Spirolactams as conformationally restricted pseudopeptides: Synthesis and conformational analysis

Article abstract of DOI:10.1021/jo025999i

The synthesis of 1-(tert-butoxycarbonyl)-7-[1-(tert-butoxycarbonyl) -3-methylbutyl]-6-oxo-1,7-diazaspiro[4.5]decanes (S,S)-1a and (S,R)-1b is described. Derivatives 17a,b and 19a are prepared for use in peptide synthesis as constrained surrogates of the Pro-Leu and Gly-Leu dipeptides. The Ac-{Gly-Leu}-Met-NH₂ derivatives (S,S,S)-2a and (S,R,S)-2b, with the tripeptidic C-terminal region present in tachykinins, are also synthesized. Conformational analyses of these tripetide analogues by NMR experiments and molecular modeling calculations show that both (S,S,S)-2a and (S,R,S)-2b epimers are γ-turn/distorted type II β-turn mimetics.

Full text of DOI:10.1021/jo025999i

Sp ir ola cta m s a s Con for m a tion a lly Restr icted P seu d op ep tid es:
Syn th esis a n d Con for m a tion a l An a lysis
M. Montserrat Ferna´ndez, Anna Diez, Mario Rubiralta, Elvira Montenegro, and
Nu´ria Casamitjana*
Laboratori de Quı´mica Orga`nica, Facultat de Farma`cia, Universitat de Barcelona,
Av. J oan XXIII, 08028 Barcelona, Spain
Marcelo J . Kogan and Ernest Giralt
Departament de Quı´mica Orga`nica, Facultat de Quı´mica, Universitat de Barcelona, c/ Martı´ i Franque`s,
08028 Barcelona, Spain
casamitj@farmacia.far.ub.es
Received May 31, 2002
The synthesis of 1-(tert-butoxycarbonyl)-7-[1-(tert-butoxycarbonyl)-3-methylbutyl]-6-oxo-1,7-
diazaspiro[4.5]decanes (S,S)-1a and (S,R)-1b is described. Derivatives 17a ,b and 19a are prepared
for use in peptide synthesis as constrained surrogates of the Pro-Leu and Gly-Leu dipeptides. The
Ac-{Gly-Leu}-Met-NH2 derivatives (S,S,S)-2a and (S,R,S)-2b, with the tripeptidic C-terminal region
present in tachykinins, are also synthesized. Conformational analyses of these tripetide analogues
by NMR experiments and molecular modeling calculations show that both (S,S,S)-2a and (S,R,S)-
2b epimers are γ-turn/distorted type II â-turn mimetics.
In tr od u ction
Gly-Leu and Pro-Leu surrogates (Figure 1). Our synthesis
starts from chiral R-allylproline derivative 3 (Schemes 1
and 2) and builds up the piperidone ring following two
strategies that allow the introduction of other amino
acids in addition to leucine. Until 1998, only 5.5-spiro-
lactams containing a pyrrolidone ring had been described
as conformationally constrained peptide analogues.4-8
More recently, a racemic synthesis of similar 5.6 spiro-
lactams, useful as inhibitors of protein-protein interac-
tions modulated by the SH3 domain and as â-turn
In 1980, Freidinger used for the first time a lactam
ring as a type II′ â-turn mimetic.1 Since then, his original
idea has been developed and several conformationally
constrained peptidomimetics containing a lactam in their
structure have been designed and synthesized.2 Substitu-
tion and size of the lactam ring allow the restriction of
up to three of the four torsion angles that define a â-turn.3
In this context, in the past decade, different spirolactams
have been reported as â-turn mimetics in which the φi+l
and ψi+1 torsion angles are restricted.4-10 With the aim
of obtaining chiral 5.6-spirolactams with the 3-amino-2-
piperidone nucleus in their structure, we report herein
a stereoselective synthesis of compounds (S,S)-1a and
(S,S)-1b, which constitute conformationally constrained
(4) (a) Genin, M. J .; J ohnson, R. L. J . Am. Chem. Soc. 1992, 114,
8778-8783. (b) Genin, M. J .; Gleason, W. B.; J ohnson, R. L. J . Org.
Chem. 1993, 58, 860-866. (c) Genin, M. J .; Ojala, W. H.; Gleason, W.
B.; J ohnson, R. L. J . Org. Chem. 1993, 58, 2334-2337. (d) Sreenivasan,
U.; Mishra, R. K.; J ohnson, R. L. J . Med. Chem. 1993, 36, 256-263.
(e) Genin, M. J .; Mishra, R. K.; J ohnson, R. L. J . Med. Chem. 1993,
36, 3481-3483. (f) Subasinghe, N. L.; Khalil, E. M.; J ohnson, R. L.
Tetrahedron Lett. 1997, 38, 1317-1320. (g) Khalil, E. M.; Ojala, W.
H.; Pradhan, A.; Nair, V. D.; Gleason, W. B.; Mishra, R. K.; J ohnson,
R. L. J . Med. Chem. 1999, 42, 628-637.
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Perlow, D. S.; Veber, D. F. J . Org. Chem. 1982, 47, 104-109.
(2) For some examples, see: (a) Nagai, U.; Sato, K. Tetrahedron Lett.
1985, 26, 647-650. (b) Kahn, M.; Chen, B. Tetrahedron Lett. 1987,
28, 1623-1626. (c) Olson, G. L.; Voss, M. E.; Hill, D. E.; Kahn, M.;
Madison, V.; Cook, C. M. J . Am. Chem. Soc. 1990, 112, 323-333. (d)
Hanessian, S.; McNaughton-Smith, G.; Lombart, H.-G.; Lubell, W. D.
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S. J .; Davies, J . E.; Holmes, A. B. Tetrahedron Lett. 1998, 39, 9309-
9312. (g) Estiarte, M. A.; Rubiralta, M.; Diez, A.; Thormann, M.; Giralt,
E. J . Org. Chem. 2000, 65, 6992-6999. (h) Angiolini, M.; Araneo, S.;
Belvisi, L.; Cesarotti, E.; Checchia, A.; Crippa, L.; Manzoni, L.;
Scolastico, C. Eur. J . Org. Chem. 2000, 2571-2581. (i) Weber, K.;
Ohnacht, U.; Gmeiner, P. J . Org. Chem. 2000, 65, 7406-7416. (j)
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S. J .; J ordan, C. C.; Porter, B.; Ross, B. C.; Ward, P. In Molecular
Recognition: Chemical and Biochemical Problems; Roberts, S. M., Ed.;
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G. R.; Moeller, K. D. J . Med. Chem. 1996, 39, 1571-1574.
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Lett. 1998, 39, 3685-3688.
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Trav. Chim. Pays Bas 1994, 113, 1-63.
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A.; Gonzalez, J . J . Org. Chem. 2001, 66, 6333-6338.
10.1021/jo025999i CCC: $22.00 © 2002 American Chemical Society
Published on Web 10/03/2002
J . Org. Chem. 2002, 67, 7587-7599
7587

Ferna´ndez et al.
SCHEME 1. Syn th esis of Sp ir ola cta m 1a a
F IGURE 1. Spirolactams as conformationally restricted
pseudopeptides.
mimetics,9 was described, as well as the synthesis of
5.5.6- and 5.6.5-spirolactamic Pro-Leu-Gly peptidomi-
metics, which present a thiazolidine ring condensed to
the lactam ring.4f Lately, the synthesis of a new type of
spiro â-lactams that adopt a â-turn conformation was also
achieved.l0
a
Reagents and conditions: (a) CH3I, K2CO3, acetone, reflux,
92%; (b) 9-BBN, THF, rt; then H2O2, 3 N NaOH, rt, 98%; (c) For
6: Ph3P, NBS, CH2Cl2, 0 °C; then, K2CO3, reflux, 71%. For 7:
Ph3P, imidazole, I2, CH2Cl2, rt, 68%; (d) H-Leu-OtBu, K2CO3, DMF,
reflux, 61% from 6 and 81% from 7; (e) TBAH, THF-H2O, rt (not
purified); (f) DCC, CH2Cl2, 0 °C, 23%.
In connection with our studies on substance P inhibi-
tors,11 we chose to study compounds 1a and 1b as
surrogates of the Gly-Leu dipeptide of the Gly-Leu-Met-
NH2 C-terminal region of tachykinins,6c,12 which is known
to be crucial for the biological activity of these neuropep-
tides. Previous works demonstrate that the use of (S)-
5.5-spirolactams improves the inhibitory activity on NK1-
receptors of the native peptide, whereas the (R)-epimer
does not.6 It was also shown that the (S)-5.5-spirolactam
adopts a type-II′ â-turn conformation, whereas the (R)-
epimer does not.6c We were therefore interested in
studying the conformational preferences of our 5.6-
spirolactams for which we prepared the Ac-{Gly-Leu}-
Met-NH2 derivatives (S,S,S)-2a and (S,R,S)-2b (Figure
1).
17
presence of Br2 or ICl18 was unsuccessful. However,
hydroboration of 4 with 9-BBN followed by oxidation with
H2O2 and NaOH19 gave the hydroxy derivative 5 in
excellent yield, which was converted into bromide 6 and
20
iodide 7 by treatment with NBS-PPh3 or I2-PPh3-
imidazole,21 respectively (Scheme 1).
When we tried to obtain spirolactam 1a in a one-pot
procedure, as described for the synthesis of five-mem-
bered lactams,22 by substitution of the halide atom in 6
or 7 by Leu-OtBu, followed by intramolecular lactamiza-
tion under a variety of experimental conditions, amine 8
was the only product isolated. The best results for amine
8 were obtained by treatment of iodide 7 with K2CO3 in
DMF at 110 °C. Direct lactamization of amino ester 8
under either acidic (SiO2)23 or basic (Et3N)24 conditions
failed. Cyclization of 8 was achieved by hydrolysis of the
ester function with tetrabutylammonium hydroxide
(TBAH)25 and intramolecular coupling of the resulting
amino acid 9 using DCC as the activating agent, which
yielded the target lactam 1a , although in a low yield.
Resu lts a n d Discu ssion
Syn th esis a n d Str u ctu r a l Assign m en t. The starting
R-allylproline ester 4 was prepared from (R)-R-allylpro-
line13,14 as described by J ohnson et al.4a,15 with slight
modifications in the N-protection16 and esterification
steps. Conversion of the N-Boc methyl ester 4 into the
corresponding bromide 6, or iodide 7, by an anti-Mark-
ovnikov addition to the double bond using 9-BBN in the
(17) Kabalka, G. W.; Sastry, K. A. R.; Hsu, H. C.; Hylarides, M. D.
J . Org. Chem. 1981, 46, 3113-3115.
(11) Diez, A.; Voldoire, A.; Lo´pez, I.; Rubiralta, M.; Segarra, V.;
Page`s, Ll.; Palacios, J . M. Tetrahedron 1995, 51, 5143-5156.
(12) (a) Deal, M. J .; Hagan, R. M.; Ireland, S. J .; J ordan, C. C.;
McElroy, A. B.; Porter, B.; Ross, B. C.; Stephens-Smith, M.; Ward, P.
J . Med. Chem. 1992, 35, 4195-4204 and references therein. (b)
Howson, W. DN&P 1995, 8, 97-103.
(13) (a) Seebach, D.; Boes, M.; Naef, R.; Schweider, B. J . Am. Chem.
Soc. 1983, 105, 5390-5398. (b) Beck, A. K.; Blank, S.; J ob, K.; Seebach,
D. Org. Synth. 1995, 72, 62-73. (c) Seebach, D.; Sting, A. R.; Hoffman,
M. Angew. Chem., Int. Ed. Engl. 1996, 35, 2708-2748.
(14) Better results were obtained starting from N-trimethylsilyl-
proline trimethylsilyl ester as described by: Annunziata, R.; Ferrari,
M.; Papeo, G.; Resmini, M.; Sisti, M. Synth. Commun. 1997, 27, 23-
38.
(18) Gooch, E. E.; Kabalka, G. W. Synth. Commun. 1981, 11, 521-
525.
(19) Zydowsky, T. M.; Dellaria, J . F.; Nellans, H. N. J . Org. Chem.
1988, 53, 5607-5616.
(20) (a) Hanessian, S.; Ponpipom, M. M.; Lavallee, P. Carbohydr.
Res. 1972, 24, 45. (b) J a¨ger, V.; Ha¨fele, B. Synthesis 1987, 801-806.
(21) (a) Garegg, J .; Samuelsson, B. J . Chem. Soc., Perkin Trans. 1
1980, 2866-2869. (b) Wu, I.; Ahlberg, P. Synthesis 1994, 463-464.
(c) Bennett, S. M.; Biboutou, R. K.; Zhou, Z.; Pion, R. Tetrahedron 1998,
54, 4761-4786.
(22) Trost, B. M.; Fleming, I. Comprehensive Organic Synthesis;
Pergamon Press: Oxford, 1991; Vol. 6, pp 407-408.
(23) Baiocchi, L.; Picconi, G. Tetrahedron: Asymmetry 1991, 2, 231-
234.
(15) Genin, M. J .; Baures, P. W.; J ohnson, R. L. Tetrahedron Lett.
1994, 35, 4967-4968.
(16) Khalil, E. M.; Subasinghe, N. L.; J ohnson, R. L. Tetrahedron
Lett. 1996, 37, 3441-3444.
(24) Mart´ın-Lo´pez, M. J .; Rodr´ıguez, R.; Bermejo, F. Tetrahedron
1998, 54, 11623-11636.
(25) Abdel-Magid, A. F.; Cohen, J . H.; Maryanoff, C. A.; Shah, R.
D.; Villani, F. J .; Zhang, F. Tetrahedron Lett. 1998, 39, 3391-3394.
7588 J . Org. Chem., Vol. 67, No. 22, 2002

Spirolactams as Conformationally Restricted Pseudopeptides
SCHEME 2. Syn th esis of Sp ir ola cta m s 1a a n d 1b a
the hydroxyl group by DEAD, instead of the expected
lactam 1a . Treatment of alcohol 13a with NBS or MsCl
yielded bromide 14a or mesylate 15a , respectively. In-
tramolecular alkylation29,30 with NaH of both 14a and
15a gave the expected spirolactam 1a . Similarly, spiro-
lactam 1b was prepared from spirolactone 11 by reaction
with D-Leu-OtBu to give hydroxy amide 13b, which was
converted to its mesylate 15b and cyclized with NaH in
the presence of crown ether.
Spirolactams 1a and 1b were identified from their
1
analytical data. The splitting of signals in the H NMR
and 13C NMR spectra indicated the presence of slow-
converting rotamers, which was confirmed for compound
1a by a variable temperature experiment (29-65 °C)
using CD3CN as the solvent. When the temperature was
raised to 65 °C, the signal patterns for the tert-butyl, the
2-position, and the R-position protons were considerably
simplified. Thus, the tert-butyl protons collapsed from two
singlets at 29 °C to only one singlet at 65 °C, the H-2
protons changed from one multiplet at 29 °C to a triplet
at 65 °C, and the R-proton collapsed from two doublets
at 29 °C to only one doublet at 65 °C. Compound 1b also
showed split signals, but their coalescence was not
observed in any of the solvents used (CDCl3, CD3CN, and
DMSO-d6).
Spirolactams 1a and 1b were then converted into
several derivatives to be used as Pro-Leu and Gly-Leu
surrogates in peptide synthesis, both in Fmoc/OtBu and
in Boc/OBn strategies. Thus, N-Boc-spirolactams 1a and
1b were N-deprotected with TMSOTf and 2,6-lutidine31
to give 16a and 16b and protected again with Fmoc-Osu32
to give the desired Fmoc/OtBu derivatives 17a and 17b
(Scheme 3). The preparation of the Boc/OBn derivatives
was not straightforward. Treatment of compound 1a with
TFA33 furnished the trifluoroacetic salt of amino acid 18a ,
which by reaction with Boc2O and TMAH afforded the
N-Boc-protected acid 19a in a 33% yield. Compound 19a
was also prepared from spirolactone 11 following the
same synthetic sequence used for the preparation of 1a ,
using Leu-OBn to obtain amide 20a . Interestingly, during
the final cyclization step the debenzylation of the inter-
mediate ester 21a also took place, to give directly 19a .
Once pseudopeptides 1a and 1b were obtained, we
prepared Ac-{Gly-Leu}-Met-NH2 derivatives 2a and 2b.
The required N-acetylated dipeptide 23a was prepared
from spirolactam 16a by N-acetylation and hydrolysis of
the resulting tert-butyl ester 22a (Scheme 4). Alterna-
a
Reagents and conditions: (a) C6F5OH, DCC, THF, 0 °C, 75%;
(b) 9-BBN, THF, rt; then, H2O2, 3 N NaOH, rt; (c) C6H5OH, DCC,
THF, 0 °C, 30% from 3; (d) 9-BBN, THF, rt; then, H2O2, 3 N NaOH,
rt, 54%; or BH3, THF, rt; then TMANO, diglyme, rt and reflux;
and then DIPEA, CH2Cl2, rt, 46%; (e) Leu-OtBu or D-Leu-O’Bu,
2-hydroxypyridine, toluene, reflux, 65% for 13a and 68% for 13b;
(f) For 14a : NBS, Ph3P, CH2Cl2, 0 °C; then, K2CO3, reflux, 43%.
For 15a and 15b: MsCl, pyridine, CH2Cl2, rt, (not purified); (g)
From 14a : NaH, 18-crown-6, THF, rt, 48%. From 15a and 15b:
NaH, 15-crown-5, THF, rt, 77% from 15a and 54% from 15b.
The lack of reactivity of amino ester 8 and the instabil-
ity of compound 9 prompted us to develop a second route
to obtain diazaspiro compounds 1a ,b on the basis of the
formation of the N7-C8 bond as the key step (Scheme
2). For this route, carboxylic acid 3 was esterified with
pentafluorophenol26 to yield the corresponding activated
ester 10, which after hydroboration and oxidation, either
in basic or neutral conditions, yielded spirolactone 11.
Lactone 11 was also obtained, although in a lower yield,
when first performing the functionalization of the double
bond to give the hydroxy acid 12, followed by esterifica-
tion with pentafluorophenol.
(27) (a) Strekowski, L.; Visnick, M.; Battiste, M. A. J . Org. Chem.
1986, 51, 4836-4839. (b) Kumazawa, T.; Harakawa, H.; Obase, H.;
Oiji, Y.; Tanaka, H.; Shuto, K.; Ischii, A.; Oka, T.; Nakamizo, N. J .
Med. Chem. 1988, 31, 770-785.
Conversion of spirolactone 11 into spirolactams 1a ,b
was planned by opening of the lactone ring with the
corresponding amine, followed by cyclization of the
resulting hydroxy amides 13. Thus, treatment of com-
pound 11 with Leu-OtBu in the presence of 2-hydroxy-
pyridine27 yielded hydroxy amide 13a . Cyclization of
13a under Mitsunobu reaction conditions (DEAD and
Ph3P)4b,c,28 gave a product resulting from substitution of
(28) Hughes, D. L. Org. React. 1992, 42, 335-376.
(29) (a) Aebi, J . D.; Guillaume, D.; Dunlap, B. E.; Rich, D. H. J . Med.
Chem. 1988, 31, 1805-1815. (b) Lee, J . P.; Dunlap, B.; Rich, D. H.
Int. J . Peptide Protein Res. 1990, 35, 481-494.
(30) (a) Bonjoch, J .; Casamitjana, N.; Quirante, J .; Rodriguez, M.;
Bosch, J . J . Org. Chem. 1987, 52, 267-275. (b) Alves, J . C. F.; Simas,
A. B. C.; Costa, P. R. R.; D′Angelo, J . Tetrahedron: Asymmetry 1997,
8, 1963-1966.
(31) (a) Sakaitani, M.; Ohfune, Y. J . Org. Chem. 1990, 55, 870-
876. (b) Zhang, A. J .; Russell, D. H.; Zhu, J .; Burgess, K. Tetrahedron
Lett. 1998, 39, 7439-7442.
(32) (a) Paquet, A. Can. J . Chem. 1982, 60, 976-980. (b) Bardaji,
E.; Torres, J . L.; Clape´s, P.; Albericio, F.; Barany, G.; Rodr´ıguez, R.
E.; Sacrista´n, M. P.; Valencia, G. J . Chem. Soc., Perkin Trans. 1 1991,
1755-1759.
(33) Toshima, H.; Maru, K.; Saito, M.; Ichihara, A. Tetrahedron
1999, 55, 5793-5808.
(26) (a) Schmidt, U.; Lieberknecht, A.; Griesser, H.; Talbiersky, J .
J . Org. Chem. 1982, 47, 3261-3264. (b) Kisfaludy, L.; Schon, I.
Synthesis 1983, 325-327. (c) Torisawa, Y.; Hosaka, T.; Tanabe, K.;
Suzuki, N.; Motohashi, Y.; Hino, T.; Nakagawa, M. Tetrahedron 1996,
52, 10597-10608.
J . Org. Chem, Vol. 67, No. 22, 2002 7589

Ferna´ndez et al.
SCHEME 3. Syn th esis of P r o-Leu a n d Gly-Leu
Su r r oga tesa
SCHEME 4. Syn th esis of Ac-{Gly-Leu }-Met-NH2
Der iva tivesa
a
Reagents and conditions: (a) AcCl, pyridine, CH2Cl2, rt, 72%
for 22a , 95% for 22b, 59% for 23a from 18a , and 68% for 23b from
18b; (b) H2O-AcOH-iPrOH, 100 °C, 60% for 23a and 88% for
23b; (c) H-Met-NH2‚HCl, DCC,HOBt, DMF, rt, 70% for 2a and
42% for 2b.
of â-turns show a characteristic hydrogen bond between
the oxygen atom of the carbonyl function of the first
amino acid (i) and the NH amide proton of the fourth (i
+ 3). If the distance between the oxygen and the
hydrogen atoms involved is less than 2.5 Å, a hydrogen
bond is present,38 while if it is between 2.5 and 4 Å,35c,39
there is a significant interaction between them. The
major â-turns are classified according to the torsion
angles of the second (φ1, ψ1: φ1 ) -60° ( 30° and ψ1 )
120° ( 30°) and third amino acids (φ2, ψ2: φ2 ) 80° (
30° and ψ2 ) 0° ( 45°).3 We shall maintain this clas-
sification for the â-turn mimetics of the tripeptides type
2a and 2b. The main features characterizing a classical
γ-turn are the torsion angles of the second amino acid
(φ2, ψ2: φ2 ) 70°/85° and ψ2 ) -60°/-70°) and the
distance between the carbonyl carbon atom of the second
amino acid (i + l) and the nitrogen atom of the amino
group of the third residue (i + 3) with a preferential value
of 3 Å (Figures 2-4).35a
a
Reagents and conditions: (a) TMSOTf, 2,6-lutidine, CH2Cl2,
rt, (not purified); (b) Fmoc-Osu, NaHCO3, acetone, rt, 48% for 17a
and 60% for 17b; (c) TFA, rt, 79% for 18a and quantitative for
18b; (d) Boc2O, TMAH, CH3CN, rt, 33%; (e) H-Leu-OBn, 2-hy-
droxypyridine, toluene, reflux, 50%; (f) MsCl, pyridine, CH2Cl2,
rt, (not purified); (g) NaH, 15-crown-5, THF, rt, 42%.
tively, compound 23a was also obtained by acetylation
of the amino acid salt 18a . The coupling of 23a with Met-
NH2 using DCC and HOBt yielded the pseudotripeptide
2a in 70% yield. Compound 2b was obtained from 16b
and 18b following a similar sequence.
Con for m a tion a l Stu d ies of 2a a n d 2b. To determine
whether compounds 2a and 2b can promote a â- or a
γ-turn conformation, we performed conformational stud-
ies using NMR experiments and molecular modeling
calculations. â- and γ-turns are nonperiodic peptide
segments that reverse the orientation of the peptide
chain.34,35 The most general features of a â-turn are the
distance R (<7 Å) between the CRi and CRi+3 and the
dihedral angle τ (-90° < τ < 90°) formed by the four CR
atoms belonging to the tetrapeptide.36 The major types37
In our case, the first relevant observation was that in
1
the H NMR spectra of 2a and 2b, both in CDCl3 and in
DMSO-d6, each of the methionine amide protons NHa
showed four signals (Table 1).40
This fact was in accordance with the presence of
several slow-converting conformations in equilibrium in
solution.
NMR Stu d ies. 1. NOESY. In both compounds, the
amide proton NHa was correlated with the 8-H proton
of the piperidine ring and with the acetyl methyl group
(34) (a) Nowick, J . S.; Smith, E. M.; Pairish, M. Chem. Soc. Rev.
1996, 25, 401-415. (b) Lippens, G.; Hallega, K.; Van Belle, D.; Wodak,
S. J .; Nirmala, N. R.; Hill, P.; Reussell, K. C.; Smith, D. D.; Hruby, V.
J . Biochemistry 1993, 32, 9423-9434.
(35) (a) Brickman, K.; Yuan, Z.; Sethon, I.; Somfai, P.; Kihlberg, J .
Chem. Eur. J . 1999, 5, 2241-2253. (b) Lindvall, M. K.; Rissanen, K.;
Hakala, J . M. L.; Koskinen, A. M. P. Tetrahedron. Lett. 1999, 40, 7427-
7430. (c) Belvisi, L.; Gennari, C.; Mielgo, A.; Potenza, D.; Scolastico,
C. Eur. J . Org. Chem. 1999, 389-400. (d) Belvisi, L.; Gennari, C.;
Madder, A.; Mielgo, A.; Potenza, D.; Scolastico, C. Eur. J . Org. Chem.
2000, 695-699. (e) Belvisi, L.; Bernardi, A.; Manzoni, L.; Potenza, D.;
Scolastico, C. Eur. J . Org. Chem. 2000, 2563-2569.
(38) Geffrey, G. A. Introduction to Hydrogen Bond; Oxford Univer-
sity Press: New York, 1997.
(39) Takeuchi, Y.; Marshall, G. R. J . Am. Chem. Soc. 1998, 120,
5363-5372.
(40) To facilitate the location of the different NH amide protons,
letters a, b, and c have been assigned to each of them. Different
conformations are distinguished by a numerical subscript.
(36) Perczel, A.; McAllister, M. A.; Csaszar, P.; Csizmadia, I. G. J .
Am. Chem. Soc. 1993, 115, 4849-4858.
(37) Vencatachalam, C. M. Biopolymers 1968, 6, 1425-1436.
7590 J . Org. Chem., Vol. 67, No. 22, 2002

Spirolactams as Conformationally Restricted Pseudopeptides
TABLE 1. Ch em ica l Sh ifts, ∆δ, u p on Ad d ition of DMSO a n d Tem p er a tu r e Coefficien ts for 2a a n d 2b
compd 2a
compd 2b
|∆δ/∆T|
(ppb/K)
(CDCl3)
|∆δ/∆T|
(ppb/K)
(DMSO)
|∆δ/∆T|
(ppb/K)
(CDCl3)
|∆δ/∆T|
(ppb/K)
(DMSO)
δ(CDCl3)
(ppm)
δ(DMSO)
(ppm)
∆δ
(ppm)
δ(CDCl3)
(ppm)
δ(DMSO)
(ppm)
∆δ
(ppm)
Ha l
Ha 2
Ha 3
Ha 4
Hbl
Hb2
Hb3
Hb4
Hc1
Hc2
Hc3
Hc4
7.90
7.76
7.67
7.52
7.11
7.11
7.11
6.80
5.32
5.29
7.62
7.61
7.49
7.46
7.31
7.31
7.22
7.22
7.07
7.04
6.99
6.92
-0.28
-0.15
-0.18
-0.06
0.20
0.20
0.11
0.42
1.75
3.5
3.0
3.5
2.5
3.0
5.0
5.0
5.0
4.5
2.8
2.7
7.90
7.77
7.69
7.49
7.63
7.60
7.48
7.46
7.31
7.31
7.22
7.22
7.07
7.04
7.00
6.92
-0.27
-0.17
-0.21
-0.03
3.5
3.0
4.0
2.5
3.6
2.9
3.1
4.1
5.0
5.0
3.0
4.0
5.5
5.0
7.15
7.11
6.70
5.20
5.18
5.18
5.08
0.16
0.11
0.52
1.87
1.86
1.82
1.84
5.0
4.0
6.7
6.0
9.0
5.0
3.5
3.0
3.0
3.0
8.0
8.0
1.75
5.19
1.73
4.0
5.0
Leu residue. This indicated the difference in conforma-
tional space due to the leucine configuration.
NMR Stu d ies. 2. Ch em ica l Sh ift, Ad d ition of a
Com p etitive Solven t, a n d Tem p er a tu r e Coefficien t.
The temperature coefficients for proton NHc in DMSO
for compounds 2a (|∆δ/∆T| ) 5-9 ppb/K) and 2b (|∆δ/
∆T| ) 5-8 ppb/K) clearly indicated that this amide
proton was exposed to the solvent2g,4a (Table 1).
In contrast, proton NHa was probably intramolecularly
hydrogen bonded (|∆δ/∆T| ) 2.7-3.1 ppb/K for 2a and
|∆δ/∆T| ) 2.9-3.6 ppb/K for 2b). However, temperature
coefficients in DMSO for NHb amide proton (|∆δ/∆T| )
4-5 ppb/K) suggested that this proton was in an equi-
librium between hydrogen-bonded and non-hydrogen-
bonded states.35c Other criteria that have been applied
to study the presence of hydrogen bonds in peptide
F IGURE 2. Hydrogen bonds stabilize γ- and â-turns in
spirolactam 2a .
1
molecules are the H NMR chemical shift of the amide
protons in CDCl3 and the ∆δ upon addition of a good
hydrogen bond forming solvent like DMSO. The 1H NMR
chemical shift values of protons NHa and NHb in CDCl3
were similar for 2a and 2b around δ 7 ppm, suggesting
that both protons were intramolecularly hydrogen-
bonded. However, proton NHc resonated at a lower
chemical shift value (δ around 5 ppm), indicating that it
was non-hydrogen-bonded.35c Similarly, when 1H NMR
spectra were run in a concentration gradient of DMSO
in CDCl3, the chemical shift of protons NHa and NHb in
both 2a and 2b scarcely changed (∆δNHa ) 0.06-0.37 ppm
and ∆δNHb ) 0.11-0.52 ppm), whereas proton NHc
underwent a strong downfield shift (∆δ ) 1.73-1.87
ppm), which confirmed that protons NHa and NHb were
intramolecularly hydrogen-bonded while proton NHc was
non-hydrogen-bonded.41,42 The overall NMR analysis
suggested that the molecules adopted folded conforma-
tions and that, in DMSO, the amide proton NHc was non-
(41) Andre´, F.; Vicherat, A.; Boussard, G.; Aubry, A.; Marraud, M.
J . Peptide Res. 1997, 50, 372-381.
F IGURE 3. NOESY correlations in 2a and 2b.
(42) In our case, the temperature coefficient values in CDCl3 were
less reliable. Only in some of the 2a and 2b conformers were the
temperature coefficients low enough, according to the criteria fixed by
Scolastico (∆δ/∆T < 2.6 ppb/K),35c to be able to conclude without any
doubt the existence of intramolecular hydrogen bonding between NHa
and COi+1. In all the other cases, we are in the two situations described
by these authors as non-hydrogen-bonded amide protons or equilibrium
between hydrogen-bonded and non hydrogen-bonded states. It is worth
mentioning that proton NHb can be considered non hydrogen-bonded,
but surprisingly in this case proton NHc appears to be in equilibrium
between the hydrogen-bonded and non hydrogen-bonded state. This
could only be explained if this latter hydrogen-bond involves a third
(Figure 3). These correlations implied that in space the
acetyl and the methionine were next to each other and
that the molecule adopted folded conformations in which
the pseudodihedral angle τ was probably close to 0°.
Moreover, these correlations suggested proximity of
proton NHa to the carbonyl residue i, and to the carbonyl
residue i + 1, which was important for the adoption of a
â- or a γ-turn conformation. In addition, in compound 2b
proton NHa was also correlated with the R-proton of the
carbonyl group which can only be leucine COi+2
.
J . Org. Chem, Vol. 67, No. 22, 2002 7591

Ferna´ndez et al.
TABLE 2. P er cen ta ges of Min im a En er gy Con for m er s
of th e P seu d op ep tid es Wh ich Exh ibit Differ en t F ea tu r es
of â-II a n d /or γ-Tu r n s (Mon te Ca r lo Con for m a tion a l
Sea r ch )
epimer
2a
81
2b
89
turn type
R (CR - CRi+3) < 7 Å
-90° < τ > +90°
NHa‚‚‚OiC < 2.5 Å
â
83
87
30 (30a)
23 (23a)
+70° < Φ2 > +85°
-60° < Ψ2 > -70°
NHa‚‚‚Oi+1C < 2.5 Å
52
57
γ
34
35
65 (59b)
63 (54b)
-90° < Φ1 > -30°
+90° < Ψ1 > +150°
+50°< Φ2 > +110°
-45° < Ψ2 > +45°
NHa‚‚‚OiC < 4 Å
NHa‚‚‚Oi+1C < 4 Å
NHb‚‚‚OiC < 2.5 Å
NHc‚‚‚Oi+2C < 2.5 Å
ω0 trans/cis
100
54
83
39
69
100
41
25
100
34
78
37
81
100
47
15
âII
F IGURE 4. Dihedral angles for structures 2a and 2b.
92/5
95/8
a
Percentage of conformers that form hydrogen bonds: the
distances NHa‚‚‚OiC are less than 2.5 Å, the NHa‚‚‚Oi bond angle
is larger than 120°, and the NHa‚‚‚OidC angle is larger than 90°.
hydrogen-bonded, proton NHa was involved in a hydrogen-
bond, and proton NHb was in an equilibrium between a
hydrogen-bonded and a non-hydrogen-bonded state.
Molecu la r Mod elin g. 1. Mon te Ca r lo Ca lcu la tion s.
To determine the conformational space available to
compounds 2a and 2b, we performed both Monte Carlo
and molecular dynamics with iterative simulated an-
nealing43 calculations. For the Monte Carlo protocol,
molecular models for compounds 2a and 2b were initially
constructed using the model-building facility imple-
mented in Spartan 5.0. Coordinates for these structures
were used as input for the Monte Carlo protocol.44
Rotation was allowed around the dihedral angles, which
constitute the main determinants of the structure (Figure
4). A conformational search of 2a and 2b was carried out
with Spartan version 5.0,45a starting from different
extended conformations. Structures corresponding to the
energy minima (as calculated with the MMF94 force
field45b) within a 10 kcal/mol window above the global
minimum were analyzed on the basis of all the geometric
turn diagnosis parameters, which are diagnostic for the
presence of γ- and â-turns. The corresponding distribu-
tion profiles are summarized in Table 2 and the Sup-
porting Information.
b
Percentage of conformers that form hydrogen bonds: the dis-
tances NHa‚‚‚Oi+1C are less than 2.5 Å, the NHa‚‚‚Oi+l bond angle
is larger than 120°, and the NHa‚‚‚Oi+ldC angle is larger than
90°.
considerable percentage of the population. Another hy-
drogen bond between NHb‚‚‚COi was probably present
in about half of the most stable conformers.38
To determine the presence of cis and trans conformers
for the acetamide group, we analyzed the dihedral angle
ω0 (Figure 4). In the majority of the conformer popula-
tions of both epimers (Table 2) we found a trans disposi-
tion for the C-N bond and a small proportion of cis. This
1
result was in agreement with the H NMR observation
of two rotamers (the acetyl methyl group was split; see
the Experimental Section).
A more detailed study of the conformers obtained in
the Monte Carlo search allowed us to classify the minima
energy conformers of each epimer into families (Table 3,
Figure 5). Epimers 2a and 2b yielded four families of
conformers. This result agreed with the observation of
four signals for the methionine NHa amide proton of 2a
and 2b in the 1H NMR spectra (in CDCl3 and DMSO-
d6). All conformers met the general criteria (R and τ) for
a â-turn. Compound 2a showed distances allowing a
hydrogen bond38 between NHa and COi+1 for the families
of conformations 2a 1, 2a 2, and 2a 4, which corresponded
to γ-turn conformations. In contrast, family 2a 3 showed
a hydrogen bond distance between NHa and COi which
corresponded to a â-turn. In addition, families 2a 1 and
2a 2 had a supplementary hydrogen bond between NHb
and COi. However, family 2a 4 showed a remarkably long
distance between NHb and COi but a short distance
between NHc and COi+2 (1.82 Å). A closer analysis of the
dihedral angles Ψ1, Φ1, Ψ2, and Φ2 confirmed that
families 2a 1, 2a 2, and 2a 4 were γ-turn conformations,
whereas 2a 3 was a â-II-type turn. A similar analysis of
the data for epimer 2b indicated that in this case all four
families of conformations were γ-turns. Families 2b1-3
had a supplementary hydrogen bond between NHb and
COi. Family 2b4 showed a long distance between NHb
A general analysis of the Monte Carlo results (Table
2) showed that two characteristic geometric features of
the â-turn (R and τ) were fulfilled by most of the stable
conformers of 2a and 2b. However, the distance con-
straint for NHa‚‚‚COi was fulfilled by approximately 30%
of the population of conformers of 2a and 2b. On the other
hand, the distance between NHa and COi+i corresponded
to that of a hydrogen bond, and the angle values φ2 and
ψ2 agreed well with the values expected for a γ-turn in a
(43) Corcho, F.; Filizola, M.; Peres J . J . Chem. Phys. Lett. 2000, 319,
65-70.
(44) To validate this approach on the mimetics of spiro compounds,
we ran a test calculation (3000 steps) on compound 5 of ref 47 allowing
rotation around all dihedral angles which constitute the main deter-
minants of the structure. The starting conformation was totally
extended. As a result we obtained similar distribution profiles to those
reported by Muller et al. in ref 47 for compound 5 (data not shown).
(45) (a) Hehre, W. J .; Huang, W. W.; Klunzinger, P. E.; Deppmeier,
B. J .; Driessen, A. J . A Spartan Tutorial; Wavefunction, Inc.: Irvine,
CA, 1997; p 85-93. (b) (MMFF 94) Halgren, T. A. J . Comput. Chem.
1996, 17, 490-519.
and COi, but proton NHc was close to COi+2
.
7592 J . Org. Chem., Vol. 67, No. 22, 2002

Spirolactams as Conformationally Restricted Pseudopeptides
TABLE 3. Ch a r a cter istics of En er gy Min im a Con for m er s (Mon te Ca r lo) for Com p ou n d s 2a a n d 2b
2a
2b
family
1
2
3
4
1
2
3
4
âIIa
γa
∆E (kcal/mol)
R (Å)
τ (deg)
NHa‚‚‚OiC (Å)
NHa‚‚‚Oi+1C (Å)
φ1 (deg)
ψ1 (deg)
φ2 (deg)
ψ2 (deg)
NHb‚‚‚OiC (Å)
0
6.2
11
3.7
1.8
-65
162
74
2
5.8
18
3.0
1.8
-57
157
76
2.2
4.7
5
2.0
2.7
-58
134
61
4.1
6.1
11
4.0
1.8
-68
165
72
0
6,3
22
3,6
2,0
-65
164
85
1.3
5.9
24
3.2
1.9
-63
161
84
3.2
5.9
7
4.3
1.9
-70
158
75
3.7
5,6
23
3.2
1.9
-59
157
84
<7
0 ( 90
<2.5
<2.5
-60 ( 30
120 ( 30
80 ( 30
0 ( 45
70/85
-60/-70
-68
2.0
-54
1.8
21
3.1
-78
6.4
-76
1.9
-66
1.8
-95
1.9
-70
6.0
a
Expected values for canonical â-II and γ turns.
even though many of the criteria for the formation of
â-turn were also met, indicating that families 2a 1-2, 2a 4,
and 2b1-4 presented the so-called “γ-turn/distorted type-
II â-turn” structure. In these structures, the presence of
the NHa and COi+1 hydrogen bond defines a seven-
membered pseudocycle more entropically favored than
the typical 10 membered ring defined by NHa and COi
in â-turns.
Molecu la r Mod elin g. 2. Molecu la r Dyn a m ics Ca l-
cu la tion s. The above results were confirmed performing
a parallel conformational search using molecular dynam-
ics with Iterative Simulated Annealing.43 This protocol46
was run five times, employing fully extended starting
structures of both compounds (2a and 2b) with AMBER
(implemented in DISCOVER 3) as the force field. The
profile for the main parameters characterizing γ- and
â-turns were similar to those found using the Monte
Carlo protocol (data not shown).
To study the dynamic behavior of the most stable
family of conformations of pseudopeptides 2a and 2b, the
global minima conformers of both compounds (2a 1 and
2b1) were taken as the starting points for molecular
dynamics calculations. We analyzed the geometric â- and
γ-turn diagnosis parameters of the structures generated
during the molecular dynamics simulation for 1 ns at 300
K in CDCl3 as an explicit solvent (Table 4). The distances
R found in the resulting conformers of both epimers 2a 1
and 2b1 during the molecular dynamics remained below
7 Å. Both conformers, 2a 1 and 2b1, showed no major
dihedral angle τ fluctuations with respect to those of the
starting structures for the molecular dynamic simula-
tions. The distances found for NHa‚‚‚OiC were not
compatible with the presence of a hydrogen bond (which
would stabilize the â-turn) in either of the conformers
2a 1 or 2b1. It is worth mentioning that during the
molecular dynamics simulation the distances correspond-
ing to the hydrogen bond NHb‚‚‚OiC were not maintained
either. In contrast, the distance values corresponding to
NHa and COi+1 of both families indicated that these
atoms established a hydrogen bond and therefore that
the γ-turn conformation was maintained for both epimers
2a and 2b. These distances remained below 2.5 Å for 32
and 63% of the population for 2a 1 and 2b1, respectively.
The participation of proton NHa in an intramolecular
F IGURE 5. Stick conformational representation of minimum
energy conformers of epimers 2a and 2b obtained after the
Monte Carlo conformational search.
(46) To validate the Iterative Simulated Annealing method on spiro
compound mimetics, we ran a test calculation on compound 5 described
in ref 47. The starting conformations were totally extended. The results
obtained have a distribution profile similar to those reported by Muller
et al. in ref 47 for compound 5 (data not shown).
In summary, our Monte Carlo calculations allowed us
to conclude that in both epimers 2a and 2b the adoption
of γ-turn conformations was predominant (except for 2a 3),
J . Org. Chem, Vol. 67, No. 22, 2002 7593

Ferna´ndez et al.
TABLE 4. P er cen ta ges of Con for m er s of th e
to internal TMS. Mass spectra were determined either by
chemical ionization (MS-CI) or electronic impact (MS-EI).
Flash column chromatography was carried out on silica gel
60 (40-63 MM, Sds), unless otherwise indicated. Analytical
TLC was performed on silica gel (F254, Macherey-Nagel) and
developed with the solvent described in each case for flash
chromatography. The spots were located by UV light and
KMnO4 reagent. Purification of reagents and solvents was
effected according to standard methods.48 All extracts were
dried (Na2SO4) prior to concentration under reduced pressure.
Elemental analyses were performed at the Serveis Cientifi-
cotecnics of Universitat de Barcelona.
P seu d op ep tid es Th a t Exh ibit Differ en t F ea tu r es of â-II
a n d /or γ-Tu r n s fr om 1 n s, 300 K Molecu la r Dyn a m ics
conformer
2a l
71
2b1
85
95
2 (1a)
turn type
R (CR-CRi+3) < 7 Å
-90° < τ > +90°
NHa‚‚‚OiC < 2.5 Å
â
90
16 (0a)
+70° < Φ2 > +85°
-60° < Ψ2 > -70°
NHa‚‚‚Oi+1C < 2.5 Å
30
71
γ
45
40
32 (29b)
63 (53b)
(R)-N-(ter t-Bu toxyca r bon yl)-2-(3-h yd r oxyp r op yl)p r o-
lin e Meth yl Ester (5). To a stirred solution of carboxylic acid
34a,15 (1 g, 4 mmol) in dry acetone (30 mL) at room temperature
was added K2CO3 (595 mg, 4.4 mmol), and stirring was
continued for 1 h. Next, MeI (0.318 mL, 5.2 mmol) was added,
and the mixture was heated to reflux overnight. After being
cooled to room temperature, the mixture was concentrated and
the residue was purified by flash chromatography (hexane/
AcOEt, 8/2) to afford (R)-N-(ter t-bu toxyca r bon yl)-2-a llyl-
p r olin e m eth yl ester (4) (966 mg, 92%, rotamers were
observed in the NMR spectra whose data were coincidental
-90° < Φ1 > -30°
+90° < Ψ1 > +150°
+50°< Φ2 > +110°
-45°< Ψ2 > +45°
NHa‚‚‚OiC < 4 Å
NHa‚‚‚Oi+1C < 4 Å
NHb‚‚‚OiC < 2.5 Å
NHc‚‚‚Oi+2C < 2.5 Å
w0 trans/cis
95
90
50
7
69
81
8
96
88
96
3
41
80
15
1
âII
0
100/0
100/0
a
Percentage of conformers that form hydrogen bonds: the
distances NHa‚‚‚OiC are less than 2.5 Å, the NHa‚‚‚Oi bond angle
is larger than 120°, and the Ha‚‚‚OidC angle is larger than 90°.
with those described by J ohnson4a), [R]22 ) +21.9 (c 0.8,
D
CHCl3). Next, to a stirred solution of alkene 4 (950 mg, 3.5
mmol) in dry THF (35 mL) at room temperature was added
9-BBN (10.6 mL, 5.25 mmol), and the mixture was stirred for
2 h. Another 10.6 mL (5.25 mmol) of 9-BBN was added, and
stirring was continued overnight. Then, H2O (0.7 mL), 3 N
NaOH (10.7 mL, 31.5 mmol), and H2O2 (10.7 mL) were added.
After 45 min, the reaction was quenched by the addition of
NaCl and extracted with Et2O. The organic extracts were dried
and concentrated to afford a pale yellow oil that was chro-
matographed to obtain alcohol 5 (992 mg, 98%, rotamers were
observed): [R]22D ) +7.0 (c 0.7, CHCl3); IR (NaCl) 3446, 1742,
1686 cm-1; 1H NMR (500 MHz) δ 1.33 and 1.37 (2s, 9 H), 1.45
(m, 1H), 1.54 (m, 1H), 1.75 (m, 2H), 1.80-2.05 (m, 3H), 2.11
(ddd, J ) 13.7, 12, 5 Hz, 1H, one rotamer) and 2.24 (ddd, J )
13.7, 12, 5 Hz, 1H, one rotamer), 3.33 (m, 1H), 3.52-3.68 (m,
3H), 3.63 (s, 3H); 13C NMR δ 22.6 and 23.0, 26.8, 28.2 and
28.3, 30.2 and 31.4, 36.1 and 37.3, 48.4 and 48.6, 52.0, 62.6
and 62.7, 67.2 and 67.8, 79.6 and 80.0, 153.7, 175.4; CIMS m/z
305 (M+ + 18, 84), 288 (M+ + 1, 100), 287 (M+, 1), 272 (36),
249 (42), 188 (21), 170 (47), 128 (30). Anal. Calcd for
b
Percentage of conformers that form hydrogen bonds: the dis-
tances NHa‚‚‚Oi+1C are less than 2.5 Å, the NHa‚‚‚Oi+l bond angle
is larger than 120°, and the Ha‚‚‚Oi+1dC angle is larger than 90°.
hydrogen bond was evidenced by the 1H NMR experi-
ments (NOESY and temperature coefficients in CDCl3
and in DMSO-d6).
Concerning the torsion angles of compounds 2a 1 and
2b1 during the molecular dynamics calculations, the
values of Φ1, Ψ1, and Ψ2 corresponded to a â-II-turn
conformation. However, the values of Φ2 were in ac-
cordance with the values expected for the two kinds of
turns (â and/or γ) especially for epimer 2b1 (Table 4). In
conclusion, our 300 K molecular dynamics calculation
showed that for both epimers the starting global mini-
mum γ-turn conformations remained stable within a 1
ns simulation.
C
14H25O5N: C, 58.52; H, 8.77; N, 4.87. Found: C, 58.60; H,
While NMR results showed the participation of the
NHa proton in an intramolecular hydrogen bond, molec-
ular modeling allowed us to establish that it was involved
in a hydrogen bond with Oi+1C stabilizing the formation
of a γ-turn. However, most of the parameters involved
in the formation of â-turn were also satisfied (the distance
Ci-Ci+l and Φ1, Φ2, ψ1 angles, not defined for γ-turns),
indicating that pseudotripeptides 2a and 2b adopted a
“γ-turn/distorted type-II â-turn” conformation.
In contrast with the results reported by Ward et al.,6
in which the (S)-5.5-spirolactam induced a classical type-
II′ â-turn conformation whereas the (R)-spirolactam
favored an extended one, we have shown that pseudot-
ripeptides 2a and 2b derived from (S)-5.6-spirolactams
adopt “γ-turn/distorted type-II â-turn” conformations.
This may alter the biological activity of pseudopeptides
containing these surrogates of the native Gly-Leu dipep-
tide.
9.27; N, 4.47.
(R)-N-(ter t-Bu t oxyca r b on yl)-2-(3-b r om op r op yl)p r o-
lin e Meth yl Ester (6). Triphenylphosphine (493 mg, 1.92
mmol) was added to a stirred solution of alcohol 5 (450 mg,
1.6 mmol) and NBS (335 mg, 1.92 mmol) in dry CH2Cl2 (30
mL). The mixture was stirred at 0 °C during 4 h. Then, solid
K2CO3 (128 mg) was added, and the mixture was heated to
reflux for 15 min. After being cooled to room temperature, the
mixture was filtered, the filtrate was concentrated, and the
residue was purified by flash chromatography (hexane/AcOEt,
10/0 to 8/2) to afford bromo ester 6 (388 mg, 71%, rotamers
were observed): [R]22D ) +10.9 (c 0.5, CHCl3); IR (NaCl) 1738,
1
1690 cm-1; H NMR δ 1.42 and 1.45 (2s, 9 H), 1.75-1.95 (m,
4H), 1.90-2.20 (m, 3H), 2.26-2.38 (m, 1H), 3.36-3.48 (m, 3H),
3.57-3.78 (m, 1H), 3.70 and 3.71 (2s, 3H); 13C NMR δ 22.6
and 23.1, 27.2 and 27.8, 28.2 and 28.3, 33.2, 33.8 and 34.0,
36.2 and 37.5, 48.4 and 48.5, 52.1, 66.9 and 67.5, 79.6 and 80.2,
153.5, 175.0; CIMS m/z 368 (M+ + 18, 23), 366 (24), 351 (M+
+ 1, 57), 350 (M+, 10), 349 (59), 313 (25), 311 (28), 240 (100),
170 (39), 138 (25), 121 (58). Anal. Calcd for C14H24O4NBr: C,
48.01; H, 6.91; N, 4.00. Found: C, 47.76; H, 6.77; N, 3.94.
(R)-N-(ter t-Bu t oxyca r b on yl)-2-(3-iod op r op yl)p r olin e
Meth yl Ester (7). Triphenylphosphine (611 mg, 2.25 mmol),
Exp er im en ta l Section
Gen er a l Meth od s. 1H and 13C NMR spectra were recorded
on a 200 or 300 MHz instrument, and 2D NMR COSY
experiments were performed on a 500 MHz instrument. Unless
otherwise noted, NMR spectra were registered in CDCl3, and
chemical shifts are expressed in parts per million (δ) relative
(47) Muller, G.; Hessler, G.; Decornes, H. Y. Angew. Chem., Int. Ed.
2000, 39, 894-896.
(48) Perrin, D. D.; Armarego, W. F. L.; Perrin, D. R. Purification of
Laboratory Chemicals; Pergamon Press: Oxford, 1980.
7594 J . Org. Chem., Vol. 67, No. 22, 2002

Spirolactams as Conformationally Restricted Pseudopeptides
imidazole (317 mg, 4.5 mmol), and I2 (552 mg, 2.1 mmol) were
added, sequentially and in small portions, to a stirred solution
of alcohol 5 (446 mg, 1.5 mmol) in dry CH2Cl2 (25 mL). The
resulting mixture was stirred at room temperature during 2
h and then filtered over silica gel. The filtrate was concentrated
and the residue purified by flash chromatography (hexane/
AcOEt, 10/0 to 8/2) to afford iodo ester 7 (420 mg, 68%,
rotamers were observed): IR(NaCl) 1734, 1686 cm-1; 1H NMR
δ 1.43 and 1.45 (2s, 9H), 1.70-2.20 (m, 7H), 2.21-2.37 (m,
1H), 3.20 (ddd, J ) 13, 6.5, 3.3 Hz, 2H), 3.41 (dt, J ) 10.6, 5
Hz, 1H), 3.56-3.65 and 3.67-3.80 (2m, 1H), 3.70 and 3.72 (2s,
3H); 13C NMR δ 6.61, 22.6 and 23.1, 27.9 and 28.6, 28.2, 35.4
and 36.2, 36.1 and 37.6, 48.4 and 48.5, 52.1, 66.8 and 67.5,
79.6 and 80.2, 153.5, 175.0; CIMS m/z 397 (M+, 1), 325 (14),
170 (100), 156 (61).
45 min, 1 equiv of oxidant was added, and the mixture was
stirred overnight. The reaction was quenched with brine and
extracted with Et2O. The organic extracts were dried and
concentrated to afford (R)-N-(ter t-bu toxyca r bon yl)-2-(3-
h yd r oxyp r op yl)p r olin e (12) as a pale yellow oil. Next, to
an ice-cooled and stirred solution of crude acid 12 and
pentafluorophenol (181 mg, 0.98 mmol) in THF (5 mL) was
added DCC (202 mg, 0.98 mmol), and stirring was continued
overnight. After gradual warming to room temperature, the
reaction mixture was filtered, the filtrate was concentrated,
and the residue was purified by flash chromatography (neutral
Al2O3, CH2Cl2/hexane, 9/1 to 10/0) to give lactone 11 (76 mg,
30% from 3). Meth od B. To a stirred solution of alkene 10
(500 mg, 1.2 mmol) in dry THF (10 mL) at room temperature
was added BH3 (0.475 mL, 1.44 mmol), and the resulting
mixture was stirred for 3 h. Another 0.475 mL (1.44 mmol) of
BH3 was added, and stirring was continued for an additional
2 h. The mixture was concentrated, the resulting organoborane
was dissolved in 2 mL of diglyme, and TMANO (160 mg, 1.44
mmol) was added all at once to this solution. The reaction
mixture was gently stirred at reflux temperature for 3 h and
at room temperature overnight. The reaction was quenched
with brine and extracted with Et2O. The organic extracts were
dried and concentrated to afford a pale yellow oil, which was
next dissolved in CH2Cl2 (60 mL) and DIPEA (0.23 mL, 1.32
mmol). After being stirred at room temperature overnight, the
mixture was concentrated and the residue was purified by
flash chromatography (neutral Al2O3, CH2Cl2/hexane, 9/1-10/
0) to give lactone 11 (140 mg, 46%). Meth od C. To an ice-
cooled and stirred solution of alkene 10 (1 g, 2.4 mmol) in dry
THF (35 mL) was added 9-BBN (7 mL, 3.6 mmol), and the
mixture was stirred at room temperature for 2 h. Another 7
mL (3.6 mmol) of 9-BBN was added, and stirring was contin-
ued for an additional 2 h. Next, 3 N NaOH (0.8 mL, 2.4 mmol)
and H2O2 (0.8 mL, 7.2 mmol) were added. After 45 min, 1 equiv
of oxidant was added, and the mixture was stirred overnight.
The reaction was quenched by the addition of solid NaCl and
extracted with Et2O. The organic extracts were dried and
concentrated to afford a pale yellow oil that was chromato-
graphed (neutral Al2O3, CH2Cl2/hexane, 9/1-10/0) to obtain
Meth yl (2S)-1-(ter t-Bu toxyca r bon yl)-2-[N-[(1S)-1-(ter t-
bu toxyca r bon yl)-3-m eth ylbu tyll-3-a m in o]p r op yl]p yr r o-
lid in e-2-ca r boxyla te (8). Meth od A. To a stirred solution
of bromo ester 6 (200 mg, 0.57 mimol) in DMF (1 mL) were
added H-Leu-OtBu (214 mg, 0.9 mmniol) and K2CO3 (95 mg,
0.684 mmol), and the resulting mixture was heated to reflux
for 15 h. After being cooled to room temperature, the mixture
was concentrated and the residue was purified by flash
chromatography (hexane/AcOEt, 8/2 to 7/3) to afford amine 8
(160 mg, 61%, rotamers were observed). Meth od B. Operating
as above, from iodo ester 7 (180 mg, 0.45 mmol) in DMF (2
mL), H-Leu-OtBu (170 mg, 1.14 mmol), and K2CO3 (75 mg,
0.694 mmol), a residue was obtained that was purified by flash
chromatography (hexane/AcOEt, 8/2 to 6/4) to afford amine 8
(167 mg, 81%): [R]22 ) +2.2 (c ) 0.6, CH3OH); IR (NaCl)
D
2956, 1740, 1728, 1699 cm-1; H NMR δ 0.90 and 0.93 (2d, J
1
) 6.7 Hz, 6H), 1.40 and 1.47 (2s, 9H each), l.39-1.48 (m, 4H),
1.64-1.80 (m, 1H), 1.75-2.20 (m, 4H), 2.48 (m, 1H), 2.60 (m,
1H), 3.12 (t, J ) 7.3 Hz, 1H), 3.4 (m, 1H), 3.7 (m, 1H), 3.70 (s,
3H); 13C NMR δ 22.4 and 22.6; 23.1 and 24.4; 24.5 and 24.7;
24.9; 28.0, 28.2 and 28.3; 31.6 and 32.8; 36.0 and 37.3; 43.0;
48.2 and 48.3; 48.4 and 48.5; 52.0; 60.6 and 60.8; 67.3 and 67.8;
79.6, 79.7 and 80.7; 153.6; 175.4; CIMS m/z 457 (M+ + 1, 100),
401 (83), 355 (63), 345 (29), 57 (24). Anal. Calcd for C24H44O6N2‚
1/2H2O: C, 61.91; H, 9.74; N, 6.02. Found: C, 61.83; H, 9.98;
N, 6.15.
lactone 11 (290 mg, 54%, rotamers were observed): [R]22
)
D
(R)-N-(ter t-Bu toxyca r bon yl)-2-a llylp r olin e P en ta flu o-
r op h en yl Ester (10). To an ice-cooled and stirred solution of
acid 3 (1 g, 319 mmol) and pentafluorophenol (720 mg, 3.9
mmol) in THF (32 mL) was added DCC (808 mg, 3.9 mmol),
and stirring was continued overnight. After gradual warming
to room temperature, the reaction mixture was filtered, the
filtrate was concentrated, and the residue was purified by flash
chromatography (neutral Al2O3, CH2Cl2/hexane, 9/1-10/0) to
l
+10.8 (c 1, CH3OH); IR (NaCl) 1739, 1693 cm-1; H NMR δ
1.46 and 1,48 (2s, 9H), 1.75 and 1.81 (2m, 1H), 1.78-2.10 (m,
4H), 2.21-2.38 (m, 2H), 2.40 (tdm, J ) 12.3, 5 Hz, 1H, one
rotamer), 2.55 (tdm, J ) 12.3, 5 Hz, 1H, one rotamer), 3.57
(m, 2H), 4.40 (m, 2H); 13C NMR 22.4 and 23.4, 28.3 and 28.4,
32.5 and 33.2, 39.1 and 40.2, 47.8 and 47.9, 64.7 and 64.8, 69.7
and 69.8, 79.9 and 80.9, 153.0 and 154.0, 173.6 and 173.8;
CIMS m/z 273 (M+ + 18, 100), 256 (M+ + 1, 8), 255 (M+, 1),
217 (57), 156 (30). Anal. Calcd for C13H21O4N: C, 61.16; H,
8.26; N, 5.49. Found: C, 61.17; H, 8.78; N, 5.99.
give ester 10 (1.22 g, 75%, rotamers were observed): [R]22
)
D
1
+7.3 (c 0.7, CH3OH); IR (NaCl) 1787, 1691 cm-1; H NMR δ
1.47 and 1.48 (2s, 9 H), 1.85-2.10 (m, 2H), 2.20-2.38 (m, 2H),
2.71 (dd, J ) 14, 8 Hz, 1H, one rotamer), 2.70 (dd, J ) 14, 8
Hz, 1H, one rotamer), 3.14 (dd, J ) 14.2, 6.3 Hz, 1H, one
rotamer), 3.22 (dd, J ) 14, 6.7 Hz, 1H, one rotamer), 3.44 (m,
1H), 3.67 (ddd, J ) 10.5, 7.6, 5.7 Hz, 1H, one rotamer), 3.79
(ddd, J ) 10.6, 7.8, 4.4 Hz, 1H, one rotamer), 5.20 (m, 2H),
5.77 (m, 1H); 13C NMR (CDCl3) δ 22.3 and 23.4, 28.1 and 28.3,
35.8 and 37.6, 38.2 and 39.1, 48.3 and 48.6, 66.9 and 67.4, 80.2
and 81.1, 119.6 and 120.1, 131.8 and 132.5, 136.0-142.9, 153.1
and 153.6, 170.5 and 170.6; CIMS m/z 439 (M+ + 18, 21), 422
(M+ + 1, 19), 421 (M+, 1), 382 (21), 310 (43), 273 (100), 256
(61), 255 (26), 250 (26), 240 (20), 217 (51), 110 (35). Anal. Calcd
for C19H20O4NF5: C, 54.16; H, 4.78; N, 3.32. Found: C, 53.75;
H, 4.93; N, 3.28.
(2R)-1-(ter t-Bu t oxyca r b on yl)-N-[(1S)-1-(ter t-b u t oxy-
car bon yl)-3-m eth ylbu tyl]-2-(3-h ydr oxypr opyl)pyr r olidin e-
2-ca r boxa m id e (13a ). A solution of lactone 11 (410 mg, 1.6
mmol), H-Leu-OtBu (900 mg, 4.8 mmol), and 2-hydroxypyri-
dine (167 mg, 1.76 mmol) in 1 mL of toluene was heated at
reflux temperature for 72 h under an argon atmosphere. After
being cooled to room temperature, the mixture was concen-
trated and the residue was purified by flash chromatography
(hexane/AcOEt, 2/1 to 1/1) to afford amide 13a (448 mg, 65%,
rotamers were present): [R]22 ) -26.5 (c 0.6, CHCl3); IR
D
(NaCl) 3500, 1735, 1693, 1681 cm-1; 1H NMR (500 MHz, CD3-
CN, 65 °C): δ 0.94 (dt, J ) 10.5, 3 Hz, 6H), 1.44, 1.45 and
1.46 (4s, 18H), 1.25-1.62 (m, 5H), 1.75-1.84 (m, 2H), 1.88-
1.98 (m, 2H), 2.02-2.17 (m, 2H), 2.40-2.52 (m, 1H), 3.45-
3.68 (m, 2H), 3.50-3.65 (m, 1H), 4.24-4.36 (m, 1H); 13C NMR
(CD3CN, 29 °C): δ 21.6 and 21.8; 23.1; 23.2 and 23.3; 25.2
and 25.6; 28.1, 28.5 and 28.6; 31.9; 33.1 and 33.7; 35.5 and
36.3; 41.1; 49.9; 52.4; 62.7; 71.3; 80.5 and 81.9; 154.5 and 156.0;
172.9; 174.8 and 175.6; CIMS m/z 460 (M+ + 18, 9), 444 (M+
+ 2, 27), 443 (M+ + 1, 100), 273 (39), 217 (23). Anal. Calcd for
(R)-1-(ter t-Bu toxycar bon yl)-6-oxo-7-oxa-1-azaspir o[4.5]-
d eca n e (11). Meth od A. To an ice-cooled and stirred solution
of alkene 3 (250 mg, 0.97 mmol) in dry THF (11 mL) was added
9-BBN (2.9 mL, 1.5 mmol), and the mixture was stirred at
room temperature for 2 h. Another 2.9 mL (1.5 mmol) of 9-BBN
was added, and stirring was continued for an additional 2 h.
Next, 3 N NaOH (2.9 mL) and H2O2 (2.9 mL) were added. After
J . Org. Chem, Vol. 67, No. 22, 2002 7595

Ferna´ndez et al.
C23H42O6N2‚1/3H2O: C, 61.59; H, 9.59; N, 6.25. Found: C, 61.62;
(12 mg, 48%). Meth od C. To an ice-cooled solution of amide
13a (180 mg, 0.4 mmol) in dry CH2Cl2 (2 mL), under argon
atmosphere, were added pyridine (0.036 mL, 0.44 mmol) and
MsCl (0.065 mL, 1.2 mmol). The resulting mixture was stirred
at room temperature for 6 h. The reaction was quenched by
the addition of 4% aqueous citric acid, and the organic layer
was successively washed with 4% aqueous citric acid and 5%
aqueous NaHCO3 solution. The organic extracts were dried
and concentrated to afford (2R)-1-(ter t-bu toxyca r bon yl)-N-
[(1S)-1-(ter t-bu toxyca r bon yl)-3-m eth ylbu tyl]-2-[3-(m eth -
a n esu lfon yl)p r op yl]p yr r olid in e-2-ca r boxa m id e (15a ) as
a pale yellow oil (190 mg). Next, to a stirred solution of NaH
(60% in mineral oil, 35 mg, 0.79 mmol) in dry THF (2 mL)
was added the crude mesylate 15a (190 mg, 0.36 mmol) in THF
(2 mL) followed by 15-crown-5 (0.08 mL, 0.4 mmol). The
resulting mixture was stirred at room temperature for 3 h.
Another 35 mg of NaH (60% in mineral oil, 0.4 mmol) was
added, and stirring was continued for 4 h. The reaction was
quenched by the addition of H2O and extracted with AcOEt.
The organic extracts were dried and concentrated to afford a
pale yellow oil that was chromatographed (hexane/AcOEt, 10/
H, 9.61; N. 6.29.
(2R)-1-(ter t-Bu t oxyca r b on yl)-N-[(1R)-1-(ter t-b u t oxy-
car bon yl)-3-m eth ylbu tyl]-2-(3-h ydr oxypr opyl)pyr r olidin e-
2-ca r boxa m id e (13b). Operating as above in the preparation
of 13a , from lactone 11 (770 mg, 3 mmol), H-D-Leu-OtBu (1.69
g, 9 mmol) and 2-hydroxypyridine (291 mg, 3.3 mmol) in 3 mL
of toluene/Et2O (1:1), amide 13b (900 mg, 68%, rotamers were
present) was obtained after flash chromatography (hexane/
AcOEt, 2/1-1/1): [R]22D ) -27 (c 1.1, CHCl3); IR (NaCl) 3580,
1734, 1679 cm-1; 1H NMR (CD3OD): δ 0.88-0.96 (m, 6H), 1.45
and 1.47 (2s, 18H), 1.52-1.64 (m, 3H), 1.65-1.76 (m, 2H),
1.78-1.88 (m, 1H), 1.90-2.04 (br s, 2H), 2.08-2.30 (br s, 3H),
3.26-3.36 (m, 1H), 3.46-3.64 (m, 2H), 3.68-3.82 (m, 1H),
4.28-4.42 (m, 1H); 13C NMR (CD3OD) δ 21.7, 23.0, 23,5, 25.8,
27.7, 28.3 and 28.7, 31.5 and 32.0, 39.1, 41.1 and 41.3, 50.4,
52.9, 62.9, 69.7 and 70.5, 81.2, 81.8, 155.7, 173.3, 177.5; CIMS
m/z 460 (M+ + 18, 11), 445 (28), 444 (M+ + 2, 100), 442 (M+,
5). Anal. Calcd for C23H42O6N2‚1/3H2O: C, 61.59; H, 9.59; N,
6.25. Found: C, 61.57; H, 9.61; N, 6.15.
(2R)-1-(ter t-Bu t oxyca r b on yl)-N-[(1S)-1-(ter t-b u t oxy-
ca r bon yl)-3-m eth ylbu tyl]-2-(3-br om op r op yl)p yr r olid in e-
2-ca r boxa m id e (14a ). Triphenylphosphine (43 mg, 0.17
mmol) was added to a stirred solution of amide 13a (60 mg,
0.14 mmol) and NBS (29 mg, 0.17 mmol) in dry CH2Cl2 (5 mL).
The mixture was stirred at 0 °C during 4 h, solid K2CO3 (114
mg, 0.82 mmol) was added, and stirring was continued at
reflux temperature for 15 min. After being cooled to room
temperature, the mixture was filtered, the filtrate was con-
centrated, and the residue was purified by flash chromatog-
raphy (hexane/AcOEt,10/0-1/1) to afford bromo amide 14a (27
mg, 43%, rotamers were observed): [R]22D ) -11.2 (c 0.4, CH3-
OH); IR (NaCl) 1736, 1681 cm-1; 1H NMR (500 MHz, CD3CN,
65 °C) δ 0.95 (d, J ) 6 Hz, 3H), 0.96 (d, J ) 6.6 Hz, 3H), 0.97
(d, J ) 6.3 Hz, 3H), 0.98 (d, J ) 6.9 Hz, 3H), 1.48 (s, 9H), 1.49
(s, 9H), 1.59-1.65 (m, 2H), 1.66-1.87 (m, 3H), 1.96-2.00 (m,
2H), 2.17-2.54 (br s, 4H), 3.33-3.41 (m, 1H), 3.52 (d, J ) 6.6
Hz, 2H), 3.58-3.72 (m, 1H), 4.27-4.38 (m, 1H); 13C NMR (CD3-
CN, 29 °C) δ 21.9 and 22.2, 23.0 and 23.4, 23.5, 25.6 and 25.9,
28.5 and 28.9, 34.6, 35.4 and 35.6, 35.7 and 36.1, 36.9 and 37.2,
41.5 and 41.9, 50.2, 52.8, 69.5 and 70.8, 81.1 and 82.1, 154.2
and 154.8, 173.2, 175.1; ElMS m/z 236 (21), 234 (21), 190 (15),
57 (100). Anal. Calcd for C23H41O5N2Br: C, 54.64; H, 8.43; N,
5.94. Found: C, 54.64; H, 8.30; N, 5.84.
(5S)-1-(ter t-Bu toxyca r bon yl)-7-[(1S)-(ter t-bu toxyca r bo-
n yl)-3-m eth ylbu tyl]-6-oxo-1,7-d ia za sp ir o[4.5]d eca n e (1a ).
Meth od A. To a solution of amine 8 (100 mg, 0.22 mmol) in
THF-H2O (15:1) was added TBAH (142 mg, 0.22 mmol) at 0
°C, and the resulting mixture was stirred at room temperature
for 48 h. The reaction was quenched by the addition of water
and extracted with AcOEt. The aqueous layer was acidified
with citric acid (pH 3-4) and extracted with AcOEt. The
organic extracts were dried and concentrated to afford (2S)-
1-(ter t-bu toxycar bon yl)-2-[N-[(1S)-1-(ter t-bu toxycar bon yl)-
3-m et h ylb u t yl]-3-a m in op r op yl]p yr r olid in e-2-ca r b oxyl-
ic a cid (9). To a stirred solution of the above crude acid 9 in
CH2Cl2 (1 mL) was added DCC (40 mg, 0.22 mmol) at 0 °C,
and the mixture was stirred for 6 h. The reaction mixture was
quenched by addition of water and extracted with AcOEt. The
organic extracts were dried and concentrated to afford a
residue that was purified by flash chromatography (hexane/
AcOEt, 10/0-8/2) to give spirolactam 1a (21 mg, 23%, rotamers
were observed). Meth od B. To a stirred solution of NaH (60%
in mineral oil, 3 mg, 0.122 mmol) was added the bromo amide
14a (27 mg, 0.06 mmol) in THF (2 mL) followed by 18-crown-6
(15 mg, 0.058 mmol), and stirring was continued at room
temperature for 3 h. Another 3 mg of NaH (60% in mineral
oil, 0.122 mmol) was added, and stirring was continued for 4
h. The reaction was quenched by the addition of H2O and
extracted with AcOEt. The organic extracts were dried and
concentrated to afford a pale yellow oil that was chromato-
graphed (hexane/AcOEt, 10/0 to 8/2) to yield spirolactam 1a
0-8/2) to yield spirolactam 1a (120 mg, 77% from 13a ): [R]22
D
) -23.2 (c 0.4, CH3OH); IR (NaCl) 1735, 1696, 1689 cm-1; 1H
NMR (500 MHz) δ 0.88 (dd, J ) 9, 6 Hz, 3H), 0.94 (t, J ) 6.6
Hz, 3H), 1.38 and 1.40 (2s, 18H), 1.42-2.04 (m, 9H), 2.18-
2.32 (m, 1H), 2.51 (td, J ) 12, 6 Hz, 1H), 3.06-3.14 (m, 1H),
3.42-3.53 (m, 2H), 3.66 (dt, J ) 12, 6 Hz, 1H, one rotamer),
5.22-5.27 (m, 1H, one rotamer), 5.32 (dd, J ) 11.5, 4.5 Hz,
1H, one rotamer); 13C MNR δ 21.1 and 22.0, 21.5 and 21.6,
22.2 and 23.3, 22.7 and 23.7, 23.9 and 24.9, 27.9 and 28.4, 32.7
and 33.4, 37.0 and 37.2, 38.9 and 41.2, 43.7 and 43.8, 48.6,
54.6 and 55.1, 65.4 and 65.6, 78.9 and 79.6, 80.8 and 81.2,
153.7 and 153.8, 171.0 and 171.4, 172.6 and 172.8; EIMS m/z
424 (M+, 3), 267 (21), 110 (28), 57 (100). Anal. Calcd for
C23H40O5N2‚3/2H2O: C, 61.05; H, 9.60; N, 6.19. Found: C, 61.05;
H, 9.12; N, 6.00.
(5S)-1-(ter t-Bu t oxyca r b on yl)-7-[(1R)-(ter t-b u t oxyca r -
b on yl)-3-m et h ylb u t yl]-6-oxo-1,7-d ia za sp ir o[4.5]d eca n e
(1b). Operating as for the preparation of 1a (method C), from
amide 13b (900 mg, 2.05 mmol), pyridine (0.18 mL, 2.3 mmol),
and MsCl (0.53 mL, 6.15 mmol) in dry CH2Cl2 (10 mL) was
obtained (2R)-1-(ter t-bu toxyca r bon yl)-N-[(1R)-1-(ter t-bu -
t oxyca r b on yl)-3-m et h ylb u t yl]-2-[3-(m et h a n esu lfon yl)-
p r op yl]p yr r olid in e-2-ca r boxa m id e (15b) (938 mg) as a
pale yellow oil. Next, from crude mesylate 15b (938 mg, 1.8
mmol), NaH (60% in mineral oil, two times 82 mg, 0.18 mmol)),
and 15-crown-5 (0.04 mL, 0.22 mmol) in dry THF (20 mL) was
obtained a pale yellow oil that was purified by flash chroma-
tography (hexane/AcOEt, 10/0-8/2) to give spirolactam 1b (462
mg, 54% from 13b): [R]22 ) +8.7 (c 0.5, CH3OH); IR (NaCl)
D
1730, 1693, 1648 cm-1; 1H NMR δ 0.85-0.96 (m, 6H), 1.42 and
1.44 (2s, 9H), 1.44 and 1.46 (2s, 9H), 1.53-2.02 (m, 9H), 2.10-
2.32 (m, 1H), 2.50-2.67 (m, 1H), 3.47-3.55 (m, 2H), 3.58-
3.66 (m, 2H), 4.55 (dd, J ) 10, 4.5 Hz, 1H, one rotamer), 5.05
(t, J ) 7.5 Hz, 1H, one rotamer); 13C NMR δ 21.4 and 21.5,
22.0, 22.2 and 22.5, 23.2 and 23.3, 23.5, 24.7 and 25.1, 32.6
and 33.7, 36.9 and 38.2, 39.0 and 41.1, 43.6 and 45.9, 48.6,
55.2 and 57.7, 65.6 and 65.8, 78.9 and 79.9, 80.9 and 81.1,
153.7 and 154.1, 170.5 and 171.1, 172.7 and 173.5; CIMS m/z
442 (M+ + 18, 44), 425 (M+ + 1, 100), 424 (M+, 1), 283 (23).
Anal. Calcd for C23H40O5N2: C, 65.07; H, 9.50; N, 6.60.
Found: C, 65.17; H, 9.64; N, 6.53.
(5S)-7-[(1S)-1-(ter t-Bu toxyca r bon yl)-3-m eth ylbu tyl]-1-
[(flu or en -9-yl)m et h oxyca r b on yl]-6-oxo-1,7-d ia za sp ir o-
[4.5]d eca n e (17a ). To a solution of spirolactam 1a (190 mg,
0.45 mmol) in dry CH2Cl2 (3 mL) were added 2,6-lutidine (0.11
mL, 0.9 mmol) and TMSOTf (0.12 mL, 0.68 mmol) at room
temperature, and the resulting mixture was stirred under
argon atmosphere for 15 min. The reaction was quenched by
the addition of two drops of saturated aqueous NH4Cl solution.
Next, saturated aqueous Na2CO3 solution (10 mL) was added,
and the mixture was extracted with Et2O. The organic extracts
7596 J . Org. Chem., Vol. 67, No. 22, 2002

Spirolactams as Conformationally Restricted Pseudopeptides
were dried and concentrated to afford (5R)-7-[(1S)-1-(ter t-
b u t oxyca r b on yl)-3-m et h ylb u t yl]-6-oxo-1,7-d ia za sp ir o-
[4.5]d eca n e (16a ) (145 mg) as a pale yellow oil. To a stirred
solution of the above crude amine 16a (110 mg, 0.36 mmol) in
acetone (1 mL) were added solid NaHCO3 (46 mg, 0.54 mmol)
and Fmoc-OSu (183 mg, 0.54 mmol), and the resulting mixture
was stirred at room temperature overnight. The mixture was
concentrated, and the residue was purified by flash chroma-
tography (hexane/AcOEt 1/0-l/1) to afford Fmoc-spirolactam
etate (162 mg, quantitative): IR (NaCl) 2960, 1726, 1666 cm-1
;
1H NMR (CD3OD) δ 0.99 (d, J ) 6.3 Hz, 3H), 1.07 (d, J ) 7
Hz, 3H), 1.56-1.66 (m, 1H), 1.80-2.01 (m, 2H), 1.88 (dd, J )
10.2, 5.4 Hz, 1H), 1.94 (dd, J ) 10.5, 4.5 Hz, 1H), 2.16-2.33
(m, 6H), 3.43-3.56 (m, 2H), 3.58-3.67 (m, 2H), 5.09 (dd, J )
10.5, 5 Hz, 1H); 13C NMR (CD3OD) δ 20.7, 21.3, 23.6, 24.7,
26.2, 31.5, 37.6, 38.4, 44.3, 47.5, 56.3, 69.2, 171.2, 173.8; ElMS
m/z 268 (M+, 3), 226 (11), 153 (35), 110 (39), 96 (100), 83 (69),
69 (20).
17a (88 mg, 48% from la , rotamers were observed): [R]22
)
D
(2R)-N-[(1S)-1-(Ben zyloxyca r bon yl)-3-m eth ylbu tyl]-1-
(ter t-b u t oxyca r b on yl)-2-(3-h yd r oxyp r op yl)p yr r olid in e-
2-ca r boxa m id e (20a ). Operating as above in the preparation
of 13a , from lactone 11 (350 mg, 1.4 mmol), H-Leu-OBzl (905
mg, 4.1 mmol), and 2-hydroxypyridine (167 mg, 1.76 mmol)
in 5 mL of toluene was obtained a residue that was purified
by flash chromatography (hexane/AcOEt, 2/1-1/1) to yield
-27.6 (c 0.5, CH3OH); IR (NaCl) 1728, 1702, 1648 cm-1 1H
;
NMR δ 0.88 and 0.94 (2d, J ) 6.3 Hz, 6H), 1.46 and 1.45 (2s,
9H), 1.62-1.68 (dd, J ) 10, 4 Hz, 2H), 1.66 (td, J ) 10, 6 Hz,
2H), 1.72-1.78 (m, 3H), 1.82-1.96 (m, 1H), 1.98-2.14 (m, 1H),
2.31-2.42 (m, 1H), 2.51-2.63 (m, 1H), 3.14-3.22 (m, 1H),
3.29-3.40 (m, 1H), 3.64-3.81(m, 2H), 4.10 (td, J ) 8, 2 Hz,
1H), 4.27 (t, J ) 7 Hz, 1H), 4.44 (dd, J ) 10, 7 Hz, 1H), 5.34
(dd, J ) 10, 5.7 Hz, 1H), 7.32 (ddd, J ) 7.5, 2.3, 1.5 Hz, 2H),
7.39 (t, J ) 7.5 Hz, 2H), 7.60-7.66 (2d, J ) 7.5 Hz, 2H), 7.75
(d, J ) 7.5 Hz, 2H); 13C NMR δ 21.2, 21.3, 22.4 and 23.0, 23.4,
23.5, 27.9, 32.4, 37.0, 38.9, 43.9, 47.0, 48.3, 54.8, 66.1, 66.9
and 67.0, 81.0, 119.7, 125.0 and 125.4, 126.8 and 126.9, 127.4,
141.0 and 141.1, 143.8 and 144.4, 154.0, 171.3, 172.3; CIMS
m/z 564 (M+ + 18, 22), 563 (57), 547 (M+ + 1, 37), 546 (M+,
100), 491 (18), 473 (20), 325 (35), 178 (23); Anal. Calcd for
C33H42O5N2‚1/4H2O: C, 71.92; H, 7.77; N, 5.08. Found: C, 71.92;
H, 7.65; N, 4.86.
amide 20a (314 mg, 50%, rotamers were observed): [R]22
)
D
-19.9 (c 0.51, CHCl3); IR (film) 1739, 1675, 1649 cm-1; 1H NMR
(CD3CN, 65 °C) δ 0.91 and 0.92 (2d, J ) 6.6 Hz, 3H each),
1.44 and 1.46 (2s, 9H), 1.60-1.68 (m, 2H), 1.69-1.77 (m, 3H),
2.01-2.15 (br s, 3H), 2.39-2.49 (br s, 1H), 3.3 (td, J ) 10.5,
6.3 Hz, 1H), 3.46-3.52 (br s, 2H), 3.53-3.55 (m, 1H), 4.43-
4.49 (m, 1H), 5.16 (s, 2H), 7.39 (m, 5H); 13C NMR (CD3CN (65
°C)) δ 21.6, 22.7, 23.2, 25.5, 28.1 and 28.5, 31.9, 35.5, 38.5,
41.2, 49.9 and 50.2, 51.8 and 51.9, 62.6, 67.3, 69.4 and 71.2,
80.4 and 81.7, 129.1, 129.2, 129.5, 137.1, 156.3 and 157.0,
173.5, 175.2 and 175.9; EIMS m/z 494 (M+ + 18, 8), 478 (M+
+ 2, 100), 477 (M+ + 1, 1). Anal. Calcd for C26H40O6N2: C,
65.54; H, 8.40; N, 5.88; Found: C, 65.36; H, 8.60; N, 5.80.
(5S)-7-[(1R)-1-(ter t-Bu toxyca r bon yl)-3-m eth ylbu tyl]-1-
[(flu or en -9-yl)m et h oxyca r b on yl]-6-oxo-1,7-d ia za sp ir o-
[4.5]d eca n e (17b). Operating as above for the preparation of
17a , from spirolactam 1b (60 mg, 0.14 mmol), 2,6-lutidine
(0.033 mL, 0.28 mmol), and TMSOTf (0.04 mL, 0.21 mmol) in
1 mL of dry CH2Cl2 was obtained (5R)-7-[(1R)-1-(ter t-bu -
toxyca r bon yl)-3-m eth ylbu tyl]-6-oxo-1,7-d ia za sp ir o[4.5]-
d eca n e (16b) (34 mg). Next, from crude amine 16b (30 mg,
0.09 mmol), solid NaHCO3 (12 mg, 0.14 mmol), and Fmoc-OSu
(47 mg, 0.135 mmol) in 1 mL of dry acetone was obtained
Fmoc-spirolactam 17b (30 mg, 60% from 1b, rotamers were
observed): [R]22D ) +32.3 (c 0.7, CHCl3); IR (NaCl) 1732, 1696,
(5S)-1-(ter t-Bu toxyca r bon yl)-7-[(1S)-1-ca r boxy-3-m eth -
ylbu tyl]-6-oxo-1,7-d ia za sp ir o[4.5]d eca n e (19a ). Meth od A.
To a suspension of spirolactam 18a (53 g, 0.17 mmol) in dry
acetonitrile (2 mL) was added TMAH (33 mg, 0.17 mmol), and
the mixture was stirred at room temperature for 30 min. Then,
Boc2O (54 mg, 0.26 mmol) was added, and the mixture was
stirred at room temperature for 2 days. Afterward, another
18 mg of Boc2O was added, and stirring was continued for 1
day. The mixture was concentrated and extracted with water
and Et2O. The aqueous layer was acidified to pH 3-4 with
citric acid and extracted with AcOEt. The organic extracts were
dried and concentrated to afford acid 19a (20 mg, 33%,
rotamers were observed). Meth od B. Operating as in the
preparation of 1a (method C), from amide 20a (105 mg, 0.22
mmol), pyridine (0.02 mL, 0.24 mmol), and MsCl (0.05 mL,
0.66 mmol) in dry CH2Cl2 (1 mL) was obtained (2R)-1-(ter t-
bu toxyca r bon yl)-N-[(1S)-1-(ben zyloxyca r bon yl)-3-m eth -
ylbu tyl]-2-[3-(m eth a n esu lfon yl)p r op yl]p yr r olid in e-2-ca r -
boxa m id e (21a ) (110 mg) as a pale yellow oil. Next, from
crude mesylate 21a (110 mg, 0.20 mmol), NaH (60% in mineral
oil, two times 19 mg, 0.44 mmol), and 15-crown-5 (0.04 mL,
0.22 mmol) in THF (20 mL) was obtained a pale yellow oil
that was chromatographed (hexane/acetate, 10/0-8/2) to afford
1
1647 cm-1; H NMR δ 0.89 and 0.98 (m, 6H), 1.46 and 1.48
(2s, 9H), 1.64-2.10 (m, 9H), 2.20-2.32 (m, 1H), 2.62 (td, J )
13.5, 3 Hz, 1H), 3.14-3.25 (m, 1H), 3.52 (td, J ) 12, 4 Hz,
1H), 3.68-3.80 (m, 2H), 3.98 (td, J ) 9.5, 5.5 Hz, 1H), 4.09
(dd, J ) 10, 2 Hz, 1H), 4.13-4.29 (m, 1H), 4.39 (dd, J ) 7.2,
4.2 Hz, 1H, one rotamer), 4.72-4.85 (m, 1H), 7.31 (dt, J )
7.5, 1.5 Hz, 2H), 7:39 (t, J ) 7.5 Hz, 2H), 7.62 (dd, J ) 15, 7.5
Hz, 2H), 7.76 (d, J ) 7.5 Hz, 2H); 13C NMR δ 21.5 and 21.7,
21.9, 22.8 and 23.3, 23.5, 24.5 and 25.0, 28.0, 32.5, 37.1, 38.8,
44.9, 47.3, 48.4, 55.2 and 56.8, 66.1, 67.0, 81.2 and 82.7, 119.8,
125.1 and 125.3, 126.9, 127.5, 141.0 and 141.1, 143.8 and 144.4,
153.9, 170.2, 172.4; CIMS m/z 564 (M+ + 18, 52), 547 (M+
+
1, 21), 326 (21), 325 (100), 284 (13), 283 (94), 179 (18). Anal.
Calcd for C33H42O5N2‚7/4H2O: C, 68.55; H, 7.93; N, 4.84.
Found: C, 68.22; H, 7.75; N, 5.00.
spirolactam 19a (31 mg, 42%): [R]22 ) -8 (c 0.45, CH3OH);
D
IR (NaCl) 1683, 1646 cm-1; 1H NMR (500 MHz, CD3OD) δ 0.90
and 0.95 (2d, J ) 6.5, 3H), 0.97 and 0.98 (2d, J ) 6 Hz, 3H),
1.42, 1.43 and 1.44 (3s, 9H), 1.68-1.86 (m, 2H), 1.64 (ddd, J
) 14.5, 11, 5 Hz, 2H), 1.87-1.96 (m, 3H), 1.99-2.03 (m, 1H,
one rotamer), 2.17 (dddd, J ) 12, 10.5, 7, 1 Hz, 1H, one
rotamer), 2.10 (td, J ) 12, 6.5 Hz, 2H, one rotamer), 2.25 (dt,
J ) 12.5, 7.5 Hz, 1H), 2.41 (td, J ) 12, 6.5 Hz, 1H, one
rotamer), 2.55 (td, J ) 13, 4.5 Hz, 1H, one rotamer), 3.22-
3.55 (m, 2H), 3.46-3.49 (m, 2H, one rotamer), 3.58 (dt, J )
10.5, 7 Hz, 2H, one rotamer), 5.12 (dd, J ) 9, 6.5 Hz, 1H, one
rotamer), 5.37 (dd, J ) 11.5, 4.5 Hz, 1H, one rotamer); 13C
NMR (CD3OD) δ 21.8, 22.2, 23.2, 23.9 and 24.3, 25.2, 28.8,
33.3 and 33.9, 38.3 and 39.8, 42.0, 45.1 and 45.8, 49.3, 55.6
and 55.7, 67.2, 80.6 and 81.7, 155.2, 174.6 and 175.2, 182.0;
CIMS m/z 386 (M+ + 18, 20), 369 (M+ + 1, 23), 368 (M+, 100),
269 (32). Anal. Calcd for C19H32O5N2‚1/2H2O: C, 60.46; H, 8.81;
N, 7.42. Found: C, 60.41; H, 8.68; N, 7.03.
(5R )-7-[(1S )-1-C a r b o x y -3-m e t h y lb u t y l]-6-o x o -1,7-
d ia za sp ir o[4.5]d eca n e (18a ). A solution of spirolactam 1a
(200 mg, 0.47 mmol) and TFA (1.4 mL) was stirred at room
temperature for 30 min. The reaction was quenched by the
addition of water (3 mL) and the mixture was lyophilized to
afford spirolactam 18a trifluoroacetate (141 mg, 79%): IR
1
(NaCl) 2961, 1726, 1670 cm-1; H NMR (CD3OD) δ 1.05 (d, J
) 6.3 Hz, 3H), 1.07 (d, J ) 6.6 Hz, 3H), 1.61-1.70 (m, 1H),
1.89 (dd, J ) 9, 5.4 Hz, 1H), 1.94 (dd, J ) 10.5, 4.5 Hz, 1H),
2.12-2.27 (m, 6H), 2.28-2.45 (m, 2H), 3.44-3.53 (m, 3H), 3.63
(dt, J ) 11.5, 6.5 Hz, 1H), 5.07 (dd, J ) 10.5, 5 Hz, 1H); 13C
NMR (CD3OD) δ 20.6, 21.9, 23.6, 24.7, 26.2, 31.3, 37.3, 38.2,
45.8, 47.6, 57.3, 69.1, 170.0, 173.9; EIMS m/z 268 (M+, 4), 226
(11), 212 (14), 153 (46), 110 (60), 96 (100), 83 (46).
(5R )-7-[(1R )-1-C a r b o x y -3-m e t h y lb u t y l]-6-o x o -1,7-
d ia za sp ir o[4.5]d eca n e (18b). Operating as above for the
preparation of 18a , from spirolactam 1b (181 mg, 0.42 mmol)
and TFA (1.1 mL) was obtained spirolactam 18b trifluoroac-
(5S)-1-Acetyl-7-[(1S)-1-(ter t-bu toxyca r bon yl)-3-m eth yl-
bu tyl]-6-oxo-1,7-d ia za sp ir o[4.5] d eca n e (22a ). To a solution
J . Org. Chem, Vol. 67, No. 22, 2002 7597

Ferna´ndez et al.
of amine 16a (77 mg, 0.24 mmol) in dry CH2Cl2 (0.5 mL) were
added pyridine (38 mL, 0.48 mmol) and acetyl chloride (34 mL,
0.48 nmmol). The mixture was stirred at room temperature
for 6 h under argon atmosphere. The reaction was quenched
by the addition of 0.1 N HCl and extracted with CH2Cl2. The
organic extracts were washed with saturated aqueous NaHCO3
solution and brine, dried, and concentrated to afford acetyl
spirolactam 22a (62 mg, 72%, rotamers were observed) as a
mmol) was obtained acid 23b (53 mg, 68%) as a yellow oil.
Meth od B. Operating as above for the preparation of 23a
(method B), from spirolactam 22b (80 mg, 0.22 mmol) in 10.5
mL of H2O-AcOH-iPrOH (1:2:1) was obtained acid 23b (37
mg, 88%, rotamers were observed): [R]22D ) +79 (c 1, CHCl3);
IR (NaCl) 3319, 1625 cm-1 1H NMR δ 0.88 and 0.92 (2d,
;
J ) 6.3 Hz, 3H each), 1.34-1.48 (m, 1H), 1.60-2.10 (m, 10H),
2.00 and 2.02 (2s, 3H), 3.24 (tdm, J ) 11.7, 3 Hz, 2H), 3.63
(dd, J ) 8.5, 4.5 Hz, 2H), 5.44 (dd, J ) 10.6, 5 Hz, 1H); 13C
NMR δ 21.1, 21.4 and 21.6, 22.8, 23.4 and 23.5, 23.8 and 23.9,
24.9, 31.8, 35.9, 37.1, 42.5, 49.5, 54.8, 66.5, 170.0, 172.2; EIMS
m/z 310 (M+, 12), 266 (24), 221 (40), 195 (25), 180 (36), 138
(38), 110 (85), 96 (100), 83 (73). Anal. Calcd for C16H26O4N2‚
2/3H2O: C, 59.60; H, 8.55; N, 8.69. Found: C, 59.55; H, 8.21;
N, 8.33.
pale yellow oil: [R]22 ) -21 (c 0.51, CHCl3); IR (NaCl) 1727,
D
1648 cm-1; 1H NMR δ 0.93 and 0.96 (2d, J ) 6.6 Hz, 3H each
one), 1.44 and 1.47 (2s, 9H), 1.60-1.69 (m, 1H), 1.70-2.00 (m,
7H), 2.03 (s, 3H), 2.19-2.37 (m, 1H), 2.47-2.58 (m, 1H), 3.11-
3.23 (m, 1H), 3.29-3.38 (m, 1H), 3.50-3.70 (m, 2H), 4.70 (t, J
) 8 Hz, 1H, one rotamer), 5.34 (dd, J ) 9.5, 7 Hz, 1H, one
rotamer); 13C NMR δ 20.4, 20.5, 22.3, 22.5, 22.9, 23.2, 27.0,
30.9, 36.3, 37.4, 42.8, 48.4, 53.8, 65.0, 80.0, 167.0, 170.4, 170.9;
ElMS m/z 366 (M+, 9), 310 (29), 265 (100), 237 (68), 110 (84),
96 (46), 83 (39). Anal. Calcd for C20H34O4N2: C, 65.54; H, 9.35;
N, 7.64. Found: C, 65.88; H, 9.50; N, 7.49.
Tr ip ep tid e An a logu e {Ac-Gly-Leu -Met-NH2} 2a . To a
solution of acid 23a (35 mg, 0.11 mmol) in DMF (1 mL) were
added DCC (26 mg, 0.13 mmol) and HOBt (18 mg, 0.13 mmol),
and the resulting mixture was stirred at room temperature
for 10 min. Next, H-Met-NH2‚HCl (21 mg, 0.11 mmol) and TEA
(0.03 mL, 0.17 mmol) were added, and stirring was continued
at room temperature for 16 h. The mixture was concentrated,
and the residue was purified by flash chromatography (hexane/
AcOEt, 70/30-0/100) to obtain compound 2a (37 mg, 70%,
(5S)-1-Acetyl-7-[(1R)-1-(ter t-bu toxyca r bon yl)-3-m eth yl-
bu tyl]-6-oxo-1,7-d ia za sp ir o[4.5]d eca n e (22b). Operating as
above for the preparation of 22a , from amine 16b (82 mg, 0.24
mmol) in dry CH2Cl2 (0.5 mL), pyridine (38 mL, 0.48 mmol),
and acetyl chloride (34 mL, 0.48 mmnol) was obtained acetyl
spirolactam 22b (82 mg, 95%, rotamers were observed) as a
rotamers were observed): [R]22 ) +23 (c 1.2, CH3OH); IR
D
pale yellow oil: [R]22 ) +32 (c 1, CHCl3); IR (NaCl) 1726,
(NaCl) 3316, 1680, 1667, 1649 and 1623 cm-1; H NMR (500
1
D
1
1646, 1605 cm-1; H NMR δ 0.90 and 0.93 (2d, J ) 6 Hz, 3H
MHz) δ 0.86 (d, J ) 6.5 Hz, 3H, one rotamer), 0.90 (d, J ) 6.5
Hz, 3H, one rotamer), 0.91 (d, J ) 7 Hz, 3H, one rotamer),
0.94 (d, J ) 6.5 Hz, 3H, one rotamer), 1.50-1.56 (m, 1H), 1.57-
1.74 (m, 3H), 1.83-1.91 (m, 2H), 1.93-1.98 (m, 3H), 1.99 and
2.01 (2s, 3H), 2.07 and 2.08 (2s, 3H), 2.09-2.14 (m, 2H), 2.17-
2.28 (m, 2H), 2.38 (td, J ) 13, 3.5 Hz, 1H, one rotamer), 2.45
(dd, J ) 8.5, 3.5 Hz, 1H, one rotamer), 2.48 (dd, J ) 9, 3.5 Hz,
1H, one rotamer) and 2.53-2.59 (m, 1H, one rotamer), 3.15-
3.23 (m, 2H), 3.68-3.77 (m, 2H), 4.36 (dd, J ) 9.5, 5.5 Hz,
1H, one rotamer), 4.37 (dd, J ) 8.5, 5 Hz, 1H, one rotamer),
4.49-4.63 (m, 1H, one rotamer), 5.17 and 5.26 (2 br s, 1H),
5.36 (dd, J ) 12, 4 Hz, 1H), 6.70 and 7.15 (2 br s, 1H) and
7.69 (d, J ) 7.5 Hz, 1H, one rotamer), 7.76 (d, J ) 7.5 Hz, 1H,
one rotamer), 7.77 (d, J ) 7 Hz, 1H, one rotamer) and 7.90 (d,
J ) 8.5 Hz, 1H, one rotamer); 13C NMR δ 15.3 and 15.4; 20.9,
21.1, 21.2 and 21.3; 21.9 and 21.8; 22.8, 22.9 and 23.1; 23.3,
23.4 and 23.5; 24.0, 24.8, 25.3 and 25.4; 30.3, 30.4, 30.5 and
30.6; 30.8, 31.0, 31.3 and 31.4; 31.5 and 32.0; 37.4 and 37.2;
37.9 and 37.8; 41.5 and 41.8; 49.3, 49.4, 51.2 and 52.2; 53.2,
53.3, 53.4, 53.6, and 54.9; 66.4, 66.5 and 66.6; 169.3, 169.4 and
169.5; 170.5, 170.6, 170.9 and 171.0; 172.4, 172.7, 173.1 and
173.3; 174.3, 174.6 and 175.1; HRMS calcd for C21H36O4N4S
440.2457, found 440.2454.
each), 1.45 and 1.47 (2s, 9H), 1.52-1.74 (m, 1H), 1.75-2.10
(m, 8H), 2.03 (s, 3H), 2.10-2.36 (m, 1H), 2.47-2.62 (m, 1H),
3.11-3.23 (m, 1H), 3.26-3.40 (m, 1H), 3.50-3.70 (m, 2H), 4.70
(t, J ) 8 Hz, 1H, one rotamer), 5.35 (dd, J ) 10, 6 Hz, 1H, one
rotamer); 13C NMR δ 21.5 and 21.8, 21.6 and 22.0, 23.3, 23.9,
24.0 and 24.3, 25.0, 28.1, 31.9 and 32.0, 37.3 and 37.4, 38.3
and 38.5, 43.9 and 45.3, 49.5, 54.9 and 57.3, 66.2, 81.1, 168.1
and 168.3, 170.2, 171.5 and 171.8; ElMS m/z 366 (M+, 12), 310
(31), 266 (26), 265 (100), 238 (15), 237 (87), 226 (29), 195 (26),
110 (80), 96 (31), 83 (29). Anal. Calcd for C20H34O4N2‚1/2H2O:
C, 63.97; H, 9.39; N, 7.46. Found: C, 63.77; H, 9.31; N, 7.17.
(5S)-1-Acetyl-7-[(1S)-ca r boxy-3-m eth ylbu tyl]-6-oxo-1,7-
d ia za sp ir o[4.5]d eca n e (23a ). Meth od A. Pyridine (38 mL,
0.46 mmol) and acetyl chloride (25 mL, 0.35 mmol) were added
to a solution of spirolactam 18a trifluoroacetate (88.6 mg, 0.23
mmol) in dry CH2Cl2 (0.4 mL), and the resulting mixture was
stirred at room temperature for 3 h. The reaction was
quenched by addition of 0.1 N HCl and extracted with CH2-
Cl2. The organic extracts were washed with more 0.1 N HCl
and saturated aqueous Na2CO3 solution, dried, and concen-
trated to furnish acid 23a (43 mg, 59%) as a yellow oil. Meth od
B. A solution of spirolactam 22a (70 mg, 0.19 mmol) in 10.5
mL of H2O-AcOH-iPrOH (1:2.5:1) was heated at 100 °C for
48 h. After being cooled to room temperature, the mixture was
concentrated, and the residue was washed with aqueous 0.1
N HCl (3 mL) and extracted with AcOEt. The organic extracts
were dried and concentrated, and the residue was purified by
flash chromatography (hexane/AcOEt, 8/2-6/4) to afford acid
Tr ip ep tid e An a logu e {Ac-Gly-Leu -Met-NH2} 2b. Oper-
ating as above for the preparation of 2a , from acid 23b (46
mg, 0.15 mmol) in 1 mL of dry DMF, DCC (34 mg, 0.17 mmol),
HOBt (30 mg, 0.17 mmol), and H-Met-NH2‚HCl (22 mg, 0.15
mmol) was obtained tripeptide analogue 2b (38 mg, 42%,
rotamers were observed) after purification by flash chroma-
23a (35 mg, 60%, rotamers were observed): [R]22 ) -25 (c 1,
tography (hexane/AcOEt, 70/30-0/100): [R]22 ) -44 (c 1.1,
D
D
CHCl3); IR (NaCl) 3357, 1624 cm-l; H NMR δ 0.91 and 0.95
l
CD3OD); IR (NaCl) 3316, 1681, 1667, 1649 and 1621 cm-1; 1H
NMR (500 MHz, CD3OD) δ 0.88 (d, J ) 7 Hz, 3H), 0.96 (d, J
) 6.5 Hz, 3H), 1.39-1.44 (m, 1H), 1.72 (dd, J ) 5.5, 2.5 Hz,
1H), 1.74 (dd, J ) 6.5, 2 Hz, 1H), 1.76-1.86 (m, 1H), 1.88-
1.97 (m, 2H), 2.00-2.19 (m, 6H), 2.01 and 2.03 (2s, 3H), 2.07
and 2.09 (2s, 3H), 2.26 and 2.37 (m, 1H), 2.44-2.53 (m, 1H),
2.55-2.62 (m, 1H), 3.23 (dt, J ) 12, 2 Hz, 1H), 3.41 (td, J )
12, 3.5 Hz, 1H), 3.59-3.64 (m, 1H), 3.68-3.73 (m, 1H), 4.29
(dd, J ) 10, 5.5 Hz, 1H, one rotamer), 4.44 (dd, J ) 10.5, 5
Hz, 1H, one rotamer), 4.46 (dd, J ) 9.5, 5 Hz, 1H, one rotamer),
5.38 (dd, J ) 9.5, 2.5 Hz, 1H); 13C NMR (CDCl3) δ 15.3, 21.7,
22.0, 22.9 and 23.0, 23.8, 24.7 and 24.8, 25.9 and 26.2, 31.1
and 31.6, 32.2 and 32.4, 32.5 and 32.7, 36.2, 38.1 and 38.2,
43.2, 50.2 and 50.4, 54.6, 55.3, 67.8 and 68.0, 170.7, 172.5,
174.9 and 177.0; HRMS calcd for C21H36O4N4S 440.2457, found
440.2466.
(2d, J ) 6.6, 3H each), 1.40 (m, 1H), 1.60-2.50 (m, 8H) 1.84-
1.89 (m, 1H), 1.94-2.00 (m, 1H), 2.05 and 2.08 (2s, 6H), 3.14-
3.32 (m, 1H), 3.20-3.33 (m, 1H), 3.56-3.70 (m, 1H), 3.63 (dd,
J ) 8.5, 4.5 Hz, 1H), 5.43 (dd, J ) 11, 5 Hz, 1H); 13C NMR δ
21.3 and 21.4, 22.8, 23.3, 23.4, 23.8, 24.4 and 24.8, 31.6 and
31.7, 35.8, 37.1 and 37.3, 44.7, 49.4, 54.8 and 55.4, 66.3 and
66.5, 168.1 and 169.8, 172.3, 174.7; EIMS m/z 310 (M+, 39),
266 (34), 226 (70), 180 (50), 153 (45), 138 (37), 110 (100), 96
(73), 83 (53). Anal. Calcd for C16H26O4N2‚1/5TFA: C, 58.96; H,
7.90; N, 8.37. Found: C, 58.97; H, 8.07; N, 8.04.
(5S)-1-Acetyl-7-[(1R)-ca r boxy-3-m eth ylbu tyl]-6-oxo-1,7-
d ia za sp ir o[4.5]d eca n e (23b). Meth od A. Operating as above
for the preparation of 23a (method A), from spirolactamn 18b
trifluoroacetate (96 mg, 0.25 mmol) in CH2Cl2 (0.4 mL),
pyridine (40 mL, 0.5 mrnol), and acetyl chloride (27 mL, 0.25
7598 J . Org. Chem., Vol. 67, No. 22, 2002

Spirolactams as Conformationally Restricted Pseudopeptides
Molecu la r Mod elin g Stu d ies. Con for m a tion a l Sea r ch .
All calculations were run on an SGI workstation (R4000, 128
MB RAM, 19 GB hard disk) under an Irix 5.3 operating
system. Molecular mechanics calculations were carried out
with Spartan v. 5.0 and InsightII discover 3.0 v.1997.
between the carbonyl function of the second amino acid (i +
1) and the amide group of the fourth amino acid (i+3), and
the torsion angles of the second amino acid (φ2, ψ2: φ1) 70/85
and ψ1 ) -60/-70).
Iter a tive Sim u la ted An n ea lin g. The calculations were
carried out within the molecular mechanics using the AMBER
force field implemented in DISCOVER v. 97. They were
conducted under vacuum with a distant dependent dielectric
constant (4r) and a cutoff of 13 Å. Starting from extended
structures of the epimers, the structure is minimized and
subsequently heated to 900 K in a very short period of time.
The structure is then cooled slowly to 100 K and minimized.
In our case the heating was carried out in steps. At each step
the temperature was raised by 100 K in 0.1 ps, and then the
system was allowed to stay 1 ps at the new temperature. The
system was allowed to stand for 10 ps at 900 K and then cooled
in steps. At each step the temperature was lowered by 100 K
in 0.1 ps, and after cooling, the system was allowed to stay 10
ps at the new temperature. This structure is the starting
conformation for another cycle, creating a library of conforma-
tions that are rank ordered by energy every 150 cycles. The
procedure is repeated until no new conformations appear after
a predetermined number of cycles (in our case 5 times) within
a 5 kcal/mol energy range with respect to the lowest energy
structure already found. Heating has to be carried out rapidly
in order to make the molecule jump to a different region in
the space. In contrast, cooling is slow in order to obtain the
lowest energy minimum of the region. This protocol was run
five times, employing fully extended starting structures of both
compounds (2a and 2b).
Op tim ized Mon te Ca r lo Sea r ch a n d En er gy Min im iza -
tion . In this paper, we used the MMFF94 force field imple-
mented in Spartan v 5.0. By default, atomic partial charges
were calculated from data in the molecular mechanics force
field chosen. The conformational search of epimers 2a and 2b
was carried out with an optimized Monte Carlo method
employing the MMFF94 force field as implemented in Spartan
5.0. Energy minimization used a conjugated gradient method,
with a final gradient of 0.0001 kcal/Å mol as the convergence
criteria. All conformers within a 10 kcal/mol window above
the global minimum were used to determine the profiles. To
determine the conformer families, atoms belonging to the
spirolactam skeleton and peptide backbones were considered.
We employed a similar protocol for the conformational search
of compounds 2a and 2b starting from extended structures.
Eight degrees of freedom were considered here (see Figure 4).
Each run included 3000 steps. The structures obtained were
clustered by conformation and energy. The resulting lowest
energy conformations (2a 1 and 2b1) were the starting points
for molecular dynamics studies.
Molecu la r Dyn a m ics. The resulting lowest energy con-
formers of compounds 2a and 2b (resulting from the Monte
Carlo search and Iterative simulated annealing) constituted
the starting points for molecular dynamics studies using a
distant dependent dielectric (4r) constant and an explicit
solvent model in chloroform. NVT calculations at 300 K were
performed using cubic boxes of 48 Å side length and 433
chloroform molecules. Periodical bounded conditions were
applied. After adequate heating and equilibration of the system
for 50 ps, evolution times were 1 ns. 1000 structures were
saved periodically from each profile distribution for further
analyses. Profile distributions were analyzed for conforma-
tional preferences measuring the γ and/or â-turn descriptors.
For the â-turn, these are the distance R (R < 7 Å) between
the CRi, and the CRi+l, the dihedral angle τ (-90 < τ < 90)
formed by the four CR atoms, and the distance between the
carbonyl function of the first amino acid (i) and the amide
group of the fourth (i + 3). The major â-turns are classified
according to the torsion angles of the second (φ1, ψ1: φ1) -60
( 30 and ψ1 ) 120 ( 30) and third amino acids (φ2, ψ2: φ1 )
80 ( 30 and ψ1 ) 0 ( 45). For the γ-turn, they are the distance
Ack n ow led gm en t. Support for this research has
been provided by the CIRIT (Generalitat de Catalunya)
through Grant Nos. QFN95-4703, 1995SGR-000429,
1997SGR-00075, and 1999SGR-00077 and by the DGY-
CIT (Ministerio de Educacio´n y Cultura, Spain) through
Grant Nos. PB97-0976 and 2FD97-0293. We also thank
the Universitat de Barcelona (Divisio´n de Ciencies de
la Salut) for a fellowship given to M.M.F.
Su p p or tin g In for m a tion Ava ila ble: Distribution profiles
for Monte Carlo Calculations of compounds 2a and 2b. This
material is available free of charge via the Internet at
http://pubs.acs.org.
J O025999I
J . Org. Chem, Vol. 67, No. 22, 2002 7599