Facile synthesis of N-Boc-(2S,5R)-5-(1′-hydroxy-1′-methylethyl)proline

Article abstract of DOI:10.1016/S0040-4020(01)00537-3

Coupling of chiral 1-O-benzylglycerol-2,3-bistriflate with trilithiated chiral 2-N-Boc-3-phenylsulfonylpropan-1-ol derivative constitutes an efficient route to chiral C-5 substituted prolines which can potentially induce cis Xaa-Pro peptide bond conformation. A straightforward seven-step synthesis of the title compound is described.

Full text of DOI:10.1016/S0040-4020(01)00537-3

TETRAHEDRON
Pergamon
Tetrahedron 57 82001) 6455±6462
Facile synthesis of N-Boc-ꢀ2S,5R)-5-
ꢀ10-hydroxy-10-methylethyl)proline
p
Qian Wang, Marie-Elise Tran Huu Dau, N. Andre Sasaki and Pierre Potier
Â
Institut de Chimie des Substances Naturelles, CNRS, Avenue de la Terrasse, 91198 Gif-sur-Yvette, France
Received 2 February 2001; revised 7 March 2001; accepted 2 April 2001
AbstractÐCoupling of chiral 1-O-benzylglycerol-2,3-bistri¯ate with trilithiated chiral 2-N-Boc-3-phenylsulfonylpropan-1-ol derivative
constitutes an ef®cient route to chiral C-5 substituted prolines which can potentially induce cis Xaa-Pro peptide bond conformation. A
straightforward seven-step synthesis of the title compound is described. q 2001 Elsevier Science Ltd. All rights reserved.
1. Introduction
cis-amide geometry for acquiring detailed knowledge of
the bioactive conformation and for the rational design of
therapeutics. For this purpose, two types of proline surro-
gates that induce cis-amide bond have recently been
developed. The ®rst approach is the construction of
constrained cis-peptidyl prolinamides which are synthe-
sized by tethering the C-2of Xaa to the C-2of proline,
or 4,5-fused 1,2,5-triazepine 3,6-diones,14 as shown in
Fig. 1.
Unlike other peptide amide bonds which adopt predomi-
nantly more stable trans conformation, Xaa-Pro peptide
8Xaaˆany proteinogenic amino acid) bond has approxi-
mately the same energy in both cis and trans conformer.
A substantial barrier for isomerization separates these two
isomers. The distribution of cis±trans-Xaa-Pro conformers
is determined by the steric interactions between the C-2
centers of the two amino acid residues in the cis rotamer,
and between the C-2center of Xaa and the functionality on
the C-5 of proline in the trans-isomer 8Scheme 1).
10±13
The second is based on the obvious assumption that intro-
duction of alkyl group8s) at the C-5 of proline may disfavor
a trans-amide isomer due to the steric interactions between
the C-5 substituent and Xaa residue. For example, while the
cis-isomer population was found to be about the same with
that of common proline-containing oligopeptides in most
solvents for N-acetyl-5-methylproline N0-methylamide, it
increased 20% for the C-5 diastereomer in nonpolar
solvents.15 In the case of 5,5-dimethyl proline, the ratio of
the cis/trans-isomer was reported to be 9:1 in N-Boc-l-Phe-
DL-5,5-dimethyl proline methyl ester.16 More recently,
an ef®cient methodology for the synthesis of all four
This cis±trans-isomerism of Xaa-Pro peptide bonds can
exert a signi®cant in¯uence upon a number of biologically
important processes, including protein folding, peptidyl-
propyl isomerases 8PPIases) activity and protein-ligand
recognition.1±4 In fact, the occurrence of cis-Xaa-Pro
peptide bond has been recognized as an important structural
feature in peptides and proteins.5±8 In particular, the
frequent observation of proline in b-turn,9 arisen from a
cis-peptidyl linkage, has evoked considerable interest
in the synthesis of proline analogs that speci®cally favor
Scheme 1. cis- and trans-Xaa-Pro peptide conformers.
Keywords: cis conformer; C-5 substituted proline.
Corresponding author. Tel.: 1330169823101; fax: 1330169077247;
p
Figure 1. Conformation of type VI-b-turn and some representative cis-
Xaa-Pro mimetics.
e-mail: andre.sasaki@icsn.cnrs-gif.fr
0040±4020/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4020801)00537-3

6456
Q. Wang et al. / Tetrahedron 57 ,2001) 6455±6462
increasing the cis-isomer population. In this report, we
describe a straightforward asymmetric synthesis of one of
the four diastereoisomers of 5-810-hydroxy-10-methylethyl)-
proline which is selected, based on molecular calculation, as
a potential cis-conformation inducer of the Xaa-Pro amide
bond.
Figure 2. 5-Alkylprolines.
2. Results and discussion
2.1. Modeling investigation
stereoisomers of 5-tert-butylproline has been reported by
Lubell and coworkers 8Fig. 2).17
2.1.1. Computational procedure. Taking into account the
number of degrees of freedom of the 82S,5R)-N-acetyl-5-810-
hydroxy-10-methylethyl)proline N0-methylamid 8IA) and its
85S)-diastereomer 8IB), 4000 conformations of each studied
compound were generated by random search Monte Carlo
method26 and optimized by TNCG Truncated Newton mini-
mization27 using the Macromodel 8Version 5.5) program28
with the AMBERp force ®eld29 and GB/SA water solvation.
Duplicate conformations as well as those that had chirality
change were discarded. From these conformational
searches, all the possible conformations within 3 kcal/mol
from the global minimum were considered. The structures
are shown in Figs. 3 and 4. The calculated steric energy
differences 8AMBER), as well as the most relevant
geometrical parameters of these structures are summarized
in Table 1.
When incorporated into dipeptides in the form of N-acetyl-
l-Xaa-82S,5R)-5-tert-butylproline N0-methylamides in
which Xaa represents Ala, Met, Leu, Val and Phe residues,
the cis-isomer population in peptides has increased to 74±
90% in water. Moreover, analyses by NMR, X-ray and CD
have con®rmed that these dipeptides adopt a type VI b-turn
conformation.18 It is reported that the Cys-Pro peptide bond
cis-isomer population in oxytocin has increased from 10 to
35% by replacing the proline residue with 82S,5R)-tert-
butylproline.19
We have been interested in developing ef®cient method-
ologies for the synthesis of conformationally constrained
amino acids that can be easily incorporated into bioactive
peptides starting from a common chiral synthon.20 These
conformationally constrained amino acids are illustrated in
Scheme 2.21±25
2.1.2. Energy analysis. The calculated steric energy analy-
sis indicates:
As for a proline surrogate that can induce Xaa-Pro amide
bond to adopt predominantly cis-amide bond conformation,
it occurred to us to introduce 1-hydroxy-1-methylethyl
group at the C-5 position of proline in a highly stereo-
selective manner. Our initial interest is to investigate
whether, in addition to the steric bulkiness of this tertiary
alcohol substituent, there can be any other effect such as
intramolecular hydrogen bonding caused by the inherent
hydroxyl group for stabilizing the cis-conformer and
² that in both cases 8IA, IB), the most stable conformation
corresponds to a cis-isomer;
² that the energy differences between the most stable cis-
and trans-isomers are 2.45 kcal/mol for IA and 3.75 kcal/
mol for IB.
These results reveal that the relative populations of the
amide cis- and trans-isomers of Xaa-Pro amide bond are
98.5/1.5 and 100/0 for IA and IB, respectively. According
Scheme 2. Conformationally constrained amino acids from the common chiral synthon.

Q. Wang et al. / Tetrahedron 57 ,2001) 6455±6462
6457
Figure 4. Conformations of the cis- and trans-isomers of IB.
Table 1. Energy differences and relevant parameters of the most stable cis-
and trans-isomers of IA and IB
cis
IA
trans
IA
IB
IAa
IB
DE 8kJ/mol)
DE 8kcal/mol)
0
0
0
0
10.23
2.45
12.74
3.05
15.66
3.75
Dihedral angles 8degrees)
C50 ±C30 ±N1±C28 v1)
10.68 217.12 2155.25 2158.91 153.95
C30 ±N1±C2±C60 8f2) 278.19 255.22 287.84 286.84 244.33
N1±C2±C60 ±N0 8c2) 212.78 227.56
3.47 34.32 238.32
49.59 240.57 43.23
O8OH)± C10 ±C2±N1 244.21
52.97
Ê
Distances 8A)
N1±H8OH)
Figure 3. Conformations of the cis- and trans-isomers of IA.
2.42
2.22
1.84
5.66
3.42
2.86
2.50
2.34
1.78
6.04
3.12
4.68
2.31
2.21
3.99
3.36
2.60
2.88
2.11
2.45
3.33
4.325.48
1.97
3.70
2.23
2.47
3.17
N1±H8N0H)
O40 ±H8OH)
O70 ±H8OH)
O40 ±H8N'H)
O8OH)± H8N'H)
to the calculations by Lubell and coworkers,18a the
relative populations of the amide cis-isomer are 46 and
77% for 82S,5R)-N-acetyl-5-tert-butylproline N0-methyl-
amide and 82S,5S)-N-acetyl-5-tert-butylproline N0-methyl-
amide, respectively.
3.28
4.90
a
The trans isomer of IA presenting an hydrogen bond between the hydro-
gen of the N0-methylamide with the oxygen atom of the carbonyl group of
the acetyl function. The dihedral angles are in degrees and the distances
2.1.3. Conformational analysis of IA. The conformational
analysis reveals:
Ê
are in A.
the carbonyl group of the acetyl function. Moreover, in
this conformation, the hydrogen of the N0-methylamide
Ê
is close to the N-1 atom 82.22 A) by stabilizing its lone-
pair electron;
² that the most stable cis-isomer of IA gives rise to hydrogen
Ê
bond 81.84 A) between the hydrogen of the hydroxyl
group and the oxygen atom of the carbonyl group
attached to the nitrogen atom N-1 of the pyrrolidine
ring. The value of O8OH)-C-10 ±C-5±N-1 dihedral
angle is 244.28 indicating that the hydroxyl group is
outside of the pyrrolidine ring and is pointed toward
² that in the most stable trans-isomer of IA, the value of
O8OH)±C-10 ±C-5±N-1 dihedral angle is 49.68, that is to
say, the hydroxyl group is pointed toward inside of the

6458
Q. Wang et al. / Tetrahedron 57 ,2001) 6455±6462
pyrrolidine ring. In this conformation, the hydrogen of
the N0-methylamide as well as the hydrogen of the
hydroxyl group are close to the N-1 atom 82.21 and
mations of IB, these values are c2 ˆ 227:6 and 238.38,
respectively.
Ê
2.31 A, respectively) by stabilizing its lone-pair electron.
The calculations also indicate that this trans-isomer is
more stable than the trans-isomer which gives rise to
Based on these conformational analyses, we commenced
our study by synthesizing ®ve-substituted proline derivative
8, the pivotal precursor of IA, with anticipation that this
amino acid can induce proline containing peptides to
adopt cis conformation at Xaa-810-hydroxy-10-methylethyl)-
Pro amide bond.
Ê
hydrogen bond 81.97 A) between the hydrogen of the
N0-methylamide and the O-40 atom of the carbonyl
group attached to the N-1 atom. Their energy difference
is 0.60 kcal/mol. It means that the last one hardly exists.
2.2. Synthesis
Recently, we have described a concise one-step synthesis
of enantiopure 2,5-disubstituted pyrrolidine derivatives.30
The speci®c feature of this method is that any desired
diastereomer out of the four possible isomers can be
prepared from readily available chiral synthons in the
same ef®ciency without separation of diastereoisomers.
Synthesis of the title compound 8 was shown in Scheme
3. The pivotal component of our synthetic strategy is the
one-step construction of enantiopure 2,5-dihydroxymethyl-
pyrrolidine derivative in which the two hydroxymethyl
groups are differentially protected. The next sequence is
the transformation of one hydroxyl group to 1-hydroxy-1-
methylethyl group followed by the oxidation of the other.
Thus treatment of chiral synthon 8S)-1 with 3 equiv. of BuLi
generated the corresponding trianion which was then
coupled with 82S)-1-O-benzylglycerol-2,3-bistri¯ate30 to
give pyrrolidine 2. The stereochemistry of C-3 was of no
consequence as it will be destroyed at the later stage. Selec-
tive removal of TBDMS group under mild conditions
8TBAF, THF) followed by desulfonylation 86% Na±Hg in
MeOH) gave compound 4 in 62% overall yield 8three steps).
On the contrary, desulfonylation of comopound 2 followed
by removal of TBDMS group gave inferior results. The
oxoammonium salt mediated oxidation of primary alcohol
using TEMPO-NaOCl gave the acid that was immediately
treated with CH2N2 to afford the methyl ester 5 in 90%
yield. Nucleophilic addition of MeMgBr to the ester func-
tion gave the tertiary alcohol 6 in 98% yield without
epimerization. Hydrogenolysis followed by Jones oxidation
afforded carboxylic acid 8. It is worthy noting that TEMPO
oxidation of 7 in this case gave only 18% of the acid 8, the
major product being the lactone 9 871%). The presence of
tertiary alcohol adjacent to the amino function was known to
cause problem in the N-deprotection step under acidic
conditions. Easy formation of oxazolidinone is a well-
established side reaction.31 In our hands, the desired depro-
tection was realized using 1 M solution of HCl in AcOEt to
provide amino alcohol in 96% yield. Compound 11 was
treated with Ac2O/Et3N in CH2Cl2 for 48 h to give
compound 12. NMR analysis shows that the ratio of cis
and trans-isomers is about 50/50 in CDCl3.
2.1.4. Conformational analysis of IB. The conformational
analysis reveals:
² that the most stable cis-isomer of IB gives rise to hydro-
Ê
gen bond 81.78 A) between the hydrogen of the hydroxyl
and the oxygen atom of the carbonyl group of the acetyl
function. The value of O8OH)±C-10 ±C-5±N-1 dihedral
angle is 53.08. This indicates that the hydroxyl group is
outside of the pyrrolidine ring and is pointed toward the
carbonyl group of the acetyl function. In this conform-
ation, the hydrogen of the N0-methylamide is close to the
Ê
N-1 atom 82.34A) by stabilizing its lone-pair electron;
² that in the most stable trans-isomer of IB, the value of
O8OH)±C-10 ±C-5±N-1 dihedral angle is 43.28, that is to
say, the hydroxyl group is outside of the pyrrolidine ring
and is pointed toward the carbonyl group of the acetyl
function. In this conformation, the hydrogen of the
Ê
hydroxyl group is close to the N-1 atom 82.23 A) by
stabilizing its lone-pair. However, the important energy
difference with the most stable cis-isomer indicates that
this trans-isomer does not exist. From these comparisons,
it is suggested that when there is no hydrogen bond, the
hydroxy group is placed inside of the pyrrolidine ring.
This conformation minimizes the steric interaction and
makes it energetically a more favorable one.
2.1.5. Modeling conclusion. On the one hand, owing to the
partial double-bond character, it is normally possible for v
to assume values only in the neighborhood of 0 or 1808
which is the value generally observed 8i.e. the trans confor-
mation). A fully extended polypeptide is characterized by
f ˆ c ˆ v ˆ 1808: Moreover, in proline-containing poly-
peptide, the corresponding dihedral angle f 8C-30 ±N-1±C-
2±C-60) is 271.68. On the other hand, the values for the
dihedral angles of an ideal typeVI b-turn are f1 ˆ 2608;
c1 ˆ 1208; v1 ˆ 08; f2 ˆ 2908; c2 ˆ 08:
Taking into account of these ideal values ꢀv1 ˆ 08; f2 ˆ
C-30±N-1±C-2±C-60 ˆ 271:68 and c2 ˆ 08† and the pos-
sible hydrogen bonding, the comparison of the dihedral
angles of the most stable cis-isomer of IA ꢀv1 ˆ 10:78;
f2 ˆ C-30±N-1±C-2±C-60 ˆ 278:28 and c2 ˆ 212:88†
with those of the trans-isomer of IA ꢀv1 ˆ 2155:38; f2 ˆ
C-30±N-1±C-2±C-60 ˆ 287:88 and c2 ˆ 3:58† allows a
better understanding of their relative stability. The most
stable trans-isomer of IA is less stable than its cis-isomer
because it gives rise to no hydrogen bond. Moreover, in the
most stable cis and trans conformations of IA, the values of
the dihedral angle c2 are close to 08 8c2 ˆ 212:8 and 3.58,
respectively) while in the most stable cis and trans confor-
In conclusion, one of the four stereoisomers of 5-810-
hydroxy-10-methylethyl)proline derivative 8 has been
chosen as a potential cis conformation inducer in the Xaa-
Pro amide bond. An ef®cient synthesis of 8 has been
effected from readily available chiral synthons using one-
step construction of enantiopure 2,5-disubstituted pyrro-
lidine as a key component of this approach. Further appli-
cations of this methodology to the synthesis of the
C-5 diastereomer of 8 and the incorporation of these C-5

Q. Wang et al. / Tetrahedron 57 ,2001) 6455±6462
6459
Scheme 3. Reagents and conditions: 8a) BuLi 83 equiv.), THF, 2708C, 30 min then 82S)-1-O-benzyglycerol-2,3-bistri¯ate, 2708C, 3 h; 8b) Bu4NF, THF, rt,
2h, 64% for two steps; 8c) 6% Na±Hg, Na 2HPO4, MeOH, 08C, 24 h, 92%; 8d) 81) TEMPO, NaOCl, KBr, 5% aq. NaHCO3, acetone, 08C, 2h; 82) CH 2N2,
CH3OH±Et2O, 90%; 8e) CH3MgBr, THF, 08C, 30 min, rt, 1 h, 98%; 8f) H2 81 atm), 10% Pd±C, EtOAc, rt, 5 h, 95%; 8g) Jones reagent, acetone, 08C, 8 h, 92%;
8h) CH3NH2´HCl, DIEA, TBTU, CH3CN, rt, 36 h, 75%; 8i) 1 M HCl±EtOAc, rt, 2h, 96%; 8j) Ac 2O, Et3N, CH2Cl2, rt, 48 h, 85%.
substituted proline analogs into peptides are currently under
way in our laboratory.
THF 870 mL) was added dropwise BuLi 81.6 M, 14.6 mL,
23.43 mmol, 3 equiv.) at 2788C. After stirring for 30 min at
the same temperature, a solution of 82S)-1-O-benzylgly-
cerol-2,3-bistri¯ate30 84.87 g, 10.93 mmol, 1.4 equiv.) in
THF 88 mL) was added dropwise, and stirring was contin-
ued for 3 h at 2788C then at rt for 1 h. The reaction was
quenched by addition of satd. NH4Cl. The reaction mixture
was extracted with ether. The ether extracts were washed
with brine, dried and evaporated. Column chromatography
on silica gel eluting with heptane/EtOAc 810/1 then 8/1)
afforded a mixture of 2 and the starting material 1 which
was used directly for the next reaction. Pure compound 2
was obtained by preparative TLC 8silica gel, toluene/
3. Experimental
3.1. General
Infrared 8IR) spectra were recorded on a Perkin±Elmer
Spectrum BX spectrometer. 1H and 13C NMR spectra
were measured on Brucker AM-300 and AC-250 8300 and
250 MHz, respectively) spectrometers with tetramethyl-
silane as internal standard 8d ppm). Flash chromatography
was performed using Kieselgel 60 8230±400 mesh, E.
Merck) and usually employed a stepwise solvent polarity
gradient, correlated with TLC mobility. Solvents and
reagents were puri®ed according to standard laboratory
techniques. Optical rotations were determined on a
Perkin±Elmer 241 polarimeter at room temperature. Mass
spectra were run on an AEI MS-9 spectrometer 8CI), and
Nawigator Thermo quest spectrometer 8ESI), respectively.
Elemental analyses were carried out at the ICSN. All reac-
tions requiring anhydrous conditions or in an inert atmos-
phere were conducted under an atmosphere of Argon.
EtOAcˆ6/1) as
a
single isomer 8two rotamers):
[a]Dˆ210 8c 4.0, CHCl3); IR 8CHCl3) 3029, 3012, 2957,
2931, 2859, 1690, 1472, 1448, 1394, 1369, 1308,
1
1257 cm21; H NMR 8250 MHz, CDCl3) d 7.91±7.89 8m,
2H), 7.69±7.54 8m, 3H), 7.32±7.26 8m, 5H), 4.51±4.40 8m,
2H), 4.29 8m, 1H), 4.04 8dq, Jˆ3.17, 7.19 Hz, 1H), 3.80
8quintet, Jˆ4.1 Hz, 1H), 3.67±3.33 8m, 4H), 2.69±2.25
8m, 2H), 1.48, 1.41 8two s, 9H), 0.80, 0.76 8two s, 9H),
20.069, 20.10, 20.128three s, 6H);
13C NMR
862.5 MHz, CDCl3) d 154.0, 153.7, 138.1, 138.0, 137.6,
133.8, 129.3, 128.7, 128.6, 128.3, 127.5, 127.4, 80.1, 73.1,
70.8, 70.1, 63.5, 62.7, 62.4, 61.7, 60.2, 56.9, 28.4, 28.3,
28.0, 27.3, 25.8, 18.1, 25.7; MS 8ESI) m/z 576 [M1H]1.
HRMS Calcd for C30H46NO6SSi: 576.28149; Found:
576.28255.
3.1.1. ꢀ2S,5S)-1-tert-Butoxycarbonyl-2-tert-butyldimethyl-
silyloxymethyl-3-phenylsulfonyl-5-benzyloxymethyl
pyrrolidine ꢀ2). To a solution of 1 83.35 g, 7.81 mmol) in

6460
Q. Wang et al. / Tetrahedron 57 ,2001) 6455±6462
3.1.2. ꢀ2S,5S)-1-tert-Butoxycarbonyl-2-hydroxymethyl-
3-phenylsulfonyl-5-benzyloxy-methyl pyrrolidine ꢀ3).
To a solution of 2 obtained above 8contaminated with
compound 1) 83.88 g, 6.75 mmol) in THF 860 mL) was
added tetrabutylammonium ¯ouride 81 M solution in THF,
7.5 mL, 7.5 mmol) at room temperature. The reaction
mixture was stirred at room temperature for 1 h. The solvent
was evaporated, and the residue was separated in EtOAc and
aq. NaHCO3. The aqueous layer was extracted with EtOAc.
The EtOAc extracts were washed with brine, dried and
evaporated. Column chromatography on silica gel 8hep-
tane/EtOAcˆ2/1) gave 3 as an oil 82.31 g, 64% for two
steps): [a]Dˆ237 8c 2.6, CHCl3); IR 8CHCl3) 3425, 3029,
3013, 2981, 2932, 2871, 1692, 1478, 1448, 1392, 1369,
®ed to pH 6 with 10% KHSO4 and extracted with EtOAc.
The combined organic extracts were dried, concentrated and
treated with excess diazomethane in ether, then evaporated.
The residue was puri®ed by ¯ash chromatography 8silica
gel, heptane/EtOAcˆ10/1 then 5/1) gave compound 5
8oil) as two rotamers 8924 mg, 90%): [a]Dˆ26 8c 2.4,
CHCl3); IR 8CHCl3) 3024, 3013, 2982, 2955, 2868, 1749,
1
1691, 1478, 1455, 1438, 1396, 1368, 1167, 1096 cm21; H
NMR 8250 MHz, CDCl3) d 7.34±7.33 8m, 5H), 4.62±4.49
8m, 2H), 4.33±4.17 8m, 2H), 3.86±3.76 8m, 1H), 3.70 8s,
3H), 3.49±3.36 8m, 1H), 2.25±1.91 8m, 4H), 1.41 8s, 9H);
13C NMR 875 MHz, CDCl3) d 173.5, 173.3, 153.9, 153.4,
138.4, 138.3, 128.1, 127.3, 79.9, 73.0, 70.7, 70.3, 59.8, 59.4,
57.3, 51.8, 51.7, 28.7, 28.1, 28.1, 27.8, 27.2; MS 8ESI) m/z
350 [M1H]1, 372[M 1Na]1, 388 [M1K]1, 72
[2M1Na]1, 737 [2M1K]1. Anal. Calcd for C19H27NO5:
C, 65.31; H, 7.79; N, 4.01; Found: C, 65.25; H, 7.91; N,
3.85.
1
1308, 1150 cm21; H NMR 8250 MHz, CDCl3) d 7.91±
1
7.89 8m, 2H), 7.70±7.55 8m, 3H), 7.30±7.25 8m, 5H),
4.51±4.47 8m, 3H), 3.98±3.32 8m, 7H), 2.53±2.43 8m,
2H), 1.48, 1.39 8two s, 9H); 13C NMR 862.5 MHz, CDCl3)
d 154.2, 153.5, 137.0, 136.9, 133.9, 129.3, 128.5, 128.3,
127.8, 127.6, 80.4, 73.1, 70.1, 69.6, 65.0, 64.5, 60.7, 57.0,
28.6, 28.1; MS 8ESI) m/z 462[M 1H]1, 484 [M1Na]1,
500 [M1K]1. Anal. Calcd for C24H31NO6S: C, 62.45; H,
6.77; N, 3.03; S, 6.95; Found: C, 62.44; H, 6.91; N, 3.01; S,
6.89.
3.1.5. ꢀ2S,5R)-1-tert-Butoxycarbonyl-2-benzyloxymethyl-
5-ꢀ10-hydroxy-10-methylethyl)-pyrrolidine ꢀ6). To a solu-
tion of 5 8665 mg, 1.90 mmol) in THF 819 mL) at 08C was
added a solution of MeMgBr in ether 83.0 M, 1.5 mL,
4.56 mmol). The reaction mixture was stirred at 08C for
30 min, then at room temperature for 1 h. The reaction
was quenched by addition of saturated aqueous NH4Cl at
08C, and the reaction mixture was extracted with EtOAc.
The organic layers were washed with brine, dried and
evaporated. Column chromatography on silica gel 8hep-
tane/EtOAcˆ5/1) afforded compound 6 8650 mg, 98%):
[a]Dˆ2128 c 4.1, CHCl3); IR 8CHCl3) 3410, 2982, 2869,
3.1.3. ꢀ2S,5R)-1-tert-Butoxycarbonyl-5-hydroxymethyl-
2-benzyloxymethyl pyrrolidine ꢀ4). To a solution of the
sulfone 3 8990 mg, 2.15 mmol) in HPLC grade MeOH
843 mL) containing Na2HPO4 83.66 g, 25.77 mmol,
12equiv.) was added 6% Na±Hg 87.41 g, 19.33 mmol,
9 equiv.) at 08C. The mixture was vigorously stirred at
08C for 24 h. Mercury was removed by decanting the reac-
tion mixture. After evaporation of MeOH in vacuo, the resi-
due was dissolved in water and EtOAc. The aqueous layer
was extracted with EtOAc. The EtOAc extracts were
washed with brine, dried and evaporated. Flash chroma-
tography on silica gel 8heptane/EtOAcˆ4/1) afforded
compound 4 as an oil 8635 mg, 92%): [a]Dˆ22 5 c8 2.2,
CHCl3); IR 8CHCl3) 3415, 3030, 3010, 2980, 2871, 1683,
1674, 1455, 1393, 1368, 1171, 1111 cm21 1H NMR
;
8250 MHz, CDCl3) d 7.37±7.328m, 5H), 4.54 8s, H2 ),
4.00 8m, 1H), 3.828m, 1H), 3.60 8m, 1H), 3.47±3.43 8m,
1H), 2.12±1.85 8m, 4H), 1.42 8s, 9H), 1.19 8s, 3H), 1.09 8s,
3H); 13C NMR 875 MHz, CDCl3) d 156.5, 137.2, 128.1,
127.6, 127.5, 79.6, 72.9 72.5, 70.3, 67.7, 58.6, 28.0, 27.8,
26.6, 26.4, 24.5; MS 8ESI) m/z 372[M 1Na]1, 388
[M1K]1. Anal. Calcd for C20H31NO4: C, 68.74; H, 8.94;
N, 4.01; Found: C, 68.86; H, 9.03; N, 3.91.
1477, 1455, 1402, 1368, 1167, 1117 cm21 1H NMR
;
8250 MHz, CDCl3) d 7.328m, 5H), 4.58±4.48 8m, 2H),
4.00±3.46 8m, 7H), 1.98±1.68 8m, 4H), 1.43 8s, 9H); 13C
NMR 862.5 MHz, CDCl3) d 156.3, 137.9, 128.3, 127.6,
127.6, 80.2, 73.2, 71.0, 67.2, 61.2, 58.3, 28.3, 27.2,
26.9; MS 8ESI) m/z 322 [M1H]1, 344 [M1Na]1, 360
[M1K]1, 665 [2M1Na]1. Anal. Calcd for C18H27NO4:
C, 67.26; H, 8.47; N, 4.36; Found: C, 67.25; H, 8.55; N,
4.31.
3.1.6. ꢀ2S,5R)-1-tert-Butoxycarbonyl-2-hydroxymethyl-
5-ꢀ10-hydroxy-10-methylethyl)-pyrrolidine ꢀ7). A suspen-
sion of 6 8864 mg, 2.48 mmol) and 10% Pd/C 8132 mg) in
EtOAc 825 mL) was hydrogenated at 1 atm for 5 h. The
reaction mixture was ®ltered through a short pad of Celite,
the ®ltrate was evaporated in vacuo and puri®ed by ¯ash
chromatography on silica gel 8heptane/EtOAcˆ1/1) to
afford compound 7 8608 mg, 95%): [a]Dˆ13.4 8c 4.4,
CHCl3); IR 8CHCl3) 3376, 2981, 1676, 1393, 1368, 1255,
1169, 1112cm 21; 1H NMR 8250 MHz, CDCl3) d 4.35 8br s,
2H), 3.97±3.84 8m, 3H), 3.58 8dd, Jˆ10.6, 3.2Hz), 2.07±
1.87 8m, 4H), 1.47 8s, 9H), 1.22 8s, 3H), 1.13 8s, 3H); 13C
NMR 862.5 MHz, CDCl3) d 156.6, 80.0, 73.2, 67.2, 62.7,
60.4, 28.2, 27.9, 26.4, 26.1, 25.1; MS 8ESI) m/z 260
[M1H]1, 282 [M1Na]1, 298 [M1K]1, 519 [2M1H]1,
541 [2M1Na]1, 557 [2M1K]1. Anal. Calcd for
C13H25NO4: C, 60.21; H, 9.72; N, 5.40; Found: C, 60.29;
H, 9.96; N, 5.12.
3.1.4. ꢀ2S,5R)-1-tert-Butoxycarbonyl-2-benzyloxymethyl-
5-methoxycarbonyl-pyrrolidine ꢀ5). To a solution of 4
8945 mg, 2.94 mmol) in acetone 822 mL) was added an
aqueous 5% NaHCO3 solution 87.6 mL). This hetero-
geneous mixture was cooled to 08C and treated sequentially
with KBr 835 mg, 0.294 mmol) and TEMPO 8506 mg,
3.24 mmol). Sodium hypochlorite 85% solution in water,
5.5 mL, 3.70 mmol) was then added dropwise while the
mixture was vigorously stirred and maintained at 08C.
After 1 h, additional NaOCl 85% solution in water,
4.3 mL, 2.94 mmol) was added, and stirring was continued
at 08C for another hour followed by addition of 5% NaHCO3
solution. When acetone was removed on a rotary evapo-
rator, the aqueous layer was washed with ether twice, acidi-
3.1.7. ꢀ2S,5R)-1-tert-Butoxycarbonyl-2-hydroxycarbonyl-
5-ꢀ10-hydroxy-10-methylethyl)-pyrrolidine ꢀ8). Jones
reagent 82.67 M, 2.5 mL, 6.56 mmol) was added dropwise

Q. Wang et al. / Tetrahedron 57 ,2001) 6455±6462
6461
to a solution of 7 8425 mg, 1.64 mmol) in acetone 816 mL)
at 08C. The mixture was stirred at 08C for 8 h. Acetone was
evaporated, and the residue was dissolved in aq. NaHCO3,
extracted with ether 82£), acidi®ed with 10% KHSO4 to
pHˆ6, extracted with EtOAc. The EtOAc extracts were
dried and evaporated to give the carboxylic acid 8
8410 mg, 92%), which was puri®ed by column chroma-
tography 8silica gel, CH2Cl2/MeOHˆ95/5): [a]Dˆ118 8c
2.5, CHCl3); IR 8CHCl3) 3343, 3175, 3026, 2983, 2937,
3.1.11. ꢀ2S,5R)-1-Acetyl-2-methylaminocarbonyl-5-ꢀ10-
hydroxy-10-methylethyl)-pyrrolidine ꢀ12). A mixture of
11 8110 mg, 0.50 mmol), Et3N 8152mg, 210 mL,
1.5 mmol), AC2O 8102mg, 95 mL, 1.0 mmol) in dichloro-
methane was stirred at rt for 48 h, then evaporated to
dryness. The residue was puri®ed by ¯ash column chroma-
tography 8silica gel, CH2Cl2/MeOHˆ95/5) to give 12
897 mg, 85%) as white solid: [a]Dˆ150.6 8c 2.0, MeOH);
IR 8nujol) 3452, 3331, 3062, 3015, 1667, 1632, 1538, 1403,
1207, 1046 cm21; 1H NMR 8250 MHz, CDCl3) d 7.78 8br q,
Jˆ4.3 Hz, 0.5 H), 7.48 8br q, Jˆ4.7 Hz, 0.5 H), 5.70 8br s,
0.5 H), 5.28 8br s, 0.5 H), 4.50 8t, Jˆ8.8 Hz, 0.5 H), 4.36 8t,
Jˆ8.1 Hz, 0.5 H), 4.28 8br d, Jˆ6.4 Hz, 0.5 H), 3.83 8d,
Jˆ7.7 Hz, 0.5 H), 2.82 8d, Jˆ4.7 Hz, 0.5 H), 2.78 8d,
Jˆ4.7 Hz, 0.5 H), 2.42±1.78 8m, 4 H), 2.14 8s, 0.5 H),
2.04 8s, 0.5 H), 1.32, 1.28, 1.23, 1.21 84S, 6H); 13C NMR
862.5 MHz, CDCl3) d 175.4, 174.8, 172.3, 171.6, 73.5, 72.7,
68.5, 66.7, 63.0, 61.1, 31.0, 29.2, 28.6, 28.3, 27.6, 27.1,
26.4, 26.3, 22.8, 22.7; MS 8CI) m/z 229 [M1H]1. HRMS
Calcd for C11H21N2O3: 229.1552; Found: 229.1525.
1
1701, 1682, 1476, 1455, 1369, 1168, 1145, 1116 cm21; H
NMR 8250 MHz, CDCl3) d 7.33 8br s, 1H), 4.41±4.38 8m,
1H), 3.93 8m, 1H), 2.37±1.88 8m, 4H), 1.45 8s, 9H), 1.30 8s,
3H), 1.22 8s, 3H); 13C NMR 862.5 MHz, CDCl3) d 177.4,
155.9, 81.2, 74.5, 67.0, 61.0, 29.2, 28.5, 28.1, 27.0, 25.0;
MS 8ESI) m/z 274 [M1H]1, 296 [M1Na]1. Anal. Calcd for
C13H23NO5: C, 57.13; H, 8.48; N, 5.12; Found: C, 57.25; H,
8.69; N, 4.91.
3.1.8. Lactone ꢀ9). [a]Dˆ114 8c 3.0, CHCl3); IR 8CHCl3)
3027, 2985, 1740, 1698, 1423, 1392, 1370, 1332, 1310,
1
1282, 1153, 1125 cm21; H NMR 8250 MHz, CDCl3) d
4.70 8br s, 1H), 4.20 8br s, 1H), 2.16±1.89 8m, 4H), 1.47
8s, 12H), 1.39 8s, 3H); 13C NMR 862.5 MHz, CDCl3) d
168.7, 153.0, 85.6, 81.0, 58.7, 57.1, 29.2, 28.2, 27.2, 24.6,
22.4; MS 8ESI) m/z 256 [M1H]1, 278 [M1Na]1, 533
[2M1Na]1. Anal. Calcd for C13H21NO4: C, 61.16; H,
8.29; N, 5.49; Found: C, 61.56; H, 8.42; N, 5.21.
References
1. Scholz, C.; Scherer, G.; Mayr, L. M.; Schlindler, T.;
Fischer, G.; Schmid, F. X. Biol. Chem. 1998, 379, 361±365.
2. Weiwad, M.; KuÈllertz, G.; Schutkowski, M.; Fischer, G. FEBS
Lett. 2000, 478, 39±42.
3.1.9. ꢀ2S,5R)-1-tert-Butoxycarbonyl-2-methylaminocar-
bonyl-5-ꢀ10-hydroxy-10-methyl-ethyl)-pyrrolidine ꢀ10).
To a solution of 8 8330 mg, 1.20 mmol) in acetonitrile
812mL) was added DIEA 81.24 g, 1.7 mL, 9.6 mmol),
MeNH2´HCl 8194 mg, 2.88 mmol) and TBTU 8694 mg,
2.16 mmol). The reaction mixture was stirred at room
temperature for 36 h, and partitioned between brine and
EtOAc. The aqueous layer was extracted with EtOAc
83£). The organic extracts were washed with 0.1 M HCl,
water, 5% NaHCO3 and brine, dried and evaporated. Flash
column chromatography on silica gel 8heptane/EtOAcˆ1/3
then EtOAc) afforded compound 10 8257 mg, 75%):
[a]Dˆ12 6 c8 3.5, CHCl3); IR 8CHCl3) 3453, 3344, 2982,
3. Fischer, G. Chem. Soc. Rev. 2000, 29, 119±127.
4. Konno, M.; Ito, M.; Hayano, T.; Takahashi, N. J. Mol. Biol.
1996, 256, 897±908.
5. 8a) Larive, C. K.; Guerra, L.; Rabenstein, D. L. J. Am. Chem.
Soc. 1992, 114, 7331±7337. 8b) Larive, C. K.; Rabenstein,
D. L. J. Am. Chem. Soc. 1993, 115, 2833±2836.
6. Yamazaki, T.; Ro, S.; Goodman, M.; Chung, N. N.; Schiller,
P. W. J. Med. Chem. 1993, 36, 708±719.
7. Adjadj, E.; Naudat, V.; Mispelter, J.; Quiniou, E.; Tartar, A.;
Craescu, C. T. Toxicon 1996, 34, 1082.
8. Jabs, A.; Weiss, M. S.; Hilgenfeld, R. J. Mol. Biol. 1999, 286,
291±304.
1
2936, 1673, 1539, 1393, 1378, 1369, 1169, 1149 cm21; H
9. Hutchinson, E. G.; Thornton, J. M. Protein Sci. 1994, 3 812),
2207±2216.
NMR 8250 MHz, CDCl3) d 4.25 8t, Jˆ8.3 Hz, 1H), 3.89, 8d,
Jˆ7.8 Hz, 1H), 2.82 8d, Jˆ4.8 Hz, 3H), 2.29±1.81 8m, 4H),
1.45 8s, 9H), 1.29 8s, 3H), 1.18 8s, 3H); 13C NMR
862.5 MHz, CDCl3) d 176.0, 155.6, 80.5, 73.0, 66.8, 61.7,
29.6, 28.8, 28.2, 26.9, 26.5, 26.2; MS 8ESI) m/z 287
[M1H]1. Anal. Calcd for C14H26N2O4: C, 58.72; H, 9.15;
N, 9.78; Found: C, 58.31; H, 9.32; N, 9.63.
10. Gramberg, D.; Weber, C.; Beeli, R.; Inglis, J.; Bruns, C.;
Robinson, J. A. Helv. Chim. Acta 1995, 78, 1588±1606.
11. Kim, K.; Dumas, J.-P.; Germanas, J. P. J. Org. Chem. 1996,
61, 3138±3144.
12. Curran, T. P.; McEnaney, P. M. Tetrahedron Lett. 1995, 36,
191±194.
13. Tong, Y.; Olczak, J.; Zabrocki, J.; Gershengorn, M. C.;
Marshall, G. R.; Moeller, K. D. Tetrahedron 2000, 56,
9791±9800.
3.1.10. ꢀ2S,5R)-2-Methylaminocarbonyl-5-ꢀ10-hydroxy-
10-methylethyl)-pyrrolidine hydrochloride ꢀ11). A solu-
tion of 10 8149 mg, 0.52mmol) in 1 M HCl±EtOAc was
stirred at room temperature for 2h. The solvent was
removed in vacuo to give compound 11 8110 mg, 95%):
[a]Dˆ239 8c 2.1, MeOH); IR 8nujol) 3236, 2923, 1660,
1576, 1461, 1378, 1284 cm21; 1H NMR 8300 MHz,
CD3OD) d 4.34 8dd, Jˆ10.0, 3.8 Hz, 1H), 3.63 8dd,
Jˆ11.0, 6.2 Hz, 1H), 2.81 8s, 3H), 2.51±2.37 8m, 1H),
2.16±1.99 8m, 2H), 1.94±1.79 8m, 1H), 1.35 8s, 3H), 1.29
8s, 3H); 13C NMR 862.5 MHz, CD3OD) d 170.4, 70.7, 69.0,
59.9, 31.1, 28.8, 26.8, 25.7; MS 8ESI) m/z 187 [M1H]1.
HRMS Calcd for C9H19N2O2: 187.14468; Found: 187.1459.
14. Lenman, M. M.; Ingham, S. L.; Gani, D. J. Chem. Soc., Chem.
Commun. 1996, 85±87.
15. Delaney, N. G.; Madison, V. Int. J. Pept. Protein Res. 1982,
19, 543±548.
16. Magaard, V. W.; Sanchez, R. M.; Bean, J. W.; Moore, M. L.
Tetrahedron Lett. 1993, 34, 381±384.
Ã
Â
17. Beausoleil, E.; L'Archeveque, B.; Belec, L.; Atfani, M.;
Lubell, W. D. J. Org. Chem. 1996, 61, 9447±9454.
18. 8a) Beausoleil, E.; Lubell, W. D. J. Am. Chem. Soc. 1996, 118,
12,902±12,908. 8b) Halab, L.; Lubell, W. D. J. Org. Chem.
1999, 64, 3312±3321.

6462
Q. Wang et al. / Tetrahedron 57 ,2001) 6455±6462
Â
19. Belec, L.; Slaninova, J.; Lubell, W. D. J. Med. Chem. 2000,
26. Chang, G.; Guida, W. C.; Still, W. C. J. Am. Chem. Soc. 1989,
111, 4379±4386.
43, 1448±1455.
20. Sasaki, N. A.; Hashimoto, C.; Potier, P. Tetrahedron Lett.
1987, 28, 6069±6072.
27. Ponder, J. W.; Richards, F. M. J. Comput. Chem. 1987, 8,
1016±1024.
28. Mohamadi, F.; Richards, N. J. G.; Guida, W. C.; Liskamp, R.;
Lipton, M. C.; Cau®eld, M.; Chang, G.; Hendrickson, T.;
Still, W. C. J. Comput. Chem. 1990, 11, 440±467.
29. Allinger, N. L. J. Am. Chem. Soc. 1977, 99, 8127±8134.
30. Wang, Q.; Sasaki, N. A.; Riche, C.; Potier, P. J. Org. Chem.
1999, 64, 8602±8607.
21. Pauly, R.; Sasaki, N. A.; Potier, P. Tetrahedron Lett. 1994, 35,
237±240.
22. Sasaki, N. A.; Pauly, R.; Fontaine, C.; Chiaroni, A.; Riche, C.;
Potier, P. Tetrahedron Lett. 1994, 35, 241±244.
23. Sagnard, I.; Sasaki, N. A.; Chiaroni, A.; Riche, C.; Potier, P.
Tetrahedron Lett. 1995, 36, 3149±3152.
31. Delair, P.; Einhorn, C.; Einhorn, J.; Luche, J. L. J. Org. Chem.
1994, 59, 4680±4682.
24. Sasaki, N. A.; Dockner, M.; Chiaroni, A.; Riche, C.; Potier, P.
J. Org. Chem. 1997, 62, 765±770.
25. Wang, Q.; Sasaki, N. A.; Potier, P. Tetrahedorn 1998, 54,
15,759±15,780.