Synthesis of Peptides and Pseudopeptides Incorporating an endo-(2S,3R)-Norborn-5-ene Residue as a Turn Inducer

Article abstract of DOI:10.1021/jo9717651

The desymmetrization of endo-norborn-5-ene-2,3-dicarboxylic anhydride (5) by proline derivatives is used to prepare peptides and pseudopeptides incorporating an endo-(2S,3R)-2-amino-3-carboxynorborn-5-ene (1) residue. The peptides contain a single conformationally constrained β-amino acid residue, while the pseudopeptides also contain a urea linkage and two peptide chains running in parallel directions. The key step in the synthesis is a Curtius rearrangement on the amido acids 6a,b to generate an isocyanate that is then directly reacted with suitably protected amino acids and peptides to give the peptides and pseudopeptides. The synthesis of the peptide analogue 4 is also described; in this compound, the two peptide chains run parallel to one another, and the stereochemistry of the norbornene unit within compound 4 was determined by X-ray analysis of the related peptide analogue 23.

Full text of DOI:10.1021/jo9717651

1496
J . Org. Chem. 1998, 63, 1496-1504
Syn th esis of P ep tid es a n d P seu d op ep tid es In cor p or a tin g a n
en d o-(2S,3R)-Nor bor n -5-en e Resid u e a s a Tu r n In d u cer
David E. Hibbs,† Michael B. Hursthouse,† Iwan G. J ones,‡ Wyn J ones,§
K. M. Abdul Malik,† and Michael North*,‡
Department of Chemistry, University of Wales, Bangor, Gwynedd LL57 2UW, U.K. Peboc Division of
Eastman Chemical (UK) Ltd., Industrial Estate, Llangefni, Anglesey, Gwynedd LL77 7YQ, U.K., and
Department of Chemistry, University of Wales, Cardiff, P.O. Box 912, Park Place, Cardiff CF1 3TB, U.K.
Received September 22, 1997
The desymmetrization of endo-norborn-5-ene-2,3-dicarboxylic anhydride (5) by proline derivatives
is used to prepare peptides and pseudopeptides incorporating an endo-(2S,3R)-2-amino-3-carbox-
ynorborn-5-ene (1) residue. The peptides contain a single conformationally constrained â-amino
acid residue, while the pseudopeptides also contain a urea linkage and two peptide chains running
in parallel directions. The key step in the synthesis is a Curtius rearrangement on the amido
acids 6a ,b to generate an isocyanate that is then directly reacted with suitably protected amino
acids and peptides to give the peptides and pseudopeptides. The synthesis of the peptide analogue
4 is also described; in this compound, the two peptide chains run parallel to one another, and the
stereochemistry of the norbornene unit within compound 4 was determined by X-ray analysis of
the related peptide analogue 23.
In tr od u ction
â-sheets, while the synthesis of constraints imposing a
parallel â-sheet has by comparison been neglected.
A template for the synthesis of parallel and antiparallel
â-sheets would be valuable in view of the role played by
this secondary structure in the biological activity of
proteins. It is well-known that proteases bind their
substrates and inhibitors by generating â-sheets or
â-strands, and this conformational requirement has been
influential in the design of inhibitors of renin7 and of
HIV-1 protease.8 Moreover, it has been reported that
protein-DNA interactions can occur with the protein
interface in a â-strand conformation.9
The main goal of the work described in this paper was
to develop the endo-(2S,3R)-2-amino-3-carboxynorborn-
5-ene residue (1) as a â-sheet inducer to allow the
formation of well-defined parallel and antiparallel â-sheets.
There is currently much interest in the synthesis and
applications of 2-amino-3-carboxynorborn-5-ene deriva-
tives; racemic syntheses of both the endo- and exo-cis-
isomers of this â-amino acid have been reported along
with methodology for their resolution.10 In addition, a
chiral auxiliary controlled Diels-Alder reaction has been
â-Sheets are a widespread element of protein struc-
ture;1 however, while a number of initiators for R-helix
formation2 and conformationally constrained analogues
of the various turns1,3 have been reported, relatively little
work has been done on the synthesis of initiators for
â-sheet formation. Two approaches can be adopted in
the synthesis of conformationally constrained peptides;
either most of the peptide backbone can be retained and
constrained by the introduction of peptidomimetics4,5 or
the peptide backbone can be completely replaced by a
conformationally rigid unit as exemplified by the work
of Smith et al.5 The first example of the former approach
was due to Kemp,6 who employed the epindolidione
nucleus as a molecular fragment to induce â-sheet
formation. The work that has been done on the design
of â-sheet mimics has largely concentrated on antiparallel
* To whom correspondence should be addressed. E-mail: m.north@
bangor.ac.uk.
† University of Wales, Cardiff.
‡ University of Wales, Bangor.
§ Peboc Division of Eastman Chemical (UK) Ltd.
(1) Moore, M. L. In Synthetic Peptides a Users Guide; Grant, G. A.,
Ed.; Freeman: New York, 1992; Chapter 2. Chemistry of Biomolecules
An Introduction; Simmonds, R. J ., Ed.; The Royal Society of Chemis-
try: Cambridge, 1992; Chapter 4. Understanding Enzymes; Palmer,
T., Ed.; Ellis Horwood: Chichester, 1991; Chapter 2.
(2) Kemp, D. S.; Curran, T. P.; Boyd, J . G.; Allen, T. J . J . Org. Chem.
1991, 56, 6683.
(3) See, for examples: Kahn, M., Ed. Peptide Secondary Structure
Mimetics: Tetrahedron Symposia in Print Number 50. Tetrahedron
1993, 49, 3433-3689. Ernest, I.; Kalvoda, J .; Rihs, G.; Mutter, M.
Tetrahedron Lett. 1990, 31, 4011. Kemp, D. S.; Stites, W. E. Tetrahe-
dron Lett. 1988, 29, 5057 and references therein.
(7) Kleinert, H. D.; Rosenberg, S. H.; Baker, W. R.; Stein, H. H.;
Klinghofer, V.; Barlow, J .; Spina, K.; Polakowski, J .; Kovar, P.; Cohen,
J .; Denissen, J . Science 1992, 257, 1940.
(8) For leading references see: Young, S. D.; Payne, L. S.; Thompson,
W. J .; Gaffin, N.; Lyle, T. A.; Britcher, S. F.; Graham, S. L.; Schultz,
T. H.; Deana, A. A.; Darke, P. L.; Zugay, J .; Schleif, W. A.; Quintero,
J . C.; Emini, E. A.; Anderson, P. S.; Huff, J . R. J . Med. Chem. 1992,
35, 1702. Thompson, W. J .; Fitzgerald, P. M. D.; Holloway, M. K.;
Emini, E. A.; Darke, P. L.; McKeever, B. M.; Schleif, W. A.; Quintero,
J . C.; Zugay, J .; Tucker, T. J .; Schwering, J . E.; Homnick, C. F.;
Nunberg, J .; Springer, J . P.; Huff, J . R. J . Med. Chem. 1992, 35, 1685.
(9) Somers, W. S.; Phillips, S. E. V. Nature 1992, 359, 387.
(4) See, for examples: Tsang, K. Y.; Diaz, H.; Graciani, N.; Kelly,
J . W. J . Am. Chem. Soc. 1994, 116, 3988 and references therein. Earlier
work is reviewed in: Giannis, A.; Kolter, T. Angew. Chem., Int. Ed.
Engl. 1993, 32, 1244.
(5) Smith, A. B., III; Guzman, M. C.; Sprengeler, P. A.; Keenan, T.
P.; Holcomb, R. C.; Wood, J . L.; Carroll, P. J .; Hirschmann, R. J . Am.
Chem. Soc. 1994, 116, 9947 and references therein.
(6) Kemp, D. S.; Bowen, B. R. Tetrahedron Lett. 1988, 29, 5077.
Kemp, D. S.; Bowen, B. R. Tetrahedron Lett. 1988, 29, 5081. Kemp, D.
S.; Bowen, B. R.; Muendal, C. C. J . Org. Chem. 1990, 55, 4650.
(10) Kanerva, L. T.; Csomos, P.; Sundholm, O.; Bernath, G.; Fulop,
F. Tetrahedron Asymm. 1996, 7, 1705. Canonne, P.; Akssira, M.;
Dahdouh, A.; Kasmi, H.; Boumzebra, M. Tetrahedron 1993, 49, 1985.
Patek, M.; Drake, B.; Lebl, M. Tetrahedron Lett. 1994, 35, 9169. Stajer,
G.; Szabo, A. E.; J anos, P.; Gabor, B.; Pal, S. J . Chem. Soc., Perkin
Trans. 1 1985, 2483. Stajer, G.; Laszlo, M.; Szabo, A. E.; Ferenc, F.;
Gabor, B.; Pal, S. Tetrahedron 1984, 40, 2385. Stajer, G.; Szabo, A. E.;
Ferenc, F.; Gabor, B. Synthesis 1984, 345. Moriconi, E. J .; Crawford,
W. C. J . Org. Chem. 1968, 33, 370. Kazuhiko, S.; Yoshiyuki, O.; Sumio,
W.; Tomonri, H.; Hiroyuki, N. Chem. Lett. 1981, 857.
S0022-3263(97)01765-9 CCC: $15.00 © 1998 American Chemical Society
Published on Web 02/18/1998

Synthesis of Peptides and Pseudopeptides
J . Org. Chem., Vol. 63, No. 5, 1998 1497
Sch em e 1a
a
Reagents: (i) Et3N; (ii) H2SO4/NaN3; (iii) EtOCOCl/Et3N/NaN3; (iv) H2CdCMeOCOCl/Et3N/NaN3; (v) ∆/C6H6; (vi) THF/H2O (1:1);
(vii) R′OH; (viii) Bu4NF/THF.
reported for the asymmetric synthesis of the trans-
isomer.11 Here, we describe an asymmetric synthesis of
the cis-diastereomer of compound 1, incorporated into
peptides and pseudopeptides.12 In recent publications,
we have reported the facile desymmetrization of meso-
anhydrides utilizing methyl prolinate13 as a chiral re-
agent.14 In this paper, we show how this methodology
can be applied to the synthesis of conformationally
constrained peptides 2 and pseudopeptides 3 containing
an endo-(2S,3R)-2-amino-3-carboxynorborn-5-ene residue
as well as to the synthesis of peptide analogues 4
containing an endo-(2S,3R)-2,3-dicarboxynorborn-5-ene
residue.
unsuccessful, but activation of the acid functionality via
Resu lts a n d Discu ssion
a mixed anhydride10,11,18 gave encouraging results. Reac-
tion of acid 6a with ethyl chloroformate and triethy-
lamine in tetrahydrofuran at -30 °C followed by addition
Treatment of endo-norborn-5-ene-2,3-dicarboxylic an-
hydride 5 with methyl prolinate hydrochloride in the
presence of triethylamine resulted in the stereoselective
formation of amido acid 6a as previously reported14
(Scheme 1). Conversion of amido acid 6a into the
corresponding acyl azide 7a under classical conditions
proved to be problematic. In particular, treatment of 6a
with concentrated sulfuric acid and sodium azide15 gave
lactone 8, while attempted formation via the acid chloride
using thionyl chloride or oxalyl chloride16 gave only
anhydride 5. Use of diphenyl phosphorazidate17 was also
(12) A preliminary account of some of this work has been pub-
lished: J ones, I. G.; J ones, W.; North, M. Synlett 1997, 63.
(13) Throughout this paper, all R-amino acids and their derivatives
are of the (S)-configuration.
(14) North, M.; Zagotto, G. Synlett 1995, 639. Albers, T.; Biagini,
S. C. G.; Hibbs, D. E.; Hursthouse, M. B.; Malik, K. M. A.; North, M.;
Uriarte, E.; Zagotto, G. Synthesis 1996, 393; J ones, I. G.; J ones, W.;
North, M.; Teijeira, M.; Uriarte, E. Tetrahedron Lett. 1997, 38, 889.
(15) Werner, N. W.; Casanova, J . Organic Syntheses; Wiley: New
York, 1973; Collect. Vol. V, p 273.
(16) Forster, M. O. J . Chem. Soc. 1909, 95, 433.
(17) Shioiri, T. In Comprehensive Organic Synthesis; Trost, B. M.,
Fleming, I., Winterfeldt, E., Eds.; Pergamon: Oxford, 1991; Vol. 3, p
806.
(11) Furuta, K.; Hayashi, S.; Miwa, Y.; Yamamoto, H. Tetrahedron
Lett. 1987, 28, 5841.

1498 J . Org. Chem., Vol. 63, No. 5, 1998
Hibbs et al.
Sch em e 2a
a
Reagents: (i) Boc-Ala-OH/Et3N/∆; (ii) Boc-Pro-OH/Et3N/∆; (iii) 17/Et3N/∆; (iv) (a) TFA, (b) H-Phe-Gly-OMe/EDC/HOBt.
of aqueous sodium azide afforded the desired acyl azide
7a , but this was always accompanied by 9% of the
corresponding ethyl ester 9. The formation of compound
9 can be explained by attack of the ethanol byproduct
upon acyl azide 7a , and this could not be eliminated even
at lower temperatures. Similarly, with the more steri-
cally hindered isobutyl chloroformate, ester formation
again occurred. However, the use of isopropenyl chloro-
formate19 produced the desired acyl azide 7a as the only
isolated product in 57% yield. The success of this
chloroformate can be attributed to the fact that the only
byproduct, acetone, is non-nucleophilic. The IR spectrum
column chromatography. The cyclization could not be
prevented even at 0 °C, and at temperatures below this
the deprotection failed. Cleavage of the urethane under
acidic conditions was also unrewarding.
We then reasoned that a larger proline ester, such as
a tert-butyl ester, might hinder this intramolecular
cyclization. Thus, desymmetrization of anhydride 5
using tert-butyl prolinate21 proceeded in 68% yield (Scheme
1), yielding amido acid 6b as an 8:1 ratio of diastereomers
that were separable by trituration with diethyl ether.
Acyl azide 7b, isocyanate 10b, and urethane 14b were
all obtained with the same conditions developed for the
methyl ester analogues, but treatment of 14b with
tetrabutylammonium fluoride again resulted in cycliza-
tion to give 15. Similarly, all attempts to hydrolyze
isocyanate 10b led to the formation of urea 12b.
of azide 7a showed a distinct peak at 2136 cm-1
,
characteristic of acyl azides. Having accomplished the
synthesis of 7a in reasonable yield, the acyl azide was
brought to reflux for 2 h in anhydrous benzene, affording
isocyanate 10a in quantitative yield. The coupling
constants between Ha and Hb in compounds 7a and 10a
were of comparable magnitude (9.4 Hz in 7a ; 8.8 Hz in
10a ), showing that as expected the Curtius rearrange-
ment had occurred with complete retention of configu-
ration at the migrating center.16
To circumvent the inability to prepare amine 11a ,b,
the direct reaction of isocyanate 10a ,b with N-protected
amino acids was investigated.22 Thus, reaction of isocy-
anate 10a with N-Boc-Ala-OH and triethylamine in
refluxing toluene for 2 h led directly to the desired
tripeptide 16a in 50% yield after purification by flash
chromatography (Scheme 2). Urea 12a was a byproduct
of this reaction produced in 21% yield. The disappear-
ance of the -NdCdO (2264 cm-1) stretch in the IR
spectrum allowed the progress of the reaction to be
monitored. Tripeptide 16b has also been prepared from
isocyanate 10b by reaction with N-Boc-Pro-OH, though
this reaction took considerably longer (80 °C for 16 h),
probably due to the effects of steric hindrance. Notice-
All attempts to directly hydrolyze isocyanate 10a to
amine 11a were unsuccessful. Decomposition occurred
under acidic or basic conditions, while neutral conditions
(water/tetrahydrofuran) resulted in dimerization to give
urea 12a , as evidenced by the distinct urea carbonyl peak
at 156 ppm in the 13C NMR and by mass spectrometry.
Isocyanate 10a could, however, be converted into ure-
thane 13 simply by reaction with excess methanol, and
this encouraged us to investigate a two-step procedure
for the hydrolysis, whereby isocyanate 10a was trapped
and subsequently deprotected. Following preliminary
experiments, â-(trimethylsilyl)ethanol was chosen as a
suitable urethane forming reagent in preference to other
possibilities such as thiophenol and p-methoxybenzyl
alcohol, which gave adducts that were difficult to purify.
Thus, addition of â-(trimethylsilyl)ethanol to a benzene
solution of isocyanate 10a followed by heating at 80 °C
gave urethane 14a in quantitative yield. Rapid removal
of the [â-(trimethylsilyl)ethoxy]carbonyl protecting group
occurred upon treatment with tetrabutylammonium fluo-
ride20 in THF at room temperature. However, the
resulting amine 11a could not be isolated; instead,
assisted by the basic reaction conditions, it cyclized onto
the methyl ester to give the seven-membered ring, bis-
lactam 15, which was isolated after purification by flash
1
ably, 16b exhibited a complex H NMR spectrum, owing
to the presence of cis and trans rotamers about the amide
bonds.
Having demonstrated that peptides 16a ,b could be
prepared by the reaction of isocyanates 10a ,b with simple
amino acids, the reaction of the isocyanates with peptide
derivatives was investigated. The amino acids used
within the peptide chains were chosen purely for ease of
(18) Arai, Y.; Kawanami, S.; Koizumi, T. J . Chem. Soc., Perkin
Trans. 1 1991, 2969.
(19) Zeggaf, C.; Poncet, J .; J ouin, P.; Dufour, M.-N.; Castro, B.
Tetrahedron 1989, 45, 5039.
(20) Carpino, L. A.; Sau, A. C. J . Chem. Soc., Chem. Commun. 1979,
514.
(21) Anderson, G. W.; Callahan, F. M. J . Am. Chem. Soc. 1960, 82,
3359.
(22) Blagbrough, I. S.; Mackenzie, N. E.; Ortiz, C.; Scott, A. I.
Tetrahedron Lett. 1986, 27, 1251.

Synthesis of Peptides and Pseudopeptides
J . Org. Chem., Vol. 63, No. 5, 1998 1499
Sch em e 3a
a
Reagents: (i) DCC/HOBt; (ii) Bu4NF/THF.
Sch em e 4a
a
Reagents: (i) H-Ala-OMe/Et3N; (ii) H-Pro-OCMe3/Et3N; (iii) H-Pro-Phe-Phe-OMe/Et3N; (iv) TFA; (v) H-Ala-Val-OMe/Et3N/EDC/HOBt.
synthesis; no preference was given to amino acids known
to stabilize â-sheets. The tripeptide Z-Leu-Ala-Pro-OH
(17) was prepared from â-(trimethylsilyl)ethyl prolinate23
(18) as shown in Scheme 3. Thus, condensation of
compound 18 with Z-Leu-Ala-OH24 gave the protected
tripeptide 19, which upon treatment with tetrabutylam-
monium fluoride gave the desired tripeptide derivative
17. Compound 17 was synthesized in this way to avoid
the use of acidolysis reactions, since preliminary experi-
ments had shown that traces of strong acids induced the
formation of urea 12b from isocyanate 10b. Reaction of
compound 17 with isocyanate 10b gave the desired
pentapeptide 16c. The tert-butyl ester could be removed
from peptide 16c by treatment with trifluoroacetic acid,
and subsequent coupling to H2N-Phe-Gly-OMe25 gave
heptapeptide 2 in 50% yield.
Isocyanates 10a ,b were also found to react with esters
of amino acids and peptides as shown in Scheme 4. Thus,
reaction of 10a with alanine methyl ester gave pseudopep-
tide 20a , while reaction of 10b with proline tert-butyl
ester and HN-Pro-Phe-Phe-OMe26 gave pseudopeptides
20b and 20c respectively. In the latter case, deprotection
of the tert-butyl ester was successfully accomplished with
trifluoroacetic acid to give acid 21 in quantitative yield.
Coupling of acid 18 to the dipeptide H2N-Ala-Val-OMe27
using water-soluble carbodiimide and HOBt gave pseudo-
heptapeptide 3.
two peptide chains running in parallel directions but not
offset from one another was also prepared. Thus, reac-
tion of endo-norborn-5-ene 2,3-dicarboxylic anhydride
with an N-terminal-proline peptide, HN-Pro-Phe-Phe-
OMe,26 resulted in ring opening to give the corresponding
amido acids 22a ,b in 81% yield (Scheme 5). This reaction
occurred with a complete lack of asymmetric induction,
giving a 1:1 ratio of diastereomers 22a ,b. The reason
for the lack of asymmetric induction in this case is not
clear, but it may be that the Phe-Phe-OMe group is
sufficiently large and flexible to hinder both faces of the
proline ring. This would remove the factor that is
thought to be responsible for the asymmetric induction
when simple proline esters are used.14 However, one of
the diastereomers, 22a , could be separated from the other
by flash column chromatography in 31% overall yield.
Subsequent coupling of 22a to HN-Pro-Ala-Val-OMe28
was accomplished via activation of compound 22a as its
N-hydroxy succinimide active ester. The isolated active
ester was used without purification, and the subsequent
coupling reaction with HN-Pro-Ala-Val-OMe proceeded
better in dimethylformamide than in dichloromethane,
giving pseudoheptapeptide 4 in 50% yield. Compound 4
exists as a mixture of two rotamers in a 10:1 ratio
1
according to H and 13C NMR spectra data in CDCl3. In
DMSO, however, two equally populated conformations
are observed, presumably due to cis and trans rotamers
about one of the proline amide bonds.
Compound 3 consists of two peptide chains running in
parallel directions but slightly offset from one another
by the presence of a urea linkage in one of the chains.
For comparison, peptide analogue 4 consisting of a
(2S,3R)-endo-2,3-dicarboxynorborn-5-ene residue with
At this stage it was not clear which of the two possible
diastereomers of compounds 22a /4 had been formed.
However, reaction of the N-hydroxy succinimide ester of
compound 22a with alanine methyl ester gave pseudo-
pentapeptide 23. Compound 23 was obtained as a clear
crystalline solid in 69% yield, and X-ray analysis showed
the absolute configuration of 23 to be as shown in Figure
1. From this, the absolute configuration of peptide
analogues 22a and 4 could be determined. In addition
to allowing the determination of the absolute configura-
(23) Allen, M. C.; Brandish, D. E.; Fullerton, J . D.; Wade, R. J .
Chem. Soc., Perkin Trans. 1, 1986, 989.
(24) Polgase, W. J .; Smith, E. L. J . Am. Chem. Soc. 1949, 71, 3081.
(25) Finn, F. M.; J ohns, P. A.; Nishi, N.; Hofmann, K. J . Biol. Chem.
1976, 251, 3576.
(26) Weiland, T.; Birr, C.; Burgermeister, W.; Trietsch, P.; Rohr, G.
J ustus Liebigs Ann. Chem. 1974, 24.
(27) Shin, C. G.; Yonezawa, Y.; Yamada, T. Chem. Pharm. Bull.
1984, 32, 3934.
(28) Krois, D.; Lehner, H. J . Chem. Soc., Perkin Trans. 2 1990, 1745.

1500 J . Org. Chem., Vol. 63, No. 5, 1998
Hibbs et al.
Sch em e 5a
a
Reagents: (i) H-Pro-Phe-Phe-OMe/Et3N; (ii) (a) NHS/DCC, (b) HN-Pro-Ala-Val-OMe/Et3N; (iii) (a) NHS/DCC, (b) H2N-Ala-OMe/Et3N.
creation of peptide mimics. It has also been shown that
the peptides and pseudopeptides incorporating 1 are
capable of undergoing further peptide chain extension
under standard solution-phase peptide synthesis condi-
tions. Heptapeptide 2 and pseudoheptapeptide 3 have
the potential to form antiparallel and parallel â-sheets,
respectively, with the norbornene residue acting as a
â-sheet inducer. Pseudopeptide 3 differs from peptide 2
in that the urea unit slightly offsets the two peptide
chains from one another, which may assist in interchain
hydrogen bonding. The related peptide analogue 4 has
also been prepared and has the potential to nucleate
parallel â-sheet formation. The conformational analysis
of compounds 2-4 will be discussed in the following
paper.
Exp er im en ta l Section
1H NMR spectra were recorded at 250 MHz on a Bruker
1
AM250 spectrometer fitted with a H-13C dual probe and were
recorded at 293 K in CDCl3 unless otherwise stated. Spectra
were internally referenced either to TMS or to the residual
solvent peak, and peaks are reported in ppm downfield of TMS.
Multiplicities are reported as singlet (s), doublet (d), triplet
(t), quartet (q), some combination of these, broad (br), or
multiplet (m). Coupling constants are reported in hertz. 13C
NMR spectra were recorded at 62.5 MHz on the same
F igu r e 1. Solid-state structure of 23 showing the intramo-
spectrometer as 1H NMR spectra, at 293 K and in CDCl3 unless
lecular hydrogen bonding. Thermal elipsoids are drawn at 50%
otherwise stated. Spectra were referenced to the solvent peak
probability.
and are reported in ppm downfield of TMS. Infrared spectra
were recorded on a Perkin-Elmer 1600 series FTIR spectrom-
tion of pseudopeptides 22, 23, and 4, the X-ray structure
of compound 23 also showed the presence of two in-
tramolecular hydrogen bonds involving the amides of the
Pro-Phe-Phe chain, which each form a 10-membered ring
â-turn structure.1 The conformation of compound 23 will
be discussed in more detail in the following paper.
eter; only characteristic absorptions are reported. Mass
spectra were recorded using the FAB technique (Cs+ ion
bombardment at 25 kV) on a VG Autospec spectrometer or by
chemical ionization (CI) with ammonia on either a VG model
12-253 quadrupole spectrometer or a VG Quattro II triple
quadrupole spectrometer. Only significant fragment ions are
reported, and only molecular ions are assigned. High-resolu-
tion mass measurements were made on a VG ZAB-E spec-
trometer. Optical rotations were recorded on an Optical
Activity Ltd. Polar 2001 polarimeter and are reported along
with the solvent and concentration in g/100 mL. Melting
points are uncorrected. Elemental analyses were performed
within the Chemistry department on a Carlo Erba model 1106
or model 1108 analyzer.
Con clu sion s
We have developed a short synthetic procedure for the
synthesis of peptides 2 and pseudopeptides 3 incorporat-
ing the conformationally constrained â-amino acid endo-
(2S,3R)-2-amino-3-carboxynorborn-5-ene (1) as a turn
inducer. The key intermediates in our synthesis are
isocyanates 10a ,b, and the reactions of these with
nucleophiles such as amines, acids, and alcohols renders
these compounds particularly attractive as synthetic
building blocks for combinatorial chemistry and the
Flash chromatography29 was carried out on 40-60 mm mesh
silica; thin-layer chromatography was carried out on aluminum-
backed silica plates (0.25 mm depth of silica containing
UV254), and the plates were visualized with UV light and/or
(29) Still, W. C.; Kahn, M.; Mitra, A. J . Org. Chem. 1978, 43, 2923.

Synthesis of Peptides and Pseudopeptides
J . Org. Chem., Vol. 63, No. 5, 1998 1501
dodecaphosphomolybdic acid as appropriate. All yields refer
to isolated, purified material, and are unoptimized. THF was
dried by distillation from sodium immediately prior to use.
Toluene and benzene were dried over sodium wire. Other
solvents were used as supplied.
roformate, in addition to acyl azide 7a ester 9 (9%) was always
obtained as a white solid. Mp: 70-74 °C. [R]22D: -69.0 (c )
1, CHCl3). IR: 1736, 1648. 1H NMR: 1.19 (t, 3, J ) 7.2), 1.35
(d, 1, J ) 8.5), 1.49 (d, 1, J ) 8.5), 1.9-2.3 (m, 4), 3.1-3.3 (m,
3), 3.40 (dd, 1, J ) 9.5, 3.0), 3.71 (s, 3), 4.05 (dq, 2, J ) 7.2,
2.6), 4.40 (dd, 1, J ) 8.4, 4.1), 6.23 (dd, 1, J ) 5.7, 3.5), 6.35
(dd, 1, J ) 5.7, 3.5). 13C NMR: 14.2, 24.9, 29.1, 46.4, 46.4,
46.6, 47.7, 48.4, 48.7, 52.1, 60.2, 60.2, 133.7, 135.9, 170.9, 172.3,
173.1. CI-MS m/ e (relative intensity): 322 (M+ + 1, 100).
HRMS (CI, NH3) m/ e: 322.1654 (MH+ C17H24NO5 requires
322.1654).
Am id o Acid 6b. A solution of endo-norborn-5-ene-2,3-
dicarboxylic anhydride 5 (4.16 g, 25 mmol) and tert-butyl
prolinate21 (4.22 g, 28 mmol) in CH2Cl2 (50 mL) was stirred
at room temperature for 18 h. The reaction mixture was
subsequently washed with 0.5 M HCl (40 mL) and water (2 ×
40 mL) and dried (MgSO4) and the solvent evaporated in vacuo
to leave a yellow oil. Trituration with Et2O gave 4.9 g (57%)
of a white crystalline solid. Mp: 108-113 °C. [R]22D: -132.1
(c ) 1, CHCl3). IR: 3500-2500, 1778, 1731, 1646. 1H NMR:
1.3-1.5 (m, 2), 1.48 (s, 9), 1.9-2.3 (m, 4), 3.2-3.4 (m, 4), 3.6-
3.8 (m, 2), 4.37 (dd, 1, J ) 8.2, 3.8), 6.23 (dd, 1, J ) 5.6, 2.7),
6.28 (dd, 1, J ) 5.6, 2.7). 13C NMR: 24.7, 28.0, 29.0, 46.9,
47.2, 47.3, 48.4, 48.5, 50.0, 59.9, 81.3, 134.7, 135.4, 171.2, 172.4,
175.6. CI-MS m/ e (relative intensity): 336 (M+ + 1, 23), 172
(100). HRMS (CI, NH3) m/ e: 336.1811 (MH+ C18H26NO5
requires 336.1811). Anal. Calcd for C18H25NO5: C, 64.46; H,
7.51; N, 4.18. Found: C, 64.70; H, 7.84; N, 3.80.
All X-ray crystallographic measurements were made at 150
K using a Delft Instruments FAST area detector diffractometer
positioned at the window of a rotating anode generator using
Mo KR radiation (λ ) 0.710 69 Å) by following previously
described procedures.30 Crystal data: C37H44N4O8‚2CH2Cl2
(FW 842.61); orthorhombic; space group P212121; a ) 11.238(2)
Å, b ) 18.043(2) Å and c ) 20.065(2) Å; V ) 4068.8(10) Å3; Z
) 4; Dc ) 1.376 Mg m-3; λ(Mo KR) ) 0.347 mm-1; F(000) )
1768. The structure was solved by direct methods
(SHELXS86)31 and refined on F2 by full-matrix least-squares
(SHELXL93)32 using all unique data corrected for Lorentz and
polarization factors but not for absorption. The structure was
finally refined (499 parameters) to R [on F, Fo > 4σ(Fo)] and
wR [on F2, all 6118 data)] values of 0.11(9) and 0.1232,
respectively. The Flack parameter in SHELXL93 refined to
a final value of 0.11(9), confirming that the absolute structure
had been determined correctly. Further details of data col-
lection and structure refinement, atom coordinates, thermal
coefficients, hydrogen atom parameters, and bond lengths and
angles are available from the Cambridge Crystallographic
Data Centre.33
La cton e 8. To a two-necked round-bottomed flask equipped
with a reflux condenser and a powder funnel were added
CHCl3 (8 mL), amido acid 6a 14 (0.5 g, 1.7 mmol), and
concentrated H2SO4 (1 mL). The flask was heated to 40-50
°C, and NaN3 (0.22 g, 3.4 mmol) was added over a period of
1.5 h, followed by heating for a further 1.5 h at 50 °C. To the
cooled reaction mixture were added H2O (30 mL) and EtOAc
(40 mL). The EtOAc layer was washed with saturated aqueous
K2CO3 (3 × 30 mL), and the solvent was dried (MgSO4),
filtered, and evaporated in vacuo to afford an orange oil that
was subjected to flash chromatography using neat EtOAc as
eluent to give (Rf ) 0.18) 0.26 g (52%) of a white solid. Mp
73-76 °C. [R]22D: -43.1 (c ) 1, CHCl3). IR: 1773, 1742, 1647.
1H NMR: 1.6-1.8 (m, 4), 1.8-2.2 (m, 4), 2.5-2.8 (m, 3), 3.3-
3.4 (m, 1), 3.6-3.8 (m, 2), 3.72 (s, 3), 4.49 (dd, 1, J ) 8.6, 3.5),
4.8-4.9 (m, 1). 13C NMR: 24.9, 28.9, 32.8, 37.6, 39.4, 41.2,
46.5, 47.7, 47.9, 52.2, 58.8, 80.7, 169.1, 172.9, 178.5. EI-MS
m/ e (relative intensity): 293 (M+, 3), 70 (100). HRMS (EI)
m/ e: 293.1263 (M+ C15H19NO5 requires 293.1263).
Acyl Azid e 7a . Isopropenyl chloroformate (0.41 mL, 3.75
mmol) was added to a mixture of amido acid 6a 14 (1 g, 3.41
mmol) and Et3N (1 mL) in dry THF (15 mL) at -20 °C. An
aqueous solution of NaN3 (0.55 g, 8.5 mmol) was added at -10
°C. The temperature was gradually raised to room tempera-
ture, and stirring was continued for 1 h. The reaction mixture
was diluted with H2O (30 mL) and extracted with EtOAc (3 ×
30 mL). The organic phase was washed with aqueous Na2CO3
solution (30 mL), water (30 mL), and brine (30 mL), dried
(MgSO4), filtered, and concentrated in vacuo to leave 0.57 g
(53%) of a white powder. Mp: 110-113 °C. [R]22D: -5.4 (c )
1, CHCl3). IR: 2136, 1738, 1644. 1H NMR: 1.35 (d, 1, J )
8.6), 1.49 (d, 1, J ) 8.6), 1.9-2.4 (m, 4), 3.1-3.3 (m, 3), 3.48
(dd, 1, J ) 9.4, 3.4), 3.5-3.8 (m, 2), 3.69 (s, 3), 4.49 (dd, 1, J )
8.5, 3.9), 6.25 (dd, 1, J ) 5.5, 2.9), 6.35 (dd, 1, J ) 5.5, 2.8).
13C NMR: 24.9, 29.0, 46.2, 46.5, 46.7, 48.3, 48.4, 51.0, 52.1,
58.7, 134.1, 135.3, 170.1, 172.8, 179.6. FAB-MS m/ e (relative
intensity): 319 (M+ + 1, 100), 225 (100). HRMS (FAB) m/ e:
319.1403 (MH+ C15H18N4O4 requires 319.1406).
Acyl Azid e 7b. Isopropenyl chloroformate (0.87 mL, 8.04
mmol) was added to a mixture of amido acid 6b (2.45 g, 7.3
mmol) and Et3N (2 mL) in dry THF (25 mL) at -20 °C. An
aqueous solution of NaN3 (1.19 g, 18.3 mmol) was added at
-10 °C. The temperature was gradually raised to room
temperature, and stirring was continued for 1 h. The reaction
mixture was diluted with water (30 mL) and extracted with
EtOAc (3 × 30 mL). The organic phase was washed with
aqueous Na2CO3 solution (30 mL), water (30 mL), and brine
(30 mL), dried (MgSO4), filtered, and concentrated in vacuo
to leave 1.5 g (57%) of a white solid. Mp: 107-109 °C. [R]22
:
D
-19.5 (c ) 1, CHCl3). IR: 2138, 1729, 1647. 1H NMR: 1.32
(d, 1, J ) 8.6), 1.44 (s, 9), 1.47 (d, 1, J ) 8.6), 1.8-2.3 (m, 4),
3.1-3.3 (m, 3), 3.43 (dd, 1, J ) 9.4, 3.3), 3.5-3.8 (m, 2), 4.40
(dd, 1, J ) 8.0, 3.4), 6.19 (dd, 1, J ) 5.5, 2.9), 6.31 (dd, 1, J )
5.5, 2.7). 13C NMR: 24.6, 28.0, 29.1, 46.2, 46.5, 46.6, 48.3, 48.3,
51.1, 59.3, 81.0, 134.0, 135.4, 169.7, 171.6, 179.5. FAB-MS
m/ e (relative intensity): 361 (M+ + 1, 83), 211 (100). HRMS
(FAB) m/ e: 333.1816 (MH - N2)+ C18H25N2O4 requires
333.1814).
Isocya n a te 10a . Acyl azide 7a (0.55 g, 1.73 mmol) was
dissolved in anhydrous benzene (10 mL) and heated at reflux
for 2 h. The solvent was evaporated in vacuo to leave 0.5 g
(100%) of a white solid. Mp 62-65 °C. [R]22D: -51.6 (c ) 1,
CHCl3). IR: 2264, 1744, 1644. 1H NMR: 1.34 (d, 1, J ) 9.2),
1.59 (d, 1, J ) 9.2), 1.9-2.3 (m, 4), 3.1-3.2 (m, 2), 3.26 (dd, 1,
J ) 8.8, 2.8), 3.78 (s, 3), 3.5-3.7 (m, 2), 4.29 (dd, 1, J ) 8.8,
3.7), 4.49 (dd, 1, J ) 8.3, 4.0), 6.08 (dd, 1, J ) 5.3, 3.0), 6.79
(dd, 1, J ) 5.7, 3.6). 13C NMR: 24.7, 29.0, 45.6, 46.3, 46.6,
49.1, 51.9, 52.2, 56.1, 58.7, 122.5, 129.9, 141.1, 169.3, 173.0.
CI-MS m/ e (relative intensity): 308 (M+ + 18, 4), 291 (M+
+
1, 100). HRMS (CI, NH3) m/ e: 291.1345 (MH+ C15H18N2O4
requires 291.1345).
Isocya n a te 10b. Acyl azide 7b (1.5 g, 4.15 mmol) was
dissolved in anhydrous benzene (15 mL), and heated at reflux
for 2 h. The solvent was evaporated in vacuo to leave 1.37 g
(100%) of a white solid. Mp: 61-62 °C. [R]22D: -3.20 (c ) 1,
CHCl3). IR: 2263, 1735, 1647. 1H NMR: 1.31 (d, 1, J ) 9.1),
1.47 (s, 9), 1.49 (d, 1, J ) 9.1), 1.9-2.3 (m, 4), 3.1-3.2 (m, 2),
3.2-3.3 (dd, 1, J ) 9.0, 2.9), 3.5-3.7 (m, 2), 4.24 (dd, 1, J )
9.1, 3.8), 4.38 (dd, 1, J ) 8.9, 3.5), 6.0 (dd, 1, J ) 5.6, 2.9),
6.79 (dd, 1, J ) 5.6, 3.3). 13C NMR: 24.5, 28.0, 29.2, 45.6,
46.3, 46.6, 49.1, 52.0, 56.1, 59.5, 81.2, 128.3, 129.8, 139.1, 169.0,
171.6. EI-MS m/ e (relative intensity): 332 (M+, 6), 70 (100).
HRMS (EI) m/ e: 332.1736 (M+ C18H24N2O4 requires 332.1736).
Anal. Calcd for C18H24N2O4: C, 65.04; H, 7.28; N, 8.43.
Found: C, 64.88; H, 7.56; N, 8.61.
Ester 9. When the preparation of acyl azide 7a was carried
out using ethyl chloroformate rather than isopropenyl chlo-
(30) Darr, J . A.; Drake, S. R.; Hursthouse, M. B.; Malik, K. M. A.
Inorg. Chem. 1993, 32, 5704.
(31) Sheldrick, G. M. Acta Crystallogr. 1990, A46, 467.
(32) Sheldrick, G. M. SHELXL93 Program for Crystal Structure
Refinement; University of Gottingen: Germany, 1993.
(33) The authors have deposited the atomic coordinates for 23 with
the Cambridge Crystallographic Data Centre. The coordinates can be
obtained, on request, from the Director, Cambridge Crystallographic
Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK.

1502 J . Org. Chem., Vol. 63, No. 5, 1998
Hibbs et al.
Ur ea 12a . Isocyanate 10a (1 g, 3.4 mmol) was dissolved in
a 1:1 solution of water/THF (10 mL). After the solution was
stirred for 5 h, the solvent was evaporated in vacuo to yield a
yellow oil. Purification by flash chromatography using 5%
MeOH/95% EtOAc as eluent afforded (Rf ) 0.11, 2% MeOH/
98% EtOAc) 0.9 g (87%) of a yellow solid. Mp: 92-93 °C.
Bis-la cta m 15. Carbamate 14a (0.4 g, 0.98 mmol) was
stirred with 1 M aqueous tetrabutylammonium fluoride solu-
tion (4.14 mL, 14.7 mmol) in THF (5 mL) at room temperature
for 24 h. Solvent was removed in vacuo, and the residue was
dissolved in CH2Cl2 (10 mL). Water (10 mL) was added, and
the layers were mixed by rapid stirring for approximately 15
min. The CH2Cl2 layer was extracted with saturated aqueous
NH4Cl solution (10 mL) and dried over MgSO4. After filtra-
tion, solvent was evaporated in vacuo to afford a clear oil.
Purification by flash chromatography using 80% EtOAc/20%
MeOH as eluent yielded (Rf ) 0.34) 0.15 g (66%) of a white
solid. Mp: 60-61 °C. [R]22D: +51.8 (c ) 1, CHCl3). IR: 3218,
3001, 1682, 1630. 1H NMR: 1.42 (d, 1, J ) 9.0), 1.54 (d, 1, J
) 9.0), 1.8-2.5 (m, 4), 3.1-3.7 (m, 5), 4.2-4.4 (m, 2), 6.05 (dd,
1, J ) 5.7, 2.9), 6.40 (s, 1), 6.51 (dd, 1, J ) 5.7, 3.0). 13C NMR:
23.1, 26.1, 45.7, 45.8, 45.9, 47.3, 53.3, 55.4, 56.2, 131.7, 141.2,
168.7, 173.2. CI-MS m/ e (relative intensity): 233 (M+ + 1,
100). HRMS (CI, NH3) m/ e: 233.1290 (MH+ C13H17N2O2
requires 233.1290). Anal. Calcd for C13H16N2O2‚0.2 H2O: C,
66.19; H, 7.01; N, 11.88. Found: C, 66.15; H, 7.01; N, 11.70.
[R]22
:
-50.1 (c ) 1, CHCl3). IR: 3375, 1742, 1638. 1H
D
NMR: 1.35 (d, 1, J ) 8.8), 1.43 (d, 1, J ) 8.8), 1.8-2.2 (m, 4),
2.8-3.2 (m, 2), 3.25 (dd, 1, J ) 9.3, 3.1), 3.6-3.9 (m, 2), 3.75
(s, 3), 4.29 (dd, 1, J ) 8.2, 5.0), 4.68 (d, 1, J ) 9.8), 4.80 (dt, 1,
J ) 9.9, 3.6), 6.05 (dd, 1, J ) 5.6, 3.0), 6.56 (dd, 1, J ) 5.5,
3.1). 13C NMR: 25.0, 29.2, 46.1, 46.7, 47.7, 47.8, 49.4, 52.2,
52.8, 58.7, 130.9, 140.0, 156.5, 171.1, 172.8. CI-MS m/ e
(relative intensity): 555 (M+ + 1, 8). HRMS (CI, NH3) m/ e:
555.282 (MH+ C29H39N4O7 requires 555.282). Anal. Calcd for
C
29H38N4O7‚0.5 THF: C, 63.02; H, 7.17; N, 9.49. Found: C,
63.19; H, 7.50; N, 9.83.
Ur ea 12b. Isocyanate 10b (50 mg, 0.15 mmol) was dis-
solved in a 1:1 solution of water/THF (1 mL). After the
solution was stirred for approximately 5 h, the solvent was
evaporated in vacuo to yield 52 mg (100%) of a white solid.
Mp: 104-105 °C. [R]22D: -43.4 (c ) 1, CHCl3). IR: 3346,
1734. 1H NMR: 1.3-1.5 (m, 2), 1.46 (s, 9), 1.7-2.2 (m, 4),
2.8-3.1 (m, 2), 3.19 (dd, 1, J ) 8.9, 2.9), 3.5-3.9 (m, 2), 4.21
(dd, 1, J ) 8.0, 3.9), 4.7-4.9 (m, 1), 4.75 (d, 1, J ) 10.3), 5.99
(dd, 1, J ) 5.6, 2.8), 6.48 (dd, 1, J ) 5.6, 2.9). 13C NMR: 24.7,
28.0, 29.2, 46.1, 46.7, 47.6, 47.7, 48.9, 52.9, 59.5, 81.1, 131.1,
139.6, 156.6, 170.8, 171.5. CI-MS m/ e (relative intensity): 639
(M+ + 1, 40), 70 (100). HRMS (CI, NH3) m/ e: 639.3757 (MH+
C35H51N4O7 requires 639.3757).
Ur eth a n e 13. Isocyanate 10a (0.4 g, 1.37 mmol) was
dissolved in MeOH (10 mL) and stirred for 5 h. The MeOH
was then evaporated in vacuo to yield 0.44 g (100%) of a
colorless oil. [R]22D: -64.3 (c ) 1, CHCl3). IR: 3407, 3018,
1741, 1715. 1H NMR: 1.35 (d, 1, J ) 8.9), 1.50 (d, 1, J ) 8.9),
1.8-2.3 (m, 4), 3.0-3.2 (m, 2), 3.31 (dd, 1, J ) 9.3, 3.1), 3.60
(s, 3), 3.73 (s, 3), 3.5-3.8 (m, 2), 4.29 (dd, 1, J ) 8.0, 4.6), 4.64
(dt, 1, J ) 9.7, 3.9), 5.45 (d, 1, J ) 9.9), 6.07 (dd, 1, J ) 5.6,
3.0), 6.50 (dd, 1, J ) 5.6, 3.0). 13C NMR: 24.9, 29.1, 46.2, 46.7,
47.5, 47.6, 48.3, 51.9, 52.2, 54.0, 58.7, 131.4, 139.3, 156.6, 171.0,
172.7. CI-MS m/ e (relative intensity): 323 (M+ + 1, 100).
HRMS (CI, NH3) m/ e: 323.1607 (MH+ C16H23N2O5 requires
323.1607).
Ur eth a n e 14a . â-(Trimethylsilyl)ethanol (0.75 mL, 5.2
mmol) and isocyanate 10a (1.5 g, 5.16 mmol) were heated at
reflux in anhydrous benzene (20 mL) for 24 h. Solvent was
subsequently removed in vacuo to afford 2.1 g (100%) of a white
solid. Mp: 79-80 °C. [R]22D: -48.3 (c ) 1, CHCl3). IR: 3419,
3015, 1735, 1702, 1637. 1H NMR: 0.20 (s, 9), 0.8-1.0 (m, 2),
1.37 (d, 1, J ) 8.9), 1.49 (d, 1, J ) 8.9), 1.8-2.2 (m, 4), 3.0-
3.2 (m, 2), 3.31 (dd, 1, J ) 9.4, 3.0), 3.5-3.7 (m, 2), 3.74 (s, 3),
4.0-4.2 (m, 2), 4.46 (dd, 1, J ) 7.9, 4.6), 4.66 (dt, 1, J ) 9.9,
3.8), 5.27 (d, 1, J ) 9.9), 6.07 (dd, 1, J ) 5.6, 3.0), 6.54 (dd, 1,
J ) 5.5, 3.0). 13C NMR: -1.5, 17.7, 24.9, 29.1, 46.2, 46.7, 47.5,
47.6, 48.7, 52.2, 53.8, 58.7, 62.8, 131.3, 139.6, 156.3, 171.0,
172.8. CI-MS m/ z (relative intensity): 409 (M+ + 1, 39), 291
(100). HRMS (CI, NH3) m/ e: 409.2159 (MH+ C20H33N2O5Si
requires 409.2159). Anal. Calcd for C20H32N2O5Si‚0.25 H2O:
C, 58.15; H, 7.93; N, 6.78. Found: C, 57.95; H, 7.60; N, 6.70.
Ur eth a n e 14b. â-(Trimethylsilyl)ethanol (0.65 mL, 4.5
mmol) and isocyanate 10b (1.37 g, 4.13 mmol) were heated at
reflux in anhydrous benzene (20 mL) for 24 h. Solvent was
subsequently removed in vacuo to afford 1.85 g (100%) of a
white solid. Mp: 64-66 °C. [R]22D: -37.6 (c ) 1, CHCl3). IR:
3420, 3016, 1737, 1636. 1H NMR: -0.30 (s, 9), 1.31 (d, 1, J )
9.0), 1.40 (s, 9), 1.45 (d, 1, J ) 9.0), 1.7-2.2 (m, 4), 3.0-3.1
(m, 2), 3.22 (dd, 1, J ) 9.4, 3.0), 3.5-3.7 (m, 2), 4.0-4.2 (m, 2),
4.22 (dd, 1, J ) 7.2, 3.4), 4.51 (dt, 1, J ) 9.8, 3.7), 5.39 (d, 1,
J ) 9.8), 6.00 (dd, 1, J ) 5.6, 3.0), 6.40 (dd, 1, J ) 5.6, 3.0).
13C NMR: 1.6, 17.6, 24.6, 27.9, 29.1, 46.2, 46.7, 47.5, 48.2, 53.8,
59.4, 59.5, 62.6, 81.0, 131.4, 139.2, 156.3, 170.7, 171.4. CI-
MS m/ e (relative intensity): 451 (M+ + 1, 100). HRMS (CI,
NH3) m/ e: 451.2628 (MH+ C23H39N2O5Si requires 451.2628).
Anal. Calcd for C23H38N2O5Si: C, 61.30; H, 8.50; N, 6.22.
Found: C, 61.37; H, 8.48; N, 6.39.
Tr ip ep tid e 16a . A solution of N-BOC-alanine (0.097 g,
0.51 mmol), isocyanate 10a (0.1 g, 0.35 mmol), and Et3N (0.072
mL, 0.51 mmol) in dry toluene (1 mL) was heated to 60 °C in
an argon atmosphere for 2 h. To the cooled reaction mixture
was added EtOAc (20 mL), which was subsequently washed
with 0.5 M HCl (10 mL), aqueous Na2CO3 (10 mL), and water
(10 mL). The solvent was dried (MgSO4) and evaporated in
vacuo to afford an orange oil that was subjected to flash
chromatography using EtOAc as eluent to give (Rf ) 0.3) 0.065
g (50%) of a white solid. Mp: 61-63 °C. [R]22D: -43.6 (c )
0.5, CHCl3). IR: 3313, 1745, 1712, 1644. 1H NMR: 1.25 (d,
3, J ) 6.0), 1.40 (d, 1, J ) 8.9), 1.45 (s, 9), 1.51 (d, 1, J ) 8.9),
1.8-2.3 (m, 4), 3.1-3.2 (m, 2), 3.26 (dd, 1, J ) 9.3, 6.1), 3.6-
3.7 (m, 2), 4.0-4.1 (m, 1), 4.40 (dd, 1, J ) 8.1, 4.2), 4.82 (dt, 1,
J ) 8.5, 3.0), 5.05 (d, 1, J ) 8.5), 6.13 (dd, 1, J ) 5.4, 2.9), 6.45
(dd, 1, J ) 5.4, 2.8), 7.05 (d, 1, J ) 8.3). 13C NMR: 19.3, 24.8,
28.3, 29.0, 46.5, 46.9, 46.9, 47.2, 47.5, 47.7, 50.1, 52.2, 58.6,
79.7, 132.1, 138.7, 155.0, 171.1, 172.1, 172.6. CI-MS m/ e
(relative intensity): 436 (M+ + 1, 4), 74 (100). HRMS (CI,
NH3) m/ e: 436.2448 (MH+ C22H34N4O6 requires 436.2448).
Tr ip ep tid e 16b. A solution of N-BOC-proline (0.58 g, 2.7
mmol), isocyanate 10b (0.60 g, 1.8 mmol), and Et3N (0.4 mL,
2.7 mmol) in dry toluene (6 mL) was heated to 80 °C in an
argon atmosphere for 24 h. To the cooled reaction mixture
was added EtOAc (30 mL), which was subsequently washed
with 0.5 M HCl (15 mL), aqueous Na2CO3 (15 mL), and water
(15 mL). The solvent was dried with MgSO4 and evaporated
in vacuo to leave a brown/black solid that was subjected to
flash chromatography using 1:1 EtOAc/petrol as eluent to give
(Rf ) 0.22 EtOAc) 0.40 g (46%) of a white powder. Mp: 42-
44 °C. [R]22D: -94.3 (c ) 1, CHCl3). IR: 3324, 3018, 1732,
1689. 1H NMR (DMSO-d6): 1.32 (s, 9), 1.41 (s, 9), 1.3-1.5
(m, 2), 1.6-2.2 (m, 8), 2.8-3.7 (m, 7), 4.0-4.1 (m, 2), 4.6-4.8
(m, 1), 5.8-6.5 (m, 2), 7.26 (d, 1, J ) 9.3), 7.45 (d, 1, J ) 9.1).
13C NMR (DMSO-d6) (only peaks corresponding to the major
conformer are reported): 24.3, 24.6, 29.1, 29.9, 28.0, 28.0, 46.1,
46.3, 46.4, 46.7, 46.8, 47.5, 52.0, 59.4, 59.5, 79.9, 81.1, 131.3,
139.1, 156.3, 170.8, 171.6, 171.8. CI-MS m/ e (relative inten-
sity): 504 (M+ + 1, 10), 70 (100). HRMS (CI, NH3) m/ e:
504.307 (MH+ C27H42N3O6 requires 504.307). Anal. Calcd for
C27H41N3O6‚2.5H2O: C, 59.09; H, 8.45; N, 7.66. Found: C,
58.98; H, 8.18; N, 7.29.
Tr ip ep tid e 19. To a suspension of Z-Leu-Ala-OH24 (3.6 g,
10.7 mmol) in CH2Cl2 (80 mL) were added DCC (2.87 g, 13.9
mmol) and HOBt (1.90 g, 13.9 mmol), followed by HN-Pro-
23
CO2CH2CH2SiMe3 (3.0 g, 13.9 mmol). The mixture was
stirred at room temperature for 14 h before the precipitate
was removed by filtration. The solution was washed with 1
M HCl (2 × 40 mL), saturated aqueous Na2CO3 (2 × 40 mL),
and water (2 × 40 mL). The organic phase was separated,
dried (MgSO4), and evaporated to dryness. The crude product
was purified by flash chromatography using 1:1 EtOAc/
petroleum ether to give (Rf ) 0.2) 4.0 g (70%) of a clear oil.
[R]23D: -43.5 (c ) 1, CHCl3). IR: 3293, 3013, 1731, 1643. 1H

Synthesis of Peptides and Pseudopeptides
J . Org. Chem., Vol. 63, No. 5, 1998 1503
NMR: 0.42 (s, 9), 0.9-1.1 (m, 8), 1.20-1.40 (m, 10), 3.6-3.8
(m, 2), 4.1-4.8 (m, 5), 5.05-5.35 (m, 3), 6.38 (d, 1, J ) 7.0),
6.97 (d, 1, J ) 7.0), 7.26-7.34 (m, 5). 13C NMR (only peaks
corresponding to major conformer are reported): -1.6, 17.2,
17.9, 21.8, 23.0, 24.6, 24.8, 28.8, 41.9, 46.6, 46.8, 53.5, 58.9,
63.4, 66.7, 127.9, 128.4, 136.4, 156.1, 170.8, 170.9, 171.8. CI-
MS m/ e (relative intensity): 534 (M+ + 1, 7), 90 (100). HRMS
(CI, NH3) m/ e: 534.300 (MH+ C27H44N3O6Si requires 534.300).
Anal. Calcd for C27H43N3O6Si‚0.33H2O: C, 60.08; H, 8.16; N,
7.79. Found: C, 60.13; H, 8.14; N, 7.76.
Tr ip ep tid e 17. Tripeptide 19 (3 g, 5.63 mmol) was stirred
with 1 M tetra-n-butylammonium fluoride (16.2 mL, 56.3
mmol) in THF (60 mL) at room temperature for 24 h. Solvent
was removed in vacuo, and the residue was dissolved in CH2Cl2
(80 mL). The CH2Cl2 layer was washed with 1 M HCl (2 × 40
mL) and dried over MgSO4. After filtration, solvent was
evaporated in vacuo to afford the crude product, which was
purified by flash chromatography using EtOAc as eluent to
afford (Rf ) 0.17) 2.24 g (93%) of a white powder. Mp: 57-64
°C. [R]23D: -48.6 (c ) 1, CHCl3). IR: 3500-2500, 3302, 3017,
1717, 1634. 1H NMR: 0.92 (m, 6), 1.1-2.4 (m, 10), 3.5-3.8
(m, 2), 4.25-4.9 (m, 3), 5.1-5.2 (m, 2), 5.63 (d, 1, J ) 8.4),
5.97 (d, 1, J ) 8.5), 7.30-7.36 (m, 5), 8.69 (br, 1). 13C NMR:
17.4, 20.3, 23.0, 24.6, 24.9, 28.9, 41.9, 46.7, 47.1, 53.4, 59.1,
66.9, 127.9, 128.5, 136.3, 156.2, 171.9, 172.4, 174.1. CI-MS
m/ e (relative intensity): 434 (M+ + 1, 57). HRMS (CI, NH3)
m/ e: 434.229 (MH+ C22H32N3O6 requires 434.229). Anal.
Calcd for C22H31N3O6‚0.5 H2O: C, 59.70; H, 7.29; N, 9.50.
Found: C, 59.80; H, 7.66; N, 9.60.
P en ta p ep tid e 16c. A solution of tripeptide 17 (0.68 g, 1.58
mmol), isocyanate 10b (0.35 g, 1.05 mmol), and Et3N (0.3 mL,
2.10 mmol) in dry toluene (5 mL) was heated to 60 °C in an
argon atmosphere for 16 h. To the cooled reaction mixture
was added EtOAc (20 mL), which was subsequently washed
with 0.5 M HCl (10 mL), saturated aqueous Na2CO3 (10 mL),
and water (10 mL). The solvent was dried (MgSO4) and
evaporated in vacuo to afford an orange solid that was
subjected to flash chromatography using EtOAc as eluent to
give (Rf ) 0.17) 0.2 g (26%) of a white powder. Mp: 83-87
°C. [R]23D -34.6 (c ) 0.5, CHCl3). IR: 3401, 3297, 3013, 1724,
1644. 1H NMR: 0.95 (m, 6), 1.40 (d, 1, J ) 9.1), 1.47 (s, 9),
1.47-2.2 (m, 15), 3.14-3.24 (m, 2), 3.23 (dd, 1, J ) 9.2, 3.4),
3.61-3.66 (m, 4), 4.27-4.43 (m, 3), 4.65-4.80 (m, 1), 4.78 (dt,
1, J ) 8.5, 3.4), 5.15 (s, 2), 5.54 (d, 1, J ) 8.6), 6.08 (dd, 1, J
) 5.2, 2.8), 6.4 (dd, 1, J ) 5.2, 2.8), 7.0 (d, 1, J ) 6.9) 7.3-7.4
(m, 6). 13C NMR: 17.8, 21.6, 23.1, 24.4, 24.6, 24.7, 27.9, 29.0,
29.1, 41.8, 46.4, 46.8, 46.9, 46.9, 47.1, 47.4, 47.5, 52.5, 53.5,
59.7, 60.5, 60.8, 81.0, 127.9, 128.4, 132.0, 136.4, 138.4, 156.1,
170.6, 170.8, 171.6, 171.6, 171. CI-MS m/ e (relative inten-
sity): 722.7 (M+ + 1, 100), 656.6 (65). HRMS (CI, NH3) m/ e:
722.413 (MH+ C39H56N5O8 requires 722.413). Anal. Calcd for
saturated aqueous Na2CO3 (5 mL), and H2O (5 mL), and dried
(MgSO4). The solvent was evaporated in vacuo and the residue
subjected to flash chromatography using 5% MeOH/95%
EtOAc as eluent to afford (Rf ) 0.12) 0.15 g (66%) of a white
powder. Mp: 68-71 °C. [R]23D: -65.4 (c ) 1, CHCl3). IR:
3411, 3326, 1708, 1657, 1631. 1H NMR data are given in the
following paper in this issue. 13C NMR (150 MHz): 17.6, 21.4,
23.1, 24.7, 24.9, 25.0, 28.8, 28.9, 36.8, 41.0, 41.4, 46.4, 47.0,
47.3, 47.5, 48.0, 49.8, 51.5, 52.2, 52.9, 54.0, 60.5, 61.3, 66.7,
127.0, 127.9, 128.4, 128.6, 129.5, 130.2, 136.4, 136.5, 140.4,
156.3, 170.0, 170.2, 171.2, 171.9, 172.1, 172.2. FAB-MS m/ e
(relative intensity): 906 (M+ + 23, 55), 884 (M+ + 1, 50), 107
(100). HRMS (FAB) m/ e: 884.4517 (MH+ C47H62N7O10 re-
quires 884.4558). Anal. Calcd for C47H61N7O10‚3 CH3OH: C,
61.25; H, 7.51; N, 10.01. Found: C, 61.04; H, 7.43; N, 9.89.
P seu d ot r ip ep t id e 20a . Et3N (0.5 mL) was added to a
cooled (0 °C) suspension of isocyanate 10a (0.5 g, 1.72 mmol)
and alanine methyl ester hydrochloride (0.36 g, 2.58 mmol)
in CH2Cl2 (8 mL). The reaction mixture was stirred at room
temperature for 18 h and subsequently washed with 0.5 M
HCl (5 mL), aqueous Na2CO3 (5 mL), and H2O (5 mL) and dried
(MgSO4). The solvent was evaporated in vacuo and the residue
subjected to flash chromatography using EtOAc as eluent to
give (Rf ) 0.33) 0.4 g (61%) of a white solid. Mp: 54-55 °C.
[R]22D: -53.1 (c ) 1, CHCl3). IR: 3420, 3018, 1742, 1672, 1632.
1H NMR: 1.32 (d, 3, J ) 7.2), 1.35 (d, 1, J ) 8.8), 1.48 (d, 1, J
) 8.8), 1.9-2.2 (m, 4), 3.0-3.2 (m, 2), 3.28 (dd, 1, J ) 9.1,
3.1), 3.5-3.8 (m, 2), 3.70 (s, 6), 4.35 (dd, 1, J ) 8.2, 4.9), 4.39
(pent, 1, J ) 7.3), 4.74 (d, 1, J ) 7.1), 4.81 (dt, 1, J ) 9.5, 3.8),
5.28 (d, 1, J ) 9.5), 6.10 (dd, 1, J ) 5.4, 3.0), 6.49 (dd, 1, J )
5.5, 3.0). 13C NMR: 18.8, 24.9, 29.2, 46.1, 46.7, 47.6, 47.7, 48.6,
48.9, 52.2, 52.3, 52.9, 58.7, 131.7, 139.2, 156.7, 171.5, 172.8,
174.4. CI-MS m/ e (relative intensity): 394 (M+ + 1, 100).
HRMS (CI, NH3) m/ e: 394.1978 (MH+ C19H28N3O6 requires
394.1978).
P seu d otr ip ep tid e 20b. A solution of isocyanate 10b (0.5
g, 15 mmol) and tert-butyl prolinate21 (0.28 g, 16.5 mmol) in
CH2Cl2 (10 mL) was stirred at room temperature for 18 h. The
reaction mixture was subsequently washed with 0.5 M HCl
(10 mL), saturated aqueous Na2CO3 (10 mL), and H2O (10 mL)
and dried (MgSO4) and the solvent evaporated in vacuo to
leave 0.68 g of a yellow oil. Purification by flash chromatog-
raphy using EtOAc as eluent yielded (Rf ) 0.28) 0.55 g (73%)
of a white solid. Mp: 42-44 °C. [R]23D: -57.5 (c ) 1, CHCl3).
IR: 3347, 1735, 1636. 1H NMR: 1.40 (s, 18), 1.3-1.6 (m, 2),
1.75-2.2 (m, 8), 3.0-3.5 (m, 4), 3.21 (dd, 1, J ) 9.0, 2.8), 3.4-
3.7 (m, 2), 4.1-4.3 (m, 2), 4.78 (dt, 1, J ) 9.5, 3.6), 5.15 (d, 1,
J ) 9.5), 6.05 (dd, 1, J ) 5.3, 3.1), 6.41 (dd, 1, J ) 5.3, 3.5).
13C NMR: 22.3, 24.2, 27.9, 28.0, 29.1, 29.7, 45.4, 46.3, 47.2,
47.4, 47.6, 48.4, 53.2, 59.5, 59.5, 80.8, 81.0, 131.8, 139.6, 155.8,
171.1, 171.3, 172.31. CI-MS m/ e (relative intensity): 504 (M+
+ 1,100). HRMS (CI, NH3) m/ e: 504.3074 (MH+ C27H42N3O6
requires 504.3073). Anal. Calcd for C27H41N3O6‚1.25 H2O: C,
61.62; H, 8.34; N, 7.99. Found: C, 61.62; H, 8.03; N, 8.18.
C
39H55N5O8: C, 62.50; H, 7.86; N, 9.38. Found: C, 62.55; H,
7.81; N, 9.35.
Dep r otection of 16c. Pentapeptide 16c (0.19 g, 0.26
mmol) was dissolved in a solution of CH2Cl2 (2 mL) and
trifluoroacetic acid (2 mL) and allowed to stir at room tem-
perature overnight. The solvent was evaporated in vacuo to
P seu d op en t a p et id e 20c. Et3N (1 mL) was added to a
cooled (0 °C) suspension of isocyanate 10b (1.15 g, 3.5 mmol)
and CF3CO2H‚HN-Pro-Phe-Phe-OMe26 (2.41 g, 4.5 mmol) in
CH2Cl2 (10 mL). The reaction mixture was stirred at room
temperature for 18 h, subsequently washed with 0.5 M HCl
(5 mL), saturated aqueous Na2CO3 (5 mL), and H2O (5 mL),
and dried (MgSO4). The solvent was evaporated in vacuo to
leave a yellow solid. Trituration with EtOAc yielded 2.05 g
(78%) of a white solid. Mp: 60-61 °C. [R]23D: -95.1 (c ) 1,
CHCl3). IR: 3407, 3313, 3016, 1734, 1636. 1H NMR: 1.3-
1.5 (m, 2), 1.46 (s, 9), 1.6-2.2 (m, 8), 2.9-3.2 (m, 8), 3.25 (dd,
1, J ) 9.0, 3.0), 3.67 (s, 3), 3.6-3.7 (m, 2), 4.2-4.3 (m, 2), 4.6-
4.9 (m, 3), 5.97 (d, 1, J ) 7.3), 6.10 (dd, 1, J ) 5.8, 3.3), 6.37
(dd, 1, J ) 5.8, 2.5), 7.0-7.4 (m, 12). 13C NMR: 24.6, 24.6,
27.8, 28.0, 28.1, 37.1, 37.7, 45.9, 46.6, 46.6, 47.0, 47.6, 47.8,
52.2, 53.4, 53.6, 53.7, 59.5, 60.4, 81.2, 126.7, 126.8, 128.4, 128.4,
129.2, 132.4, 138.2, 136.4, 136.9, 157.3, 170.8, 171.2, 171.4,
171.6, 172.3. CI-MS m/ e (relative intensity): 756 (M+ + 1,
12), 424 (100). HRMS (CI, NH3) m/ e: 756.397 (MH+
leave 0.18 g (100%) of a yellow powder. Mp: 61-65 °C. [R]23
:
D
-32.9 (c ) 1, CHCl3). IR: 3302, 3017, 1720, 1630. 1H NMR:
0.95 (d, 6, J ) 5.9), 1.29-1.57 (m, 16), 3.16-3.20 (m, 2), 3.28
(dd, 1, J ) 9.5, 3.1), 3.50-3.85 (m, 4), 4.2-4.9 (m, 5), 5.13 (d,
2, J ) 3.7), 5.50 (d, 1, J ) 7.9), 6.03-6.05 (m, 1), 6.40-6.43
(m, 1) 7.20-7.45 (m, 5). 13C NMR: 17.4, 21.0, 21.5, 23.0, 24.6,
24.8, 28.8, 29.0, 41.4, 46.5, 47.5, 52.7, 53.4, 59.2, 60.5, 67.0,
127.9, 128.7, 128.4, 132.1, 136.2, 138.7, 156.4, 171.5, 171.8,
172.3, 173.0, 174.6. CI-MS m/ e (relative intensity): 666 (M+
+ 1, 70). HRMS (CI, NH3) m/ e: 666.350 (MH+ C35H48N5O8
requires 666.350). Anal. Calcd for C35H47N5O8‚CH2Cl2: C,
57.60; H, 6.58; N, 9.33. Found: C, 57.70; H, 6.77; N, 9.13.
Hep ta p ep tid e 2. Et3N (0.10 mL, 0.76 mmol) was added
to a cooled (0 °C) suspension of the acid prepared above (0.17
g, 0.32 mmol), EDC (0.063 g, 0.33 mmol), HOBt (0.045 g, 0.33
mmol), and CF3CO2‚H3N-Phe-Gly-OMe25 (0.18 g, 0.51 mmol)
in CH2Cl2 (10 mL). The reaction mixture was stirred at room
temperature for 24 h, washed with 0.5 M HCl (5 mL),

1504 J . Org. Chem., Vol. 63, No. 5, 1998
Hibbs et al.
C42H54N5O8 requires 756.397). Anal. Calcd for C42H53N5O8‚1.5
H2O: C, 64.43; H, 7.21; N, 8.95. Found: C, 64.75; H, 7.23; N,
8.67.
solved in CH2Cl2 (10 mL), and CF3CO2H‚HN-Pro-Ala-Val-
OMe28 (1.15 g, 2.87 mmol) and Et3N (0.4 mL, 2.87 mmol) were
added at 0 °C. The solution was stirred at room temperature
for 9 h and then washed sequentially with 1 M HCl (5 mL),
saturated aqueous Na2CO3 (5 mL), and H2O (5 mL). The
organic phase was dried (MgSO4), filtered, and evaporated to
dryness in vacuo, affording a white foamy solid. Flash
chromatography using 2% MeOH/98% EtOAc as eluent gave
(Rf ) 0.11) 0.41 g (50%) of a white powder. Mp: 96-99 °C.
[R]22D: -87.6 (c ) 1, CHCl3). IR: 3420, 3293, 1738, 1659, 1642,
1632. 1H NMR data are given in the following paper in this
issue. 13C NMR (150 MHz): 17.3, 17.7, 18.8, 24.2, 24.9, 28.6,
28.6, 31.1, 36.2, 37.9, 46.3, 46.7, 46.8, 47.0, 47.6, 47.7, 48.3,
49.4, 50.0, 51.9, 52.0, 54.2, 56.9, 59.5, 60.8, 126.5, 126.9, 128.2,
128.5, 128.9, 129.3, 131.2, 136.6, 137.7, 138.2, 171.5, 171.6,
172.0, 172.1, 172.1, 172.3, 172.5, 172.7. FAB-MS m/ e (relative
intensity): 891 (M+ + 23, 100), 869 (M+ + 1, 20). HRMS (FAB)
m/ e: 869.4976 (MH+ C47H61N6O10 requires 869.4449). Anal.
Calcd for C47H60N6O10‚ H2O: C, 63.64; H, 7.05; N, 9.47.
Found: C, 63.71; H, 7.23; N, 9.55.
Acid 21. Pseudopentapeptide 20c (1.90 g, 2.5 mmol) was
dissolved in a solution of CH2Cl2 (10 mL) and trifluoroacetic
acid (3 mL) and allowed to stir at room temperature overnight.
The solvent was evaporated in vacuo to leave 1.76 g (100%) of
a yellow powder. Mp: 42-44 °C. [R]23D: -62.0 (c ) 1, CHCl3).
IR: 3286, 1716, 1633. 1H NMR: 1.40 (d, 1, J ) 8.4), 1.55 (d,
1, J ) 8.4), 1.6-2.3 (m, 8), 2.9-3.2 (m, 8), 3.25 (dd, 1, J ) 9.1,
5.0), 3.6-3.70 (m, 2), 3.69 (s, 3), 4.2-4.4 (m, 1), 4.4-4.5 (m,
1), 4.5-4.8 (m, 3), 6.10 (dd, 1, J ) 5.7, 3.3), 6.28 (dd, 1, J )
5.7, 3.1), 6.88 (d, 1, J ) 7.7), 7.0-7.3 (m, 12), 8.05 (br, 1). 13C
NMR: 24.2, 24.7, 28.5, 29.2, 37.3, 37.6, 45.8, 46.1, 46.8, 47.5,
47.6, 47.9, 52.4, 53.7, 54.2, 54.3, 59.2, 60.5, 127.1, 128.6, 129.2,
133.1, 137.6, 135.8, 136.1, 157.5, 171.2, 171.3, 173.0, 173.1,
174.9. CI-MS m/ e (relative intensity): 700 (M+ + 1, 13), 550
(100). HRMS (CI, NH3) m/ e: 700.335 (MH+ C38H46N5O8
requires 700.335). Anal. Calcd for C38H45N5O8‚1.5 H2O: C,
57.35; H, 5.85; N, 8.47. Found: C, 57.35; H, 5.62; N, 8.19.
P seu d oh ep ta p ep tid e 3. Et3N (0.4 mL, 2.8 mmol) was
added to a cooled (0 °C) suspension of acid 21 (0.4 g, 0.57
mmol), water-soluble carbodiimide (0.143 g, 0.75 mmol), HOBt
(0.1 g, 0.75 mmol), and CF3CO2H‚H2N-Ala-Val-OMe27 (0.24 g,
0.75 mmol) in CH2Cl2 (8 mL). The reaction mixture was
stirred at room temperature for 24 h, subsequently washed
with 0.5 M HCl (5 mL), saturated aqueous Na2CO3 (5 mL),
and H2O (5 mL), and dried (MgSO4). The solvent was
evaporated in vacuo and the residue subjected to flash chro-
matography using 4% MeOH/96% EtOAc as eluent to give (Rf
P seu d op en ta p ep tid e 23. To a suspension of amido acid
22a (0.5 g, 0.85 mmol) and DCC (0.21 g, 1.02 mmol) in CH2Cl2
(20 mL) was added NHS (0.11 g, 1.02 mmol) over a period of
5 min at 0 °C. After being stirred for 16 h, the byproduct was
removed by filtration. The filtrate was washed with water (3
× 10 mL) and the organic layer dried (MgSO4), filtered, and
evaporated to dryness in vacuo, yielding 0.52 g of crude
N-hydroxysuccinimide ester. The ester was then redissolved
in CH2Cl2 (10 mL), and HCl‚H2N-Ala-OMe (0.58 g, 4.1 mmol)
and Et3N (0.57 mL) were added at 0 °C. The solution was
stirred at room temperature for 9 h, and then washed
sequentially with 1 M HCl (5 mL), saturated aqueous Na2CO3
(5 mL), and H2O (5 mL). The organic phase was dried
(MgSO4), filtered, and evaporated to dryness in vacuo, afford-
ing a white foamy solid. Flash chromatography using 2%
MeOH/98% EtOAc as eluent afforded (Rf ) 0.17) 0.36 g (69%)
of a white powder. Mp: 209-213 °C. [R]23D: -97.4 (c ) 1,
CHCl3). IR: 3420, 3316, 3054, 1744, 1664. 1H NMR: 1.37
(d, 3, J ) 7.2), 1.39 (d, 1, J ) 8.6), 1.53 (d, 1, J ) 8.6), 1.7-2.1
(m, 4), 3.28-3.55 (m, 10), 3.59 (s, 3), 3.73 (s, 3), 4.38 (dd, 1, J
) 6.0, 2.1), 4.42 (pent., 1, J ) 7.2), 4.60 (q, 1, J ) 7.9), 4.73
(ddd, 1, J ) 12.5, 9.3, 3.5), 6.05 (dd, 1, J ) 5.5, 2.8), 6.41 (d, 1,
J ) 7.3), 6.66 (dd, 1, J ) 5.5, 3.0), 7.10-7.30 (m, 10), 7.53 (d,
1, J ) 7.7), 8.14 (d, 1, J ) 9.3). 13C NMR: 18.1, 24.2, 29.3,
36.3, 38.3, 46.8, 47.0, 47.8, 48.2, 48.2, 48.6, 49.8, 51.9, 52.5,
54.3, 54.4, 61.5, 126.3, 126.5, 128.2, 128.2, 128.7, 129.3, 130.8,
137.2, 138.7, 139.4, 171.5, 171.6, 171.94, 172.2, 173.0, 173.2.
) 0.23) 0.34 g (66%) of a white foam. Mp: 68-72 °C. [R]23
:
D
1
-109.6 (c ) 1, CHCl3). IR: 3419, 3302, 3048, 1743, 1643. H
NMR: 0.89 (d, 3, J ) 6.8), 0.91 (d, 3, J ) 6.8), 1.39 (d, 3, J )
7.0), 1.3-2.5 (m, 11), 2.8-3.2 (m, 8), 3.2 (dd, 1, J ) 9.1, 3.3),
3.6-3.75 (m, 2), 3.66 (s, 3), 3.73 (s, 3), 4.2-4.9 (m, 7), 5.81 (d,
1, J ) 8.9), 6.15 (dd, 1, J ) 5.8, 3.0), 6.33 (dd, 1, J ) 6.0, 3.0),
6.60 (d, 1, J ) 8.5), 7.0 (d, 1, J ) 7.9), 7.1-7.3 (m, 12). 13C
NMR (150 MHz): 17.5, 17.6, 18.9, 24.5, 24.9, 27.6, 28.1, 31.0,
37.1, 37.7, 45.8, 46.9, 47.0, 47.4, 47.7, 47.8, 49.0, 52.1, 52.1,
53.4, 53.7, 53.7, 57.0, 59.9, 60.3, 126.7, 126.8, 128.3, 128.4,
129.1, 129.1, 132.6, 136.2, 136.8, 138.0, 157.1, 170.6, 171.1,
171.5, 171.8, 172.0, 172.1, 173.1. FAB-MS m/ e (relative
intensity): 907 (M+ + 23, 100), 885 (M+ + 1, 56), 461 (43).
HRMS (FAB) m/ e: 884.4627 (MH+ C47H62N7O10 requires
884.4558).
Am id o Acid 22a . Et3N (4.24 mL, 30 mmol) was added to
a cooled (0 °C) suspension of endo-norborn-5-ene-2,3-dicar-
boxylic anhydride 5 (0.99 g, 6.0 mmol) and CF3CO2H‚HN-Pro-
Phe-Phe-OMe26 (3.6 g, 6.7 mmol) in CH2Cl2 (40 mL). The
reaction mixture was stirred at room temperature for 24 h,
subsequently washed with 0.5 M HCl (20 mL) and water (20
mL), and dried (MgSO4) and the solvent evaporated in vacuo
to leave a yellow solid. Purification by flash chromatography
using EtOAc as eluent gave (Rf ) 0.34, 10% MeOH/90%
CI-MS m/ e (relative intensity): 691 (M+ + 18, 2), 673 (M+
+
1, 13), 250 (100). HRMS (CI, NH3) m/ e: 673.3237 (MH+
C37H45N4O8 requires 673.3237). Anal. Calcd for C37H44N4O8‚0.2
EtOAc: C, 65.75; H, 6.66; N, 8.12. Found: C, 66.09; H, 7.03;
N, 8.00.
EtOAc) 1.02 g (31%) of a white solid. Mp: 96-98 °C. [R]22
:
D
-93.3 (c ) 1, CHCl3). IR: 3500-2500, 3328, 3011, 1732, 1634.
1H NMR: 1.30 (d, 1, J ) 8.6), 1.42 (d, 1, J ) 8.6), 1.6-2.0 (m,
4), 2.8-3.1 (m, 4), 3.0-3.5 (m, 6), 3.58 (s, 3), 4.35 (dd, 1, J )
8.5, 3.9), 4.5-4.8 (m, 2), 6.10 (dd, 1, J ) 5.3, 2.9), 6.35 (dd, 1,
J ) 5.5, 2.9), 7.0-7.3 (m, 11), 7.40 (d, 1, J ) 9.1). 13C NMR:
23.9, 29.1, 36.3, 37.9, 46.8, 46.8, 47.4, 47.8, 48.7, 49.2, 52.1,
54.3, 54.4, 61.2, 126.4, 126.8, 128.2, 128.4, 129.1, 129.3, 132.8,
137.6, 137.6, 137.9, 171.5, 171.9, 172.0, 173.0, 174.9. CI-MS
m/ e (relative intensity): 588 (M+ + 1, 10), 424 (100). HRMS
(CI, NH3) m/ e: 588.271 (MH+ C33H38N3O7 requires 588.271).
Anal. Calcd for C33H37N3O7‚EtOAc: C, 65.76; H, 6.71; N, 6.22.
Found C, 65.41; H, 6.88; N, 6.29.
P seu d oh ep ta p ep tid e 4. To a suspension of amido acid
22a (0.56 g, 0.95 mmol) and DCC (0.3 g, 1.43 mmol) in CH2Cl2
(15 mL) was added NHS (0.16 g, 1.43 mmol) over a period of
5 min at 0 °C. After the mixture was stirred for 16 h, the
byproduct was removed by filtration. The filtrate was washed
with water (3 × 10 mL) and the organic layer dried (MgSO4)
and evaporated to dryness in vacuo yielding 0.6 g of N-
hydroxysuccinimide ester. The crude ester was then redis-
Ack n ow led gm en ts. The authors thank Peboc Divi-
sion of Eastman Chemical (UK) Ltd. and the EPSRC
for a studentship to I.G.J . and the EPSRC for access to
the national mass spectrometry, 600 MHz NMR, and
X-ray crystallography services.
Su p p or tin g In for m a tion Ava ila ble: Tables of crystal
data, atomic coordinates, bond lengths and angles, anisotropic
displacement parameters, hydrogen coordinates and isotropic
displacement parameters, and possible hydrogen bonds and
short contacts for C37H44N4O8‚2(CH2Cl2) (9 pages). This mate-
rial is contained in libraries on microfiche, immediately follows
this article in the microfilm version of the journal, and can be
ordered from the ACS; see any current masthead page for
ordering information.
J O9717651