Stereoselective formation of a functionalized dipeptide isostere by zinc carbenoid-mediated chain extension

Article abstract of DOI:10.1021/jo801993k

The application of a zinc carbenoid-mediated chain-extension reaction to a functionalized peptide isostere is reported. The cleavage site of human CVM protease was utilized as a target for testing the synthetic methodology. The utility of this chain-exten

Full text of DOI:10.1021/jo801993k

Stereoselective Formation of a Functionalized Dipeptide Isostere by
Zinc Carbenoid-Mediated Chain Extension†
Weimin Lin,‡ Cory R. Theberge,‡ Timothy J. Henderson,‡ Charles K. Zercher,*,‡
Jerry Jasinski,§ and Ray. J. Butcher|
Department of Chemistry, UniVersity of New Hampshire, Durham, New Hampshire 03824, Department of
Chemistry, Keene State College, 229 Main Street, Keene, New Hampshire 03435, and Department of
Chemistry, Howard UniVersity, 525 College Street NW, Washington, D.C. 20059
ckz@cisunix.unh.edu
ReceiVed September 15, 2008
The application of a zinc carbenoid-mediated chain-extension reaction to a functionalized peptide isostere
is reported. The cleavage site of human CVM protease was utilized as a target for testing the synthetic
methodology. The utility of this chain-extension reaction is demonstrated in the preparation of an amino
acid-derived R-unsubstituted γ-keto ester, which is incorporated into a framework that mimics a
tetrapeptide. The identification of a suitable protecting group strategy facilitated the application of a
tandem reaction for the incorporation of an R-side chain, and the use of an oxazolidinone auxiliary provided
excellent diastereocontrol in a tandem chain-extension-aldol reaction. Stereoselectivity of the tandem
chain-extension-aldol reaction was determined through application of a CAN-mediated oxidative cleavage
reaction.
Introduction
sor to the hydroxyethylene isostere, another peptide isostere that
has been explored extensively.6 The inhibition of protease targets
with peptide isosteres is a widely utilized strategy in medicinal
chemistry. Aspartic acid protease targets like HIV-protease and
renin7 and cysteine proteases8 have been inhibited by ketom-
ethylene and hydroxyethylene isosteres. The successful utiliza-
tion of isosteric replacements with serine9 proteases has also
A variety of functional groups have been used to mimic the
peptide bond, including (E)-alkenes,1 silanols,2 and dihydroxy-
ethylene.3 The ketomethylene group (2) has been utilized
successfully as a peptide isostere in a variety of contexts,4 and
the isolation5 from nature of a ketomethylene-containing dipep-
tide mimic provides further support for its utility in protease
inhibition. The ketomethylene isostere is also a suitable precur-
(5) (a) Ohuchi, S.; Suda, H.; Naganawa, H.; Takita, T.; Aoyagi, T.; Umezawa,
H.; Nakamura, H.; Iitaka, Y. J. Antibiot. 1983, 36, 1576–1580. (b) Umezawa,
H.; Aoyagi, T.; Ohuchi, S.; Okuyama, A.; Suda, H.; Takita, T.; Hamada, M.;
Takeuchi, T. J. Antibiot. 1983, 36, 1572–5.
† Dedicated to Professor Marvin J. Miller in honor of his 60th birthday.
‡ University of New Hampshire.
§ Keene State College.
(6) (a) Benedetti, F.; Berti, F.; Norbedo, S. J. Org. Chem. 2002, 67, 8635–
8643. (b) Kath, J. C.; DiRico, A. P.; Gladue, R. P.; Martin, W. H.; McElroy,
E. B.; Stock, I. A.; Tylaska, L. A.; Zheng, D. Bioorg. Med. Chem. Lett. 2004,
9, 2163–2167. (c) Tao, J.; Hoffman, R. V. J. Org. Chem. 1997, 62, 6240–6244.
(7) (a) Dondoni, A.; Perrone, D. Tetrahedron Lett. 1992, 33, 7259–7262.
(b) Young, S. D.; Payne, L. S.; Thompson, W. J.; Gaffin, N.; Lyle, T. A.; Brichter,
S. F.; Graham, S. L.; Schulz, T. H.; Deana, A. A.; Darke, P. L.; Zugay, J.;
Schlerf, W. A.; Quntero, J. C.; Emini, E. A.; Anderson, P. A.; Huff, J. R. J. Med.
Chem. 1992, 35, 1702–1709. (c) 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. A.; Tucker, T. J.; Scherring, J. E.; Homnick, C. F.;
Nunberg, J.; Springer, J. P.; Huff, J. R. J. Med. Chem. 1992, 35, 1685–1701. (d)
Askin, D.; Wallace, M. A.; Vacca, J. P.; Reamer, R. A.; Volante, R. P.; Shinkai,
I. J. Org. Chem. 1992, 57, 2771–2773.
| Howard University.
(1) (a) Wipf, P.; Henninger, T. C.; Geib, S. J. J. Org. Chem. 1998, 63, 6088–
6089. (b) Oishi, S.; Kamano, T.; Niida, A.; Odagaki, Y.; Hamanaka, N.;
Yamamoto, M.; Ajito, K.; Tamamura, H.; Otaka, A.; Fujii, N. J. Org. Chem.
2002, 67, 6162–6173. (c) Jingbo, X.; Weisblum, B.; Wipf, P. J. Am. Chem. Soc.
2005, 127, 5742–5743.
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Kim, J.; Glekas, A.; Sieburth, S. M. Bioorg. Med. Chem. Lett. 2002, 12, 3625–
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10.1021/jo801993k CCC: $40.75
Published on Web 12/04/2008
2009 American Chemical Society
J. Org. Chem. 2009, 74, 645–651 645

Lin et al.
FIGURE 1. Peptide backbone and ketomethylene isosteric peptide
mimic.
FIGURE 2. HCMV protease cleavage site and proposed ketomethylene
mimics.
SCHEME 1
conditions that are not suitable with easily epimerizable amino
acid-derived products. The application of this chain-extension
method to the synthesis of diverse ketomethylene systems
requires that both N and C termini be available for further
modification and that the R-side chain be stereoselectively
incorporated.
As a target for the methodology development, a potential
inhibitor for human cytomegalovirus (HCMV) protease was
proposed. Human cytomegalovirus is a ubiquitous pathogen that
presents a significant health risk to neonates and immunocom-
promised patients.16 Since replication and assembly of HCMV
is dependent upon a self-processing HCMV protease,17 the
targeted inhibition of this serine protease is an attractive strategy
for disruption of this virus’ life cycle. This self-processing
protease contains three active cleavage sites, although the m-site
(maturation site: Ala 643-Ser 644) is cleaved 30 times more
quickly than the other two sites. The common feature of all
three cleavage sites is the involvement of an alanine-serine (Ala-
Ser) bond, with the P2 site being occupied by asparagine,
glutamine, or lysine, although the most efficient cleavage takes
place with asparagine.18
We report herein investigations into the application of a zinc
carbenoid-mediated chain-extension reaction for the generation
of a ketomethylene-based peptide-mimic of the HCMV protease
cleavage site. One potential target (10) is illustrated in Figure
2. Since modestly acidic NH-amide protons would quench the
intermediate enolate and thereby prevent application of tandem
reaction strategies, a related target compound (11) was designed
to contain an N,N-dimethylasparagine unit at the N-terminus.
N,N-Dimethylasparagine had been incorporated at this P-2
position within active peptide isosteres in a complementary
investigation to HCMV protease inhibition.19 A synthetic
approach was eventually utilized in which the N,N-dimethylas-
paragine was incorporated subsequent to the tandem chain-
extension reaction, which made the use of dimethylasparagine
inconsequential with respect to quenching the proposed orga-
nometallic intermediate (4). The strategy that was eventually
utilized would be broadly applicable to the vast majority of
amino acids, even those that contain acidic protons.
been reported. However, preparation of ketomethylene peptide
mimics has often relied upon lengthy and resource intensive
synthetic sequences.10 The availability of a simple, efficient
reaction that facilitates the preparation of functionalized, amino
acid-derived γ-keto esters/amides would find utility in the
preparation of these isosteres, regardless of the inhibition target.
The aim of our study was the continued development of a
one-pot, zinc-mediated chain-extension reaction method, as it
applies to the formation of ketomethylene isosteres. This simple
and efficient conversion of ꢀ-keto esters (3),11 amides,12 and
phosphonates13 to their γ-keto homologues (5) (Scheme 1)
through treatment with the electrophilic Furukawa zinc car-
benoid has provided access to amino acid-derived dipeptide
isosteres. Previous studies in our group have demonstrated that
a variety of amino acids, with diverse side chains, can be
converted to ꢀ-keto esters or amides, which are good substrates
for the chain-extension reaction.14 Many different protecting
groups are tolerated during the chain extension, and no
epimerization is observed, even with easily epimerizable amino
acids like phenylglycine.
Tandem reactions, which rely upon the nucleophilic character
of the organometallic intermediate (4), can be performed to
incorporate side chains at the position R-to the ester or amide.15
For example, when compound 6 is exposed to the chain-
extension reaction conditions and trapped with benzaldehyde
in a tandem chain-extension-aldol reaction, good diastereo-
control is realized in the formation of 7. Side chains incorporated
during a tandem chain-extension-aldol reaction have been
manipulated to provide hydrocarbon side chains equivalent to
those found in natural amino acids.
Yet our studies on the stereoselective incorporation of the
R-side chain mimic have been limited to substrates, i.e., 6, in
which no N-terminus is present. Furthermore, since control of
stereochemistry was influenced through proline-based auxilia-
ries, hydrolysis of an amide bond for the release of the carboxyl
group would be required. This process requires harsh reaction
Results and Discussion
The key step in the assembly of the isostere is a zinc
carbenoid-mediated chain-extension reaction that proceeds
through a putative donor-acceptor cyclopropane to provide an
(16) de Jong, M. D.; Galasso, G. J.; Gazzard, B.; Griffiths, P. D.; Jabs, D. A.;
Kern, E. R.; Spector, S. A. AntiViral Res. 1998, 39, 141–162.
(9) Garrett, G. S.; Emge, T. J.; Lee, S. C.; Fischer, E. M.; Dyehouse, K.;
McIver, J. M. J. Org. Chem. 1991, 56, 4823–4826.
(17) Matusick-Kumar, L.; McCann, P. J., III; Robertson, B. J.; Newcomb,
W. W.; Brown, J. C.; Gao, M. J. Virol. 1995, 69, 7113–7121.
(10) Hoffman, R. V.; Kim, H. O. Tetrahedron Lett. 1992, 33, 3579–3582.
(11) Brogan, J. B.; Zercher, C. K. J. Org. Chem. 1997, 62, 6444–6446.
(12) Hilgenkamp, R.; Zercher, C. K. Tetrahedron 2001, 57, 8793–8800.
(13) Verbicky, C. A.; Zercher, C. K. J. Org. Chem. 2000, 65, 5615–5622.
(14) Theberge, C. R.; Zercher, C. K. Tetrahedron 2003, 59, 1521–1527.
(15) (a) Lin, W.; Tryder, N.; Su, F.; Zercher, C. K.; Jasinski, J. P.; Butcher,
R. J. J. Org. Chem. 2006, 71, 8140–8145. (b) Hilgenkamp, R.; Zercher, C. K.
Org. Lett. 2001, 3, 3037–3040.
(18) (a) Bonneau, P. R.; Plauffe, C.; Pellatier, A.; Wernic, D.; Poupart, M.-
A. Anal. Biochem. 1997, 155, 59–65.
(19) Ogilvie, W.; Bailey, M.; Poupart, M.-A.; Abraham, A.; Bhavsar, A.;
Bonneau, P.; Bordeleau, J.; Bousquet, Y.; Chabot, C.; Duceppe, J.-S.; Fazal,
G.; Goulet, S.; Grand-Maitre, C.; Guse, I.; Halmos, T.; Lavallee, P.; Leach, M.;
Malenfant, E.; O’Meara, J.; Plante, R.; Plouffe, M.; Poirier, M.; Soucy, F.;
Yoakim, C.; Deziel, R. J. Med. Chem. 1997, 40, 4113–4135.
646 J. Org. Chem. Vol. 74, No. 2, 2009

Formation of a Functionalized Dipeptide Isostere
SCHEME 2
SCHEME 3
intermediate zinc organometallic intermediate (4). The assembly
of a tetrapeptide mimic could proceed via one of two strategies.
The first would involve the formation of a dipeptide isostere,
followed by sequential derivitization of the two termini. This
approach would offer the possibility of library assembly, perhaps
through application of a solid-phase mediated approach.20
A
second, complementary approach could involve the formation
of a dipeptide, followed by chain extension of the C-terminus
and peptide coupling on the C-terminus. Both approaches were
utilized in the study and offer opportunities for rapid assembly
of the peptide isostere.
Rapid formation of a tetrapeptide mimic is described in
Scheme 2. The coupling between Boc-asparagine (12) and
benzyl-protected alanine (13) proceeded cleanly. Deprotection
of the carboxy terminus via hydrogenolysis provided the
carboxylic acid 15, which was converted to the ꢀ-keto ester 16
via a mixed Claisen variation reported initially by Masamune.21
Treatment of the ꢀ-keto ester with 5 equiv of the Furukawa
reagent provided the chain-extension product 17. The use of
excess Furukawa reagent was necessary due to the presence of
the acidic NH protons. Compound 17 could be easily converted
to the carboxylic acid 18 by hydrogenolysis. Peptide coupling
with the alanine methyl ester 19 provided the tetrapeptide mimic
20 as a single diastereoisomer. While the assembly of this
tetrapeptide mimic is quite efficient, the absence of a substitutent
that could serve as a mimic of the amino acid’s R-side chain
limits the desirability of this direct approach.
coupling at the two termini would then complete preparation
of a tetrapeptide mimic.
Protection of an alanine’s amine functionality to remove its
nucleophilic character (due to the use of the electrophilic
carbenoid and aldehyde), basic character, and NH acidity was
required. The phthalimide protecting group offered the added
attraction of removing rotameric forms within the protected
tertiary amine (Scheme 3). In order to test the viability of the
phthalimide protecting group, valine was converted to 21 by a
literature procedure.22 Formation of the ꢀ-keto ester 22 was
accomplished through the modified Claisen reaction described
by Masamune.21 Exposure of 22 to the zinc carbenoid-mediated
chain-extension reaction conditions resulted in the formation
of tricycle 24 in 58% by spontaneous cyclization of the
organozinc intermediate (23) onto the phthalimide. Stereochem-
ical assignments were made by X-ray structural analysis. The
electrophilic character of the imide functionality was, therefore,
incompatible with the reaction conditions. An alternate protect-
ing group strategy that utilized two different protecting groups
was mandated, even though double protection of the amino acid
was expected to complicate NMR analysis through the appear-
ance of rotameric forms.
Application of the tandem reaction strategies15 that have been
developed in our research program for the incorporation of the
R-substitutent would be difficult, if not impossible, due to the
presence of the modestly acidic N-H protons that would quench
the organometallic intermediate. The use of a protecting group
was deemed necessary for removal of the NH-protons. In order
to minimize the number of functional groups that required
protection, a strategy was developed in which the dipeptide
isostere would be prepared. Derivitization of the dipeptide by
(20) Almquist, R. G.; Chao, W.-R.; Judd, A. K.; Mitoma, C.; Rossi, D. J.;
Panesovich, R. E.; Matthews, R. J. J. Med. Chem. 1988, 31, 561–567.
(21) Brooks, D. W.; Lu, L. D.-D.; Masamune, S. Angew. Chem., Int. Ed.
Engl. 1979, 18, 72–73.
(22) Casimir, J. R.; Guichard, G.; Briand, J. P. J. Org. Chem. 2002, 67, 3764–
3768.
J. Org. Chem. Vol. 74, No. 2, 2009 647

Lin et al.
SCHEME 4
SCHEME 6
SCHEME 5
2927 for the formation of ꢀ-keto imide 30 (Scheme 5). The
typical reaction protocol for a tandem chain-extension aldol
reaction involves exposure of the ꢀ-keto carbonyl to the chain-
extension reaction conditions, followed by addition of the
aldehyde. When compound 30 was chain extended and paraform-
aldehyde was added after 30 min, the chain-extended cyclo-
propane 32 was generated as a single diastereoisomer, presum-
ably through chain extension, formation of an intermediate
homoenolate 31 by capture of excess carbenoid, and cyclization.
The capture of the excess carbenoid, which resulted in the
formation of homoenolate 31, occurred so quickly that no chain-
extended enolate was present to capture formaldehyde. The
stereochemical assignment of the putative homoenolate 31 and
of the cyclopropane 32 has not been made. Previous studies on
diastereoselective homoenolate formation through the chain-
extension protocol has revealed a similar enolate-facial selectiv-
ity to that observed in the chain-extension aldol reaction;15
therefore, the stereochemistry at the R-carbon of 31 is tentatively
assigned as S on the basis of the aldol stereochemistry, which
was determined as described below. The carbinol center of 32
is not assigned.
Two equivalents of the carbenoid are, in theory, required for
enolate formation and chain extension. Attempts to perform the
chain extension of 30 with only 2 equiv of carbenoid, thereby
removing the excess carbenoid responsible for homoenolate (31)
formation, resulted in successful chain extension and capture
of the aldehyde, although significant amounts of unreacted
starting material (30) remained in the reaction mixture. There-
fore, the procedure was modified to incorporate the source of
formaldehyde prior to exposure of the ꢀ-keto imide to the
carbenoid in an effort to ensure that the organometallic
intermediate captured an aldehyde instead of excess carbenoid.
Under these modified conditions, only the aldol product 33 was
formed. The stereochemistry of the R-position was assigned
through formation of the (R) -γ-lactone 3428 by application of
a CAN-mediated oxidative cleavage reaction we reported
previously.25 Efforts to determine the diastereoslectivity of the
aldol reaction were hindered by the presence of hemiacetal and
rotameric forms. A single γ-lactone stereoisomer was observed
by NMR after the CAN oxidation, which suggests the diaste-
reoslectivity of the aldol reaction to be >9:1.
Alanine was derivatized with a p-methoxybenzyl (Pmb) group
through a reductive alkylation reaction to provide 25,23 onto
which a Cbz group was added (Scheme 4). Using the method
developed by Masamune, conversion of the double-protected
amino acid 26 to a ꢀ-keto ester 27 was accomplished by use of
carbonyl diimidazole and the magnesium salt of the monoallyl
ester of malonic acid.24 A tandem chain-extension-aldol
reaction, using formaldehyde as the electrophile, was performed
on 27, which generated a mixture of diastereoisomers (28) in
approximately a 1:1 ratio.25 Not surprisingly, the remote
stereocenter of the alanine was not effective in controlling facial
selectively on the intermediate enolate. It was clear that an
alternate stereodirecting group would need to be incorporated.
The successful use of an oxazolidinone functionality in a
stereocontrolled tandem chain-extension aldol reaction26 sug-
gested that an amino acid-derived ꢀ-keto imide could be a useful
substrate for generation of the desired functionalized ketom-
ethylene group. Extensive experimentation led to the identifica-
tion of a reproducible mixed Claisen reaction between 25 and
(23) Quitt, P.; Hellerbach, J.; Vogler, K. HelV. Chim. Acta 1963, 46, 327–
333.
(24) Padwa, A.; Dean, D. C.; Fairfax, D. J.; Xu, S. L. J. Org. Chem. 1993,
58, 4646.
(25) The assignment of the diastereomeric ratio was not possible at this point,
due to the combined appearance of rotameric and hemiacetal forms. Protection
of the hydroxyl group and removal of the Pmb-protecting group by treatment
with CAN provided the support for the assigned diastereoselectivity. Jacobine,
A. M.; Lin, W.; Walls, B.; Zercher, C. K. J. Org. Chem. 2008, 73, 7409–7412.
(26) Lai, S.-J.; Zercher, C. K.; Jasinski, J. P.; Reid, S. N.; Staples, R. J. Org.
Lett. 2001, 3, 4169–4171.
While the strategy described above provided excellent dias-
tereocontrol and complete regiocontrol in the assembly of a
substituted peptide isostere, the hydroxymethyl group of 33
possessed the opposite stereochemistry required to mimic an
(27) Furuno, H.; Inoue, T.; Abiko, A. Tetrahedron Lett. 2002, 43, 8297.
(28) Comini, A.; Forzato, C.; Nitti, P.; Pitacco, G.; Valentin, E. Tetrahedron:
Asymmetry 2004, 15, 617–625.
648 J. Org. Chem. Vol. 74, No. 2, 2009

Formation of a Functionalized Dipeptide Isostere
SCHEME 7
L-amino acid. In an effort to mimic the L-amino acid stereo-
chemistry within the peptide isostere, the enantiomeric acetylated
oxazolidinone was reacted with compound 25 to provide
diastereomeric ꢀ-keto imide 35 (Scheme 6). Exposure to the
chain-extension aldol reaction conditions provided 36 in a good
yield. The aldol stereocenter was assigned on the basis of a
CAN-mediated oxidative cleavage and hydrolysis to provide
the known (S)-lactone 34. This result provided further confirma-
tion that stereocontrol was predominantly influenced by the
oxazolidine and not by the remote alanine stereocenter.
Protection of the alcohol functionality of 36 with TBDPSCl
provided a substrate 38 (Scheme 7) that was then treated with
lithium hydroperoxide in an effort to remove the oxazolidinone.
Cleavage of the oxazolidinone was not successful, most likely
due to a competitive Bayer-Villiger reaction. While cleavage
of the oxazolidinone through a hydroxide-mediated process was
contemplated, the Pmb group was first removed in order to
simplify NMR analysis. Removal of a Pmb group from an amide
nitrogen can be accomplished through treatment with ceric
ammonium nitrate (CAN).29 The p-methoxybenzyl (Pmb) group
was excised cleanly by treatment of 38 with CAN, providing
support for the required intermediacy of a hemiacetal in the
oxidative cleavage reactions leading to γ-lactones 34 and 37.
It is worth noting that removal of the Pmb group from 38 is
performed in an acidic, aqueous environment, yet no oxidative
cleavage through the intermediacy of a ketone hydrate is
observed during the CAN treatment. The oxazolidinone was
removed from 39 through exposure to LiOH. Careful control
of time, temperature and stoichiometry was necessary to avoid
formation of diastereomers, most likely through the epimeriza-
tion of the alanine stereocenter. The Cbz-protected dipeptide
isostere 40 provided a substrate that presented opportunities to
functionalize both the C and N termini. This was accomplished
through a sequence involving EDC coupling, hydrogenolysis
in the presence of acid (to prevent diketopiperizine formation),
and a second EDC coupling step with 43 to generate the
tetrapeptide mimic 44. Removal of the silyl group with
tetrabutylammonium provided the final compound 11, which
exists predominantly in the hemiacetal form.
The predominance of the tetrahedral hemiacetal form due to
the presence of the R-hydroxylmethylene group (serine-mimic)
may make this isostere less desirable for serine protease
inhibition, although a small concentration of the ketone func-
tionality may be all that is needed for inhibition.30 The cyclic
nature of the final compound (11) also suggests that the synthetic
chemistry described above could be useful for the development
of a nonhydrolyzable, conformationally rigid, transition state
mimic based upon the tetrahydrofuranyl substructure.4a Nev-
ertheless, the predominance of a tetrahedral hemiacetal form is
not a dead end for potential application to ketomethylene peptide
isostere formation. We have reported previously that the
hemiacetal products generated in the tandem chain-
(30) We would like to thank the reviewer who brought the following
references to our attention. (a) Semple, J. E.; Rowley, D. C.; Brunck, T. K.;
Ha-Uong, T.; Minami, N. K.; Owens, T. D.; Tamura, S. Y.; Goldman, E. A.;
Siev, D. V.; Ardecky, R. J.; Carpenter, S. H.; GE, Y.; Richard, B. M.; Nolan,
T. G.; Ha˚kanson, K.; Tulinsky, A.; Nutt, R. F.; Ripka, W. C J. Med. Chem.
1996, 39, 4531–4536. (b) Schultz, R. M.; Varma-Nelson, P.; Ortiz, R.; Kozlowski,
K. A.; Orawski, A. T.; Pagast, P.; Frankfater, A. J. Biol. Chem. 1989, 264, 1497–
507.
(29) Yamaura, M.; Suzuki, T.; Hashimoto, H.; Yoshimura, J.; Okamoto, T.;
Shin, C. Bull. Chem. Soc. Jpn. 1985, 58, 1413–1420.
J. Org. Chem. Vol. 74, No. 2, 2009 649

Lin et al.
J ) 7.2 Hz); 13C NMR (90 MHz, CDCl3) δ 207.1, 173.4, 172.3,
171.1, 155.7, 135.7, 128.5, 128.3, 128.2, 80.3, 66.5, 54.2, 51.0,
36.9, 33.6, 28.3, 27.7, 16.9; HRMS (FAB+) m/z calcd for
C22H32N3O7 [M + H] 450.2240, found [M + H] 450.2249.
extension-aldol reactions of amino acid systems can be
deoxygenated to provide aliphatic side chains; therefore, access
to ketomethylene groups without hemiacetal contamination is
also possible using the chain-extension methodology.15 More
recent studies in our laboratory have demonstrated the utility
of a tandem chain extension-iodomethylation reaction for the
incorporation of functionalized side chains.31 This series of
studies has established that tandem chain-extension reactions,
when applied to amino acid systems, are capable of providing
stereocontrolled access to a wide variety of peptide isosteres.
Furthermore, the dipeptide isosteres formed through the strategy
illustrated above are easily derivatized from either the C or N
terminus, which makes the methodology available for library
generation and/or solid-phase peptide synthesis.
(1R,4S,10bS)-Benzyl
10b-Hydroxy-4-isopropyl-3,6-dioxo-
1,2,3,4,6,10b-hexahydropyrido[2,1-a]isoindole-1-carboxylate (24).
A 10-mL oven-dried, round-bottomed flask, equipped with a septum
with a flow of nitrogen through a needle and a stir bar, was charged
with methylene chloride (4 mL) and diethylzinc (1.0 M in hexanes,
0.5 mL, 0.5 mmol). The solution was cooled to 0 °C, and methylene
iodide (0.04 mL, 0.5 mmol) was added slowly by syringe. After
the solution was stirred for 10 min, compound 22 (38 mg, 0.1 mmol,
in 0.5 mL of methylene chloride) was added by syringe to the
resulting white suspension. The mixture was stirred for 30 min.
After TLC analysis (hexanes/ethyl acetate ) 10:1, Rf ) 0.20)
indicated that the starting material was consumed, the solution was
quenched by cautious addition of saturated aqueous ammonium
chloride (5 mL). The mixture was extracted with diethyl ether (2
× 5 mL), washed with brine (10 mL), and dried over anhydrous
sodium sulfate. The resulting liquid was filtered and concentrated
under reduced pressure. The residue was chromatographed on silica
(hexanes/ethyl acetate ) 5:1, Rf ) 0.10) to yield 22 mg (58%) of
Experimental Section
Benzyl (4S)-4-{[N-2-(tert-butoxycarbonyl)-L-asparaginyl]amino}-
3-oxopentanoate (16). Into a 10-mL round-bottomed flask was
weighed 148 mg (0.48 mmol) of N-2-(tert-butoxycarbonyl)-L-
asparaginyl-L-alanine 15 and the solid dissolved in 2 mL of
anhydrous DMF. Carbonyldiimidazole (CDI) 80 mg (0.48 mmol)
was added and the solution stirred until gas evolution ceased
(approximately 15 min). In a separate 25-mL round-bottomed flask
which contained a solution of monobenzyl malonate (284 mg, 1.44
mmol) in 10 mL of anhydrous THF was added 284 µL (0.72 mmol)
of a 1 M hexane solution of dibutylmagnesium at 0 °C. The clear,
colorless solution was allowed to warm to room temperature. The
acyl imidazole solution was transferred to the flask that contained
the magnesium salt, and the reaction was monitored by TLC. The
solution was allowed to stir for 8 h and quenched by the addition
of 20 mL of satd aq NH4Cl solution. The solution was extracted
three times with 20 mL of EtOAc. The combined organics were
washed with satd NaHCO3, water, and brine before being dried
with MgSO4 and concentrated. The colorless oil was placed on a
high vacuum for 2 h, and then crystallization of the material was
induced by the addition of a small amount of THF. The crystalline
material was dried under vacuum to yield 172 mg (81%) of benzyl
(4S)-4-{[N-2-(tert-butoxycarbonyl)-L-asparaginyl]amino}-3-oxopen-
1
24 as a white solid: mp ) 134-136 °C; H NMR (400 MHz,
CDCl3) δ 7.80 (d, J ) 5.3 Hz, 1H), 7.52-7.34 (m, 7H), 7.21 (d,
J ) 5.3 Hz, 1H), 5.34-5.22 (m, 2H), 4.94 (s, 1H), 4.39 (d, J )
11.0 Hz), 3.27 (dd, J ) 11.8, 12.5 Hz, 1H), 2.99 (dd, J ) 3.5, 12.5
Hz, 1H), 2.64 (dd, J ) 3.5, 11.8 Hz, 1H), 2.47 (m, 1H), 1.05 (d, J
) 6.8 Hz, 3H), 1.00 (d, J ) 6.8 Hz, 3H); 13C NMR (100 MHz,
CDCl3) δ 203.1, 172.9, 166.2, 145.8, 134.4, 133.0, 130.3, 129.0,
128.9, 124.3, 121.9, 86.1, 68.3, 66.2, 49.4, 36.8, 31.7, 19.9, 19.7;
HRMS (FAB+) m/z calcd for C23H22O4N [M - OH] 376.1549,
found [M - OH] 376.1534.
Benzyl (S)-5-((R)-4-Benzyl-2-oxooxazolidin-3-yl)-3,5-dioxopen-
tan-2-yl(4-methoxybenzyl)carbamate (35). An oven-dried 100-mL
round-bottomed flask was equipped with a stir bar and septum and
placed under an inert atmosphere using nitrogen flow through the
septum. The round-bottomed flask was charged with THF (30 mL)
and diisopropylamine (0.44 mL, 3.1 mmol). The reaction flask was
cooled to 0 °C using an ice bath, n-BuLi (2.5 M in hexanes, 1.2
mL, 3.0 mmol) was added, and LDA was allowed to form for 15
min while maintaining the ice bath. The reaction flask was then
cooled to -78 °C using a dry ice/acetone bath. (R)-3-Acetyl-4-
benzyloxazolidin-2-one27 (0.665 g, 3.0 mmol, in 15 mL THF) was
added dropwise using a syringe pump over the course of 1.5 h. An
hour into the addition, an oven-dried 15-mL round-bottomed flask
was equipped with a stir bar, septum, and a nitrogen atmosphere
and charged with (S)-2-((benzyloxycarbonyl)(4-methoxybenzy-
l)amino)propanoic acid (0.275 g, 0.81 mmol, in 5 mL THF) and
carbonyldiimidazole (0.143 g, 0.88 mmol) in the indicated order.
The solution was allowed to stir for the remainder of the acyl
oxazolidinone addition. The acyl imidazole mixture was transferred
to the enolate solution using a cannula. The reaction mixture was
allowed to stir for 1.5 h maintaining the dry ice/acetone bath and
was quenched with HCl (10 mL, 1 M solution). The product was
extracted with diethyl ether (3 × 50 mL) and washed with HCl
(25 mL, 1 M), saturated sodium bicarbonate (50 mL), and brine
(50 mL). The combined organic layers were dried over anhydrous
sodium sulfate, filtered by gravity, and concentrated in vacuo. The
product was purified by flash chromatography on silica (hexanes/
ethyl acetate ) 3:1; Rf ) 0.1) to yield 0.164 g (38%) of 35 as a
colorless oil and a mixture of rotomers (1:1): 1H NMR (500 MHz,
CDCl3) δ 7.38-7.12 (m, 12H), 6.89-6.75 (m, 2H), 5.27-5.11 (m,
2H), 4.75-4.55 (m, 2H), 4.45-4.25 (m, 1.5H), 4.20-4.00 (m, 3H),
3.89 (d, 0.5H, J ) 15.8 Hz), 3.80-3.65 (m, 3.5H), 3.56 (d, 0.5H,
J ) 16.3 Hz), 3.36 (t, 1H, J ) 15 Hz), 2.73 (t, 1H, J ) 11.6 Hz),
1.35-1.19 (m, 3H); 13C NMR (125.67 MHz, CDCl3) δ 201.3, 166.8,
166.6, 159.3, 159.1, 155.9, 155.8, 153.7, 153.6, 136.3, 135.8, 135.3,
135.3, 130.1, 129.5, 129.3, 129.0, 128.6, 128.5, 128.3, 128.2, 127.3,
114.1, 114.1, 68.0, 67.9, 67.8, 66.4, 64.4, 62.0, 62.0, 60.4, 55.3,
1
tanoate 16 as a white solid: mp ) 173 - 175 °C dec; H NMR
(360 MHz, DMSO-d6) δ 8.30 (m, 1H), 7.40-7.30 (m, 5H), 7.26
(m, 1H), 6.93-6.90 (m, 2H), 5.10 (s, 2H), 4.26-4.20 (m, 2H),
3.65 (s, 2H), 2.43-2.32 (m, 2H), 1.35 (s, 9H), 1.17 (d, 3H, J )
6.9 Hz); 13C NMR (90 MHz, DMSO-d6) δ 203.2, 171.7, 171.4,
167.2, 155.2, 135.8, 128.4, 128.0, 127.9, 78.2, 65.9, 54.1, 51.2,
45.1, 36.9, 28.1, 15.3; HRMS (FAB+) m/z calcd for C21H30N3O7
[M + H] 436.2084, found [M + H] 436.2072.
Benzyl (5S)-5-{[N-2-(tert-butoxycarbonyl)-L-asparaginyl]amino}-
4-oxohexanoate (17). A 25-mL round-bottomed flask was charged
with 12 mL of anhydrous CH2Cl2, and 150 µL (1.82 mmol) of
methylene iodide was added. The solution was cooled to 0 °C, and
1.20 mL (1.20 mmol) of a 1 M solution of diethylzinc in hexanes
was added slowly. The ice bath was removed, and a white
precipitate formed rapidly. After the mixture was stirred for 2 min,
106 mg (0.24 mmol) of solid benzyl (4S)-4-{[N-2-(tert-butoxycar-
bonyl)-L-asparaginyl]amino}-3-oxopentanoate 16 was added in one
portion and the reaction stirred for 2 min. The reaction was diluted
with 20 mL of CH2Cl2 and quenched with 25 mL of satd aq NH4Cl
solution. The organic portion was dried with MgSO4, filtered, and
evaporated under reduced pressure. Column chromatography on
silica using 10% MeOH/CH2Cl2 yielded 70 mg (64%) of benzyl
(5S)-5-{[N-2-(tert-butoxycarbonyl)-L-asparaginyl]amino}-4-oxohex-
anoate 17 as a colorless oil: 1H NMR (360 MHz, CDCl3) δ
7.38-7.28 (m, 5H), 6.11 (m, 1H), 5.74 (m, 1H), 5.10 (s, 2H),
4.54-4.49 (m, 2H), 2.91-2.55 (m, 6H), 1.45 (s, 9H), 1.34 (d, 3H,
(31) Pu, Q.; Wilson, E.; Zercher, C. K. Tetrahedron 2008, 64, 8045–8051.
650 J. Org. Chem. Vol. 74, No. 2, 2009

Formation of a Functionalized Dipeptide Isostere
55.2, 55.1, 51.6, 50.4, 47.4, 47.3, 37.5, 37.4, 30.7, 29.7, 21.1, 21.0,
(R)-4-Benzyl-3-((S)-5-oxotetrahydrofuran-3-carbonyl)oxazolidin-
2-one (37). A 25-mL round-bottomed flask was equipped with a
stir bar and charged with THF (10 mL), water (2.5 mL), and
compound 36 (0.35 g, 0.6 mmol). To this solution was added ceric
ammonium nitrate (1.3 g, 2.4 mmol), and the mixture was allowed
to stir for 30 min at room temperature. Water (5.0 mL) was added
to the solution, which was then extracted with Et2O (3 × 10 mL).
The combined organic extracts were dried carefully over anhydrous
sodium sulfate and concentrated in vacuo. The residue was
chromatographed on silica (hexanes/ethyl acetate ) 3:1; Rf ) 0.20)
to yield 0.13 g (75%) of (R)-4-benzyl-3-((S)-5-oxotetrahydrofuran-
19.2, 14.3, 14.0, 13.8, 13.3; [R]25 ) - 47.6 (c ) 0.03 g/mL,
D
CH2Cl2); HRMS (FAB+) m/z calcd for C31H33N2O7 [M + H+]
545.2288, found [M + H+] 545.2291.
Benzyl (2S,5S)-6-((R)-4-benzyl-2-oxooxazolidin-3-yl)-5-(hydroxy-
methyl)-3,6-dioxohexan-2-yl(4-methoxybenzyl)carbamate (36). A
25-mL oven-dried, round-bottomed flask, equipped with a septum
with a flow of nitrogen through a needle and a stir bar, was charged
with dichloromethane (4 mL) and diethylzinc (1.0 M in hexanes,
2.0 mL, 2.0 mmol). The solution was cooled to 0 °C in the ice
bath, and diiodomethane (0.16 mL, 2.0 mmol) was added dropwise.
After the soltuion was stirred for 10 min, paraformaldehyde (0.5
g, 6 mmol) and benzyl (S)-5-((R)-4-benzyl-2-oxooxazolidin-3-yl)-
3,5-dioxopentan-2-yl(4-methoxybenzyl)carbamate 35 (0.40 mmol,
in 0.5 mL of dichloromethane) were added to the resulting white
suspension in order. The mixture was stirred for 1 h. After TLC
analysis (hexanes/ethyl acetate ) 3:1; Rf ) 0.20) indicated that
the starting material was consumed, the solution was quenched by
cautious addition of saturated aqueous ammonium chloride (4 mL)
and the mixture extracted with diethyl ether (3 × 5 mL). The
combined organic layers were washed with brine (5 mL) and dried
over anhydrous sodium sulfate. The resulting liquid was filtered
and concentrated under reduced pressure. The residue was chro-
matographed on silica (hexanes/ethyl acetate ) 1:1; Rf ) 0.20) to
yield 0.16 g (68%) of 36 as a colorless oily mixture of two rotamers
with a ratio of 1:1: [R]25D ) - 48.0 (c ) 0.006 g/mL, CHCl3); IR
(neat) cm-1 3350 (b), 3028, 2960, 1735, 1675; 1H NMR (400 MHz,
CDCl3) δ 7.38-7.14 (m, 12H), 6.88-6.79 (m, 2H), 5.29-4.89 (m,
2H), 4.69-4.02 (m, 6H), 3.85-3.37 (m, 5H), 3.22 (m, 1H),
2.89-2.03 (m, 5H), 1.26-1.19 (2d, 3H, J ) 6.9, 6.9 Hz); 13C NMR
(100 MHz, CDCl3) δ 201.2, 166.8, 159.4, 159.3, 159.1, 155.8,
153.6, 135.3, 130.1, 129.5, 129.3, 129.0, 128.6, 128.5, 128.2, 127.3,
114.1, 114.0, 67.9, 67.8, 66.3, 62.0, 55.3, 55.1, 51.6, 50.3, 47.4,
42.2, 37.5, 13.9, 13.5, 13.2; HRMS (FAB+) m/z calcd for
C25H26N2O6 [M - OH] 450.1791, found [M - OH] 450.1784.
3-carbonyl)oxazolidin-2-one (37) as a viscous colorless oil: [R]25
D
) -5.8 (c ) 0.002 g/mL, CHCl3); IR (neat) cm-1 3075, 2997,
1
1748, 1670; H NMR (500 MHz, CDCl3) δ 7.36-7.17 (m, 5H),
4.71 (m, 1H), 4.62 (t, 3H, J ) 7.9 Hz), 4.42-4.26 (m, 4H), 3.24
(dd, 1H, J ) 3.3, 13.6 Hz), 3.03 (dd, 1H, J ) 5.1, 17.7 Hz), 2.88
(dd, 1H, J ) 9.1, 13.5 Hz), 2.72 (dd, 1H, J ) 9.1, 17.9 Hz); 13C
NMR (125 MHz, CDCl3) δ 175.4, 170.7, 153.3, 134.6, 129.4, 129.1,
127.7, 69.3, 67.0, 55.2, 40.1, 37.8, 30.1; HRMS (FAB+) m/z calcd
for C15H16NO5 [M + H+] 290.1028, found [M + H+] 290.1028.
Acknowledgment. We thank the National Institutes of Health
(R15 GM060967-02), Merck Pharmaceutical Research, and the
University of New Hampshire for their financial support.
Supporting Information Available: Experimental proce-
dures are available for the preparation of compounds 14, 15,
1
18, 20, 22, 26-28, 30, 32-34, 38-44, and 11. H NMR and
13C NMR spectra are available for 14-18, 20, 22, 24, 26-28,
30, 32-44, and 11. This material is available free of charge
via the Internet at http://pubs.acs.org.
JO801993K
J. Org. Chem. Vol. 74, No. 2, 2009 651