Systematic study of the synthesis of macrocyclic dipeptide β-turn mimics possessing 8-, 9-, and 10- membered rings by ring-closing metathesis

Article abstract of DOI:10.1021/jo0477648

A systematic study was performed to establish general synthesis protocols for forming enantiomerically pure macrocyclic dipeptide lactams. Focusing on macrocycles of 8-, 9-, and 10-membered rings, effective syntheses were achieved by a sequence featuring

Full text of DOI:10.1021/jo0477648

Systematic Study of the Synthesis of Macrocyclic Dipeptide
â-Turn Mimics Possessing 8-, 9-, and 10- Membered Rings by
Ring-Closing Metathesis
Ramesh Kaul, Simon Surprenant, and William D. Lubell*
De´partement de Chimie, Universite´ de Montre´al, C.P. 6128, Succursale Centre Ville,
Montre´al, Que´bec, Canada H3C 3J7
lubell@chimie.umontreal.ca
Received December 21, 2004
A systematic study was performed to establish general synthesis protocols for forming enantio-
merically pure macrocyclic dipeptide lactams. Focusing on macrocycles of 8-, 9-, and 10-membered
rings, effective syntheses were achieved by a sequence featuring peptide coupling of allyl- and
homoallyl-glycine building blocks followed by ring-closing metathesis. The 8-membered lactam-
possessing cis-amide and cis-olefin geometry as well as 9- and 10-membered lactams having trans-
amide and trans-olefin configurations were effectively prepared by a general strategy employing
the respective protected dipeptide, the first generation Grubbs’ catalyst, and temporary protection
of the central amide as a benzyl derivative.
Introduction
LHRH.6 Mimics of â-turns are thus desirable tools for
studying the structure-activity relationships responsible
for protein and peptide biology.7
â-turns in peptides and proteins are of interest because
of their structure and biological activity.1 Polar in nature,
these secondary structures generally occupy the surfaces
of protein molecules, where they are involved in recogni-
tion and binding.2 â-turns have also been shown to be
important for receptor affinity in biologically active
peptides, such as somatostatin,3 MSH,4 bradykinin,5 and
Strategies to design â-turn mimics have often con-
strained the peptide backbone dihedral angles. For
example, fused bicyclic systems, such as azabicyclo[X,Y,0]-
alkanone amino acids 1-4, of varying ring sizes have
been used as rigid dipeptide surrogates7 that structurally
constrain three contiguous dihedral angles, φ, ψ, and ω,
of a â-turn segment within the body of the heterocycle
(Figure 1). Conformational analysis of bicyclic lactams
1-4 has demonstrated their propensity to favor type II
and II′ â-turns8,9 contingent on their stereochemistry, as
evident from X-ray diffractometry, IR, and NMR spec-
troscopy as well as theoretical calculations.10 Because
differences in the type II turn dihedral angle have been
(1) (a) Venkatachalam, C. M. Biopolymers 1968, 6, 1425. (b) Suat,
K. K.; Seetharama, D. S. J. Curr. Pharm. Des. 2003, 9, 1209.
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(b) Lewis, P. N.; Momany, F. A.; Scheraga, H. A. Proc. Natl. Acad.
Sci. U.S.A. 1971, 68, 2293.
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Leahy, E. M.; Sprengler, P. A.; Furst, G.; Smith, A. B.; Strader, C. D.;
Casieri, M. A.; Candelore, M. R.; Donaldson, G.; Maechler, L. J. Am.
Chem. Soc. 1992, 114, 9217. (b) Veber, D. F.; Freidinger, R. M.; Perlow,
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Arison, B. H.; Homnick, C.; Randall, W. C.; Glitzer, M. S.; Saperstein,
R.; Hirschmann, R. Nature 1981, 292, 55.
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Biochem. 1998, 51, 251. (b) Zhang, J.; Xiong, C.; Ying, J.; Wang, W.;
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Saperstein, R. Science 1980, 210, 656.
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Lubell, W. D. Tetrahedron 1997, 53, 12789. (b) Cluzeau, J.; Lubell, W.
D. Biopolymers (Peptide Sci.) 2005, in press.
(5) (a) Miskolzie, M.; Yamamoto, H.; York, E. J.; Stewart, J. M.;
Kotovych, G. J. Biomol. Struct. Dyn. 2000, 17, 947.(b) Sejbal, J.; Cann,
J. R.; Stewart, J. M.; Gera, L.; Kotovych, G. J. Med. Chem. 1996, 39,
1281.
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2000, 55, 101.
(9) Gillespie, P.; Cicariello, J.; Olson, G. L. Biopolymers (Peptide Sci.)
1997, 43, 191.
10.1021/jo0477648 CCC: $30.25 © 2005 American Chemical Society
Published on Web 03/22/2005
3838
J. Org. Chem. 2005, 70, 3838-3844

Synthesis of Macrocyclic Dipeptide â-Turn Mimics
spectroscopy and the X-ray crystal structure of (S,S)-5.14
Although the type VI â-turn did not possess proline, the
eight-membered macrocycle forced the amide between the
i + 1 and i + 2 residues to adopt the cis-isomer geometry,
as illustrated by X-ray structural analysis, which also
revealed a cis-olefin in a folded twist-boat-boat conforma-
tion.14 The saturated analogue of eight-membered lactam
(R,R)-5 possessed a cis-amide geometry and type VIb
â-turn conformation; however, dimerization took place
over time in solution and in the solid state, as revealed
by NMR dilution experiments and X-ray crystallographic
studies.15,16 Their cis-amide geometry and their propen-
sity to favor type VI â-turns makes eight-membered
lactams (5) structurally comparable to other type VI
â-turn mimics, such as 5-tert-butylproline-containing
peptides.17
Among the few syntheses of nine-membered macrocyle
dipeptide lactams, an Ugi-multicomponent reaction fol-
lowed by ring-closing metathesis has been used to
prepare unsaturated nine-membered lactam 6 (R1 ) CH3-
CO, R2 ) PhCH2, X ) OEt), albeit as a racemic isomeric
mixture.18 The ring geometry and conformational prefer-
ences of nine-membered macrocyclic lactams such as 6-8
have, however, yet to be characterized.
The 10-membered macrocycle dipeptide lactam (9, R
) Boc, X ) OMe) has also been synthesized by ring-
closing metathesis and shown to possess a trans-olefin
geometry and φ, ψ, and ω torsional angles similar to that
of an ideal type I â-turn, as demonstrated by X-ray and
NMR analyses.19
With the precedent that 8- and 10-membered macro-
cyclic peptide lactams adopted type VI and type I â-turns,
respectively, and with potential for their nine-membered
counterparts to serve similarly as turn mimics, we sought
to develop a general means for constructing the set of
heterocyclic dipeptides (5 and 7-9, R ) Boc or Fmoc, X
) OH). We envisioned that this set will serve in studies
of biologically active peptides as well as intermediates
for trans-annular cyclizations to prepare azabicyclo-
[X,Y,0]alkanone amino acids related to 1-4.
Medium-sized macrocyclic lactams of 8-, 9-, and 10-
members have been traditionally harder to form than
their smaller and larger-sized counterparts;13 however,
ring-closing metathesis has significantly advanced their
synthesis. For example, N-alkyl lactams of 6-10 mem-
bers have been prepared by RCM.20 Transient N-alkyla-
tion of the central amide with a 2,4-dimethoxy benzyl
group (Dmb) was also shown to be essential to favor a
cis-amide geometry and facilitate metathesis in the
synthesis of eight-membered lactams (5).14,15 Moreover,
FIGURE 1. Representative â-turn structure as well as bicyclic
and macrocyclic constrained dipeptide â-turn mimics.
observed after variation of the ring sizes in these bicyclic
mimics, application of sets of related azabicycloalkane
amino acids has thus proven effective for studying the
effect of turn geometry on the biological activity of native
peptides.11,12
Alternative scaffolds are needed to cover a wider range
of turn geometries in order to better mimic natural
diversity and to enhance success in peptide mimicry.
Macrocyclic dipeptide lactams of 8-, 9-, and 10-members
have been less well investigated relative to their bicyclic
cousins, in part due to the difficulties inherent in
synthesizing such medium-sized ring systems.13
The syntheses of eight-membered macrocycles, such as
5, have been accomplished by ring-closing metathesis.
Constrained dipeptides (S,S)-5 and (R,S)-5, (R ) Ac, X
) NHMe) were shown to adopt conformations similar to
an ideal type VIa â-turn, as demonstrated by NMR
(14) Creighton, C. J.; Leo, G. C.; Du, Y.; Reitz, A. B. Bioorg. Med.
Chem. 2004, 12, 4375.
(15) (a) Derrer, S.; Davies, J. E.; Holmes, A. D. J. Chem. Soc., Perkin
Trans 1 2000, 2943. (b) Creighton, C. J.; Reitz, A. B. Org. Lett. 2001,
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(b) Halab, L.; Lubell, W. D. J. Org. Chem. 1999, 64, 3312.
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C. Eur. J. Org. Chem. 1999, 389. (b) Belvisi, L.; Gennari, C.; Madder,
A.; Mielgo, A.; Potenza, D.; Scolastico, C. Eur. J. Org. Chem. 2000,
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C. J. Org. Chem. 2000, 65, 2563.
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L.; Simonin, F.; Lubell, W. D. J. Med. Chem. 2002, 45, 5353.
(12) Belvisi, L.; Bernardi, A.; Checchia, A.; Manzoni, L.; Potenza,
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J. Org. Chem, Vol. 70, No. 10, 2005 3839

Kaul et al.
SCHEME 1. General Scheme for the Synthesis of
Macrocyclic Dipeptides
SCHEME 2. Synthesis of Allylglycine Components
13 and 14
RCM has been used effectively to make larger ring
systems that have constrained peptide conformations.21
Considering these results and the need for material
on a suitable scale and in a protected form for peptide
synthesis, we undertook a detailed study to prepare
macrocyclic dipeptide lactams of 8-10 members by a
general strategy featuring RCM (Scheme 1).
SCHEME 3. Synthesis of Homoallylglycine
Components 16 and 17
Results and Discussion
Dipeptide Precursor Synthesis. The construction
of 8-, 9-, and 10-membered ring sizes necessitated the
synthesis and coupling of allylglycine and homoallylgly-
cine units prior to RCM. Allylglycine had been synthe-
sized by a variety of methods;22 however, our expertise
with R-tert-butyl N-(PhF)aspartate â-aldehyde (10, PhF
) 9-phenylfluren-9-yl)23 served as motivation to employ
this chiral educt in the synthesis of allylglycine (Scheme
2). Aldehyde 10 was synthesized on a 7-g scale from
23b,24
aspartic acid
in five steps and 47% overall yield.
N-(PhF)Allylglycine tert-butyl ester 11 was obtained in
95% yield from the Wittig reaction of aldehyde 10 and
the ylide generated from treatment of methyltriph-
enylphosphonium bromide with KHMDS in THF.25 Al-
lylglycine hydrochloride 12 was quantitatively prepared
from the treatment of 11 with TFA, which cleaved both
the tert-butyl ester and PhF groups, followed by conver-
sion of the trifluoroacetate salt to the hydrochloride using
aq HCl and lyophilization. N-(Boc)- and N-(Fmoc)allylg-
lycines 13a and 13b were then prepared by the protection
of 12 with (Boc)2O26a and FmocOSu,26b respectively, under
basic conditions. Allylglycine methyl ester hydrochloride
14 was quantitatively obtained by the treatment of 12
with methanol and thionyl chloride. The higher N-(Boc)-
and N-(Fmoc)-protected homologues (homoallylglycines
16a and 16b) were synthesized respectively in five steps
from serine using reported procedures (Scheme 3).27
Briefly, serine was converted to its methyl ester hydro-
chloride, which was protected using Boc2O or FmocOSu
and transformed to N-(Boc)- or N-(Fmoc)iodoalanine
methyl ester using iodine, PPh3, and imidazole.28 The
iodide was then converted to a zincate and coupled to
allyl chloride
(21) (a) Trinka, T. M.; Grubbs, R. H. Acc. Chem. Res. 2001, 34, 18.
(b) Grubbs, R. H.; Blackwell, H. E. Angew. Chem., Int. Ed. 1998, 37,
3281. (c) Ripka, A. S.; Bohacek, R. S.; Rich, D. H. Bioorg. Med. Chem.
Lett. 1998, 8, 357. (d) Stymiest, J. L.; Mitchell, B. F.; Wong, S.; Vederas,
J. C. Org. Lett. 2003, 5, 47. (e) Reichwein, J. F.; Versluis, C.; Liskamp,
R. M. J. J. Org. Chem. 2000, 65, 6187. (f) Schafmeister, C. E.; Po, J.;
Verdine, G. L. J. Am. Chem. Soc. 2000, 122, 5891.
(22) (a) Xu, P.-F.; Lu, T.-J. J. Org. Chem. 2003, 68, 658. (b) Na´jera,
G. G. A. C. J. Org. Chem. 2000, 65, 7310. (c) Myers, A. G.; Gleason, J.
L. Org. Synth. 1999, 76, 57. (d) Yeh, T.-L.; Liao, C.-C.; Uang, B.-J.
Tetrahedron 1997, 53, 11141. (e) Katagiri, N.; Okada, M.; Morishita,
Y.; Kaneko, C. Tetrahedron 1997, 53, 5725. (f) Hanessian, S.; Yang,
R.-Y. Tetrahedron Lett. 1996, 37, 8997. (g) Workman, J. A.; Garrido,
N. P.; Sancon, J.; Roberts, E.; Wessel, H. P.; Sweeney, J. B. J. Am.
Chem. Soc. 2005, 127, 1066.
(23) (a) Gosselin, F.; Lubell, W. D. J. Org. Chem. 1998, 63, 7463.
(b) Swarbrick, M. E.; Gosselin, F.; Lubell, W. D. J. Org. Chem. 1999,
64, 1993.
(24) Gmeiner, P.; Feldman, P. L.; Chu-Moyer, M. Y.; Rapoport, H.
J. Org. Chem. 1990, 55, 3068.
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Lett. 2001, 42, 3319.
(26) (a) Tarbell, D. S.; Yamamoto, Y.; Pope, B. M. Proc. Natl. Acad.
Sci., U.S.A. 1972, 69, 730. (b) Paquet, A. Can. J. Chem. 1982, 60, 976.
(27) Rodr´ıguez, A.; Miller, D. D.; Jackson, R. F. W. Org. Biomol.
Chem. 2003, 1, 973.
(28) Trost, B. M.; Rudd, M. T. Org. Lett. 2003, 5, 4599.
3840 J. Org. Chem., Vol. 70, No. 10, 2005

Synthesis of Macrocyclic Dipeptide â-Turn Mimics
SCHEME 4. Synthesis of N-Benzyl Components 18
and 19
2,4-dimethoxybenzyl tertiary amide bond in yields vary-
ing between 71% and 86%. Changing from Boc protection
to Fmoc did not affect coupling, and dipeptides 21c and
22c were obtained in 85% and 82% yields, respectively.
The use of HATU could be avoided by employing N-
(Hmb)homoallylglycine methyl ester 19b and the sym-
metrical anhydride generated from N-(Boc)homoallylg-
lycine 16a, respectively, in situ to furnish 23c in 86%
yields. Attempts to apply the symmetrical anhydride
conditions using N-(Hnb)allylglycine 18b gave 22d in
87% yield; however, this route was not pursued further
because of the difficulties in making the requisite 2-hy-
droxy-6-nitrobenzaldhyde.33
Ring-Closing Metathesis. The importance of the rate
of amide bond isomerization on the ring-closing metath-
esis was realized in the synthesis of nine-membered
macrocyles, which could not be formed without a tertiary
amide. When cyclization by olefin metathesis was exam-
ined on dipeptides bearing no benzyl group at the amide
nitrogen, secondary amides 21a and 22a failed to cyclize
using both the first and second generation Grubbs’
catalyst in a variety of solvents at reflux and room
temperature. Dipeptide 21a also did not cyclize to a nine-
membered ring in the presence of Ti(O-iPr)4, which has
been used previously to prevent the formation of a cyclic
chelate between the catalyst and an amide.34 As re-
ported,19 10-membered macrocyclic dipeptide 27a was
formed on cyclization of dipeptide 23a using the first
generation of Grubbs’ catalyst at high dilution (0.7 mM).
Macrocycles of eight and nine members were synthe-
sized from dipeptides bearing only a benzyl group on the
nitrogen of the amide. Dmb-analogues 20b, 21b and c,
22b and c, and 23b reacted with the first generation
Grubbs’ catalyst to provide 8-10 membered macrocycles
24a, 25b and c, 26b and c, and 27b, respectively, in 71-
81% yields (Table 2). The 10-membered macrocyle (27b)
was formed in 87% yield from tertiary amide 23b
employing the same conditions used to cyclize secondary
amide 23a at a concentration four and a half times
higher. The phenol of the Hmb group was also tolerated
by the catalyst, and 10-membered macrocyle 27c was
prepared in 86% yield. The tertiary amide facilitated
amide bond rotation and favored conformers in which the
ring-closing metathesis was possible. In the smaller-ring
cases, without the benzyl group, the cis-amide isomer was
disfavored energetically, and the barrier for isomerization
was higher such that macrocylization was inhibited.
using CuBr‚DMS. N-(Boc)amino ester 15a was hydro-
lyzed with LiOH and deprotected with 50% TFA in CH2-
Cl2 to provide N-(Boc)homoallylglycine 16a and homoal-
lylglycine methyl ester trifluoroacetate 17, respectively,
as components for peptide coupling. N-(Fmoc)Homoal-
lylglycine 16b was obtained by hydrolysis of N-(Fmoc)-
amino ester 15b using 0.8 M calcium chloride and 0.5 M
sodium hydroxide in iPrOH/H2O (7:3).29
Considering that rotation around the amide bound
would influence the rate of RCM of the dipeptides, we
synthesized an array of secondary benzyl amino esters
to prepare tertiary amides having a lower barrier for
isomerization about the amide bond (Scheme 4). This
hypothesis was supported by earlier studies of the
synthesis of eight-membered lactams that demonstrated
the utility of the acid labile 2,4-dimethoxybenzyl (Dmb)
group in the RCM step.15b The related 2-hydroxy-4-
methoxybenzyl (Hmb) group has been used to surmount
difficult couplings in solid-phase peptide synthesis,30 and
we envisioned that secondary amino esters bearing this
group could have better reactivity than their Dmb
cousins. Similary, the 2-hydroxy-6-nitrobenzyl (Hnb)
group was considered as a photocleavable protection
reported to favor coupling by O to N acyl transfer in a
similar way as the Hmb auxiliary.31 Reductive amina-
tions of imine prepared with the respective benzaldehyde
and allyl and homoallylglycine methyl esters (14 and 17)
were performed using NaBH(OAc)3 to furnish Dmb-,
Hmb-, and Hnb-protected amino esters 18 and 19 in
yields varying between 52% and 79%.
Coupling of the Fragments. The set of dipeptides
21a-23a was assembled first by combining allylglycine
and homoallylglycine methyl ester salts 14 and 17 as
amine components and N-(Boc)allyl and N-(Boc)homoal-
lylglycines 13a and 16a as acid components using TBTU
as the coupling agent in 85-87% yields (Table 1).
Coupling of the Dmb secondary amino esters proved more
difficult, and many coupling conditions were tried;32
however, only HATU as the coupling agent with N-
ethylmorpholine as the base was found to be effective for
making N-Boc-protected dipeptides 20b-23b bearing a
The Dmb and Hmb groups were reported to be easily
cleaved under acidic conditions.30,35 However, N-Boc-
protected-9- and 10-membered lactams 25b, 26b, and
(32) Couplings were unsuccessful using (a) TBTU/DIEA/DCM,
Knorr, R.; Trzeciak, A.; Bannwarth, W.; Gillessen, D. Tetrahedron Lett.
1989, 30, 1927. (b) BOPCl/DIEA/CH2Cl2, Castro, B.; Dormoy, J.-R.;
Evin, G.; Selve, C. Tetrahedron Lett. 1975, 1219. (c) HBTU/DIEA/CH2-
Cl2, Dourtoglou, C.; Ziegler, J. C.; Gross, B. Tetrahedron Lett. 1978,
1269. (d) DCC/HOBT/DIEA/CH2Cl2, Sheehan, J. C.; Hess, G. P. J. Am.
Chem. Soc. 1955, 77, 1067; Konig, W.; Geiger, R. Chem. Ber. 1970,
103, 788.
(33) Harayama, T.; Nakatsuka, K.; Nishioka, H.; Murakami, K.;
Ohmori, Y.; Takeuchi, Y.; Ishii, H.; Kenmotsu, K. Heterocycles 1994,
38, 2729.
(34) Fu¨rstner, A.; Langemann, K. J. Am. Chem. Soc. 1997, 119,
9130.
(35) Schlessinger, R. H.; Bebernitz, G. R.; Lin, P.; Poss, A. J. J. Am.
Chem. Soc. 1985, 107, 1777.
(29) Pascal, R.; Sola, R. Tetrahedron Lett. 1998, 39, 5031.
(30) Johnson, T.; Quibell, M.; Sheppard, R. C. J. Pept. Sci. 1995, 1,
11.
(31) Miranda, L. P.; Meutermans, W. D. F.; Smythe, M. L.; Alewood,
P. F. J. Org. Chem. 2000, 65, 5460.
J. Org. Chem, Vol. 70, No. 10, 2005 3841

Kaul et al.
TABLE 1. Dipeptide Assembly
acid
m
P
amine
n
R
conditionsa
amide
yield (%)
13a
13a
13a
13b
16a
16a
16b
16a
16a
16a
16a
1
1
1
1
2
2
2
2
2
2
2
Boc
Boc
Boc
Fmoc
Boc
Boc
Fmoc
Boc
Boc
Boc
Boc
18a
17
19a
19a
14
18a
18a
18b
17
19a
19b
1
2
2
2
1
1
1
1
2
2
2
Dmb
H
Dmb
Dmb
H
Dmb
Dmb
Hnb
H
Dmb
Hmb
A
B
A
A
B
A
A
C
B
A
C
20b
21a
21b
21c
22a
22b
22c
22d
23a
23b
23c
71
86
86
85
85
81
82
87
87
74
86
a Condition A: HATU, N-ethylmorpholine, CH2Cl2, rt; condition B: TBTU, DIEA, CH2Cl2, rt; condition C: symmetric anhydride, benzylic
amino ester, CH2Cl2, rt.
TABLE 2. Macrocycle Formation
amide
P
m
n
R
macrocycle
yield (%)
20b
21a
21b
21c
22a
22b
22c
23a
23b
23c
Boc
1
1
1
1
2
2
2
2
2
2
1
2
2
2
1
1
1
2
2
2
Dmb
H
Dmb
Dmb
H
Dmb
Dmb
H
Dmb
Hmb
24a
25a
25b
25c
26a
26b
26c
27a
27b
27c
78
0
80
75
0
81
71
77
87
86
Boc
Boc
Fmoc
Boc
Boc
Fmoc
Boc
Boc
Boc
27b and c bearing Dmb and Hmb groups were recovered
as deprotected amines without the loss of the benzylic
tertiary amide after prolonged treatment with 50% TFA
in CH2Cl2. Alternatively, both the Boc and Dmb groups
were removed upon exposure of eight-membered lactam
24a to the same TFA in CH2Cl2 conditions as reported.15b
In contrast to the N-Boc-protected lactams, N-Fmoc-
protected lactams 25c and 26c were quantitatively
converted in 30 min to their secondary amide derivatives
28 and 29 by treatment with 50% TFA in CH2Cl2
(Scheme 5). This behavior confirms the hypothesis36 that
difficulties in cleaving Dmb groups are due to the
formation of a neighboring positively charged ammonium
ion upon deprotection of the Boc moiety, which inhibits
protonation of the amide and shuts down the reaction.
Conversion of N-Boc-protected macrocycles 25b and 26b
to N-Fmoc-derivatives 25c and 26c gave average yields
of 35-45%, illustrating an advantage in starting with
N-Fmoc-protected amino acids. Finally, esters 28 and 29
were hydrolyzed using 0.5 M NaOH and 0.8 M CaCl2 in
iPrOH/water (7:3) to obtain the protected amino acids
(30 and 31) suitable for incorporation into peptides by
solid-phase synthesis.29
SCHEME 5. Synthesis of Macrocycle
N-(Fmoc)-dipeptides 30 and 31
a Reagents and conditions: (a) 50% TFA/CH2Cl2; (b) FmocOSu,
Na2CO3, acetone/H2O; (c) 0.8 M CaCl2/0.5 M NaOH, i-PrOH/H2O.
The olefin geometry was determined based on the
vicinal coupling constant of the vinyl protons. In the case
of eight-membered lactam (S, S)-5, a coupling-constant
value of 10.4 Hz was observed and assigned to cis-olefin
(36) Dinsmore, C. J.; Bergman, J. M.; Bogusky, M. J.; Culberson, J.
C.; Hamilton, K. A.; Graham, S. L. Org. Lett. 2001, 3, 865.
3842 J. Org. Chem., Vol. 70, No. 10, 2005

Synthesis of Macrocyclic Dipeptide â-Turn Mimics
was completed, Et3SiH (7 mL, 43.8 mmol) was added to the
solution, which was stirred for 15 h. The volatiles were
evaporated, and the residue was dissolved in 3:1 hexanes/Et2O
(50 mL) and treated with 0.5 N HCl (25 mL). The phases were
separated, and the organic layer was extracted twice with 0.5
N HCl. The combined aqueous layers were lyophilized to give
amino acid hydrochloride 12 (2.50 g, 94% yield) as a white
solid: mp 205-207 °C; 1H NMR (CD3OD): δ 5.79 (m, 2H), 5.29
(m, 2H), 4.06 (dd, 1H, J ) 7.1, 5.1 Hz), 2.68 (m, 2H); 13C NMR
(CD3OD): δ 169.4, 130.1, 119.7, 51.6, 34.0. MS (ESI, m/z):
116.0 (MH)+.
geometry. In the case of 9- and 10-membered lactams
7-9, coupling-constant values ranged from 18.5 to 19.1
Hz and were assigned to trans-olefins.
The amide isomer geometry of 8- and 10-membered
lactams 5 and 9 were assigned cis and trans, respectively,
based on analogy to literature compounds 514 and 9,19
for which X-ray structures were obtained. The assign-
ment of the amide isomers of nine-membered lactams 30
and 31 was made based on 2D NMR experiments.
Initially, COSY spectra of 30 and 31 were performed in
pyridine-d5 and CDCl3, respectively, to assign the protons
for each compound. Subsequently, NOESY spectra were
performed to measure long-distance transfers of magne-
tization. In the NOESY spectrum of both 30 and 31, no
transfer of magnetization between the CRH protons was
observed using a variety of mixing times. Because an
amide cis isomer would be expected to exhibit strong
NOE between the neighboring CRH protons, the lack of
such a transfer of magnetization leads us to assign the
amide trans isomer geometry for 30 and 31.
(2S)-2-[(tert-Butoxycarbonyl)amino]pent-4-enoic Acid
(13a). A 1:1 dioxane/H2O solution (33 mL) containing hydro-
chloride 12 (750 mg, 4.95 mmol) was treated with NaOH (436
mg, 9.90 mmol). After the NaOH dissolved, Boc2O (1.30 g, 5.94
mmol) was added to the mixture in three portions over 10 min.
The mixture was stirred for 18 h. The dioxane was removed
by evaporation. The crude mixture was diluted with H2O and
acidified to pH 3-4 with a 10% KHSO4 solution. This aqueous
solution was extracted with Et2O (three times). The combined
organic layers were washed with brine, dried over MgSO4,
filtered, and concentrated to give N-(Boc)allylglycine 13a (1.03
g, 97% yield) as a thick clear oil: [R]20D +14.5° (c 1.28, CHCl3);
1
The H and 13C NMR were consistent with the literature.22c
Conclusions
HRMS calcd for C10H17NO4Na, 238.10498; found, 238.10538.
(2S)-2-[(9H-Fluoren-9-ylmethoxycarbonyl)amino]pent-
4-enoic Acid (13b). A 1:1 acetone/H2O solution (67 mL)
containing hydrochloride 12 (1.5 g, 10 mmol) was treated with
Na2CO3 (2.1 g, 20 mmol) and FmocOSu (3.3 g, 10 mmol). The
mixture was stirred for 18 h at room temperature and
concentrated. The crude mixture was diluted with H2O and
acidified to pH 3-4 with a 10% KHSO4 solution. This aqueous
solution was extracted with EtOAc (three times). The combined
organic layers were washed with brine, dried over MgSO4,
filtered, and concentrated to give N-(Fmoc)allylglycine 13b
A general methodology has been developed for synthe-
sizing enantiomerically pure 8-, 9-, and 10-membered
macrocyclic dipeptides. This methodology has provided
the first syntheses of nine-membered lactams 7 and 8,
as well as a more practical route for making 10-
membered lactam 9. The importance of transient N-
alkylation of the central amide was demonstrated to
facilitate ring-closing metathesis, such that higher con-
centrations (3.15 mM) could be used without evoking
dimer or polymer formation. As reported, the cis-olefin
geometry was found in eight-membered lactam 5. The
9- and 10-membered lactams (7-9) possessed trans-olefin
geometry. The amide bond geometry was shown to be cis
in the case of 5 and trans for macrocycles 7-9. A more
detailed investigation of the conformation of 7 and 8 in
peptide analogues is presently under study. In light of
the ability of these constrained dipeptide surrogates to
adopt conformations similar to natural â-turns, this
practical methodology should be of general utility for
research in peptide science and medicinal chemistry.
(2.36 g, 70% yield): mp 134-135 °C; [R]20 +10.6° (c 0.93,
D
1
CHCl3); H NMR (CD3OD): δ 7.68 (d, 2H, J ) 9.9), 7.59 (m,
2H), 7.32 (t, 2H, J ) 9.7), 7.23 (t, 2H, J ) 9.9), 5.64 (m, 1H),
5.09 (m, 3H), 4.25 (m, 2H), 4.12 (m, 2H), 2.57 (m, 1H), 2.44
(m, 1H); 13C NMR (CD3OD): δ 175.2, 158.5, 145.3, 145.1, 142.5,
134.7, 128.8, 128.1, 126.3, 120.8, 118.7, 68.0, 55.1, 37.1; HRMS
calcd for C20H19NO4Na, 360.12063; found, 360.12080.
Methyl (2S)-2-Aminopent-4-enoate Hydrochloride (14).
SOCl2 (0.55 mL, 19.8 mmol) was added dropwise to a solution
of allylglycine 12 (2.0 g, 13.2 mmol) in MeOH (22 mL) at 0 °C.
The mixture was allowed to warm to room temperature, stirred
overnight, and evaporated to give methyl ester 14 (2.16 g, 99%
yield) as a white solid: mp 91-92 °C; [R]20D +8.3° (c 1.07, CH3-
OH); 1H NMR (CD3OD): δ 5.76 (m, 1H), 5.28 (m, 2H), 4.15
(dd, 1H, J ) 6.8, 5.5), 3.83 (s, 3H), 2.69 (m, 2H); 13C NMR
(CD3OD): δ 168.8, 130.1, 120.1, 52.1, 52.0, 34.1; HRMS (MH)+
calcd for C6H11NO2, 130.08626; found, 130.08636.
(2S)-2-[(tert-Butoxycarbonyl)amino]hex-5-enoic Acid
(16a). A solution of N-(Boc)homoallylglycine methyl ester (15a,
400 mg, 1.64 mmol prepared according to ref 27) in 1:1 H2O/
dioxane (16 mL) was treated with LiOH‚H2O (103 mg, 2.47
mmol), stirred for 3 h, and evaporated to a residue that was
partitioned between H2O (20 mL) and EtOAc (20 mL). The
aqueous phase was acidified with 0.1 M HCl to pH 4 and
extracted twice with EtOAc (20 mL). The combined organic
extracts were washed with brine, dried over MgSO4, filtered,
and concentrated to afford acid 16a (0.364 g, 97% yield) as a
colorless oil: [R]20D -1.1° (c 1.31, CH3OH); 1H NMR (400 MHz,
CD3OD): δ 5.82 (m, 1H), 5.03 (m, 3H), 4.08 (m, 1H), 2.15 (m,
2H), 1.88 (m, 1H), 1.73 (m, 1H), 1.44 (s, 9H); 13C NMR (75
MHz, CD3OD): δ 175.2, 157.1, 137.4, 115.0, 79.4, 53.2, 31.1,
30.0, 27.7; HRMS calcd for C11H19NO4Na, 252.12063; found,
252.12089.
Experimental Section
tert-Butyl (2S)-2-[(N-(PhF)Amino]pent-4-enoate (11). A
rt solution of methyltriphenylphosphonium bromide (13.2 g,
37 mmol) in THF (103 mL) was treated with a 0.5 M solution
of KHMDS in toluene (67 mL, 33 mmol), stirred for 30 min,
and treated dropwise with aldehyde 10 (7.64 g, 18.5 mmol,
prepared according to ref 23) in THF (103 mL) over 10 min.
The reaction mixture was stirred for 1 h, quenched with 200
mL of a saturated solution of NH4Cl, and the phases were
separated. The aqueous layer was extracted twice with 150
mL of Et2O. The combined organic layers were washed with
brine, dried over MgSO4, filtered, and concentrated. Chroma-
tography (7% EtOAc/hexanes) afforded allylglycine 11 (7.22
g, 95% yield) as a clear oil: [R]20D +163.7° (c 1.0, CHCl3), lit.37
[R]20D 168.7° (c 1.0, CHCl3). The 1H and 13C NMR spectral data
were consistent with the literature.37
(2S)-2-Aminopent-4-enoic Acid Hydrochloride (12).
TFA (60 mL) was added dropwise to a solution of allylglycine
11 (7.22 g, 17.5 mmol) in CH2Cl2 (60 mL). Once the addition
General Procedure A: Reductive Aminations. Allyl-
glycine methyl ester hydrochloride 14 or homoallylglycine
methyl ester trifluoroacetate 17 (3 mmol) was treated with
20 mL of a saturated NaHCO3 solution and extracted with 3
(37) Kaul, R.; Broullette, Y.; Sajjadi, Z.; Hansford, K. A.; Lubell, W.
D. J. Org. Chem. 2004, 69, 6131.
J. Org. Chem, Vol. 70, No. 10, 2005 3843

Kaul et al.
× 25 mL of CH2Cl2. The combined organic layers were washed
with 20 mL of brine and dried over MgSO4, filtered, and
concentrated to a volume of 30 mL. The selected benzaldehyde
derivative (3.3 mmol) and NaBH(OAc)3 (4.5 mmol) were added
to the mixture, which was stirred for 18 h at room tempera-
ture, treated with 20 mL of saturated NaHCO3, and stirred
for 30 min. The aqueous layer was separated and washed with
3 × 20 mL of CH2Cl2. The combined organic layers were
washed with brine, dried over MgSO4, filtered, and concen-
General Procedure D: Removal of Dmb. A stirred
solution of Fmoc-protected dipeptide lactam (0.2 mmol) in CH2-
Cl2 (8 mL) was treated dropwise with TFA (2 mL), stirred for
18 h, and evaporated to a residue that was purified by
chromatography.
Methyl
(3S,9S)-3-N-(Fmoc)Amino-2-oxo-2,3,4,7,8,9-
hexahydro-1H-azonine-9-carboxylate (28). Chromatogra-
phy of the product from 25c (0.5 mmol) using EtOAc as the
eluant gave 28 (95% yield) as a brown gum: 1H NMR: δ 7.76
(d, 2H, J ) 7.4), 7.60 (d, 2H, J ) 7.3), 7.39 (t, 2H, J ) 7.4),
7.30 (t, 2H, J ) 7.4), 6.35 (d, 1H, J ) 7.0), 6.25 (d, 1H, J )
11.6), 6.08 (dd, 1H, J ) 8.8, 18.9), 5.65 (ddd, 1H, J ) 6.04,
10.95, 10.84), 4.39-4.38 (m, 2H), 4.25 (m, 3H), 3.73 (s, 3H),
2.70 (m, 1H), 2.30 (dd, 2H, J ) 8.5, 8.7), 2.12 (m, 1H), 1.87
(m, 1H), 1.75 (m, 1H); 13C NMR: δ 172.7, 172.0, 155.2, 143.7,
143.6, 141.1, 130.1, 128.7, 127.5, 126.9, 124.9, 119.8, 66.8, 52.5,
52.1, 47.0, 34.1, 33.8, 22.5; HRMS (MH)+ calcd for C25H27N2O5,
435.19145; found, 435.19184.
General Procedure E: Methyl Ester Hydrolysis. A
stirred solution of methyl ester (0.5 mmol) in 0.8 M CaCl2 in
a 7:3 i-PrOH/H2O solution (10 mL) was treated with 0.5 M
NaOH solution (2 mL). After 2 h, ether was added, and the
phases were separated. The aqueous layer was acidified with
1.0 N HCl and extracted with EtOAc (three times). The
combined organic layers were washed with brine, dried over
MgSO4, filtered, and concentrated to give the acid.
(E,3S,9S)-3-N-(Fmoc)Amino-2-oxo-2,3,4,7,8,9-hexahydro-
1H-azonine-9-carboxylic Acid (30). Hydrolysis of 28 (0.4
mmol) gave 30 (99% yield) as a white solid: mp 190-193 °C;
1H NMR (400 MHz, pyridine-d5): δ 8.90 (d, 1H, J ) 11.0), 8.38
(d, 1H, J ) 7.1), 7.8 (m, 3H), 7.7 (t, 1H, J ) 7.5), 7.35 (m, 3H),
7.25 (t, 1H, J ) 9.0), 6.04 (dd, 1H, J ) 8.6, 18.5), 5.75 (bs,
1H), 5.56 (dd, 1H, J ) 8.7, 18.5), 4.84 (t, 1H, J ) 7.8), 4.69 (m,
1H), 4.55 (d, 2H, J ) 7.16), 4.33 (t, 1H, J ) 6.8), 2.95-2.88
(m, 1H), 2.5 (m, 1H), 2.35 (m, 1H), 2.20 (m, 1H), 1.9 (m, 2H);
13C NMR: δ 175.3, 174.1, 156.7, 145.2, 144.9, 142.1, 131.5,
129.3, 128.5, 127.9, 126.1, 120.8, 67.3, 52.9, 48.2, 41.4, 34.8,
31.0; HRMS (MH)+ calcd for C24H25N2O5, 421.17580; found,
421.17599.
trated to
graphy.
a residue that was purified by chromato-
Methyl (2S)-2-[(2,4-Dimethoxybenzyl)amino]pent-4-
enoate (18a). Chromatography of the product from 14 (20
mmol) using 30:70 EtOAc/hexanes as the eluant gave 18a as
a yellow oil (74% yield): 1H NMR: δ 7.10 (d, 1H, J ) 7.8),
6.39 (m, 2H), 5.69 (m, 1H), 5.08 (m, 2H), 3.81-3.64 (m, 11H),
3.11 (t, 1H, J ) 6.6), 2.39 (t, 2H, J ) 6.5), 2.22 (bs, 1H); 13C
NMR: δ 174.8, 160.0, 158.4, 133.6, 130.2, 120.0, 117.6, 103.5,
98.2, 60.0, 55.1, 55.0, 51.4, 46.8, 37.5. MS (ESI, m/z): 280.1
(MH)+.
General Procedure B: Peptide Coupling using HATU.
The selected N-protected amino acid (1.5 equiv) and N-benzyl
amino ester (1.0 equiv) were dissolved in CH2Cl2 (0.07 M),
treated with N-ethylmorpholine (1.5 equiv) and HATU (1.5
equiv), stirred for 24h, and diluted with water. The aqueous
layer was extracted with CH2Cl2 (3 times). The combined
organic layers were washed with brine, dried over MgSO4,
filtered, and concentrated to a residue that was purified by
chromatography.
N-(Fmoc)-L-Allylglycinyl-N-(2,4-dimethoxybenzyl)-L-
homoallylglycine Methyl Ester (21c). Chromatography of
the product from 13b (1.5 mmol) and 19a (1.0 mmol) using
30:70 EtOAc/hexanes as the eluant gave 21c (85% yield):1H
NMR: δ 7.76 (d, 2H, J ) 7.5), 7.62 (dd, 2H, J ) 3.3, 3.4), 7.39
(t, 2H, J ) 7.5), 7.30 (t, 2H, J ) 7.4), 7.07 (d, 1H, J ) 8.0),
6.40 (dd, 2H, J ) 2.1, 2.3), 5.81-5.65 (m, 3H), 5.14 (m, 2H),
5.05 (m, 1H), 4.98 (m, 2H), 4.67 (d, 1H, J ) 15.8), 4.46-4.29
(m, 3H), 4.24-4.22 (t, 1H, J ) 7.1), 4.13 (m, 1H), 3.76 (s, 6H),
3.57 (s, 3H), 2.59 (m, 1H), 2.45 (m, 1H), 2.11 (m, 1H), 2.03 (m,
2H), 1.80 (m, 1H); 13C NMR: δ 171.6, 171.0, 160.9, 158.6,
155.3, 143.7, 141.1, 137.3, 132.3, 130.2, 127.5, 126.8, 125.0,
124.9, 119.8, 118.7, 115.8, 115.4, 103.5, 98.3, 66.7, 57.7, 55.2,
55.0, 51.8, 50.7, 47.0, 37.8, 30.4, 28.1. MS (ESI, m/z): 635.2
(MNa)+.
Acknowledgment. This work was supported by
grants from Fonds Que´be´cois de la Recherche sur la
Nature et les Technologies (FQRNT), Valorisation-
Recherche Que´bec (VRQ), the Natural Sciences and
Engineering Research Council of Canada (NSERC), the
Canadian Society for Chemistry (CSC), Merck Frosst
Canada, Boehringer Ingelheim Recherche Inc., Shire
Biochem., and AstraZeneca Canada for financial sup-
port. We thank Hassan Iden for assistance in making
starting materials, Dr. Alexandra Frutos and Mr. Dalbir
Sekhon for mass spectral analysis, and Sylvie Bilodeau
and Dr. M. T. Pham Viet of the Regional High-field
NMR Laboratory for their assistance in running 2D
NOESY/COSY experiments.
General Procedure C: Ring-Closing Metathesis. In a
flame dried flask, dipeptide (1.0 equiv) was dissolved in dry
CH2Cl2 (3 mM). The mixture was heated for 10 min at 35 °C,
treated with bis(tricyclohexylphosphonium)benzylidine ruthe-
nium (IV) dichloride (RuCl2(dCHPh)(PCy3)2, 20 mol %), heated
at reflux for 72h, and concentrated. The crude residue was
purified by chromatography to afford the unsaturated lactam.
Methyl (E, 3S, 9S)-3-N-(Fmoc)Amino-1-(2,4-dimethoxy-
benzyl)-2-oxo-2,3,4,5,8,9-hexahydro-1H-Azonine-9-car-
boxylate (25c). Chromatography of the product from 21c (0.4
mmol) using 20:80 EtOAc/hexanes as the eluant gave 25c (75%
yield) as a brown solid: mp 96-101 °C; [R]20 -37.8° (c 0.93,
D
CHCl3); 1H NMR: δ 7.76 (d, 2H, J ) 7.5), 7.62 (d, 2H, J )
7.4), 7.40 (t, 2H, J ) 7.4), 7.30 (t, 2H, J ) 7.3), 7.23(d, 1H, J
) 8.3), 6.49 (d, 1H, J ) 6.6), 6.45-6.36 (m, 2H), 6.09 (dd, 1H,
J ) 9.1, 18.0), 5.61 (dd, 1H, J ) 9.1, 18.0), 4.66-4.35 (m, 5H),
4.23 (t, 1H, J ) 7.1), 3.79 (s, 3H), 3.77 (s, 3H), 3.46 (s, 3H),
2.67 (m, 1H), 2.30-2.15 (m, 2H), 1.90 (m, 2H), 1.75-1.65 (m,
2H); 13C NMR: δ 173.3, 170.4, 159.8, 157.4, 155.2, 143.8, 141.1,
130.9, 129.9, 129.0, 127.5, 126.9, 125.0, 125.0 119.8, 117.5,
104.0, 97.9, 66.8, 56.9, 55.1, 52.0, 51.7, 47.0, 39.9, 35.0, 27.8,
22.0; HRMS (MH)+ calcd for C34H37N2O7, 585.25953; found,
585.25918.
Supporting Information Available: General experimen-
tal section, general procedures for TBTU and symmetric
anyhydride couplings, 1H and 13C NMR data for 18b, 19a and
b, 21a and b, 22a-d, 23b and c, 25b, 26b and c, 27b and c,
29, and 31, 1H and 13C NMR spectra of 12, 13b, 14, 16a, 18a
and b, 19a and b, 21a-c, 22a-d, 23b and c, 25b and c 26b
and c, 27b and c, and 28-31, 2D COSY and NOESY spectra
of 30 and 31, and HPLC profiles of 30 and 31. This material
is available free of charge via the Internet at http://pubs.acs.org.
JO0477648
3844 J. Org. Chem., Vol. 70, No. 10, 2005