Design, synthesis, and dopamine receptor modulating activity of spiro bicyclic peptidomimetics of L-prolyl-L-leucyl-glycinamide

Article abstract of DOI:10.1021/jm980525q

In the present study, the synthesis of the 5.5.6. and 5.6.5. spiro bicyclic lactam PLG peptidomimetics, compounds 3 and 4, respectively, was undertaken. These peptidomimetics were designed to examine the following: (1) the effect that changing the size of

Full text of DOI:10.1021/jm980525q

628
J . Med. Chem. 1999, 42, 628-637
Design , Syn th esis, a n d Dop a m in e Recep tor Mod u la tin g Activity of Sp ir o
Bicyclic P ep tid om im etics of L-P r olyl-L-leu cyl-glycin a m id e
Ehab M. Khalil,† William H. Ojala,‡ Ashish Pradhan,§ Venugopalan D. Nair,§ William B. Gleason,‡
Ram K. Mishra,§ and Rodney L. J ohnson*,†
Department of Medicinal Chemistry and The Biomedical Engineering Center and Department of Laboratory Medicine and
Pathology, University of Minnesota, Minneapolis, Minnesota 55455, and Department of Psychiatry and Behavioural
Neuroscience, McMaster University, Hamilton, Ontario L8N 3Z5, Canada
Received September 17, 1998
In the present study, the synthesis of the 5.5.6. and 5.6.5. spiro bicyclic lactam PLG
peptidomimetics, compounds 3 and 4, respectively, was undertaken. These peptidomimetics
were designed to examine the following: (1) the effect that changing the size of the thiazolidine
and lactam ring systems would have on the ability of these systems to mimic the type-II â-turn
and (2) the effect that these structural perturbations would have on the ability of the
peptidomimetics to modulate dopamine receptors. Through the use of the [3H]spiroperidol/N-
propylnorapomorphine (NPA) dopamine D2 receptor competitive binding assay, 3 and 4, at a
concentration of 100 nM, decreased the dissociation constant of the high-affinity state of the
dopamine receptor for the agonist. These effects were observed when either Gpp(NH)p was
absent or present and they were comparable to those produced by PLG at a concentration of
1 µM. Peptidomimetics 3 and 4 also increased the percentage of D2 receptors that existed in
the high-affinity state. Even with Gpp(NH)p present, 3 and 4 were able to return the RH/RL
ratios to values observed in the respective controls where Gpp(NH)p was absent. Furthermore,
both peptidomimetics were able to attenuate the Gpp(NH)p-induced shift to the low-affinity
state to a greater extent than PLG. Peptidomimetics 3 and 4 were evaluated in vivo as
modulators of apomorphine-induced rotational behavior in the 6-hydroxydopamine-lesioned
rat model of hemiparkinsonism, and each affected the rotational behavior in a bell-shaped
dose-response relationship producing increases of 95 ( 31% (0.01 mg/kg, ip) and 88 ( 14%
(0.001 mg/kg, ip), respectively. In comparison, the previously reported 5.5.5. spiro bicyclic lactam
2 increased rotational behavior by 25 ( 11% (0.01 mg/kg, ip).
L-Prolyl-L-leucyl-glycinamide (PLG, 1) is an endog-
enously derived peptide that has been shown to modu-
late dopaminergic neurotransmission within the central
nervous system. Through in vitro studies, PLG and its
peptidomimetic analogues have been shown to increase
the percentage of D2 receptors that exist in the high-
affinity state and to increase the affinity of the high-
affinity state of the D2 receptor for agonists.1-5 They
also have been shown to potentiate the contralateral
rotational behavior induced by apomorphine in rats with
unilateral 6-hydroxydopamine lesions of the nigrostri-
atal pathway.5-9
These peptidomimetics were designed to examine the
effect that changing the size of the thiazolidine and
lactam ring systems would have on the ability of these
systems to mimic the type-II â-turn and the effect that
these structural perturbations would have on the ability
of the peptidomimetics to modulate dopamine receptors.
In an attempt to elucidate the bioactive conformation
of PLG, numerous conformationally constrained ana-
logues of PLG have been synthesized and tested.4,5,10-13
One such analogue, 2, incorporated the highly rigid
5.5.5. spiro bicyclic system previously designed by us
as a type-II â-turn mimic.14 This analogue was found
to possess the same ability as PLG with respect to
enhancing the binding of [3H]ADTN to dopamine recep-
tors.13 In the present study, the synthesis of the 5.5.6.
and 5.6.5. spiro bicyclic lactam PLG peptidomimetics,
compounds 3 and 4, respectively, was undertaken.
Syn th esis
† Department of Medicinal Chemistry, University of Minnesota.
‡ The Biomedical Engineering Center and Department of Laboratory
Medicine and Pathology, University of Minnesota.
§ Department of Psychiatry and Behavioral Neuroscience, McMaster
University.
The synthesis of the 5.5.6. spiro bicyclic lactam PLG
peptidomimetic 3 was carried out as outlined in Scheme
1. The synthesis started with the stereoselective allyl
alkylation of L-Pro-OH by Seebach’s method of self-
10.1021/jm980525q CCC: $18.00 © 1999 American Chemical Society
Published on Web 01/29/1999

Synthesis of Spiro Bicyclic Peptidomimetics
J ournal of Medicinal Chemistry, 1999, Vol. 42, No. 4 629
Sch em e 1
Sch em e 2
failed to produce any product in this case. In another
attempt, the deprotected primary amide derivative of 8
was condensed with Boc-L-Pro-OH with excess PyBroP
as the condensing reagent, since this reagent had been
reported to give good results for the coupling of sterically
demanding amino acids.21,22 In our hands, only a modest
yield (21%) of 10 was obtained when this reagent was
used. The best results (81% yield) were obtained by
deprotecting 8 with HCl in dioxane and coupling the
deprotected amine with Boc-L-Pro-OH using excess
dicyclohexylcarbodiimide (DCC) and 1-hydroxybenzo-
triazole (HOBt) and a long reaction time (3 days). Ester
9 was smoothly converted to the primary amide 10,
which was deprotected to give PLG peptidomimetic 3
as a hygroscopic hydrochloride salt.
In building the 5.6.5. spiro bicyclic ring system, we
initially envisioned applying the protocol of Brown et
al.23,24 for the oxidation of terminal olefins to their
corresponding aldehydes to 5. As outlined in Scheme 2,
hydroboration of 5 followed by oxidation with pyri-
dinium chlorochromate gave the desired aldehyde 11
in modest yields (25-30%). Condensation of this alde-
hyde with D-cysteine gave a diastereoisomeric mixture
of thiazolidines (12). However, in contrast with the 5.5.6.
spiro bicyclic system, formation of the lactam to give
5.6.5. spiro bicyclic system 13 did not take place under
thermal reaction conditions.
To overcome the poor yields associated with the
formation of 11 from 5 and to overcome the obstacle
encountered in trying to form the 5.6.5. spiro bicyclic
lactam ring system under thermal conditions, an alter-
nate route to 4 was developed as outlined in Scheme 3.
In a modification of Seebach’s original procedure,15
alkylation of oxazolidinone 14 with the relatively un-
activated electrophile 4-bromo-1-butene was achieved
in the presence of HMPA to give 15.25 This alkylated
oxazolidinone was deprotected to 16 with the procedure
of Genin et al.17 This was followed by protection of the
amino group under the conditions of Khalil et al.18
Following the procedure of Ono et al.,26 the carboxylic
acid functionality of 17 was protected as the benzyl
ester. Oxidative cleavage of 18 provided aldehyde 19
from which the benzyl ester was removed by palladium-
catalyzed hydrogenolysis to give 20. Condensation of
this material with D-cysteine methyl ester hydrochloride
afforded a mixture of epimeric thiazolidines (21).
In initial experiments, closure of 21 to the corre-
sponding lactams was attempted with either DCC or
reproducing chirality.15,16 The alkylated oxazolidinone
thus formed was cleanly deprotected to (R)-R-allyl-
proline under the mild conditions previously developed
in our laboratory.17 Amino protection of (R)-R-allyl-
proline as the N-tert-butoxycarbamate was done as
reported by Khalil et al.,18 and this step was followed
by esterification with diazomethane. Oxidative cleavage
of the double bond of 5 with OsO4 and NaIO4 furnished
the previously reported aldehyde 6.14
Aldehyde 6 was condensed with D-homocysteine (pre-
pared by the reduction of D-homocystine with PPh3 in
the presence of HCl as described by Overman et al.19)
under the conditions reported previously by Genin and
J ohnson14 to give a diastereoisomeric mixture of thiazi-
nanes 7. Conversion of 7 to spiro bicyclic lactam 8 was
achieved by heating the diastereomeric mixture in DMF
for 3 days followed by treatment of the crude spiro
bicyclic carboxylic acid with diazomethane. Under the
thermal reaction conditions, only the spiro bicyclic
lactam diastereoisomer in which the C4′ and C8′a
hydrogens are in an anti relationship was obtained. This
result is in agreement with that seen with the 5.5.5
spiro bicyclic lactam system.14 This stereochemical
relationship was confirmed through the X-ray crystal
structure of 8 (see Supporting Information).20
A number of routes were explored for the attachment
of the prolyl residue to the deprotected derivative of 8.
In one route, 8 was converted to the primary amide with
NH3 in MeOH and the amino deprotected species then
reacted with the pentafluorophenyl ester of Boc-L-
proline. Although this method had proved useful in
coupling other bicyclic lactams to Boc-L-proline,12,21 it

630 J ournal of Medicinal Chemistry, 1999, Vol. 42, No. 4
Khalil et al.
Sch em e 3
PyBroP as the condensing agent. Both reactions re-
sulted in complex mixtures that, after chromatography,
gave very poor yields (<15%) of the desired product.
Excellent results were ultimately achieved with the
pyridinium-based condensing agent developed by Mu-
kaiyama et al.27 The epimeric lactams 22a and 22b were
obtained in a 1:1 ratio (83% yield) after chromatographic
separation. Delineation of the stereochemistry of these
two isomers was accomplished by obtaining an X-ray
crystal structure of 22b (see Supporting Information).20
Isomer 22b was carried onto 4 by use of the same set of
reaction conditions used to make 3. This isomer also was
converted to the model dipeptide isostere 25 by convert-
ing 22b to the methylamide followed by acidolytic
removal of the tert-butoxycarbonyl group and then
acylation with acetic anhydride.
of the high-affinity receptor state from a control value
of 0.122 nM to a value 0.064 nM. For 4, the dissociation
constant went from a control value of 0.196 nM to a
value of 0.09 nM. No significant change in the dissocia-
tion constant of NPA for the low-affinity state of the
receptor was observed. These effects on the dissociation
constant were comparable to those produced by PLG at
a concentration of 1 µM.
Peptidomimetics 3 and 4 also increased the percent-
age of D2 receptors that existed in the high-affinity state,
an effect that was seen both in the presence and absence
of Gpp(NH)p. In the absence of Gpp(NH)p, treatment
with 3 increased the ratio of the percentage of receptors
in the high- and low-affinity states (RH/RL) from 0.92
to 1.77, while treatment with 4 resulted in an increase
in RH/RL from 1.21 to 2.12. In the control experiment
with Gpp(NH)p present, there is a significant decrease
in the RH/RL ratio. The low-affinity state of the receptor
becomes the predominant species. Both 3 and 4, how-
ever, were able to attenuate significantly the Gpp(NH)p-
induced shift to the low-affinity state. Treatment with
3 resulted in a change in the ratio from 0.17 to 1.12,
while in the case of 4 the change in the ratio was from
0.2 to 1.22. Thus, whereas PLG partially reverses the
Gpp(NH)p-induced shift to the low-affinity state, pep-
tidomimetics 3 and 4 were able to return the RH/RL
ratios to values observed in their respective controls
where Gpp(NH)p was absent.
P h a r m a cologica l Stu d ies
The novel spiro bicyclic lactam PLG peptidomimetics
3 and 4 were initially evaluated in a [3H]spiroperidol/
N-propylnorapomorphine (NPA) D2 receptor competition
binding assay.2,5 The assay was carried out in either the
presence or the absence of 5′-guanylylimidodiphosphate
(Gpp(NH)p), a nonhydrolyzable analogue of GTP. The
kinetic binding data obtained in such a study, along
with that previously obtained for PLG, are summarized
in Table 1. At a concentration of 100 nM, both 3 and 4
decreased the dissociation constant of the high-affinity
state of the dopamine receptor for the agonist NPA,
thereby increasing the affinity of the dopamine receptor
for agonists. This effect was seen when either Gpp(NH)p
was absent or present. In the presence of Gpp(NH)p,
compound 3 decreased the agonist dissociation constant
Peptidomimetics 3 and 4, along with the previously
reported spiro bicyclic lactam peptidomimetic 2, also
were evaluated in vivo as modulators of apomorphine-
induced rotational behavior in the 6-hyroxydopamine-
lesioned rat model of hemiparkinsonism.5,8,9 In this

Synthesis of Spiro Bicyclic Peptidomimetics
J ournal of Medicinal Chemistry, 1999, Vol. 42, No. 4 631
Ta ble 1. Modulation of [3H]Spiroperidol/N-Propylnorapomorphine Binding Competition by 3 and 4a
binding parameters
experiment
control for 3
KH (nM)
KL (nM)
RH (%)
RL (%)
RH/RL
-Gpp(NH)p
+Gpp(NH)p
0.071 ( 0.008
0.122 ( 0.01
196 ( 16
246 ( 18
52 ( 7
15 ( 2
48 ( 3.2
85 ( 5.8
1.08 ( 0.7
0.17 ( 0.008
pretreatment with 3 (100 nM)
-Gpp(NH)p
0.031 ( 0.002b
0.064 ( 0.005b
168 ( 12
202 ( 26
64 ( 5b
53 ( 4b
36 ( 2.4
47 ( 3.5
1.77 ( 0.52b
1.12 ( 0.22b
+Gpp(NH)p
control for 4
-Gpp(NH)p
+Gpp(NH)p
0.067 ( 0.004
0.196 ( 0.017
190 ( 17
260 ( 19
53 ( 6
17 ( 3
47 ( 3.5
83 ( 6.7
1.13 ( 013
0.2 ( 0.03
pretreatment with 4 (100 nM)
-Gpp(NH)p
0.05 ( 0.002b
0.09 ( 0.006b
188 ( 22
211 ( 24
68 ( 7b
55 ( 4b
32 (4
45 (6
2.12 ( 0.24b
1.22 ( 0.13b
+Gpp(NH)p
control for 1d
-Gpp(NH)p
0.09 ( 0.005
0.13 ( 0.01
73.2 ( 5.5
60.0 ( 8.6
49 ( 5
19 ( 2
51 ( 3
82 ( 5
0.96 ( 0.05
0.23 ( 0.01
+Gpp(NH)p
pretreatment with 1 (1 µM)d
-Gpp(NH)p
+Gpp(NH)p
0.04 ( 0.002c
0.06 ( 0.006c
60.1 ( 5.4
78.0 ( 9.9
61 ( 4b
32 ( 3c
41 ( 2
72 ( 6
1.48 ( 0.06c
0.44 ( 0.03c
a
Competition data were obtained as described in ref 5. KH and KL represent the inhibitor constant (Ki) of agonist calculated for the
high- and low-affinity components of the [3H]spiroperidol binding, respectively. RH and RL are percentages of receptors in the high- and
low-affinity form for the agonist, respectively. Values for each preparation are the mean ( SEM of three to four separate experiments
with each experiment carried out in duplicate or triplicate. Concentration of Gpp(NH)p was 100 µM. b,c Statistical difference from the
d
respective control group indicated as follows: (b) p < 0.05, (c) p < 0.01. Data for 1 from ref 2.
Discu ssion
Both spiro bicyclic lactam peptidomimetics 3 and 4
were found in this study to exhibit a pharmacological
profile similar to PLG in terms of their activity in the
[3H]spiroperidol/N-propylnorapomorphine D2 receptor
competition binding assay and the 6-hydroxydopamine-
lesioned animal model of Parkinson’s disease. In the
former assay, 3 and 4 induced a change in the D2
receptor N-propylnorapomorphine dissociation constant
comparable to that induced by PLG. Both peptidomi-
metics, however, produced a greater shift from the low-
F igu r e 1. Effect of ip administered PLG peptidomimetics 2,
3, and 4 on contralateral behavior of ip administered apomor-
phine (0.25 mg/kg) in rats (n ) 4) with unilateral lesions.
Number of contralateral rotations in 10 min expressed as a
percentage change from control rotational response. Dosage
expressed as log (mg/kg) from -5 or 0.00001 mg/kg to 0 or 1
mg/kg. Each point represents the mean percent change (
SEM. Statistical significance: *p < 0.05, **p < 0.005.
affinity state to the high-affinity state of the dopamine
D2 receptor than did PLG.
The 5.5.6. and 5.6.5. spiro bicyclic lactam peptidomi-
metics, compounds 3 and 4, respectively, were found to
be much more effective in inducing apomorphine-
induced rotational behavior in the 6-hydroxydopamine-
lesioned animal model of Parkinson’s disease than the
5.5.5. spiro bicyclic lactam peptidomimetic 2. Further-
more, these two peptidomimetics are more active than
PLG, as this peptide was shown previously to increase
apomorphine-induced rotations 30% at a dose of 1.0 mg/
kg, ip.8 As with PLG, peptidomimetics 2-4 all produced
bell-shaped dose-response curves. The underlying basis
for such dose-response curves observed for PLG and its
analogues is not known. Several models, however, have
been proposed to account for such a phenomenon.28
model, a unilateral lesion is created in the substantia
nigra of the rat with 6-hydroxydopamine, thereby lead-
ing to destruction of the dopaminergic efferents from
the substantia nigra to the striatum. Loss of these
neurons leads to a development of postsynaptic super-
sensitivity on the lesioned side which leads to a con-
tralateral rotational response by the animal upon
administration of a direct acting dopaminergic agonist
like apomorphine.
The effectiveness of 3 and 4 with respect to PLG may
be due to the ability of these spiro bicyclic lactams to
mimic a type-II â-turn, which has been postulated to
be the bioactive conformation of PLG.10-13 A comparison
of the torsion angles of the spiro bicyclic lactam systems
with those for an ideal type-II â-turn are shown in Table
2. The values for the 5.5.5. system of 2 were obtained
previously by computational methods.14 Those for the
5.5.6. and 5.6.5. systems of 3 and 4 were obtained from
the X-ray crystal structures of the respective amide
derivatives 10 (Figure 2) and 25 (Figure 3).20 Both 10
and 25 were found to exist in a type-II â-turn. A
comparison of the three torsional angles (φ2, ψ2, and φ3)
The spiro bicyclic lactam PLG analogues were tested
at a number of doses for their ability to modify the
rotational response to apomorphine. The results are
summarized in Figure 1. The peptidomimetics when
administered with apomorphine were seen to affect the
rotational behavior in a bell-shaped dose-response
relationship. The maximal response to 2 occurred at
0.01 mg/kg, ip, resulting in a 25 ( 11% increase in
contralateral rotations. Peptidomimetic 3 also had its
maximum effect at 0.01 mg/kg, ip, but it produced an
increase of 95 ( 31% in contralateral rotations. Com-
pound 4, on the other hand, produced a maximal change
of 88 ( 14% at a dose of 0.001 mg/kg, ip.

632 J ournal of Medicinal Chemistry, 1999, Vol. 42, No. 4
Khalil et al.
Ta ble 2. Torsion Angle Comparisons of the Peptidomimetic
Spiro Bicyclic Lactam Systems
system
φ2
ψ2
φ3
ψ3
5.5.5.a
5.5.6. (9)
5.6.5. (21)
-40.3
-48.9
-56
108.1
134.1
133.7
77.7
116.0
97.3
-16.7
-29.8
-19.9
ideal type-II â-turnb -60 ( 30 120 ( 30
80 ( 30
0 ( 30
a
Data for the 5.5.5. spiro bicyclic lactam system are from ref
b
14. Values for the ideal type-II â-turn are from ref 29.
F igu r e 4. Overlay of the spiro bicyclic lactam ring systems
of peptidomimetics 2 (green), 3, (red), and 4 (blue) in which
the N- and C-terminal ends of the ring systems have been
substituted with the acetyl and N-methylcarboxamide moi-
eties, respectively: A, frontal view; B, side view. The starting
structures for the superimposition were obtained by compu-
tational methods in the case of 214 and from X-ray structures
in the case of 3 and 4. The superimposition was done by
calculating the best fit for all backbone atoms (10 atoms) using
Insight II (release 95.0). Calculated RMS values were as
follows: 5.6.5 and 5.5.6. ) 0.26 Å; 5.5.5. and 5.5.6. ) 0.59 Å;
5.5.5. and 5.6.5. ) 0.47 Å.
F igu r e 2. ORTEP representation of 10 at the 50% probability
level with the crystallographic numbering system. Hydrogen
atoms are drawn as spheres of arbitrary radius. A molecule
of water was found to associate with the carboxamide func-
tionality.
explain the differences seen between peptidomimetic 2
and peptidomimetics 3 and 4.
In summary, this study shows that changing the ring
sizes of the spiro bicyclic lactam peptidomimetic systems
does result in changes in the torsional angles which in
turn produce changes in the pharmacological activity
of the PLG peptidomimetics. These spiro bicyclic lactam
systems should also prove to be valuable tools in
studying the bioactive conformation of other biologically
active peptides, since the ability to produce subtle
differences in backbone torsional angles should be useful
given the heterogeneous nature of â-turn secondary
structures.
F igu r e 3. ORTEP representation of 25 at the 40% probability
level with the crystallographic numbering system. Hydrogen
atoms are drawn as spheres of arbitrary radius.
contrained by the 5.5.6. and 5.6.5. spiro bicyclic lactam
systems found in 3 and 4, respectively, shows the φ2 and
ψ2 torsion angles have similar values. In contrast, the
ψ2 and φ3 values for the 5.5.5. system are quite different
from those seen for the 5.5.6 and 5.6.5. systems. The
structural ramifications of these different ψ2, and φ3
torsion angles for the 5.5.5. system can be seen in the
overlay of the three structures (Figure 4). There appear
to be two main differences. One is the position of the
lactam carbonyl where it is seen that that of the 5.5.5.
system projects out from the ring in a different direction
than do the carbonyls of the 5.5.6. and 5.6.5. spiro
bicyclic systems. A second difference is the direction that
the carboxamide moiety projects out from the spiro
bicyclic systems. That of the 5.5.5. system projects in a
significantly different direction than the carboxamide
moieties of the 5.5.6. and 5.6.5. systems. Since the
lactam carbonyl and carboxamide moiety are believed
to be important pharmacophoric groups, this may
Exp er im en ta l Section
Gen er a l Asp ects. Thin-layer chromatography was per-
formed on Analtech 250 µm silica gel HLF Uniplates and were
visualized by UV, I2, ninhydrin spray (amines), 2,6-dichlo-
rophenol indophenol spray (acids), and 2,4-dinitrophenyl-
hydrazine (aldehydes). Chromatographic purification on silica
gel (Merck, grade 60, 240-400 mesh, 60 Å) was done by flash
or gravity methods and semipreparative HPLC was done with
a Waters Associates 25 × 100 mm PrepPak cartridge. Optical
rotations were measured on a Rudolph Research Autopol III
polarimeter at the 589 nm Na D-line. Melting points were
obtained on a Thomas-Hoover melting point apparatus and
are uncorrected. Elemental analyses were performed by M-
H-W Laboratories, Phoenix, AZ. 1H and 13C NMR spectra were
measured in CDCl3 at 300 and 75.5 MHz, respectively, with
CDCl3 as the internal reference for 1H (δ 7.26) and 13C (δ
77.06).

Synthesis of Spiro Bicyclic Peptidomimetics
J ournal of Medicinal Chemistry, 1999, Vol. 42, No. 4 633
(2RS,4R)-2-[[2′-(R)-N-(ter t-Bu t oxyca r b on yl)-2′-(m et h -
oxyca r bon yl)p yr r olid in yl]m eth yl]th ia zin a n e-4-ca r boxy-
lic Acid (7). D-Homocysteine‚HCl (0.705 g, 4.13 mmol) was
dissolved in H2O (10 mL). NaOH (165 mg, 4.13 mEq) in H2O
(3.4 mL) was added followed by a solution of 614 (1.12 g, 4.13
mmol) in EtOH (95%, 20 mL). The reaction mixture was stirred
overnight at room temperature and then concentrated in vacuo
to a white solid which was partitioned between H2O (70 mL)
and EtOAc (100 mL). The aqueous layer was extracted with
additional portions of EtOAc (2 × 100 mL). The combined
organic layers were dried with MgSO4, and the solvent was
removed in vacuo to give 1.21 g (75%) of the diastereomeric
mixture as a white solid. This material was used without
further purification: mp 94-97 °C; FAB MS m/z 389 [MH]+.
Anal. (C17H28N2O6S) C, H, N, S.
carbamate bond in approximately a 1:1 ratio observed, CDCl3)
δ 1.41 and 1.43 (s, 9 H, Boc CH3), 1.62-2.21 (m, 10 H, Pro
γ-CH2, Pro â-CH2, 4-CH2, 3′-CH2, and 3-CH2), 2.36-2.41 (m,
1 H, 8′-CH2), 2.57-2.66 (m, 1 H, 8′-CH2), 2.96-3.13 (m, 2 H,
2′-CH2), 3.32-3.62 and 3.94-4.01 (m, 4 H, 5-CH2 and Pro
δ-CH2), 3.77 and 3.78 (s, 3 H, OCH3), 4.35 and 4.46 (dd, J )
7.4 and 3.6 Hz, 1 H, Pro R-CH), 5.06 (d, J ) 4.0 Hz, 1 H, 4′-
CH), 5.37 (dd, J ) 7.4, 6.0 Hz, 1 H, 8′a-CH); 13C NMR (75 MHz,
CDCl3) δ 24.23 and 25.00 (Pro γ-C), 24.69 (4-C), 25.74 (3′-C),
27.64 (3-C), 29.08 (Boc CH3), 29.90 and 30.83 (Pro â-C), 37.09
(8′-C), 38.84 (2′-C), 47.40 and 47.57 (Pro δ-C), 48.32 (5-C), 51.92
(OCH3), 52.97 and 53.00 (Pro R-C), 54.78 (4′-C), 58.34 (8′a-C),
67.36 (7′-C), 79.94 (Boc C-O), 154.2 and 156.2 (Boc CdO),
170.74 and 170.77 (CO2CH3), 171.5, 171.6, 173.7, and 173.9
(6′-CdO and Pro CdO); FAB MS m/z 468 [MH]+. Anal.
(C22H33N3O6S) C, H, N, S.
Meth yl [4′R-(4′r,7′r,8′a r)]-1-(ter t-Bu toxyca r bon yl)tet-
r a h yd r o-6′-oxosp ir o-[p yr r olid in e-2,7′-(6′H)-p yr r olo[2,1-b]-
th ia zin e]-4′-ca r boxyla te (8). In an oven-dried 250 mL round-
bottom flask, the thiazinane diastereomeric mixture 7 (1.19
g, 3.06 mmol) was dissolved in dry DMF (100 mL). To the flask
was added NEt3 (0.43 mL, 3.06 mol), and the solution was
heated to 70 °C and stirred for 3 days under N2. The solution
turned a deep golden color with time. The DMF was removed
in vacuo to give a dark yellow oil which was dissolved in MeOH
(10 mL) and then cooled to 0 °C in an ice bath. CH2N2 in Et2O
was added in portions until the evolution of gas ceased. Excess
CH2N2 and solvent were removed in vacuo. The resulting oil
was dissolved in EtOAc (100 mL) and extracted with 10% citric
acid (50 mL) and saturated NaCl solution and dried with
MgSO4. The solvent was removed in vacuo to give 1.1 g (97%
crude yield) of a yellow oil which was chromatographed on a
silica gel column using EtOAc/hexane (1:1) as eluent. The
product was obtained as a colorless oil in a yield of 303 mg
(27%). This oil was crystallized from Et2O to give 8 as colorless
crystals: mp 156-158 °C; [R]2D5 +126.1 (c 1.2, CHCl3); 1H
NMR (presence of rotamers about the carbamate bond in a
ratio of 3:2 observed, CDCl3) 1.36 and 1.41 (s, 9 H, Boc CH3),
1.71-2.05 (m, 4 H, 4-CH2, 8′-CH2, and 3′-CH2), 2.13-2.43 (m,
3 H, 3-CH2 and 3′-CH2), 2.61 (dt, J ) 14.6 and 3.0 Hz, 1 H,
2′-CH2), 2.82 (dd, J ) 13.4 and 8.5 Hz, 1 H, 8′-CH2), 2.93-
3.05 (m, 1 H, 2′-CH2), 3.45-3.55 (m, 2 H, 5-CH2), 3.72 and
3.76 (s, 3 H, OCH3), 5.07-5.12 (m, 1 H, 4′-CH), 5.30 and 5.41
(d, J ) 8.7, 1 H, 8′-CH); 13C NMR (CDCl3) δ 23.24 and 24.07
(4-C), 25.57 and 25.87 (3′-C), 27.63 and 28.00 (3-C), 29.01 and
29.19 (Boc CH3), 37.22 and 37.55 (8′-C), 39.57 and 40.32 (2′-
C), 48.29 and 48.42 (5-C), 51.91 and 51.99 (OCH3), 52.90 and
53.23 (4′-C), 53.87 and 54 65 (8′a-C), 66.28 and 66.45 (7′-C),
80.30 and 81.25 (Boc C-O), 153.77 and 153.80 (Boc CdO),
171.00 (6′-CdO), 174.71 and 175.04 (CO2CH3); FAB MS m/z
371 (40%) [MH]+, 271 (100%) [M-(CH3)3COCO]+. Anal.
(C17H26N2O5S) C, H, N, S.
Meth yl [4′R-(4′r,7′r,8′a r)]-1-[[1-(ter t-Bu toxyca r bon yl)-
2(S)-p yr r olid in yl]ca r bon yl]tetr a h yd r o-6′-oxosp ir o[p yr -
r olid in e -2,7′-(6′H )-p yr r olo[2,1-b]t h ia zin e ]-4′-ca r b oxy-
la te (9). Spiro bicyclic ester 8 (730 mg, 1.97 mmol) was
deprotected in 4 N HCl in dioxane (10 mL) under Ar for 1 h.
Solvent and excess HCl were removed in vacuo. Traces of HCl
were removed with the aid of CH2Cl2 to give a white solid
which was dissolved in DMF (40 mL). To that solution, Boc-
L-Pro-OH (1.02 g, 4.73 mmol, 2.4 equiv), DCC (0.98 g, 4.73
mmol, 2.4 equiv), HOBt‚H2O (0.64 g, 4.73 mmol, 2.4 equiv),
and Et3N (0.27 mL, 1.97 mmol, 1 equiv) were added. The
reaction was stirred at room temperature under Ar for 3 days.
The DMF was removed in vacuo and the residue partitioned
between H2O (50 mL) and EtOAc (200 mL). The aqueous layer
was extracted with EtOAc (2 × 50 mL). The combined organic
extracts were washed with H2O and brine and dried with
MgSO4, and the solvent was removed in vacuo. The crude
product was isolated as an oil which was crystallized from
EtOAc to give 644 mg of 9 as white needles. The mother liquor
was chromatographed on silica gel (2% MeOH in EtOAc)
affording an additional 100 mg of product (combined yield
81%): mp 216-217 °C; [R]D25 +56.1 (c 0.26, CHCl3), +25.6 (c
0.70, MeOH); 1H NMR (presence of rotamers about the
[4′R-(4′r,7′r,8′a r)]-1-[[(ter t-Bu toxyca r bon yl)-2(S)-p yr -
r olid in yl]ca r bon yl]tetr a h yd r o-6′-oxosp ir o[p yr r olid in e-
2,7′-(6′H)-pyr r olo[2,1-b]th iazin e]-4′-car boxam ide (10). Spiro
bicyclic ester 9 (550 mg, 1.48 mmol) was treated with a
concentrated solution of ammonia in MeOH (10 mL) at room
temperature. After the mixture was stirred for 3 h, the solvent
and excess ammonia were removed in vacuo to give a white
foam which was chromatographed on a silica gel column using
EtOAc/hexanes (1:1) as the eluting solvent. The product was
isolated as a white foam which was crushed to a fine white
powder (520 mg, 98%): mp 104-106 °C; [R]2D5 +107.0 (c 96,
MeOH); 1H NMR (CDCl3) δ 1.41 (s, 9 H, Boc CH3), 1.72-2.01
(m, 4 H, 4-CH2, 3′-CH2), 2.06-2.15 (m, 1 H, 3-CH2), 2.32-2.41
(m, 1 H, 3-CH2), 2.65 (dt, J ) 14.0 and 3.7 Hz, 1 H, 2′-CH2),
2.76-2.83 (m, 2 H, 8′-CH2), 2.96 (dt, J ) 14.0 and 2.4 Hz, 1
H, 2′-CH2), 3.48 (dd, J ) 7.8 and 4.8 Hz, 5-CH2), 4.93 (dd, J )
5.4 and 1.8 Hz, 1 H, 4′-CH), 4.99 (dd, J ) 9.2 and 3.2 Hz, 1 H,
8′a-CH), 5.39 (br s, 1 H, cis-CONH2), 7.74 (br s, 1 H, trans-
CONH2); 13C NMR (CDCl3) δ 24.54 (4-C), 25.65 (3′-C), 26.70
(3-C), 29.08 (Boc CH3), 38.51 and 39.70 (8′-C and 4′-CH2), 48.29
(5-C), 52.69 (4′-C), 55.38 (8′a-C), 65.84 (7′-C), 81.43 (Boc C-O),
154.88 (Boc CdO), 171.72 and 173.80 (CONH2 and 6′-CdO);
FAB MS m/z 356 (10%) [MH]+, 256 (100%) [M-(CH3)3COCO]+.
Anal. (C16H25N3O4S) C, H, N, S.
[4′R-(4′r,7′r,8′ar)]-1-[2(S)-P yr r olidin ylcar bon yl]tetr ah y-
d r o-6′-oxosp ir o [p yr r olid in e-2,7′-(5′H)-p yr r olo[2,1-b]th i-
a zin e]-4′-ca r b oxa m id e Hyd r och lor id e (3). Spiro bicyclic
amide 10 (210 mg, 0.46 mmol) was deprotected in 4 N HCl in
dioxane for 8 h under N2. The solvent and excess HCl were
removed in vacuo. The resulting residue was twice suspended
in CH2Cl2 (5 mL) and then evaporated to a white solid. This
material was lyophilized to give 3‚HCl as a hygroscopic white
solid (180 mg, 100%): mp 139-140 °C; [R]2D5 +83.0 (c 1.45,
1
MeOH); H NMR (DMSO-d6) δ 1.54-1.74 and 1.80-1.99 (m,
8 H, 4-CH2, Pro γ-CH2, 3-CH2, and 3′-CH2), 2.31-2.49 (m, 3
H, 8′-CH2, Pro â-CH2), 2.65-2.88 (m, 3 H, 8′-CH2 and 2′-CH2),
3.09-3.13 (m, 2 H, 5-CH2), 3.38-3.44 (m, 1 H, Pro δ-CH2),
3.74-3.79 (m, 1 H, Pro δ-CH2), 4.42 (t, J ) 7.6 Hz, 1 H, Pro
R-CH), 4.62 (d, J ) 4.9 Hz, 1 H, 4′-CH), 5.13 (d, J ) 8.6 Hz, 1
H, 8′a-CH), 6.98 (br s, 1 H, cis-CONH2), 7.53 (br s, 1 H, trans-
CONH2), 8.62 (br s, 1 H, +NH2), 10.12 (br s, 1 H, +NH2); 13C
NMR (DMSO-d6) δ 23.41 and 23.77 (4-C and Pro γ-C), 25.03
and 25.47 (3′-C and 3-C), 27.77 (Pro â-C), 36.02 and 37.25 (8′-C
and 2′-CH2), 45.44 and 47.54 (Pro δ-C and 5-C), 51.14 (Pro
R-C), 54.09 (4′-C), 58.09 (8′a-C), 66.62 (7′-C), 166.62, 170.39,
and 171.53 (CONH2, 6′-CdO, and Pro CdO); FAB HRMS m/z
353.1645 (C16H24N4O3S + H+ requires 353.1649). Anal.
(C16H24N4O3S‚HCl‚3H2O) C, H, N, S.
Meth yl (R)-ter t-Bu toxyca r bon yl-2-(3′-for m yleth yl)p r o-
lin a te (11). To an oven-dried 500 mL round-bottom flask
charged with 40 mL of distilled CH2Cl2 was transferred under
N2 a 2.0 M solution of borane-methyl sulfide complex in THF
(1.55 mL, 3.1 mmol). Compound 5 (2.48 g, 9.22 mmol) was
dissolved in 5 mL of dry CH2Cl2, and this solution was
transferred dropwise to the reaction flask at room tempera-
ture. The solution was stirred under Ar for 1 h, after which
time the mixture was concentrated in vacuo. The resulting
trialkylborane was suspended in CH2Cl2 (10 mL) and then

634 J ournal of Medicinal Chemistry, 1999, Vol. 42, No. 4
Khalil et al.
transferred dropwise to a well-stirred suspension of pyridinium
chlorocarbonate (3.9 g, 18.4 mmol) in CH2Cl2 (40 mL). After
the addition was complete, the suspension was refluxed for 4
h with stirring. The reaction mixture was allowed to cool to
room temperature, diluted with 100 mL of Et2O, and filtered
through a pad of silica gel. The filtrate was concentrated in
vacuo, and the residue was subjected to silica gel chromatog-
raphy (EtOAc/hexanes, 1:3) to give 0.65 g (25%) of product as
a tan oil: [R]2D5 +24.3 (c 0.4, CHCl3); FAB MS m/z 286 [MH]+.
Anal. (C14H23NO5) C, H, N.
(2RS,4S)-2-[[2′(R)-ter t-Bu t oxyca r b on yl)-2′-(m et h oxy-
ca r b on yl)p yr r olid in yl]et h yl]t h ia zolid in e-4-ca r b oxylic
Acid (12). To a solution of D-cysteine hydrochloride monohy-
drate (0.4 g, 2.29 mmol) dissolved in H2O (5 mL) was added a
solution of NaOH (92 mg, 2.29 mmol) in H2O (1.9 mL). A
solution of 5 (0.65 g, 2.29 mmol) in EtOH (5 mL) was added,
and the reaction mixture was stirred overnight at room
temperature. The reaction mixture was diluted with 15 mL of
H2O and then extracted with EtOAc (2 × 100 mL). The pH of
the aqueous layer was adjusted to 5 with 1 N NaHCO3, and
then the aqueous layer was extracted with an additional
portion of EtOAc (75 mL). The combined organic layers were
dried with MgSO4, and the solvent was removed in vacuo to
give 0.17 g (80%) of diastereoisomers as a white foam, which
was used without further purification: FAB MS m/z 389
[MH]+.
â-C), 46.21 (δ-C), 74.51 (R-C), 115.97 (CHdCH2), 137.40 (CHd
CH2), 175.29 (CdO); FAB MS m/z 170 [MH]+, 339 [2M + H]+.
Anal. (C9H15NO2) C, H, N.
(S)-N-ter t-Bu t oxyca r bon yl-2-(3′-b u t en yl)p r olin e (17).
(R)-2-(3′-Butenyl)proline (16, 4.0 g, 23.6 mmol) and tetra-
methylammonium hydroxide pentahydrate (4.28 g, 23.6 mmol)
were added to CH3CN (100 mL) which had been freshly
distilled from CaH2. The mixture was stirred at room temper-
ature until a solution was formed. Boc2O (7.73 g, 35.4 mmol)
was then added, and stirring was continued for 2 days. On
the third day, another 0.5 equiv of Boc2O (2.6 g, 11.8 mmol)
was added, and the mixture was stirred for another day. The
CH3CN was removed in vacuo, and the residue was partitioned
between H2O and Et2O. The aqueous layer was washed with
an additional portion of Et2O and then acidified with solid
citric acid to pH 3-4. The aqueous solution was extracted 3
times with EtOAc. The combined organic extracts were washed
with H2O and dried with MgSO4, and the EtOAc was removed
in vacuo to give 17 as a white solid (6.2 g, 98%), which was
recrystallized from hot Et2O: mp 90-91 °C; [R]2D5 +362 (c
2.13, MeOH). 1H and 13C NMR show the presence of 2 rotamers
about the carbamate bond in a ratio of 3:2: 1H NMR (CDCl3)
δ 1.42 and 1.49 (s, 9 H, Boc CH3), 1.7-2.3 (m, 7 H, â-CH2,
γ-CH2, and CH2CH2CHdCH2), 2.70-2.78 (m, 1 H, CH2CH2-
CHdCH2), 3.26-3.78 (m, 2 H, δ-CH2), 4.94-5.06 (m, 2 H,
CHdCH2), 5.70-5.86 (m, 1 H, CHdCH2), 11.05 (br s, 1 H,
COOH); 13C NMR (CDCl3) δ 23.35 (γ-C), 28.56, 29.02 (Boc
CH3), 34.20, 34.66, 35.74, 38.15, and 38.20 (â-C and CH2CH2-
CHdCH2), 49.22, 49.95 (δ-C), 70.92 (R-C), 82.73 (Boc C-O),
115.31, 115.83 (CHdCH2), 137.87, 138.69 (CHdCH2), 157.73
(Boc CdO), 175.39 (COOH); FAB MS m/z 270 [MH]+, 214
[M-(CH3)3C]+. Anal. (C14H23NO4) C, H, N.
(2R,5S)-5-(3′-Bu ten yl)-2-ter t-bu tyl-1-a za -3-oxa bicyclo-
[3.3.0]-octa n -4-on e (15). Freshly distilled diethylamine (28.4
mL, 0.274 mol) and THF (300 mL) were transferred via
cannula to a dry 1 L round-bottom flask under Ar. The solution
was cooled to -78 °C, and then a 1.6 M solution of n-BuLi in
hexanes (158 mL, 0.253 mol) was added via a syringe while
the reaction was stirred. After 10 min, a solution of freshly
prepared oxazolidinone 14 in dry THF (150 mL) was trans-
ferred to the reaction vessel with the aid of a cannula, while
maintaining the temperature at -78 °C. The resulting solution
was stirred for 20 min and then HMPA (44 mL, 0.253 mol)
was added. After an additional 30 min, 4-bromo-1-butene was
added dropwise in 60 mL of THF over ca. 1 h. The reaction
was stirred for 12 h at -76 °C and for 5 more hours at -40 °C
(CH3CN/dry ice bath). The reaction mixture was concentrated
in vacuo to an oil which was partitioned between H2O (250
mL) and CH2Cl2 (400 mL). The aqueous layer was washed with
an additional portion of CH2Cl2 (400 mL). The combined
organic layers were dried with MgSO4, and the solvent was
removed to give an orange oil which was purified by Kugelrohr
distillation (bp 90-120 °C, 0.2 mmHg) to give 15 as a clear oil
(25.4 g, 59%). 1H and 13C NMR indicated the presence of about
13% HMPA. This material was used without further purifica-
(S)-N-ter t-Bu toxyca r bon yl-2-(3′-bu ten yl)p r olin e Ben z-
yl Ester (18). By use of the procedure of Ono et al.26 17 (4.3
g, 16.0 mmol) was dissolved in benzene (60 mL). 1,8-
Diazabicyclo[5.4.0]undec-7-ene (2.43 mL, 16.0 mmol) was
added followed by a solution of benzyl bromide (2.3 mL, 19.2
mmol) in benzene (5 mL). A white precipitate appeared after
ca. 10 min. The mixture was refluxed, and reaction progress
was monitored by TLC. After 4.5 h, additional benzyl bromide
was added (0.57 mL, 4.8 mmol), and the reaction was refluxed
for 4 more hours. The mixture was cooled to room temperature,
and the precipitate was removed by filtration with suction.
The white solid was washed with EtOAc. The combined filtrate
and EtOAc wash were extracted with H2O (50 mL), 10% citric
acid (70 mL), 1 N NaHCO3 (50 mL), and again with H2O (50
mL). The organic layer was dried with MgSO4, and the solvent
was removed in vacuo to give a pale yellow oil which was
subjected to silica gel chromatography (EtOAc/hexanes, 1:10).
The pure product was isolated as a clear oil in 93% yield (5.27
g): [R]2D5
1
tion. H NMR (CDCl3) δ 0.88 (s, 9 H, (CH3)3C), 1.54-1.91 (m,
5 H, 7-CH2, 6-CH2, and CH2CH2CHdCH2), 2.04-2.32 (m, 3
H, CH2CH2CHdCH2, and CH2CH2CHdCH2), 2.80-3.03 (m, 2
H, 8-CH2), 4.22 (s, 1 H, 2-CH), 4.92-5.05 (m, 2 H, CHdCH2),
5.74-5.87 (m, 1 H, CHdCH2); 13C NMR (CDCl3) δ 24.94
(C(CH3)3), 25.46 (7-C), 28.79 (CH2CH2CHdCH2), 36.34, 37.04,
and 37.53 (CH2CH2CHdCH2, C(CH3)3, and 6-C), 58.09 (8-C),
72.31 (5-C), 105.71 (2-C), 115.35 (CHdCH2), 138.67 (CHdCH2),
179.01 (4-C).
+8.4 (c 1.1, CHCl3); TLC Rf ) 0.21 (EtOAc/hexanes,
1:10). 1H and 13C NMR show the presence of two rotamers
about the carbamate bond in a ratio of 2:1: 1H NMR (CDCl3)
δ 1.30 and 1.38 (s, 9 H, Boc CH
3), 1.73-2.08 (m, 7 H, â-CH2,
γ-CH
2, and CH2CH2CHdCH2), 2.22-2.50 (m, 1 H, CH2CH2-
CHdCH
2), 3.27-3.74 (m, 2 H, δ-CH2), 4.88-5.18 (m, 4 H,
CHdCH2 and OCH2Ph), 5.70-5.83 (m, 1 H, CHdCH2), 7.24-
7.32 (m, 5 H, Ph); 13C NMR (75 MHz, CDCl3) δ 23.71 and 23.86
(γ-C), 28.56, 28.88, 29.03, 33.91, 34.84, 36.78, and 38.06
(CH2CH2CHdCH2, Boc CH3, and â-C), 49.25 (δ-C), 67.26,
67.35, 67.95, and 68.42 (R-C and OCH2Ph), 80.02 and 80.63
(Boc C-O), 115.03 and 115.29 (CHdCH2), 128.56, 128.60,
128.72, 128.86, 129.04, 129.22, 136.39, and 136.76 (Ph), 138.75,
139.11 (CHdCH2), 154.35 and 154.57 (Boc CdO), 175.01 and
175.14 (CO2Bn); FAB MS m/z 360 [MH]+; 304 [M-(CH3)3C]+;
260 [M-(CH3)3COCO]+. Anal.(C21H29NO4) C, H, N.
(S)-N-ter t-Bu t oxyca r b on yl-2-(3′-for m ylet h yl)p r olin e
Ben zyl Ester (19). Alkene 18 (5.2 g, 14.46 mmol) was
dissolved in THF/H2O (250 mL, 4:1). To that solution OsO4 (5
mol %, 184 mg, 0.723 mmol) was added while the reaction was
stirred under Ar. After 5 min, NaIO4 (7.73 g, 36.15 mmol) was
added in several portions. The reaction mixture changed in
color from black to lavender-purple upon addition of NaIO4.
The reaction was stirred overnight at room temperature, and
(S)-2-(3′-Bu ten yl)p r olin e (16). Oxazolidinone 15 (21.2 g,
89.7 mmol) was dissolved in 220 mL of MeOH/H2O (6:1). Silica
gel (22 g, 200-400 mesh, 60 Å) was added to that solution,
and the resulting suspension was stirred at room temperature
for 48 h. The reaction mixture was filtered on a fritted Buchner
funnel, and the filtrate was evaporated in vacuo to a yellowish
solid which was triturated with copious amounts of Et2O to
give 14.3 g (94%) of the pure free amino acid 16 as a white
solid: mp 281-282 °C (dec); [R]2D5 -73.6 (c 0.88, MeOH). 1H
NMR (DMSO-d6) δ 1.5-2.1 (m, 7 H, â-CH2, γ-CH2, CH2CH2-
CHdCH2, and CH2CH2CHdCH2), 2.19-2.24 (m, 1 H, CH2CH2-
CHdCH2), 2.93-3.15 (m, 1 H, δ-CH2), 3.16-3.23 (m, 1 H,
δ-CH2), 4.89-4.99 (m, 2 H, CHdCH2), 5.67-5.82 (m, 1 H, CHd
CH2), 8.34 (br s, 1 H, +NH2), 8.74 (br s, 1 H, +NH2); 13C NMR
(75 MHz, CDCl3 with trace CD3OD) δ 24.20 (γ-CH2), 30.04
(CH2CH2CHdCH2), 35.85 and 36.47 (CH2CH2CHdCH2 and

Synthesis of Spiro Bicyclic Peptidomimetics
J ournal of Medicinal Chemistry, 1999, Vol. 42, No. 4 635
then the mixture was gravity-filtered to remove a white
precipitate. The clear filtrate was concentrated in vacuo to
about 1/4 of its volume, and then it was extracted with EtOAc
(2 × 200 mL). The organic layer was dried (MgSO4), and the
solvent was removed under vacuum to give a brown oil. This
oil was chromatographed on a silica gel column using EtOAc/
hexanes (1:1 f 3:1) as the eluent to give 4.3 g (82%) of the
clean product as pale yellow oil: [R]2D5 +11.5 (c 1.7, CHCl3).
1H and 13C NMR show the presence of 2 rotamers about the
addition of NEt3 (1.2 mL, 8.88 mmol) in 20 mL of CH2Cl2. The
resulting solution was refluxed for 6 h. The reaction mixture
was allowed to cool, and then was extracted with 10% citric
acid, 1 N NaHCO3, and brine. Drying with MgSO4 and
subsequent removal of the solvents under vacuum gave 1.5 g
of a mixture of two epimers with respect to the bridgehead
carbon (8′a-C) in a ratio of 1:1. These epimers were readily
separated by silica gel chromatography (EtOAc/hexanes, 1:1
f 3:1). Of the two epimers, 22b, which has the higher Rf value
on TLC, was recrystallized from MeOH/EtOAc to give white
needles. The bridgehead stereochemistry was tentatively as-
signed based on NOE experiments and subsequently confirmed
by X-ray structure determination of 22b.
1
carbamate bond in a ratio of 1:1. H NMR (CDCl3) δ 1.32 and
1.38 (s, 9 H, Boc CH3), 1.7-2.3 (m, 5 H, â-CH2, γ-CH2, and
CH2CH2CHO), 2.3-2.72 (m, 3 H, CH2CH2CHO and â-CH2),
3.27-3.75 (m, 2 H, δ-CH2), 5.02-5.22 (m, 2 H, OCH2Ph), 7.27-
7.34 (m, 5 H, Ph), 9.67 and 9.76 (s, 1 H, CHO); 13C NMR
(CDCl3) δ 22.26 and 22.87 (γ-C), 27.26 and 27.33 (CH2CH2-
CHO), 27.93 and 27.99 (Boc CH3), 36.12 and 37.07 (â-C), 38.68
and 39.02 (CH2CH2CHO), 48.18 and 48.26 (δ-C), 66.60 and
66.65 (OCH2Ph), 66.70 and 67.12 (R-C), 79.69 and 80.30 (Boc
C-O), 127.73, 127.83, 128.07, 128.17, 128.34. 135.16, and
135.53 (Ph), 153.35 and 154.03 (Boc CdO), 173.51 and 174.03
(CO2Bn), 200.99 and 202.59 (CHO); FAB MS m/z 362 [MH]+.
Anal. (C20H27NO5) C, H, N.
22a : Obtained 0.63 g (42%) of a clear oil; [R]2D5 +11.5 (c
0.61, CHCl3); [R]25 +33.1 (c 0.61, CHCl3); TLC Rf ) 0.27
365
1
(EtOAc/hexanes, 3:1), 0.38 (CH2Cl2/MeOH, 20:1). H and 13C
NMR show the presence of two rotamers about the carbamate
bond in a ratio of ca. 1:1: 1H NMR (CDCl3, 328 K) δ 1.44 and
1.49 (s, 9 H, Boc CH3), 1.69-2.35 (m, 7 H, 4-CH2, 7′-CH2, 8′-
CH2, and 3-CH2), 2.69 (ddd, J ) 13.4, 7.3, and 2.4 Hz, 1 H,
8′-CH2), 3.08-3.17 (m, 1 H, 2′-CH2), 3.33-3.62 (m, 3 H, 2′-
CH2 and 5-CH2), 3.73 (s, 3 H, OCH3), 4.62-4.73 (m, 1 H, 3′-
CH) 4.80 (dd, J ) 11.0 and 2.4 Hz, 8′a-CH); 1H NMR (CD3CN)
δ 1.42 and 1.43 (s, 9 H, Boc CH3), 1.70-1.84 (m, 2 H, 4-CH2),
1.92-2.15 (m, 5 H, 7′-CH2, 8′-CH2, and 3-CH2), 2.50-2.57 (m,
1 H, 8′-CH2), 3.08 (d, J ) 12.2 Hz, 1 H, 2′-CH2), 3.36-3.46 (m,
3 H, 2′-CH2 and 5-CH2), 3.63 and 3.67 (s, 3 H, OCH3), 4.56 (d,
J ) 7.3 Hz, 3′-CH), 4.84-4.88 (m, 1 H, 8′a-CH); 13C NMR (CD3-
CN) δ 22.47 and 23.16 (4-C), 28.17 and 28.40 (Boc CH3), 28.52
and 28.95 (7′-C), 33.22, 33.32, 35.77, and 36.42 (3-C and 8′-
C), 42.22 and 44.00 (2′-C), 48.79 and 48.84 (5-C), 52.25 and
52.51 (OCH3), 62.60, 63.02, and 63.28 (3′-C and 8′a-C), 64.62
and 64.78 (6′-C), 79.21 and 80.12 (Boc C-O), 154.68 (Boc
CdO), 170.69 and 171.67 (CO2CH3 and 5′-CdO); FAB MS m/z
371 [MH]+, 315 [M-(CH3)3C]+, 271 [M-(CH3)3COCO]+. Anal.
(C17H26N2O5S) C, H, N.
(R)-N-ter t-Bu t oxyca r b on yl-2-(3′-for m ylet h yl)p r olin e
(20). Compound 19 (1.95 g, 5.4 mmol) was dissolved in benzene
(25 mL) in an Ar-purged pressure bottle. To that solution, 10%
Pd/C (200 mg) catalyst was added, and the mixture was placed
on a Parr shaker under 35 psi of H2 for 34 h. The mixture was
gravity-filtered, and the filtrate was passed through a 0.45 µm
PTFE filter disk. Concentration of the resulting clear solution
under vacuum gave 1.36 g (93%) of 20 as a white foam, which
was used without further purification: [R]2D5 -4.6 (c 0.87,
25
MeOH); [R] -26.4 (c 0.87, MeOH). 1H and 13C NMR show
365
the presence of two rotamers about the carbamate bond in a
ratio of 1:1: 1H NMR (CDCl3) δ 1.28 and 1.30 (s, 9 H, Boc CH3),
1.68-1.93 (m, 3 H, γ-CH2 and CH2CH2CHO), 2.07-2.40 (m, 5
H, CH2CH2CHO, CH2CH2CHO, and â-CH2), 3.2-3.6 (m, 2 H,
δ-CH2), 9.62 and 9.78 (m, 2 H, CHO and COOH); 13C NMR
(75 MHz, CDCl3) δ 23.12 and 23.03 (γ-C), 27.69 and 28.07 (CH2-
CH2CHO), 28.88 and 28.98 (Boc CH3), 37.03 and 37.97 (â-C),
39.3 (CH2CH2CHO), 49.02 and 49.20 (δ-C), 67.37 and 67.98
(R-C), 81.18 and 81.64 (Boc C-O), 154.48 and 155.48 (Boc
CdO), 177.95 and 179.20 (CO2H), 203.84 (CHO); FAB MS m/z
272 [MH]+. Anal. (C13H21NO5) C, H, N.
22b: Obtained 0.62 g (41%) of a white solid; mp 181.5-182.0
°C; [R]2D5 +139.1 (c 0.78, CHCl3); TLC Rf ) 0.49 (EtOAc/
hexanes, 3:1); 0.62 (CH2Cl2/MeOH, 20:1). 1H and 13C NMR
show the presence of two rotamers about the carbamate bond
1
in a ratio of ca. 1:1. H NMR (CDCl3) δ 1.43 and 1.49 (s, 9 H,
Boc CH3), 1.72-2.04 (m, 5 H, 4-CH2, 8′-CH2, and 3-CH2), 2.18-
2.35 (m, 2 H, 3-CH2 and 7′-CH2), 2.67-2.80 (m, 1 H, 7′-CH2),
3.12-3.33 (m, 2 H, 2′-CH2), 3.48-3.70 (m, 2 H, 5-CH2), 3.74
and 3.76 (s, 3 H, OCH3), 4.91-5.11 (m, 2 H, 3′-CH and 8′a-
CH); 13C NMR (75 MHz, CDCl3) δ 23.17 and 24.17 (4-C), 28.56,
28.84, 29.05, and 29.11 (7′-C and Boc CH3), 32.38, 32.49, 32.98,
and 33.26 (3-C and 8′-C), 39.34 and 41.68 (2′-C), 49.17 and
49.33 (5-C), 53.05 and 53.17 (OCH3), 62.88, 63.05, 63.09, and
63.25 (3′-C and 8′a-C), 65.26 and 65.40 (6′-C), 80.07 and 81.30
(Boc C-O), 154.26 and 154.30 (Boc CdO), 171.13, 171.29,
171.85, and 172.17 (CO2CH3 and 5′-CdO); FAB MS m/z 371
[MH]+, 315 [M-(CH3)3C]+, 271 [M-(CH3)3COCO]+. Anal.
(C17H26N2O5S) C, H, N, S.
(2RS,4S)-2-[[2′-(R)-N-(ter t-Bu toxyca r bon yl)-2′-ca r boxy-
p yr r olid in yl]eth yl]th ia zolid in e-4-ca r boxylic Acid Meth -
yl Ester (21). Aldehyde 20 (1.3 g, 4.79 mmol) was dissolved
in 95% EtOH (20 mL), and the solution cooled to -15 °C in an
ice/H2O/NaCl bath. A solution of NaHCO3 (374 mg, 4.45 mmol)
in 20 mL of H2O was added while the reaction was stirred. To
the resulting mixture was added D-cysteine methyl ester
hydrochloride (763 mg, 4.45 mmol) in one portion. The pH of
the solution was adjusted to ca. 7 using NaHCO3, and the
reaction was allowed to warm to room temperature. After 16
h, the solvents were removed under vacuum, and the residue
was redissolved in H2O (25 mL). The solution was extracted
with EtOAc (100 mL). The pH of the aqueous layer was
readjusted to ca. 6 using 1 N HCl, and then the aqueous layer
was extracted with EtOAc (100 mL). To facilitate partitioning
of the thiazolidine mixture into organic solvents, NaCl was
added to the aqueous layer followed by extractions into CHCl3.
This process was repeated 3 times with adjustment of the pH
to 6 between extractions. The organic layers were combined
and dried with MgSO4. Removal of the solvent under vacuum
gave 1.8 g (97%) of the diastereomeric mixture 21 as a white
foam: FAB MS m/z 389 [MH]+, 333 [M-(CH3)3C]+. Anal.
(C17H28N2O6S) C, H, N, S.
Meth yl [3′S-(3′r,6′r,8′a r)]-1-[[1-(ter t-Bu toxyca r bon yl)-
2(S)-p yr r olid in yl]ca r bon yl]-5′-oxosp ir o[p yr r olid in e-2,6′-
th ia zolid in o[3,2-a ]p ip er id in e]-3′-ca r boxyla te (23). Spiro
bicyclic ester 22b (520 mg, 1.59 mmol) was deprotected in 4
N HCl in dioxane under Ar for 3 h. Solvent and excess HCl
were removed in vacuo. Trace HCl was removed with the aid
of CH2Cl2 to give a white solid which was dissolved in DMF
(40 mL). To that solution were added Boc-L-Pro-OH (1.37 g,
6.37 mmol), DCC (1.31 g, 6.37 mmol), HOBt‚H2O (0.80 g, 6.37
mmol), and Et3N (0.22 mL, 1.59 mmol), and the reaction was
stirred at room temperature under Ar for 3 days. The DMF
was removed in vacuo, and the residue was dissolved in EtOAc
(120 mL). The solution was filtered to remove undissolved
material. The filtrate was successively washed with 50 mL of
each of the following solutions: 1 N NaHCO3, H2O, 10% citric
acid, 1 N NaHCO3, H2O, and brine. The organic layer was
dried with MgSO4 and evaporated in vacuo to give a crude oil
which was chromatographed on a 3.5 × 44 cm silica gel column
using 2% MeOH in EtOAc solution as the eluent. The product
was isolated as a white foam in 76% yield (500 mg). For
Meth yl [3′S-(3′r,6′r,8′a â)]-1-(ter t-Bu toxyca r bon yl)-5′-
oxosp ir o[p yr r olid in e-2,6′-th ia zolid in o[3,2-a ]p ip er id in e]-
3′-ca r boxyla te (22a ) a n d Meth yl [3′S-(3′r,6′r,8′a r)]-1-(ter t-
Bu t oxyca r b on yl)-5′-oxosp ir o[p yr r olid in e-2,6′-t h ia zol-
id in o[3,2-a ]p ip er id in e]-3′-ca r boxyla te (22b). To a solution
of the thiazolidine mixture 21 (1.6 g, 4.12 mmol) in freshly
distilled CH2Cl2 (300 mL) was added 2-chloro-1-methylpyri-
dinium iodide (1.13 g, 4.44 mmol). This was followed by

636 J ournal of Medicinal Chemistry, 1999, Vol. 42, No. 4
Khalil et al.
analysis, a sample of 23 was further purified on an HPLC
column (Waters radial compression module, 25 × 10 cm Prep-
Pak) using CH2Cl2/MeOH (30:1) as the mobile phase. The
27.52, 27.77, 29.97, and 30.44 (Pro â-C, 8′-C, 7′-C, and 3-C),
36.06 (2′-CH2), 45.49 and 48.15 (Pro δ-C and 5-C), 57.86 (Pro
R-C), 60.94 and 62.25 (3′-C and 8′a-C), 66.34 (6′-C), 166.82,
168.94, and 170.54 (CONH2, 6′-CdO, and Pro CdO); FAB
HRMS m/z 353.1671 (C16H24N4O3S + H+ requires 353.1649).
Anal. (C16H24N4O3S‚HCl‚1.5H2O): C, H, N, S, Cl.
[3′S-(3′r,6′r,8′a r)]-1-Acetyl-5′-oxospir o[p yr r olid in e-2,6′-
t h ia zo lid in o [3,2-a ]p ip e r id in e ]-3′-N -m e t h y lc a r b o x -
a m id e (25). A concentrated solution of methylamine in MeOH
was added to 22b (143 mg, 0.386 mmol) in a round-bottom
flask equipped with a magnetic stirrer. The reaction vessel
was sealed, and the reaction was stirred at room temperature
for 7 h. The mixture was concentrated under vacuum to give
a white foam which was chromatographed on a 2.5 × 33 cm
silica gel column with EtOAc/hexanes (3:1) as the eluting
solvents. The pure product was obtained as a clear oil (132
mg, 92%) which was crystallized from Et2O to give [3′S-
(3′R,6′R,8′aR)]-1-(tert-butoxycarbonyl)-5′-oxospiro[pyrrolidine-
2,6′-thiazolidino[3,2-a]piperidine]-3′-N-methylcarboxamide as
white needles: mp 153-154 °C; [R]2D5 +142.4° (c 0.79, MeOH;
FAB MS m/z 370 [MH]+. Anal. (C17H27N2O4S) C, H, N, S.
product had a tR ) 10.2 min: [R]25 +58.0 (c 0.55, CHCl3); TLC
D
Rf ) 0.64 (CH2Cl2/MeOH, 10:1); 0.28 (2% MeOH in EtOAc).
1H and 13C NMR show the presence of two rotamers about the
carbamate bond in a ratio of ca. 1:1: 1H NMR (CDCl3) δ 1.39
and 1.42 (s, 9 H, Boc CH3), 1.70-2.32 (m, 11 H, 8′-CH2, Pro
γ-CH2, Pro â-CH2, 4-CH2, 3-CH2, 7′-CH2), 2.72-2.87 (m, 1 H,
7′-CH2), 3.14-3.92 (m, 6 H, 2′-CH2, 5-CH2, and Pro δ-CH2),
3.71 (s, 3 H, OCH3), 4.33 (dd, J ) 7.8 and 3.0 Hz, 0.5 H, Pro
R-CH), 4.47 (dd, J ) 7.2 and 3.6 Hz, 0.5 H, Pro R-CH), 4.90-
4.97 (m, 1 H, 3′-CH), 5.07-5.13 (m, 1 H, 8′a-CH); 13C NMR
(75 MHz, CDCl3) δ 23.97, 24.54, and 24.94 (Pro γ-C and 4-C),
28.75 and 28.79 (7′-C), 29.08 and 29.16 (Boc CH3), 30.12 (Pro
â-C), 31.74, 31.85, and 32.31 (3-C and 8′-C), 38.24 and 38.27
(2′-C), 47.31, 47.50,48.82, and 48.86 (Pro δ-C and 5-C), 53.00
and 53.08 (OCH3), 58.52 (Pro R-C), 62.78, 62.92, and 62.97 (3′-C
and 8′a-C), 66.61 and 66.71 (6′-C), 79.85 and 79.99 (Boc C-O),
154.27 and 155.12 (Boc CdO), 170.93, 171.05, 171.19, 171.60,
173.46, and 173.95 (CdO); FAB HRMS m/z 468.2172 [MH]+
(C22H33N3O6S + H+ requires 468.2170).
The above carboxamide (83 mg, 0.225 mmol) was dissolved
in 4 N HCl in dioxane (3 mL), after which the reaction was
stirred for 4 h. The solvent and excess HCl were removed in
vacuo, and fresh HCl in dioxane (3 mL) was added. After
another 2 h of stirring, the mixture was concentrated under
vacuum and the resulting hydrochloride salt was dissolved in
pyridine (10 mL). Acetic anhydride (2 mL) was added under
N2, and the mixture was stirred overnight. The reaction
mixture was evaporated under vacuum, and residual pyridine
was twice azeotroped with MeOH (5 mL). The resulting crude
product was loaded on a 2.5 × 33 cm silica gel column and
eluted with CH2Cl2/MeOH (20:1), affording 61 mg (87%) of the
product as a clear oil. Crystallization from EtOAc/hexanes gave
25 as white needles: mp 153-154 °C; [R]2D5 +195.8 (c 0.92,
MeOH); 1H NMR (CDCl3) δ 1.64-1.80 (m, 2 H, 8′-CH2), 1.90-
2.15 (m, 3 H, 4-CH2 and 3-CH2), 2.02 (s, 3 H, COCH3), 2.20-
2.40 (m, 3 H, 3-CH2 and 7′-CH2), 2.75, d, J ) 3.6 Hz, 3 H,
NCH3), 3.20 (dd, J ) 11.1 and 7.5 Hz, 1 H, pro R 2′-CH2), 3.50
(dd, J ) 11.1 and 3.9 Hz, 1 H, pro S 2′-CH2), 3.61 (dd, J ) 9.2
and 4.3 Hz, 2 H, 5-CH2), 4.81 (dd, J ) 9.6 and 3.6 Hz, 1 H
8a′-CH), 5.30 (dd, J ) 7.5 and 3.7 Hz, 3′-CH), 7.73 br s, 1 H,
NH; 13C NMR (CDCl3) δ 23.07, (COCH3), 24.02 (4-C), 26.59
(NCH3), 28.67, 30.77, and 31.00 (7′-C, 3-C, and 8′-C), 36.81
(2′-C), 49.46 (5-C), 60.60 and 62.61 (3′-C and 8a′-C), 65.59 (6′-
C), 169.20, 169.54, and 169.97 (CdO); FAB MS m/z 312 [MH]+.
Anal. (C14H21N3O3S) C, H, N, S.
[3′S-(3′r,6′r,8′a r)]-1-[[1-(ter t-Bu toxyca r bon yl)-2(S)-p yr -
r olid in yl]ca r bon yl]-5′-oxosp ir o[p yr r olid in e-2,6′-t h ia zo-
lid in o[3,2-a ]p ip er id in e]-3′-ca r boxa m id e (24). Compound
23 (475 mg, 1.0 mmol) was treated with
a solution of
concentrated NH3 in MeOH (10 mL) at 0 °C. After 5 min, the
reaction was allowed to warm to room temperature, and
stirring was continued until starting material disappeared by
TLC. After 10 h, the solvent and excess NH3 were removed in
vacuo, and the resulting residue was chromatographed on a
silica gel column (2 × 40 cm, CH2Cl2/MeOH, 20:1) to give 440
mg (96%) of the product as a white foam. For analysis, a
sample of 24 was further purified on an HPLC column (Waters
radial compression module, 25 × 10 cm Prep-Pak) using CH2-
Cl2/MeOH (20:1) as the mobile phase. The product had a tR
)
11.2 min: [R]2D5 +36.0 (c 0.78, CHCl3); TLC Rf ) 0.29 (CH2-
Cl2/MeOH, 20:1). 1H and 13C NMR show the presence of 2
rotamers about the carbamate bond in a ratio of ca. 1:1: 1H
NMR (CDCl3) δ 1.40 and 1.43 (s, 9 H, Boc CH3), 1.71-2.35
(m, 11 H, 8′-CH2, Pro γ-CH2, Pro â-CH2, 4-CH2, 3-CH2, 7′-CH2),
2.40-2.55 (m, 1 H, 7′-CH2), 3.27 (dd, J ) 11.0 and 7.8 Hz, 1
H, 2′-CH2), 3.33-3.64 (m, 4 H, 2′-CH2, 5-CH2, and Pro δ-CH2),
3.82 (t, J ) 8.0 Hz, 0.5 H, Pro δ-CH2), 3.98 (t, J ) 7.4 Hz, 0.5
H, Pro δ-CH2), 4.35 (dd, J ) 8.6 and 3.0 Hz, 0.5 H, Pro R-CH),
4.46 (dd, J ) 7.8 and 2.4 Hz, 0.5 H, Pro R-CH), 4.92 (dd, J )
10.8 and 3.6 Hz, 1 H, 3′-CH), 5.27-5.30 (m, 2 H, cis-NH and
8′a-CH) 7.70 and 7.77 (s, 1 H, trans-NH); 13C NMR (75 MHz,
CDCl3) δ 24.19, 24.87, and 24.95 (Pro γ-C and 4-C), 29.05 and
29.11 (Boc CH3), 29.18, 29.24, 30.04, 30.35, 31.54, 31.58, and
31.69 (Pro â-C, 7′-C, 3-C, and 8′-C), 37.03 (2′-C), 47.34, 47.63,
and 48.91 (Pro δ-C and 5-C), 58.48 (Pro R-C), 61.42, 61.50,
63.02, and 63.05 (3′-C and 8′a-C), 66.99 (6′-C), 80.19 and 80.35
(Boc C-O), 154.27 and 155.20 (Boc CdO), 170.38, 170.67,
172.19, 172.42, 172.84 (CdO); FAB HRMS m/z 453.2196
(C21H32N4O5S + H+ requires 453.2174). Anal. (C21H42N4O5S)
C, H, N, S.
X-r a y Diffr a ction . Colorless prisms of 8 were grown from
Et2O, while colorless plates of 10 were grown from MeOH/
hexanes. Colorless needles of 22b and 25 were grown from
MeOH/EtOAc and EtOAc/hexanes, respectively. All measure-
ments were made on a Rigaku AFC6S diffractometer with
graphite monochromated Cu KR radiation (λ ) 1.54178 Å) as
described previously.5
P h a r m a cologica l Assa ys. The [3H]spiroperidol/N-propyl-
norapomorphine (NPA) dopamine D2 receptor competition
binding assay was carried out as described previously.2,5 The
procedures used to measure the apomorphine-induced rota-
tional behavior in the 6-hyroxydopamine-lesioned rat model
of hemiparkinsonism were the same as that described by us
in previous work.5,8,9
[3′S -(3′r,6′r,8′a r)]-1-[2(S )-P yr r olid in ylca r b on yl]-5′-
oxosp ir o[p yr r olid in e-2,6′-th ia zolid in o[3,2-a ]p ip er id in e]-
3′-car boxam ide Hydr och lor ide (4‚HCl). Spiro bicyclic amide
20 (268 mg, 0.59 mmol) was deprotected in 4 N HCl in dioxane
for 8 h under N2. The solvent and excess HCl were removed
in vacuo. The resulting residue was twice suspended in CH2-
Cl2 (5 mL) and then evaporated to a white solid. This material
was lyophilized to give 4‚HCl as a hygroscopic white solid (215
mg, 93%): mp 158-159 °C; [R]2D5 +55.3 (c 0.47, MeOH); 1H
NMR (DMSO-d6) δ 1.67-2.05 and 2.21-2.29 (m, 12 H, 3-CH2,
4-CH2, Pro â-CH2, Pro γ-CH2, 7′-CH2, and 8′a-CH2), 3.12-3.13
(m, 4 H, 5-CH2 and 2′-CH2), 3.38-3.44 (m, 1 H, Pro δ-CH2),
3.81-3.86 (m, 1 H, Pro δ-CH2), 4.47 (m, 1 H, Pro R-CH), 4.80
(dd, J ) 10.99 and 4.9 Hz, 1 H, 3′-CH), 4.95 (dd, J ) 7.33 and
4.88 Hz, 1 H, 8′a-CH), 6.92 (s, 1 H, cis-CONH2), 7.35 (s, 1 H,
trans-CONH2), 8.66 (br s, 1 H, +NH2), 10.23 (br s, 1 H, +NH2);
13C NMR (DMSO-d6) δ 23.08 and 23.61 (4-C and Pro γ-C),
Ack n ow led gm en t. This work was supported in part
by an NIH grant (NS20036) to R.L.J . The authors thank
Tom Crick for mass spectral analysis and Dr. J ulio
Camarero for assistance with the Insight software.
Su p p or tin g In for m a tion Ava ila ble: X-ray crystallo-
graphic data including tables of positional parameters, bond
distances, and bond angles for 8, 10, 22b, and 25. This
material is available free of charge via the Internet at
http://pubs.acs.org.

Synthesis of Spiro Bicyclic Peptidomimetics
J ournal of Medicinal Chemistry, 1999, Vol. 42, No. 4 637
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