Use of Steric Interactions to Control Peptide Turn Geometry. Synthesis of Type VI β-Turn Mimics with 5-tert-Butylproline

Article abstract of DOI:10.1021/jo990294a

The influences of steric interactions on peptide geometry were studied to develop a novel means for generating type VIa β-turn mimics. (2S,5R)-5-tert-Butylproline and L-proline were respectively introduced at the C-terminal residue of N-(acetyl)dipeptide N′-methylamides 1 and 2. The relative populations of prolyl cis- and trans-amide isomers in dipeptides 1 and 2 were measured in chloroform, DMSO, and water by proton NMR spectroscopy. Although the trans-amide isomer was favored in prolyl peptide 2, the Xaa-Pro peptide bond adopted preferably the cis-amide isomer in the case of 5-tert-butylprolyl peptide 1. Measurements of the influence of solvent and temperature on the chemical shift values for the amide proton signals of 1 in the cis-amide conformer indicated that the N′-methylamide was engaged in a hydrogen bond with the acetamide carbonyl in a type VIa β-turn conformation. Analysis of N-(acetyl)leucyl-5-tert-butylproline N′-methylamide (1d) in the solid state by X-ray diffraction showed the cis-amide conformer which adopted a geometry characteristic of the central, i + 1 and i + 2 residues of an ideal type VIa β-turn. In contrast to prolyl peptides 2b and 2d, N-(acetyl)alanyl- and N-(acetyl)leucyl-5-tert-butylproline N′-methylamides (1b and 1d) maintained ordered β-turn conformations in solution that were shown to be independent of solvent composition by a comparison of their circular dichroism spectra obtained in water and acetonitrile. The NMR, X-ray, and CD data all confirm that the steric interactions of the 5-tert-butylprolyl residue induced dipeptide 1 to adopt a type VIa β-turn conformation.

Full text of DOI:10.1021/jo990294a

3312
J . Org. Chem. 1999, 64, 3312-3321
Use of Ster ic In ter a ction s To Con tr ol P ep tid e Tu r n Geom etr y.
Syn th esis of Typ e VI â-Tu r n Mim ics w ith 5-ter t-Bu tylp r olin e
Liliane Halab and William D. Lubell*
De´partement de chimie, Universite´ de Montre´al, C. P. 6128, Succursale Centre Ville,
Montre´al, Que´bec, Canada H3C 3J 7
Received February 17, 1999
The influences of steric interactions on peptide geometry were studied to develop a novel means
for generating type VIa â-turn mimics. (2S,5R)-5-tert-Butylproline and L-proline were respectively
introduced at the C-terminal residue of N-(acetyl)dipeptide N′-methylamides 1 and 2. The relative
populations of prolyl cis- and trans-amide isomers in dipeptides 1 and 2 were measured in
chloroform, DMSO, and water by proton NMR spectroscopy. Although the trans-amide isomer was
favored in prolyl peptide 2, the Xaa-Pro peptide bond adopted preferably the cis-amide isomer in
the case of 5-tert-butylprolyl peptide 1. Measurements of the influence of solvent and temperature
on the chemical shift values for the amide proton signals of 1 in the cis-amide conformer indicated
that the N′-methylamide was engaged in a hydrogen bond with the acetamide carbonyl in a type
VIa â-turn conformation. Analysis of N-(acetyl)leucyl-5-tert-butylproline N′-methylamide (1d ) in
the solid state by X-ray diffraction showed the cis-amide conformer which adopted a geometry
characteristic of the central, i + 1 and i + 2 residues of an ideal type VIa â-turn. In contrast to
prolyl peptides 2b and 2d , N-(acetyl)alanyl- and N-(acetyl)leucyl-5-tert-butylproline N′-methylamides
(1b and 1d ) maintained ordered â-turn conformations in solution that were shown to be independent
of solvent composition by a comparison of their circular dichroism spectra obtained in water and
acetonitrile. The NMR, X-ray, and CD data all confirm that the steric interactions of the 5-tert-
butylprolyl residue induced dipeptide 1 to adopt a type VIa â-turn conformation.
In tr od u ction
The spatial requirements for peptide biology may be
elucidated by employing constrained analogues of native
secondary structures to probe relationships between
conformation and activity. Because of the importance of
â-turns in protein folding and recognition,1 considerable
effort has focused on developing conformationally re-
stricted mimics of the backbone geometry, intramolecular
hydrogen bonding, and side-chain orientations exhibited
by these secondary structures.2 Among such designs, the
use of modified prolines has often led to successful
surrogates because of the high frequency of this amino
acid at the central residues of the â-turn conformation.2,3
We report now the use of the steric interactions of 5-tert-
butylproline to generate conformationally rigid mimics
of the type VIa â-turn.
F igu r e 1. Central, i + 1 and i + 2 residues of type VIa and
VIb turn conformations found respectively in ribonuclease S5b
and Bence-J ones protein.5c Only amide protons shown (C,
black; N, dark gray; O, light gray; H, white).
Two classes of type VI â-turns have been defined based
on the backbone dihedral angle values of their central, i
+ 1 and i + 2 residues (Figure 1).1,4 In the type VIa
â-turn, the proline ψ-dihedral angle is near 0° and an
intramolecular hydrogen bond exists between the carbo-
nyl oxygen of the i and amide hydrogen of the i + 3
The type VI â-turn is a unique secondary structure that
features an amide cis-isomer N-terminal to a prolyl
residue situated at the i + 2 position of the peptide bend.4
(1) Reviews and leading articles on â-turns include the following:
(a) Wilmot, C. M.; Thornton, J . M. J . Mol. Biol. 1988, 203, 221. (b)
Rose, G. D.; Gierasch, L. M.; Smith, J . A. Adv. Protein Chem. 1985,
37, 1. (c) Richardson, J . S. Adv. Protein Chem. 1981, 34, 167. (d) Lewis,
P. N.; Momany, F. A.; Scheraga, H. A. Biochim. Biophys. Acta 1973,
303, 211. (e) Venkatachalam, C. M. Biopolymers 1968, 6, 1425.
(2) Reviewed in: (a) Hanessian, S.; McNaughton-Smith, G.; Lom-
bart, H.-G.; Lubell, W. D. Tetrahedron 1997, 53, 12789. (b) Gillespie,
P.; Cicariello, J .; Olson, G. L. Biopolym., Pept. Sci. 1997, 43, 191.
(3) Examples include: (a) Takeuchi, Y.; Marshall, G. R. J . Am.
Chem. Soc. 1998, 120, 5363. (b) Baures, P. W.; Ojala, W. H.; Gleason,
W. B.; J ohnson, R. L. J . Pept. Res. 1997, 50, 1. (c) Lombart, H.-G.;
Lubell, W. D. J . Org. Chem. 1996, 61, 9437. (d) Chalmers, D. K.;
Marshall, G. R. J . Am. Chem. Soc. 1995, 117, 5927. (e) Genin, M. J .;
Mishra, R. K.; J ohnson, R. L. J . Med. Chem. 1993, 36, 3481. (f) Hinds,
M. G.; Welsh, J . H.; Brennand, D. M.; Fisher, J .; Glennie, M. J .;
Richards, N. G. J .; Turner, D. L.; Robinson, J . A. J . Med. Chem. 1991,
34, 1777.
(4) Mu¨ller, G.; Gurrath, M.; Kurz, M.; Kessler, H. Proteins: Struct.,
Funct. Genet. 1993, 15, 235.
(5) Reviewed in ref 4. (a) Werner, M. H.; Wemmer, D. E. Biochem-
istry 1992, 31, 999. (b) Kim, E. E.; Varadarajan, R.; Wyckoff, H. W.;
Richards, F. M. Biochemistry 1992, 31, 12304. (c) Epp, O.; Lattman,
E. E.; Schiffer, M.; Huber, R.; Palm, W. Biochemistry 1975, 14, 4943.
(6) Recent examples include the following: (a) segetalin A, Morita,
H.; Yun, Y. S.; Takeya, K.; Itokawa, H.; Shiro, M. Tetrahedron 1995,
51, 5987. (b) cycloleonurinin, Morita, H.; Gonda, A.; Takeya, K.;
Itokawa, H.; Hirano, T.; Oka, K.; Shirota, O. Tetrahedron 1997, 53,
7469. (c) aureobasidin E, Fujikawa, A.; In, Y.; Inoue, M.; Ishida, T.;
Nemoto, N.; Kobayashi, Y.; Kataoka, R.; Ikai, K.; Takesako, K.; Kato,
I. J . Org. Chem. 1994, 59, 570. (d) evolidine, Eggleston, D. S.; Baures,
P. W.; Peishoff, C. E.; Kopple, K. D. J . Am. Chem. Soc. 1991, 113, 4410.
(e) phakellistatin, Pettit, G. R.; Xu, J .-P.; Cichacz, Z. A.; Williams, M.
D.; Dorsaz, A.-C.; Brune, D. C.; Boyd, M. R.; Cerny, R. L. Bioorg. Med.
Chem. Lett. 1994, 4, 2091.
10.1021/jo990294a CCC: $18.00 © 1999 American Chemical Society
Published on Web 04/07/1999

Synthesis of â-Turn Mimics with 5-tert-Butylproline
J . Org. Chem., Vol. 64, No. 9, 1999 3313
residues. The proline ψ-dihedral angle is situated around
150° in the type VIb geometry which cannot form an
intramolecular hydrogen bond. Identified on the surfaces
of globular proteins,5 type VI â-turns are commonly
located in cyclic peptides possessing prolyl residues.6 In
addition, the minor cis-amide conformers of certain linear
peptides, particularly those possessing aromatic amino
acids N-terminal to proline and pipecolate residues, have
been observed to adopt type VI â-turn geometry.7-10
Type VI â-turn conformations play important roles in
the recognition and reactivity of bioactive peptides and
proteins. For example, a type VI â-turn conformation has
been suggested as a requirement for thrombin-catalyzed
cleavage of the V3 loop of HIV gp120, a prerequisite to
viral infection.11 Furthermore, peptidyl prolyl isomerases
(PPIases) which catalyze the isomerization of Xaa-Pro
amide bonds and thereby accelerate the folding of par-
ticular proteins have been proposed to bind preferably
to peptides possessing type VI â-turn conformations.12,13
Computational analysis revealed that the PPIase FKBP
bound N-acetyl-Leu-Pro-Phe-methylamide in a type VIa
â-turn.12 In the solid state, the tetrapeptide substrate,
influence peptide turn recognition.14 A second approach
for replicating the type VIa â-turn, which may allow
diversification at the i + 1 position, has employed
azaproline analogues in which the prolyl R-carbon is
replaced by nitrogen.17 In the solid state, azaproline
analogues adopted a type VIa conformation as demon-
strated by X-ray diffraction.17a Furthermore, spectro-
scopic studies of N-(BOC)alanyl-azaprolinyl-alanine N′-
isopropylamides by NMR in solution indicated that
intramolecular hydrogen bonding was maintained in a
type VIa â-turn conformation as solvent composition was
changed from chloroform to DMSO.17b Although azapro-
line is not chiral and the configurations of the neighbor-
ing residues may influence the ring puckering and
ψ-dihedral angle of this prolyl residue in peptides, this
approach may provide a variety of type VI â-turn mimics
should the pyrazolidine moiety be efficiently introduced
into peptide structures. Type VI â-turn surrogates may
also be procured from alternative strategies for replicat-
ing cis-amide geometry, such as cyclo-cystine18 and cyclo-
lanthione19 derivatives, diazabicycloalkane amino acids,20
and heterocycle21 olefin22 and fluoro-olefin23 amide bond
replacements; however, less is known about the influence
of these constraints on the overall peptide conformation.
Moreover, approaches involving amide isosteres may not
be readily amenable to the construction Xaa-Pro dipep-
tide surrogate libraries by diversification of the N-
terminal amino acid.
N-acetyl-Ala-Ala-Pro-Ala-amidomethylcoumarin
was
shown by X-ray diffraction to adopt a type VIb â-turn
geometry when bound to the PPIase cyclophilin.13 In
addition, the nature of the amino acid (Xaa) N-terminal
to the prolyl residue was found to influence PPIase
activity.14
Toward an approach for generating libraries of con-
formationally constrained type VI â-turn mimics, we have
employed 5-alkylprolines to control the prolyl amide
isomer geometry. The steric interactions between a 5-tert-
butyl substituent and the N-terminal residue disfavor the
Xaa-Pro peptide bond trans-isomer and increase the cis-
isomer population.24 By studying analogues of N-(acetyl)-
proline N′-methylamide, we demonstrated that incorpo-
ration of 5-tert-butylproline into this model peptide
increased the cis-isomer population, influenced the en-
ergy barrier for prolyl amide isomerization, and restricted
the proline ψ-dihedral angle.24 We have now incorporated
(2S,5R)-5-tert-butylproline at the C-terminal of a series
of N-(acetyl)dipeptide N′-methylamides 1 in order to
Among the cis-amide prolyl peptide surrogates, com-
petent replacements for the backbone geometry of the
central residues of the type VIa â-turn have been
synthesized by tethering the R-carbon of the N-terminal
amino acid residue to the proline 2-position in a dipeptide
lactam.15,16 These azabicycloalkane amino acids have
been effectively used in constrained analogues of peptides
that require a type VI â-turn for bioactivity.15b In model
peptides, these dipeptide lactams have oriented the N-
and C-terminal amides to form intramolecular hydrogen
bonds in 10-member â-turn and 14-member â-hairpin
secondary structures.16c,d Although they may replicate the
backbone and hydrogen-bonding elements of type VI
â-turns, because of difficulties in appending substituents
onto the N-terminal amino acid residue of dipeptide
lactams, these azabicycloalkane amino acids do not
effectively mimic side-chain pharmacophores that may
(17) (a) Didierjean, C.; Del Duca, V.; Benedetti, E.; Aubry, A.;
Zouikri, M.; Marraud, M.; Boussard, G. J . Pept. Res. 1997, 50, 451. (b)
Zouikri, M.; Vicherat, A.; Aubry, A.; Marraud, M.; Boussard, G. J . Pept.
Res. 1998, 52, 19.
(18) (a) Brady, S. F.; Paleveda, W. J ., J r.; Arison, B. H.; Saperstein,
R.; Brady, E. J .; Raynor, K.; Reisine, T.; Veber, D. F.; Freidinger, R.
M. Tetrahedron 1993, 49, 3449. (b) Cumberbatch, S.; North, M.;
Zagotto, G. Tetrahedron 1993, 49, 9049. (c) Cumberbatch, S.; North,
M.; Zagotto, G. J . Chem. Soc., Chem. Commun. 1993, 641. (d) Horne,
A.; North, M.; Parkinson, J . A.; Sadler, I. H. Tetrahedron 1993, 49,
5891.
(19) O¨ sapay, G.; Zhu, Q.; Shao, H.; Chadha, R. K.; Goodman, M.
Int. J . Pept. Protein Res. 1995, 46, 290.
(20) (a) Curran, T. P.; McEnaney, P. M. Tetrahedron Lett. 1995, 36,
191. (b) Lenman, M. M.; Ingham, S. L.; Gani, D. Chem. Commun. 1996,
85.
(21) (a) Zabrocki, J .; Dunbar, J . B.; Marshall, K. W.; Toth, M. V.;
Marshall, G. R. J . Org. Chem. 1992, 57, 202. (b) Garofolo, A.; Tarnus,
C.; Remy, J .-M.; Leppik, R.; Piriou, F.; Harris, B.; Pelton, J . T. In
Peptides: Chemistry, Structure and Biology; Rivier, J . E., Marshall,
G. R., Eds.; ESCOM Science Publishers B.V.: Leiden, The Netherlands,
1990; pp 833-834. (c) Beusen, D. D.; Zabrocki, J .; Slomczynska, U.;
Head, R. D.; Kao, J . L.-F.; Marshall, G. R. Biopolymers 1995, 36, 181.
(d) Abell, A. D.; Foulds, G. J . J . Chem. Soc., Perkin Trans. 1 1997,
2475.
(22) (a) Hart, S. A.; Sabat, M.; Etzkorn, F. A. J . Org. Chem. 1998,
63, 7580. (b) Andres, C. J .; Macdonald, T. L.; Ocain, T. D.; Longhi, D.
J . Org. Chem. 1993, 58, 6609.
(23) Welch, J . T.; Lin, J . Tetrahedron 1996, 52, 291.
(24) Beausoleil, E.; Lubell, W. D. J . Am. Chem. Soc. 1996, 118,
12902.
(7) Recent examples include the following: (a) tuftsin, Valdeavella,
C. V.; Blatt, H. D.; Pettitt, B. M. Int. J . Pept. Protein Res. 1995, 46,
372. (b) decorin/DS-PGII, Wang, Y.; Scott, P. G.; Sejbal, J .; Kotovych,
G. Can. J . Chem. 1996, 74, 389.
(8) (a) Yao, J .; Bru¨schweiler, R.; Dyson, H. J .; Wright, P. E. J . Am.
Chem. Soc. 1994, 116, 12051. (b) Dyson, H. J .; Rance, M.; Houghten,
R. A.; Lerner, R. A.; Wright, P. E. J . Mol. Biol. 1988, 201, 161.
(9) Oka, M.; Montelione, G. T.; Scheraga, H. A. J . Am. Chem. Soc.
1984, 106, 7959.
(10) (a) Wu, W.-J .; Raleigh, D. P. Biopolymers 1998, 45, 381. (b) Wu,
W.-J .; Raleigh, D. P. J . Org. Chem. 1998, 63, 6689.
(11) J ohnson, M. E.; Lin, Z.; Padmanabhan, K.; Tulinsky, A.; Kahn,
M. FEBS Lett. 1994, 337, 4.
(12) Fischer, S.; Michnick, S.; Karplus, M. Biochemistry 1993, 32,
13830.
(13) Kallen, J .; Walkinshaw, M. D. FEBS Lett. 1992, 300, 286.
(14) Reviewed in: (a) Fischer, G. Angew. Chem., Int. Ed. Engl. 1994,
33, 1415. (b) Liu, J .; Chen, C.-M.; Walsh, C. T. Biochemistry 1991, 30,
2306.
(15) (a) Gramberg, D.; Robinson, J . A. Tetrahedron Lett. 1994, 35,
861. (b) Gramberg, D.; Weber, C.; Beeli, R.; Inglis, J .; Bruns, C.;
Robinson, J . A. Helv. Chim. Acta 1995, 78, 1588.
(16) (a) Dumas, J .-P.; Germanas, J . P. Tetrahedron Lett. 1994, 35,
1493. (b) Kim, K.; Dumas, J .-P.; Germanas, J . P. J . Org. Chem. 1996,
61, 3138. (c) Kim, K.; Germanas, J . P. J . Org. Chem. 1997, 62, 2847.
(d) Kim, K.; Germanas, J . P. J . Org. Chem. 1997, 62, 2853.

3314 J . Org. Chem., Vol. 64, No. 9, 1999
Halab and Lubell
and solid-phase strategies.26 In the context of the present
project, dipeptides 6 were coupled to methylamine using
benzotriazol-1-yl-1,1,3,3-tetramethyluronium tetrafluo-
roborate (TBTU)27 in acetonitrile to furnish N-BOC-Xaa-
5-tert-butylproline N′-methylamides 7. The respective
N-acetyl-Xaa-5-tert-butylproline N′-methylamides 1 were
synthesized from 7 by solvolysis of the BOC group with
trifluoroacetic acid and N-acetylation with acetic anhy-
dride and potassium carbonate in dichloromethane.
In the second route, N-(BOC)-5-tert-butylproline (3)
was coupled to methylamine using TBTU in acetonitrile
to provide N-(BOC)-5-tert-butylproline N′-methylamide
(8).24 Solvolysis of the BOC group with TFA in dichlo-
romethane and coupling to N-(BOC)-amino acids pro-
vided N-BOC-Xaa-5-tert-butylproline N′-methylamides 7
that were acetylated as described above. Among the
reagents explored for coupling to the N-terminal of 5-tert-
butylproline allyl ester and 5-tert-butylproline N′-meth-
ylamide, N,N′-bis(2-oxo-3-oxazolidinyl)phosphonic chlo-
ride (BOPCl)28 in CH2Cl2 at 5 °C gave the best yields of
protected dipeptides 5 and 7.29 For comparison with
dipeptides possessing natural L-proline, we synthesized
N-(acetyl)alanylproline N′-methylamide (2b ) and N-
(acetyl)leucylproline N′-methylamide (2d ) by respectively
coupling N-(BOC)alanine and N-(BOC)leucine to proline
N′-methylamide, followed by BOC group solvolysis and
acetylation as described in the Experimental Section.
Con for m a tion a l An a lysis of P r olyl Dip ep tid es 1
a n d 2 by NMR Sp ectr oscop y. The relative populations
of the amide cis- and trans-isomers N-terminal to the
prolyl residues of peptides 1 and 2 were ascertained by
NMR spectroscopy in chloroform, dimethyl sulfoxide, and
water. The amide populations of 1 and 2 could be
determined for all peptides in water; however, coales-
cence of signals prevented their measurement for N-acetyl-
glycyl-5-tert-butylproline N′-methylamide (1a ) in chloro-
form and DMSO at room temperature. The cis-isomer
was assigned on the basis of observation of the cross-
peak arising from the nuclear Overhauser effect between
the N-terminal amino acid and proline R-hydrogens in
the NOESY and ROESY spectra in DMSO. The popula-
tions of the amide isomers were measured by integration
of the isomeric tert-butyl singlets and N′-methyl doublets
Sch em e 1. Syn th esis of N-(Acetyl)d ip ep tid e
N′-Meth yla m id es 1
examine if the constraints on the prolyl ω- and ψ-dihedral
angles would stabilize the type VI â-turn geometry.
Conformational analyses of dipeptides 1 by NMR experi-
ments, X-ray diffraction and circular dichroism spectros-
copy, and comparison of 1 with dipeptide counterparts 2
possessing natural proline all have shown that (2S,5R)-
5-tert-butylproline stabilized the type VIa â-turn geom-
etry possessing an intramolecular hydrogen bond be-
tween the N′-methylamide proton and the acetamide
carbonyl. By simply coupling a variety of different amino
acid residues to the N-terminal of (2S,5R)-5-tert-butyl-
proline, we have demonstrated the means for generating
a library of dipeptide surrogates that mimic the backbone
geometry, hydrogen bonding, and side-chain elements of
type VIa â-turns. These 5-tert-butylproline surrogates
may thus reproduce both the structural and recognition
elements of type VI â-turns.
1
in the H NMR spectra of 1 and 2. The tert-butyl singlet
of the amide trans-isomer appeared always downfield
from that of the cis-isomer. The ratios of amide isomers
in 1b-f and 2 for each solvent are listed as the percent
of cis-isomer in Table 1.
As is typically observed for linear prolyl peptides,7-11
the major conformer of peptides 2 possessed the trans-
amide geometry N-terminal to the prolyl residue. On the
other hand, the major conformer of peptides 1 adopted
the cis-amide geometry N-terminal to the 5-tert-butyl-
prolyl residue. N-Acetyl-glycyl-5-tert-butylproline N′-meth-
Resu lts
Syn th esis of Ac-Xa a -P r o-NHMe Dip ep tid es 1 a n d
2. (2S,5R)-N-(BOC)-5-tert-butylproline (3) was synthe-
sized in seven steps from glutamic acid as an inexpensive
chiral educt using our acylation/diastereoselective reduc-
tive amination sequence.25 Two routes were investigated
to introduce 3 into N-acetyl-Xaa-5-tert-butylproline N′-
methylamides 1 (Scheme 1). In the first route, N-(BOC)-
5-tert-butylproline allyl ester (4) was synthesized by
alkylation of acid 3 with allyl bromide and DIEA in
dichloromethane. Solvolysis of the BOC group with HCl
gas in dichloromethane and coupling to N-(BOC)amino
acids provided the N-(BOC)-dipeptide allyl esters 5 that
were converted to N-(BOC)-dipeptides 6 by palladium-
catalyzed ester cleavage. We have introduced N-BOC-
Xaa-5-tert-butylprolines 6 into peptide structures by
conventional coupling techniques using both solution-
(26) Halab, L.; Beausoleil, E.; Belec, L.; L’Archeveˆque, B.; Gosselin,
F.; Lubell, W. D. Unpublished results.
(27) (a) Knorr, R.; Trzeciak, A.; Bannwarth, W.; Gillessen, D.
Tetrahedron Lett. 1989, 30, 1927. The crystal structure of HBTU, the
corresponding PF6 salt, substantiates a guanidinium-1-N-oxide salt for
TBTU: (b) Abdelmoty, I.; Albericio, F.; Carpino, L. A.; Foxman, B.;
Kates, S. A. Lett. Pept. Sci. 1994, 1, 57.
(28) (a) Van Der Auwera, C.; Anteunis, M. J . O. Int. J . Pept. Protein
Res. 1987, 29, 574. (b) Tung, R. D.; Rich, D. H. J . Am. Chem. Soc.
1985, 107, 4342.
(29) Preliminary results were reported in part in the following:
Halab, L.; Lubell, W. D. In Peptides 1998 (Proceedings of the 25th
European Peptide Symp.); Bajusz, S., Hudecz, F., Eds.; Akade´mia
Kiado´: Budapest, Hungary, 1999, in press.
(25) Beausoleil, E.; L’Archeveˆque, B.; Be´lec, L.; Atfani, M.; Lubell,
W. D. J . Org. Chem. 1996, 61, 9447.

Synthesis of â-Turn Mimics with 5-tert-Butylproline
J . Org. Chem., Vol. 64, No. 9, 1999 3315
Ta ble 1. In flu en ce of Solven t on th e Ch em ica l Sh ifts a n d Am id e Isom er Equ ilibr iu m of 1 a n d 2a
% cis-isomer ( 3%
(CDCl3)
(CDCl3 f DMSO)
(CDCl3 f D2O)
entry
R
R1
D2O
DMSO
CDCl3
δ NHXaa
NHMe
∆δ NHXaa
NHMe
∆δ NHXaa
NHMe
1b
1c
1d
1e
1f
CH3
t-Bu
t-Bu
t-Bu
t-Bu
t-Bu
H
79
74
81
81
90
14
19
79
72
67
73
79
30
17
83
73
85
89
89
19
20
6.07
6.41
5.97
6.18
6.09
6.33
6.03
8.30
8.27
8.27
8.48
8.37
6.61
6.68
2.39
2.08
2.43
2.13
2.53
1.79
2.00
0.28
0.32
0.22
0.32
0.35
1.06
1.65
2.22
1.95
2.27
1.86
0.28
0.16
0.12
0.13
0.26
1.20
1.10
CH2CH2SCH3
CH2CH(CH3)2
CH(CH3)2
CH2Ph
CH3
CH2CH(CH3)2
2b
2d
1.82
2.13
H
a
Values are for the major conformer at 5 mM concentration.
ylamide (1a ) exhibited only 55% cis-isomer population
in water, which was similar to the amount of cis-isomer
(48%) previously observed with (2S,5R)-N-acetyl-5-tert-
butylproline N′-methylamide and indicated that the
additional acetamide group had a limited influence on
the prolyl amide equilibrium.24 The presence of an alkyl
substituent at the R-position of the N-terminal amino acid
residue augmented significantly the cis-isomer popula-
tion in peptides 1b-f (Table 1). Additional alkyl branch-
ing at the â- and γ-positions gave a relatively minor
increase to the cis-isomer population. As previously noted
in prolyl peptides,8-10 the presence of an aromatic amino
acid N-terminal to proline caused a notable increase in
the population of the cis-isomer in water and N-acetyl-
phenylalanyl-5-tert-butylproline N′-methylamide (1f) ex-
hibited the largest amounts of cis-amide among the
examples studied.
In the major cis-amide conformer of peptides 1, the
signal for the N′-methylamide proton was observed
downfield relative to the signal for the acetamide proton
in all three solvents. This downfield shift was most
evident in chloroform, in which the N′-methylamide
proton signal appeared between 8.27 and 8.48 ppm at
the same time the acetamide proton signal came between
5.97 and 6.41 ppm (Table 1). The downfield shifted amide
proton signal was indicative of an intramolecular hydro-
gen bond between the N′-methylamide proton and the
acetamide carbonyl in a type VI â-turn conformation.30
The signal for the N′-methylamide proton was much
less affected by changes in solvent relative to the signal
for the acetamide proton in the NMR spectra for the cis-
amide conformer of dipeptides 1. The N′-methylamide
proton signal was shifted 0.22-0.35 ppm downfield on
switching solvent from chloroform to DMSO and 0.12-
0.28 ppm downfield on changing solvent from chloroform
to water (Table 1). On the contrary, the signal for the
acetamide proton was shifted 2.08-2.53 ppm downfield
on switching solvent from chloroform to DMSO and 1.86-
2.27 ppm downfield on changing solvent from chloroform
to water. The influence of solvent on the chemical shifts
of the amide proton signals supported a type VI â-turn
conformation for the cis-amide conformer of 1 by indicat-
ing that the N′-methylamide proton was engaged in an
intramolecular hydrogen bond.30
Ta ble 2. In flu en ce of Tem p er a tu r e on th e N-H
Ch em ica l Sh ifts of N-(Acetyl)d ip ep tid e N′-Meth yla m id es
1 a n d 2 in DMSO
∆δ/∆T (-ppb/K)
entry
R
R1
isomer
NHXaa
NHMe
1b
CH3
CH3
t-Bu
cis
trans
cis
trans
cis
trans
cis
5.6
4.9
6.3
5.7
5.3
4.3
6.1
5.6
3.7
4.4
4.3
4.4
3.0
4.4
4.1
5.3
2b
1d
2d
H
CH2CH(CH3)2
CH2CH(CH3)2
t-Bu
H
trans
A comparison of the measured temperature coefficients
for the amide protons in peptides 1b, 1d , 2b, and 2d in
DMSO provided additional evidence for a type VI â-turn
conformation in 1 (Table 2). In the case of the cis- and
trans-conformers of Ac-Ala-Pro-NHMe (2b) and Ac-Leu-
Pro-NHMe (2d ), all of the amide proton signals exhibited
chemical shift temperature coefficients that were less
than -4 ppb/K, within the range for unstructured pep-
tides.31 Similarly, in the case of the corresponding tert-
butylprolyl peptides 1b and 1d , the amide signals for the
minor trans-amide conformer and the acetamide proton
signal of the cis-amide conformer all possessed chemical
shift temperature coefficients less than -4 ppb/K in
DMSO. Only the N′-methylamide proton signal for the
major cis-amide conformer of tert-butylprolyl peptides 1b
and 1d exhibited a temperature coefficient that was
greater than -4 ppb/K in DMSO. Although values
greater than -3 ppb/K have been suggested to indicate
a solvent-shielded amide proton engaged in an intramo-
lecular hydrogen bond in DMSO,31 such temperature
coefficients are usually measured on cyclic peptides and
peptide structures larger than those studied in this
report. The respective values of -3.7 and -3.0 ppb/K for
the N′-methylamide proton signal of the major cis-amide
conformer of dipeptides 1b and 1d fall into the region
between temperature coefficients associated with hydro-
gen-bonded and solvent-exposed amide; however, their
size is most probably due to the inability of the acetamide
and N′-methyl groups in model peptides 1 to provide
adequate solvent shielding of the hydrogen-bound N′-
methylamide proton.
(30) Reviewed in: Dyson, H. J .; Wright, P. E. Annu. Rev. Biophys.
Biophys. Chem. 1991, 20, 519.
(31) Reviewed in: Kessler, H. Angew Chem., Int. Ed. Engl. 1982,
21, 512.

3316 J . Org. Chem., Vol. 64, No. 9, 1999
Halab and Lubell
Smaller values were consistently measured for the
vicinal coupling constant (3J NH,R) between the amide and
R-protons of the N-terminal amino acid residue in the
major cis-amide conformer of tert-butylprolyl peptides
1b-f than were observed for the minor trans-amide
conformer of 1 and the cis- and trans-amide conformers
3
of 2. For example, J NH,R values were 0.8-3.5 Hz lower
for the cis-amide than for the trans-amide conformer of
3
1b-f in DMSO. The J NH,R values varied from 5.2 to 6.4
Hz for the cis-amide conformers of 1b-f in DMSO. Since
the ideal φ-dihedral angle of -60° for the i + 1 residue
3
in a type VIa â-turn corresponds to a J NH,R of 4.2 Hz,
the observed reduced coupling constant values support
the hypothesis that the major cis-amide conformer of
peptides 1b-f adopts a significant population of type VI
â-turn in solution.32
Evidence for the presence of a twisted amide geometry
in solution was obtained by examining the chemical shift
value for the carbonyl carbon of the N-terminal amino
acid residue in peptide 1b.33 After assigning the amide
carbonyl resonances by using two-dimensional HMBC
NMR experiments,34 we observed that 5-tert-butylprolyl
peptide 1b exhibited carbonyl chemical shift values for
the N-terminal amino acid residue that were 3.1 ppm
downfield relative to those of prolyl peptide 2b in water.
Because inhibition of amide resonance by factors that
distort the N-C(O) bond deshields the carbonyl carbon,33a
the downfield-shifted 13C NMR chemical shift value
indicated that the prolyl amide bond of 1b was twisted
from planarity by steric interactions between the N-
terminal residue and 5-tert-butyl substituent.
X-r a y Cr ysta llogr a p h ic An a lysis of N-Acetyl-L-
leu cyl-5-ter t-bu tylpr olin e N′-Meth ylam ide (1d). Crys-
tals of N-acetyl-L-leucyl-5-tert-butylproline N′-methyla-
mide (1d ) were grown from a mixture of ether and
hexane. Crystallographic analysis of 1d by X-ray diffrac-
tion demonstrated the presence of the amide cis-isomer
N-terminal to the 5-tert-butylprolyl residue (Figure 2).35
Furthermore, the dihedral angles of peptide 1d re-
sembled those of the central, i + 1 and i + 2 residues of
a type VIa â-turn. For comparison, the dihedral values
for the crystal structure of 1d are listed in Table 3 with
those of an ideal type VIa geometry4 and the values for
the Leu-Pro residues found in the central positions of the
type VIa â-turn in the X-ray structure of the cyclic
peptide evolidine.6d An intramolecular hydrogen bond
between the N′-methylamide proton and the acetamide
F igu r e 2. ORTEP view of Ac-Leu-5-t-BuPro-NHMe 1d .
Ellipsoids drawn at 40% probability level. Hydrogens repre-
sented by spheres of arbitrary size.35
Ta ble 3. Com p a r ison of th e Dih ed r a l An gles of Id ea l
Typ e VIa â-Tu r n a n d X-r a y Str u ctu r e of
N-(Acetyl)leu cyl-5-ter t-bu tylp r olin e N′-Meth yla m id e 1d
entry
φ2
ψ2
ω
φ3
ψ3
0°
ideal type VIa â-turn4
-60° 120°
0° -90°
Ac-Leu-5-t-BuPro-NHMe 1d 35 -61° 139° 17° -95° 19°
Leu-Pro residues in X-ray
-65° 151°
2° -93° 13°
of evolidine6d
carbonyl oxygen was clearly inferred from the interatomic
distance of 2.13 Å in the X-ray structure of 1d .
The ω, ψ, and φ values for the dihedral angles of the
5-tert-butylprolyl residue in the X-ray structure of peptide
1d were similar to those calculated for the energy
minimum of the cis-isomer of N-acetyl-5-tert-butylproline
N′-methylamide.24 A twisted amide conformation N-
terminal to the 5-tert-butylprolyl residue was observed
on measuring the ω-dihedral angle value of 17.3° for 1d
in the X-ray structure and indicated that the bulky
5-position substituent skewed the amide bond away from
planarity.33 The twisted amide geometry was substanti-
ated by the 1.36 Å carbonyl carbon to nitrogen bond
distance for the amide N-terminal to the 5-tert-butylpro-
lyl residue, which was longer than the 1.33 Å bond
lengths for the other amides.33b The measured ψ-dihedral
angle value of 18.5° placed the N′-methylamide hydrogen
at a 2.45 Å interatomic distance from the prolyl nitrogen.
These constraints account for the observed acceleration
of amide isomerization N-terminal to (2S,5R)-5-tert-
butylproline. Ground-state destabilization results from
the bulky 5-tert-butyl substituent distorting the amide
bond away from planarity. Stabilization of the pyrami-
dalized amide transition state arises from the N′-meth-
ylamide hydrogen interacting with the nitrogen lone pair
of the rotating prolyl amide.12,36 In the case of N-(acetyl)-
glycyl-5-tert-butylproline N′-methylamide (1a ), such an
acceleration of the rate of prolyl amide isomerization was
illustrated by the coalescence of the isomeric cis- and
trans-amide signals in DMSO and chloroform at room
(32) (a) Karplus, M. J . Am. Chem. Soc. 1963, 85, 2870. (b) Bystrov,
V. F.; Ivanov, V. T.; Portnova, S. L.; Balashova, T. A.; Ovchinnikov,
Yu. A. Tetrahedron 1973, 29, 873.
(33) The use of carbon NMR spectroscopy to study twisted amide
geometry is presented in: (a) Bennet, A. J .; Somayaji, V.; Brown, R.
S.; Santarsiero, B. D. J . Am. Chem. Soc. 1991, 113, 7563. The
relationship between bond length and twisted amide geometry is
described in: (b) Shao, H.; J iang, X.; Gantzel, P.; Goodman, M. Chem.
Biol. 1994, 1, 231 and the characterization and reactivity of other
distorted amides is discussed in refs 15-19 therein. (c) Bennet, A. J .;
Wang, Q.-P.; Slebocka-Tilk, H.; Somayaji, V.; Brown, R. S. J . Am.
Chem. Soc. 1990, 112, 6383. (d) Kirby, A. J .; Komarov, I. V.; Feeder,
N. J . Am. Chem. Soc. 1998, 120, 7101.
(34) Bax, A.; Summers, M. F. J . Am. Chem. Soc. 1986, 108, 2093.
(35) The structure of 1d was solved at l’Universite´ de Montre´al X-ray
facility using direct methods (SHELXS96) and refined with NRCVAX
and SHELXL96: C18H33N3O3; Mr ) 339.474; orthorhombic, colorless
crystal; space group P212121; unit cell dimensions (Å) a ) 9.110 (3), b
) 10.860 (5), c ) 20.435 (10); volume of unit cell (Å3) 2021.7 (15); Z )
4; R1 ) 0.0505 for I >2 σ(I), wR2 ) 0.1140 for all data; GOF ) 0.986.
The author has deposited the atomic coordinates for the structure of
1d with the Cambridge Crystallographic Data Center. The coordinates
can be obtained, on request, from the Cambridge Crystallographic Data
Center, 12 Union Road, Cambridge, CB2 1EZ, U.K.
(36) (a) Fischer, S.; Dunbrack, J r. R. L.; Karplus, M. J . Am. Chem.
Soc. 1994, 116, 11931. (b) Cox, C.; Young, J r. V. G.; Lectka, T. J . Am.
Chem. Soc. 1997, 119, 2307. (c) Beausoleil, E.; Sharma, R.; Michnick,
S.; Lubell, W. D. J . Org. Chem. 1998, 63, 6572. (d) Cox, C.; Lectka, T.
J . Am. Chem. Soc. 1998, 120, 10660.

Synthesis of â-Turn Mimics with 5-tert-Butylproline
J . Org. Chem., Vol. 64, No. 9, 1999 3317
F igu r e 4. Circular dichroism spectra of N-(acetyl)alanyl- and
N-(acetyl)leucyl-proline N′-methylamides (2b and 2d ) in water
and acetonitrile.
F igu r e 3. Circular dichroism spectra of N-(acetyl)alanyl- and
N-(acetyl)leucyl-5-tert-butylproline N′-methylamides (1b and
1d ) in water and acetonitrile.
temperature, because amide isomerization N-terminal to
proline proceeded faster in the nonprotic and compara-
tively nonpolar solvent than in water which stabilized
the polar amide ground states relative to the less polar
transition state.37
Discu ssion
Steric interactions have been employed to restrain
peptide geometry and favor particular secondary struc-
tures. For example, amino isobutyric acid (Aib) residues
can create steric interactions with neighboring residues
that favor 310- and R-helical geometries such as those
found in antibiotic peptides such as althemethicin.41 By
similar interactions, the related R,R-dialkyl glycines
induce 310-helices and type III â-turn geometries in linear
peptides.42 In pioneering studies of substituted polypro-
line oligomers, the steric effects of 2-methylproline were
found to stabilize polyproline type II helical conforma-
tion.43 Recently, the combination of A1,3- and A1,2-strain
of alkyl-substituted olefin amide bond isosteres has been
used to induce the formation of â-turn conformations
exhibiting 10-member intramolecular hydrogen bonds44
and a â-hairpin mimic possessing a 14-member intramo-
lecular hydrogen bond.45 On the other hand, disruption
of the γ-turn conformation has been caused by the
placement of alkyl substituents at the 3-position of a
central prolyl residue.46
Con for m a tion a l An a lysis of Dip ep tid es 1 a n d 2
by Cir cu lar Dich r oism Spectr oscopy. Circular dichro-
ism (CD) spectra of 1b, 1d , 2b, and 2d were measured
in both water and acetonitrile to examine the influence
of solvent composition on peptide conformation. The CD
spectra of 1b and 1d in acetonitrile exhibited a strong
negative band at 188 nm, a strong positive band at 209
nm, and a weak negative band at 227 nm (Figure 3). This
type of CD curve shape has previously been assigned to
â-turn conformations in studies of model peptides in
water.38 Aside from a slight blue shift,39 the shape of the
CD curves for 1b and 1d remained constant as solvent
was changed from acetonitrile to water. The type VIa
â-turn conformation adopted by 5-tert-butylprolyl peptide
1 was thus shown to be independent of solvent composi-
tion. On the other hand, the CD spectral characteristics
of prolyl peptides 2 were similar to those reported for
N-(acetyl)proline N′-methylamide and varied signifi-
cantly with changes in solvent composition exhibiting a
n-π* band near 225 nm in acetonitrile that shifted to a
significant π-π* band near 195 nm in water (Figure 4).40
In this report, we have demonstrated that the steric
interactions of 5-tert-butylproline can induce the forma-
(42) For representative examples, see: (a) Benedetti, E.; Di Blasio,
B.; Iacovino, R.; Menchise, V.; Saviano, M.; Pedone, C.; Bonora, G. M.;
Ettorre, A.; Graci, L.; Formaggio, F.; Crisma, M.; Valle, G.; Toniolo,
C. J . Chem. Soc., Perkin Trans. 2 1997, 2023. (b) Toniolo, C.; Crisma,
M.; Formaggio, F.; Benedetti, E.; Santini, A.; Iacovino, R.; Saviano,
M.; Di Blasio, B.; Pedone, C.; Kamphuis, J . Biopolymers 1996, 40, 519.
(c) Prasad, S.; Rao, R. B.; Balaram, P. Biopolymers 1995, 35, 11. (d)
Toniolo, C.; Benedetti, E. Macromolecules 1991, 24, 4004 and references
therein.
(37) Reviewed in: Stein, R. L. Adv. Protein Chem. 1993, 44, 1.
(38) (a) Deslauriers, R.; Evans, D. J .; Leach, S. J .; Meinwald, Y. C.;
Minasian, E.; Ne´methy, G.; Rae, I. D.; Scheraga, H. A.; Somorjai, R.
L.; Stimson, E. R.; Van Nispen, J . W.; Woody, R. W. Macromolecules
1981, 14, 985. (b) Reviewed in: Brahms, S.; Brahms, J . J . Mol. Biol.
1980, 138, 149.
(39) Lambert, J . B.; Shurvell, H. F.; Lightner, D. A.; Cooks, R. G.
Organic Structural Spectroscopy; Prentice Hall: Upper Saddle River,
NJ , 1998; pp 281-283.
(40) Madison, V.; Kopple, K. D. J . Am. Chem. Soc. 1980, 102, 4855.
(41) (a) Pispisa, B.; Palleschi, A.; Amato, M. E.; Segre, A. L.; Venanzi,
M. Macromolecules 1997, 30, 4905. (b) Fox, R. O., J r.; Richards, F. M.
Nature 1982, 300, 325.
(43) Overberger, C. G.; J on, Y. S. J . Polym. Sci.: Polym. Chem. Ed.
1977, 15, 1413.
(44) (a) Gardner, R. R.; Liang, G.-B.; Gellman, S. H. J . Am. Chem.
Soc. 1995, 117, 3280. (b) Wipf, P.; Henninger, T. C.; Geib, S. J . J . Org.
Chem. 1998, 63, 6088.
(45) Schopfer, U.; Stahl, M.; Brandl, T.; Hoffmann, R. W. Angew
Chem., Int. Ed. Engl. 1997, 36, 1745.

3318 J . Org. Chem., Vol. 64, No. 9, 1999
Halab and Lubell
tion of type VI â-turn conformations. Although 5-alkyl-
prolines and their related 4-thia- and 4-oxaproline ana-
logues have previously been employed for augmenting
the prolyl amide cis-isomer population in peptide struc-
tures,24,47,48 prior to our examination, the influence of the
5-alkyl substituent on the prolyl peptide turn conforma-
tion had not been reported.
the X-ray structure of dipeptide 1d . Contortion from
planarity is the primary force diminishing the barrier for
5-tert-butylprolyl amide isomerization. For example,
amide isomerization N-terminal to (2S,5R)-N-acetyl-5-
tert-butylproline N′-methylamide was reduced by 3.7
kcal/mol compared to the barrier for N-(acetyl)proline N′-
methylamide in water.24 However, the 5-tert-butyl sub-
stituent influences the prolyl carboxylate to adopt a
ψ-dihedral angle around ψ ≈ 0°, which may also lead to
a lower barrier for amide isomerization by enabling
stabilization of the pyramidalized transition state via
interactions between the C-terminal amide NH with the
nitrogen lone pair of the rotating N-terminal amide.12,36
In conclusion, we have developed a novel approach for
mimicking type VIa â-turns that features the employ-
ment of (2S,5R)-5-tert-butylproline to constrain the con-
formational liberty of N-(acetyl)dipeptide N′-methyla-
mides 1. An ensemble of spectroscopic and crystallographic
data verified the presence of the type VIa â-turn geom-
etry in both solution and the solid state. Because (2S,5R)-
5-tert-butylproline can now be conveniently introduced
into peptide structures, we are presently generating type
VIa â-turn libraries in order to explore the importance
of this structure in peptide chemistry and biology.26
We synthesized N-(acetyl)dipeptide N′-methylamides
1 and 2 possessing respectively (2S,5R)-5-tert-butylpro-
line and L-proline at the C-terminal residue. Conforma-
tional analysis of dipeptides 1 and 2 by NMR spectros-
copy showed that the Xaa-Pro peptide bond adopted
preferably the cis-amide isomer in 5-tert-butylprolyl
peptides 1 in contrast to prolyl dipeptides 2 which
preferred the trans-amide isomer in solution. The limited
influence of solvent composition and temperature on the
chemical shift value of the N′-methylamide in the cis-
amide conformer of dipeptide 1 indicated that it was
engaged in an intramolecular hydrogen bond with the
acetamide carbonyl in a type VIa â-turn conformation.
The presence of significant type VI â-turn populations
for tert-butylprolyl peptides 1b-f in solution was also
supported by the reduced value for the vicinal coupling
constant between the R and amide protons of the N-
terminal amino acid residue in the major cis-amide
conformer. Furthermore, in the solid state, N-acetyl-L-
leucyl-5-tert-butylproline N′-methylamide (1d ) existed in
a type VIa â-turn geometry as shown by X-ray diffraction.
Finally, because they exhibited circular dichroism spectra
characteristic of â-turn conformation in both water and
acetonitrile, tert-butylprolyl peptides 1b and 1d were
shown to adopt type VIa â-turn geometry independent
of solvent composition.
Steric interactions between the bulky 5-position sub-
stituent and the N-terminal residue contorted the (2S,5R)-
5-tert-butylprolyl amide away from the planar sp2 hy-
bridized geometry as illustrated by spectroscopic and
crystallographic data. In the 13C NMR spectrum of
N-acetyl-L-alanyl-5-tert-butylproline N′-methylamide 1b,
the carbonyl carbon signal of the N-terminal residue was
downfield shifted 3.1 ppm relative to the amide carbon
resonance of its prolyl dipeptide counterpart 2b in water.
The tert-butylprolyl peptide bond exhibited a 17° ω-di-
hedral angle and an extended N-C(O) bond length in
Exp er im en ta l Section
Gen er a l. Unless otherwise noted, all reactions were run
under a nitrogen atmosphere and distilled solvents were
transferred by syringe. THF was distilled from sodium/
benzophenone, CH2Cl2 was distilled over P2O5, CH3CN was
distilled over CaH2, and DIEA was distilled over ninhydrin
and CaH2. Final reaction mixture solutions were dried over
Na2SO4. Chromatography was on 230-400 mesh silica gel, and
TLC was on aluminum-backed silica plates. Melting points are
uncorrected. Mass spectral data, HRMS (EI and FAB), were
obtained at the Universite´ de Montre´al Mass Spectroscopy
facility.
NMR Mea su r em en ts. 1H and 13C NMR experiments were
performed on Bruker DMX600 and ARX400 spectrometers.
The chemical shifts are reported in ppm (δ units) downfield
of the internal tetramethylsilane ((CH3)4Si). Coupling con-
stants are in hertz. The chemical shifts for the carbons and
the protons of the minor isomers are respectively reported in
parentheses and in brackets. COSY, NOESY, and ROESY
spectra were obtained with 2048 by 512 data points. A mixing
time of 500 ms was used for the NOESY and ROESY spectra.
The temperature coefficients of the amide proton chemical
shifts in DMSO-d6 were measured for at least five different
temperatures in 5 deg steps by varying the temperature
between 298 and 328 K. The value of the temperature
coefficient was obtained by a linear least-squares fit of the
data.
(46) (a) Beausoleil, E.; Sharma, R.; Michnick, S.; Lubell, W. D. In
Peptides 1996 (Proceedings of the 24th European Peptide Symposium);
Ramage, R., Epton, R., Eds.; ESCOM, Leiden: The Netherlands, 1997;
pp 241-242. (b) Delaney, N. G.; Madison, V. J . Am. Chem. Soc. 1982,
104, 6635. (c) Samanen, J .; Zuber, G.; Bean, J .; Eggleston, D.; Romoff,
T.; Kopple, K.; Saunders: M.; Regoli, D. Int. J . Pept. Protein Res. 1990,
35, 501.
Cir cu la r Dich r oism Mea su r em en ts. CD spectra of 0.1
mM solutions in H2O and CH3CN were measured on a J asco
J -710 spectropolarimeter using a circular quartz cell with a
path length of 1 mm at 23 °C. Spectra were run with a
bandwidth of 1 nm, a response time of 0.25 s, and a scan speed
of 100 nm min-1. Each measurement was the average result
of 10 repeated scans in steps of 0.2 nm. Baseline spectra of
the solvents were subtracted.
(47) 5-Alkylproline examples include: (a) Delaney, N. G.; Madison,
V. Int. J . Pept. Protein Res. 1982, 19, 543. (b) Magaard, V. W.; Sanchez,
R. M.; Bean, J . W.; Moore, M. L. Tetrahedron Lett. 1993, 34, 381. (c)
Overberger, C. G.; David, K.-H. Macromolecules 1972, 5, 373. (d)
Overberger, C. G.; Han, M. J . J . Polym. Sci.: Polym. Chem. Ed. 1975,
13, 2251. (e) Overberger, C. G.; Han, M. J . Pure Appl. Chem. 1974,
39, 33. (f) Ahn, K.-D.; Overberger, C. G. J . Polym. Sci.: Polym. Chem.
Ed. 1983, 21, 1699. (g) Yang, W. W.-Y.; Overberger, C. G.; Venkatacha-
lam, C. M. J . Polym. Sci.: Polym. Chem. Ed. 1983, 21, 1741. (h) Yang,
W. W.-Y.; Overberger, C. G.; Venkatachalam, C. M. J . Polym. Sci.:
Polym. Chem. Ed. 1983, 21, 1751. (i) McGrady, K. A. W.; Overberger,
C. G. Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 1987, 28,
429.
(48) 5-Alkyl-4-thiaproline analogues: (a) Nefzi, A.; Schenk, K.;
Mutter, M. Protein Pept. Lett. 1994, 1, 66. (b) Savrda, J . In Peptides
1976; Loffet, A., Ed.; Edition de l’Universite´ de Bruxelles: Brussels,
1976; p 653. 5-Alkyl-4-thia- and 4-oxaproline analogues: (c) Keller,
M.; Sager, C.; Dumy, P.; Schutkowski, M.; Fischer, G. S.; Mutter, M.
J . Am. Chem. Soc. 1998, 120, 2714. (d) Dumy, P.; Keller, M.; Ryan, D.
E.; Rohwedder, B.; Wo¨hr, T.; Mutter, M. J . Am. Chem. Soc. 1997, 119,
918.
(2S,5R)-N-(BOC)-5-ter t-bu tylp r olin e Allyl Ester (4). A
solution of (2S,5R)-N-(BOC)-5-tert-butylproline (3, 0.71 g, 2.62
mmol, prepared according to ref 25) in CH2Cl2 (26 mL) was
treated with DIEA (1.0 mL, 5.76 mmol) and allyl bromide (2.3
mL, 26.2 mmol), heated to a reflux, stirred for 18 h, cooled to
room temperature, and evaporated. The residue was dissolved
in EtOAc (50 mL), and the solution was washed with cold 0.1
M HCl (2 × 10 mL) and a phosphate buffer solution at pH 9.5
(15 mL), dried, and evaporated to give 4 (0.28 g, 98%) as an
oil: [R]20 -29.7° (c 0.5, CHCl3); 1H NMR (CDCl3) δ 0.88 (s, 9
D

Synthesis of â-Turn Mimics with 5-tert-Butylproline
J . Org. Chem., Vol. 64, No. 9, 1999 3319
H), 1.35 (s, 9 H), 1.85 (m, 3 H), 2.17 (m, 1 H), 3.76 (m, 1 H),
4.24 (m, 1 H), 4.55 (m, 2 H), 5.30 (m, 2 H), 5.86 (m, 1 H); 13C
NMR (CDCl3) δ 26.4, 27.2, 28.0, 29.5, 36.2, 61.4, 65.1, 66.5,
79.7, 118.2, 131.8, 155.8, 172.8; HRMS calcd for C17H30O4N
(MH+) 312.2175, found 312.2193.
N-(BOC)-(2S)-p h e n yla la n yl-(2S,5R)-5-ter t-b u t ylp r o-
lin e a llyl ester (5f) was obtained in 94% yield as a white
solid: mp 118-119 °C; [R]20 -67.4° (c 0.84, CHCl3); 1H NMR
D
(CDCl3) δ 0.81 (s, 7.4 H) [0.96 (s, 1.6 H)], [1.28 (s, 1.6 H)] 1.42
(s, 7.4 H), 1.51-1.65 (m, 2 H), 1.83 (m, 1 H), 2.05 (m, 1 H),
2.83 (m, 1 H), 3.02 (m, 1 H), 3.48 (t, 1 H, J ) 8.4), 4.07 (d, 0.8
H, J ) 8.5) [4.29 (d, 0.2 H, J ) 7.9)], 4.62 (m, 3 H), 4.91 (br s,
1 H), 5.29 (m, 2 H), 5.84 (m, 1 H), 7.19-7.27 (m, 5 H); 13C
NMR (CDCl3) δ 25.4, 27.3 (27.5), (27.4) 28.0, (28.1) 28.2, (35.6)
36.0, (38.3) 41.0, (52.0) 53.7, 59.5 (60.5), (65.3) 66.2, 66.6, 79.2
(79.5), (118.1) 118.8, (126.4) 126.9, (128.2) 128.6, 129.3, 131.4
(131.8), 136.4 (136.9), 154.4 (154.9), 171.4 (171.5), 172.8
(173.7); HRMS calcd for C26H39O5N2 (MH+) 459.2859, found
459.2872.
(2S,5R)-5-ter t-Bu tylp r olin e Allyl Ester Hyd r och lor id e.
A solution of (2S,5R)-N-(BOC)-5-tert-butylproline allyl ester
(4, 1.91 g, 6.14 mmol) in CH2Cl2 (60 mL) was saturated with
HCl (g) bubbles at 0 °C, stirred for 2 h at room temperature,
and evaporated to provide (2S,5R)-5-tert-butylproline allyl
ester hydrochloride in 99% (1.51 g) yield as a white precipi-
tate: [R]20D -24.6° (c 0.7, CHCl3); 1H NMR (CD3OD) δ 1.12 (s,
9 H), 1.79 (m, 1 H), 2.13 (m, 1 H), 2.3-2.4 (m, 2 H), 3.55 (dd,
1 H, J ) 6.3, 12.0), 4.58 (dd, 1 H, J ) 3.8, 9.2), 4.77 (m, 2 H),
5.33 (m, 2 H), 6.00 (m, 1 H); 13C NMR (CD3OD) δ 25.7, 26.7,
29.5, 33.1, 60.6, 68.4, 72.9, 120.0, 132.5, 170.1; HRMS calcd
for C12H22O2N (MH+) 212.1651, found 212.1656. Anal. Calcd
for C12H22O2NCl: C, 58.17; H, 8.95; N, 5.65. Found: C, 58.20,
H, 9.34, N, 5.65. (2S,5R)-5-tert-Butylproline allyl ester tri-
fluoroacetate was prepared by stirring a solution of allyl ester
4 (0.84 g, 2.70 mmol) in 1:3 TFA:CH2Cl2 at room temperature
for 2 h to give an oil (0.86 g, 98%) after evaporation of the
volatiles: 1H NMR (CDCl3) δ 1.08 (s, 9 H), 1.65 (m, 1 H), 2.04
(m, 1 H), 2.33 (m, 1 H), 2.50 (m, 1 H), 3.65 (m, 1 H), 4.59 (dd,
1 H, J ) 2.1, 9.9), 4.74 (m, 2 H), 5.36 (m, 2 H), 5.91 (m, 1 H).
Gen er a l P r oced u r e for P ep tid e Cou p lin g to 5-ter t-
Bu tylp r olin e Resid u es. A solution of (2S,5R)-5-tert-butyl-
proline allyl ester hydrochloride (355 mg, 1.43 mmol), N-(BOC)-
amino acid (1.72 mmol), and DIEA (1.0 mL, 5.72 mmol) in
CH2Cl2 (14 mL) was cooled to 0 °C, treated with BOP-Cl (430
mg, 1.72 mmol), stirred for 1 h ,and allowed to warm to room
temperature with stirring for 18 h. Brine (5 mL) was added
to the reaction solution which was extracted with CH2Cl2 (2
× 10 mL). The combined organic layers were washed with 0.1
M HCl (2 × 5 mL), 5% NaHCO3 (2 × 5 mL), and brine (10
mL), dried, and evaporated to a residue that was purified by
chromatography on silica gel using 35% EtOAc in hexane as
eluant. Evaporation of the collected fractions afforded N-
(BOC)dipeptide allyl ester 5. The same protocol was used to
couple N-(BOC)amino acids and (2S,5R)-5-tert-butylproline N′-
methylamide trifluoroacetate to provide N-(BOC)dipeptide N′-
methylamides 7.
N-(BOC)-glycyl-(2S,5R)-5-ter t-b u t ylp r olin e N′-m et h -
yla m id e (7a ) was obtained in 87% yield as a white solid: mp
1
63-66 °C; [R]20 -113.7° (c 0.86, CHCl3); H NMR (CDCl3) δ
D
0.92 (s, 9 H), 1.45 (s, 9 H), 1.85 (m, 2 H), 2.04 (m, 0.7 H) [2.20
(m, 0.3)], [2.35 (m, 0.3 H)] 2.63 (m, 0.7 H), 2.80 (m, 3 H), 3.71
(d, 0.7 H, J ) 8.1) [3.85 (m, 0.3 H)], 4.11 (m, 1 H), [4.30 (m,
0.3 H)] 4.68 (t, 0.7 H, J ) 8.5), 5.40 (br s, 1 H), [6.81 (br s, 0.3
H)] 7.24 (br s, 0.7 H); 13C NMR (CDCl3) δ 24.7 (25.6), 26.1,
26.5, 27.4, 28.1, 35.6 (35.9), (42.9) 43.3, 61.7, (67.2) 67.6, 79.6,
155.6, (171.4) 171.7, 172.2; HRMS calcd for C17H32O4N3 (MH+)
342.2393, found 342.2399.
N-(BOC)-(2S)-a la n yl-(2S,5R)-5-ter t-b u t ylp r olin e N′-
m eth yla m id e (7b) was obtained in 75% yield as a white
1
solid: mp 100-101 °C; [R]20 -81.5° (c 0.6, CHCl3); H NMR
D
(CDCl3) δ 0.81 (s, 7.6 H) [0.84 (s, 1.4 H)], 1.16 (d, 2.6 H, J )
6.8) [1.27 (d, 0.4 H, J ) 6.6)], 1.34 (s, 9 H), 1.75 (m, 2 H), 2.15
(m, 1 H), 2.33 (m, 1 H), [2.72 (d, 0.4 H, J ) 6.5)] 2.75 (d, 2.6 H,
J ) 4.6), 4.07 (m, 1 H), 4.22 (m, 2 H), 5.29 (d, 1 H, J ) 3.8),
8.32 (br s, 1 H); 13C NMR (CDCl3) δ 16.2 (18.2), (24.9) 25.2,
26.2, 27.4, 28.0, (26.3) 29.1, (35.1) 35.6, (46.6) 48.9, (61.5) 61.9,
67.0 (67.5), (79.7) 80.3, 156.4, 171.7, 175.6; HRMS calcd for
C
18H34O4N3 (MH+) 356.2549, found 356.2556.
N-(BOC)-(2S)-m eth ion yl-(2S,5R)-5-ter t-bu tylp r olin e N′-
m eth yla m id e (7c) was obtained in 64% yield as a white
1
solid: mp 102-103 °C; [R]20 -88.4° (c 0.9, CHCl3); H NMR
D
(CDCl3) δ 0.86 (s, 6.4 H) [0.93 (s, 2.6 H)], 1.42 (s, 9 H), 1.83
(m, 3 H). 2.01-2.09 (m, 5 H), 2.56 (m, 3 H), [2.78 (d, 0.9 H, J
) 4.7)] 2.83 (d, 2.1 H, J ) 4.4), 4.28 (m, 2 H), 4.44 (m, 1 H),
5.12 (d, 1 H, J ) 7.8), 8.29 (br s, 1 H); 13C NMR (CDCl3) δ
(14.0) 15.6, (25.1) 25.2, 26.1 (26.2), (26.3) 27.1, (27.4) 28.0, 28.7,
30.3, 30.7, (35.1) 35.6, (50.5) 52.5, (61.5) 61.8, 67.0 (67.5), (80.0)
80.6, (155.1) 157.0, 171.3 (171.9), 174.6 (175.2); HRMS calcd
for C20H38O4N3S (MH+) 416.2583, found 416.2593.
N-(BOC)-(2S)-a la n yl-(2S,5R)-5-ter t-bu tylp r olin e a llyl
ester (5b) was obtained in 68% yield as an oil: [R]20 -92.4°
D
(c 0.85, CHCl3); 1H NMR (CDCl3) δ [0.83 (s, 3.7 H)] 0.93 (s,
5.3 H), [1.17 (d, 1.2 H, J ) 6.4)] 1.26 (d, 1.8 H, J ) 6.5), 1.33
(s, 5.3 H) [1.35 (s, 3.7 H)], 1.75-1.98 (m, 3 H), 2.25 (m, 1 H),
4.12 (d, 1 H, J ) 7.9), 4.31 (m, 1 H), 4.55-4.65 (m, 3 H), 4.95
(d, 1 H, J ) 8.8), 5.27 (m, 2 H), 5.83 (m, 1 H); 13C NMR (CDCl3)
δ 18.1 (19.0), 25.6, (27.3) 27.5, 28.0 (28.1), 28.8, 35.7 (36.0),
46.5 (47.9), (59.8) 60.4, 65.1 (66.3), 66.6, (79.0) 79.4, 117.9
(118.9), (131.3) 131.8, (154.3) 154.9, (171.2) 171.5, (173.7)
174.8; HRMS calcd for C20H35O5N2 (MH+) 383.2546, found
383.2560.
N-(BOC)-(2S)-va lyl-(2S,5R)-5-ter t-b u t ylp r olin e
N′-
m eth yla m id e (7e) was obtained in 35% yield as a white
1
solid: mp 56-58 °C; [R]20 -80.8° (c 0.57, CHCl3); H NMR
D
(CDCl3) δ 0.86 (m 9 H), 0.96 (d, 3 H, J ) 6.9), 1.03 (d, 3 H, J
) 6.7), 1.43 (s, 9 H), 1.61 (m, 1 H), 1.84 (m, 3 H), 2.57 (m, 1
H), 2.81 (d, 3 H, J ) 4.6), 3.95 (t, 1 H, J ) 8.0), 4.32 (t, 1 H,
J ) 7.3), 4.46 (dd, 1 H, J ) 3.5, 8.1), 4.99 (d, 1 H, J ) 8.5),
8.48 (br s, 1 H);13C NMR (CDCl3) δ (17.9) 18.3, 19.5 (20.0),
(24.9) 25.2, 26.0, (26.2) 27.1, (27.6) 28.0, 28.1, 30.5 (31.1), (35.0)
35.6, (56.9) 58.6, (61.3) 61.7, 66.7 (66.9), (79.8) 80.4, 156.9,
N-(BOC)(2S)-leu cyl-(2S,5R)-5-ter t-bu tylp r olin e a llyl es-
ter (5d ) was obtained in 82% yield as an oil: [R]20 -91.8° (c
D
1, CHCl3); 1H NMR (CDCl3) δ 0.89-1.01 (s, 15 H), 1.41 (m, 10
H), 1.70-1.93 (m, 3 H), 2.04 (m, 1 H), 2.30-2.35 (m, 2 H), 4.19
(d, 1 H, J ) 8.6), 4.38 (m, 1 H), 4.63-4.74 (m, 4 H), 5.21-5.35
(m, 2 H), 5.92 (m, 1 H); 13C NMR (CDCl3) δ 21.5 (21.7), (25.8)
27.3, 27.4 (29.1), (23.4) 27.6, (24.2) 28.0, (24.3) 28.1, 35.6 (36.1),
40.5 (43.3), 49.4 (50.4), (59.9) 60.4, (66.4) 65.2, 66.5 (66.6),
(78.8) 79.3, 118.0 (118.9), (131.5) 131.8, (154.7) 155.2, (171.3)
171.1 (172.0), 175.0; HRMS calcd for
384.2862, found 384.2872.
C
20H38O4N3 (MH+)
Gen er a l P r oced u r e for Allyl Ester Rem ova l. A solution
of allyl ester 5 (1.0 g, 2.18 mmol) in THF (22 mL) was treated
with morpholine (1.9 mL, 21.8 mmol) and Pd(PPh3)4 (25 mg,
0.2 mmol), stirred for 3 h at room temperature, and evapo-
rated. The residue was dissolved in CH2Cl2 (40 mL), washed
with 0.1 M HCl (2 × 15 mL) and brine (2 × 15 mL), dried,
and evaporated to a residue that was purified by chromatog-
raphy on silica gel using 5% MeOH in CHCl3 as eluant.
Evaporation of the collected fractions afforded N-(BOC)-
dipeptide 6.
171.5, (173.9) 174.6; HRMS calcd for
425.3015, found 425.3023.
C
23H41O5N2 (MH+)
N-(BOC)-(2S)-va lyl-(2S,5R)-5-ter t-bu tylp r olin e a llyl es-
ter (5e) was obtained in 43% yield as an oil: [R]20 -72.5° (c
D
0.88, CHCl3); 1H NMR (CDCl3) δ 0.89-1.01 (m, 15 H), 1.41 (s,
9 H), 1.75-2.32 (m, 5 H), 4.20 (m, 1 H), 4.42 (m, 1 H), 4.55-
4.75 (m, 3 H), 5.33 (m, 2 H), 5.89 (m, 1 H); 13C NMR (CDCl3)
δ 17.5 (17.9), 19.4 (19.8), 26.0 (28.9), 27.4 (27.5), (27.7) 28.1,
28.2, (31.1) 32.5, (35.6) 36.2, 56.5 (56.6), 60.0 (60.1), (65.4) 66.4,
(66.2) 66.5, 78.9 (79.6), 118.3 (118.9), (131.6) 131.8, 155.0
(155.3), 171.4 (171.7), 173.2 (173.7); HRMS calcd for C22H39O5N2
(MH+) 411.2859, found 411.2852.
N-(BOC)-(2S)-leu cyl-(2S,5R)-5-ter t-bu tylpr olin e (6d) was
isolated in 79% yield as a white solid: mp 152-153 °C; [R]20
D
1
-170.4° (c 0.6, CHCl3); H NMR (CDCl3) δ 0.86-0.91 (m, 15
H), 1.36 (s, 9 H), 1.42 (m, 1 H), 1.66-1.86 (m, 3 H), 2.01 (m, 1
H), 2.34 (m, 2 H), [4.19 (m, 0.3 H)] 4.44 (m, 1.2 H), 4.62 (m, 1
H), 5.21 (d, 0.6 H, J ) 9.5) [5.68 (d, 0.4 H, J ) 8.9)]; 13C NMR

3320 J . Org. Chem., Vol. 64, No. 9, 1999
Halab and Lubell
(CDCl3) δ 21.4 (21.6), 23.4, 24.1 (24.3), (25.6) 26.0, 26.5 (29.0),
(27.3) 27.5, 28.1, 35.3 (35.9), 40.1 (41.9), 49.5 (51.1), (60.2) 61.4,
(66.9) 67.4, 79.6 (80.0), 155.3 (156.0), 172.8 (173.0), (174.5)
176.8; HRMS calcd for C20H37O5N2 (MH+) 385.2702, found
385.2710.
62.0, 67.4 (68.0), (169.8) 171.5, 171.7 (171.9), 175.4 (176.3);
HRMS calcd for C15H28O3N3 (MH+) 298.2131, found 298.2120.
N-Acetyl-(2S)-m eth ion yl-(2S,5R)-5-ter t-bu tylpr olin e N′-
m eth yla m id e (1c) was isolated in 99% yield as a white
solid: mp 53-56 °C; [R]20 -71.7° (c 0.4, CHCl3); 1H NMR
D
N-(BOC)-(2S)-p h e n yla la n yl-(2S,5R)-5-ter t-b u t ylp r o-
lin e (6f) was isolated in 64% yield as a white solid: mp 149-
(CDCl3) δ 0.85 (s, 6.6 H) [0.93 (s, 2.4 H)], 1.82 (m, 2 H), 1.96
(m, 4 H), 2.07 (s, 5 H), 2.53 (m, 3 H), [2.76 (d, 0.8 H, J ) 4.6)]
2.83 (d, 2.2 H, J ) 4.6), 4.23 (dd, 1 H, J ) 5.3, 8.2), 4.48 (m, 2
H), 7.72 (d, 1 H, J ) 6.2), 8.41 (br s, 1 H); 13C NMR (CDCl3) δ
(15.5) 15.6, 22.2 (22.5), 25.1 (25.5), (26.2) 26.3, 27.2, (27.4) 28.4,
30.3, 30.5, (35.1) 35.5, (49.7) 52.1, (61.6) 61.7, 67.4 (67.8),
(170.1) 171.0, (171.8) 172.2, 174.5 (175.2); HRMS calcd for
151 °C; [R]20 -66.7° (c 0.88, CHCl3); 1H NMR (CDCl3) δ 0.90
D
(s, 6.3 H) [1.03 (s, 2.7 H)], [1.29 (s, 2.7 H)] 1.39 (s, 6.3 H), 1.48
(m, 1 H), 1.76-1.96 (m, 3 H), 2.87 (m, 2 H), 3.75 (t, 1 H, J )
8.7), 4.06 (d, 1 H, J ) 8.5), 4.35 (d, 1 H, J ) 7.5), 5.60 (m, 1
H), 7.24 (m, 5 H); 13C NMR (CDCl3) δ 26.7 (28.0), 28.3, (28.6)
28.8, (28.5) 30.2, (36.7) 37.2, (38.7) 41.0, (54.2) 55.3, 62.8 (63.8),
68.0 (68.3), (80.4) 80.9, (127.5) 128.0, (129.3) 129.7, (130.5)
130.6, 138.0 (138.9), 157.1 (157.4), 174.5 (176.0), 179.5.
Gen er a l P r oced u r e for Am id a tion of th e Dip ep tid es.
A solution of N-(BOC)dipeptide 6 (104 mg, 0.26 mmol) in CH3-
CN (2.6 mL) was treated with DIEA (181 µL, 1.0 mmol),
methylamine hydrochloride (21 mg, 0.31 mmol), and TBTU
(100 mg, 0.31 mmol), stirred at room temperature for 18 h,
and partitioned between brine (2 mL) and EtOAc (10 mL). The
organic phase was washed with 0.1 M HCl (2 × 3 mL), 5%
NaHCO3 (2 × 3 mL), and brine (4 mL), dried, and evaporated
to yield the N-(BOC)dipeptide methylamide 7.
C
17H32O3N3 (MH+) 358.2131, found 358.2152.
N-Acet yl-(2S)-leu cyl-(2S,5R)-5-ter t-b u t ylp r olin e N′-
m eth yla m id e (1d ) was isolated in 98% yield as a white
1
solid: mp 172-173 °C; [R]20 -88.2° (c 0.3, CHCl3); H NMR
D
(CDCl3) δ 0.88 (s, 7.7 H) [0.91 (s, 1.3 H)], 0.92 (d, 3 H, J )
4.0), 0.97 (d, 3 H, J ) 6.6), 1.29 (m, 1 H), 1.58-1.72 (m, 3 H),
1.85 (m, 1 H), [1.98 (s, 0.4 H)] 2.04 (s, 2.6 H), 2.17 (m, 1 H),
2.48 (m, 1 H), [2.79 (d, 0.4 H, J ) 4.6)] 2.87 (d, 2.6 H, J ) 4.6),
4.29 (m, 3 H), 5.96 (d, 1 H, J ) 6.9), 8.26 (br s, 1 H); 13C NMR
(CDCl3) δ 21.2 (21.4), 22.4 (22.7), 23.3 (23.5), (24.4) 24.8, 25.2,
26.3 (26.4), 27.3 (27.6), 28.7, (35.0) 35.6, 39.5 (40.7), (48.6) 51.5,
(61.5) 61.8, 67.3 (67.9), 171.2, (171.8) 172.0, 174.8; HRMS calcd
for C18H34O3N3 (MH+) 340.2600, found 340.2611.
N-(BOC)-(2S)-leu cyl-(2S,5R)-5-ter t-b u t ylp r olin e
N′-
m eth yla m id e (7d ) was isolated in 97% yield as a white
N-Acet yl-(2S)-va lyl-(2S,5R)-5-ter t-b u t ylp r olin e
N′-
solid: mp 152-153 °C; [R]20 -95.9° (c 0.75, CHCl3); 1H NMR
m eth yla m id e (1e) was isolated in 97% yield as a white
D
1
(CDCl3) δ 0.87-0.94 (m, 15 H), 1.25 (m, 1 H), 1.42 (s, 9 H),
1.72-1.84 (m, 4 H), 2.17 (m, 1 H), 2.44 (m, 1 H), [2.78 (d, 0.5
H, J ) 4.7) 2.83 (d, 2.5 H, J ) 4.6), 4.12 (m, 1 H), 4.28 (m, 2
H), 4.92 (d, 1 H, J ) 7.7), 8.32 (br s, 1 H); 13C NMR (CDCl3) δ
21.3 (21.5), 23.4 (23.5), (24.3) 24.5, (24.8) 25.2, 26.1 (26.2), 27.2
(27.5), 28.0, 29.0 (29.6), (35.0) 35.7, 40.0 (41.0), (49.5) 51.9,
(61.5) 61.7, 67.0 (67.5), (79.8) 80.4, (155.1) 157.1, 171.5 (172.0),
175.2 (176.2); HRMS calcd for C21H40O4N3 (MH+) 398.3019,
found 398.3031.
solid: mp 166-167 °C; [R]20 -93.8° (c 0.3, CHCl3); H NMR
D
(CDCl3) δ 0.86 (s, 8 H), 0.97 (m, 4 H), 1.05 (d, 3 H, J ) 6.7),
1.81-2.06 (m, 7 H), 2.59 (m, 1 H), [2.80 (d, 0.3 H, J ) 4.6)]
2.84 (d, 2.7 H, J ) 4.6), 4.18 (t, 1 H, J ) 7.5), 4.30 (t, 1 H, J
) 7.3), 4.50 (m, 1 H), 6.18 (d, 1 H, J ) 6.2), 8.48 (br s, 1 H);
13C NMR (CDCl3) δ (18.0) 18.7, 19.3 (19.9), 22.6, (25.0) 25.2,
26.2 (26.5), 27.2, 27.9, 30.5, (35.0) 35.5, (55.6) 58.1, (61.3) 61.8,
67.0 (67.2), (169.2) 170.9, 171.5 (171.8), 174.3 (175.1); HRMS
calcd for C17H32O3N3 (MH+) 326.2444, found 326.2455.
N-(BOC)-(2S)-p h e n yla la n yl-(2S,5R)-5-ter t-b u t ylp r o-
lin e N′-m eth yla m id e (7f) was isolated in 49% yield as a
N-Acet yl-(2S)-p h en yla la n yl-(2S,5R)-5-ter t-b u t ylp r o-
lin e N′-m eth yla m id e (1f) was isolated in 94% yield as a
white solid: mp 81-82 °C; [R]20D -32.2° (c 1, CHCl3); 1H NMR
(CDCl3) δ 0.83 (s, 9 H), 1.16 (m, 1 H), 1.51-1.67 (m, 2 H), [1.90
(s, 0.3 H)] 2.00 (s, 2.7 H), 2.13 (m, 1 H), 2.79 (d, 3 H, J ) 4.6),
2.97-3.09 (m, 2 H), 3.71 (dd, 1 H, J ) 4.5, 8.9), 4.25 (dd, 1 H,
J ) 4.9, 8.7), 4.57 (m, 1 H), 6.72 (d, 1 H, J ) 5.6), 7.20-7.35
(m, 5 H), 8.37 (br s, 1 H); 13C NMR (CDCl3) δ 22.6 (22.7), 24.9
(25.1), 26.2 (26.4), 27.2 (27.5), 27.8, 35.5, 38.1 (38.4), 54.0, (61.5)
61.7, 67.3 (67.9), (127.0) 127.6, (128.4) 129.1, 129.2 (129.3),
white solid: mp 51-52 °C; [R]20 -28.6° (c 0.5, CHCl3); 1H
D
NMR (CDCl3) δ 0.82 (s, 9 H), 1.15 (m, 1 H), [1.36 (s, 1.4 H)]
1.43 (s, 7.6 H), 1.59 (m, 2 H), 2.08 (m, 1 H), 2.76 (d, 3 H, J )
4.6), 2.92 (m, 2 H), 3.57 (dd, 1 H, J ) 4.4, 8.9), 4.25 (dd, 1 H,
J ) 5.2, 8.6), 4.37 (m, 1 H), 5.12 (d, 1 H, J ) 7.0), 7.17-7.33
(m, 5 H), 8.38 (br s, 1 H); 13C NMR (CDCl3) δ 25.0, 25.9, 27.1,
27.9, 28.0, 35.6, 38.4, 54.5, 61.6, 66.9, 80.7, 127.5, 129.0, 129.3,
135.2, 156.2, 171.3, 174.6; HRMS calcd for C24H38O4N3 (MH+)
432.2862, found 432.2871.
Gen er a l P r oced u r e for Aceta m id e Syn th esis. A solution
of N-(BOC)dipeptide N′-methylamide 7 (62 mg, 0.16 mmol) in
1:3 TFA:CH2Cl2 (1.6 mL) was stirred at room temperature for
2 h and evaporated on a rotary evaporator. The resulting
dipeptide N′-methylamide trifluoroacetate was dissolved in
CH2Cl2 (1.6 mL), treated with K2CO3 (0.22 mg, 1.6 mmol) and
Ac2O (148 µL, 1.6 mmol), stirred for 18 h, filtered, washed with
CH2Cl2 (3 × 3 mL), and evaporated to give the N-acetyl-
dipeptide N′-methylamide 1.
134.5, 171.2, 173.9; HRMS calcd for
374.2444, found 374.2449.
C
21H32O3N3 (MH+)
N-Acet yl-(2S)-a la n ylp r olin e N′-m et h yla m id e a n d N-
a cetyl-(2S)-leu cylp r olin e N′-m eth yla m id e were synthe-
sized in solution phase from N-(BOC)-L-proline N′-methyla-
mide using the TBTU coupling and acetylation conditions as
described above.
N-Acetyl-(2S)-a la n ylp r olin e N′-m eth yla m id e (2a ): mp
176-177 °C; [R]20 -145.0° (c 0.5, CHCl3); 1H NMR (CDCl3) δ
D
[1.31 (d, 0.6 H, J ) 7.0)], 1.36 (d, 2.4 H, J ) 6.9), [1.78 (m, 0.2
H)] 1.92 (m, 0.8 H), 2.00 (s, 2.4 H) [2.01 (s, 0.6 H)], 2.03 (m, 1
H), [2.09 (m, 0.2 H)] 2.14 (m, 0.8 H), 2.34 (m, 0.8 H) [2.55 (m,
0.2 H)], 2.79 (d, 2.4 H, J ) 3.9) [2.85 (d, 0.7 H, J ) 3.8)], 3.57
(m, 1 H), 3.68 (m, 1 H), [4.31 (m, 0.2 H, J ) 8.2)] 4.53 (dd, 0.8
H, J ) 2.9, 8.1), [4.25 (m, 0.2 H)] 4.76 (m, 0.8 H), 6.53 (d, 0.8
H, J ) 6.3) [6.65 (br s, 0.2 H)], 6.72 (br s, 0.8 H) [7.58 (br s,
0.2 H)]; 13C NMR (CDCl3) δ (16.3) 17.8, (22.3) 22.8, (21.8) 24.8,
26.0 (26.4), 27.7 (31.3), 46.5 (48.0), (46.7) 47.2, 59.8 (60.7), 169.5
(170.9), (171.1) 171.5, (172.0) 172.4; HRMS calcd for C11H20O3N3
(MH+) 242.1505, found 242.1498.
N-Acet yl-glycyl-(2S,5R)-5-ter t-b u t ylp r olin e N′-m et h -
yla m id e (1a ) was isolated in 93% yield as a white solid: mp
71-73 °C; [R]20 -99.5° (c 0.46, CHCl3); mixture of 1:1
D
1
rotamers H NMR (CDCl3) δ 0.93 (s, 9 H), 1.27 (m, 1 H), 1.64
(m, 2 H), 1.85 (m, 1 H), 2.05 (s, 3 H), 2.82 (m, 3 H), 3.74 (d, 1
H, J ) 9.5), 4.20 (m, 2 H), 4.66 (m, 1 H), 6.43 (br s, 1 H), 7.14
(br s, 1 H); 13C NMR (CDCl3) δ 13.9, 22.5, 22.7, 25.0, 25.7, 26.2,
26.5, 27.4, 29.5, 30.2, 31.4, 35.6, 36.0, 42.2, 42.4, 61.7, 61.9,
67.4, 67.7, 170.2, 170.9, 171.1, 171.7, 171.9; HRMS calcd for
C
14H26O3N3 (MH+) 284.1974, found 284.1968.
N-Acet yl-(2S)-a la n yl-(2S,5R)-5-ter t-b u t ylp r olin e N′-
N-Acetyl-(2S)-leu cylp r olin e N′-m eth yla m id e (2b): mp
90-91 °C; [R]20D -130.3° (c 1, CHCl3); 1H NMR (CDCl3) δ 0.94
(m, 6 H), 1.53 (m, 1 H), 1.67 (m, 1 H), 1.88 (m, 3 H), 2.00 (s,
2.4 H) [2.02 (s, 0.6 H)], 2.15 (m, 1 H), 2.35 (m, 1 H), 2.77 (d,
2.4 H, J ) 4.8) [2.85 (d, 0.6 H, J ) 4.7)], 3.56 (m, 1 H), 3.76
(m, 1 H), 4.50 (dd, 1 H, J ) 2.7, 8.1), 4.81 (m, 1 H), 6.18 (d, 0.8
H, J ) 8.4) [6.29 (d, 0.2 H, J ) 6.1)], 6.72 (br s, 0.8 H) [7.52
(br s, 0.2 H)]; 13C NMR (CDCl3) δ (21.1) 21.6, (22.3) 22.8, 23.2
(23.3), 24.6 (24.7), (21.8) 24.8, 26.0 (26.5), 27.4 (31.2), 39.8
m eth yla m id e (1b) was isolated in 97% yield as a white
solid: mp 119-121 °C; [R]20 -60.3° (c 0.36, CHCl3); 1H NMR
D
(CDCl3) δ 0.89 (s, 7.5 H) [0.93 (s, 1.5 H)], 1.31 (d, 2.5 H, J )
6.9) [1.39 (d, 0.5 H, J ) 6.6)], 1.85 (m, 2 H), [1.97 (s, 0.5 H)]
2.01 (s, 2.5 H), 2.22 (m, 1 H), 2.45 (m, 1 H), [2.80 (d, 0.5 H, J
) 4.7)] 2.86 (d, 2.5 H, J ) 4.6), 4.32 (m, 3 H), 6.54 (d, 1 H, J
) 6.1), 8.35 (br s, 1 H); 13C NMR (CDCl3) δ (15.8) 16.0, 22.1
(22.6), (24.5) 25.2, 26.2 (26.4), 27.3 (27.5), 28.9, 35.6, 48.5, (61.6)

Synthesis of â-Turn Mimics with 5-tert-Butylproline
J . Org. Chem., Vol. 64, No. 9, 1999 3321
(41.4), (46.7) 47.2, 49.0 (50.9), 59.6 (60.8), 170.0 (170.6), 171.5
(171.6), (172.0) 172.8; HRMS calcd for C14H26O3N3 (MH+)
284.1974, found 284.1966.
sistance. The crystal structure analysis of compound 1d
was performed by Francine Be´langer-Garie´py at l′Uni-
versite´ de Montre´al X-ray facility.
Su p p or t in g In for m a t ion Ava ila b le: 1H and 13C NMR
spectra of 1a -f, 2b, and 2d ; COSY and NOESY spectra and
plots of temperature versus amide N-H chemical shift for 1b
and 2b, and crystallographic data for 1d . This material is
availabl free of charge via the Internet at http://pubs.acs.org.
Ack n ow led gm en t. This research was supported in
part by the Natural Sciences and Engineering Research
Council of Canada, the Ministe`re de l’EÄ ducation du
Que´bec, and the DuPont Educational Aid Program. We
thank Sylvie Bilodeau and Dr. M. T. Phan Viet of the
Regional High-Field NMR Laboratory for their as-
J O990294A