Pseudo-A(1,3) strain as a key conformational control element in the design of poly-L-proline type II peptide mimics

Article abstract of DOI:10.1021/ja972494e

A strategy for the synthesis of peptide mimics of the poly-L-proline type II secondary structure from 4-substituted prolines is presented. Dimeric and trimeric oligomers composed of 4-substituted prolines are shown by NMR to preferentially populate the poly-L-proline type II secondary structure in both CDCl₃ and D₂O. Oligomers composed of 4-substituted prolines thus imitate the desired backbone conformation and are able to incorporate non- prolyl side chains on the proline backbone.

Full text of DOI:10.1021/ja972494e

3894
J. Am. Chem. Soc. 1998, 120, 3894-3902
Pseudo-A(1,3) Strain as a Key Conformational Control Element in the
Design of Poly-L-proline Type II Peptide Mimics
Rui Zhang, Floyd Brownewell, and Jose S. Madalengoitia*
Contribution from the Department of Chemistry, UniVersity of Vermont, Burlington, Vermont 05405
ReceiVed July 23, 1997
Abstract: A strategy for the synthesis of peptide mimics of the poly-L-proline type II secondary structure
from 4-substituted prolines is presented. Dimeric and trimeric oligomers composed of 4-substituted prolines
are shown by NMR to preferentially populate the poly-L-proline type II secondary structure in both CDCl3
and D2O. Oligomers composed of 4-substituted prolines thus imitate the desired backbone conformation and
are able to incorporate non-prolyl side chains on the proline backbone.
Introduction
Recent years have seen the emergence of a wealth of NMR
and X-ray crystal structure data which demonstrates that kinases,
SH3 domains, and MHC class II molecules as well as other
proteins bind peptide ligands in the poly-L-proline type II (PPII)
conformation or in extended conformations closely resembling
this geometry.1 In addition, a survey of protein X-ray crystal
structures has shown that short spans of poly-L-proline type II
helices occur regularly in globular proteins where they are
predominantly located on the protein surface,2 a position where
their extended structure makes them ideal for interacting with
binding clefts on other proteins. The PPII secondary structure
thus emerges as a critical recognition element in mediating
protein-protein interactions and therefore regulating cellular
signaling, a role which makes the design of PPII mimics highly
desirable as potential chemotherapeutic agents.
The poly-L-proline type II secondary structure is defined as
an extended left-handed helix with 3.3 amino acid residues per
turn and φ ) -75°, ψ ) 145°, and ω ) 180°. While this
conformation was first identified in polyproline, a peptide strand
possessing no prolines but described by these φ, ψ, and ω angles
is defined as a PPII helix. Furthermore, while many PPII helices
contain proline amino acids which serve to stabilize the
secondary structure, often it is the side chains of the non-proline
amino acids in the PPII helix which are critical for receptor
recognition of the PPII motif. Thus, the challenge in the
development of mimics of the PPII secondary structure is the
design of a framework which imitates the correct peptide
backbone conformation and can incorporate non-prolyl side
chains required for recognition of the PPII mimic by the receptor
site. This paper describes work in our laboratory toward this
Figure 1. Minimization of pseudo-A(1,3) strain defines ψ ∼ 145°.
goal by the design and synthesis of oligomers containing proline-
templated amino acids.
We have recently reported that appropriately protected all-
proline oligomers (n ) 2-5) adopt the all-trans amide bond
conformation in solution, as required for oligopeptides in the
PPII secondary structure.3 This work further implies that
proline-containing oligomers are conformationally constrained
given (1) the φ angle is defined as ∼-75° by the constraints of
the proline pyrrolidine ring, (2) the ψ angle (∼145°) is defined
by minimization of pseudo-A(1,3) strain in which the amide
bond represents a double-bond surrogate (Figure 1), and (3) the
amide s-trans conformation (ω ) 180°) is favored by 2.3-5
kcal mol.4 Due to the presence of these conformational control
elements, in an all-proline oligomer, the PPII secondary structure
is non-cooperative (i.e., a critical chain length is not required
to conformationally bias the peptide); therefore, an oligomer
unit as small as an appropriately protected proline dimer is
conformationally constrained.3
Having established the stability of the amide trans conforma-
tion in proline-containing oligomers (n ) 2-5), we now report
the design and synthesis of novel oligopeptides which mimic
the PPII secondary structure. The basic strategy involves the
synthesis of peptides from substituted proline analogues (proline-
templated amino acids, PTAAs, see Figure 2) which possess a
non-prolyl side chain functionality (natural or unnatural) on the
proline backbone. These PTAA-derived oligopeptides therefore
furnish molecules which not only preferentially populate the
PPII conformation in solution but also possess the amino acid
side chain functionality which corresponds to that found on
hydrophobic, basic, and acidic amino acids (for a representative
example, see Figure 3). An additional advantage of this
(1) (a) For a recent review, see: Siligardi, G.; Drake, A. F. Peptide Sci.
1995, 37, 281. (b) For recent examples of proteins which bind PPII helices,
see: Feng, S.; Chen, J. K.; Yu, H.; Simon, J. A.; Schreiber, S. L. Science
1994, 266, 1241. (c) Yu, H.; Chen, J. K.; Feng, S.; Dalgarno, D. C.; Brauer,
A. W.; Schreiber, S. L. Cell 1994, 76, 933. (d) Gorina, S.; Pavletich, N. P.
Science 1996, 274, 1001. (e) Raj, P. A.; Marcus, E.; Edgerton, M.
Biochemistry 1996, 35, 4314. (f) Lee, C.-H.; Saksela, K.; Mirza, U. A.;
Chait, B. T.; Kuriyan, J. Cell 1996, 85, 931. (g) Feng, S.; Kasahara, C.;
Rickles, R. J.; Schreiber, S. L. Proc. Natl. Acad. Sci. U.S.A. 1995, 92, 12408.
(h) Jardetzky, T. S.; Brown, J. H.; Gorga, J. C.; Stern, L. J.; Urban, R. G.;
Strominger, J. L.; Wiley, D. C. Proc. Natl. Acad. Sci. U.S.A. 1996, 93,
734. (i) Zeile, W. L.; Purich, D. L.; Southwick, F. S. J. Cell Biol. 1996,
133, 49.
(3) Zhang, R. ; Madalengoitia, J. S. Tetrahedron Lett. 1996, 37, 6235.
(4) McDonald, Q. D.; Still, W. C. J. Org. Chem. 1996, 61, 1385.
(2) Adzhubei, A. A.; Sternberg, M. J. Mol. Biol. 1992, 229, 472.
S0002-7863(97)02494-3 CCC: $15.00 © 1998 American Chemical Society
Published on Web 04/11/1998

Design of Poly-L-proline Type II Peptide Mimics
J. Am. Chem. Soc., Vol. 120, No. 16, 1998 3895
Scheme 1a
Figure 2. PTAA and oligoPTAA as PPII mimic.
Figure 3. Representative amino acid and corresponding PTAA
analogues.
approach is that the introduction of the pyrrolidine ring creates
a new stereocenter which allows definition of the side chain
orientation. Therefore, oligopeptides composed of PTAAs
should possess not only well-defined φ, ψ, and ω angles but
also defined side chain angles (ø1 and ø2) useful in mapping
the side chain disposition required for occupancy of receptor
sites. While some substituted prolines (PTAAs) have been
synthesized and incorporated into polypeptides, the idea that
conformational control of the peptide backbone can be ac-
complished through coupling these modified prolines together
has thus far not been explored.5
The focus of these initial studies involves an evaluation of
the conformational properties of oligopeptides composed of
PTAAs. In particular, we sought to establish that the fidelity
of the PPII conformation is maintained in oligomers possessing
alternate side chain orientations and that this fidelity is also
maintained in various solvents, including water, in which these
agents would serve as bioactive agents. To this end, we have
synthesized and studied a series of oligomers (n ) 2, 3)
composed from 2,4-trans- and 2,4-cis-3-methylbutyl-L-proline
as well as 2,4-trans-carboxymethyl-L-proline. Although these
proline substitutions do not exactly correspond to any of the
side chains of the natural amino acids, they serve to evaluate
the effect of a proline substituent on the conformational
properties of these peptides. Furthermore, the PPII mimics
composed of 2,4-trans-carboxymethylproline are designed to
provide water-soluble peptides for evaluation in aqueous
medium. In addition, although conformational evaluation of a
dimer will provide evidence of the PPII forming potential of
the PTAAs, a trimer was chosen as a key unit to study since it
represents one turn of a PPII helix. Finally, the N-terminus
was protected as the pivalamide (or was deblocked) to avoid
any of the spectral complications which would arise from the
use of other protecting groups capable of existing as the cis/
trans N-terminal amide bond rotamers.3,6
a (a) LDA, THF, -78 °C; prenyl bromide; (b) (i) LAH; (ii) H2, Pd-
C; (iii) Boc2O; (c) Jones oxidation; (d) DIC, DMAP, EtOH; (e) HCl/
HOAc; (f) 5, Et3N, DIC, HOBt, DMAP, CH2Cl2; (g) (i) HCl/HOAc;
(ii) pivaloyl choride, Et3N; (h) (i) LiOH, dioxane/H2O; (ii) 7, Et3N,
DIC, HOBt, DMAP, CH2Cl2.
of 4-cyclohexyl-L-proline (Scheme 1).7 Functionalization of the
O,N-acetal 1 was accomplished through enolization (LDA) and
alkylation with 4-bromo-2-methyl-2-butene to give the diaster-
eomers 2 and 3 in 57% and 21% yields, respectively. Reduction
of the 2,4-trans-substituted acetal 2 with LAH gave the prolinol
in 88% yield. Hydrogenolysis of the benzyl group and reduction
of the double bond (H2, Pd-C) was achieved simultaneously
to afford the deprotected amine which was reprotected with
Boc2O to give the alcohol 4. Jones oxidation of the alcohol
then afforded the PTAA 5 in 63% yield. The acid was
converted to the ethyl ester 6 with DIC/DMAP in 94% yield.
Boc deprotection (1 M HCl/HOAc) followed by coupling with
the PTAA 5 furnished the Boc dimer 8. To avoid diketopip-
erazine formation which is especially troublesome with proline
dimers, the oligomers were synthesized in an N-terminal to
C-terminal sense. Thus, the N-terminus was Boc-deprotected
(1 M HCl/HOAc) and reprotected as the pivalamide with
trimethylacetyl chloride/Et3N to give the dimer 9, which
Results and Discussion
1
exhibited a single conformation by H NMR.8 Finally, hy-
PPII Mimic Synthesis. The synthesis of the hydrophobic
PTAAs was achieved by a modified procedure for the synthesis
drolysis of the ester (LiOH, dioxane/H2O) and coupling with
the PTAA salt 7 (DIC, HOBt, DMAP, Et3N) afforded the trimer
(5) (a) Mosberg, H. I.; Lomize, A. L.; Wang, C.; Kroona, H.; Heyl, D.
L.; Sobczyc-Kojiro, K.; Ma, W.; Mousigian, C.; Porreca, F. J. Med. Chem.
1994, 37, 4371-83. (b) Krapcho, J.; Turk, C.; Cushman, D. W.; Powell, J.
R.; DeForrest, J. M.; Spitzmiller, E. R.; Karanewsky, D. S.; Duggan, M.;
Rovnyak, G.; Schwartz, J.; Natarajan, S.; Godfrey, J. D.; Ryono, D. E.;
Neubeck, R.; Atwal, K. S.; Petrillo, E. W. J. Med. Chem. 1988, 31, 1148-
60.
(7) Thottahil, J. K.; Moniot, J. L.; Mueller, R. H.; Wong, M. K. Y.;
Kissick, T. P. J. Org. Chem. 1986, 51, 3140.
(8) Whereas reaction of Cl-H2+-PTAA-PTAA-OEt with PivCl/Et3N
afforded Piv-PTAA-PTAA-OEt, reaction of Cl-H2+-PTAA-PTAA-OEt with
Boc-PTAA-OH/DIC/HOBt/Et3N was slow relative to diketopiperazine
formation. Some PTAA-trimer was formed via reaction with Boc-PTAA-
F/Et3N; however, this transformation remains unoptimized.
(6) Liang, G.-B.; Rito, C. J.; Gellman, S. H. Biopolymers 1992, 32, 293.

3896 J. Am. Chem. Soc., Vol. 120, No. 16, 1998
Zhang et al.
Scheme 2a
of the aldehyde to the acid with PDC in DMF and methylation
of the resultant acid with MeI/NaHCO3 in DMF gave the diester
18 in 82% yield. Again, to avoid diketopiperazine formation,
the synthesis of these oligomers was performed in an N-terminal
to C-terminal sense. Therefore, 18 was Boc-deprotected and
reprotected as the pivalamide with PivCl/Et3N. Hydrogenolysis
of the benzyl ester allowed selective R-carboxy deprotection to
give the PTAA 19. The fully protected dimer was obtained by
Boc deprotection of 18 (HCl/HOAc) and DIC coupling with
19, which furnished the dimer-triester 20 in 84% yield. Finally,
hydrolysis of 20 with LiOH gave the dimer triacid 21 in 91%
yield. The tetracid 24 was obtained in a similar manner from
20 through the hydrogenolysis, coupling, and hydrolysis se-
quence used to make 21.
Conformational Studies. While circular dichroism is most
commonly used to assign the PPII secondary structure, this
method can be less reliable for short peptides. We instead
decided to use the nuclear Overhauser effect to investigate the
conformational properties of these PPII mimics. It was expected
that, in the PPII conformation, a positive NOE would be evident
between RH(i) and δH(i+1) since the minimization of pseudo-
A(1,3) strain would place RH(i) in close proximity to δH(i+1).
Furthermore, this NOE would be evident in the amide trans
conformation (ω ∼ 180°) as required for the PPII secondary
structure (Figure 4). Of note, all peptides utilized in this study
exhibited the predominance of a single conformation by 1H and
13C NMR spectroscopy. Thus, these oligomers do not exist as
a random mixture of cis/trans proline amide bond rotamers, but
exist in a single conformation (within our detection limits).
Figure 4 summarizes the results of our NOE experiments with
the dimeric and trimeric PPII mimics. The NOESY spectrum
of the 2,4-trans-substituted dimer 9 in CDCl3 exhibits the desired
NOEs between the R-hydrogen of the N-terminal PTAA and
the δ-hydrogens of the C-terminal PTAA, in agreement with
results expected for PTAA oligomers in the PPII conformation.
In addition, the trimer 10 also exhibited analogous NOEs
between RH(i) and δH(i+1) and between RH(i+1) and δH-
(i+2). Although these NOEs are not considered diagnostic for
the PPII conformation, a qualitative statement can be made
regarding the conformational properties of these peptides from
a comparison of the intensity of these NOEs with additional
inter-residue NOEs.
a (a) LDA, THF, -78 °C; prenyl bromide; (b) (i) Et3BHLi, THF,
-78 °C; (ii) BF3OEt2, Et3SiH; (c) O3, Ph3P; (d) (i) PDC, DMF; (ii)
MeI, NaHCO3, DMF; (e) (i) TFA; (ii) pivaloyl chloride, Et3N; (iii) H2,
Pd-C; (f) (i) first, 18, TFA; (ii) then, RCO2H, Et3N, DIC, HOBt,
DMAP, CH2Cl2; (g) LiOH; (h) H2, Pd-C.
For these molecules for which rotation about ψ is possible,
the NOE intensity between δH(i+1) and xH(i) (where xH(i)
denotes additional (i)-residue protons) will be a function of the
average distance between these protons as influenced by the
population of the ψ-rotamers. Since different ψ-rotamers will
exhibit δH(i+1)-xH(i) distances equal or greater than the δH-
(i+1)-RH(i) distance at ψ ∼ 145°, a comparison of NOE
intensities between δH(i+1) and xH(i) can provide a qualitative
measure of the most populated ψ-rotamer. This is best observed
by examination of the NOESY rows at the shift corresponding
to the δ-protons. Figure 5 shows the NOESY rows of the trimer
10 at the shifts corresponding to δtH(i), δtH(i+1), δtH(i+2),
δcH(i)/δcH(i+2), and δcH(i+1) (cis/trans (c/t) is defined
relative to RH). The row corresponding to the shift of δtH(i)
shows a strong geminal NOE with δcH(i) and a medium vicinal
NOE with γtH(i) and, not surprisingly, no measurable NOEs
with xH(i+1). The analogous intra-residue NOEs can also be
observed in the row corresponding to the shift of δtH(i+1).
However, δtH(i+1) row also exhibits three inter-residue
NOEs: a medium NOE with RH(i) and two weak NOEs with
âtH(i) and âcH(i). The row corresponding to the shift of δcH-
(i+1) also shows a medium NOE with RH(i) and no additional
10. The 2,4-cis-substituted analogues 11 and 12 were synthe-
sized from 3 in an analogous fashion with the exception that
the PTAA was protected as the benzyl ester and deprotection
of this ester was carried out through hydrogenolysis.
The synthesis of the water-soluble PPII mimics was ac-
complished through elaboration of another synthon derived from
pyroglutamic acid (Scheme 2).9 Enolization (LHMDS) and
alkylation of the protected pyroglutamate10 13 with 4-bromo-
2-methyl-2-butene afforded a 5:1 mixture of diastereomers 14
(63% yield) and 15 (Scheme 2). Lactam 14 was reduced to
the pyrrolidine by the two-step sequence involving superhydride
reduction of the lactam to the hemiaminal and subsequent
reduction of the hemiaminal with Et3SiH/BF3OEt2 in 82% yield
for two steps as described by Ruano.11 Ozonolysis of the alkene
16 afforded the aldehyde 17 in 90% yield. Subsequent oxidation
(9) Ezquerra, J.; Pedregal, C.; Yruretagoyena, B.; Rubio, A.; Carreno,
M. C.; Escribano, A.; Garcia Ruano, J. J. Org. Chem. 1995, 60, 2925.
(10) (a) Baldwin, J. E.; Miranda, T.; Moloney, M. Tetrahedron 1989,
45, 7459. (b) Ezquerra, J.; Pedregal, C.; Rubio, A.; Yruretagoyena, B.;
Escribano, A.; Sanchez-Ferrando, F. Tetrahedron 1993, 49, 8665.
(11) Pedregal, C.; Ezquerra, J.; Escribano, A.; Carreno, M. C.; Garcia
Ruano, J. L. Tetrahedron Lett. 1994, 35, 2053.

Design of Poly-L-proline Type II Peptide Mimics
J. Am. Chem. Soc., Vol. 120, No. 16, 1998 3897
Figure 5. NOESY rows of 10 at the δ corresponding to δtH(i), δtH-
(i+1), δtH(i+2), δcH(i)/δcH(i+2), and δcH(i+1). Cis and trans (c/t)
protons are defined relative to RH.
Figure 4. RH(i)-δH(i+1) and RH(i+1)-δH(i+2) NOEs for dimeric
and trimeric PPII mimics. NOESY spectra for 9-12 were obtained in
CDCl3 and for 21, 24, and 25 in D2O.
trimer 12 also exhibit the desired NOE intensity profile in
CDCl3.12 Thus, peptides composed of 2,4-cis-substituted
PTAAs also preferentially populate the PPII conformation in
solution. Since peptides can exhibit different conformational
behavior in different solvents, it is necessary to determine if
inter-residue NOEs of equal magnitude. From this data it is
apparent that the strongest δH(i+1)-xH(i) NOEs are between
δH(i+1) and RH(i), consistent with a significantly populated
conformation in which δH(i+1) and RH(i) are in close proximity
(i.e., when ψ ∼ 145°) as expected for the minimization of
pseudo-A(1,3) strain. Analysis of the rows corresponding to
the shifts of δtH(i+2) and δcH(i+2) also reveals that the
strongest inter-residue NOEs involve δH(i+2)-RH(i+1), again
consistent with a populated conformation in which ψ ∼ 145°.
The cumulative NOE data presented for the trimer 10 demon-
strates that this PPII mimic, composed of 2,4-trans-substituted
PTAAs, is conformationally constrained by the minimization
of pseudo-A(1,3) strain and preferentially populates the PPII
conformation. Analogous results are also observed for the dimer
9, e.g. the NOESY spectrum of 9 also exhibits the strongest
inter-residue NOEs between δH(i+1) and RH(i) (data not
shown). These data also illustrate that the minimization of
pseudo-A(1,3) strain can conformationally bias a unit as small
as a dimer.
(12) For the PPII mimics 11 and 12, the NOEs between the N-terminal
R-Hs and the cis and trans δ-Hs (cis and trans are assigned relative to the
proline carbonyl) are of unequal intensity with the NOE between the trans
δ-H and the R-H being more pronounced. Furthermore, the NOE between
the tert-butyl group and the N-terminal trans δ-H is also greater than
between the tert-butyl group and the N-terminal cis δ-H. Although
pyrrolidine ring conformations are difficult to assign due to the small energy
differences between conformers and the low energy of activation for five-
member ring conformational interconversion, these observed NOEs can be
tentatively assigned to a populated Cγ exo pyrrolidine ring conformation.
The Cγ exo pucker thus places the trans δ-H into closer spacial relationship
to the R-H of an N-terminally positioned PTAA (or to the tert-butyl group
in the N-terminal residue).
Analysis of the NOESY spectra of the additional PPII mimics
synthesized for this study reveals similar trends. For example,
the NOESY spectra of the 2,4-cis-substituted dimer 11 and

3898 J. Am. Chem. Soc., Vol. 120, No. 16, 1998
Zhang et al.
the potential application of this approach to the design of
bioactive agents is especially attractive since these agents
preferentially populate the PPII conformation in aqueous
medium. Further work is now ongoing to expand the number
of PTAAs which serve to form PPII mimics. Additional work
is ongoing to synthesize PPII mimics which target specific
signaling pathways in which the PPII secondary structure has
been implicated.
Figure 6. NOEs of cis and trans amide bond rotamers of dimer 26.
Experimental Section
the PTAA approach will provide agents which preferentially
populate the PPII conformation in aqueous medium. To this
end, 2D NOE studies of the PPII mimics 21 and 24 possessing
hydrophilic side chains were conducted in D2O. The NOESY
spectra of 21 and 24 in D2O also exhibited the diagnostic NOE
intensity profile, thus confirming the PPII conformation for these
peptides in water. Finally, to ascertain the resistance of PTAA
oligomers possessing hydrocarbon chains to hydrophobic col-
lapse in aqueous medium, the HCl salt 25 was also investigated
in D2O (the trimer was not studied due to difficulties with
solubility in D2O). This PPII dimer also showed the desired
NOEs indicative of the PPII conformation. Thus, the confor-
mational control elements which stabilize an all-proline helix
still function in aqueous medium with a PTAA dimer which
possess hydrophobic side chains. For all peptides used in this
study, correlation between the R-H and δ-H resonances of
individual residues was determined through COSY or TOCSY
spectroscopy. Sequence assignments were made by first
identifying δH(i) from the NOE with the pivaloyl tert-butyl
group and then performing the (i), (i+1), and (i+2) sequence
assignments through the RH(i)-δH(i+1) NOEs.
To establish the validity of the positive NOEs in assigning
the amide trans conformation, we sought to study a system
which possessed a cis amide bond rotamer.13 Previously
Naider13 and co-workers had suggested that the Pro-Pro amide
bond in the trimer Boc-Pro-Pro-Gly-OMe existed as a mixture
of cis-trans rotamers and that the cis conformation was
stabilized by hydrogen bonding of the Gly amide to the Boc
carbonyl oxygen. Analogously, it was found that the dimer 26
(Figure 6) exists as a ∼1:1 mixture of conformations in CDCl3.
Subsequent 2D NOE experiments indicated that one rotamer
exhibits an NOE between RH(i) and δH(i+1), while the other
rotamer (cis) does not, thus establishing that the RH(i) and δH-
(i+1) NOEs are valid in assigning the trans conformation.
Furthermore, the cis conformer exhibited an NOE between RH(i)
and RH(i+1) as expected. In addition, an NOE was also evident
between the N-terminal R-hydrogen and the hydrogen-bonded
NH (Figure 6), further confirming the conformational assign-
ment. It should be noted that stabilization of the cis conforma-
tion by intramolecular hydrogen bonding is more predominant
in CDCl3 than in more polar solvents. In a hydrogen-bonding
solvent such as CD3OD for example, the population of the cis
conformation diminishes to afford an 85:15 trans/cis mixture
of rotamers. It is thus expected that, in aqueous medium, a
natural amino acid incorporated on the C-terminal end of a PPII
mimic will not significantly contribute to this intramolecular
hydrogen bonding.
General Procedure. Unless otherwise specified, all reagents were
obtained from commercial sources and were used without further
purification. THF was distilled from sodium benzophenone ketyl, and
CH2Cl2 was distilled from CaH2. All reactions were carried out under
an N2 atmosphere unless otherwise specified. 1D and 2D spectra were
collected on a Bruker ARX 500-MHz NMR spectrometer. 2D NMR
spectra were collected using standard pulse sequences offered by Bruker.
All TOCSY spectra were obtained using a 60-ms mlevtp spin lock and
2-s repetition delay except the TOCSY spectrum of 11 which was
obtained with a 100-ms mlevtp spin lock. NOESY spectra were
obtained using a 2-s repetition delay and a 400-ms mixing time. All
2D spectra were obtained in CDCl3 except the 2D spectra of 21, 24,
and 25, which were collected in D2O.
Alkylation of O,N-Acetal 1. A solution of 1 (5.98 g, 29.4 mmol)
in THF (75 mL) was cooled to -78 °C, and LDA (14.7 mL, 29.4 mmol)
was added dropwise as a 2.0 M solution in heptane/THF/benzene. After
30 min, 2-methyl-4-bromobutene (3.7 mL, 32 mmol) was added
dropwise. The reaction was maintained at -78 °C for 15 min and
then quenched by the addition of water (15 mL). The resulting
suspension was allowed to warm to room temperature (rt) and diluted
with EtOAc (50 mL). The layers were separated, and the organic layer
was washed with water (20 mL) and brine (30 mL), dried (Na2SO4),
and concentrated. Flash chromatography (25% EtOAc/hexanes) af-
forded 4.56 g (57%) of 2 and 1.64 g (21%) of 3 as colorless oils. 2:
1H NMR (500 MHz, CDCl3) δ 7.43-7.41 (m, 2H), 7.37-7.27 (m,
3H), 6.30 (s, 1H), 5.14 (m, 1H), 4.17 (dd, app t, J ) 6.9 Hz, 2H), 4.01
(m, 1H), 3.37 (dd, app t, J ) 8.4 Hz, 1H), 2.70 (m, 1H), 2.46 (m, 1H),
2.34 (m, 1H), 2.03 (dd, app t, J ) 6.6 Hz, 1H), 1.69 (s, 3H), 1.62 (s,
3H); 13C NMR (125 MHz, CDCl3) 180.8, 139.6, 135.1, 128.9, 128.8,
126.4, 120.8, 87.7, 71.9, 57.9, 45.8, 31.0, 28.2, 26.4, 18.4 ppm; IR
(film) 1705, 1377, 1350, 1253, 1221, 1027 cm-1; MS (CI) m/z 272
(MH), 203, 165, 97, 69; [R]25 ) +196.2° (c ) 0.13, EtOAc). Anal.
D
Calcd for C17H21NO2: C, 75.25; H, 7.80; N, 5.16. Found: C, 75.07;
H, 7.77; N, 5.08. 3: 1H NMR (500 MHz, CDCl3) δ 7.51-7.29 (m,
5H), 6.33 (s, 1H), 5.10 (m, 1H), 4.23 (dd, app t, J ) 6.4 Hz, 1H), 4.06
(m, 1H), 3.48 (dd, app t, J ) 7.8 Hz, 1H), 2.92 (m, 1H), 2.58-2.48
(m, 2H), 2.17 (m, 1H), 1.71 (s, 3H), 1.64 (s, 3H), 1.58 (m, 1H); 13C
NMR (125 MHz, CDCl3) 178.9, 139.5, 134.7, 129.1, 129.0, 126.6,
121.5, 87.4, 73.0, 57.3, 46.1, 32.0, 29.6, 26.4, 18.6 ppm; IR (film) 2915,
1700, 1576, 1260; MS (CI) m/z 272 (MH), 203, 194; [R]25D ) +198.4°
(c ) 0.19, EtOAc). Anal. Calcd for C17H21NO2: C, 75.25; H, 7.80;
N, 5.16. Found: C, 75.06; H, 7.88; N, 5.10.
trans-4-(3-Methylbut-2-enyl)-N-benzyl-L-prolinol. A suspension
of LAH (0.69 g, 18 mmol) in THF (20 mL) was brought to a gentle
reflux, and 2 (3.28 g, 12.1 mmol) was added dropwise as a solution in
THF (10 mL). The suspension was maintained at reflux for 1 h and
then cooled to 0 °C. Saturated aqueous Na2SO4 was added slowly until
a white granular precipitate formed. The mixture was diluted with
EtOAc (30 mL), filtered through Celite, and concentrated to afford
3.26 g of a colorless oil. Flash chromatography (5% MeOH/20%
EtOAc hexanes) afforded 2.75 g (88%) of product as a colorless oil:
1H NMR (CDCl3, 500 MHz) δ 7.33-7.23 (m, 5H), 5.05 (m, 1H), 3.93
(d, J ) 13.0 Hz, 1H), 3.61 (dd, J ) 10.7, 3.4 Hz, 1H), 3.97-3.35 (m,
2H), 3.05 (dd, J ) 8.7, 6.2 Hz, 1H), 2.81 (m, 1H), 2.13-1.92 (m, 5H),
1.65 (s, 3H), 1.62-1.57 (m, 4H); 13C NMR (CDCl3, 125 MHz) 139.9,
132.7, 129.3, 128.9, 127.7, 123.4, 64.8, 62.9, 61.3, 59.4, 38.6, 35.3,
33.0, 26.3, 18.4 ppm; IR (film) 3400, 2924, 1653, 1454, 1375, 1139
The PTAA approach has thus been successfully used to
synthesize peptide mimics of the PPII secondary structure which
can incorporate nonprolyl side chain functionality. The feasibil-
ity of this approach is highlighted by the fact that a unit as
small as a dimer is conformationally constrained. Furthermore,
cm-1; MS (CI) m/z 260 (MH), 228, 91; [R]25 ) -90.0° (c ) 0.15,
D
(13) (a) Deslauriers, R.; Becker, J. M.; Steinfeld, A. S.; Naider, F.
Biopolymers 1979, 18, 523. (b) Cung, M. T.; Vitoux, B.; Marraud, M. New
J. Chem. 1987, 11, 503.
EtOAc). Anal. Calcd for C17H25NO: C, 78.72; H, 9.71; N, 5.40.
Found: C, 78.83; H, 9.82; N, 5.48.

Design of Poly-L-proline Type II Peptide Mimics
J. Am. Chem. Soc., Vol. 120, No. 16, 1998 3899
trans-4-(3-Methylbutyl)-N-(tert-butoxycarbonyl)-L-prolinol (4).
The N-benzylprolinol (2.28 g, 8.79 mmol) was dissolved in 2:1 acetic
acid/EtOAc (45 mL), and 10% Pd-C (0.47 g) was added. The mixture
was shaken under H2 (60 PSI) for 3 h and then filtered through Celite.
The filtrate was concentrated and made basic to pH 13 with aqueous
30% KOH. The aqueous layer was extracted with CH2Cl2 (2 × 100
mL), and the combined organic layers were dried (Na2SO4) and
concentrated to afford 1.40 g of a pale yellow oil which was used
without further purification: 1H NMR (CDCl3, 500 MHz) δ 3.51 (dd,
J ) 10.2, 3.8 Hz, 1H), 3.37-3.29 (m, 2H), 3.07 (dd, J ) 14.8, 7.4 Hz,
1H), 2.54 (dd, J ) 10.1, 7.9 Hz, 1H), 2.02 (m, 1H), 1.65 (m, 1H),
1.52-1.41 (m, 2H), 1.33-1.28 (m, 2H), 1.19-1.16 (m, 2H), 8.6 (d, J
) 6.6 Hz, 6H); 13C NMR (CDCl3, 125 MHz) 65.5, 59.6, 53.2, 39.9,
38.3, 34.8, 32.6, 28.7, 23.2, 23.1 ppm; IR (film) 3273, 2953, 1540,
1366, 1048 cm-1; MS (CI) m/z 172 (MH), 154, 140. The deprotected
amine (1.11 g, 6.48 mmol) was dissolved in CH2Cl2 (10 mL) and cooled
to 0 °C. Di-tert-butyl pyrocarbonate (1.40 g, 6.46 mmol) in CH2Cl2
(10 mL) was added dropwise, and the resulting solution was stirred at
0 °C for 20 min. The solution was concentrated under reduced pressure,
and the resulting oil was subjected to flash chromatography (5% MeOH/
20% EtOAc/hexanes) to afford 1.38 g of product as a colorless oil:
1H NMR (CDCl3, 500 MHz) δ 4.31 (sb, 1H), 4.04 (sb, 1H), 3.60 (m,
2H), 3.48 (m, 1H), 2.96 (t, J ) 8.5 Hz, 1H), 2.10 (m, 1H), 1.67 (m,
1H), 1.58-1.46 (m, 11H), 1.34-1.29 (m, 2H), 1.21-1.13 (m, 2H),
0.86 (d, J ) 6.6 Hz, 1H); 13C NMR (CDCl3, 125 MHz) 157.9, 80.9,
68.7, 60.3, 53.7, 38.1, 38.0, 35.3, 31.8, 29.2, 28.8, 23.3 ppm; IR (film)
3422, 1699, 1674, 1398, 1169 cm-1; MS (CI) m/z 272 (MH), 240, 216,
removed under reduced pressure (2 × 15 mL) to remove residual HOAc
and H2O. This oil was dissolved in CH2Cl2 (3 mL) and added to a
separate flask containing 5 (0.296 g, 1.04 mmol), DIC (0.33 mL, 2.1
mmol), 1-hydroxybenzotriazole (HOBt) (0.16 g, 1.0 mmol), DMAP
(0.128 g, 1.05 mmol), and Et3N (0.43 mL, 3.1 mmol) in CH2Cl2 (3
mL). The mixture was maintained at rt overnight and concentrated.
Purification by flash chromatography (30% EtOAc/hexanes afforded
0.437 g (86%) of product as a colorless oil: 1H NMR (CDCl3, 500
MHz) δ 4.61 (d, J ) 8.8 Hz, 0.6H), 4.58 (d, J ) 8.7 Hz, 0.4H), 4.49
(d, J ) 8.7 Hz, 0.6H), 4.39 (d, J ) 8.0 Hz, 0.4H), 4.19-4.11 (m, 2H),
3.82-3.70 (m, 2H), 3.29 (dd, app t, J ) 9.4 Hz, 0.6H), 3.15 (dd, app
t, J ) 8.9 Hz, 0.4H), 2.97 (dd, app t, J ) 9.3 Hz, 0.4H), 2.92 (dd, app
t, J ) 9.6 Hz, 0.6H), 2.44-2.30 (m, 2H), 2.13-2.05 (m, 2H), 1.82-
1.74 (m, 2H), 1.55-1.13 (m, 22H), 0.92-0.85 (m, 12H); 13C NMR
(CDCl3, 125 MHz) 172.0, 171.8, 171.0, 170.7, 154.3, 153.5, 79.1, 79.0,
60.8, 60.7, 58.6, 58.5, 57.6, 52.3, 52.0, 51.9, 51.7, 38.24, 38.19, 37.2,
37.1, 37.0, 36.2, 35.7, 35.1, 34.6, 34.5, 31.1, 31.0, 30.7, 30.5, 30.3,
28.3, 28.2, 27.90, 27.88, 27.8, 22.30, 22.28, 22.2, 13.9 ppm; IR (film)
2955, 1743, 1700, 1668, 1397, 1184 cm-1; MS (CI) m/z 481 (MH),
381, 240, 184, 140; [R]25 ) -34.0° (c ) 0.1, EtOAc).
D
Synthesis of trans-4-(3-Methylbutyl)-Piv-dimer 9. A solution of
8 (154 mg, 0.330 mmol) in 1 M HCl/HOAc (2 mL) was maintained at
rt for 1 h. The solvent was removed under reduced pressure. Toluene
was added and removed under reduced pressure (2 × 15 mL) to
azeotropically remove residual HOAc and H2O. The resultant oil was
dissolved in CH2Cl2 (1 mL). To this solution was added pivaloyl
chloride (0.81 mL, 0.66 mmol) followed by Et3N (0.14 mL, 0.99 mmol).
After 15 min, the mixture was diluted with 20% EtOAc/hexanes (20
mL) and washed with H2O (2 mL), dried (Na2SO4), and concentrated.
Flash chromatography (20% hexanes/EtOAc) afforded 133 mg (87%)
of product as a thick colorless oil: 1H NMR (CDCl3, 500 MHz) δ
4.62 (dd, J ) 8.9, 2.7 Hz, 1H), 4.57 (d, J ) 8.9 Hz, 1H), 4.07 (m, 2H),
3.91 (dd, J ) 9.4, 7.3 Hz, 1H), 3.68 (dd, app t, J ) 8.7 Hz, 1H), 3.27
(dd, app t, J ) 9.5 Hz, 1H), 3.21 (dd, app t, J ) 9.3 Hz, 1H), 2.42 (m,
1H), 2.25 (m, 1H), 2.01-1.95 (m, 2H), 1.72 (ddd, J ) 12.3, 9.3, 9.3
Hz, 1H), 1.64 (ddd, J ) 12.2, 9.1, 9.1 Hz, 1H), 1.48-1.05 (m, 22H),
0.81-0.79 (m, 12H); 13C NMR (CDCl3, 125 MHz) 177.2, 172.9, 171.6,
61.4, 60.2, 59.2, 54.6, 52.5, 39.4, 39.2, 38.9, 37.9, 37.8, 35.4, 33.5,
31.6, 31.3, 27.7, 27.9, 13.0, 13.7 ppm; IR (film) 2870, 1744, 1662,
184; [R]25 ) -23.7° (c ) 0.13, EtOAc). Anal. Calcd for C15H29-
D
NO3: C, 66.38; H, 10.77; N, 5.16. Found: C, 66.13; H, 10.71; N,
5.07.
trans-4-(3-Methylbutyl)-N-(tert-butoxycarbonyl)-L-proline (5). The
alcohol 4 (1.15 g, 4.24 mmol) in acetone (3 mL) was added dropwise
over 3 h to a solution of Jones reagent (3.1 mL, prepared by dissolving
27 g of CrO3, in 23 mL of H2SO4, and diluting to 100 mL with H2O)
in acetone (3 mL) at 0 °C. After the addition was complete, the mixture
was maintained at 0 °C for 1 h. Isopropyl alcohol (0.5 mL) was added,
and after 30 min, the solvent was decanted and concentrated to afford
a green oil. Purification by flash chromatography (10% MeOH/20%
EtOAc in hexanes) afforded 0.766 g of product as a colorless oil
(63%): 1H NMR (CDCl3, 500 MHz) δ 4.35 (d, J ) 8.5 Hz, 0.5H),
4.24 (d, J ) 8.2 Hz, 0.5H), 3.71 (dd, app t, J ) 9.1 Hz, 0.5H), 3.57
(dd, app t, J ) 8.4 Hz, 0.5H), 2.95 (dd, app t, J ) 9.3 Hz, 0.5H), 2.87
(dd, app t, J ) 9.8 Hz, 0.5H), 2.30-2.13 (m, 2H), 1.84 (m, 0.5H),
1.68 (m, 0.5H), 1.53-1.10 (m, 15H), 0.84 (d, J ) 6.3 Hz, 6H); 13C
NMR (CDCl3, 125 MHz) 179.3, 176.6, 156.5, 154.5, 81.6, 80.9, 59.8,
59.6, 53.0, 52.4, 38.4, 37.9, 37.4, 37.3, 35.6, 31.4, 29.0, 28.9, 28.7,
23.0 ppm; IR (film) 3212, 2956, 1701, 1653 cm-1; MS (CI) m/z 286
1623 cm-1; MS (CI) m/z 465 (MH), 379, 268, 252, 224, 140; [R]25
)
D
-41.9° (c ) 0.09, CHCl3). Anal. Calcd for C27H48N2O4: C, 69.79;
H, 10.41; N, 6.03. Found: C, 69.70; H, 10.50; N, 6.00.
Ester Hydrolysis of 9. A solution of the ester 9 (0.123 g, 0.272
mmol) and LiOH (42 mg, 1.0 mmol) in 2:1 dioxane/H2O (3 mL) was
maintained at rt for 6 h. The solution was cooled to 0 °C, made acidic
to pH 2 with aqueous 10% HCl, and extracted with EtOAc (3 × 5
mL). The combined organic extracts were dried and concentrated to
afford 0.141 g of a pale yellow oil. Purification by flash chromatog-
raphy (5% MeOH/CH2Cl2) afforded 78 mg (67%) of product as a
colorless oil: 1H NMR (CDCl3, 500 MHz) δ 4.68 (d, J ) 8.8 Hz, 1H),
4.65 (dd, J ) 9.0, 3.0 Hz, 1H), 3.98 (dd, J ) 9.5, 7.2 Hz, 1H), 3.65
(dd, app t, J ) 8.3 Hz, 1H), 3.35-3.28 (m, 2H), 2.51 (m, 1H), 2.41-
2.34 (m, 2H), 1.96 (m, 1H), 1.74-1.66 (m, 2H), 1.56-1.14 (m, 19H),
0.88-0.85 (m, 12H); 13C NMR (CDCl3, 125 MHz) 177.7, 174.5, 173.7,
60.5, 60.2, 54.7, 53.3, 39.6, 39.4, 39.2, 38.0, 37.9, 34.0, 33.7, 31.5,
31.0, 28.8, 28.7, 27.9, 27.8, 23.2, 23.1 ppm; IR (film) 3136, 2968, 1734,
1653, 1456 cm-1; MS (CI) m/z 437 (MH), 252, 224; [R]25D ) -72.3°
(c ) 0.4, CH2Cl2).
Synthesis of trans-4-(3-Methylbutyl)-Piv-trimer 10. Boc depro-
tection of 6 (73.1 mg, 234 mmol) was accomplished as described for
the synthesis of 8. The hydrochloride salt was dissolved in CH2Cl2 (2
mL) and added to a separate reaction flask containing DIC (0.40 mL,
0.26 mmol), Et3N (0.65 mL, 0.47 mL), and the dimer-acid (0.102 g,
0.233 mmol) in CH2Cl2 (2 mL) at 0 °C. The mixture was allowed to
warm to rt and then maintained at rt overnight. The mixture was
concentrated and purified by flash chromatography (25% EtOAc/
hexanes) to afford 0.102 g (74%) of 10 as a colorless oil: 1H NMR
(CDCl3, 500 MHz) δ 4.74 (d, J ) 8.7 Hz, 1H), 4.64 (d, J ) 8.8 Hz,
1H), 4.54 (d, J ) 8.9 Hz, 1H), 4.15-4.05 (m, 2H), 3.92 (dd, apparent
t, J ) 7.5 Hz, 1H), 3.75 (dd, app t, J ) 8.3 Hz, 1H), 3.67 (dd, app t,
J ) 8.2 Hz, 1H), 3.29 (dd, app t, J ) 9.0 Hz, 1H), 3.24-3.21 (m, 2H),
(MH), 230, 214, 186, 184, 140; [R]25 ) -25.0° (c ) 0.1, EtOAc).
D
Anal. Calcd for C21H40NO4 (cyclohexylamine salt; mp 166-171 °C):
C, 65.56; H, 10.48; N, 7.28. Found: C, 65.36; H, 10.51; N, 7.28.
trans-4-(3-Methylbutyl)-N-(tert-butoxycarbonyl)-L-proline Ethyl
Ester (6). The acid 5 (196 mg, 0.687 mmol) and DMAP (42 mg, 0.34
mmol) were dissolved in anhydrous ethanol (2 mL), and diisopropyl-
carbodiimide (DIC) (0.13 mL, 0.82 mmol) was added dropwise. The
mixture was maintained at rt overnight and was then concentrated and
purified by flash chromatography (15% EtOAc/hexanes) to afford 185
mg (94%) of product as a colorless oil: 1H NMR (CDCl3, 500 MHz)
δ 4.31 (d, J ) 8.5 Hz, 0.4H), 4.21 (dd, J ) 9.0, 1.8 Hz, 0.6H), 4.18-
4.09 (m, 2H), 3.71 (dd, J ) 10.2, 7.8 Hz, 0.6H), 3.64 (dd, J ) 9.7, 8.2
Hz, 0.4H), 2.95 (dd, app t, J ) 9.1 Hz, 0.6H), 2.89 (dd, app t, J ) 9.5
Hz, 0.4H), 2.19 (m, 1H), 2.04 (m, 1H), 1.76 (m, 1H), 1.56-1.21 (m,
17H), 0.85-0.83 (m, 6H); 13C NMR (CDCl3, 125 MHz) 173.9, 173.6,
155.0, 154.5, 80.4, 61.51, 61.46, 59.9, 59.6, 52.8, 52.5, 38.3, 38.0, 37.43,
37.40, 36.6, 31.6, 31.5, 29.1, 29.0, 28.8, 28.7, 23.2, 23.1, 15.0, 14.9;
IR (film) 1749, 1705, 1395, 1188 cm-1; MS (CI) m/z 314 (MH), 258,
240, 214, 184, 140; [R]25D ) -23.7° (c ) 0.12, CHCl3). Anal. Calcd
for C17H31NO4: C, 65.14; H, 9.97; N, 4.47. Found: C, 65.22; H, 10.05;
N, 4.47.
Synthesis of trans-4-(3-Methylbutyl)-Boc-dimer 8. A solution of
6 (0.327 g, 1.05 mmol) in 1 M HCl/HOAc (4 mL) was maintained at
rt for 1 h. The solution was concentrated. Toluene was added and

3900 J. Am. Chem. Soc., Vol. 120, No. 16, 1998
Zhang et al.
2.48 (m, 1H), 2.40 (m, 1H), 2.26 (m, 1H), 2.05-1.99 (m, 3H), 1.77-
1.63 (m, 3H), 1.49-1.10 (m, 27H), 0.83-0.81 (m, 18H); 13C NMR
(CDCl3, 125 MHz) 177.2, 172.8, 171.4, 171.1, 61.5, 60.4, 59.4, 58.4,
54.7, 53.1, 52.6, 39.4, 39.2, 38.9, 38.5, 38.0, 37.9, 37.8, 35.5, 34.8,
33.6, 31.7, 31.3, 28.74, 28.69, 28.0, 23.1, 23.0, 14.7 ppm; IR (film)
2955, 1744, 1653, 1625, 1185 cm-1; MS (CI) m/z 632 (MH), 252, 143,
0.84 g (64%) of product as a colorless solid: mp 69-72 °C; 1H NMR
(CDCl3, 500 MHz) δ 4.31-4.16 (m, 1H), 3.77-3.71 (m, 1H), 2.98-
2.92 (m, 1H), 2.45-2.33 (m, 1H), 2.07-1.95 (m, 1H), 1.60-1.40 (m,
12H), 1.25-1.12 (m, 3H), 0.88 (d, J ) 6.5 Hz, 6H); 13C NMR (CDCl3,
125 MHz) 179.5, 177.4, 156.1, 154.4, 81.4, 81.0, 60.0, 59.8, 53.3, 52.7,
39.4, 39.0, 38.0, 37.9, 36.2, 31.1, 30.3, 29.0, 28.9, 28.6, 23.0 ppm; IR
(film) 2927, 1734, 1702, 1635, 1435 cm-1; MS (CI) m/z 286 (MH)
230, 214, 186, 140; [R]25D ) -56.7° (c ) 0.07, EtOAc). Anal. Calcd
for C15H27NO4: C, 63.13; H, 9.53; N, 4.91. Found: C, 63.35; H, 9.26;
N, 4.90.
101, 87; [R]25 ) -41.3° (c ) 0.16, CHCl3). Anal. Calcd for
D
C37H65N3O5: C, 70.32; H, 10.37; N, 6.65. Found: C, 70.17; H, 10.38;
N, 6.63.
Synthesis of trans-4-(3-Methylbutyl)-Piv-dimer 26. A solution of
2,4-trans-PTAA-PTAA-OH (51.9 mg, 0.122 mmol) and 1-(3-(dimeth-
ylamino)propyl)-3-ethylcarbodiimide hydrochloride (28.5 mg, 0.145
mmol) in CH2Cl2 (1 mL) was maintained at 0 °C for 20 min, and
methylamine (46 µL, 0.37 mmol) was added as an 8 M solution in
EtOH and allowed to warm to rt. After 6 h, the suspension was diluted
with EtOAc (10 mL) and washed with 10% aqueous HCl (2 × 1 mL)
and saturated aqueous NaHCO3. The organic layer was dried (Na2-
SO4) and concentrated. Purification by flash chromatography (5%
MeOH/CH2Cl2) afforded 18 mg (33%) of product as a colorless oil:
1H NMR (CDCl3, 500 MHz) δ 8.21 (qb, J ) 4.5 Hz, 0.5H), 7.02 (sb,
0.5H), 4.67 (dd, J ) 8.7, 3.3 Hz, 0.5H), 4.63 (d, J ) 8.2 Hz, 0.5H),
4.35 (dd, J ) 8.9, 2.7 Hz, 0.5H), 4.27 (d, J ) 8.1 Hz, 0.5H), 4.02 (dd,
J ) 9.6, 7.1 Hz, 0.5H), 3.98 (dd, J ) 9.8, 7.1 Hz, 0.5H), 3.68 (dd, J
) 11.7, 7.8 Hz, 0.5H), 3.61 (dd, app t, J ) 8.4 Hz, 0.5H), 3.36-3.28
(m, 1H), 3.23 (dd, app t, J ) 9.0 Hz, 0.5H), 3.08 (dd, app t, J ) 11.0
Hz, 0.5H), 2.82 (d, J ) 4.7 Hz, 1.5H), 2.74 (d, J ) 4.8 Hz, 1.5H),
2.65 (dd, J ) 12.2, 5.7 Hz, 0.5H), 2.52-2.42 (m, 2H), 2.10 (m, 0.5H),
1.89-1.81 (m, 1H), 1.76-1.12 (m, 21H), 0.92-0.85 (m, 12H); 13C
NMR (CDCl3, 125 MHz) 177.6, 177.4, 173.7, 172.8, 172.0, 171.8, 62.1,
60.9, 60.5, 60.3, 54.9, 54.7, 53.5, 53.0, 39.8, 39.7, 39.3, 38.3, 38.1,
38.0, 37.9, 36.3, 34.0, 33.7, 33.6, 31.5, 31.41, 31.38, 28.78, 28.76, 28.73,
28.70, 28.0, 27.9, 27.2, 26.7, 23.21, 23.19, 23.17, 23.10 ppm; IR (CH2-
Cl2) 3455, 3324, 1654, 1609 cm-1; MS (CI) m/z 450 (MH), 252, 224;
cis-4-(3-Methylbutyl)-N-(tert-butoxycarbonyl)-L-proline Benzyl
Ester. A solution of cis-4-(3-methylbutyl)-N-(tert-butoxycarbonyl)-
L-proline (0.447 g, 1.56 mmol), diisopropylcarbodiimide (0.49 mL, 3.1
mmol), benzyl alcohol (0.24 mL, 2.4 mmol), and DMAP (95 mg, 0.78
mmol) in CH2Cl2 (3 mL) was stirred at rt overnight. The suspension
was filtered and concentrated. Flash chromatography (10% EtOAc/
hexanes) afforded 0.512 g (87%) of product as a colorless oil: 1H NMR
(CDCl3, 500 MHz) δ 7.36-7.30 (m, 5H), 5.27 (d, J ) 12.5 Hz, 0.4H),
5.15 (s, 1.2H), 5.07 (d, J ) 12.5 Hz, 0.4H), 4.31 (dd, app t, J ) 8.9
Hz, 0.4H), 4.23 (dd, app t, J ) 8.1 Hz, 0.6H), 3.78 (dd, J ) 10.3, 7.5
Hz, 0.6H), 3.66 (dd, J ) 10.0, 7.6 Hz, 0.4H), 3.01-2.95 (m, 1H), 2.43-
2.38 (m, 1H), 2.09-2.04 (m, 1H), 1.55-1.18 (m, 13H), 1.15-1.12
(m, 2H), 0.85 (d, J ) 6.6 Hz, 6H); 13C NMR (CDCl3, 125 MHz) 173.8,
173.5, 155.1, 154.3, 136.7, 136.3, 129.3, 129.1, 129.0, 128.7, 80.6,
80.5, 67.2, 60.2, 59.9, 53.2, 52.8, 39.6, 38.9, 38.0, 37.9, 36.9, 31.3,
31.2, 29.1, 28.9, 28.7, 28.6, 23.1 ppm; IR (film) 2955, 1750, 1704,
1401, 1160 cm-1; MS (CI) m/z 376 (MH), 348, 320, 276, 240, 184,
140, 119, 91, 57; [R]25D ) -68.3° (c ) 0.1, CHCl3). Anal. Calcd for
C22H33NO4: C, 70.37; H, 8.86; N, 3.73. Found: C, 70.46; H, 8.93; N,
3.76.
Synthesis of cis-4-(3-Methylbutyl)-Boc-dimer. Synthesized as
described for 8 (82% yield): 1H NMR (CDCl3, 500 MHz) δ 7.33-
7.27 (m, 5H), 5.23 (d, J ) 12.3 Hz, 0.5H), 5.22 (d, J ) 12.3 Hz, 0.5H),
5.04 (d, J ) 12.3 Hz, 0.5H), 5.02 (d, J ) 12.3 Hz, 0.5H), 4.54 (dd,
app t, J ) 8.2 Hz, 0.5H), 4.47-4.43 (m, 1H), 4.36 (dd, J ) 9.2, 7.6
Hz, 0.5H), 4.02 (dd, app t, J ) 7.9 Hz, 0.5H), 3.89 (dd, app t, J ) 7.9
Hz, 0.5H), 3.75 (dd, J ) 10.1, 7.4 Hz, 0.5H), 3.66 (dd, J ) 10.2, 7.4
Hz, 0.5H), 3.08-2.96 (m, 2H), 2.42-1.98 (m, 5H), 1.55-1.08 (m,
20H), 0.87-0.84 (m, 12H); 13C NMR (CDCl3, 125 MHz) 172.2, 171.8,
171.4, 171.0, 154.3, 153.4, 135.8, 135.7, 128.42, 128.39, 128.2, 128.1,
128.07, 128.04, 79.4, 79.3, 66.7, 66.6, 59.4, 59.2, 58.0, 57.7, 52.7, 52.5,
52.2, 39.7, 39.4, 38.5, 37.4, 37.25, 37.21, 36.5, 35.4, 35.2, 30.5, 30.3,
30.2, 28.5, 28.3, 28.1, 27.9, 22.5, 22.42, 22.40, 22.37 ppm; IR (film)
2955, 1749, 1700, 1664, 1404, 1169 cm-1; MS (CI) m/z 543 (MH),
[R]25 ) -65.6° (c ) 0.16, CHCl3).
D
cis-4-(3-Methylbutyl)-N-(tert-butoxycarbonyl)-L-prolinol. A sus-
pension of LAH (0.85 g, 22 mmol) in THF (30 mL) was brought to a
gentle reflux, and 3 (4.05 g, 14.9 mmol) was added dropwise as a
solution in THF (10 mL). The suspension was maintained at reflux
for 1 h and then cooled to 0 °C. Saturated aqueous Na2SO4 was added
slowly until a white granular precipitate formed. The mixture was
diluted with EtOAc (40 mL), filtered through Celite, and concentrated
to afford 3.86 g of a colorless oil. This oil was dissolved in 2:1 acetic
acid/EtOAc (60 mL), and 10% Pd-C (0.79 g) was added. The mixture
was shaken under H2 (60 PSI) for 3 h and then filtered through Celite.
The filtrate was concentrated and made basic to pH 13 with 30% KOH.
The aqueous layer was extracted with CH2Cl2 (2 × 100 mL), and the
combined organic layers were dried (Na2SO4) and concentrated to afford
2.49 g of a pale yellow oil which was used without further purification.
This oil was dissolved in CH2Cl2 (20 mL) and cooled to 0 °C. Di-
tert-butyl pyrocarbonate (3.30 g, 15.1 mmol) in CH2Cl2 (20 mL) was
added dropwise, and the resulting solution was stirred at 0 °C for 20
min. The solution was concentrated under reduced pressure, and the
resulting oil was subjected to flash chromatography (5% MeOH/20%
EtOAc/hexanes to afford 3.13 g (77% from 3) of product as a colorless
oil: 1H NMR (CDCl3, 500 MHz) δ 5.25 (d, J ) 8.4 Hz, 1H), 3.90 (m,
1H), 3.65 (m, 2H), 3.55 (m, 1H), 2.77 (dd, apparent t, J ) 10.3 Hz,
1H), 2.13 (m, 1H), 1.96 (m, 1H), 1.54-1.40 (m, 11H), 1.33-1.30 (m,
2H), 1.20-1.27 (m, 2H), 0.86 (d, J ) 6.6 Hz, 6H); 13C NMR (CDCl3,
125 MHz) 157.7, 81.0, 68.6, 62.0, 54.0, 38.4, 38.1, 36.3, 31.4, 29.2,
28.8, 23.2; IR (film) 3412, 2956, 1699, 1669, 1418 cm-1; MS (CI) m/z
471, 443, 240, 184, 140, 91, 57; [R]25 ) -89.9° (c ) 0.15, CHCl3).
D
Anal. Calcd for C32H50N2O5: C, 70.81; H, 9.23; N, 5.16. Found: C,
70.65; H, 9.20; N, 5.15.
Synthesis of cis-4-(3-Methylbutyl)-Piv-dimer 11. The synthesis
of 11 was accomplished as described for 9 (87%) from the
2,4-cis-Boc-PTAA-PTAA-OBn-dimer: mp 85-86 °C; 1H NMR (CDCl3,
500 MHz) δ 7.33-7.29 (m, 5H), 5.25 (d, J ) 12.4 Hz, 1H), 5.02 (d,
J ) 12.4 Hz, 1H), 4.64 (dd, app t, J ) 9.0 Hz, 1H), 4.53 (dd, J ) 9.1,
7.8 Hz, 1H), 4.14 (dd, app t, J ) 8.3 Hz, 1H), 3.99 (dd, J ) 9.5, 6.9
Hz, 1H), 3.22 (dd, app t, J ) 10.5 Hz, 1H), 3.09 (dd, app t, J ) 9.5
Hz, 1H), 2.40 (m, 1H), 2.35-2.23 (m, 2H), 2.06 (m, 1H), 1.57-1.08
(m, 21H), 0.88 (d, J ) 6.6 Hz, 6H), 0.85 (d, J ) 5.9 Hz, 6H); 13C
NMR (CDCl3, 125 MHz) 177.0, 172.9, 172.1, 136.6, 129.1, 128.8,
128.7, 67.3, 60.5, 59.9, 55.1, 53.4, 41.4, 40.1, 39.1, 38.1, 38.0, 36.1,
34.2, 31.3, 30.1, 28.9, 28.7, 28.0, 23.19, 23.16, 23.0 ppm; IR (film)
2954, 1748, 1662, 1623, 1410, 1171 cm-1; MS (CI) m/z 527 (MH),
252, 224; [R]25 ) -80.0° (c ) 0.1, CHCl3). Anal. Calcd for
272 (MH) 240, 216, 200, 184, 172, 140, 58; [R]25 ) -32.4° (c )
D
D
C32H50N2O4: C, 72.96; H, 9.57; N, 5.32. Found: C, 72.93; H, 9.54;
N, 5.25.
0.14, CHCl3). Anal. Calcd for C15H29NO3: C, 66.38; H, 10.77; N,
5.16. Found: C, 66.13; H, 10.66; N, 5.07.
Synthesis of cis-4-(3-Methylbutyl)-Piv-trimer 12. A suspension
of Pd-C (29 mg) and 11 (0.130 g, 0.253 mmol) in EtOAc (2 mL) was
stirred under a hydrogen balloon for 6 h. The suspension was filtered
through Celite and concentrated to afford 107 mg of the acid as a
colorless oil which was homogenous by TLC. The trimer 12 was
obtained as described for 10 from the 2,4-cis-Piv-PTAA-PTAA-OH-
dimer. Purification by flash chromatography (30% EtOAc/hexanes)
afforded 153.6 mg (89% from 11) of product as a colorless glassy
solid: 1H NMR (CDCl3, 500 MHz) δ 7.33-7.29 (m, 5H), 5.24 (d, J )
cis-4-(3-Methylbutyl)-N-(tert-butoxycarbonyl)-L-proline. Oxida-
tion of the 2,4-cis-4-(3-methylbutyl)-N-(tert-butoxycarbonyl)-L-prolinol
(1.26 g, 4.64 mmol) was accomplished as described for the synthesis
of 5. NMR of the crude reaction mixture revealed the presence of
∼10% cis-4-(3-methylbut-2-enyl)-N-(tert-butoxycarbonyl)-L-proline as
a byproduct which was inseparable by chromatography. The crude
reaction mixture was hydrogenated (0.49 g of 10% Pd-C, H2, 1 atm)
and purified by flash chromatography (5% MeOH/CH2Cl2) to afford

Design of Poly-L-proline Type II Peptide Mimics
J. Am. Chem. Soc., Vol. 120, No. 16, 1998 3901
12.4 Hz, 1H), 5.02 (d, J ) 12.4 Hz, 1H), 4.69 (dd, app t, J ) 8.7 Hz,
1H), 4.64 (dd, app t, J ) 8.9 Hz, 1H), 4.53 (dd, app t, J ) 8.5 Hz,
1H), 4.11 (dd, app t, J ) 8.0 Hz, 1H), 4.07 (dd, app t, J ) 8.3 Hz,
1H), 3.98 (dd, J ) 10.6, 6.9 Hz, 1H), 3.20 (dd, app t, J ) 10.6 Hz,
1H,) 3.12 (dd, app t, J ) 9.8 Hz, 1H), 3.05 (dd, app t, J ) 9.6 Hz,
1H), 2.41-2.22 (m, 5H), 2.04 (m, 1H), 1.54-1.07 (m, 27H), 0.87-
0.84 (m, 18H); 13C NMR (CDCl3, 125 MHz) 176.9, 172.8, 171.5, 171.3,
136.5, 129.1, 128.8, 67.2, 60.7, 59.7, 58.7, 55.1, 53.5, 53.2, 41.3, 40.3,
40.0, 39.1, 38.04, 38.00, 37.9, 35.9, 35.2, 34.2, 31.2, 31.0, 30.6, 28.8,
28.7, 28.6, 28.0, 23.1, 23.07, 23.0 ppm; IR (film) 2955, 1749, 1652,
trans-1-(tert-Butoxycarbonyl)-4-(2-oxoethyl)-L-proline Benzyl Es-
ter (17). The alkene 16 (2.06 g, 5.52 mmol) in CH2Cl2 (20 mL) was
cooled to -78 °C, and ozone was bubbled through the solution until a
blue color persisted. Nitrogen was then bubbled through the solution
for 15 min to drive off excess ozone. Triphenylphosphine (2.17 g,
8.28 mmol) was added, the resulting mixture was warmed to room
temperature over 2 h, and the solvent was removed under reduced
pressure. Flash chromatography on silica gel (2:1 hexanes-ethyl
acetate) gave 17 (1.73 g, 90%) as pale yellow oil: [R]25D ) -36.3° (c
1
) 0.4, CHCl3); H NMR (CDCl3, 500 MHz) δ 9.71 (s, 1H), 7.35 (m,
1623, 1456, 1170 cm-1; MS (CI) m/z 694 (MH), 101, 87, 69; [R]25
)
5H), 5.15 (m, 2H), 4.42 (dd, J ) 2.9, 8.8 Hz, 0.4H), 4.31 (dd, J ) 2.9,
8.9 Hz, 0.6H), 3.83 (dd, J ) 7.8, 10.4 Hz, 0.6H), 3.79 (dd, J ) 7.9,
10.1 Hz, 0.4H), 3.04 (dd, J ) 8.3, 10.2 Hz, 0.6H), 2.98 (dd, J ) 8.3,
10.2 Hz, 0.4H), 2.75 (m, 1H), 2.52 (m, 2H), 2.20 (m, 1H), 1.88 (m,
1H), 1.45 (s, 3.6H), 1.33 (s, 5.4H); 13C NMR (CDCl3, 125 MHz) 199.8,
199.7, 172.3, 172.1, 153.9, 153.3, 135.5, 135.4, 128.4, 128.3, 128.2,
128.1, 127.9, 127.7, 79.8, 66.5, 58.6, 58.4, 51.3, 51.0, 46.7, 46.6, 36.0,
35.1, 31.2, 30.4, 28.2, 28.0 ppm; IR (film) 2975, 1746, 1724, 1698,
1455, 1398, 1367, 1258, 1181, 1159, 1129, 740, 699 cm-1; MS (CI)
m/z 348 (MH), 308, 292, 276, 264, 248, 228, 212, 202, 184, 172, 156,
129, 119, 112, 91, 68, 59, 58, 57. Anal. Calcd for C19H25O5N: C,
65.69; H, 7.25; N, 4.03. Found: C, 65.44; H, 7.26; N, 4.09.
D
-75.4° (c ) 0.5, CH2Cl2). Anal. Calcd for C42H67N3O5: C, 72.69;
H, 9.73; N, 6.05. Found: C, 72.46; H, 9.80; N, 6.02.
Benzyl (2S,4R)-1-(tert-Butoxycarbonyl)-4-(2-methylbut-2-enyl)-
pyroglutamate (14). The pyroglutamic acid derivative 13 (6.36 g,
19.9 mmol) in THF (40 mL) was added dropwise to a 1.0 M solution
of LHMDS in THF (19.9 mL, 19.9 mmol) at -78 °C. After 30 min,
a cold solution (0 °C) of 4-bromo-2-methyl-2-butene (2.55 mL, 19.9
mmol) in THF (10 mL) was added dropwise, and the resulting solution
was maintained at -78 °C for 2 h. Saturated aqueous NaHCO3 (50
mL) was then added, and the resulting suspension was allowed to warm
to room temperature. The layers were separated, and the aqueous layer
was extracted with 1:1 hexanes-ethyl acetate (3 × 50 mL). The
combined organic fractions were dried (Na2SO4) and concentrated. 1H
NMR of the crude product indicated the ratio of 5:1 of trans/cis product.
Flash chromatography of the residue on silica gel (3:1 hexanes-ethyl
trans-1-(tert-Butoxycarbonyl)-4-((methoxycarbonyl)methyl)-L-
proline Benzyl Ester (18). PDC (3.62 g, 9.62 mmol) was added to a
solution of 17 (1.67 g, 4.81 mmol) in DMF (15 mL), the resulting
suspension was stirred at rt for 9 h. H2O (150 mL) was added, and
the aqueous layer was extracted with Et2O (6 × 120 mL). The
combined organic fractions were dried (Na2SO4) and concentrated. The
crude acid was used without further purification. NaHCO3 (0.81 g,
9.62 mmol) and iodomethane (2.99 mL, 48.1 mmol) were added to
the crude acid in DMF (15 mL), and the resulting suspension was stirred
at room temperature for 20 h. Et2O (100 mL) and H2O (100 mL) were
then added, and the organic layer was washed with H2O (3 × 100 mL),
dried (Na2SO4), and concentrated. Flash chromatography of the residue
acetate) afforded 14 (4.86 g, 63%) as colorless oil: [R]25 ) -35.3°
D
(c ) 0.75, CHCl3); 1H NMR (CDCl3, 500 MHz) δ 7.35 (m, 5H), 5.21
(d, J ) 12.2 Hz, 1H), 5.18 (d, J ) 12.2 Hz, 1H), 5.04 (m, 1H), 4.58
(dd, J ) 1.5, 9.7 Hz, 1H), 2.65 (m, 1H), 2.51 (m, 1H), 2.17 (m, 2H),
1.98 (m, 1H), 1.69 (s, 3H), 1.60 (s, 3H), 1.47 (s, 9H); 13C NMR (CDCl3,
125 MHz) 174.5, 171.2, 149.4, 135.1, 134.7, 128.6, 128.5, 128.4, 119.7,
83.4, 67.2, 57.1, 41.9, 28.4, 27.8, 27.7, 25.7, 17.8 ppm; IR (film) 2978,
2931, 1793, 1752, 1718, 1457, 1393, 1368, 1317, 1251, 1186, 1155,
965, 853, 776, 751, 699, 668 cm-1; MS (CI) m/z 332, 316, 288, 219,
196, 152, 119, 91, 57. Anal. Calcd for C22H29O5N: C, 68.20; H, 7.54;
N, 3.62. Found: C, 68.32; H, 7.58; N, 3.62.
on silica gel (4:1 hexanes-ethyl acetate) gave 18 (1.48 g, 82%) as
1
yellowish oil: [R]25 ) -30.8° (c ) 0.5, CHCl3); H NMR (CDCl3,
D
500 MHz) δ 7.35 (m, 5H), 5.15 (m, 2H), 4.43 (dd, J ) 2.6, 8.8 Hz,
0.4H), 4.31 (dd, J ) 2.6, 8.8 Hz, 0.6H), 3.81 (dd, J ) 8.0, 10.4 Hz,
0.6H), 3.77 (dd, J ) 8.0, 10.4 Hz, 0.4H), 3.66 (s, 3H), 3.10 (dd, J )
8.1, 10.4 Hz, 0.6H), 3.03 (dd, J ) 8.1, 10.4 Hz, 0.4H), 2.71 (m, 1H),
2.38 (m, 2H), 2.18 (m, 1H), 1.93 (m, 1H), 1.45 (s, 3.6H), 1.33 (s, 5.4H);
13C NMR (CDCl3, 125 MHz) 172.5, 172.2, 172.0, 171.9, 154.1, 153.5,
135.7, 135.5, 128.5, 128.4, 128.3, 128.2, 128.0, 127.9, 80.0, 79.9, 66.6,
58.8, 58.6, 51.6, 51.5, 51.2, 37.1, 36.1, 35.3, 33.7, 32.9, 28.3, 28.0
ppm; IR (film) 2976, 2878, 1741, 1701, 1479, 1456, 1438, 1400, 1367,
1258, 1173, 1127, 1004, 891, 773, 699 cm-1; MS (CI) m/z 378 (MH),
322, 306, 278, 242, 214, 186, 142, 119, 91, 68, 59, 58, 57, 52. Anal.
Calcd for C20H27O6N: C, 63.65; H, 7.21; N, 3.71. Found: C, 63.41;
H, 7.16; N, 3.73.
trans-1-(tert-Butoxycarbonyl)-4-(2-methyl-2-butenyl)-L-proline Ben-
zyl Ester (16). A 1.0 M solution of lithium triethylborohydride in
THF (13.0 mL, 13.0 mmol) was added to a solution of 14 (4.16 g,
10.7 mmol) in THF (10 mL) at -78 °C. After 30 min, the reaction
mixture was quenched with saturated aqueous NaHCO3 (40 mL) and
warmed to 0 °C. Then 30% H2O2 (0.5 mL) was added, and the resulting
mixture was stirred at 0 °C for 20 min. The organic solvent was
removed under reduced pressure, and the aqueous layer was extracted
with CH2Cl2 (3 × 40 mL). The combined organic fractions were dried
(Na2SO4) and concentrated. The crude product was used without further
purification. A solution of the crude product and triethylsilane (3.47
mL, 21.5 mmol) in CH2Cl2 (40 mL) was cooled to -78 °C, and boron
trifluoride etherate (2.92 mL, 23.6 mmol) was then added dropwise.
After 2 h, the reaction mixture was quenched with saturated aqueous
NaHCO3 (40 mL) and warmed to room temperature, the layers were
separated, and the aqueous layer was then extracted with CH2Cl2 (3 ×
40 mL). The combined organic fractions were dried (Na2SO4) and
concentrated. Flash chromatography of the residue on silica gel (5:1
trans-1-(2,2-Dimethylpropanoyl)-4-((methoxycarbonyl)methyl)-
L-proline Benzyl Ester. 18 (600 mg, 1.59 mmol) was treated with
TFA (0.5 mL), and the resulting mixture was stirred at room temperature
for 30 min. TFA was evaporated, and the residue was dried in Vacuo
overnight and then dissolved in CH2Cl2 (3.0 mL). Triethylamine (0.55
mL, 4.0 mmol) and trimethylacetyl chloride (0.22 mL, 1.8 mmol) were
successively added, and the resulting mixture was stirred at room
temperature for 1 h. After dilution with CH2Cl2 (10 mL), the reaction
mixture was washed with 0.25 N HCl (10 mL), 5% NaHCO3 (10 mL),
and saturated aqueous NaCl (10 mL). The organic layer was dried
(Na2SO4) and concentrated. Flash chromatography of the residue on
silica gel (3:1 hexanes-ethyl acetate) afforded benzyl ester 19 (546
mg, 95%) as a yellowish oil: [R]25D ) -51.8° (c ) 0.25, CHCl3); 1H
NMR (CDCl3, 500 MHz) δ 7.34 (m, 5H), 5.23 (d, J ) 12.4 Hz, 1H),
5.07 (d, J ) 12.4 Hz, 1H), 4.60 (br d, J ) 5.8, 1H), 4.02 (dd, J ) 7.8,
8.7 Hz, 1H), 3.68 (s, 3H), 3.41 (dd, J ) 7.9, 9.5 Hz, 1H), 2.79 (m,
1H), 2.40 (dd, J ) 6.5, 15.9 Hz, 1H), 2.36 (dd, J ) 8.1, 15.9 Hz, 1H),
2.04 (m, 1H), 1.85 (td, J ) 9.0, 13.0 Hz, 1H), 1.24 (s, 9H); 13C NMR
(CDCl3, 125 MHz) 176.8, 172.2, 171.9, 135.7, 128.4, 128.1, 128.0,
66.6, 60.3, 53.0, 51.7, 38.6, 36.7, 35.2, 33.1, 27.1 ppm; IR (film) 2957,
1740, 1627, 1478, 1457, 1437, 1406, 1361, 1172, 1082, 1003, 756,
699, 668 cm-1; MS (CI) m/z 362 (MH), 254, 226, 142, 119, 91, 87,
hexanes-ethyl acetate) yielded 16 (3.28 g, 82%) as a colorless oil:
1
[R]25 ) -33.0° (c ) 0.75, CHCl3); H NMR (CDCl3, 500 MHz) δ
D
7.34 (m, 5H), 5.15 (m, 2H), 5.10 (m, 0.4H), 5.07 (m, 0.6H), 4.32 (dd,
J ) 2.1, 8.8 Hz, 0.4 H), 4.30 (J ) 2.6, 8.8 Hz, 0.6H), 3.69 (dd, J )
7.6, 10.3 Hz, 0.6H), 3.62 (dd, J ) 7.7, 10.1 Hz, 0.4 H), 3.05 (dd, J )
8.3, 10.2 Hz, 0.6H), 2.99 (dd, J ) 8.5, 10.2 Hz, 0.4H), 2.28 (m, 1H),
2.03 (m, 3H), 1.90 (m, 1H), 1.69 (s, 1.2H), 1.68 (s, 1.8H), 1.58 (m,
3H), 1.46 (s, 3.6H), 1.34 (s, 5.4H); 13C NMR (CDCl3, 125 MHz) 172.9,
172.7, 154.3, 153.7, 135.8, 135.6, 133.2, 128.5, 128.4, 128.3, 128.2,
128.0, 127.9, 121.6, 121.5, 79.7, 79.6, 66.5, 59.1, 58.8, 51.5, 51.2, 37.8,
36.9, 36.2, 35.3, 31.1, 31.0, 28.4, 28.1, 25.6, 17.7 ppm; IR (film) 2974,
2930, 1749, 1702, 1478, 1456, 1398, 1366, 1258, 1178, 1124, 970,
892, 772, 751, 698, 668 cm-1; MS (CI) m/z 374 (MH), 318, 302, 274,
238, 210, 182, 138, 121, 119, 91, 68, 59, 58, 57. Anal. Calcd for
C22H31O4N: C, 70.75; H, 8.37; N, 3.75. Found: C, 70.68; H, 8.43; N,
3.71.

3902 J. Am. Chem. Soc., Vol. 120, No. 16, 1998
Zhang et al.
85, 68, 59, 58, 57, 52. Anal. Calcd for C20H27O5N: C, 66.46; H,
7.53; N, 3.88. Found: C, 66.46; H, 7.63; N, 3.82.
Dimer 22. 20 (250.0 mg, 0.47 mmol) in MeOH (2 mL) was treated
with 10% Pd-C (25 mg), and the resulting mixture was stirred under
hydrogen balloon overnight, then filtered through Celite, and washed
with MeOH (15 mL). The filtrate was concentrated and dried in Vacuo,
trans-1-(2,2-Dimethylpropanoyl)-4-((methoxycarbonyl)methyl)-
L-proline (19). Benzyl ester 19 (450 mg, 1.25 mmol) in MeOH (2
mL) was treated with 10% Pd-C (45 mg), and the resulting mixture
was stirred under hydrogen balloon overnight and then filtered through
Celite. The filtrate was concentrated, and crystallization of the residue
giving 22 as a white solid (206.5 mg, 100%): mp 57-59 °C; [R]25
)
D
1
-69.8° (c ) 0.3, CHCl3); H NMR (CDCl3, 500 MHz) δ 4.68 (br d,
J ) 8.7 Hz, 1H), 4.65 (dd, J ) 4.0, 8.7 Hz, 1H), 4.07 (dd, J ) 6.8,
10.0 Hz, 1H), 3.86 (t, J ) 8.9 Hz, 1H), 3.70 (s, 3H), 3.68 (s, 3H), 3.47
(m, 2H), 2.97 (m, 1H), 2.79 (m, 1H), 2.57 (dd, J ) 5.4, 16.3 Hz, 1H),
2.40 (m, 3H), 2.06 (m, 1H), 1.82 (m, 2H), 1.25 (s, 9H); 13C NMR
(CDCl3, 125 MHz) 177.2, 173.6, 172.4, 172.1, 172.0, 59.3, 58.9, 53.3,
52.2, 51.8, 38.6, 37.0, 36.7, 35.3, 34.6, 33.0, 32.5, 27.2 ppm; IR (film)
3158 (br), 2957, 2873, 1736, 1661, 1622, 1587, 1436, 1368, 1203, 1172,
in 1:2 hexanes-ethyl acetate gave 19 (308 mg, 91%) as colorless
1
needles: mp 150-151 °C; [R]25 ) -112.8° (c ) 0.5, CHCl3); H
D
NMR (CDCl3, 500 MHz) δ 10.36 (br s, 1H), 4.60 (br d, J ) 7.3 Hz,
1H), 4.03 (dd, J ) 8.1, 8.8 Hz, 1H), 3.70 (s, 3H), 3.41 (dd, J ) 8.8,
9.4 Hz, 1H), 2.82 (m, 1H), 2.49 (dd, J ) 5.8, 15.9 Hz, 1H), 2.37 (dd,
J ) 8.6, 15.9 Hz, 1H), 2.29 (m, 1H), 1.80 (dt, J ) 9.2, 12.7 Hz, 1H),
1.27 (s, 9H); 13C NMR (CDCl3, 125 MHz) 178.8, 174.8, 172.0, 60.9,
53.3, 51.8, 39.0, 36.7, 35.2, 32.5, 27.1 ppm; IR (film) 3100 (br), 2967,
1736, 1734, 1718, 1653, 1635, 1559, 1540, 1507, 1457, 1419, 1172,
668 cm-1; MS (CI) m/z 272 (MH), 254, 227, 226, 216, 202, 188, 185,
142, 85, 68, 59, 58, 57. Anal. Calcd for C13H21O5N: C, 57.55; H,
7.80; N, 5.16. Found: C, 57.55; H, 7.79; N, 5.13.
1082, 999, 858, 734 cm-1
.
trans-4-((Methoxycarbonyl)methyl)-trimer 23. The synthesis of
23 was accomplished from 18 (151 mg, 0.40 mmol) and 22 (175.8
mg, 0.40 mmol) as described for the synthesis of 20. Flash chroma-
tography of the residue on silica gel (ethyl acetate) gave 23 (240 mg,
86%) as white solid: mp 134-135 °C; [R]25 ) -54.7° (c ) 0.3,
D
trans-4-((Methoxycarbonyl)methyl)-dimer 20. The PTAA 18 (340
mg, 0.90 mmol) was treated with TFA (0.5 mL), and the resulting
mixture was stirred at room temperature for 30 min and concentrated
under reduced pressure. The residue was dried in Vacuo overnight. In
a separate flask, 1,3-diisopropylcarbodiimide (0.29 mL, 1.80 mmol)
was added to a solution of 19 (245 mg, 0.90 mmol), DMAP (110 mg,
0.90 mmol), and HOBt (138 mg, 0.90 mmol) in CH2Cl2 (2 mL) at 0
°C, and the resulting mixture was stirred at 0 °C for 0.5 h. A solution
of the Boc-deprotected PTAA and triethylamine (0.16 mL, 1.10 mmol)
in CH2Cl2 (1 mL) was added, and the reaction mixture was warmed to
rt and stirred for 15 h. CH2Cl2 (8 mL) was added, and the reaction
mixture was washed with aqueous 0.25 N HCl (10 mL), aqueous 5%
NaHCO3 (10 mL), and saturated aqueous NaCl (10 mL). The organic
layer was dried (Na2SO4) and concentrated. Flash chromatography of
the residue on silica gel (1:1.5 hexanes-ethyl acetate) gave 20 (401
mg, 84%) as colorless oil: [R]25D ) -64.5° (c ) 0.3, CHCl3); 1H NMR
(CDCl3, 500 MHz) δ 7.33 (m, 5H), 5.22 (d, J ) 12.3 Hz, 1H), 5.04 (d,
J ) 12.3 Hz, 1H), 4.72 (dd, J ) 1.8, 9.2 Hz, 1H), 4.66 (dd, J ) 3.7,
8.8 Hz, 1H), 4.06 (dd, J ) 6.9, 10.0 Hz, 1H), 3.89 (dd, J ) 8.0, 9.3
Hz, 1H), 3.69 (s, 3H), 3.67 (s, 3H), 3.45 (m, 2H), 2.88 (m, 1H), 2.77
(m, 1H), 2.46 (dd, J ) 6.4, 16.3 Hz, 1H), 2.29 (dd, J ) 9.0, 15.6 Hz,
1H), 2.40 (dd, J ) 8.2, 16.3 Hz, 1H), 2.39 (dd, J ) 5.6, 15.6 Hz, 1H),
2.16 (ddd, J ) 1.8, 6.5, 13.0 Hz, 1H), 1.98 (ddd, J ) 3.9, 6.4, 12.5
Hz, 1H), 1.90 (ddd, J ) 9.3, 11.2, 13.0 Hz, 1H), 1.76 (td, J ) 8.6,
12.6 Hz, 1H), 1.26 (s, 9H); 13C NMR (CDCl3, 125 MHz) 176.8, 172.1,
172.0, 171.7, 170.9, 135.6, 128.5, 128.2, 128.1, 66.9, 58.9, 58.3, 53.3,
51.7, 51.6, 51.5, 38.6, 37.2, 36.9, 35.1, 34.3, 32.4, 27.2 ppm; IR (film)
2955, 1736, 1662, 1622, 1436, 1408, 1361, 1172, 1001, 756, 700, 668
cm-1; MS (CI) m/z 531 (MH), 254, 226, 91, 85, 58. Anal. Calcd for
C28H38O8N2: C, 63.38; H, 7.22; N, 5.28. Found: C, 63.11; H, 7.29;
N, 5.18.
1
CHCl3); H NMR (CDCl3, 500 MHz) δ 7.32 (m, 5H), 5.22 (d, J )
12.3 Hz, 1H), 5.04 (d, J ) 12.3 Hz, 1H), 4.75 (brd, J ) 8.6 Hz, 1H),
4.66 (brd, J ) 8.5 Hz, 2H), 4.04 (dd, J ) 6.9, 9.9 Hz, 1H), 3.93 (t, J
) 8.6 Hz, 1H), 3.83 (t, J ) 8.5 Hz, 1H), 3.68 (s, 6H), 3.67 (s, 3H),
3.45 (t, J ) 7.7 Hz, 1H), 3.44 (t, J ) 9.3 Hz, 1H), 3.36 (t, J ) 9.4 Hz,
1H), 2.87 (m, 2H), 2.76 (m, 1H), 2.42 (m, 4H), 2.30 (m, 2H), 2.17
(ddd, J ) 1.4, 6.5, 12.9 Hz, 1H), 2.10 (ddd, J ) 3.8, 6.4, 12.8 Hz,
1H), 2.02 (ddd, J ) 1.6, 6.8, 12.6 Hz, 1H), 1.84 (m, 3H), 1.25 (s, 9H);
13C NMR (CDCl3, 125 MHz) 176.7, 172.1, 172.0, 171.9, 171.5, 170.6,
170.2, 135.5, 128.5, 128.2, 128.1, 66.9, 59.0, 58.4, 57.2, 53.4, 51.9,
51.7, 51.6, 51.4, 38.6, 37.3, 37.2, 36.9, 35.1, 34.3, 34.2, 33.9, 33.5,
32.4, 27.2 ppm; IR (film) 2955, 2876, 1737, 1651, 1621, 1435, 1172,
1089, 1004, 748, 705 cm-1; MS (CI) m/z 700 (MH), 475, 447, 423,
254, 226, 143, 115, 107, 101, 92, 91, 87, 85, 69, 60, 59, 52. Anal.
Calcd for C36H49O11N3: C, 61.79; H, 7.06; N, 6.00. Found: C, 61.86;
H, 7.12; N, 5.93.
trans-4-(Carboxymethyl)-trimer 24. As described for the synthesis
of 21. 24 was obtained (66 mg, 90%) from 23 (88.0 mg, 0.13 mmol)
as a white solid, which was crystallized from MeOH-EtOAc, yielding
colorless cubic crystals (56 mg, 76%): mp 159-161 °C; [R]25
)
D
1
-71.5° (c ) 0.2, EtOH); H NMR (D2O, 500 MHz) δ 4.64 (m, 2H),
4.40 (dd, J ) 1.3, 8.5 Hz, 1H), 3.88 (dd, J ) 6.6, 10.5 Hz, 1H), 3.80
(t, J ) 8.7 Hz, 2H), 3.52 (dd, J ) 6.0, 10.5 Hz, 1H), 3.38 (t, J ) 7.8
Hz, 1H), 3.37 (t, J ) 8.8 Hz, 1H), 2.68 (m, 3H), 2.46 (d, J ) 7.0 Hz,
2H), 2.45 (d, J ) 7.0 Hz, 2H), 2.38 (m, 2H), 2.14 (m, 1H), 2.08 (m,
1H), 1.92 (m, 4H), 1.12 (s, 9H); 13C NMR (CD3SOCD3, 125 MHz)
174.7, 173.4, 173.2, 173.1, 173.0, 170.0, 169.7, 58.7, 58.1, 56.9, 51.5,
51.0, 37.9, 37.0, 36.9, 36.4, 34.7, 34.0, 33.8, 33.1, 31.9, 27.1 ppm; IR
(film) 3452 (br), 3089 (br), 2972, 2883, 2612 (br), 1729, 1617, 1431,
1365, 1172, 876. Anal. Calcd for C26H37O11N3: C, 55.02; H, 6.57;
N, 7.40. Found: C, 55.02; H, 6.61; N, 7.37.
trans-4-(Carboxymethyl)-dimer 21. 20 (53.1 mg, 0.10 mmol) was
treated with a 1 M solution of LiOH in 3:1 H2O-MeOH (1.0 mL),
and the resulting mixture was stirred at rt for 3 h. H2O (2 mL) was
then added, and the organic solvent was evaporated in Vacuo. The
resulting solution was applied to a column of Dowex-50W HCR-W2
Acknowledgment. Support for this work has been provided
by the American Cancer Society Vermont Division and NSF
Vermont EPSCoR OSR9350540 and CA22435 from the Na-
tional Cancer Institute. Its contents are solely the responsibility
of the authors and do not necessarily represent the official views
of the National Cancer Institute.
resin (H2O) and eluted with H2O affording 21 (37 mg, 91%) as white
1
solid: mp 118-121 °C; [R]25 ) -43.0 (c ) 0.1, EtOH); H NMR
D
(D2O, 500 MHz) δ 4.59 (m, 1H), 4.39 (dd, J ) 3.0, 9.1 Hz, 1H), 3.90
(dd, J ) 6.6, 10.7 Hz, 1H), 3.82 (dd, J ) 7.4, 10.1 Hz, 1H), 3.52 (dd,
J ) 6.2, 10.8 Hz, 1H), 3.34 (dd, J ) 8.7, 10.0 Hz, 1H), 2.68 (m, 2H),
2.45 (d, J ) 7.3 Hz, 2H), 2.38 (d, J ) 7.4 Hz, 2H), 2.14 (ddd, J ) 3.0,
6.3, 13.1 Hz, 1H), 1.95 (m, 3H), 1.11 (s, 9H); 13C NMR (CD3OD, 125
MHz) δ 178.9, 175.7, 175.6, 175.3, 173.0, 60.8, 60.1, 54.8, 52.9, 39.7,
37.9, 37.7, 36.7, 35.7, 35.4, 33.2, 27.7 ppm; IR (film) 3431 (br), 2967,
1731, 1623, 1587, 1425, 1366, 1232, 1175, 869 cm-1; MS (CI) m/z
413 (MH), 311, 258, 240, 213, 212, 188, 171, 156, 143, 128, 103, 87,
85, 73, 69, 59, 58, 57.
Supporting Information Available: 1H and 13C NMR
spectra for 2-6, 8-12, 14, 16-24, and 26 and TOCSY and
NOESY spectra for 9-12, 21, 24, and 26 (60 pages, print/PDF).
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JA972494E