Asymmetric synthesis of 3-substituted proline chimeras bearing polar side chains of proteinogenic amino acids

Article abstract of DOI:10.1021/jo048762q

The amino-zinc-ene-enolate cyclization reaction is a straightforward route to the synthesis of 3-substituted prolines. Herein we report the application of this reaction to the syntheses of proline chimeras of lysine, glutamic acid, glutamine, arginine, an

Full text of DOI:10.1021/jo048762q

Asymmetric Synthesis of 3-Substituted Proline Chimeras Bearing
Polar Side Chains of Proteinogenic Amino Acids
Jean Quancard, Aure´lie Labonne, Yves Jacquot, Ge´rard Chassaing, Solange Lavielle, and
Philippe Karoyan*
Synthe`se, Structure et Fonction de Mole´cules Bioactives, CNRS/UMR 7613, Universite´ Pierre et Marie
Curie, 4 Place Jussieu, 75252 Paris Cedex 05, France
karoyan@ccr.jussieu.fr
Received July 20, 2004
The amino-zinc-ene-enolate cyclization reaction is a straightforward route to the synthesis of
3-substituted prolines. Herein we report the application of this reaction to the syntheses of proline
chimeras of lysine, glutamic acid, glutamine, arginine, and serine. All these compounds were
obtained in enantiomerically pure form and suitably protected for peptide synthesis.
The control of the tridimensional structure of peptides
and proteins by chemical modification of proteinogenic
amino acids has been the keystone of peptide research
for the past few decades. The aim of these studies is the
development of compounds with improved selectivity,
bioavailability, stability, and permeability.1 The major
difficulty remains in inducing the correct folding of the
peptide while retaining the critical recognition elements
involved in the interaction.2 Among the various ap-
proaches (disulfide bridging, lactam cyclization, N-me-
thylation, ...) described in the de novo design of peptides
with a high propensity to fold with predetermined
secondary structure, the replacement of a native residue
by proline chimeras has been widely used.3 These cyclic
amino acids constrain the peptide backbone Φ value
around -60°, while insertion of the side chain of natural
amino acids on the pyrrolidine ring might give data on
both its conformation and the importance of the informa-
tion it carries. With this aim, we have recently demon-
strated that, if the insertion of proline in a heterochiral
peptide sequence remains the easiest strategy to induce
a â-turn,4 incorporation of 3-substituted prolines is a
valuable approach to mimic natural â-turn type I, II, and
II′ found in proteins with retention of the side chain
functionality in the i + 1 position of the turn.5 These
secondary structures are known to play several roles such
as stabilizing the tertiary structure, initiating folding,
and facilitating intermolecular recognition.6 Recently,
polyproline II (PPII) helices have received much atten-
tion. They are believed to be the dominant conformation
of proline-rich sequences7 and are often involved in
mediating protein-protein interactions.8 The introduc-
tion of chemical diversity into polyproline II helices
through proline chimeras may lead to compounds able
to inhibit protein-protein interactions by creating new
stabilizing interactions as suggested by Chmieliewski.9
Several methods for the synthesis of 3-substituted
prolines have been reported, but there is still a need for
a general stereoselective route to these compounds.10 We
have already reported the amino-zinc-ene-enolate
cyclization as a versatile strategy for the asymmetric
synthesis of cis-3-alkylprolines.11 We report here the
extension of this methodology for the synthesis of proline
chimeras bearing polar side chains of natural amino acids
and suitably protected for peptide synthesis.
Results and Discussion
The asymmetric syntheses of cis-3-prolinolysine, cis-
3-prolinoglutamine, cis-3-prolinoglutamic acid, cis-3-pro-
linoarginine, and cis-3-prolinohomoserine were investi-
gated through functionalization of the zinc intermediate
2 obtained after the amino-zinc-enolate cyclization of
* To whom correspondence should be addressed. Phone: 33-144-
27-38-42. Fax: 33-144-27-38-43.
(1) (a) Giannis, A.; Kolter, T. Angew. Chem., Int. Ed. Engl. 1993,
32, 1244. (b) Hruby, V. J.; Balse, P. M. Curr. Med. Chem. 2000, 7,
945. (c) Marshall, G. R. Biopolymers 2003, 60, 246.
(2) Schafmeister, C. E.; Po, J.; Verdine, G. L. J. Am. Chem. Soc.
2000, 122, 5891.
(5) Quancard, J.; Karoyan, P.; Lequin, O.; Wenger, E.; Aubry, A.;
Lavielle, S.; Chassaing, G. Tetrahedron Lett. 2004, 45, 623.
(6) Gibbs, A. C. Bjorndahl, T. C. Hodges, R. S. Wishart, D. S. J.
Am. Chem. Soc. 2002, 124, 1203.
(7) Williamson, M. P. Biochem. J. 1994, 297, 249.
(8) Kay, B. K.; Williamson, M. P. Sudol, M. FASEB J. 2000, 14, 231.
(9) Chang, E.; Roberts, D.; Fillon, Y.; Chmieliewski, J. Biopolymers
2003, 71, P417.
(10) Angle, S. R.; Belanger, D. S. J. Org. Chem. 2004, 69, 4361 and
references therein.
(11) (a) Karoyan, P.; Quancard, J.; Vaissermann, J.; Chassaing, G.
J. Org. Chem. 2003, 68, 2256. For the extension of this reaction and a
mechanistic point of view, see: (b) Denes, F.; Chemla, F.; Normant,
J.-F. Eur. J. Org. Chem. 2002, 3536. (c) Sliwinski, E.; Prian, F.; Denes,
F.; Chemla, F.; Normant, J.-F. C. R. Acad. Sci., Ser. IIc: Chim. 2003,
6, 67. (d) Lorthiois, E.; Marek, I.; Normant, J.-F. J. Org. Chem. 1998,
63, 566.
(3) (a) Sugase, K.; Horikawa, M.; Sugiyama, M.; Ishiguro, M. J. Med.
Chem. 2004, 47, 489. (b) Cai, M.; Cai, C.; Mayorov, A. V.; Xiong, C.;
Cabello, C. M.; Soloshonok, V. A.; Swift, J. R.; Trivedi, D.; Hruby, V.
J. J. Pept. Res. 2004, 63, 116. (c) Paradisi, M. P.; Mollica, A.; Cacciatore,
I.; Di Stephano, A.; Pinnen, F.; Caccuri, A. M.; Ricci, G.; Dupre, S.;
Spirito, A.; Lucente, G. Biooorg. Med. Chem. 2003, 11, 1677. (d)
Quancard, J.; Karoyan, P.; Sagan, S.; Convert, O.; Lavielle, S.;
Chassaing, G.; Lequin, O. Eur. J. Biochem. 2003, 270, 2869 and
references therein.
(4) Chalmers, D. K.; Marshall, G. R. J. Am. Chem. Soc. 1995, 117,
5927.
10.1021/jo048762q CCC: $27.50 © 2004 American Chemical Society
Published on Web 10/16/2004
7940
J. Org. Chem. 2004, 69, 7940-7948

Synthesis of 3-Subsituted Proline Chimeras
SCHEME 1
SCHEME 2
olefin 1 (Scheme 1). In a typical procedure, olefin 1
(methyl ester or benzyl ester) was treated with LDA in
THF at -78 °C, and the lithium enolate was transmeta-
lated with zinc bromide. Warming the reaction mixture
to room temperature led to organozinc derivative 2, with
a cis relative stereochemistry and 2S absolute configu-
ration as previously demonstrated.11
Synthesis of Protected cis-3-Prolinolysine 8. To
our knowledge, the synthesis of the titled compound has
never been reported. The introduction of the lysine side
chain on the cyclic organozinc species 2 was first inves-
tigated by aziridine ring opening (Scheme 2). The main
interest of this approach was that an ꢀ-protected form of
3-prolinolysine would be prepared in a “one-pot” proce-
dure starting from olefin 1a. Indeed, nonsubstituted
aziridine12 can be considered as an “aminoethyl” synthon,
and the aziridine ring opening by organometallic species
(organocuprate obtained from lithium or Grignard re-
agents) has been rewiewed.13 Moreover, the protection
of the aziridine nitrogen by electron-withdrawing groups
(Boc, Z, or tosyl), stabilizing the charge formed after ring
opening, has been described as a way to activate this
synthon.13
The cyclic organozinc reagent 2 is poorly reactive14
compared to classical organozinc reagents. A chelation
of the metal by the ester function after cyclization can
explain this low reactivity (Scheme 1). Nevertheless,
transmetalation of organozinc compounds by THF-soluble
copper salts leads to copper-zinc reagents that have been
reported to exhibit reactivity similar to that of lithium
or Grignard reagent derived copper organometallics.15 In
the case of the amino-zinc-ene-enolate cylization lead-
ing to 2, this enhanced reactivity has been demonstrated
by the synthesis of a few proline chimeras.16 Despite that,
the opening of N-protected aziridines (Boc, Z, or tosyl)
by the mixed copper-zinc species was never observed,
even upon activation with BF3‚Et2O. Compound 5, re-
sulting from hydrolysis of the copper-zinc reagent upon
quenching, was the sole product of the reaction.
Confronted with the low reactivity of the organome-
tallic species, we searched for a more reactive aminoethyl
(12) For the synthesis of N-protected aziridines, see: Martino, A.
D.; Galli, C.; Gargano, P.; Mandolini, L. J. Chem. Soc., Perkin Trans.
2 1985, 1345.
(13) For a recent review on aziridine opening, see: McCoull, W.;
Davis, F. A. Synthesis 2000, 10, 1347. For recent work on aziridine
opening, see: Medina, E.; Moyano, A.; Perica`s, M. A.; Riera, A. J. Org.
Chem. 1998, 63, 8574.
(14) Karoyan, P.; Chassaing, G. Tetrahedron Lett. 2002, 43, 1221.
(15) Knochel, P.; Perea, J. J. A.; Jones, P. Tetrahedron. 1998, 54,
8275.
(16) (a) Karoyan, P.; Chassaing, G. Tetrahedron Lett. 1997, 38, 84.
(b) Karoyan, P.; Chassaing, G. Tetrahedron: Asymmetry. 1997, 8, 2025.
(c) Karoyan, P.; Chassaing, G. Tetrahedron Lett. 2002, 43, 253. (d)
Reference 11a.
J. Org. Chem, Vol. 69, No. 23, 2004 7941

Quancard et al.
SCHEME 3
TABLE 1. Copper-Zinc Addition to Nitroethylene
hydrolysis product 5. However, almost the same 4/5 ratio
was observed in each case, just as if an equilibrium
between the two organometallic species 4′ and 2′ was
governing this reaction (Scheme 3).
amt of
isolated
nitroethylene reaction
reaction
temp (°C)
yield (%)
entry
(equiv)
time
of 4 and 5
It is known that highly stabilized carbanions undergo
reversible addition to R,â-unsaturated ketones. Revers-
ible conjugate addition of organocuprates to enones has
also been reported.18 However, reversibility in the addi-
tion of copper-zinc reagents to nitroolefins has never
been observed.17 In our case, the reversibilty of the
Michael addition reaction can be considered if one admits
that the organometallic species 2′ is stabilized by chela-
tion between the metal and the ester function (Scheme
2).
1
2
3
4
5
6
1
1
1
15 min
1 h
-78
-78
-78
-78
-78
-78
4, 30; 5, 40
4, 26; 5, 35
4, 23; 5, 33
4, 26; 5, 32
4, 20; 5, 20
4, 18; 5, 22
2 h
2 at one time 15 min
3 at one time 15 min
3 at 15
intervals
1 h
7
8
9
1
1
1
1
15 min
15 min
15 min
15 min
-100
-40
0
-78 and
warming to 0
4, 27; 5, 39
4, 25; 5, NDa
4, 2; 5, NDa
4, 25; 5, 33
10
To test the reversibility of the carbon-carbon bond
forming step, the reaction mixture obtained after addition
of the nitroolefin was treated with tosyl cyanide (TsCN).
We have already reported that copper-zinc reagent 2′
reacts in good yield with this electrophile.16c Therefore,
15 min after the addition of nitroethylene (corresponding
to the conditions of entry 1), 1 equiv of TsCN was added
to the mixture, and the solution was stirred at -78 °C
for another 2 h (Scheme 2). After quenching and purifica-
tion, less than 2% of nitro adduct 4 (instead of 30%, entry
1) could be isolated along with 39% of the cyano deriva-
tive 6 and 14% of the hydrolysis product 5. No product
resulting from the addition of Michael adduct 4′ to tosyl
cyanide was detected. These results confirm the revers-
ibility of the Michael addition.
However, the reversibility of the addition is not enough
to account for the low yield observed when several
equivalents of nitroethylene were added to displace the
equilibrium toward the nitro adduct 4. In this case,
degradation and/or polymerization of the enolate result-
ing from the initial 1,4-addition have to be considered.19
Indeed, nucleophiles that contain an electron-withdraw-
ing group such as nitrile or nitro groups are among the
best nucleophiles in Michael addition. In our case, the
Michael addition of the copper-zinc reagent to nitro-
a ND ) not determined.
synthon. Nitroolefins are known to be very good Michael
acceptors, and their use for the generation of nitroalkane
is well documented.17 More interestingly, copper-zinc
reagents have been shown to add cleanly to nitroolefins.17b
Hence, the organozinc species 2 was submitted to trans-
metalation by the THF-soluble CuCN/2LiCl salts and
reacted with nitroethylene under various conditions,
some of which are reported in Table 1.
Surprisingly, whatever the reaction conditions, the
yield of the Michael adduct 4 never exceeded 30%. The
only other major product isolated from the reaction
mixture was compound 5, resulting from hydrolysis of
the copper-zinc intermediate. The best reaction condi-
tions (entry 1) were reached with 1 equiv of nitroolefin
added at -78 °C followed by 15 min of stirring before
quenching at this temperature. Indeed, a longer reaction
time (entries 2, 3, and 6), addition of more than 1 equiv
of nitroethylene (entries 4-6), or performing the reaction
at a temperature higher than -78 °C (entries 8-10) led
to lower yields of both the Michael adduct 4 and the
(17) (a) Seebach, D.; Colvin, E. W.; Lehr, F.; Weller, T. Chimia 1979,
33, 1. (b) For references on addition of organometallics to nitroolefins,
see: Jubert, C.; Knochel, P. J. Org. Chem. 1992, 57, 5431. (c) For recent
work on the addition of organozinc compounds to nitroolefins, see:
Rimkus, A.; Sewald, N. Synthesis 2004, 135. (d) For a general rewiew
on organozinc reagents, see: Knochel, P.; Millot, N.; Rodriguez, A. L.
Org. React. 2001, 58, 417.
(18) Corey, E. J.; Neil, B. W. Tetrahedron Lett. 1985, 26, 6015.
(19) (a) Namboothiri, I. N. A.; Hassner, A.; Gottlieb, H. E. J. Org.
Chem. 1997, 62, 485. (b) Corey, E. J.; Neil, B. W. Tetrahedron Lett.
1985, 26, 6019.
7942 J. Org. Chem., Vol. 69, No. 23, 2004

Synthesis of 3-Subsituted Proline Chimeras
SCHEME 4
(II) complex.20 However, this method is quite tedious and
is hardly usable for low gram or milligram scales.
Interestingly, it has been shown that saponification of
N-(benzyloxycarbonyl)aspartic acid dibenzyl ester21 and
N-(benzyloxycarbonyl)glutamic acid bis-2,4,6-trimethyl-
benzyl ester22 with LiOH occurs selectively on the R-ester,
providing the desired â- or γ-ester. This simple method
was applied to 6, via the dibenzyl ester 10 intermediate
(Scheme 6).
Hydrolysis of nitrile 6 followed by one-pot hydrogenoly-
sis and Boc protection afforded 9 in moderate yield.
Alkylation of the cesium salt of 9 with benzyl bromide
gave the desired dibenzyl ester 10. Saponification of 10
with 1 equiv of LiOH provided one major product (11)
containing a single benzyl ester. To determine which of
the R- or γ-acid group was still protected as a benzyl
ester, an HMBC (heteronuclear multiple-bond connectiv-
ity) 500 MHz NMR experiment was performed on the
isolated major product.23 A portion of the NMR spectrum
is shown in Figure 1A. The correlation of the carbon at
171 ppm with the benzylic protons at 5.18 ppm clearly
identifies this carbon as the benzyl ester carbonyl.
Correlations are also observed between this carbon and
HR (2J, 4.45 ppm) and Hâ (3J, 2.82 ppm), indicating that
this carbon belongs to the R-carbonyl group. Conversely,
the carbon at 176 ppm correlates with the methylene of
the C3-substituent. Thus, compound 11 is protected on
the R-position. This inverse selectivity in the deprotection
reaction probably reflects the difficulties encountered
upon the saponification of the R-ester function of cis-3-
substituted prolines, which often requires heating (see
below).
Thus, we searched for conditions which could preserve
the benzyl ester of 6 and convert the nitrile into an
appropriate ester. The most simple strategy was the use
of acetyl chloride in dry MeOH (Scheme 7).24 These mild
acidic conditions led in good yield to the γ-methyl ester
R-benzyl ester 12. One-pot hydrogenolysis and Boc
protection afforded 3-prolinoglutamic acid γ-methyl ester
(13), which is suitable for peptide synthesis. Although
the position of the ester was not ambiguous, we per-
formed the same HMBC experiment for comparison with
R-ester 11 (Figure 1B). The correlation of the carbon at
172 ppm with the methyl protons at 3.70 ppm identifies
this carbon as the carbonyl of the methyl ester. This
carbon shows one more correlation at 2.45 ppm which is
attributed to the methylene of the C3-substituent. The
SCHEME 5
ethylene generates a very reactive carbanion (4′), which
polymerizes and leads to degradation. This point was
exemplified (i) by a longer reaction time, which led to a
decreasing yield of both compounds 4 and 5 (entries 2
and 3), and (ii) by the use of an excess of nitroolefin
(entries 4-6): in that case, in addition to the decreasing
yield, a dinitroethyl adduct was detected by ESI+ MS
analysis in the crude reaction mixture (m/z 470.15
[MH+]). The use of (TMS)Cl, reported to increase the yield
of the Michael addition by trapping the intermediate
anion to avoid side reactions,19 had no effect in our case
(data not shown).
Despite the moderate yield (30%), this methodology
allows the one-pot preparation of compound 4, which is
a very good intermediate to prepare an orthogonally
protected 3-prolinolysine suitable for peptide synthesis
(Scheme 4). First, the nitro group was reduced to a
primary amine by treatment of 4 with zinc in acetic acid.
This primary amine was Boc-protected after filtration on
a Celite pad and alkalinization, leading to compound 7
in good yield. Special care should be taken to keep the
pH e 8; otherwise, Boc2-amine is obtained as a side
product. Nꢀ-Boc-NR-Fmoc-3-prolinolysine (8) was then
obtained after catalytic hydrogenation of 7 over pal-
ladium charcoal, followed by Fmoc protection.
Synthesis of 3-Prolinoglutamic acid, 3-Prolino-
glutamine, and 3-Prolinoarginine. The syntheses of
the titled compounds were investigated starting from a
common intermediate, i.e., the cyano derivative 6 (Scheme
5), which can be obtained in good yield in a one-pot
procedure starting from olefin 1a.16c
(a) 3-Prolinoglutamic Acid. We have previously
reported the synthesis of unprotected cis- and trans-3-
prolinoglutamic acid from 6.16c In the present study, we
have investigated different routes for the preparation of
orthogonally protected 3-prolinoglutamic acid, suitable
for peptide synthesis. Several methods are described to
obtain γ-esters of glutamic acid. The most frequently used
is the alkylation of alkali-metal salts of glutamate copper-
SCHEME 6
J. Org. Chem, Vol. 69, No. 23, 2004 7943

Quancard et al.
FIGURE 1. Portion of the HMBC spectra of compounds 11 (A) and 13 (B).
SCHEME 7
ods are described for the conversion of nitrile to amine.
Complex metal hydrides such as NaBH4 + CoCl2‚6H2O
have been reported to successfully achieve nitrile-amine
conversion,25 and these conditions can be applied to Boc-
containing compounds.26 Indeed, reduction of 14 with this
method consumed the starting material in a few hours.
However, no pure compound could be isolated, probably
due to formation of metal chelates and/or decomposition
of the products. We therefore focused on reduction of the
nitrile to amine by hydrogenation although catalytic
hydrogenation of nitriles often gives poor yields. The
difficulty seems to arise because the reduction proceeds
stepwise via the aldimine, some of which can condense
with the primary amine already formed, giving secondary
amines.27 Many studies describe the use of Raney nickel
catalysis in a basic medium under quite drastic pressure
and/or temperature conditions.28 Smoother conditions
have been described with the use of a cocatalyst such as
Pd/C29 or rhodium.30 A very attractive method is the
catalytic hydrogenation of nitriles over PtO2, in the
presence of CHCl3, which allows the in situ formation of
the amine hydrochloride, thus avoiding any side reac-
tions.31 In our case, CHCl3, which could have damaged
the Boc protecting group (by in situ HCl formation),
turned out to be needless. Indeed, the presence of the
carboxylic acid function of compound 14 was sufficient
to allow a clean reduction of the nitrile group of 14 over
other carbonyl at 176 ppm correlates with HR (2J, 4.39
ppm) and Hâ (3J, 2.90 ppm). These results confirmed the
γ-position of the methyl ester.
It is worth mentioning that the syntheses of both R-
and γ-orthogonally protected 3-prolinoglutamic acids are
achievable from the intermediate 6.
(b) 3-Prolinoglutamine and 3-Prolinoarginine. As
previously reported, one-pot debenzylation and Boc pro-
tection of the pyrrolidine ring occur cleanly on such
compounds when Boc2O is added to the hydrogenation
mixture.11 Under these conditions (Scheme 8) only the
amine and the ester functions of 6 were deprotected,
leaving intact the nitrile function. The cyano intermedi-
ate 14, obtained in good yield, turned out to be a versatile
compound for the preparation of two chimeras, i.e.,
prolinoglutamine and prolinoarginine. cis-Boc-3-prolino-
glutamine (15) was first obtained by treatment of 14 with
H2O2 and NaOH.
Further reduction of the nitrile function of compound
14 was studied to access prolinoarginine. Several meth-
SCHEME 8
7944 J. Org. Chem., Vol. 69, No. 23, 2004

Synthesis of 3-Subsituted Proline Chimeras
SCHEME 9
PtO2 in MeOH, under 5 bar of H2, to afford amine 16
with an excellent yield (96%). Guanilation of 16 using
N,N′-di-Cbz-S-methylisothiourea32 in THF provided or-
thogonally protected cis-3-prolinoarginine 17.
Conclusion
Proline chimeras are valuable tools for a wide field of
biological applications. We have reported the amino-
zinc-enolate cyclization as a straightforward method for
the asymmetric synthesis of 3-substituted prolines, and
we have extended this methodology to the synthesis of
all types of proline chimeras bearing a side chain of
natural amino acids. The strategy described in this study
allows the asymmetric synthesis, in few steps, of polar
3-substitued prolines suitably protected for peptide syn-
thesis. The introduction of these proline chimeras into
biologically active peptides is under way.
Synthesis of cis-3-Prolinohomoserine. The synthe-
sis of this compound was investigated through direct
oxidation of zinc intermediate 2 (Scheme 9). Oxidation
of organometallic compounds to alcohols has been previ-
ously studied by bubbling dry air33 or dioxygen34 in THF.
More recently, the oxidation of piperidinic organozinc
compounds has been studied.11d These procedures have
been applied to the organozinc 2 for the synthesis of
prolinohomoserine. Direct oxidation of organozinc 2 with
dry air did not take place, but bubbling O2 in the mixture
quickly afforded a mixture of lactone 18 and methyl ester
19. As lactonization occurred until protection of the
hydroxyl group, the synthesis was followed up on the
mixture. Hydrogenolysis of the mixture over Pd/C and
Boc protection in methanol afforded a mixture of lactone
20 and methyl ester 21. The mixture was then heated at
80 °C in toluene with an excess of KOH and benzylbro-
mide to give compound 22. Saponification of the benzyl
ester required heating overnight with 5 equiv of LiOH.
Epimerization (5%) of the R-center was observed by NMR
analysis of the crude product. The two diastereoisomers
were easily separated by flash chromatography, and
N-Boc-prolinohomoserine benzyl ether (23) was obtained
in 80% yield.
Experimental Section
General Considerations. Tetrahydrofuran (THF) and
diethyl ether (Et2O) were distilled from sodium benzophenone
and kept over 4 Å molecular sieves. Dry dimethylformamide
(DMF) and other reagents were commercially available. Zinc
bromide (ZnBr2) was dried by fusion under flame and nitrogen;
the fused salts were allowed to solidify under nitrogen and
then dissolved in dry Et2O. Lithium chloride (LiCl) was dried
with a heat gun under reduced pressure. After the LiCl was
cooled under nitrogen, CuCN was added, followed by dry THF,
and the mixture was then stirred until complete dissolution
(1h).Olefins1aand1bwerepreparedasdescribedpreviously.11a
Nitroethylene was prepared from nitroethanol and resublimed
phthalic anhydride and kept at -20 °C as a 1 N solution in
dry THF or toluene.35 N,N′-Di-Cbz-S-methylisothiourea was
synthesized as reported by Tian et al.32 Anhydrous reactions
were performed under an argon atmosphere; glassware was
flame-dried prior to use under a stream of nitrogen.
(2S,3R)-3-Nitropropyl-1-(1-phenylethyl)pyrrolidine-2-
carboxylic Acid Benzyl Ester (4). To a solution of 1a (3.22
g, 10 mmol) in dry THF (20 mL) at - 78 °C was added LDA (5
mL, 2 N in heptane). A solution of ZnBr2 (6.76 g, 3 equiv) in
dry Et2O (30 mL) was then added, and the mixture was
allowed to warm to rt for 4 h. A solution of CuCN (1.15 g, 1.3
equiv) and LiCl (1.15 g, 2.6 equiv) in dry THF (10 mL) was
added at 0 °C. The mixture was cooled to - 78 °C, and a
(20) (a) Van Heeswijk, W. A. R.; Eenink, M. D. J.; Feijen, J. Synthesis
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(22) Rizo, J.; Albericio, F.; Romero, G.; Garcia-Echeverria, C.; Claret,
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(23) Bax, A.; Summers, M. F. J. Am. Chem. Soc. 1986, 108, 2093.
(24) Nicolaou, K. C.; Petasi, N. A.; Li, W. S.; Ladduwahetty, T.;
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(25) Satoh, T.; Suzuki, S.; Suzuki, Y.; Miyaji, Y.; Imai, Z. Tetrahe-
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(26) Ushio-Sata, N.; Sugano, M.; Matsunaga, S.; Fusetani, N.
Tetrahedron Lett. 1999, 40, 719.
(31) Cox, R. J.; O’Hagan, D. J. Chem. Soc., Perkin Trans. 1 1991,
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(27) Adkins, H.; Shriner, R. L. In Organic synthesis, 2nd ed.; Gilman,
H., Ed.; J. Wiley and Sons: New York, 1943; Vol. I, p 809.
(28) (a) Whitmore, F. C.; Mosher, H. S.; Adams, R. R.; Taylor, R.
B.; Chaplin, E. C.; Weisel, C.; Yanko, W. J. Am. Chem. Soc. 1944, 66,
725. (b) Gould, F. E.; Johnson, G. S.; Ferris, A. F. J. Org. Chem. 1960,
25, 1658.
(32) Tian, Z.; Edwards, P.; Roeske, R. W. Int. J. Pept. Protein Res.
1992, 40, 119.
(33) Chemla, F.; Normant, J.-F. Tetrahedron Lett. 1995, 36, 3157.
(34) Klement, I.; Lu¨tjens, H.; Knochel, P. Tetrahedron Lett. 1995,
36, 3161.
(29) Klenke, B.; Gilbert, I. H. J. Org. Chem. 2001, 66, 2480.
(30) Freifelder, M. J. Am. Chem. Soc. 1960, 82, 2386-2389.
(35) Ranganathan, D.; Rao, C. B.; Ranganathan, S.; Methotra, A.
K.; Iyengar, R. J. Org. Chem. 1980, 45, 1185.
J. Org. Chem, Vol. 69, No. 23, 2004 7945

Quancard et al.
solution of nitroethylene (10 mL, 1 N in dry THF) was then
added. After 15 min of stirring at -78 °C, the mixture was
quenched with saturated aqueous NH4Cl. Et2O was added, and
the organic layer was washed with NH4Cl/NH4OH (1 M), 2/1,
until complete decoloration followed by brine, dried over
MgSO4, and concentrated in vacuo. The residue was purified
by flash chromatography (cyclohexane/ethyl acetate, 87/13) to
give 4 as a yellow oil (1.19 g, 30%) and 516a (1,26 g, 39%). Data
for 4: [R]20D -67.8 (c 1, CHCl3); 1H NMR (250 MHz, CDCl3) δ
recrystallized from Et2O/pentane to give 8 as a white solid (498
mg, 67%): mp, degradation over 80 °C; [R]20D 7.8 (c 1, CHCl3);
1H NMR (400 MHz, CDCl3) δ 7.75-7.73 (m, 2H), 7.61-7.52
(m, 2H), 7.41-7.33 (m, 2H), 7.29-7.26 (m, 2H), 6.73 + 6.42 +
4.68 (3 br s, NH, 1H, Boc and Fmoc cis-trans isomerization),
4.47-4.16 (m, 4H), 3.79-3.77 (m, 1H), 3.43-3.39 (m, 1H),
3.21-3.01 (m, 2H), 2.43-2.27 (m, 1H), 2.09-1.96 (m, 1H),
1.94-1.71 (m, 1H), 1.69-1.48 (m, 2H), 1.41 + 1.46 ) 1.43 (3s,
9H, Boc and Fmoc cis-trans isomerization), 1.47-1.27 (m, 2H);
13C NMR (100 MHz, CDCl3) δ 174.7, 174.4, 158.5, 156.1, 154.9,
154.5, 143.9, 141.3, 141.2, 141.2, 127.7, 127.1, 125.2, 125.1,
119.9, 80.9, 80.9, 79.5, 79.3, 67.7, 62.4, 62.1, 47.3, 47.2, 46.4,
46.0, 43.0, 41.6, 40.4, 29.8, 28.6, 28.4, 28.1, 27.5, 27.3, 27.1;
ESI+ MS m/z 517.16 [MNa+]. Anal. Calcd for C28H34N2O6‚
0.5H2O: C, 66.85; H, 6.96; N, 5.57. Found: C, 66.98; H, 7.27;
N, 5.45.
3
7.37-7.15, (m, 10H), 5.12-4.96 (AB, 2H, J ) 12.5 Hz), 4.08
(t, 2H, 3J ) 6.25 Hz), 3.69 (q, 1H, 3J ) 6.5 Hz), 3.39 (d, 1H, 3J
) 7.5 Hz), 3.12-2.94 (m, 2H), 2.48-2.18 (m, 1H), 2.10-1.91
(m, 1H), 1.92-1.75 (m, 2H), 1.76-1.52 (m, 1H), 1.33 (d, 3H,
3J ) 6.5 Hz), 1.32-1.12 (m, 1H); 13C NMR (62.5 MHz, CDCl3)
δ 172.6, 144.4, 135.7, 129.0, 128.5, 128.4, 128.3, 127.3, 127.1,
75.2, 66.1, 65.8, 61.3, 49.8, 41.9, 29.6, 27.7, 26.0, 22.9. Anal.
Calcd for C23H28N2O4: C, 69.68; H, 7.12; N, 7.07. Found: C,
69.52; H, 7.32; N, 6.88.
Reversibility of the Michael Addition of the Copper-
Zinc Reagent to Nitroethylene. The same protocol as for
compound 4 was followed. Fifteen minutes after the addition
of nitroethylene, TLC indicated complete formation of 4. TsCN
(1 equiv) was added, and the mixture was stirred at -78 °C
for 2 h. After the reaction was quenched with saturated
aqueous NH4Cl and workup, the residue was purified by flash
chromatography (cyclohexane/ethyl acetate, 87/13). Only 2%
of the nitro adduct 4 was isolated. A 14% yield of 5 was isolated
as well as 38% of 6, the product resulting from the reaction of
the copper-zinc reagent with TsCN.16c
(2S,3S)-3-Carboxymethyl-1-(tert-butyloxycarbonyl)-
pyrrolidine-2-carboxylic Acid (9). A mixture of 616c (1.046
g, 3 mmol) and 6 N HCl (30 mL) were refluxed for 3 h. The
solvent was evaporated and the crude product dissolved in
MeOH (15 mL). K2CO3 (830 mg, 6 mmol) and Boc2O (657 mg,
3 mmol) were added, and the mixture was stirred overnight
under H2. The mixture was filtered on a Celite pad and
concentrated in vacuo. The crude product was taken up in
water and washed with Et2O. The aqueous layer was acidified
at 0 °C with 1 N HCl and extracted with CH2Cl2 (many
extractions were required), and the combined organic layers
were dried over MgSO4 and concentrated in vacuo. The residue
was crystallized from Et2O to give 9 as a white solid (383 mg,
D
1
47%): mp 152-155 °C; [R]20 82 (c 1, MeOH); H NMR (250
MHz, CDCl3) δ 4.51 and 4.38 (2d, Boc cis-trans isomerization,
(2S,3R)-3-(N-tert-Butyloxycarbonyl)aminopropyl-1-(1-
phenylethyl)pyrrolidine-2-carboxylic Acid Benzyl Ester
(7). Zinc powder (5.21 g) was added portionwise at 0 °C to a
stirred solution of 4 (792 mg, 2 mmol) in AcOH (33 mL). After
being stirred overnight at room temperature, the mixture was
filtered on a Celite pad, which was then washed with water
(3 × 20 mL). A solution of K2CO3 in water (1 g‚mL-1) was then
added dropwise at 0 °C until pH 8. Dioxane (120 mL) was then
added followed by Boc2O (436 mg, 2 mmol). After the resulting
mixture was stirred for 2 h at room temperature, the organic
layer was separated and the aqueous layer was extracted with
AcOEt. The combined organic layers were washed with brine,
dried over MgSO4, and concentrated in vacuo. The crude
product was purified by flash chromatography (cyclohexane/
ethyl acetate, 84/16) to give 7 as a colorless oil (831 mg, 89%):
[R]20D -44.9 (c 1, CHCl3); 1H NMR (400 MHz, CDCl3) δ 7.37-
7.17, (m, 10H), 5.09-5.01 (AB, 2H), 4.33 + 1.74 (2 br s, NH,-
3
1H, J ) 8.1 and 8.1 Hz), 3.71-3.59 (m, 1H), 3.42-3.30 (m,
1H) 2.97-2.79 (m, 1H), 2.64-2.46 (m, 2H), 2.10-2.04 (m, 1H),
1.86-1.73 (m, 1H), 1.46-1.42 (2s, Boc cis-trans isomerization,
9H); 13C NMR (62.5 MHz, CDCl3) δ 176.5, 176.4, 175.2, 155.0,
154.0, 80.9, 80.7, 61.6, 61.1, 45.8, 45.4, 38.0, 37.3, 34.6, 29.7,
28.9, 28.4, 28.3. Anal. Calcd for C12H19NO6: C, 52.74; H, 7.01;
N, 5.13. Found: C, 52.45; H, 7.19; N, 5.04.
(2S,3S)-3-Benzyloxycarbonylmethyl-1-(tert-butyloxy-
carbonyl)pyrrolidine-2-carboxylic Acid Benzyl Ester
(10). To a solution of 9 (273 mg, 1 mmol) in MeOH (1 mL) and
H2O (0.1 mL) was added a 20% aqueous solution of CsCO3 until
pH 7. The solvent was removed in vacuo and the mixture taken
in DMF (2 mL). BnBr (241 µL, 2.2 mmol) was then added, and
the mixture was stirred at rt overnight. Et2O was added, and
the organic layer was washed with saturated aqueous NH4Cl.
The crude oil was purified by flash chromatography (cyclo-
hexane/ethyl acetate, 85/15) to give 10 as a colorless oil (340
mg, 75%): [R]25D 21.3 (c 1, CHCl3); 1H NMR (250 MHz, CDCl3)
δ 7.40-7.25 (m, 10H) 5.22-4.96 (m, 4H) 4.47 and 4.39 (2d,
3
1H, Boc cis-trans isomerization), 3.68 (q, 1H, J ) 6.5 Hz),
3
3.39 (d, 1H, J ) 8.1 Hz), 3.08-2.90 (m, 4H), 2.33-2.22 (m,
1H), 2.04-1.96 (m, 1H), 1.67-1.57 (m, 1H), 1.41 (s, 9H), 1.49-
1.17 (m, 3H), 1.33 (d, 3H, 3J ) 6.5 Hz), 1.09-1.02 (m, 1H); 13
C
3
NMR (100 MHz, CDCl3) δ 173.1, 155.8, 144.6, 135.9, 128.9,
128.5, 128.3, 128.2, 127.4, 127.1, 79.0, 66.5, 65.7, 61.5, 50.1,
42.2, 40.4, 29.8, 28.7, 28.4, 28.2, 22.9. Anal. Calcd for
C23H28N2O4: C, 72.07; H, 8.21; N, 6.00. Found: C, 72.22; H,
8.05; N, 6.00.
Boc cis-trans isomerization, 1H, J ) 8.3 and 8.3 Hz), 3.76-
3.58 (m, 1H), 3.41-3.25 (m, 1H) 2.94-2.78 (m, 1H), 2.49-2.08
(m, 2H), 2.08-1.56 (m, 2H), 1.45-1.32 (2s, Boc cis-trans
isomerization, 9H); 13C NMR (62.5 MHz, CDCl3) δ 171.6, 171.3,
154.4, 153.8, 135.8, 135.5, 128.8, 128.7, 128.6, 128.5, 128.4,
80.2, 80.0, 66.8, 66.6, 61.9, 61.6, 45.9, 45.5, 38.6, 37.8, 35.0,
29.9, 29.0, 28.5, 28.3. Anal. Calcd for C26H31NO6: C, 68.86; H,
6.89; N, 3.09. Found: C, 68.89; H, 7.05; N, 3.10.
(2S,3R)-3-(N-tert-Butyloxycarbonyl)aminopropyl-1-(9-
fluorenylmethyloxycarbonyl)pyrrolidine-2-carboxylic
Acid Benzyl Ester (8). A mixture of 7 (700 mg, 1.5 mmol)
and Pd/C (150 mg) in methanol (7.5 mL) was stirred overnight
under H2. The mixture was filtered on a Celite pad and
concentrated under reduced pressure to give a white solid. The
product was dissolved in H2O (24 mL), and the solution was
cooled to 0 °C. NaHCO3 (378 mg, 4.5 mmol) was then added,
and a solution of Fmoc-O-succinimide (552 mg, 1.65 mmol) in
dioxane (24 mL) was added dropwise over 15 min. After the
resulting solution was stirred for 7 h at room temperature,
the solvent was evaporated and the residue was taken up in
water. The aqueous layer was acidified with 1 N HCl until
pH 2 and extracted with AcOEt. The combined organic layers
were washed with brine, dried over MgSO4, and concentrated
under reduced pressure. The crude oil was purified by flash
chromatography (CH2Cl2/EtOH/acetic acid, 10/0.1/0.1) and
(2S,3S)-3-Carboxymethyl-1-(tert-butyloxycarbonyl)-
pyrrolidine-2-carboxylic Acid Benzyl Ester (11). To a
solution of 10 (272 mg, 0.6 mmol) in acetone/water (6 mL, 4/1)
was slowly added 56% LiOH (25 mg, 0.6 mmol) in water (1.2
mL). After the resulting solution was stirred overnight at rt,
the solvent was evaporated in vacuo and water was added.
The aqueous layer was washed with Et2O, acidified at 0 °C
with 1 N HCl until pH 2, and extracted with AcOEt. The
combined AcOEt layers were dried over MgSO4 and concen-
trated in vacuo. The crude oil was purified by flash chroma-
tography (CH2Cl2/EtOH/acetic acid, 10/0.1/0.1) to give 11 as a
D
1
colorless oil (148 mg, 68%): [R]25 13.5 (c 1, CHCl3); H NMR
(250 MHz, CDCl3) δ 7.37-7.27 (m, 5H) 5.29-5.05 (m, 2H) 4.50
3
and 4.39 (2d, Boc cis-trans isomerization, 1H, J ) 8.3 and
7946 J. Org. Chem., Vol. 69, No. 23, 2004

Synthesis of 3-Subsituted Proline Chimeras
8.0 Hz), 3.76-3.60 (m, 1H), 3.40-3.26 (m, 1H), 2.86-2.75 (m,
1H), 2.42-2.17 (m, 2H), 2.08-1.99 (m, 1H), 1.77-1.66 (m, 1H),
1.46-1.33 (2s, Boc cis-trans isomerization, 9H); 13C NMR
(62.5 MHz, CDCl3) δ 176.6, 171.4, 154.4, 153.8, 135.4, 135.2,
128.8, 128.6, 128.5, 128.3, 80.3, 80.2, 66.9, 61.7, 61.4, 45.8, 45.5,
38.3, 37.5, 34.6, 29.7, 28.8, 28.4, 28.2. Anal. Calcd for C19H25-
NO6: C, 62.80; H, 6.93; N, 3.85. Found: C, 62.76; H, 6.97; N,
3.76.
(2S,3S)-3-Acetamide-1-(tert-butyloxycarbonyl)pyrroli-
dine-2-carboxylic Acid (15). To a solution of 14 (509 mg, 2
mmol) in 0.5 N NaOH (6 mL) was added 35% H2O2 (6 mL)
followed by 2 N NaOH (4 mL). After being stirred for 30 min
at rt, the mixture was cooled to 0 °C and 1 N HCl was added
until pH 2. The aqueous layer was extracted with CH2Cl2
(many extractions were required). The combined organic layers
were dried on MgSO4 and concentrated in vacuo. The crude
product was recrystallized with Et2O/pentane to give 15 as a
(2S,3S)-3-Methyloxycarbonylmethyl-1-(1-phenylethyl)-
pyrrolidine-2-carboxylic Acid Benzyl Ester (12). Acetyl
chloride (7.42 mL, 30 equiv) was added dropwise to a solution
of 616c (1.20 g, 3.48 mmol) in 17 mL of anhydrous MeOH at 0
°C. The reaction mixture was stirred at 0 °C for 1 h and then
for 5 h at rt. The solvent was evaporated in vacuo and the
crude product taken up in water. A 10% aqueous solution of
K2CO3 was added until pH 8. After extraction with Et2O, the
organic layer was washed with brine and dried over MgSO4,
and the solvent was evaporated in vacuo. The crude oil was
purified by flash chromatography (cyclohexane/ethyl acetate,
D
white solid (327 mg, 60%): mp, degradation over 110 °C; [R]25
21.5 (c 1, CHCl3); 1H NMR (500 MHz, CDCl3) δ 9.71 (br s, 1H),
7.02 (d, 1H, 3J ) 21.0 Hz), 6,75 (d, 1H, 3J ) 19.7 Hz), 4.38
3
and 4,33 (2d, Boc cis-trans isomerization, 1H, J ) 8.1 and
8.4 Hz), 3.68-3.58 (m, 1H), 3.36-3.29 (m, 1H), 2.89-2.82 (m,
1H), 2.49-2.35 (m, 2H), 2.05-2.00 (m, 1H), 1.81-1.71 (m, 1H),
1.44 and 1.40 (2s, Boc cis-trans isomerization, 9H); 13C NMR
(100 MHz, CDCl3) δ 175.8, 174.8, 154.9, 154.1, 80.6, 80.4, 62.0,
61.6, 45.8, 45.4, 38.6, 37.8, 35.8, 35.7, 29.8, 29.0, 28.5, 28.4;
ESI- MS m/z 271.06 [M - H-].
D
9/1) to give 12 as a colorless oil (923 mg, 70%): [R]25 - 44.9
(2S,3R)-3-Aminoethyl-1-(tert-butyloxycarbonyl)pyrro-
lidine-2-carboxylic Acid (16). A mixture of 14 (509 mg, 2
mmol), PtO2 (200 mg), and MeOH (10 mL) was stirred
overnight under 5 bar of H2. The mixture was filtered on a
Celite pad, and the solvent was evaporated in vacuo. The
resulting white solid was washed with Et2O to give 16 (496
mg, 96%): mp, degradation over 200 °C; [R]25D 26 (c 0.3, H2O);
1H NMR (250 MHz, D2O) δ 4.11 and 4.10 (2d, Boc cis-trans
(c 1, CHCl3); 1H NMR (250 MHz, CDCl3) δ 7.35-7.15, (m, 10H),
5.12-4.94 (AB, 2H, 3J ) 11.25 Hz), 3.71 (q, 1H, 3J ) 7.5 Hz),
3.51 (d, 1H, 3J ) 10 Hz), 3.53 (s, 3H), 3.12-2.70 (m, 3H), 2.80-
2.60 (m, 1H), 2.37-2.00 (m, 3H), 1.32 (d, 3H, 3J ) 7.5 Hz); 13
C
NMR (62.5 MHz, CDCl3) δ 172.9, 172.5, 144.4, 135.7, 128.6,
128.5, 128.3, 127.3, 65.9, 65.7, 61.4, 51.5, 49.9, 38.1, 35.5, 29.7,
22.8. Anal. Calcd for C23H27NO4: C, 72.42; H, 7.13; N, 3.67.
Found: C, 72.28; H, 7.31; N, 3.51.
3
isomerization, 1H, J ) 8.5 and 8.5 Hz), 3.70-3.50 (m, 1H),
3.39-3.16 (m, 1H), 3.08 (t, 2H, 3J ) 8 Hz) 2.52-2.32 (m, 1H),
2.08-1.94 (m, 1H), 1.91-1.49 (m, 3H), 1.43-1.39 (2s, Boc cis-
trans isomerization, 9H); 13C NMR (62.5 MHz, D2O) δ 178.6,
156.4 81.8, 65.1, 64.6, 46.3, 45.7, 45.5, 39.6, 38.7, 38.4, 29.6,
29.1, 28.4, 28.2, 28.0 Anal. Calcd for C13H21NO6‚H2O: C, 52.1;
H, 8.60; N, 10.10. Found: C, 52.34; H, 8.31; N, 9.96.
(2S,3S)-3-Methyloxycarbonylmethyl-1-(tert-butyloxy-
carbonyl)pyrrolidine-2-carboxylic Acid (13). A mixture of
compound 12 (627 mg, 1.64 mmol), 10% Pd/C (164 mg), and
Boc2O (1.5 equiv, 378 mg) in MeOH (8 mL) was stirred
overnight at room temperature under hydrogen. After filtra-
tion over a Celite pad and evaporation of the solvent in vacuo,
the crude product was taken up in water and a 10% aqueous
solution of K2CO3 was added until pH 8. The aqueous layer
was washed with Et2O and then acidified at 0 °C with 1 N
HCl until pH 2. The aqueous layer was extracted with AcOEt,
and the combined organic layers were dried over MgSO4 and
concentrated in vacuo to give 13 as a colorless oil. White
crystals were obtained after crystallization from Et2O/pentane
(376 mg, 80%): mp 87-90 °C; [R]25D 25.0 (c 1, CHCl3); 1H NMR
(250 MHz, CDCl3) δ 4.44 and 4.37 (2d, Boc cis-trans isomer-
(2S,3R)-3-N,N′-Dicarboxybenzylguanidinoethyl-1-(tert-
butyloxycarbonyl)pyrrolidine-2-carboxylic Acid (17). To
a solution of 16 (259 mg, 1 mmol) in dry THF (5 mL) under
argon were added N,N′-di-Cbz-S-methylisothiourea32 (388 mg,
1.1 mmol) and K2CO3 (276 mg, 2 mmol). The reaction mixture
was stirred at rt for 6 days. The solvent was removed under
reduced pressure, and the crude mixture was partitioned
between water and Et2O. The aqueous layer was washed with
Et2O, acidified at 0 °C with 1 N HCl, and extracted with
EtOAc. The combined AcOEt layers were dried over MgSO4
and concentrated in vacuo. The residue was crystallized from
Et2O/pentane to give 17 as a white solid (361 mg, 61%): mp
93-94 °C; [R]25D 24.8 (c 1, CHCl3); 1H NMR (250 MHz, CDCl3)
11.72 (br s, 1H), 8.41-8.29 (m, 2H), 7.39-7.27 (m, 10H), 5.17
(s, 2H), 5.12 (s, 2H), 4.36 and 4.28 (2d, Boc cis-trans isomer-
3
ization, 1H, J ) 7.5 and 10 Hz), 3.74-3.58 (m, 1H), 3.70 (s,
3H), 3.38-3.30 (m, 1H), 2.99-2.73 (m, 1H), 2.72-2.28 (m, 2H),
2.16-2.03 (m, 1H), 1.88-1.64 (m, 1H), 1.46-1.42 (2s, Boc cis-
trans isomerization, 9H); 13C NMR (62.5 MHz, CDCl3) δ 176.6,
175.3, 172.1, 154.8, 153.8, 80.6, 61.6, 61.2, 51.9, 45.9, 45.5, 38.3,
37.4, 34.6, 29.7, 28.9, 28.4, 28.3; DCI MS m/z 288 [MH+]. Anal.
Calcd for C13H21NO6‚H2O: C, 51.43; H, 7.58; N, 4.61. Found:
C, 51.70; H, 7.30; N, 4.81.
3
ization, 1H, J ) 8.4 and 8.3 Hz), 3.76-3.42 (m, 3H), 3.33-
3.24 (m, 1H) 2.47-2.34 (m, 1H), 2.05-1.99 (m, 1H), 1.87-1.73
(m, 2H), 1.61-1.44 (m, 1H), 1.45-1.40 (2s, Boc cis-trans
isomerization, 9H); 13C NMR (62.5 MHz, CDCl3) δ 176.2, 175.0,
163.5, 156.1, 154.7, 153.9, 153.7, 136.7, 136.6, 134.5, 128.8,
128.7, 128.4, 128.1, 128.0, 80.4, 68.3, 67.2, 62.1, 61.6, 46.0, 45.6,
40.0, 39.5, 39.2, 29.7, 29.5, 29.1, 28.4, 28.3. Anal. Calcd for
C28H35N4O8: C, 60.53; H, 6.35; N, 10.08. Found: C, 60.81; H,
6.23; N, 9.77.
(2S,3S)-3-Cyanomethyl-1-(tert-butyloxycarbonyl)pyr-
rolidine-2-carboxylic Acid (14). 616c (4 g, 11.6 mmol), 10%
Pd/C (1.16 g), Boc2O (2.54 g, 11.6 mmol), and 55 mL of MeOH
were stirred overnight under H2. The reaction mixture was
then filtered on a Celite pad, and the solvent was evaporated
in vacuo. The crude product was taken up in water, and a 10%
aqueous solution of K2CO3 was added until pH 8. The aqueous
layer was washed with Et2O, acidified at 0 °C with 1 N HCl
until pH 2, and extracted with AcOEt. The combined AcOEt
layers were washed with brine and dried over MgSO4, and the
solvent was evaporated in vacuo. The crude product was
recrystallized from Et2O to give 14 as a white solid (2.21 g,
75%): mp 162 °C; [R]25D 2.3 (c 1, CHCl3); 1H NMR (250 MHz,
CDCl3) δ 4.41 and 4.35 (2d, Boc cis-trans isomerization, 1H,
3J ) 7.5 and 8.0 Hz), 3.80-3.58 (m, 1H), 3.52-3.32 (m, 1H),
2.87-2.56 (m, 2H), 2.56-2.33 (m, 1H), 2.31-2.08 (m, 1H),
2.06-1.80 (m, 1H), 1.46-1.42 (2s, Boc cis-trans isomerization,
9H); 13C NMR (62.5 MHz, CDCl3) δ 175.2, 174.0, 154.9, 153.9,
117.8, 117.6, 81.4, 81.2, 61.5, 61.1, 45.8, 45.4, 38.6, 37.8, 29.6,
28.8, 28.5, 28.4, 18.4. Anal. Calcd for C12H18N2O4: C, 56.68; H,
7.13; N, 11.02. Found: C, 56.83; H, 7.38; N, 10.83.
(2S,3R)-2-Oxotetrahydrofuran(2,3)-1-(1-phenylethyl)-
pyrrolidine (18) and (2S,3R)-3-Hydroxymethyl-1-(1-phen-
ylethyl)pyrrolidine-2-carboxylic Acid Methyl Ester (19).
To a solution of 1b (1.61 g, 5 mmol) in dry THF (10 mL) at -
78 °C was added LDA (2.5 mL, 2 N in heptane). A solution of
ZnBr2 (3.38 g, 3 equiv) in dry Et2O (15 mL) was then added,
and the mixture was allowed to warm to rt for 4 h. The solution
was cooled to 0 °C, and dioxygen was bubbled for 30 min. The
reaction was then was quenched with saturated aqueous NH4-
Cl. Et2O was added, and the organic layer was washed with
saturated aqueous NH4Cl, dried over MgSO4, and concentrated
under reduced pressure. The residue was purified by flash
chromatography (cyclohexane/ethyl acetate, 85/15) to give a
6/4 mixture of 18 and 19 (731 mg, 60%). 18 was isolated for
J. Org. Chem, Vol. 69, No. 23, 2004 7947

Quancard et al.
D
analysis as a pale yellow solid: mp 65 °C; [R]20 -62.9 (c 1,
80 °C for 8 h. After being cooled to room temperature, the
mixture was partitioned between Et2O and water. The aqueous
layer was extracted with Et2O, and the combined organic
layers were washed with brine, dried over MgSO4, and
concentrated under reduced pressure. The residue was purified
by flash chromatography (cyclohexane/ethyl acetate, 85/15) to
give 22 (672 mg, 79%): [R]20 1.1 (c 1, CHCl3); H NMR (400
MHz, CDCl3) δ 7.32-7.22, (m, 10H), 5.18-4.97 (m, 2H), 4.44-
4.29 (m, 3H), 3.73-3.61 (m, 1H), 3.46-3.24 (m, 3H) 2.78-2.63
(m, 1H), 2.07-1.82 (m, 2H), 1.45-1.34 (2s, Boc cis-trans
isomerization, 9H); 13C NMR (100 MHz, CDCl3) δ 171.6, 171.4,
154.3, 153.8, 138.0, 137.9, 135.8, 135.6, 128.7, 128.5, 128.4,
128.4, 128.4, 128.1, 127.7, 127.6, 127.5, 80.0, 79.9, 73.2, 73.1,
69.2, 69.1, 66.6, 61.1, 60.6, 45.9, 45.5, 43.2, 42.2, 28.4, 28.2,
27.6, 26.6. Anal. Calcd for C25H31NO4: C, 70.57; H, 7.34; N,
3.29. Found: C, 70.20; H, 7.67; N, 3.27.
CHCl3); 1H NMR (250 MHz, CDCl3) δ 7.42-7.24, (m, 5H), 4.31
(t, 1H, 3J ) 9.5 Hz), 4.17 (q, 1H, 3J ) 7.5 Hz), 4.11 (dd, 1H, 3J
3
) 2.5, 9.5 Hz), 3.50 (d, 1H, J ) 7.5 Hz), 2.95-2.83 (m, 1H),
2.82-2.75 (m, 2H), 2.16-2.08 (m, 1H), 1.75-1.70 (m, 1H),1.46
(d, 3H, 3J ) 7.5 Hz); 13C NMR (62.5 MHz, CDCl3) δ 176.7,
143.0, 128.2, 127.9, 127.1, 72.0, 61.8, 59.4, 50.0, 38.7, 30.87,
22.8. Anal. Calcd for C23H28N2O4: C, 72.73; H, 7.36; N, 6.06.
Found: C, 72.73; H, 7.48; N, 5.94. Data for 19: 1H NMR (250
D
1
3
MHz, CDCl3) δ 7.41-7.20, (m, 5H), 4.01 (q, 1H, J ) 7.5 Hz),
3.94 (d, 1H, 3J ) 5 Hz), 3.70 (s, 3H), 3.34 (d, 1H, 3J ) 7.5 Hz),
3.20-2.83 (m, 1H), 2.83-2.53 (m, 3H), 1.99-1.56 (m, 2H), 1.43
3
(d, 3H, J ) 7.5 Hz).
(2S,3R)-2-Oxotetrahydrofuran(2,3)-1-(tert-butyloxy-
carbonyl)pyrrolidine (20) and (2S,3R)-3-Hydroxymethyl-
1-(tert-butyloxycarbonyl)pyrrolidine-2-carboxylic Acid
Methyl Ester (21). A mixture of 18 + 19 (610 mg, 2.5 mmol)
and Pd/C (250 mg) in methanol (12.5 mL) was stirred overnight
under H2. The mixture was filtered on a Celite pad and
concentrated under reduced pressure to give a white oil. The
residue was purified by flash chromatography (cyclohexane/
ethyl acetate/acetic acid, 6/4/0.1) to give a 1/1 mixture of 20
and 21 (547 mg, 90%). 20 was further isolated for analysis as
(2S,3R)-3-Benzyloxymethyl-1-(tert-butyloxycarbonyl)-
pyrrolidine-2-carboxylic Acid (23). To a solution of 22 (596
mg, 1.4 mmol) in acetonitrile/water (4/1, 5 mL) was added
LiOH‚H2O (294 mg, 7 mmol), and the mixture was refluxed
overnight. The solvent was evaporated and the residue taken
up in water. After acidification to pH 2 with 1 N HCl, the
aqueous layer was extracted with Et2O. The combined organic
layers were washed with brine, dried over MgSO4, and
concentrated in vacuo. The residue was purified by flash
chromatography (cyclohexane/ethyl acetate/acetic acid, 8/2/0.1)
D
1
a white solid: mp 93 °C; [R]20 -114.1 (c 1, CHCl3); H NMR
(400 MHz, CDCl3) δ 4.78 4.52 (m, 1H), 4.43 (t, 1H, J ) 9.5
3
Hz), 4.14 (dd, 1H, 3J ) 2.5, 9.5 Hz), 3.60-3.45 (m, 2H), 3.22-
3.07 (m, 1H), 2.31-2.19 (m, 1H), 1.92-1.78 (m, 1H), 1.49-
1.41 (2s, Boc cis-trans isomerization, 9H); 13C NMR (100 MHz,
CDCl3) δ 174.0, 154.1, 154.0, 80.6, 69.4, 59.0, 46.8, 46.0, 39.2,
38.3, 29.9, 29.8, 28.8, 28.3. Anal. Calcd for C23H28N2O4: C,
58.14; H, 7.54; N, 6.16. Found: C, 58.12; H, 7.59; N, 6.12. Data
for 21: 1H NMR (250 MHz, CDCl3) δ 4.47-4.37 (m, 2H), 3.75
(s, 3H), 3.75-3.28 (m, 3H), 2.85-2.61 (m, 2H), 2.07-1.60 (m,
2H), 1.46-1.41 (2s, Boc cis-trans isomerization, 9H).
D
to give 23 (376 mg, 80%) as a white solid: [R]20 14.5 (c 1,
1
CHCl3); H NMR (400 MHz, CDCl3) δ 8.87 (br s, 1H), 7.35-
7.27, (m, 5H), 4.49-4.43 (m, 2H), 4.42 and 4.34 (2d, Boc cis-
3
trans isomerization, 1H, J ) 8.3 and 8.4 Hz), 3.71-3.33 (m,
4H), 2.83-2.65 (m, 1H), 2.10-1.88 (m, 2H), 1.45-1.41 (2s, Boc
cis-trans isomerization, 9H); 13C NMR (100 MHz, CDCl3) δ
177.0, 175.7, 154.9, 154.0, 138.1, 138.0, 128.6, 127.9, 127.8,
80.6, 73.5, 69.6, 69.4, 61.1, 60.6, 46.1, 45.7, 43.1, 42.1, 28.6,
28.5, 27.8, 26.8. Anal. Calcd for C18H25NO5: C, 64.46; H, 7.51;
N, 4.18. Found: C, 64,08; H, 7.62; N, 4.03.
(2S,3R)-3-Benzyloxymethyl-1-(tert-butyloxycarbonyl)-
pyrrolidine-2-carboxylic Acid Benzyl Ester (22). To a
mixture of 20 + 21 (486 mg, 2 mmol), benzyl bromide (0.956
mL, 8 mmol), and anhydrous toluene (8 mL) was added freshly
crushed KOH (476 mg, 8.5 mmol). The mixture was heated at
JO048762Q
7948 J. Org. Chem., Vol. 69, No. 23, 2004