Design and asymmetric synthesis of β-strand peptidomimetics

Article abstract of DOI:10.1021/jo9522771

We describe the asymmetric synthesis of non-peptidic compounds that feature rigid backbone conformations and present various side-chain functions. The key step in the synthesis of these compounds is the C-acylation of an appropriate ketone with a suitably protected aspartic acid derivative. The resulting dipeptide modules may be connected to form tetrapeptide mimics. Specifically is described the mimicry of a four-residue segment of CD4, the cellular receptor of HIV-1. The design was based on molecular modeling and the X-ray crystal structures of CD4 and intended to present the most important side chains and backbone elements of the Phe43-Lys46 segment.

Full text of DOI:10.1021/jo9522771

4434
J . Org. Chem. 1996, 61, 4434-4438
Design a n d Asym m etr ic Syn th esis of â-Str a n d P ep tid om im etics
Ahcene Boumendjel, J ohn C. Roberts, Essa Hu, and Peter V. Pallai*
Department of Rational Drug Design, Procept, Inc. 840 Memorial Drive, Cambridge, Massachusetts 02139
J ulius Rebek, J r.
Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
Received December 28, 1995X
We describe the asymmetric synthesis of non-peptidic compounds that feature rigid backbone
conformations and present various side-chain functions. The key step in the synthesis of these
compounds is the C-acylation of an appropriate ketone with a suitably protected aspartic acid
derivative. The resulting dipeptide modules may be connected to form tetrapeptide mimics.
Specifically is described the mimicry of a four-residue segment of CD4, the cellular receptor of
HIV-1. The design was based on molecular modeling and the X-ray crystal structures of CD4 and
intended to present the most important side chains and backbone elements of the Phe43-Lys46
segment.
In tr od u ction
One of the fundamental structural elements of proteins
and polypeptides is the â-strand. Although this motif
represents a very prominant secondary structural class,
there have been few small molecule â-strand mimics
reported.1,2 The peptidic mimetics clearly suffer from
several therapeutic limitations such as poor oral bio-
availability and short half-lives in vivo.3 Non-peptidic
mimics, however, may overcome these limitations which,
in a general sense, is the ultimate goal for most small
molecule approaches. We have sought to develop a
strategy of general utility which results in small mol-
ecules that adopt specific secondary structures relevant
to those found in protein segments or substrate-bound
peptides of interest.
In recent years there have been a wide variety of
dipeptide-sized, non-peptidic amino acids reported.4 In
general, these molecules either contain a peptide back-
bone (i.e. N-sp3C-sp2C-N-sp3C-sp2C) or are designed
to display properly oriented side chains.5 We have
focused on the development of a methodology that
provides both these features as shown in Figure 1. A
dipeptide of interest (Figure 1A) is mimicked with a
bicyclic vinylamide (Figure 1B) which arises from the
condensation of a ketone with an aspartic acid derivative
(Figure 1C). If the dipeptide and the dipeptide mimics
F igu r e 1. General approach.
are compared, it is clear that C2 and X6 are used to make
constraints in a standard fashion while C4 is utilized as
part of a vinylamide moiety. The C5 side chain (R)
originates from the ketone starting material whereas the
second side chain (R′) could arise from either the use of
an aspartic acid derivative which contains this group at
C2 or from attaching a similar group at a later stage via
the C1 carboxylate. The chosen dipeptide mimic may be
one of the four possible diastereomers.
In a previous report, cyclohexanones were used to
develop a methodology which allows for the asymmetric
synthesis of compounds such as 1a -d (Figure 1B).6
These compounds do not have an available nitrogen for
further extension, but present side chains (in this case
benzyl) in very specific orientations. In this report, we
have extended our methodology to the asymmetric syn-
X Abstract published in Advance ACS Abstracts, May 1, 1996.
(1) (a) Diaz, H.; Tsang, K. Y.; Choo, D.; Espina, J . R.; Kelly, J . W. J .
Am. Chem. Soc. 1993, 115, 3790-3791. (b) Kemp, D. S.; Bowen, B. R.;
Muendel, C. J . Org. Chem. 1990, 55, 4650-4657 and references cited
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T. P.; Holcomb, R. C.; Wood, J . L.; Carroll, P. J .; Hirshmann, R. J .
Am. Chem. Soc. 1994, 116, 9947-9962.
(2) Martin, S. F.; Austin, R. E.; Oalman, C. J .; Baker, W. R.; Condon,
S. L.; DeLara, E.; Rosenberg, S. H.; Spina, K. P.; Stein, H. H.; Cohen,
J .; Kleinert, H. D. J . Med. Chem. 1992, 35, 1710-1721.
(3) (a) Hirschmann, R. Angew. Chem., Int. Ed. Engl. 1991, 30, 1278-
1301. (b) Smith, A. B. III; Hirshmann, R.; Pasternak, A.; Akaishi, R.;
Guzman, M. C.; J ones, D. R.; Keenan, T. P.; Speengeler, P. A.; Emini,
E. A.; Holloway, M. K.; Schleif, W. A. J . Med. Chem. 1994, 37, 215-
218.
(4) Spatola, A. F. In Chemistry and Biochemistry of Amino Acids,
Peptides and Proteins; Weinstein, B., Ed., Marcel Dekker: New York,
1983; pp 267-357.
(5) (a) Dalgleish, A. G.; Beverley, P. C. L.; Clapham, P. R.; Crawford,
D. H.; Greaves, M. F.; Weiss, R. A. Nature 1985, 312, 763-767 (b)
Klatzman, D.; Champagne, E.; Chamaret, S.; Gruest, J .; Guetard, D.;
Hercend, T.; Gluckman, J . C.; Montagnier, L. Nature 1984, 312, 767-
768.
(6) Roberts, J . C.; Pallai, P. V.; Rebek, J ., J r. Tetrahedron Lett. 1995,
36, 691-694.
S0022-3263(95)02277-8 CCC: $12.00 © 1996 American Chemical Society

Asymmetric Synthesis of â-Strand Peptidomimetics
J . Org. Chem., Vol. 61, No. 13, 1996 4435
F igu r e 3. Mimicry of the Phe43-Lys46 segment of CD4: 1a
+ 2 + ethylenediamine.
Sch em e 1
F igu r e 2. Mimicry of Thr45-Lys46 segment of CD4.
theses of nitrogen-containing analogs (i.e. X ) NH) and
show that these two types of modules may be used to
construct tetrapeptide mimics.
We chose to adapt our general methodology to the
mimicry of a four-residue segment found in the first
domain of CD4, which, via binding to gp120, serves as
the cellular receptor of HIV-1.7 It has been reasoned that
successful disruption of the CD4-gp120 interaction could
lead to the prevention of HIV infection,8 and others have
also attempted to inhibit this protein-protein interaction
with small molecules.9 The â-strand segment of CD4
which we targeted includes the residues Phe43 and
Lys46, each of which are significant contributors to the
gp120 binding event as indicated by mutational analy-
sis.10
found in the available X-ray crystal structures of CD4.
The synthetic methodology permits access to a variety
of related analogs of 3, differing principally at the Lys-
and Phe-type side chains.
Our approach, based in part on X-ray crystal structures
of CD4,11 was previously used in the design of vinylamide
1a ,6,9b a Phe43-Leu44 module, and a similar process was
used to design a Thr45-Lys46 module. Since mutation
of Thr45 to Ala has no significant effect on binding there
appeared to be no need to mimic the Thr hydroxyl.10 From
a three-dimensional comparison of the Thr45-Lys46
segment and a variety of vinylamide-containing com-
pounds, we identified compound 2 (Figure 2) as having
good overlay with respect to the atoms and bonds shown
in bold. The tricyclic nucleus of this molecule ap-
propriately directs a lysine-type side chain (in this case
derived from ethylenediamine) which, although not as
flexible as a 4-aminobutyl group, can adopt the confor-
mations observed for this side chain in the crystal
structures of CD4. A subtle but important point for ridge
mimicry is that the atoms used to make constraints in
the Thr45-Lys46 module, when overlaid appropriately
with CD4, reach into the space occupied by the protein
itself, and therefore the module does not present struc-
tural elements which would sterically preclude binding
to gp120.
Resu lts a n d Discu ssion
The synthesis of vinylamide 2 required enantiomeri-
cally pure bicyclic ketone 6a , which was prepared in three
steps (Scheme 1). The aza-Diels-Alder reaction between
an imminium ion [generated in situ from (R)-(+)-R-
methylbenzylamine and formaldehyde] and cyclopenta-
diene afforded olefin 4 in 83% yield after separation from
the minor diastereoisomer. Hydroboration of olefin 4
with 2.2 equiv of BH3‚THF complex provided alcohol 5,
isolated from a 4:1 diastereomeric mixture, in 58% yield.
The regioselectivity of this hydroboration step is in accord
with related results reported for 1,2,5,6-tetrahydropyr-
idines.12 The stereochemistry of 5 was determined by an
X-ray crystal structure analysis.13 Swern oxidation of
both the hydrochloride salt of alcohol 5 and the diaster-
eomeric minor product provided ketone 6a .
The next reaction was C-acylation of ketone 6a with
an aspartic acid derivative. In the synthesis of 1a -d we
had accomplished the enolization step with mesityl-
lithium, but this amine-free base simply added to the
carbonyl of ketone 6a .14 Summarized in Table 1 is a
systematic study which concerned the acylation of 6a
(entry 1) as well as 6b (entry 2) and 6c (entry 3). The
latter two ketones, when subjected to the conditions of
method A, similarly condensed with mesityllithium to
provide the 1,2-adducts. Enolization of ketones 6a -c
with LDA (method B) does occur, but the self-condensa-
tion and bis-acylation products predominated after treat-
ment with 7. The first isolation of a monoacylation
product, contaminated with the bis-acylated compound,
Connection of module 1a , module 2, and ethylenedi-
amine would provide the tetrapeptide mimic 3 (Figure
3). The limited flexibility at this amide juncture allows
the four-residue mimic to adopt a set of conformations
of which many are close to the native conformations
(7) McDougal, J . S.; Kennedy, M. S.; Sligh, J . M.; Cort, S. P.; Mawle,
A.; Nicholson, J . K. A. Science 1986, 231, 382-385.
(8) (a) Fisher, R. AIDS Res. Rev. 1991, 1, 289-300. (b) Capon, D.
J .; Ward, R. H. R. Annu. Rev. Immunol. 1991, 9, 649-678.
(9) See for example, (a) Chen, S.; Chrusciel, R. A.; Nakanishi, H.;
Raktabutr, A.; J ohnson, M. E.; Sato, A.; Weiner, D.; Hoxie, J .; Saragovi,
H. U.; Greene, M. I.; Kahn, M. Proc. Natl. Acad. Sci U.S.A. 1992, 89,
5872-5876. (b) Ma, S.; McGregor, M. J .; Cohen, F. E.; Pallai, P. V.
Biopolymers 1994, 34, 987-1000 and references cited therein.
(10) Moebius, U.; Clayton, L. K.; Abraham, S.; Diener, A.; Yunis, J .
J .; Harrison, S. C.; Reinherz, E. L. Proc. Natl. Acad. Sci. U.S.A. 1992,
89, 12008-12012.
(12) Porthoghese, P. S.; Sepp, D. T. Tetrahedron Lett. 1973, 2253-
2256.
(13) An X-ray structure was detemined by Molecular Structure
Corporation on the hydrochloride salt of the enantiomer of 5, prepared
identically to 5 except for the use (S)-(-)- instead of (R)-(+)-R-
methylbenzylamine. The author has deposited atomic coordinates for
this structure 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.
(11) (a) Wang, J .; Yan, Y.; Garrett, T. P. J .; Liu, J .; Rodgers, D. W.;
Garlick, R. L.; Tarr, G. E.; Husain, Y.; Reinherz, E. L.; Harrison, S. C.
Nature 1990, 348, 411-418. (b) Ryu, S. E.; Kwong, P. D.; Truneh, A.;
Porter, T. G.; Arthos, J .; Rosenberg, M.; Dai, X.; Xuong, N. H.; Axel,
R.; Sweet, R. W,; Hendrickson, W. A. Nature 1990, 348, 419-426.
(14) Beck, A. K.; Hoekstra, M. S.; Seebach, D. Tetrahedron Lett.
1977, 1187-1190.

4436 J . Org. Chem., Vol. 61, No. 13, 1996
Ta ble 1. Acyla tion of En ola tes Der ived fr om Th r ee Am in o Keton es
Boumendjel et al.
Sch em e 2
resulted from treatment of ketone 6a with KHMDS at
-78 °C and transfer of the resulting enolate solution to
a solution of acid chloride 7 also cooled to -78 °C (method
C). Both with ketone 6b and 6c, however, the bis-
acylated compound proved to be the major product.
Ultimately, good yields of all the mono-acylation products
were obtained when the lithium enolates were generated
from the corresponding TMS-enolates by treatment with
methyllithium at -78 °C (method D).15
The acylation product was saponified in situ (1 N KOH)
to provide N-Cbz-protected amino acid 8 (Scheme 2). The
side chain which mimics that of lysine (in this case
ethylenediamine) was introduced by coupling acid 8 with
tert-butyl N-(2-aminoethyl)carbamate in the presence of
HOBT and EDC to give diketone 9 in 71% yield (Scheme
2). Removal of the benzyloxy carbonyl protecting group
by catalytic hydrogen transfer provided cyclic vinylamide
10 directly as a fully protected version of 2 (Figure 2).
1
The H NMR spectrum of compound 10 appeared to be
that of a single compound, consistent with the notion that
no epimerization had occurred during the synthesis.
Indeed, when a mixture of 10 and C2-epi-10 was syn-
1
thesized by utilizing racemic 7, the H NMR resonances
phenethyl group of 10 gave the corresponding secondary
amine which could be coupled with 1a (EDC, HOBT) to
provide compound 11 in 32% overall yield (Scheme 3).
When compound 11 was subjected to a conventional
method for the removal of tert-butyl carboxy protecting
groups (1:1 TFA-CH2Cl2) complete decomposition oc-
cured. Interestingly, both of the analogous compounds
derived from ketones 6b and 6c (footnote) did not display
this acid sensitivity which suggests that decomposition
of compound 11 might be via an acid catalyzed retro
Diels-Alder reaction of the bridged system. Although
for protons at C2 for the diastereomers were resolved
from each other at 250 MHz. As part of a parallel study,
all four diastereomers of the free acid derivative of 10
were synthesized independently via the route outlined
in Schemes 1 and 2 (data not shown) by choice of the
appropriate enantiomers of R-methylbenzylamine and 7.
Next, the coupling of the Phe43-Leu44 module with the
Thr45-Lys46 module was investigated. Removal of the
(15) Garst, M. E.; Bongflio, J . N.; Grudoski, D. A.; Marks, J . J . Org.
Chem. 1980, 45, 2307-2314.

Asymmetric Synthesis of â-Strand Peptidomimetics
J . Org. Chem., Vol. 61, No. 13, 1996 4437
(m, 5 H), 6.35 (dd, J ) 5.2, 1.8 Hz, 1 H), 6.18 (dd, J ) 5.6, 1.8,
Hz, 1 H), 4.15 (s, 1 H), 3.15 (q, J ) 6.5 Hz, 1 H), 2.90 (dd, J )
8.8, 3.0 Hz, 1 H), 2.86 (s, 1 H), 1.50 (m, J ) 8.3 Hz, 2 H), 1.37
(d, J ) 6.4 Hz, 3 H); 13C NMR (CDCl3) δ 147.02, 136.62, 130.55,
128.57 (2 C), 127.81 (2 C), 127.01, 63.96, 62.51, 53.02, 48.21,
44.01, 24.12; MS (CI) m/ z (rel intensity) 199 (100). Anal.
Calcd for C14H17N: C, 84.37; H, 8.59; N, 7.03. Found: C, 83.75;
H, 8.74; N, 7.23.
Sch em e 3
1-P h en eth yl-1-a za bicyclo[2.2.1]-3-h ep ta n ol (5). To a 0
°C solution of olefin 4 (5 g, 25.1 mmol) in THF (100 mL) was
added dropwise BH3‚THF (27.6 mL, 1 M solution, 1.1 equiv).
The reaction was stirred at room temperature for 30 min and
cooled again to 0 °C where another 1.1 equiv of BH3‚THF was
added dropwise. After 30 min at room temperature, the
reaction mixture was cooled to 0 °C, and NaOH (4.4 g in 5 mL
of water) was added. The resulting solution was stirred at 0
°C for 10 min and treated with H2O2 (3 mL, 30% solution in
H2O). The cooling bath was removed and the stirring was
continued at room temperature for 45 min. Ammonium
hydroxide (35 mL) was added, and the mixture was heated at
65 °C for 1 h 30 min. After cooling to room temperature, the
reaction was diluted with ethyl acetate (250 mL), washed with
NaOH (5%, 40 mL) and brine (40 mL), dried, and concentrated.
The crude material was purified by flash chromatography
eluted with EtoAC:hexane (1:1) to give 5.3 g (58%) of alcohol
relatively mild conditions (e.g. 0.1 M HCl) also led to
immediate disappearance of starting material and no
desired product, treatment of a dilute solution of 11 in
CH2Cl2 with only 1 equiv of TFA resulted in a clean
conversion to compound 3, the final target.
Con clu d in g Rem a r k s
We have described the synthesis of a tetrapeptide
mimetic of the Phe43-Lys46 segment of CD4 using a
general strategy. It should be noted that the remaining
15 diastereoisomers of 3 are all accessible through
connection of 1a , 1b, 1c, or 1d with one of the four
diastereomers of 10. The further versatility of the
strategy has been illustrated by the synthesis of 12 and
13.16 Our approach can clearly be applied to a variety of
mimetic problems and accommodate unique constraints.
We have not been able to demonstrate that compound 3,
our initial tetrapeptide mimic, binds to gp120. We are
currently analyzing the solution conformations of our
tetrapeptide mimics by NMR.
5 as a colorless oil: [R]20 ) +64° (c 1.05, CHCl3); 1H NMR
D
(CDCl3) δ 7.45 (m, 5 H), 4.15 (d, J ) 6.8 Hz, 1 H), 3.63 (q, J )
6.5 Hz, 1 H), 3.14 (s, 1 H), 2.65 (dt, J ) 6.3, 2.4 Hz, 1 H), 2.40
(s, 1 H), 2.34 (d, J ) 8.7 Hz, 1 H), 1.92 (m, 2 H), 1.61 (s, 2 H),
1.45 (d, J ) 6.2 Hz, 3 H); 13C NMR (CDCl3/CD3OD) δ 146.10,
128.96, 128.84, 127.65, 127.36, 126.58, 71.46, 64.09, 63.56,
57.93, 40.17, 36.81, 31.57, 22.97; MS (CI) m/ z (rel intensity)
218 (M + 1, 100), 200 (7). Anal. Calcd for C14H19NO: C, 77.38;
H, 8.81; N, 6.44. Found: C, 77.75; H, 8.63 ; N, 6.43.
1-P h en eth yl-3-oxo-1-a za bicyclo[2.2.1]h ep ta n e (6a ). To
a -78 °C cooled solution of alcohol 5 (5 g, 23 mmol) in
anhydrous dichloromethane (100 mL) was added HCl (23 mL,
from 1 M in Et2O, 1 equiv). In a second reaction vessel, oxalyl
chloride (4 mL, 46 mmol) was added dropwise to a -78 °C
cooled solution of DMSO (6.5 mL, 92 mmol) in anhydrous
dichloromethane (100 mL). The mixture was stirred at -78
°C for 15 min and added dropwise via cannula to the flask
containing alcohol 5. The reaction mixture was stirred for 50
min during which time the temperature rose to -50 °C. The
reaction mixture was cooled again to -78 °C and treated with
triethylamine (25.8 mL, 184.3 mmol). The cooling bath was
removed, and the reaction was stirred for 1 h, washed with
NaOH (5%, 50 mL) and brine (50 mL), dried, and concentrated.
Purification by flash chromatography eluted with hexane:
EtOAc (3:2) gave 2.77 g (56%) of the title compound as a yellow
Exp er im en ta l Section
Gen er a l P r oced u r e. All reactions were carried out in an
argon inert atmosphere in glassware that was dried in an oven
at 120 °C. 1H NMR and 13C NMR spectra were recorded on a
Bruker AC 250 NMR at 250 MHz, and 50 MHz, respectively.
Chemical shifts are reported in ppm (δ) with TMS as internal
standard. Positive and negative FAB spectra were recorded
on a J EOL J MS-SX102 instrument. CI spectra were recorded
on a J EOL J MS-AX5058 instrument. Elemental analysis was
obtained from Atlantic Microlab, Inc; Norcross, GA. Column
chromatography was performed on silica gel (60-mesh, Merck).
All reagents and dry solvents were used as is from the supplier
unless otherwise stated. Abbreviations: HOBT: 1-hydroxy-
benzotriazole hydrate, EDC: 1-[3-dimethylamino)propyl]-3-
ethylcarbodimide, KHMDS: potassium bis(trimethylsilyl)-
amide.
1-P h en eth yl-1-a za bicyclo[2.2.1]-3-h ep ten e (4). (R)- (+)-
R-methylbenzylamine (10 g, 83 mmol) was dissolved in 100
mL of water. The solution was acidified to pH ) 4 by adding
hydrochloric acid (10% by volume), and again the pH was
adjusted to 8 with methylbenzylamine. Formaldehyde (4.98
g, 2 equiv) was added dropwise, and the solution was placed
in an ice bath (0 °C) and treated with cyclopentadiene (15 mL,
2 equiv). The reaction mixture was stirred overnight at 0 °C
and washed with hexane, and the aqueous phase was sepa-
rated, treated with KOH (5 g in 15 mL of H2O), extracted with
CH2Cl2, dried (MgSO4), and concentrated. The crude product
was purified by flash chromatography eluted with hexane:
EtOAc (3:2) to give 13.7 g (83%) of the title compound as a
brown oil: [R]20D ) +23° (c 1, CHCl3); 1H NMR (CDCl3) δ 7.30
oil which solidified on standing: [R]20 ) +12° (c 1, CHCl3);
D
1H NMR (CDCl3) δ 7.36 (m, 5 H), 3.64 (s, 1 H), 3.30 (q, J ) 7.6
Hz, 1 H), 3.05 (dd, J ) 5.4, 1.5 Hz, 1 H), 2.55 (s, 1 H), 2.15 (m,
1 H), 2.15 - 1.70 (m, 4 H), 1.32 (d, J ) 7.6 Hz, 3 H); 13C NMR
(CDCl3) δ 207.7, 146.31, 128.80, 128.47, 127.97, 127.39, 126.02,
64.19, 62.8, 56.03, 44.27, 36.56, 35.21, 23.31; MS (CI) m/ z (rel
intensity) 216 (M + 1, 100), 187 (9). Anal. Calcd for C14H17
-
NO: C, 78.10; H, 7.96; N, 6.50. Found: C, 78.46; H, 7.98 ; N,
6.47.
1-P h en eth yl-4-[3-(ben zyloxycar bon yl)am in o]3-car boxy-
1-oxop r op yl]-3-oxo-1-a za b icyclo[2.2.1]h ep t a n e
(8).
Meth od D. Trimethylsilyl ether was prepared as follows. To
LHMDS (1.6 mL, 1.6 mmol) in THF (5 mL) cooled to -78 °C
was added dropwise a solution of ketone 6a (344 mg, 1.4 mmol)
in THF (3 mL). The solution was stirred at -78 °C for 30 min,
and trimethylsilyl chloride (216 mg, 2 mmol) was added. The
solution was stirred for 10 min and warmed rapidly to room
temperature, stirred for 10 min, and concentrated. The crude
material was partitioned between hexane and 10% Na2CO3,
the aqueous solution was extracted with hexane, and the
organic phases were combined, dried, and concentrated to
provide the TMS-enol ether as a clear oil 414 mg (90%) which
was used without purification: 1H NMR (CDCl3/CD3OD) δ 7.35
(m, 5 H), 4.94 (d, J ) 4 Hz, 1 H), 3.92 (s, 1 H), 3.24 (q, J ) 6.7
Hz, 1 H), 3 (m, 1 H), 2.78 (s, 1 H), 1.68 (m, 2 H), 1.4 (d, J ) 6.9
Hz, 3 H), 0.30 (s, 9 H).
(16) Compounds 12 and 13 were prepared starting from ketones 6b
and 6c, respectively, according to Schemes 1-3.

4438 J . Org. Chem., Vol. 61, No. 13, 1996
Boumendjel et al.
Methyllithium (1 mL, from 1.4 M in diethyl ether) was
concentrated under an argon atmosphere and diluted in THF
(4 mL) and cooled to -10 °C. A solution of TMS-enol ether
(403 mg, 1.4 mmol) in THF (3 mL) was added dropwise. The
solution was stirred at -10 °C for 40 min, cooled to -78 °C,
and slowly transferred via cannula to a solution of acid chloride
7 (1.4 mmol) in THF (5 mL) also cooled to -78 °C. The mixture
was stirred at this temperature for 30 min, and potassium
hydroxide (2 mL from 2 M solution) was added. The cooling
bath was removed, and the solution was stirred at room
temperature for 1 h. The aqueous solution was separated,
washed with diethyl ether, and acidified with HCl (10%) to
pH ) 5. The solution was extracted with chloroform (3 × 30
mL), and the organic layers were combined, dried, and
evaporated. The crude material was purified by flash chro-
matography eluted with CHCl3:MeOH (9:1) to give 389 mg
methanol (10 mL) containing 80 mg of Pd/C (10%) was stirred
for 24 h under 1 atm of hydrogen (60 psi). The catalyst was
filtered off and washed with methanol and chloroform. Sol-
vents were removed under reduced pressure. The crude
material (30 mg) was dissolved in CH2Cl2 (2 mL) and treated
with EDC (19 mg, 0.1 mmol) and HOBT (13 mg, 0.1 equiv).
Compound 1a (48 mg, 0.17 mmol) dissolved in CH2Cl2 (1 mL)
was added, triethylamine was added until pH ) 8.5 was
reached, and stirring was continued for 24 h. The solution
was diluted with CH2Cl2 (20 mL), washed with water and
brine, dried, and concentrated. The crude material was
purified by flash chromatography eluted with CHCl3:MeOH
(9:1) to give 35 mg (32%) of the title compound as a colorless
1
oil: [R]20 ) -6° (c 0.84, MeOH); H NMR (CDCl3/CD3OD) δ
D
7.35 (m, 5 H), 5.94 (s, 0.5 H), 5.62 (s, 1 H), 4.8 (m, 1 H), 4.25
(m, 1 H), 3.51-3.13 (m, 8 H), 3.02-2.62 (m, 9 H), 2.26 (m, 6
H), 2.24 (m, 3 H), 1.55 (m, 12 H); 13C NMR (CDCl3/CD3OD) δ
190.87, 190.85, 171.61, 171.45, 157.11, 139.95, 138.20, 132.00,
131.68, 129.70, 129.19, 128.04, 127.15, 98.56, 81.20, 55.54,
51.43, 45. 12, 40.48, 30.30, 29.07, 29.05, 22.95; MS (FAB) m/ z
(rel intensity) 640 (M + Na, 8), 541 (10), 473 (12), 451 (100).
Anal. Calcd for C34H43N5O6: C, 66.11; H, 7.01; N, 11.33.
Found: C, 66.69; H, 7.01; N, 11.79.
(42%) of the title compound as a yellow foam: [R]20 ) -57°
D
(c 1.1, MeOH); 1H NMR (CDCl3/CD3OD) δ 7.51 (m, 10 H), 6.25
(d, J ) 5.2 Hz, 1 H), 5.15 (s, 2 H), 4.55 - 4.38 (m, 2 H), 4.05 -
3.62 (m, 6 H), 3.55 (s, 1 H), 3.48 (d, J ) 7.8 Hz, 1 H), 3.10 -
2.75 (m, 4 H), 2.65 (s, 1 H), 2.15 - 1.80 (m, 2 H), 1.72 (d, J )
6.5 Hz, 3 H); 13C NMR (CDCl3/CD3OD) δ 215.45, 208.10,
174.82, 154.62, 137.05, 130.64, 130.06, 129.56 (2 C), 129.43,
128.72 (2 C), 128.32, 127.61, 127.50 (2 C), 92.66, 67.50, 60.05,
52.20, 42.66, 38.45, 37.82, 26.89, 22.29, 20.59; MS (FAB) m/ z
(rel intensity) 465 (M + 1, 23), 373 (100). Anal. Calcd for
C26H28N2O6: C, 67.22; H, 6.07; N, 6.03. Found: C, 66.96; H,
6.52; N, 6.05.
ter t-Bu tyloxyca r bon yl Rem ova l fr om 11 (3). To com-
pound 11 (15 mg, 0.024 mmol) in dry dichloromethane (10 mL)
was added TFA (2 µL, 0.026 mmol). The solution was stirred
under 1 atm of argon for 5 h. The solution was diluted further
with CH2Cl2, washed several times with saturated solution of
NaCl, dried, and concentrated. The crude material (12 mg)
was purified by preparative TLC eluted with CHCl3:MeOH (8:
2) to give 5 mg (51%) of the title compound as a yellow solid:
[R]20D ) -15° (c 0.9, MeOH); 1H NMR (CDCl3/CD3OD) δ 7.50-
7.15 (m, 5 H), 4.60 (t, J ) 6.1 Hz, 1 H), 4.12 (t, J ) 5.9 Hz, 1
H), 3.65 (m, 6 H); 3.40 (m, 6 H), 3.28-2.80 (m, 4 H), 2.50 (m,
4 H), 2.10 (m, 3 H), 1.77 (m, 5 H); 13C NMR (CDCl3/CD3OD) δ
190.87, 190.85, 171.6, 171.39, 157.11, 139.95, 138.20, 132.00,
131.72, 129.70, 129.19, 128.00, 127.15, 98.56, 81.20, 55.54,
51.43, 45.12, 40.48, 30.30, 29.07, 29.05, 22.95; MS (FAB) m/ z
(rel intensity) 616 (M - 1, 100), 416 (13), 316 (40), 274 (38).
Anal. Calcd for C29H35N5O4: C, 67.29; H, 6.81; N, 13.33.
Found: C, 67.10; H, 6.46; N, 12.89.
1-P h en et h yl-4-[2-[(b en zyloxyca r b on yl)a m in o]-4-[[2-
[(ter t-bu toxyca r bon yl)a m in o]eth yl]a m in o]-1,4-d ioxobu -
tyl]-3-oxo-1-a za bicyclo[2.2.1]h ep ta n e (9). Acid 8 (200 mg,
0.43 mmol) was dissolved in CH2Cl2 (5 mL) and treated with
HOBT (68 mg, 0.5 mmol) and EDC (96 mg, 0.5 mmol).
A
solution of tert-butyl N-(2-aminoethyl)carbamate (80 mg, 0.5
mmol) in CH2Cl2 (1 mL) was added. Triethylamine was added
until pH ) 8 was reached, and the mixture was stirred at room
temperature for 15 h. The solution was diluted with CH2Cl2
(20 mL) and washed with H2O and brine. The organic layer
was separated, dried, and concentrated. Purification of the
crude material by flash chromatography eluted with CHCl3:
MeOH (9:1) gave 186 mg (71%) of the title compound as a
white foam: [R]20D ) -62° (c 1, MeOH); 1H NMR (CDCl3/CD3-
OD) δ 7.43-7.27 (m, 10 H), 5.10 (s, 2 H), 4.31-4.16 (m, 2 H),
3.93-3.39 (m, 6 H), 3.04-2.50 (m, 5 H), 1.9 (m, 1 H), 1.50 (m,
12 H); 13C NMR (CDCl3/CD3OD) δ 210.12, 200.10, 181.34,
171.95, 158.23, 138.30, 136.62, 129.64, 129.56, 129.54, 129.21,
129.18, 129.02, 128.94, 128.83, 128.05, 127.95, 106.98, 81.04 ,
67.94, 62.91, 53.54, 51.33, 48.77, 41.92, 31.36, 29.07, 28.99;
MS (FAB) m/ z (rel intensity) 607 (M + 1, 12), 509 (100). Anal.
Calcd for C33H42N4O7: C, 65.33; H, 6.97; N, 9.23. Found: C,
65.43; H, 6.75; N, 9.56.
2-[N -[2-[(t er t -B u t y lo x y c a r b o n y l)a m in o ]e t h y l]c a r -
ba m oyl]-4-oxo-7-p h en eth yl-1,7-d ia za tr icyclo[6.2.1.0 10,11]-
11-d od ecen e (10). To a solution of cyclohexene/methanol (5
mL, 5/1) containing 100 mg of Pd/C (10%) was added compound
9 (100 mg, 0.16 mmol). The solution was refluxed for 20 min
and cooled to room temperature. The catalyst was filtered off
and washed with methanol, and solvents were evaporated. The
residue obtained was partitioned between CHCl3 and NaHCO3,
the aqueous layer was extracted with CHCl3 (2×), and the
combined organics were dried and concentrated. The crude
material was purified by column chromatography CHCl3:
MeOH (9:1) to give 43 mg (50%) of the title compound as a
clear oil.
[R]20D ) -32° (c 0.9, MeOH); 1H NMR (CDCl3/CD3OD) δ 7.35
(m, 5 H), 4.25 (t, J ) 6 Hz, 1 H), 4.19 (t, J ) 6.2 Hz, 1H this
resonance was observed when racemic 7 was used), 3.55 (q, J
) 6.5 Hz, 1 H), 3.15 (m, 2 H), 3.05 (s, 1 H), 2.82-2.50 (m, 6
H), 2.42 (s, 1 H), 2.37 (d, J ) 8.7 Hz, 1 H), 1.51 (s, 9 H), 1.34
(d, J ) 6.4 Hz, 3 H); 13C NMR (CDCl3/CD3OD) δ 194.72, 181.50,
158.21, 141.02, 138.10, 129.77, 129.69, 129.33, 129.11, 128.26,
128.01, 80.94, 71.03, 65.07, 64.38, 58.21, 40.27, 36.82, 32.10,
28.99 (5 C), 22.28; MS (FAB) m/ z (rel intensity) 455 (M + 1,
25), 413 (35), 240 (70). Anal. Calcd for C25H34N4O4: C, 66.05;
H, 7.53; N, 12.32. Found: C, 66.16; H, 7.49; N, 12.93
Hyd r ogen a tion of 10 a n d Cou p lin g w ith Mod u le 1a
(11). A solution of compound 10 (80 mg, 0.17 mmol) in
1
An a log 12: [R]20 ) -24° (c 1, MeOH); H NMR (CDCl3/
D
CD3OD) δ 7.25 (m, 5 Η), 4.17 (m, 2 Η), 3.21-2.90 (m, 7 H),
2.83-2.50 (m, 6 H), 2.35-2.00 (m, 5 H), 1.78-1.67 (m, 6 H),
1.5 (m, 3 H); 13C NMR (CDCl3/CD3OD) δ 192.41, 192.36,
172.60, 171.93, 140.05, 139.20, 132.13, 131.70, 129.46, 128.94,
128.07, 127.35, 126.62, 106.28, 56.74, 52.13, 47.02, 38.48,
30.30, 28.05, 27.14, 25.73, 22.95; MS (FAB) m/ z (rel intensity)-
506 (M + 1, 32), 477 (64), 413 (100), 335 (88). Anal. Calcd
for C28H35N5O4: C, 66.50; H, 6.97; N, 12.85. Found: C, 66.06;
H, 6.48; N, 12.89.
1
An a log 13: [R]20 ) -18° (c 1, MeOH); H NMR (CDCl3/
D
CD3OD) δ 7.25 (m, 5 Η), 4.15 (m, 2 Η), 3.30-3.06 (m, 6 H),
2.85-2.55 (m, 6 H), 2.35-2.00 (m, 4 H), 1.85-1.70 (m, 6 H),
1.5 (m, 3 H); 13C NMR (CDCl3/CD3OD) δ 190.79, 190.43,
172.42, 172.00, 141.15, 140.30, 132.32, 131.74, 129.52, 128.71,
128.00, 127.56, 126.12, 108.04, 55.95, 52.31, 46.21, 38.40,
29.45, 27.95, 27.04, 25.73, 23.00; MS (FAB) m/ z (rel intensity)
506 (M + 1, 10), 477 (45), 335 (100). Anal. Calcd for
C28H35N5O4: C, 66.50; H, 6.97; N, 12.85. Found: C, 66.10; H,
6.42; N, 12.89.
Ack n ow led gm en t. This investigation was made
possible in part by the financial support provided by the
National Institutes of Health (grant AI 27336. project
3) to whom we are grateful.
Su p p or tin g In for m a tion Ava ila ble: An ORTEP drawing
derived from the crystal structure of ent-5 (1 page). This
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