Locked conformations for proline pyrrolidine ring: Synthesis and conformational analysis of cis- and trans-4-tert-butylprolines

Article abstract of DOI:10.1021/jo050838a

The motional restrictions of the proline pyrrolidine ring allow this secondary amine amino acid to act as a turn inducer in many peptides and proteins. The pyrrolidine ring is known to exhibit two predominant pucker modes (i.e., C-4 (C^γ) exo and endo envelope conformers whose ratio can be controlled by proper substituents in the ring). In nature, the exo puckered 4(R)-hydroxy-L-proline plays a crucial role as a building block in collagen and collagen-like structures. It has been previously concluded that the electronegativity of the 4-cis-substituent increases the endo puckering while the electronegativity of the 4-trans-substituent favors the exo puckering. Here, we have introduced a sterically demanding tert-butyl group at C-4 in trans- and cis-configurations. In the case of trans-substitution, the induced puckering effect on the pyrrolidine ring was studied with X-ray crystallography and ¹H NMR spectral simulations. Both cis- and trans-4-tert-butyl groups strongly favor pseudoequatorial orientation, thereby causing opposite puckering effects for the pyrrolidine ring, cis-exo and trans-endo for L-prolines, in contrast to the effects observed in the case of electronegative C-4 substituents. The syntheses and structural analysis are presented for the conformationally constrained 4-tert-butylprolines. The prolines were synthesized from 4-hydroxy-L-proline, substitution with t-BuCuSPhLi being the key transformation. This reaction gave N-Boc-trans-4-tert-butyl-L-proline tert-butyl ester in 94% ee and 57% de. Enantioselectivity was increased to 99.2% ee by crystallization of N-Boc-trans-4-tert-butyl-L-proline in the final step of the synthesis.

Full text of DOI:10.1021/jo050838a

Locked Conformations for Proline Pyrrolidine Ring: Synthesis
and Conformational Analysis of cis- and trans-4-tert-Butylprolines
Ari M. P. Koskinen,*,† Juho Helaja,† Esa T. T. Kumpulainen,† Jari Koivisto,†
Heidi Mansikkama¨ki,‡ and Kari Rissanen‡
Laboratory of Organic Chemistry, Department of Chemical Technology, Helsinki University of Technology,
P.O. Box 6100, FIN-02015 HUT, Finland, and NanoScience Centre, Department of Chemistry, University
of Jyva¨skyla¨, P.O. Box 35, FIN-40014, JYU, Finland
ari.koskinen@hut.fi
Received April 26, 2005
The motional restrictions of the proline pyrrolidine ring allow this secondary amine amino acid to
act as a turn inducer in many peptides and proteins. The pyrrolidine ring is known to exhibit two
predominant pucker modes (i.e., C-4 (Cγ) exo and endo envelope conformers whose ratio can be
controlled by proper substituents in the ring). In nature, the exo puckered 4(R)-hydroxy-L-proline
plays a crucial role as a building block in collagen and collagen-like structures. It has been previously
concluded that the electronegativity of the 4-cis-substituent increases the endo puckering while
the electronegativity of the 4-trans-substituent favors the exo puckering. Here, we have introduced
a sterically demanding tert-butyl group at C-4 in trans- and cis-configurations. In the case of trans-
substitution, the induced puckering effect on the pyrrolidine ring was studied with X-ray
crystallography and 1H NMR spectral simulations. Both cis- and trans-4-tert-butyl groups strongly
favor pseudoequatorial orientation, thereby causing opposite puckering effects for the pyrrolidine
ring, cis-exo and trans-endo for L-prolines, in contrast to the effects observed in the case of
electronegative C-4 substituents. The syntheses and structural analysis are presented for the
conformationally constrained 4-tert-butylprolines. The prolines were synthesized from 4-hydroxy-
L-proline, substitution with t-BuCuSPhLi being the key transformation. This reaction gave N-Boc-
trans-4-tert-butyl-L-proline tert-butyl ester in 94% ee and 57% de. Enantioselectivity was increased
to 99.2% ee by crystallization of N-Boc-trans-4-tert-butyl-L-proline in the final step of the synthesis.
Introduction and Background
Due to the conformational restrictions imposed by its
pyrrolidine ring, the proteinogenic amino acid proline has
an exceptional tendency to act as a turn inducer in
peptides and proteins.1 The pyrrolidine ring exhibits two
predominant pucker modes: C-4 (Cγ) exo and endo
envelope conformers, that is, “up” and “down”, respec-
tively (Figure 1).2 In the case of unsubstituted proline,
the endo puckering mode is favored over the exo mode.
The puckering propensity can be controlled by proper
choice of ring substituents. In collagen structures, the
FIGURE 1. Proline ring pucker modes are generally referred
to C-4 (Cγ) “endo” (or “down”) and “exo” (or “up”). Notations
arise from the N1 - C4 pseudorotation angles; endo is
associated to the gauche+ and exo to the gauche- torsion
rotamer. The substitution of C-4 affects not only the endo and
exo puckering but also the trans and cis peptide bonding.
† Helsinki University of Technology.
‡ University of Jyva¨skyla¨.
nonproteinogenic amino acid, 4-(R)-hydroxyl-L-proline
(trans-substituted), is equipped with a C-4 (R) OH group
that makes the exo puckering prevailing (Y ) OH, X )
H, Figure 1).3
(1) (a) Branden, C.; Tooze, J. Introduction to Protein Structure;
Garland Publishing: New York, 1991. (b) Stryer, L. Biochemistry, 4th
ed.; W. H. Freeman and Company: New York, 1999.
(2) DeTar, D. L. F.; Luthra, N. P. J. Am. Chem. Soc. 1977, 99, 1232-
1244.
10.1021/jo050838a CCC: $30.25 © 2005 American Chemical Society
Published on Web 07/01/2005
J. Org. Chem. 2005, 70, 6447-6453
6447

Koskinen et al.
Synthetic C-4 fluoroprolines have been used to eluci-
date the puckering effect of electronegative substituent
where trans-fluoroprolines promote exo envelope and cis-
fluoroprolines promote the endo envelope conformers.3,4
Extensive computational and NMR studies have sug-
gested that the conformational effects of the electroneg-
ative substituent is dictated by inductive and stereoelec-
tronic factors. Moreover, it has been concluded that the
peptide cis/trans-isomerism in collagen triple helix struc-
ture is dictated by the stereochemistry (R/S) of the C-4
substituent (OH, F).5 It has also been suggested that the
4-hydroxyproline stabilized ring pucker is a key deter-
minant endowing collagen its stability.3c
Although 4-substituted prolines have gained consider-
able synthetic interest,6-8 the conformational restrictions
caused by this substitution have not been considered in
the context of peptide secondary structures. As an excep-
tion, Koskinen et al. have synthesized conformationally
constrained C-4 methyl prolines for peptidomimetics with
the purpose of increasing conformational stability through
only a minor chemical change on the natural amino
acid.6b,9 Secondary structure constrainment was observed
for the open chain 4-methylproline tetrapeptides (Ala-
(C-4)MePro-Gly-Ala) in a conformational NMR study.10
The sterically bulky tert-butyl group is commonly used
to lock a ring conformation, due to its tendency to orient
equatorially for spatial and entropic reasons. For pro-
lines, this approach has been applied at the C-3 and C-5
positions.11,12 Several synthetic routes toward C-4 alkyl-
ation have been explored,6-9 but to our knowledge no
synthetic pathway has been reported for 4-tert-butyl
prolines. Yet, introduction of this bulky group specifically
on carbon C-4 is attractive as it can be expected that the
steric control takes place via pyrrolidine ring while the
tert-butyl substituents at both C-3 and C-5 positions
sterically interfere with the backbone peptide bonds.
In addition to the C-4 tert-butyl prolines, a number of
4-alkyl substituted prolines have already been synthe-
sized6-9 and are widely used in pharmaceutical industry,
such as angiotensin-converting enzyme (ACE) inhibitors
and potential inhibitors of proline dehydrogenase.7h Our
primary synthetic interest was the preparation of trans-
4-tert-butyl-L-proline. We present herein the synthetic
access to all the C-4 tert-butyl and C-2 CO2-tBu epimers.
In this work, we also present a conformational study of
the C-4 tert-butylprolines based on X-ray diffraction data
1
and H NMR spectroscopy.
Results and Discussion
Among the reported syntheses toward C-4 alkyl sub-
stituted prolines, the routes involving glutamate or
pyroglutamate intermediates have proven to be success-
ful in γ-alkylations. The alkyl groups have usually been
introduced as electrophiles on glutamate enolates.6 How-
ever, the poor electrophilic nature of tert-butyl group
precludes this type of synthetic strategy for 4-tert-
butylprolines. Many alkylation routes involve synthetic
steps via R-vinylic bonds and subsequent catalytic hy-
drogenation.7 Unfortunately, this type of synthetic route
would involve unfavorable high energy intermediates
caused by the allylic strain involving the tert-butyl group.
Alkyl cuprate substitutions of prolines at the C-4 position
appealed to us as an intriguing approach.7
(3) (a) Improta, R.; Benzi, C.; Barone, V. J. Am. Chem. Soc. 2001,
123, 12568-12577. (b) DeRider, M. L.; Wilkens, S. J.; Waddell, M. J.;
Bretscher, L. E.; Weinhold, F.; Raines, R. T.; Markley J. L. J. Am.
Chem. Soc. 2002, 124, 2497-2505. (c) Jenkins, C. L.; Lin, G.; Duo, J.;
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(4) Holmgren, S. K.; Taylor, K. M.; Bretscher, L. E.; Raines, R. T.
Nature 1998, 392, 666-667.
(5) Bretscher, L. E.; Jenkins, C. L.; Taylor, K. M.; DeRider, M. L.;
Raines, R. T. J. Am. Chem. Soc. 2001, 123, 777-778.
Our synthesis began with the preparation of interme-
diate 4 starting from trans-4-hydroxyproline using lit-
erature procedures (Scheme 1).7h,k The hydroxyprolinate
4 was then submitted to a Mitsunobu-type bromination,
yielding the bromo prolinate 5, key intermediate in this
synthesis route.13 Bromide was substituted with a tert-
butyl group in a Corey-House reaction using tert-
butylcuprate as the nucleophile.14 The alkylation reaction
was attempted with several types of tert-butyl cuprates:
Gilman (t-Bu2CuLi),8,14,15 cyano (t-Bu2CuCNLi2),14 and
thiophenol (t-BuCuSPhLi)17 reagents under various reac-
tion conditions.16 We found out that the Posner tert-butyl
thiophenolcuprate procedure proved to be most efficient
in the substitution.17 The substitution reaction proceeded
(6) (a) Thottathil, J. K.; Moniot, J. L.; Mueller, R. H.; Wong, M. K.
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1998, 39, 2199-2202. (e) Zhang, R.; Brownewell, F.; Madalengoitia,
J. S. J. Am. Chem. Soc. 1998, 120, 3894-3902. (f) Zhang, R.;
Brownewell, F.; Madalengoitia, J. S. Tetrahedron Lett. 1999, 40, 2707-
2710. (g) Chen, X.; Du, D.-M.; Hua, W.-T. Tetrahedron: Asymmetry
2002, 13, 43-46. (h) Zhang, J.; Wang, W.; Xiong, C.; Hruby, V. J.
Tetrahedron Lett. 2003, 44, 1413-1415.
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DeForrest, J. M.; Spitzmiller, E. R.; Karanewsky, D. S.; Duggan, M.;
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1148-1160. (b) Thottathil, J. K. U.S. Patent 4,734,508. (c) Moody, C.
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J.; Pedregal, C.; Yruretagoyena, B.; Rubio, A. J. Org. Chem. 1995, 60,
2925-2930. (e) Feeney, J.; McCormick, J. E.; Bauer, C. J.; Birdsall,
B.; Moody, C. M.; Starkmann, B. A.; Young, D. W.; Francis, P.; Havlin,
R. H.; Arnold, W. D.; Oldfield, E. J. Am. Chem. Soc. 1996, 118, 8700-
8706. (f) Durand, X.; Hudhomme, P.; Khan, J.; Young, D. W. J. Chem.
Soc., Perkin Trans. 1 1996, 11, 1131-1139. (g) Oba, M.; Terauchi, T.;
Miyakawa, A.; Kamo, H.; Nishiyama, K. Tetrahedron Lett. 1998, 39,
1595-1598. (h) Tamaki, M.; Han, G.; Hruby, V. J. J. Org. Chem. 2001,
66, 3593-3596. (i) Qiu, X.-l.; Qing, F.-L. J. Org. Chem. 2002, 67, 7162-
7164. (j) Del Valle, J. R.; Goodman, M. Angew. Chem., Int. Ed. 2002,
41, 1600-1602. (k) Del Valle, J. R.; Goodman, M. J. Org. Chem. 2003,
68, 3923-3931.
(11) (a) Halab, L.; Lubell, W. D. J. Am. Chem. Soc. 2002, 124, 2474-
2484. (b) Walle´n, E. A. A.; Christiaans, J. A. M.; Saarinen, T. J.; Jarho,
E. M.; Forsberg, M. M.; Vena¨la¨inen, J. I.; Ma¨nnisto¨, P. T.; Gynther, J.
Bioorg. Med. Chem. 2003, 11, 3611-3619.
(12) Perni, R. B.; Farmer, L. J.; Cottrell, K. M.; Court, J. J.;
Courtney, L. F.; Deininger, D. D.; Gates, C. A.; Harbeson, S. L.; Kim,
J. L.; Lin, C.; Lin, K.; Luong, Y.-P.; Maxwell, J. P.; Murcko, M. A.;
Pitlik, J.; Rao, B. G.; Schairer, W. C.; Tung, R. D.; Van Drie, J. H.;
Wilson, K.; Thomson, J. A. Bioorg. Med. Chem. Lett. 2004, 14, 1939-
1942.
(13) Webb, T. R.; Eigenbrot, C. J. Org. Chem. 1991, 56, 3009-3016.
(14) Organocopper Reagents; Taylor, R. J. K., Ed.; Oxford University
Press: New York, 1994.
(15) Johnson, C. R.; Dutra, G. A. J. Am. Chem. Soc. 1973, 95, 7777-
7782.
(16) See Supporting Information for optimization of the reaction
conditions.
(8) (a) Thottathil, J. K.; Moniot, J. L. Tetrahedron Lett. 1986, 27,
151-154. (b) Hashimoto, K.; Shima, Y.; Shirahama, H. Heterocycles
1996, 42, 489-492. (c) Heindl, C.; Hu¨bner, H.; Gmeiner, P. Tetrahe-
dron: Asymmetry 2003, 14, 3153-3172.
(9) Nevalainen, M.; Kauppinen, P. M.; Koskinen, A. M. P. J. Org.
Chem. 2001, 66, 2061-2066.
(10) O’Connell, J. F.; Koskinen, A. M.; Krische, M.; Rapoport, H.
Unpublished results.
6448 J. Org. Chem., Vol. 70, No. 16, 2005

Locked Conformations for Proline Pyrrolidine Ring
SCHEME 1. Synthesis of N-Boc-Protected 4-tert-Butylprolines
in THF at -18 °C in acceptable yield (54%) and gave
4-tert-butylprolinate 6 in 94% ee and 57% de. The yield
was satisfactory, taking into account the reported modest
yields for tert-alkylcuprates in general. This was espe-
cially the case, while our electrophile, inactivated second-
ary halogen in the 5-ring, is a poor candidate for
substitution reactions.
The reason for the observed C-2 epimerization lies in
the applied basic reaction conditions that led to enolate
equilibrium and thus loss in diastereoselectivity. This
was especially pronounced when the reaction times were
prolonged.
determined through the corresponding Mosher esters.
The enantiopurity of 8 was also determined through the
Mosher ester. The final step in the synthesis involved
TEMPO-catalyzed oxidation of the primary alcohols back
to the corresponding substituted prolines 9 and 10.18
Although the oxidation proceeded in 80% (for 9) and 76%
(for 10) yields and gave NMR pure samples, these
products were recrystallized to improve enantiopurity,
lowering the total yields to 57 and 48%, respectively. A
small amount of proline 9 was esterified using isourea 3
to determine the enantiopurity with GC, and it was found
to be excellent (99.2% ee).
In all cuprate reactions, some ring-opened N-Boc allyl
glycine tert-butyl ester was found as a side product. This
is most likely formed through direct transmetalation, and
similar observations have been reported in the literature.8c
The diastereo- and enantioselectivities were deter-
mined by gas chromatography. Unfortunately, only the
trans-enantiomers of tert-butylproline 6 of the four
epimers were fully separable in chiral GC. The poor
chromatographic separation of the product mixture was
solved by reduction of the ester to an alcohol group with
lithium aluminum hydride to give a diastereomer mix-
ture of prolinols 7 and 8 in excellent yield (90%). These
diastereomers were separable by flash chromatography.
The absolute stereochemistries in prolinols 7 and 8 were
An X-ray structure of proline 9 proved that the tert-
butyl substituent is trans to the carboxyl group and
occupies a pseudoequatorial position in the crystal (Fig-
ure 2).
Determination of the enantiopurity also required the
other enantiomers for accurate chiral GC analysis.
Synthesis of the enantiomer for prolinate 6 was per-
formed starting from the intermediate 11, which was
prepared from trans-4-hydroxy-L-proline using literature
procedures (Scheme 2).19 The stereochemistry at C-4 was
inverted using a Mitsunobu reaction to give 12 in good
yield (82%). The fully protected hydroxyproline 12 was
hydrolyzed (1 M sodium hydroxide) to give N-Boc-trans-
4-hydroxy-D-proline ent-2 in quantitative yield. Subse-
quent esterification with isourea 3 proceeded in 79%
yield. The hydroxyprolinate ent-4 was brominated using
Mitsunobu-type reaction with inversion at C-4. This key
intermediate was then alkylated using the same condi-
tions as for its enantiomer 5 using t-BuCuSPhLi. The
reaction gave prolinate ent-6 in acceptable yield (34%)
and in selectivity similar (92% ee, 68% de) to that of its
enantiomer 6.
Pure trans-epimer of 6 was crystallized from freezer
cold (-18 °C) isooctane, and the crystallographic struc-
ture was determined (Figure 3).
(17) (a) Posner, G. H.; Whitten, C. E.; Sterling, J. J. J. Am. Chem.
Soc. 1973, 95, 7788-7800. (b) Posner, G. H.; Brunelle, D. J.; Sinoway,
L. Synthesis 1974, 9, 662-663.
(18) Epp, J. B.; Widlanski, T. S. J. Org. Chem. 1999, 64, 293-295.
(19) Sakuraba, S.; Takahashi, H.; Takeda, H.; Achiwa, K. Chem.
Pharm. Bull. 1995, 43, 738-747. (b) Merino, P.; Ruevuelta, J.; Tejero,
T.; Chiacchio, U.; Rescifina, A.; Piperno, A.; Romeo, G. Tetrahedron:
Asymmetry 2002, 13, 167-172. (c) Rosen, T.; Chu, D. T. W.; Lico, I.
M.; Fernandes, P. B.; Marsh, K.; Shen, L.; Cepa, V. G.; Pernet, A. G.
J. Med. Chem. 1988, 31, 1598-1611.
FIGURE 2. ORTEP plot of the X-ray crystal structure of
proline 9.
J. Org. Chem, Vol. 70, No. 16, 2005 6449

Koskinen et al.
FIGURE 3. ORTEP plot of the X-ray crystal structures of proline 6 and 3D plot of the RMS fit of its crystal structure (blue) and
solution structure (red).
SCHEME 2. Synthesis of the Enantiomeric Pair for the Ester 6
3
3
TABLE 1. Simulated and Calculated JH-H Couplings
TABLE 2. Simulated and Calculated JHH Couplings for
for trans-4-tert-Butylproline
cis-4-tert-Butylproline
simulated 3JH-H
3JH-H couplings (Hz) calculated
simulated 3JH-H
3JH-H couplings (Hz) calculated
couplings (Hz)a
from structural parametersb
couplings (Hz)a
from structural parametersb
coupled
protons
NMR at 25 °C
DFT exo
DFT endo
coupled NMR at NMR at
DFT endo
pucker
DFT exo
pucker
protons
25 °C
110 °C
X-ray
H2-H3a
H2-H3e
H3a-H4
H3e-H4
H5a-H4
H5e-H4
9.08
7.99
12.17
6.98
11.24
7.47
9.96
7.96
12.61
5.40
10.20
6.48
11.50
4.53
H2-H3a
H2-H3e
H3a-H4
H3e-H4
H5a-H4
H5e-H4
9.26
1.13
12.57
6.65
10.09
8.49
9.13
1.54
11.43
7.18
9.49
8.45
9.55
0.91
12.25
6.03
11.20
6.64
8.16
0.45
12.59
5.42
10.62
7.36
8.52
9.03
9.08
0.45
8.22
0.80
11.25
6.50
10.10
2.33
a Based on spectral iteration (see Experimental Section). b Cal-
culated from torsion parameters using the Haasnoot-Altona
equation (including the â-substituent correction).
a Based on spectral iteration (see Experimental Section). b Cal-
culated from torsion parameters using the Haasnoot-Altona
equation (including the â-substituent correction).
optimization (MM+ force field). As a result, unambiguous
endo puckering could be deduced for this proline. By
comparing the simulated JH-H couplings at 25 and 110
°C, we found that there is still strong preference for endo
puckering mode even at elevated temperatures (Table 1).
Solution structures were determined relying on vicinal
(3JH-H) coupling constants, which were obtained from
spectral simulations. The simulated couplings were first
converted into H-C-C-H torsion angles exploiting the
Haasnoot-Altona equation (including the â-substituent
correction) (Tables 1 and 2).20,21 The reciprocal use of this
formula yielded the estimated JH-H coupling constants
from the corresponding structural dihedral parameters
(Tables 1 and 2).
3
The 4-tert-butyl-proline ring conformations were also
studied computationally using B3LYP at 6-31G* level of
theory.22 For density functional theory (DFT) modeling,
the structures were truncated from the peripheral tert-
butyl ester and Boc-group for computational reasons
(Figures 4 and 5). In the case of trans-4-tert-butyl-L-
proline geometry optimization, both exo and endo con-
formers were found as energy minima of which the endo
mode was energetically favored by 2.5 kcal/mol over its
3
In the case of trans-4-tert-butyl-L-proline, the calcu-
lated dihedrals were set as constraints in the geometry
(20) Haasnoot, C. A. G.; de Leeuw, F. A. A. M.; Altona C. Tetrahe-
dron 1980, 36, 2783-2792.
(21) Navarro-Va´zquez, A.; Cobas, J. C.; Sardina, F. J.; Casanueva,
J.; D´ıez, E. J. Chem. Inf. Comput. Sci. 2004, 44, 1680-1685.
3
exo counterpart. On the basis of the JH-H couplings
6450 J. Org. Chem., Vol. 70, No. 16, 2005

Locked Conformations for Proline Pyrrolidine Ring
strained peptide turns. Currently we are investigating
structural effects of trans-4-tert-butylproline in â-turn
mimetics.
Experimental Section
(2S)-N-Boc-trans-4-hydroxy-L-proline tert-Butyl Ester
(4). A solution of proline 2 (3.96 g, 17.1 mmol, 100 mol %) in
60 mL of dry THF was treated with O-tert-butyl N,N-
diisopropylisourea23 3 (5.14 g, 25.7 mmol, 150 mol %) at room
temperature and then stirred for 2.5 h at 60 °C. Additional
O-tert-butyl N,N-diisopropylisourea 3 (3.43 g, 17.1 mmol, 100
mol %) was added to the mixture, and then stirring was
continued overnight. Precipitated urea was filtered off through
Celite followed by ether washings, and the filtrate was
evaporated in vacuo to give an oily white solid. The crude
product was purified by flash chromatography (30-50% ethyl
acetate/hexanes) to give the corresponding tert-butyl ester 4
as a colorless oil (3.93 g, 80%). Rf ) 0.45 (ethyl acetate); [R]D
) -57.3 (c 1.01, CHCl3); IR (thin film, cm-1): 3437, 2978, 2934,
FIGURE 4. DFT B3LYP 6-31G* geometry optimized endo
(left) and exo (right) puckering modes for N-Ac methyl trans-
4-tert-butyl-l-prolinate. The endo is the global minimum
conformer with a 2.5 kcal/mol relative energy preference.
1
1742, 1703, 1403, 1367, 1151; H NMR (400 MHz, CDCl3): δ
4.43 (br s, 1H), [4.28 (t, 7.6 Hz), 4.24 (t, 7.6 Hz) 1H], [3.59 (d,
4.5 Hz), 3.56 (d, 4.3 Hz) 1H], [3.51 (d, 11.5 Hz), 3.39 (d, 11.1
Hz) 1H], [2.62 (s), 2.55 (s) 1H], 2.35-2.17 (m, 1H), 2.10-1.97
(m, 1H), 1.48-1.41 (m, 18H); 13C NMR (100 MHz, CDCl3): δ
[172.1, 172.0], [154.4, 154.1], [81.1, 81.0], [80.1, 79.8], [70.0,
69.2], [58.5, 58.4], [54.6, 54.5], [39.1, 38.3], 28.3, [27.9, 27.8];
HRMS (ESI) calcd for C14H25NO5Na, 310.1630; found, 310.1649;
∆ ) 6.1 ppm. These data match those reported in the
literature.7h,24
FIGURE 5. DFT B3LYP 6-31G* geometry optimized exo (left)
and endo (right) puckering modes for N-Ac methyl cis-4-tert-
butyl-D-prolinate. The exo is the global minimum conformer
with a 3.3 kcal/mol relative energy preference.
(2S)-N-Boc-cis-4-bromo-L-proline tert-Butyl Ester (5).
Hydroxyproline 4 (4.61 g, 16.0 mmol, 100 mol %) and tetra-
bromomethane (16.23 g, 48.9 mmol, 305 mol %) were dissolved
in 40 mL of dry dichloromethane. The mixture was cooled to
0 °C, and triphenylphosphine (13.09 g, 49.9 mmol, 310 mol
%) was added carefully. The reaction was stirred at room
temperature for 15 h. Ethanol (4 mL) was added, and the
solution was stirred for 2 h. Ether (40 mL) was added dropwise
to precipitate the phosphine oxide, which was filtered off, the
filter cake was washed with dichloromethane (2 × 30 mL), and
the filtrate was evaporated in vacuo to give a brown oil.
Purification by flash chromatography (20-30% ether/hexanes)
gave bromide 5 (4.17 g, 74%) as a colorless oil. Rf ) 0.43 (50%
ether in hexanes); [R]D ) -38.1 (c 1.00, CHCl3); IR (thin film,
cm-1): 2978, 2932, 1746, 1704, 1395, 1367, 1151; 1H NMR (400
MHz, CDCl3): δ 4.33-4.15 (m, 2H), [4.05 (dd, 6.4 Hz, 11.8 Hz),
3.99 (dd, 6.4 Hz, 11.7 Hz) 1H], [3.69 (dd, 5.8 Hz, 12.0 Hz), 3.65
(dd, 6.0 Hz, 11.9 Hz) 1H], 2.89-2.72 (m, 1H), 2.42-2.30 (m,
1H), [1.47 (s), 1.46 (s) 9H], [1.45 (s), 1.42 (s) 9H]; 13C NMR
(100 MHz, CDCl3): δ [170.5, 170.3], [153.4, 153.2], [80.3, 80.2],
[58.8, 58.7], [55.6, 55.3], [42.2, 41.5], [41.0, 40.0], [28.3, 28.2],
27.9; HRMS (ESI) calcd for C14H24NO4NaBr, 372.0786; found,
372.0777; ∆ ) 2.4 ppm.
deduced from the X-ray structure and DFT models, it is
evident that trans-4-tert-butylproline exists predomi-
nantly in the endo puckering conformation.
The DFT modeling of the cis-4-tert-butyl-D-proline
again indicates that pseudoequatorial orientation of the
tert-butyl group is clearly preferred: the exo conformer
is 3.3 kcal/mol lower in energy than the endo one (Figure
5). It is also worth noticing that in the endo conformer
the 4-tert-butyl group is spatially in a close proximity to
the 2-carboxy group. This distorts the pyrrolidine to a
nearly planar conformation, where the nitrogen is located
3
slightly below the plane. Inspection of simulated JH-H
couplings and the calculated ones (Table 2) shows ap-
parent preference for exo puckering mode for the cis-tert-
butyl proline, although this was less pronounced than
the endo puckering in the case of the trans-substitution.
Conclusions
(2S)-N-Boc-trans-4-tert-butyl-L-proline tert-Butyl Es-
ter (6). A suspension of copper(I) thiophenolate17b (1.47 g, 8.52
mmol, 600 mol %) in 30 mL of dry THF was cooled to -20 °C
and treated with careful addition of 1.85 M tert-butyllithium
in pentane (4.6 mL, 8.52 mmol, 600 mol %). This black solution
was stirred for 20 min, and a precooled (-20 °C) solution of
bromide 5 (0.50 g, 1.42 mmol, 100 mol %) in dry THF (5 mL)
was added. The reaction was stirred at -18 °C for 16 h and
quenched by pouring into a solution of saturated aqueous NH4-
Cl (30 mL). The aqueous mixture was stirred vigorously until
the black color disappeared. Solids were filtered off, and the
phases were separated. The aqueous phase was extracted with
ether, and the combined organic phases were washed with
saturated aqueous NaHCO3 and brine and dried over Na2SO4.
We have developed selective synthetic routes to both
trans- and cis-4-tert-butylprolines from trans-4-hydroxy-
proline using t-BuCuSPhLi mediated substitution via a
1
secondary bromide. According to H NMR-based confor-
mational analysis, molecular modeling, and X-ray struc-
ture, the C-4 tert-butyl group prefers strongly the pseu-
doequatorial orientation in the pyrrolidine ring and
thereby promotes conformational ring locking for exo
puckering in the case of cis and endo puckering in the
case of trans-substituted prolines, in both solution and
solid state. As a result of demonstrated conformational
locking effect, the C-4 tert-butyl-substituted prolines offer
potentially very attractive tools to construct short con-
(23) Bergmeier, S. C.; Cobas, A. A.; Rapoport, H. J. Org. Chem. 1993,
58, 2369-2376.
(22) Essentials of Computational Chemistry: Theories and Models;
Cramer, C. J., Ed.; Wiley & Sons: West Sussex, England, 2003.
(24) Tanaka, K.-i.; Sawanishi, H. Tetrahedron: Asymmetry 1998,
9, 71-77.
J. Org. Chem, Vol. 70, No. 16, 2005 6451

Koskinen et al.
Evaporation of solvent in vacuo gave a yellow oily solid. Silica
gel chromatography (5-10% ether/hexanes) yielded prolinate
6 (0.253 g, 54%, 95% ee, 57% de) as an oily white solid. Rf )
0.54 (50% ether in hexanes).
) 0.11 (50% ethyl acetate in hexanes); mp 140-141 °C; [R]D
) +62.7 (c 1.00, CHCl3); IR (thin film, cm-1): 3500-2300
(broad band), 2960, 2894, 1702, 1663, 1478, 1419, 1366, 1257,
1168, 1137; 1H NMR (400 MHz, CDCl3): δ 10-7.60 (br s, 1H),
[4.39 (d, 8.6 Hz), 4.31 (d, 5.3 Hz) 1H], [3.62 (t, 8.2 Hz), 3.42 (t,
8.7 Hz) 1H], [3.12 (t, 9.8 Hz), 3.03 (t, 10.4 Hz) 1H], 2.35-1.90
(m, 3H), [1.48 (s), 1.41 (s) 9H], 0.90 (s, 9H); 13C NMR (100 MHz,
CDCl3): δ [178.5, 174.5], [156.7, 153.9], [81.6, 80.1], [59.5,
59.2], [48.0, 47.8], [47.3, 46.7], [31.5, 30.9], [30.8, 29.1], [28.3,
28.3], 27.4; HRMS (ESI) calcd for C14H25NO4Na, 294.1681;
found, 294.1687; ∆ ) 2.0 ppm.
(2R)-N-Boc-cis-4-tert-butyl-D-proline (10). A suspension
of prolinol 8 (28 mg, 0.108 mmol, 100 mol %), TEMPO (1.7
mg, 10.8 µmol, 10 mol %), and BAIB (77 mg, 0.23 mmol, 220
mol %) in acetonitrile (0.5 mL) and distilled water (0.5 mL)
was stirred for 21 h at room temperature. The reaction was
quenched by addition of saturated aqueous Na2SO3 solution
until color had disappeared. Acetonitrile was evaporated under
reduced pressure, and the aqueous mixture was basified (pH
≈ 10) with 1 M NaOH solution, washed twice with ether, and
acidified with 1 M HCl solution (pH ≈ 3). The acidic aqueous
phase was extracted three times with ether. The combined
organic extracts were dried over Na2SO4 and evaporated in
vacuo to yield proline 10 (22 mg, 76%) as a white solid.
Recrystallization from refluxing MTBE (0.4 mL)/hexanes (4
mL) gave white crystals (14 mg, 48%). Rf ) 0.12 (50% ethyl
acetate in hexanes); mp 166-167 °C; [R]D ) -90.8 (c 1.00,
CHCl3); IR (thin film, cm-1): 3500-2300 (broad band), 2952,
2872, 1731, 1637, 1437, 1405, 1365, 1252, 1167, 774; 1H NMR
(400 MHz, CDCl3): δ [4.30 (t, 8.0 Hz), 4.20 (t, 7.8 Hz) 1H],
3.73-3.54 (m, 1H), [3.15 (t, 10.6 Hz), 3.03 (t, 10.2 Hz) 1H],
2.35-2.15 (m, 1H), 2.10-1.65 (m, 2H), [1.48 (s), 1.42 (s) 9H],
0.91 (s, 9H); 13C NMR (100 MHz, CDCl3): δ [178.7, 175.0],
[156.5, 153.7], [81.5, 80.4], [59.6, 59.4], 48.5, [48.3, 47.7], [32.3,
30.3], 30.8, [28.3, 28.2], 27.5; HRMS (ESI) calcd for C14H25-
NO4Na, 294.1681; found, 294.1687; ∆ ) 2.0 ppm.
(2R)-N-Boc-trans-4-acetoxy-D-proline Ethyl Ester (12).
To a solution of prolinate 11 (0.50 g, 1.93 mmol, 100 mol %),
triphenyl phosphine (1.01 g, 3.86 mmol, 200 mol %), and glacial
acetic acid (0.22 mL, 0.23 g, 3.86 mmol, 200 mol %) in dry THF
(5 mL) was added 40% DEAD in toluene (1.67 g, 1.76 mL, 3.86
mmol, 200 mol %) at 0 °C. The reaction mixture was allowed
to warm to room temperature and was stirred overnight. The
solvent was evaporated, and the residue was purified by silica
gel chromatography (20-40% ether/hexanes) to give prolinate
12 (0.482 g, 82%) as a yellow oil. Rf ) 0.24 (50% ether in
hexanes); [R]D ) +44.0 (c 1.00, CHCl3); IR (thin film, cm-1):
2979, 2935, 1744, 1704, 1401, 1367, 1239, 1194, 1160, 1127;
1H NMR (400 MHz, CDCl3): δ 5.28-5.18 (m, 1H), [4.36 (t 7.6
Hz), 4.29 (8.0 Hz) 1H], 4.25-4.07 (m, 2H), 3.74-3.45 (m, 2H),
2.40-2.28 (m, 1H), 2.21-2.10 (m, 1H), 2.02 (s, 3H), [1.42 (s),
1.39 (s) 9H], [1.24 (t, 7.3 Hz), 1.23 (t, 7.4 Hz) 3H]; 13C NMR
(100 MHz, CDCl3): δ [172.5, 172.1], [170.3, 170.2], [154.0,
153.5], [80.3, 80.2], [72.6, 71.8], 61.0, [57.8, 57.5], [52.1, 51.9],
[36.5, 35.4], [28.2, 28.1], 20.9, [14.1, 14.0]; HRMS (ESI) calcd
for C14H23NO6Na, 324.1423; found, 324.1398; ∆ ) 7.7 ppm.
(2R)-N-Boc-trans-4-hydroxy-D-proline (ent-2). A solu-
tion of prolinate 12 (0.424 g, 1.40 mmol) in THF (5 mL) was
treated with aqueous 15 wt % NaOH solution and was stirred
for 3 h. Most of the THF was evaporated under reduced
pressure, and the aqueous mixture was acidified with 6 M HCl
to pH ≈ 3. The mixture was extracted with ethyl acetate (10
times) until TLC showed no product in the aqueous phase. The
combined organic extracts were dried over Na2SO4, filtered,
and evaporated in vacuo to give proline ent-2 (0.325 g, 100%)
as a solid. Rf ) 0.50 (50% methanol in ethyl acetate); IR (thin
film, cm-1): 3500-2300 (broad band), 3419, 2979, 2935, 1725,
1671, 1421, 1197; 1H NMR (400 MHz, CDCl3): δ 4.55-4.35
(m, 2H), 3.70-3.45 (m, 2H), 2.50-2.05 (m, 2H), [1.47 (s), 1.42
(s) 9H]; 13C NMR (100 MHz, CDCl3): δ [177.6, 174.1], [156.7,
154.1], [81.9, 80.9], [69.6, 69.4], 57.8, 54.6, [39.0, 37.2], 28.3;
HRMS (ESI) calcd for C10H17NO5Na, 254.1004; found, 254.1005;
trans-L-6: IR (thin film, cm-1): 2965, 1742, 1704, 1394,
1
1366, 1176, 1153, 1124; H NMR (400 MHz, CDCl3): δ [4.24
(d, 7.8 Hz), 4.15 (d, 8.6 Hz) 1H], [3.58 (dd, 8.6 Hz, 10.2 Hz),
3.48 (dd, 8.7 Hz, 10.6 Hz) 1H], [3.10 (t, 10.2 Hz), 3.03 (t, 10.2
Hz) 1H], 2.23-2.06 (m, 1H), 2.20-1.80 (m, 2H), [1.46 (s), 1.44
(s), 1.42 (s) 18H], 0.87 (s, 9H); 13C NMR (100 MHz, CDCl3): δ
[172.3, 172.2], [154.4, 154.0], [80.8, 80.8], [79.5, 79.4], [60.1,
59.9], [47.5, 47.3], [47.5, 46.6], 31.7, [30.9, 30.8], [28.4, 28.3],
28.0, 27.3; HRMS (ESI): calcd for C18H33NO4Na, 350.2307;
found, 350.2312; ∆ ) 1.4 ppm; GC tr ) 79.4 min.
cis-D-6: 1H NMR (400 MHz, CDCl3): δ [4.11 (dd, 7.3 Hz,
8.6 Hz), 4.07 (dd, 8.2 Hz,8.8 Hz) 1H], [3.66 (dd, 7.8 Hz, 9.8
Hz), 3.49 (dd, 7.5 Hz, 10.0 Hz) 1H], [3.10 (t, 11.0 Hz), 3.08 (t,
10.6 Hz) 1H], 2.29-2.18 (m, 1H), 2.06-1.91 (m, 1H), 1.65-
1.55 (m, 1H), [1.46 (s), 1.45 (s), 1.45 (s), 1.43 (s) 18 H], [0.90
(s), 0.89 (s) 9H]; 13C NMR (100 MHz, CDCl3): δ 172.4, 154.0,
80.7, 79.7, [60.0, 60.0], [49.0, 48.2], [47.9, 47.8], 32.2, [31.1,
30.9], [28.4, 28.3], [28.0, 27.9], [27.5, 27.4]; GC tr ) 94.5 min.
(2S)-N-Boc-trans-4-tert-butyl-L-prolinol (7) and (2R)-
N-Boc-cis-4-tert-butyl-D-prolinol (8). A solution of ester 6
(0.496 g, 1.51 mmol, 100 mol %) in dry THF was treated with
careful addition of lithium aluminum hydride (0.23 g, 6.06
mmol, 400 mol %) at room temperature and then was stirred
for 40 min. The reaction was finished using the three-step
quench method: addition of distilled water (0.23 mL), 15 wt
% NaOH solution (0.23 mL), and distilled water (0.69 mL) to
a precooled solution (0 °C). The mixture was allowed to warm
to room temperature and was stirred for 20 min, then dried
over Na2SO4 and filtered. The filtrate was again dried over
Na2SO4 and filtered, and the solvent was evaporated under
reduced pressure to give a yellow oil. Silica gel chromatography
(5-15% ethyl acetate/hexanes) yielded fraction A: alcohol 8
(0.045 g, 11%), fraction B: alcohols 8 and 7 (0.020 g, 5%),
fraction C: alcohol 7 (0.353 g, 74%).
7: Rf ) 0.44 (50% ethyl acetate in hexanes); [R]D ) +27.8 (c
1.00, CHCl3); IR (thin film, cm-1): 3430, 2960, 2874, 1695,
1
1673, 1404, 1366, 1174, 1127; H NMR (400 MHz, CDCl3): δ
4.07-3.97 (m, 1H), 3.63 (dd, 7.8 Hz, 10.8 Hz, 1H), 3.55 (dd,
4.3 Hz, 10.6 Hz, 1H), 3.6 (dd, 7.8 Hz, 10.6 Hz, 1H), 3.29 (br s,
1H), 2,13-2.00 (m, 1H), 1.77 (dt, 8.7 Hz, 12.6 Hz, 1H), 1.60
(dd, 6.6 Hz, 12.6 Hz, 1H), 1.46 (s, 9H), 0.87 (s, 9H); 13C NMR
(100 MHz, CDCl3): δ 156.5, 80.1, 67.4, 59.7, 48.3, 47.3, 31.0,
29.3, 28.4, 27.4; HRMS (ESI) calcd for C14H27NO3Na, 280.1889;
found, 280.1889; ∆ ) 0 ppm.
8: Rf ) 0.51 (50% ethyl acetate in hexanes); [R]D ) -28.2 (c
1.00, CHCl3); IR (thin film, cm-1): 3411, 2961, 2872, 1694,
1
1670, 1409, 1366, 1167, 1119; H NMR (400 MHz, CDCl3): δ
3.96-3.86 (m, 1H), 3.68 (dd, 1.8 Hz, 11.5 Hz, 1H), 3.63-3.43
(m, 3H), 2.95 (t, 10.9 Hz, 1H), 1.99-1.84 (m, 2H), 1.46 (s, 9H),
1.33-1.17 (m, 1H), 0.89 (s, 9H); 13C NMR (100 MHz, CDCl3):
δ 156.8, 80.4, 67.4, 61.2, 48.7, 47.6, 30.7, 30.4, 28.4, 27.5;
HRMS (ESI) calcd for C14H27NO3Na, 280.1889; found, 280.1908;
∆ ) 6.7 ppm.
(2S)-N-Boc-trans-4-tert-butyl-L-proline (9). To a flask
charged with prolinol 7 (0.263 g, 1.02 mmol, 100 mol %),
TEMPO (0.016 g, 0.102 mmol, 10 mol %), and bis-acetoxy-iodo-
benzene (BAIB) (0.724 g, 2.25 mmol, 220 mol %) was added
2.5 mL of acetonitrile and 2.5 mL of distilled water. The
reaction mixture was stirred for 2 h and quenched with
saturated aqueous Na2SO3 solution (3 mL). Acetonitrile was
evaporated under reduced pressure, and the aqueous mixture
was basified (pH ≈ 10) with 1 M NaOH solution, washed twice
with ether, and acidified with 1 M HCl solution (pH ≈ 3). The
acidic aqueous phase was extracted three times with ether.
The combined organic extracts were dried over Na2SO4,
filtered, and evaporated in vacuo to yield proline 9 (0.222 g,
80%) as a white solid. Recrystallization from refluxing MTBE
(1 mL)/hexanes (10 mL) gave white crystals (0.160 g, 57%). Rf
6452 J. Org. Chem., Vol. 70, No. 16, 2005

Locked Conformations for Proline Pyrrolidine Ring
∆ ) 0.4 ppm. These data are consistent to those reported in
the literature for the enantiomer 2.25
Supporting Information Available: Table of the opti-
mization conditions for the tert-butyl copper alkylation. Ex-
perimental procedures for ent-4, ent-5, and ent-6. Copies of
1H and 13C NMR spectra of compounds 5-10, 12. GG chro-
matograms for compounds 6, ent-6, and tert-butyl ester
derivative 9. Mosher ester analysis for compound 7 and 8.
Crystallographic collection data and tables for 6 and 9. This
material is available free of charge via the Internet at
http://pubs.acs.org.
Acknowledgment. Financial support from the Na-
tional Technology Agency of Finland, the Academy of
Finland, and EU COST action D28 is gratefully ac-
knowledged.
(25) (a) Koh, D. W.; Coyle, D. L.; Mehta, N.; Ramsinghani, S.; Kim,
H.; Slama, J. T.; Jacobson M. K. J. Med. Chem. 2003, 46, 4322-4332.
(b) Postels, H.-T.; Ko¨nig, W. A. Liebigs Ann. Chem. 1992, 12, 1281-
1287.
JO050838A
J. Org. Chem, Vol. 70, No. 16, 2005 6453