(s, 1H), 3.5 (m, 2H), 3.36 (s, 1H), 2.29 (dd, J = 18.83 Hz, 1H), 1.77
(s, 1H), 1.45 (s, 9H), 0.93 (d, J = 6.96 Hz, 3H); 13C NMR (CDCl3,
75 MHz) d 156.41, 138.04, 128.45, 127.19, 80.51, 63.40, 52.22,
48.47, 34.31, 28.26, 14.91. MS (ESI) Calcd for (C16H25NO3) m/z
279.37; found: 280.4 (M+H+); [ 25 = -16.3 (c 0.3 MeOH); Anal.
a]
D
Calcd for (C16H25NO3): C, 68.79; H, 9.02; N, 5.01; O, 17.18; found:
C, 68.76; H, 9.01; N, 5.03; O, 17.15.
1456.6, 1415.7 cmÀ1
;
1H NMR (CDCl3, 400 MHz) d 7.29 (m, 5H),
4.7. (R,E)-Ethyl 5-(benzyl(tert-butoxycarbonyl)amino)-4-methyl-
4.57 (d, J = 15.64, 1H), 4.29 (d, J = 15.52 Hz, 1H), 3.75 (br, 1H),
3.58 (q, J = 86.6 Hz, 1H), 3.36 (br, 1H), 2.92 (d, J = 14.36 Hz, 1H),
1.74 (s, 2H), 1.45 (s, 9H), 0.93 (d, J = 6.64 Hz, 3H); 13C NMR (CDCl3,
100 MHz) d 280.3, 128.84, 127.37, 79.15, 48.77, 28.47, 15.32; MS
(ESI) Calcd for (C17H25NO4) m/z 307.38; found: 308.1 (M+H+); Anal.
Calcd for (C17H25NO4): C, 66.43; H, 8.20; N, 4.56; O, 20.82; found: C,
66.42; H, 8.18; N, 6.77; O, 20.42.
pent-2-enoate 12
To a stirred solution of compound 10 (8 g, 0.029 mol) in 50 mL
of ethyl acetate, IBX (12.5 g, 0.039 mol) was added and heated at
60 °C for 3 h. The reaction mixture was filtered, concentrated under
reduced pressure, and used in the next step immediately. To a stir-
red solution of the crude aldehyde product in toluene, ethoxy car-
bonyl methylene triphenyl phosphorane Ph3P@CH-CO2Et (11.9 g,
0.034 mol, Witting salt) was added in one portion and heated to
80 °C for 12 h. Completion of the reaction was confirmed by TLC
and toluene was removed under reduced pressure. The crude pro-
duct was directly loaded onto a silica gel (60–120 mesh) column
and purified by eluting with a hexane/ethyl acetate (9.5:0.5) mix-
ture. Yield: 8.8 g, (89%); IR (NaCl): 2973.8, 1690.0, 1553.5,
4.4. (R)-tert-Butyl benzyl(3-hydroxy-2-methylpropyl)carbamate
8
To a stirred solution of compound 7 (30 g) in 250 mL of vinyl
acetate, Pseudomonas capacia (0.5 mol %) was added and the result-
ing solution was stirred at room temperature. The progress of the
formation of OAc was monitored by TLC. After 16 h, there was no
increase in the formation of OAC product. The reaction mixture
was filtered through a Celite bed. The residue was washed with
vinyl acetate and the filtrate was concentrated under reduced pres-
sure. The residue was purified by column chromatography on silica
gel (60–120 mesh) hexane/ethyl acetate (8:2). Two spots were iso-
lated, with the polar compound being the free AOH compound 9,
1162.6 cmÀ1 1H NMR (CDCl3, 400 MHz) d 7.33 (t, J = 14.36 Hz,
;
2H), 7.26 (q, J = 15.32 Hz, 1H), 7.22 (t, J = 14.52 Hz, 2H), 6.85 (m,
1H), 5.77 (t, J = 32.72 Hz, 1H), 3.26 (d, J = 12.2 Hz, 1H), 3.06 (q,
J = 19.84 Hz, 2H), 2.74 (br, 1H), 1.44 (d, J = 15.2 Hz, 9H), 1.32 (t,
J = 12.68 Hz, 3H), 1.02 (d, J = 6.6 Hz, 2H); 13C NMR (CDCl3,
100 MHz)
d 166.36, 155.75, 151.27, 137.97, 128.44, 127.64,
127.16, 121.05, 79.93, 60.15, 51.39, 50.27, 35.98, 28.24, 16.89,
Yield: 12.9 g (43%). IR (NaCl): 3427.9, 2871.0, 2361.2 cmÀ1
;
1H
14.17; MS (ESI) Calcd for (C20H29NO4) m/z 347.45; found 348.54
NMR (CDCl3, 400 MHz) 7.22 (d, J = 7.24 Hz, 2H), 7.27 (q,
d
(M+H+); [ 25 = +8.3 (c 0.3 MeOH); Anal. Calcd for (C20H29NO4): C,
a]
D
J = 13.24 Hz, 1H), 7.36 (t, J = 2.84 Hz, 2H) 4.57 (d, J = 15.56 Hz,
1H), 4.28 (d, J = 15.72 Hz, 1H), 3.55 (dd, J = 15.0 Hz, 1H), 3.47 (d,
J = 10.4 Hz, 1H), 3.37 (d, J = 10.74 Hz, 1H), 2.91 (dd, J = 17.72 Hz,
1H), 2.46 (br, 1H), 1.74 (br, 1H), 1.45 (s, 9H) 0.93 (d, J = 6.96 Hz,
3H); 13C NMR (CDCl3, 400 MHz) d 156.30, 138.02, 128.47, 127.21,
127.07, 80.54, 51.16, 48.39, 34.24, 28.26, 14.94; MS (ESI) Calcd
69.14; H, 8.41; N, 4.03; O, 18.42; found: C, 69.19; H, 8.39; N,
4.04; O, 18.41.
4.8. (S)-tert-Butyl benzyl(2-methyl-3-oxopropyl)carbamate 11
1H NMR (CDCl3, 400 MHz) d 9.64 (s, 1H), 7.27 (m, 5H), 4.49 (d,
J = 13.44 Hz, 2H), 3.57 (br, 1H), 3.23 (t, J = 12.28 Hz, 1H), 2.27 (br,
1H), 1.47 (d, J = 23.52 Hz, 9H). 0.98 (d, J = 6.64 Hz, 3H); 13C NMR
(CDCl3, 100 MHz) d 155.70, 128.53, 127.28, 46.16, 28.26, 11.73;
MS (ESI) Calcd for (C16H23NO3) m/z 277.36; found: 278.41 (M
for (C16H25NO3) m/z 279.37; found: 280.7 (M+H+), [ 25 = +16.6 (c
a]
D
0.3 MeOH); Anal. Calcd for (C16H25NO3): C, 68.79; H, 9.02; N,
5.01; O, 17.18; found: C, 68.80; H, 9.03; N, 5.11; O, 17.13.
4.5. (S)-3-(Benzyl(tert-butoxycarbonyl)amino)-2-methylpropyl-
acetate (non-polar compound) 9
+H+); [
a]
25 = À10.3 (C = 0.3 MeOH); Anal. Calcd for (C16H23NO3):
D
C, 69.29; H, 8.36; N, 5.05; O, 17.31; found: C, 69.30; H, 8.39; N,
5.03; O, 17.29.
IR (NaCl): 2971.8, 2930.1, 1738.4, 1689.6 cmÀ1; 1H NMR (DMSO,
300 MHz) d 7.36 (m, 2H), 7.20 (m, 2H) 4.34 (s, 2H), 3.27 (m, 2H),
3.17 (m, 2H), 3.05 (m, 1H), 2.27 (s, 1H), 2.11 (s, 3H), 1.14 (s, 9H),
0.81 (d, J = 8.1 Hz, 3H); 13C NMR (DMSO, 75 MHz) d 170.79,
128.88, 127.47, 79.38, 67.01, 49.69, 28.41, 27.82, 21.10, 15.17. MS
(ESI) Calcd for (C18H27NO4) m/z 321.41; found: 322.7 (M+H+);
4.9. (2S,3R,4S)-Ethyl 5-(benzyl(tert-butoxycarbonyl)amino)-2,3-
dihydroxy-4-methylpentanoate 13
Osmium tetroxide (1.05 mg, 0.01 mmol) and methane sulfon-
amide (1.23 g, 0.013 mol) were stirred in 120 mL of 1:1 t-BuOH/
H2O solvent mixture. To this mixture K4Fe(CN)6 (14.7 g,
0.04 mol), K2CO3 (5.5 g, 0.04 mol) and (DHQP)2 PHAL (0.75 mg,
0.01 m mol) were added and stirred for 15 min at À30 °C. Next
compound 12 (5 g, 0.014 mol) was added and stirred at À30 °C
for 18 h. The completion of the reaction was confirmed by TLC.
Sodium sulfite Na2SO3 (18 g) was then added and stirred at room
temperature for an hour. t-Butyl alcohol was removed under
reduced pressure and the aqueous later was extracted using ethyl
acetate. The combined organic layer was dried over anhydrous
sodium sulfate, purified by column chromatography (silica 60–
120 mesh) hexane/ethyl acetate (8:2) Yield: 4.4 g, (80%); IR (NaCl):
[a]
25 = +6.6 (c 0.3 MeOH); Anal. Calcd for (C18H27NO4): C, 67.26;
D
H, 8.47; N, 4.36; O, 19.91; found: C, 67.31; H, 8.53; N, 4.41; O, 19.16.
4.6. (S)-tert-Butyl benzyl(3-hydroxy-2-methylpropyl)carbamate
10
To a stirred compound 9 (12 g) in 50 mL of 1:1 methanol/water
solvent mixture, K2CO3 (5 equiv) was added and stirred for 6 h. The
reaction mixture was extracted with ethyl acetate, and the organic
extracts were washed with brine, dried over anhydrous sodium
sulfate, and evaporated under reduced pressure. The residue
was purified by column chromatography (silica 60–120 mesh)
2973.9, 2931.2, 1688.0, 1665.7 cmÀ1 1H NMR (DMSO, 300 MHz) d
;

266
M. Hiremath et al. / Tetrahedron: Asymmetry 27 (2016) 261–267
7.35 (m, 5H), 4.30 (m, 2H), 4.45 (m, 1H), 4.16 (m, 2H), 4.06 (m, 1H)
3.48 (m, 1H), 3.26 (m, 1H), 3.72(m, 1H), 1.91 (m, 1H), 1.23 (m, 9H),
0.80 (m, 3H); 13C NMR (DMSO, 75 MHz) d 173.52, 173.19, 128.85,
128.78, 127.54, 27.38, 79.17, 75.24, 73.04, 66.52, 49.32, 39.16,
35.31, 14.59; MS (ESI) Calcd for (C20H31NO6) m/z 381.46; found:
7.45 (m, 3H), 4.54 (d, 1H), 4.31 (q, 1H), 3.81 (s, 1H), 3.68 (d,
J = 6.2 Hz, 1H), 3.62 (d, J = 5.4 Hz, 1H), 3.41 (d, 1H), 3.33 (d,
J = 2.8 Hz, 1H), 2.34 (m, 1H), 0.92 (m, 3H); 13C NMR (DMSO
75 MHz)
d 176.14, 172.20, 171.73, 169.10, 134.27, 130.26,
128.89, 128.05, 81.03, 76.21, 75.69, 72.30, 58.31, 40.78, 37.10,
282.1 (M+H+-Boc). [
a
]
D
25 = À26.5 (c 0.3 MeOH); Anal. Calcd for
17.26, 11.09; MS (ESI) Calcd for (C13H17NO3) m/z 235.28; found:
(C20H31NO6): C, 62.97; H, 8.19; N, 3.67; O, 25.15; found: C, 62.99;
H, 8.20; N, 3.66; O, 25.18.
264.3 (M+H+).
[a
]
D
25 = À12.2 (c 0.3 MeOH); Anal. Calcd for
(C13H17NO3): C, 66.36; H, 7.28; N, 5.95; O, 20.40; found: C,
66.36; H, 7.28; N, 5.94; O, 20.41.
4.10.
(2S,3R,4S)-Ethyl-5-(benzyl(tert-butoxycarbonyl)amino)-
2,3-dihydroxy-4-methylpentanoate 14
4.13. tert-Butyl-(2R,3S,4S)-4-(methoxycarbonyl)-3,4-dihydroxy-
2-methylbutylbenzylcarbamate 17
To a stirred solution of compound 13 (4 g, 0.01 mol) and tri-
ethyl amine (7 mL, 0.05 mol) in 50 mL DCM at 0 °C, SOCl2
(1.5 mL, 0.02 mol) was added dropwise and stirring was contin-
ued at 0 °C for 30 min. The reaction mixture was diluted with
25 mL of DCM after which 50 mL of H2O was added. The organic
layer was separated, washed with 2 M HCl solution followed by
saturated brine, and dried over anhydrous sodium sulfate. DCM
was removed under reduced pressure, Yield: 4.1 g (92%); IR
Osmium tetroxide (1.05 mL, 0.00010 mol of 0.1 molar) and
(1.23 g, 0.013 mol) methane sulfanamide were added to mixture
of K2Fe(CN)6 (12.78 g, 0.039 mol) K2CO3 (5.36 g, 0.03885 mol)
and (DHQ)2AQN (0.11 g, 0.00013 mol) in tert-butanol/water
(80:80 mL) and stirred for 15 min. Compound 12 (4.5 g,
0.013 mol) was then added and stirred at 0 °C for 18 h. Comple-
tion of the reaction was confirmed by TLC. Sodium sulfite
(Na2SO3, 18 g) was added and stirred at room temperature for
an hour. The reaction mixture was extracted using ethyl acetate.
The combined organic layer was washed with brine and dried
over anhydrous Na2SO4. Ethyl acetate was then removed under
reduce pressure and the crude was purified by column chro-
matography (silica 60–120) hexane/ethyl acetate (8:2). Yield:
(NaCl): 2973.9, 2931.2, 1688.0, 1665.7 cmÀ1 1H NMR (CDCl3,
;
300 MHz) d 7.36–7.18 (m, 5H), 5.33 (d, 1H, J = 4.6), 4.29 (q,
J = 8.63 Hz, 2H), 4.24 (q, J = 8.52 Hz, 1H), 4.16 (d, J = 7.17 Hz, 2H)
3.73 (d, J = 6.36 Hz, 1H), 3. 62 (d, J = 9.69 Hz, 1H) 2.05 (s, 1H),
1.46 (d, J = 7.29 Hz, 9H), 1.30 (t, J = 7.4 Hz, 3H) 1.24 (d,
J = 7.11 Hz, 3H); 13C NMR (CDCl3, 75 MHz)
d
179.34,
159.26,137.99, 128.47, 127.17, 80.14, 28.26, 28.19, 14.08. MS
4.65 g, (92.2%); IR (NaCl): 2972.9, 2930.2, 1689.0, 1666.7 cmÀ1
;
(ESI) Calcd for (C20H29NO7S) m/z 427.51; found: 282.1 (M+H+);
1H NMR (CDCl3, 300 MHz) d 7.36 (m, 3H), 7.21 (m, 2H), 4.35 (d,
1H), 4.26 (s, 3H), 3.72 (d, 2H), 3.65 (m, 1H), 3.24 (m, 1H), 2.1
(m, 1H), 1.46 (d, 9H), 1.16 (d, 3H); 13C NMR (DMSO 75 MHz) d
173.7, 137.9, 128.49, 137.47, 127.1, 71.3, 61.7, 28.2, 28.1, 22.5,
14.0; MS (ESI) Calcd for (C20H31NO6) m/z 381.46; found: 282.3
[
a
]
25 = À9.7 (c 0.3 MeOH); Anal. Calcd for (C20H29NO7S): C,
D
56.19; H, 6.84; N, 3.28; O, 26.20, S 7.50; found: C, 56.20; H,
6.83; N, 3.27; O, 26.21, S 7.49.
4.11. (2R,3S,4S)-Methyl 1-benzyl-3-hydroxy-4-methylpyrrolidine-
(M+H+-Boc).
[a]
25 = +16.5 (c 0.3 MeOH); Anal. Calcd for
D
2-carboxylate 15
(C20H31NO6): C, 62.97; H, 8.19; N, 3.67; O, 25.15; found: C,
62.97; H, 8.21; N, 3.64; O, 25.21.
Compound 14 (3.5 g) was dissolved in 20 mL after which 4 Å
molecular seives were added. The resulting mixture was stirred
at 0 °C for 10 min, then 3 ml of BF3 etherate was added and stirred
at room temperature for 4 h. The reaction mixture was filtered and
washed with a 2 M Na2CO3 solution. The organic layer was dried
over anhydrous sodium sulfate and removed under reduced pres-
sure. The crude product was purified by column chromatography
(silica 60–120 mesh) hexane/ethyl acetate (8:2). Yield: 1.36 g,
4.14. (2S,3R,4R)-Ethyl-5-(benzyl(tert-butoxycarbonyl)amino)-
2,3-dihydroxy-4-methylpentanoate 18
This compound was synthesized according to the procedure
used for the synthesis of compound 14 IR (NaCl): 2973.9, 2931.2,
1688.0, 1665.7 cmÀ1 1H NMR (CDCl3, 400 MHz) d 7.37–7.23 (m,
;
5H), 4.34 (d, J = 4.8 Hz, 1H), 4.28 (d, J = 2.6 Hz, 2H),3.46–3.40 (m,
2H), 3.19 (t, J = 12.0 Hz, 1H) 3.17 (q, J = 9.8 Hz, 2H), 3.11 (q,
J = 7.6 Hz, 2H) 1.46–1.47 (br, 1H), 1.33 (d, J = 5.12 Hz, 9H), 1.05 (t,
J = 9.96 Hz, 3H), 0.94 (d, J = 6.56 Hz, 3H); 13C NMR (CDCl3,
100 MHz) d 158.34, 128.55, 128.41, 127.31, 80.28, 62.57, 28.54,
13.94, 9.24, 8.55; MS (ESI) Calcd for (C20H29NO7S) m/z 427.51;
(63.06%); IR(NaCl): 1726.1 cmÀ1 1H NMR (DMSO, 300 MHz) d
;
7.28 (m, 5H), 4.21–4.26 (m, 4H), 4.03 (m, 2H), 3.78 (m, 2H), 3.74
(q, 1H), 3.08 (d, J = 2.04 Hz, 1H), 2.71 (m, 1H), 1.92 (m, 1H), 1.14
(m, 3H), 1.09 (t, J = 5.4, 3H); 13C NMR (DMSO, 75 MHz) d 173.01,
139.31, 139.02, 129.09, 129.00, 128.50, 128.47, 127.30, 81.70,
76.11, 75.25, 74.06, 60.34, 59.31, 58.11, 39.71, 37.37, 14.54. MS
(ESI) Calcd for (C15H21NO3) m/z 263.33; found: 264.3 (M+H+); Anal.
Calcd for (C15H21NO3): C, 68.42; H, 8.04; N, 5.32; O, 18.23; found: C,
68.42; H, 8.03; N, 5.32; O, 18.22.
found: 282.1 (M+H+); [ 25 = +9.3 (c 0.3 MeOH); Anal. Calcd for
a]
D
(C20H29NO7S): C, 56.19; H, 6.84; N, 3.28; O, 26.20, S 7.50; found:
C, 56.21; H, 6.81; N, 3.29; O, 26.23, S 7.48.
4.15. (2R,3R,4S)-Ethyl 1-benzyl-3-hydroxy-4-methylpyrrolidine-
4.12. (2R,3S,4S)-Methyl 1-benzyl-3-hydroxy-4-methylpyrrolidine-
2-carboxylate 19
2-carboxylate 16
This compound was synthesized according to the procedure
used for the synthesis of compound 15 1H NMR (CDCl3,
300 MHz) d 7.47 (m, 2H), 7.36 (m, 3H), 4.23 (m, 1H), 4.09 (m,
1H), 3.78 (br, 2H), 3.68 (br, 1H), 3.16 (m, 1H), 3.07 (t, 1H), 2.35
(br, 1H), 1.42, (m, 2H), 1.17 (m, 2H), 1.09 (m, 3H);13C NMR (CDCl3,
75 MHz) d 130.4, 130.19, 129.52, 129.03, 128.81, 80.49, 73.99,
63.02, 59.32, 55.14, 39.74, 37.22, 18.52, 17.10, 13.87, 13.75,
12.46; MS (ESI) Calcd for (C15H21NO3) m/z 263.33; found: 264.3
A solution of compound 15 (1.0 g, 37 mol) in 10 ml of metha-
nol was stirred at room temperature with 3 ml of 2 M LiOH for
2 h. The reaction mixture was acidified with 2 M KHSO4 solution
to pH 2 and extracted with ethyl acetate. The ethyl acetate
extract was washed with saturated brine solution and dried over
anhydrous sodium sulfate. The ethyl acetate was removed under
reduced pressure and crude product was purified by column
chromatography (silica 60–120 mesh) hexane/ethyl acetate
(8:2). Yield: 0.74 g (83%); IR (NaCl): 2931.1, 2359.7, 1716.1
(M+H+); [
a]
25 = À9.2 (c 0.3 MeOH); Anal. Calcd for (C15H21NO3):
D
C, 68.42; H, 8.04; N, 5.32; O, 18.23; found: C, 68.42; H, 8.03; N,
5.32; O, 18.22.
1622.2, 1456.0 cmÀ1 1H NMR (DMSO, 300 MHz) d 7.63 (m, 2H),
;

M. Hiremath et al. / Tetrahedron: Asymmetry 27 (2016) 261–267
267
the Proline Residues in L-671, 329 and L-688, 786. In Pro. 4th Chem. Congr. N.
Am., New York NY 1991, Abstr. No. 58.
Acknowledgments
9. Hammond, M. L. In Cutaneous Antifungal Agents; Rippon, J. W., Formtling, R. A.,
Eds.; Marcel Dekker I, 1993; p 395.
K.S.R. thanks DST, Indo-Korea (Grant no. INT/Korea/dated
13.09.2011) and DST-PURSE University of Mysore for financial sup-
port. DST-Fastrack, New Delhi (Grant no. B/F/5061/2013-14 dated
31-10-2013) is gratefully acknowledged.
10. Bouffard, F. A.; Zambias, R. A.; Dropinski, J. E.; Balkovec, J. M.; Hammond, M. L.;
Gabruzzo, K.; Bartizal, K. F.; Marrinan, J. A.; Kuntz, M. B.; McFadden, D. C.;
Nollstadt, K. H.; Powles, M. A.; Schmatz, D. M. J. Med. Chem. 1994, 37, 222.
11. Watson, A. A.; Fleet, G. W. J.; Asano, N.; Molyneux, R. J.; Nash, R. J.
Phytochemistry 2001, 56, 265.
12. Kanasaki, R.; Sakamoto, K.; Hashimoto, M.; Takase, M.; Taurumi, Y.; Fuje, A.;
Hino, M.; Hashimoto, S.; Hori, H. J. Antibiot. 2006, 59, 137.
13. Katz, E.; Williams, W. K.; Mason, K. T.; Mauger, A. B. Antimicrob. Agent
Chemother. 1997, 11, 1056.
14. Capobianco, O. J.; Zakula, D.; Frost, J. D.; Goldman, C. R.; Li, L.; Klein, L. L.;
Lartey, A. P. Antimicrob. Agent Chemother. 1998, 42, 389.
15. Bartizal, K. C. J.; Gill, G. K.; Abruzzo, A. M.; Flattery, L.; Kong, P. M.; Scott, J. G.;
Smith, C. E.; Leighton, A.; Bouffard, J. F.; Dropinski, J. M. Balkovec Antimicrob.
Agents Chemother. 1997, 41, 2326.
16. Shuichi, T.; Fumiaki, I.; Katsuyuki, M.; Yoshihiko, M.; Kazumi, O.; Noriko, T.;
Toshio, G.; Masaki, T.; Hidenori, O.; Akira, Y.; Koji, K.; Hisashi, T.; Kazuo, S.;
Hirokazu, T.; Fumio, M.; Shogo, K. Antimicrob. Agent Chemother. 2000, 44, 57.
17. Petraitis, V. R.; Petraitiene, A. H.; Groll, A.; Bell, D. P.; Callender, T.; Sein, R. L.;
Schufele, C. L.; McMillian, J.; Walsh, T. J. Antimicrob. Agents Chemother. 1998, 42,
2898.
Supplementary data
Supplementary data (copies of 1H NMR, and 13C NMR, spectra
for all compounds. Chairal HPLC spectra of compounds 8 and 10.
NOSY spectra of compound 15, and 19) associated with this article
can be found, in the online version, at http://dx.doi.org/10.1016/j.
tetasy.2016.02.008.
References
1. (a) Pearsons, W. H. In Studies in Natural Product Chemistry; Atta-Ur-Rahman,
Ed.; Elsevier: New York, 1988; Vol. 1, pp 323–358; (b) Bindra, J. S. In The
Alkolides; Manske, R. H. R., Ed.; Academic Press: New York, 1973; Vol. 14, p
84121; (c) Bellina, F.; Rossi, R. Tetrahedron 2006, 62, 7213; (d) Anotnchick, A. P.;
Gerding-Reimers, C.; Catarinella, M.; Schurmann, M.; Preut, H.; Ziegler, S.;
Rauh, D.; Waldmann, H. Nat. Chem. 2010, 2, 735.
2. (a) Sung, M. L.; Fowden, L. Phytochemistry 1968, 7, 2061; (b) Szymanovicz, G.;
Mercier, O.; Randoux, A.; Borel, J. P. Biochimie 1978, 60, 499.
3. Tschessche, R.; Samuel, T. D.; Uhlendorf, J.; Fehalaber, H. W. Chem. Ber. 1972,
105, 3106.
4. Sheehan, J. C.; Mania, D.; Nakamura, S.; Stock, J. A.; Maeda, K. J. Am. Chem. Soc.
1968, 90, 462.
5. Benz, F.; Knüsel, F.; Nuesch, J.; Treichler, H.; Voser, W.; Nyfeler, R.; Keller-
Schierlin, W. W. Helv. Chim. Acta 1974, 57, 2459.
6. Noble, H. M.; Langely, D.; Sidebottom, P. J.; Lane, S. J.; Fischer, P. J. Mycol. Res.
1991, 95, 1439.
7. (a) Formtling, R. A.; Aruzzo, G. K. J. Antibiot. 1989, 42, 174; (b) Wichmann, C. F.;
Liesch, J. M.; Schwartz, R. E. J. Antibiot. 1989, 42, 168; (c) Schwartz, R. E.;
Giacobbe, R. A.; Bland, J. A.; Monaghan, R. L. J. Antibiot. 1989, 42, 163.
8. (a) Schwartz, R. E.; Sesin, D. F.; Kempf, A. J.; Joshus, H.; Wilson, K. E.; Hensens,
O. D.; Liesch, J. M.; Smith, J. L.; White, R. F.; Zitano, L.; Salmon, P. M.; Gailliot, F.
P.; Gleason, C.; Schmatz, D. M.; Powles, M. A.; Fountoulakis, J. M.; Bartizal, K.;
Abruzzo, G.; Trainor, C. The Discovery, Isolation and Biological Activity of L-688,
786 and a Series of Related Lipopetides with Potent Anti-Pneumocystis Carinii
and Anti-Candida albicans Activities. In Pro. 4th Chem. Congr. N. Am., New York
NY 1991, Abstr. No. 181.; (b) Adefarati, A. A.; Hensens, O. D.; Jones, E. T. T.;
Tkacz, J. S. Biosynthesis of Echinocandin Type Antibiotics: Distinct Origins of
18. Inaba, H.; Hiraiwa, Y.; Yoshida, S.; Tamura, M.; Kuroda, A.; Ogawa, A.; Aoyagi,
Y.; Ohta, A.; Takeya, K. Heterocycles 2005, 65, 2657.
19. Meng, W. H.; Wu, T. J.; Zhang, H. K.; Huang, P. Q. Tetrahedron: Asymmetry 2004,
15, 3899.
20. Huang, P. Q.; Meng, W.-H. Lett. Org. Chem. 2004, 1, 99.
21. Raghavan, S.; Reddy, S. R. Tetrahedron Lett. 2003, 44, 7459.
22. Jao, E.; Bogen, S.; Saksena, A. K.; Girijavallabhan, V. Synthesis 2003, 17, 2643.
23. Langlois, N.; Rakotondradany, F. Tetrahedron 2000, 56, 2437.
24. Langlois, N. Tetrahedron: Asymmetry 1998, 9, 1333.
25. Panday, S. K.; Langlois, N. Synth. Commun. 1997, 27, 1373.
26. Herdeis, C.; Aschenbrenner, A.; Kirfel, A. Tetrahedron: Asymmetry 1997, 8, 2421.
27. Kurokawa, N.; Ohfune, Y. Tetrahedron 1993, 49, 6195.
28. Mulzer, J.; Becker, R.; Brunner, E. J. Am. Chem. Soc. 1989, 111, 7500.
29. Evans, D. A.; Weber, A. E. J. Am. Chem. Soc. 1987, 109, 7151.
30. Kurokawa, N.; Ohfune, Y. J. Am. Chem. Soc. 1986, 108, 6041.
31. Mohapatra, D. K.; Mondal, D.; Gonnade, R. G.; Chorghade, M. S.; Gurjar, M. K.
Tetrahedron Lett. 2006, 47, 6031.
32. Mohapatra, D. K.; Mondal, D.; Chorghade, M. S.; Gurjar, M. K. Tetrahedron Lett.
2006, 47, 9215.
33. (a) Barrow, A. R.; Hemscheidt, T.; Liang, J.; Paik, S.; Moore, E. R.; Tius, A. M. J.
Am. Chem. Soc. 1995, 117, 2479; (b) Alexander, R. F.; Huet, F. Tetrahedron:
Asymmetry 1998, 9, 2301.
34. (a) Sharpless, K. B.; Amber, W. J. Org. Chem. 1992, 57, 2768; (b) Lolb, H. C.;
Vanniewvenhze, M. S.; Sharpless, K. B. Chem. Rev. 1994, 94, 2483.
35. Magauer, T.; Myers, A. G. Org. Lett. 2011, 13, 5584.