Synthesis of isopeptide epoxide peptidomimetics

Article abstract of DOI:10.1021/jo801907p

Two epoxide-containing peptidomimetics of the isopeptide, glutamyl-γ-glutamate, have been synthesized via a route that should be generally applicable to the synthesis of isopeptide analogues in which an oxirane replaces the scissile peptide bond. Enzymes

Full text of DOI:10.1021/jo801907p

Synthesis of Isopeptide Epoxide Peptidomimetics
Debatosh Majumdar, Matthew D. Alexander, and James K. Coward*
Departments of Medicinal Chemistry and Chemistry, UniVersity of Michigan,
Ann Arbor, Michigan 48109-1055
jkcoward@umich.edu
ReceiVed September 2, 2008
Two epoxide-containing peptidomimetics of the isopeptide, glutamyl-γ-glutamate, have been synthesized
via a route that should be generally applicable to the synthesis of isopeptide analogues in which an
oxirane replaces the scissile peptide bond. Enzymes that catalyze the hydrolysis of peptides and isopeptides
are often susceptible to inactivation by electrophilic substrate analogues. In this research, an epoxide
was installed as an electrophilic replacement of the scissile isopeptide bond. The C-terminal glutamyl
mimic was accessed by the stereospecific synthesis of suitably substituted cyclopentenes, 8 and 10, as
surrogates for either the L- or D- enantiomer. The enantiomeric cyclopentenes were further elaborated to
incorporate an appended sulfone that was reacted with a suitably protected glutamyl-γ-semialdehyde in
a Julia-Kocienski olefination reaction. This olefination afforded predominantly the desired E-olefin
isosteres of L-glutamyl-γ-D-glutamate and L-glutamyl-γ-L-glutamate, following which peracid-mediated
epoxidation and deprotection provided the epoxide-containing peptidomimetics, 4 and 5.
Introduction
the development of “active-site-directed irreversible inhibitors”5,6
and subsequently, “mechanism-based enzyme inhibitors”7 has
been a powerful tool in elucidating the mechanism of numerous
enzyme-catalyzed reactions.
Although the design and synthesis of mechanism-based
inhibitors of proteases that act on proteins and peptides involving
R-linked amino acids has been investigated extensively,1 similar
efforts to identify specific inhibitors of enzyme-catalyzed
isopeptide hydrolysis are much less common.2 The latter
reaction, although less well studied, is of critical importance in
areas such as the ubiquitination-deubiquitination cycle3 and
folate polyglutamate homeostasis involving the formation and
hydrolysis of γ-glutamyl peptides.4 The use of electrophilic
substrate analogues as enzyme inactivators and probes of
enzyme mechanism and structure has been at the forefront of
medicinal and bioorganic chemistry research for decades. Thus,
This approach has been applied to the design of selective
inactivators of proteases that are postulated (and in many cases
proven) to proceed via acyl enzyme intermediates, i.e., cysteine,
serine, and threonine proteases.1 In most of these inhibitors,
however, the pendant electrophilic moiety (e.g., halomethylke-
tone, acyloxymethylketone, epoxysuccinates, R,ꢀ-epoxyketones,
vinyl sulfones) is incorporated in place of the C-terminal
carboxyl group of the peptide substrate. The well-studied
ubiquitination pathway of protein degradation involves ATP-
dependent attachment of ubiquitin, a small protein, to a targeted
protein, followed by recycling of the ubiquitin via hydrolysis
* Corresponding author. Phone: 734-936-2843. Fax: 734-647-4865.
(1) Powers, J. C.; Asgian, J. L.; Ekici, O. D.; James, K. E. Chem. ReV. 2002,
102, 4639–4750.
(2) Love, K. R.; Catic, A.; Schlieker, C.; Ploegh, H. L. Nature Chem. Biol.
2007, 3, 697–705.
(3) Wilkinson, K. D. Cell DeV. Biol. 2000, 11, 141–148.
(4) Schneider, E.; Ryan, T. J. Clin. Chim. Acta 2006, 374, 25–32.
(5) Baker, B. R. Design of actiVe-site-directed irreVersible enzyme inhibitors;
the organic chemistry of the enzymic actiVe-site; Wiley: New York, 1967.
(6) Baker, B. R. Annu. ReV. Pharmacol. 1970, 10, 35–50.
(7) Silverman, R. B. Mechanism-based enzyme inactiVation: chemistry and
enzymology; CRC Press: Boca Raton, FL, 1988.
10.1021/jo801907p CCC: $40.75
Published on Web 12/16/2008
2009 American Chemical Society
J. Org. Chem. 2009, 74, 617–627 617

Majumdar et al.
FIGURE 1. Reaction catalyzed by γ-glutamyl hydrolase.
of the isopeptide bond between the C-terminal glycine residue
of ubiquitin and the ε-amino group of a lysine residue of the
targeted protein.3 Inhibition of the isopeptide hydrolase has been
effected by the attachment of an electrophilic moiety to the
C-terminus of ubiquitin.2 The structure of one ubiquitin C-
terminal hydrolase, UCH-L3, has been determined by X-ray
crystallography in the absence of any ligand8 and in the presence
of ubiquitin to which an electrophilic vinyl methyl ester was
attached to the C-terminal glycine.9
γ-Glutamyl hydrolase (GH, EC 3.4.19.9) is a cysteine
peptidase10 that catalyzes the hydrolysis of γ-glutamyl peptides11
which, together with the ATP-dependent ligase, folylpoly-
glutamate synthetase (FPGS, EC 6.3.2.17), controls folate
homeostasis in living cells. Our interest in GH led us to consider
the synthesis of electrophilic substrate analogues for use in
further probing the catalytic mechanism as well as for develop-
ing probes for the localization of this enzyme in complex
biological environments. Our initial focus was the synthesis of
C-terminal epoxides due to the demonstrated selectivity of the
epoxide moiety for cysteine proteases, as well as the many
reported syntheses of C-terminal epoxides and related epoxysuc-
cinyl peptides in the literature.1,12 The reaction catalyzed by
GH is shown in Figure 1 (1a f 2a + 3a) . Evaluation of a
C-terminal epoxide derivative, 1b, as an inhibitor of GH revealed
that 1b inhibited the enzyme but most, if not all of the inhibition
could be attributed to GH-catalyzed hydrolysis (1b f 2a +
3b), i.e., 1b acts an alternate substrate.13 GH does not display
product inhibition and, consistent with this fact, 2b, a compound
lacking the isopeptide bond but also containing a C-terminal
epoxide, is neither a substrate nor an inhibitor. As a result,
subsequent synthetic efforts have been aimed at the synthesis
of epoxide peptidomimetics in which the scissile isopeptide bond
is replaced by an oxirane.
There is some precedent for the replacement of a peptide bond
by an oxirane and our research builds on those initial studies.14-18
These syntheses involved either peracid-mediated epoxidation
of the corresponding E- or Z-olefins, or elaboration of the
corresponding ꢀ-hydroxy selenide to the desired epoxide via
an oxidative cyclization process. However, to the best of our
knowledge, no such epoxide peptidomimetics of isopeptides
have been synthesized. In this paper, using a peracid-mediated
epoxidation of an olefin, we describe the synthesis of two
diastereomeric epoxide peptidomimetics, 4 and 5, in which the
scissile isopeptide bond is replaced by an oxirane.
Results and Discussion
In previous research from our laboratory on the synthesis of
phosphinic acid-containing pseudopeptide inhibitors of FPGS,
the use of cyclopentene derivatives as glutarate surrogates has
been described.19 In the present work, a similar approach has
been pursued. Thus, the desired oxiranes are envisioned to be
accessible from the corresponding E-olefin, 6, as shown in
Figure 2 for the L-Glu-γ-L-Glu mimetic, 4, with the chiral centers
derived ultimately from (N,N-Boc2)-L-glutamic acid γ-semial-
(14) Jenmalm, A.; Berts, W.; Li, Y. L.; Luthman, K.; Csoregh, I.; Hacksell,
U. J. Org. Chem. 1994, 59, 1139–1148.
(15) Mann, A.; Quaranta, L.; Reginato, G.; Taddei, M. Tetrahedron Lett.
1996, 37, 2651–2654.
(8) Johnston, S. C.; Larsen, C. N.; Cook, W. J.; Wilkinson, K. D.; Hill, C.
P EMBO J. 1997, 16, 3787–3796.
(9) Misaghi, S.; Galardy, P. J.; Meester, W. J. N.; Ovaa, H.; Ploegh, H. L.;
Gaudet, R. J. Biol. Chem. 2005, 280, 1512–1520.
(10) Chave, K. J.; Galivan, J.; Ryan, T. J. Biochem. J. 1999, 343, 551–555.
(11) Alexander, J. P.; Ryan, T. J.; Ballou, D. P.; Coward, J. K. Biochemistry
2008, 47, 1228–1239.
(16) Perlman, N.; Livneh, M.; Albeck, A. Tetrahedron 2000, 56, 1505–1516.
(17) Demarcus, M.; Ganadu, M. L.; Mura, G. M.; Porcheddu, A.; Quaranta,
L.; Reginato, G.; Taddei, M. J. Org. Chem. 2001, 66, 697–706.
(18) Wiktelius, D.; Berts, W.; Jensen, A. J.; Gullbo, J.; Saitton, S.; Csoregh,
I.; Luthman, K. Tetrahedron 2006, 62, 3600–3609.
(12) Albeck, A. Drug DeV. Res. 2000, 50, 425–434.
(13) Alexander, M. D. Ph.D. Thesis, The University of Michigan, 2004.
(19) Bartley, D. M.; Coward, J. K. J. Org. Chem. 2005, 70, 6757–6774.
618 J. Org. Chem. Vol. 74, No. 2, 2009

Synthesis of Isopeptide Epoxide Peptidomimetics
SCHEME 1
reaction through the olefination of an aldehyde (e.g., 7).23,24
As outlined in Scheme 1, the critical stereoselective synthesis
of 10, via sequential dynamic kinetic resolution, xanthate
formation, and pyrolysis to form the desired cyclopentene
derivative25 proceeded in higher yield (65%, 5 steps)26 than the
synthesis of 8 via acetylation, hydrogenation of the enol acetate,
lipase-catalyzed resolution, and pyrolysis of the enantiomeric
xanthate (21%, 7 steps).19 Accordingly, the synthesis of 5 from
the more readily available enantiomer, 10, was investigated
before the synthesis of 4 from 8 (Schemes 2-5).
As shown in Scheme 2, dihydroxylation of the (1R)-ꢀ,γ-
unsaturated ethyl ester derivative 10 was performed by using
the Upjohn procedure (OsO4/N-methylmorpholine N-oxide)27
to afford the corresponding vicinal diols as an inseparable
mixture of trans and cis diastereomers in 97% yield. To protect
the free hydroxyl groups of the vicinal diols, reaction with 2,2-
dimethoxypropane/acetone/TsOH afforded the corresponding
isopropylidene derivatives (93% yield, diastereomeric ratio
(trans:cis) ) ca. 1.7:1), which were separated by flash chro-
matography to give 11 in 58% yield and the cis diastereomer,
cis-11, in 35% yield. Stereochemistry of the diastereomers was
confirmed by a nuclear Overhauser effect (NOE) between H-1
and H-2 that was observed only in the NMR spectra of the cis
diastereomer. The ethyl ester functionality of the trans isomer
was reduced by using LiAlH4 at -78 °C to obtain the
corresponding hydroxymethyl derivative in 95% yield. Mit-
sunobu coupling28 of this alcohol to 1-phenyl-1H-tetrazole-5-
thiol using PPh3/diisopropylazodicarboxylate (DIAD) (0 °C f
rt) furnished the sulfide product in 88% yield, which was
followed by oxidation (H2O2, (NH4)6Mo7O24 ·4H2O, 0 °C) to
give sulfone 12 in 87% yield.
FIGURE 2. Synthesis of oxirane- and olefin-containing analogues of
γ-glutamyl peptides: retrosynthetic analysis.
dehyde, 7, and (S)-1-(carboethoxy)cyclopent-2-ene, 8. The
L-Glu-γ-D-Glu mimetic, 5, also should be accessible via the
same route from the enantiomer of 8 (ent-8). In developing a
strategy for the synthesis of target olefin isosteres of the
γ-glutamyl dipeptide (e.g., 6), and the corresponding internal
epoxide derivatives (e.g., 4), the two important synthetic
challenges that need to be addressed are (i) stereospecific
generation of the chiral center which serves as a pseudo-R-
carbon in the C-terminal glutamic acid replacement, analogous
to the 2S or 2R stereochemistry in the parent isopeptide, and
(ii) selective formation of the E-olefin during synthesis of the
olefinic dipeptide isostere. Stereospecific generation of either
stereoisomer at C-1 of the cyclopent-2-ene derivative, 8 or ent-8
(10), from ethyl 2-oxocyclopenane carboxylate, 9, has been
achieved via lipase-mediated ester hydrolysis,13,19-21 or a
dynamic kinetic resolution,22 respectively (Scheme 1). The
desiredE-olefinismostconsistentlyachievedbytheJulia-Kocienski
The enantiomeric sulfone, 14, was obtained in an analogous
manner from the enantiomeric cyclopentene derivative, 8. Thus,
dihydroxylation of the ꢀ,γ-unsaturated ethyl ester derivative 8
by using the Upjohn procedure27 afforded the corresponding
vicinal diols as an inseparable mixture of trans and cis
diastereomers in 78% yield. Subsequent protection of the vicinal
(23) Blakemore, P. R. J. Chem. Soc., Perkin Trans 1 2002, 2563–2585.
(24) Plesniak, K.; Zarecki, A.; Wicha, J. Top. Curr. Chem. 2007, 275, 163–
250.
(20) Rozzell, J. D. Tetrahedron Lett. 1982, 23, 1767–1770.
(25) Maeda, K.; Inouye, Y. Bull. Chem. Soc. Jpn. 1994, 67, 2880–2882.
(26) Bartley, D. M.; Coward, J. K. J. Org. Chem. 2006, 71, 372–374.
(27) Vanrheenen, V.; Kelly, R. C.; Cha, D. Y. Tetrahedron Lett. 1976, 1973–
1976.
(21) Xie, Z. F.; Suemune, H.; Sakai, K. J. Chem. Soc., Chem. Commun. 1987,
838–839.
(22) Wahhab, A.; Tavares, D. F.; Rauk, A. Can. J. Chem. 1990, 68, 1559–
1563.
(28) Mitsunobu, O. Synthesis 1981, 1–28.
J. Org. Chem. Vol. 74, No. 2, 2009 619

Majumdar et al.
SCHEME 2
SCHEME 3
Previous research in our laboratory has shown that compounds
similar to 12 and 14 with the 2,3-isopropylidene substituent trans
to a bromomethyl substituent at C-1 are more reactive in SN2
reactions than the cis diastereomers.19 In the current research,
olefination reactions of sulfone 14 or cis-14 proceeded in similar
yields. Since the stereochemical distinction between these
diastereomers is lost in the oxidative cleavage of the masked
cyclopentene moiety to the glutarate targets, it would be
reasonable to proceed with the synthesis by using a mixture of
diastereomeric isopropylidenes, e.g., 14 and cis-14. However,
the 1H NMR spectra of these mixtures are very complex,
involving numerous diastereomeric and diastereotopic nuclei.
In contrast, spectra of compounds derived from a single
diastereomer, i.e., 12 or 14, are considerably simplified and more
readily interpreted. Therefore, we chose to explore the olefi-
nation reactions primarily with a single diastereomer, usually
the more readily available trans diastereomer.29 Subsequent
investigationscanbecarriedoutwiththemixtureofdiastereomers.
One of the important features of this reaction design is the
preferredformationofatransdoublebondwiththeJulia-Kocienski
olefination.23,24 Initial investigation of the reaction on a small
scale at low temperature indicated that the reaction did not go
to completion and a large proportion of unreacted sulfone 14
was recovered along with the desired product.13 However, as
depicted in Scheme 4, reaction of glutamyl semialdehyde
diols yielded the corresponding isopropylidene derivatives (91%
yield, diastereomeric ratio (trans:cis) ) ca. 1.2:1). Separation
of the diastereomers was accomplished by flash chromatography
to give 13 in 49% yield and cis-13 in 43% yield. The ethyl
ester functionality of the trans isomer was reduced by using
LiAlH4 to obtain the corresponding hydroxymethyl derivative
in 98% yield, followed by Mitsunobu coupling28 to 1-phenyl-
1H-tetrazole-5-thiol to give the corresponding thioether in 93%
yield. This product was oxidized to sulfone 14 in 98% yield.
As shown in Scheme 3, the cis diastereomer, cis-13, was
converted to the corresponding sulfone, cis-14, by identical
chemistry to that described above for the trans diastereomer.
Crystallization of cis-14 enabled the determination of the
absolute configuration of cis-14 by X-ray crystallography and
confirmed the absolute stereochemistry of all substituents in this
1
series of compounds (11-14) originally deduced based on H
(29) The trans diastereomers of the isopropylidene precursors, 11 and 13,
are predicted to have a lower dipole moment, with associated decreased polarity,
than the corresponding cis diastereomers. As a result, the trans diastereomers
(higher Rf) were readily separated from the more polar cis diastereomers in high
purity by flash chromatography.
NMR spectral data (see the Supporting Information for details
of the synthesis of cis-14 and also the crystal structure
determination).13
620 J. Org. Chem. Vol. 74, No. 2, 2009

Synthesis of Isopeptide Epoxide Peptidomimetics
SCHEME 4
derivative 730 and sulfone 12 under optimized conditions23,24
(KHMDS, -78 °C) provided the olefin product 15 in 69% yield.
Analysis of 1H NMR spectral data showed predominant forma-
tion of the desired E-olefin (E:Z ) 9:1; Jtrans )15 Hz vs Jcis
)11 Hz). Similarly, reaction of 7 and sulfone 14 under identical
conditions provided the desired olefin product 18 in 71% yield.
(80% yield). A similar series of reactions converted 18 to 20.
Thus, sequential hydrolysis to the vicinal diol (83%), oxidative
cleavage to the crude dialdehyde, further oxidation to diacid
19 under Kraus conditions (94%), esterification with TMSCHN2
(83%), and finally N-deprotection afforded 20 (84%).
Subsequent elaboration to the target epoxide peptidomimetics,
4 and 5, is shown in Scheme 5. As described in the Supporting
Information, the 1-hydroxybenzotriazole (HOBt) ester of p-
aminobenzoic acid, 21, was synthesized by DCC-mediated
coupling of 4-(N-methyl)aminobenzoic acid (N-Me-pABA) and
HOBt in 84% yield. This preactivated N-Me-pABA was coupled
with olefin isostere 17 in the presence of Et3N to give 22 (73%
yield). As observed in the synthesis of terminal epoxides, 1b
and 2b (see the Supporting Information),13 epoxidation of 22
without prior N-protection resulted in the oxidation of the
secondary amine. To avoid this problem, the amine nitrogen
atom of 22 was protected as a trifluoroacetamide (CF3CO)2O/
Et3N, 0 °C, 62% yield). Reaction of the olefinic amide with
mCPBA/NaHCO3 proceeded as expected, without facial selec-
tivity, to yield the epoxide 23 (80% yield) as a mixture of trans
epoxide diastereomers. TLC, NMR, and MS analysis of the
product confirmed the isomeric mixture was contaminant-free.
Epoxide 23 was treated with LiOH, to give, after concomitant
deprotection of the acids and the amine, the corresponding target
compound 4. Formation of the desired product 4 was confirmed
by NMR and MS analysis.33,34
1
Analysis of the H NMR spectral data indicated that 18 was
also mostly trans (E:Z ) 4:1; Jtrans ) 15 Hz vs Jcis )11 Hz).
The Julia-Kocienski reaction is known to be readily scalable
and the olefination reactions described herein are amenable to
scale-up to provide adequate quantities of 15 and 18 for further
elaboration to the desired target compounds. To remove the
isopropylidene functionality, the Julia-Kocienski olefination
product 15 was treated with 50% acetic acid to furnish the
vicinal diol in 96% yield. Oxidative cleavage of the vicinal diol
was performed with NaIO4 to give the dialdehyde as a crude
product. Without purification, the dialdehyde was oxidized under
Kraus conditions (NaClO2/NaH2PO4/2-methyl-2-butene, 0 °C)31,32
to the diacid 16 (78% yield, two steps). Esterification of 16
(TMSCHN2, 0 °C f rt) gave the corresponding triester (85%
yield), which on removal of the Boc groups gave the amine 17
(30) Padron, J. M.; Kokotos, G.; Martin, T.; Markidis, T.; Gibbons, W. A.;
Martin, V. S. Tetrahedron: Asymmetry 1998, 9, 3381–3394.
(31) Kraus, G. A.; Taschner, M. J. J. Org. Chem. 1980, 45, 1175–1176.
(32) Santhanam, B.; Worfent, M. A.; Moore, J. N.; Boons, G. J. Chem. Eur.
J. 2004, 10, 4798–4807.
J. Org. Chem. Vol. 74, No. 2, 2009 621

Majumdar et al.
SCHEME 5
the trans and cis olefin, respectively. The E:Z ratio determined for
the products of the olefination reaction, 15 (9:1) and 18 (4:1), was
assumed to be maintained during subsequent chemical transforma-
tions. Compounds 7,30 8,13,19-21 and 1022,25 were synthesized as
described in the literature.
Similarly, amine 20 was coupled with 21 to obtain 24 (68%).
Formation of the N-Tfa derivative (76%) followed by epoxi-
dation afforded epoxide 25 (97%) as a mixture of trans epoxide
diastereomers. TLC, NMR, and MS analysis of the product
confirmed the isomeric mixture contained no impurities. Epoxide
25 was treated with LiOH to give the corresponding triacid 5,
the structure of which was confirmed by NMR and mass spectral
data.35 Biochemical studies on the effect of 4 and 5 on the
reaction catalyzed by GH are in progress and will be reported
in due course.
(3aS,4R,6aR)-Ethyl 2,2-Dimethyltetrahydro-3aH-cyclopenta[d]-
[1,3]dioxole-4-carboxylate (11). A solution of N-methylmorpholine
N-oxide (7.03 g, 60 mmol) and osmium tetroxide (3 mL of 2.5 wt
% solution, 300 µmol) in water (20 mL) was added to a cooled (0
°C) stirred solution of optically pure ethyl (cyclopent-2-ene)-1-
carboxylate 10 (4.22 g, 30 mmol) in acetone/THF (15 mL/15 mL).
The solution was warmed to rt and stirring was continued for 22 h.
TLC analysis showed the complete consumption of the starting
olefin. Then the reaction mixture was diluted with DCM (70 mL)
and washed with 10% Na2SO3 (40 mL) and saturated NH4Cl (40
mL). The combined aqueous layers were extracted with DCM (2
× 50 mL). The organic layers were combined, dried over anhydrous
MgSO4, and concentrated under reduced pressure. The crude
product was purified by flash chromatography (EtOAc/hexane 2:3)
to provide an inseparable mixture of diastereomeric diols as a
Experimental Section
General Experimental Procedures. See the Supporting Infor-
mation. The selectivity of the Julia-Kocienski reaction was
consistent with literature precedent and provided primarily the E
geometry for the newly formed olefin. For compounds 15 and 18,
the diastereomeric ratio (E:Z) was determined by comparing the
relative integration values of corresponding methine protons, the
resonances of which do not overlap with any other signals.
Independent decoupling of the methine protons for both the trans
and cis olefin isomers within the mixture allowed the coupling
between the vinyl protons to be measured at J ) 15 and 11 Hz for
1
colorless oil (5.08 g, 97% yield). H NMR (400 MHz; CDCl3) δH
4.24-4.05 (m, 4H), 3.52 (br s, 0.3H), 2.98-2.84 (m, 2H), 2.63 (br
s, 0.6H), 2.19-1.71 (m, 4H), 1.30-1.26 (m, 3H). 13C NMR (100
MHz, CDCl3) δC: 175.2, 77.3, 74.0, 73.8, 72.9, 61.0, 60.8, 48.3,
46.2, 30.8, 29.8, 24.0, 23.8, 14.2, 14.1. Rf 0.16 (EtOAc/hexane 2:3).
MS (ESI) m/z (rel intensity) 197.1 ([M + Na]+, 100).
(33) The use of a stereorandom epoxidation method was chosen to obtain
the target compounds as “proof of concept” irreversible inhibitors of GH.
Following biochemical evaluation of 4 and 5, the use of chiral epoxidation
methods34 will be pursued to optimize both efficacy and selectivity.
(34) Frohn, M.; Shi, Y. Synthesis 2000, 1979–2000.
(35) All of the reactions depicted in Schemes 4 and 5 were performed multiple
times on less than 100 mg scale with consistent results. However, all of the
chemical transformations utilized here were chosen for their generality and are
known to work well on significantly larger scale. Thus, we expect the synthetic
route described here to be readily amenable to gram scale without modification.
The diastereomeric mixture of cis and trans diols (6.49 g, 37
mmol) and 2,2-dimethoxypropane (100 mL) were dissolved in
acetone (115 mL) and stirred for 5 min. TsOH·H2O (704 mg, 3.7
mmol) was then added portionwise and stirring was continued at
rt for 18 h. TLC analysis showed complete consumption of the
starting diastereomeric diols. The reaction mixture was diluted with
EtOAc (150 mL), then washed with saturated NaHCO3 (50 mL)
622 J. Org. Chem. Vol. 74, No. 2, 2009

Synthesis of Isopeptide Epoxide Peptidomimetics
and then saturated NaCl (150 mL). The aqueous layer was extracted
with EtOAc (3 × 50 mL). The combined organic layer was dried
over MgSO4 and concentrated under reduced pressure. The residue
(7.48 g) was purified by flash chromatography (EtOAc/hexane 1:9)
to give compounds cis-11 (2.67 g, 35%) and 11 (4.47 g, 58%) as
(NH4)6Mo7O24 ·4H2O (165 mg, 133 µmol) was dissolved in 30%
H2O2 (1.13 mL, 10 mmol) at 0 °C to give a yellow solution that
was added dropwise to the ethanolic solution of the thioether
precursor (222 mg, 667 µmol) in EtOH (10 mL) at 0 °C. The yellow
color of the (NH4)6Mo7O24 ·4H2O solution disappeared after addition
of each drop in the reaction mixture and a white precipitate was
observed. The resulting suspension was stirred at rt for 24 h, added
to brine (40 mL), and extracted with EtOAc (3 × 50 mL). The
combined organic layer was dried over anhydrous Na2SO4 then
filtered and the solvent was removed under reduced pressure to
afford the crude product as an oil (590 mg). Purification of the
crude product by flash chromatography (EtOAc/hexane 3:7) gave
1
a colorless oil. Cis: H NMR (500 MHz; CDCl3) δH 4.79 (t, J )
5.0 Hz, 1H), 4.66 (t, J ) 5.5 Hz, 1H), 4.24-4.13 (m, 2H),
2.61-2.59 (m, 1H), 2.14-2.10 (m, 1H), 1.94-1.90 (m, 1H),
1.76-1.71 (m, 1H), 1.46-1.41 (m, 4H), 1.28-1.25 (m, 6H). 13C
NMR (125 MHz, CDCl3) δC 171.1, 109.9, 80.9, 80.4, 60.4, 50.1,
31.4, 25.8, 24.1, 24.0, 14.3. Rf 0.26 (EtOAc/hexane 1:9). MS (ESI)
m/z (rel intensity) 237.1 ([M + Na]+, 100). Trans: 1H NMR (500
MHz; CDCl3) δH 4.83 (d, J ) 5.5 Hz, 1H), 4.71 (t, J ) 5.0 Hz,
1H), 4.12 (q, J ) 7.5 Hz, 2H), 2.89 (d, J ) 7.5 Hz, 1H), 2.11-2.08
1
the pure sulfoxide 12 as a clear, colorless oil (208 mg, 87%). H
NMR (500 MHz; CDCl3) δH 7.69-7.59 (m, 5H, Ar-H), 4.67 (t, J
) 5.5 Hz, 1H), 4.49 (dd, J ) 5.0, 2.0 Hz, 1H), 3.78 (dd, J ) 15,
7.5 Hz, 1H), 3.67 (dd, J ) 15.0, 7.0 Hz, 1H), 2.75-2.74 (m, 1H),
2.18-2.15 (m, 1H), 1.88-1.80 (m, 2H), 1.66-1.63 (m, 1H), 1.42
(s, 3H), 1.26 (s, 3H). 13C NMR (125 MHz, CDCl3) δC 154.1, 133.4,
131.9, 130.1, 125.5, 111.3, 85.3, 80.5, 57.9, 40.1, 31.2, 28.9, 26.9,
24.6. Rf 0.45 (EtOAc/hHexane 3:7). MS (ESI) m/z (rel intensity)
387.1 ([M + Na]+, 100).
(3aR,4S,6aS)-Ethyl 2,2-Dimethyltetrahydro-3aH-cyclopenta[d]-
[1,3]dioxole-4-carboxylate (13). A solution of N-methylmorpholine
N-oxide (1.86 g, 15.87 mmol) and osmium tetroxide (0.80 mL of
2.5 wt % solution, 79 µmol) in water (5 mL) was added to a cooled
(0 °C) stirred solution of optically pure cyclopent-2-ene-ethylcar-
boxylate 8 (1.12 g, 7.99 mmol) in acetone/THF (3.5 mL/3 mL).
Subsequent reaction conditions, workup, and purification employed
identical reagents and solvents proportionately to that described in
the synthesis of 11, again providing an inseparable mixture of
diastereomeric diols as a colorless oil (1.08 g, 78% yield). 1H NMR
(400 MHz; CDCl3) δH 4.19-4.04 (m, 4H), 3.36 (br d, 0.2H),
2.90-2.80 (m, 1H), 2.26-2.10 (m, 1H), 2.00-1.90 (m, 1H),
1.82-1.68 (m, 2H), 1.30-1.26 (m, 3H). 13C NMR (125 MHz,
CDCl3) δC 175.9, 174.4, 77.2, 74.4, 74.3, 73.3, 61.2, 61.0, 48.8,
46.9, 30.6, 30.4, 24.6, 23.5, 14.5, 14.4. Rf 0.33 (EtOAc/hexane 3:2).
MS (ESI) m/z (rel intensity) 197.1 ([M + Na]+, 100). HRMS-ESI
(m/z) [M + Na]+ calcd for C8H14O4Na [M + Na]+ 197.0790, found
197.0792.
(m, 1H), 1.92-1.75 (m, 3H), 1.45 (s, 3H), 1.32-1.24 (m, 6H). 13
C
NMR (125 MHz, CDCl3) δC 174.1, 110.2, 83.2, 81.4, 60.9, 50.9,
31.8, 27.0, 26.7, 24.4, 14.5. Rf 0.41 (EtOAc/hexane 1:9). MS (ESI)
m/z (rel intensity) 237.1 ([M + Na]+, 100).
5-(((3aS,4R,6aR)-2,2-Dimethyltetrahydro-3aH-cyclopenta[d][1,3]-
dioxol-4 yl)methylthio)-1-phenyl-1H-tetrazole (12). The optically pure
bicyclic ethyl ester derivative 11 (2.97 g, 13.8 mmol) was dissolved
in THF (30 mL). In a separate flask, LiAlH4 (0.73 g, 19.2 mmol)
was suspended in THF (10 mL) in an atmosphere of argon and the
mixture was cooled to 0 °C. The THF solution of the ester was
added dropwise to the LiAlH4 suspension at 0 °C. Vigorous
bubbling was observed during the addition of the ester derivative.
The reaction mixture was stirred for 4 h at 0 °C in an atmosphere
of argon and then quenched by dropwise addition of 10% NH4Cl
solution (0.8 mL). The solution was allowed to warm to rt and
stirring was continued for 60 min. The reaction mixture was filtered
through Celite with use of EtOAc and the solvent was removed
under reduced pressure to give the crude product as a clear oil (2.4
g). The crude product was purified by flash chromatography
(EtOAc/hexane 3:7) to give the desired hydroxymethyl derivative
1
as a colorless oil (2.27 g, 95%). H NMR (500 MHz; CDCl3) δH
4.65 (t, J ) 5.6 Hz, 1H), 4.47 (d, J ) 5.5 Hz, 1H), 3.53-3.45 (m,
2H), 2.22-2.21 (m, 1H), 1.98-1.96 (m, 1H), 1.82-1.71 (m, 3H),
1.49-1.46 (m, 3H), 1.30 (s, 3H). 13C NMR (125 MHz, CDCl3) δC
110.2, 83.5, 81.1, 63.6, 48.5, 31.4, 26.8, 25.6, 24.3. Rf 0.18 (EtOAc/
hexane 3:7). MS (ESI) m/z (rel intensity) 195.1 ([M + Na]+, 100).
The diastereomeric mixture of cis and trans diols (770 mg, 4.41
mmol) and 2,2-dimethoxypropane (15.0 mL) were dissolved in
acetone (10.0 mL) and stirred for 5 min. Then TsOH ·H2O (22 mg,
116 µmol) was added portionwise and stirring was continued at rt
for 30 min. Subsequent workup and purification employed identical
reagents and solvents proportionately to that described in the
synthesis of 11, providing compounds cis-13 (398 mg, 43%) and
The bicyclic hydroxymethyl derivative (890 mg, 5.1 mmol) was
dissolved in THF (25 mL) at 0 °C in an atmosphere of argon
followed by addition of Ph3P (2.0 g, 7.62 mmol) and 1-phenyl-
1H-tetrazole-5-thiol (1.45 g, 8.13 mmol). In a separate flask,
diisopropylazadicarboxylate, DIAD (2.08 g, 10.3 mmol), was
dissolved in THF (10 mL), which gave a yellow solution. This
DIAD solution was added dropwise to the reaction mixture at 0
°C. After addition of each drop of DIAD solution the reaction
mixture became yellow, then the color persisted for a second and
disappeared. When approximately half of the DIAD solution was
added, the reaction mixture remained yellow. Stirring was continued
at rt for 12 h. The TLC analysis showed the complete consumption
of the starting material. The reaction mixture was diluted with Et2O
(150 mL) and washed with saturated NaHCO3 solution (100 mL).
The aqueous layer was extracted with Et2O (3 × 100 mL). The
combined organic layer was washed with saturated NaCl solution
(100 mL) and dried over anhydrous Na2SO4, then the solvent was
removed under reduced pressure to give a yellow oil that was
purified by flash chromatography (EtOAc/hexane 1:9) to give the
1
13 (466 mg, 49%) as colorless oils. Cis: H NMR (400 MHz;
CDCl3) δH 4.79 (t, J ) 5.7 Hz, 1H), 4.66 (t, J ) 5.2 Hz, 1H),
4.25-4.11 (m, 2H), 2.63-2.57 (m, 1H), 2.18-2.06 (m, 1H),
1.94-1.89 (m, 1H), 1.77-1.70 (m, 1H), 1.49-1.41 (m, 4H),
1.28-1.25 (m, 6H). 13C NMR (125 MHz, CDCl3) δC 171.4, 110.3,
81.2, 80.8, 60.8, 50.5, 31.8, 26.2, 24.5, 24.4, 14.6. Rf 0.25 (EtOAc/
hexane 1:9). MS (ESI) m/z (rel intensity) 237.2 ([M + Na]+, 100).
HRMS-ESI (m/z) [M + Na]+ calcd for C11H18NaO4 237.1103, found
237.1104. Trans: 1H NMR (400 MHz; CDCl3) δH 4.83 (d, J ) 5.6
Hz, 1H), 4.72 (t, J ) 5.2 Hz, 1H), 4.12 (q, J ) 7.6 Hz, 2H), 2.90
(d, J ) 7.2 Hz, 1H), 2.15-2.05 (m, 1H), 1.93-1.70 (m, 3H), 1.44
(s, 3H), 1.31-1.23 (m, 6H). 13C NMR (125 MHz, CDCl3) δC 174.0,
110.2, 83.2, 81.4, 60.9, 50.9, 31.8, 27.0, 26.0, 24.4, 14.5. Rf 0.39
(EtOAc/hexane, 1:9). MS (ESI) m/z (rel intensity) 237.2 ([M +
Na]+, 100). HRMS-ESI (m/z) [M + Na]+ calcd for C11H18O4Na
237.1103, found 237.1102.
5-(((3aR,4S,6aS)-2,2-Dimethyltetrahydro-3aH-cyclopenta[d][1,3]-
dioxol-4-yl)methylthio)-1-phenyl-1H-tetrazole (14). The optically pure
ethyl ester-containing bicycle 13 (145 mg, 0.68 mmol) was
dissolved in Et2O (1 mL). In a separate flask, LiAlH4 (36 mg, 0.95
mmol) was suspended in Et2O (4 mL) and the mixture was cooled
to 0 °C. The solution of 13 was transferred to the reaction flask via
syringe along with an Et2O rinse. Vigorous bubbling was observed
1
thioether precursor of 12 as a yellow oil (1.5 g, 88%). H NMR
(500 MHz; CDCl3) δH 7.60-7.53 (m, 5H, Ar-H), 4.70 (t, J ) 5.0
Hz, 1H), 4.43 (t, J ) 5.5 Hz, 1H), 3.36 (dd, J ) 13.1, 6.5 Hz, 1H),
3.27 (dd, J ) 13.1, 7.0 Hz, 1H), 2.46-2.44 (m, 1H), 2.11-2.08
(m, 1H), 1.88-1.82 (m, 2H), 1.59-1.56 (m, 1H), 1.43 (s, 3H),
1.28 (s, 3H). 13C NMR (125 MHz, CDCl3) δC 154.3, 133.9, 130.4,
130.0, 124.1, 110.4, 85.1, 80.8, 45.3, 35.1, 31.0, 28.2, 26.6, 24.3,
22.2. Rf 0.45 (EtOAc/hexane 3:7). MS (ESI) m/z (rel intensity):
355.1 ([M + Na]+, 100).
J. Org. Chem. Vol. 74, No. 2, 2009 623

Majumdar et al.
upon addition. The ice bath was removed and the reaction allowed
to stir at rt for 1 h, at which point TLC analysis indicated the
reaction was complete (hexanes-EtOAc 2:1; 13, Rf 0.59, HCl then
KMnO4, the desired hydroxymethyl derivative, Rf 0.14, HCl then
KMnO4). The reaction was worked up according to the Fieser and
Fieser protocol.36 The mixture was cooled to 0 °C and H2O (36
µL), 15% NaOH (36 µL), and H2O (105 µL) were added in that
order. The mixture was stirred for 45 min during which time the
gray solid turned to a white solid. Filtration through Celite with
EtOAc and removal of the solvent in vacuo produced 123 mg of
clear oil. Purification by flash chromatography (1 × 3 in. silica;
hexanes-EtOAc 2:1 f 1:1) yielded pure hydroxymethyl derivative
(114 mg, 98%) as a clear, colorless oil: [R]22D + 9.7 (c 1.0, CHCl3).
(91 mg, 98%) as a colorless oil. [R]24D +13.4 (c 1.0, CHCl3). FTIR
(film, NaCl) 1498, 1340, 1153, 1044 cm-1. 1H NMR (500 MHz) δ
7.66 (m, 2 H), 7.59 (m, 3 H), 4.66 (t, J ) 5.4 Hz, 1 H), 4.48 (dd,
J ) 5.7, 1.6 Hz, 1 H), 3.77 (dd, J ) 7.4, 14.6 Hz, 1 H), 3.66 (dd,
J ) 7.0, 14.6 Hz, 1 H), 2.72 (m, 1 H), 2.16 (m, 1 H), 1.87 (m, 1
H), 1.79 (m, 1 H), 1.62 (m, 1 H), 1.41 (s, 3 H), 1.24 (s, 3 H). 13C
NMR (125 MHz) δ 153.7, 133.1, 131.6, 129.8, 125.2, 110.9, 85.0,
80.2, 57.5, 39.8, 30.9, 28.6, 26.5, 24.3. MS (ESI) m/z 387.1 (MNa+,
100). HRMS (ESI) calcd for C16H20N4NaO4S 387.1103 (MNa+),
found 387.1102.
(S,E)-Methyl 2-(Bis(tert-butoxycarbonyl)amino)-6-((3aS,4S,6aR)-
2,2-dimethyltetra-hydro-3aH-cyclopenta[d][1,3]dioxol-4-yl)hex-5-
enoate (15). KHMDS in toluene (164 µL 0.5M, 82 µmol) was added
dropwise to a solution of the sulfone 12 (25 mg, 68 µmol) in THF
(3 mL) in an atmosphere of argon at -78 °C. The resulting yellow
solution was stirred for 60 min followed by the dropwise addition
of a solution of methyl (S)-5-oxo-2-di-tert-butoxycarbonylamino-
pentaneoate (7; 43 mg, 123 µmol) in THF (2 mL). Approximately
15 min after the addition of the aldehyde, the yellow color became
lighter. The reaction mixture was stirred for 1 h at -78 °C, warmed
to rt, and stirred for an additional 20 h. The reaction mixture was
quenched with satd. NH4Cl solution (20 µL), stirred for 5 min,
and diluted with ether (10 mL), then the layers were separated and
the organic layer was washed with water (10 mL). The aqueous
layer was extracted with ether (3 × 15 mL), the combined organic
layer was dried over anhydrous Na2SO4, and the solvent was
removed under reduced pressure to afford a colorless oil that was
purified by flash chromatography (EtOAc/hexane 1:9) to give the
olefin derivative 15 as an oil (23 mg, 69%). Diastereomeric ratio
(E:Z) ) 9:1. 1H NMR (500 MHz; CDCl3) δH 5.47-5.39 (m, 1H),
5.37-5.31 (m, 0.9H), 5.23 (t, J ) 10 Hz, 0.1H), 4.90-4.83 (m,
1H), 4.66-4.62 (m, 1H), 4.40-4.38 (m, 0.9H), 4.29-4.26 (m,
0.1H), 3.71 (s, 3H), 2.93-2.90 (m, 0.1H), 2.66-2.64 (m, 0.9H),
2.20-2.13 (m, 1H), 2.06-1.72 (m, 4H), 1.79-1.72 (m, 2H),
1.53-1.41 (m, 22H), 1.29 (s, 3H). 13C NMR (75 MHz, CDCl3) δC
171.4, 152.2, 131.2, 129.3, 109.6, 85.9, 83.1, 57.5, 52.2, 47.6, 31.2,
29.8, 29.3, 28.2, 28.0, 26.4, 24.1. Rf 0.73 (EtOAc/hexane 3:7). MS
(ESI) m/z (rel intensity) 506.2 ([M + Na]+, 100). HRMS-ESI (m/
z) calcd for C25H41NO8Na [M + Na]+ 506.2730, found 506.2731.
(S)-2-((S,E)-5-(Bis(tert-butoxycarbonyl)amino)-6-methoxy-6-oxo-
hex-1-enyl)pentanedioic Acid (16). Acetonide 15 (17 mg, 35 µmol)
was dissolved in 50% AcOH (15 mL) at 0 °C and brought to rt,
then the solution was stirred at rt for 15 h. The solvent was removed
under reduced pressure. The crude reaction product (16 mg) was
purified by flash chromatography (EtOAc/hexane 2:3) to afford the
corresponding vicinal diol as a white solid (15 mg, 96%). Diol: 1H
NMR (500 MHz; CDCl3) δH 5.52-5.46 (m, 1H), 5.33-5.29 (m,
1H), 4.92-4.85 (m, 1H), 4.12-4.07 (m, 1H), 3.71 (s, 3H),
3.61-3.54 (m, 1H), 3.24 (br s, 0.8H), 2.88-2.82 (m, 0.3H),
2.61-2.48 (m, 2H), 2.27-1.87 (m, 6H), 1.70-1.63 (m, 1H), 1.50
(s, 18H), 1.32-1.24 (m, 1H). 13C NMR (125 MHz, CDCl3) δC
171.5, 152.4, 133.6, 133.1, 130.4, 130.3, 83.3, 83.2, 79.6, 78.7,
72.6, 72.4, 57.6, 56.9, 52.2, 47.2, 42.1, 30.4, 30.4, 29.8, 29.1, 28.8,
28.0, 27.4, 27.0. Rf 0.19 (EtOAc/hexane 2:3). MS (ESI) m/z (rel
intensity) 466.2 ([M + Na]+, 100).
1
FTIR (film, NaCl) 3425, 1041 cm-1. H NMR (500 MHz) δ 4.60
(t, J ) 5.5 Hz, 1 H), 4.40 (d, J ) 5.8 Hz, 1 H), 3.42 (dd, J ) 7.1,
10.8 Hz, 1 H), 3.38 (dd, J ) 8.2, 10.8 Hz, 1 H), 2.15 (m, 1 H),
2.62 (br s, 1 H), 1.91 (m, 1 H), 1.76 (m, 1 H), 1.68 (m, 1 H), 1.43
(m, 1 H), 1.40 (s, 3 H), 1.25 (s, 3 H). 13C NMR (125 MHz) δ
110.0, 83.3, 81.0, 63.1, 48.2, 31.2, 26.6, 25.3, 24.2. MS (ESI) m/z
(rel intensity) 227.2 ([M + Na + MeOH]+, 100), 195.2 ([M +
Na]+, 78). HRMS-ESI (m/z) [M + Na]+ calcd for C9H16NaO3
195.0997, found 195.1001.
The 1-hydroxymethyl derivative (104 mg, 0.60 mmol) was
dissolved in THF (1 mL) under argon before Ph3P (236 mg, 0.90
mmol) and 1-phenyl-1H-tetrazole-5-thiol (160 mg, 0.90 mmol) were
added. In a separate flask, DIAD (213 µL, 1.08 mmol) was
dissolved in THF (4 mL) to give a yellow solution. The reaction
mixture was cooled to 0 °C and the DIAD solution was added
dropwise. The yellow color persisted approximately 1 s after each
drop was added before the solution returned to colorless. After
approximately 75% of the DIAD had been added, the yellow color
remained. After 30 min of reaction, TLC analysis (hexanes-EtOAc
1:1) indicated the alcohol (Rf 0.29) had been consumed and the
sulfide (Rf 0.55) had formed. The reaction was diluted with Et2O
(50 mL) and washed with sat. NaHCO3 (20 mL). The aqueous layer
was repeatedly extracted with Et2O (3 × 40 mL) and the combined
organic layer was washed with brine (30 mL) and dried (Na2SO4).
The solvent was removed by rotary evaporation to yield 816 mg
of yellow oil that was purified by flash chromatography (Isco
CombiFlashRETRIEVEwithRediSep10gcolumn;hexanes-EtOAc
5:1; Rf 0.12) to afford 184 mg of clear, colorless oil that was pure
sulfide (93% yield). [R]24D +9.3 (c 1.0, CHCl3). FTIR (film, NaCl)
1500, 1043 cm-1. 1H NMR (500 MHz) δ 7.51 (m, 5 H), 4.64 (t, J
) 5.3 Hz, 1 H), 4.38 (d, J ) 5.7 Hz, 1 H), 3.29 (dd, J ) 13.0, 7.3
Hz, 1 H), 3.21 (dd, J ) 13.0, 9.2 Hz, 1 H), 2.03 (m, 1 H), 2.40 (m,
1 H), 1.77 (m, 1 H), 1.51 (m, 1 H), 1.37 (s, 3 H), 1.21 (s, 3 H). 13
C
NMR (125 MHz) δ 154.1, 133.6, 130.1, 129.8, 123.8, 110.0, 84.8,
80.5, 45.0, 34.8, 30.7, 27.7, 26.4, 24.1. MS (ESI) m/z 355.1 (MNa+,
100). HRMS (ESI) calcd for C16H20N4NaO2S 355.1205 (MNa+),
found 355.1209.
The sulfide (85 mg, 0.26 mmol) was dissolved in EtOH (95%,
1 mL) and the mixture was cooled to 0 °C. In a separate flask,
(NH4)6Mo7O24 ·4H2O (63 mg, 0.051 mmol) was dissolved in H2O2
(30%, 465 µL, 4.10 mmol) at 0 °C to give a yellow solution. The
oxidant solution was added to the sulfide and the reaction monitored
by TLC (hexanes-EtOAc 3:1; sulfide, Rf 0.23, UV; 14, Rf 0.23,
UV; sulfoxide, Rf 0.06, UV). The starting material and product have
identical Rf values. Therefore, the reaction progress was evaluated
based on the appearance and then disappearance of the sulfoxide
intermediate. Additionally, reaction progression was indicated by
the voluminous precipitate that formed during the last 6 h of the
24 h reaction period. The mixture was diluted with brine (20 mL)
and extracted with EtOAc (3 × 50 mL). The combined organic
layers were dried (Na2SO4) then filtered, and the solvent was
removed in vacuo to afford 113 mg of clear oil. Flash chromatog-
raphy (1 × 3 in. silica; hexanes-EtOAc 9:1 f 3:1) provided 14
The vicinal diol (19 mg, 42 µmol) was dissolved in THF/H2O
(v/v, 9/1, 3 mL) at rt and the mixture was stirred. NaIO4 (47 mg,
219 µmol) was added to the above solution and stirring was
continued for 20 h during which time a white precipitate was
observed in the reaction mixture. TLC analysis showed the complete
consumption of the starting material and formation of the desired
product. The solvent was removed under reduced pressure, then
the crude product was dissolved in DCM (12 mL) and washed with
water (10 mL). The aqueous layer was extracted with DCM (3 ×
10 mL). The combined organic layer was dried over Na2SO4, then
then solvent was removed under reduced pressure and dried under
1
vacuum to obtain the desired R,γ-dialdehyde (17 mg). H NMR
(500 MHz; CDCl3) δH 9.76 (br t, 1H), 9.54 (d, J ) 1.5 Hz, 1H),
5.69-5.63 (m, 1H), 5.31-5.27 (m, 1H), 4.88-4.85 (m, 1H), 3.71
(36) Fieser, L. F.; Fieser, M. Reagents for organic synthesis; John Wiley &
Sons, Inc.: New York, 1967; Vol. 1.
624 J. Org. Chem. Vol. 74, No. 2, 2009

Synthesis of Isopeptide Epoxide Peptidomimetics
(s, 3H), 3.01-2.96 (m, 1H), 2.50 (t, J ) 7 Hz, 2H), 2.17-1.23 (m,
24H). 13C NMR (125 MHz, CDCl3) δC 201.4, 200.8, 171.2, 152.2,
152.1, 135.9, 124.3, 83.2, 57.3, 55.0, 52.2, 40.9, 29.6, 29.4, 28.0,
20.7. MS (ESI) m/z (rel intensity) 496.1 ([M + Na + MeOH]+,
100).
continued at -78 °C for 3 h. The reaction solution was warmed to
rt and stirring was continued for 12 h during which time a white
precipitate was observed. The reaction was quenched with saturated
NH4Cl solution (200 µL). The aqueous layer was extracted with
ether (3 × 15 mL). The combined organic layer was dried over
anhydrous Na2SO4 and the solvent was removed under reduced
pressure to afford a colorless oil that was purified by flash
chromatography (EtOAc/hexane 1:9). Olefin derivative 18 was
obtained as an oil (11 mg, 71%). Diastereomeric ratio (E:Z) ) 4:1.
1H NMR (500 MHz; CDCl3) δH 5.47-5.31 (m, 1.8H), 5.23 (t, J )
11 Hz, 0.2H), 4.90-4.83 (m, 1H), 4.67-4.62 (m, 1H), 4.38 (d, J
) 5.5 Hz, 0.8H), 4.28 (d, J ) 5.5 Hz, 0.2H), 3.71 (s, 3H),
2.93-2.90 (m, 0.2H), 2.70-2.62 (m, 0.8H), 2.26-1.70 (m, 8H),
1.64-1.40 (m, 21H), 1.30-1.18 (m, 3H). 13C NMR (100 MHz,
CDCl3) δC 171.3, 152.1, 131.1, 130.7, 129.8, 129.2, 109.5, 86.6,
85.8, 83.0, 80.7, 80.6, 57.6, 57.4, 52.1, 47.4, 43.3, 31.4, 31.2, 29.9,
29.8, 29.6, 29.4, 29.3, 28.0, 27.9, 26.4, 24.3, 24.0. Rf 0.73 (EtOAc/
hexane 3:7). MS (ESI) m/z (rel intensity) 506.2 ([M + Na]+, 100).
HRMS-ESI (m/z) calcd for C25H41NO8Na [M + Na]+ 506.2730,
found 506.2731.
The R,γ-dialdehyde (17 mg, 38 µmol) was dissolved in THF (2
mL) at 0 °C followed by addition of 2-methyl-2-butene (0.9 mL
2M). In a separate vial NaClO2 (18 mg, 199 µmol), NaH2PO4 (5
mg, 41 µmol), and 2-methyl-2-butene (1.0 mL 2M) were dissolved
in t-BuOH/H2O (v/v, 7/3, 4 mL) at 0 °C. This solution was added
dropwise to the reaction mixture at 0 °C, and the stirring was
continued at 0 °C for 3 h. Excess NaClO2 was quenched by addition
of 2-propanol (1.0 mL). Solvents were removed under reduced
pressure and the crude product was purified by flash chromatog-
raphy (EtOAc/hexane 2:3, few drops of AcOH) to give the desired
1
olefinated glutaric acid 16 as an oil (14 mg, 78%, two steps). H
NMR (500 MHz; CDCl3) δH 5.63-5.57 (m, 1H), 5.50-5.39 (m,
1H), 4.88-4.84 (m, 1H), 4.04 (br s, 2H), 3.72 (br s, 3H), 3.37-3.36
(m, 1H), 3.01-2.97 (m, 1H), 2.32 (t, J ) 8 Hz, 2H), 2.23-1.79
(m, 6H), 1.63-1.46 (m, 18H). 13C NMR (125 MHz, CDCl3) δC
176.6, 175.9, 171.7, 152.3, 132.7, 131.8, 128.4, 83.9, 83.8, 58.0,
57.8, 52.4, 52.4, 31.7, 30.2, 29.8, 29.4, 28.0, 28.0, 27.4, 27.6, 27.4,
24.6. Rf 0.14 (EtOAC/hexane 2:3). MS (ESI) m/z (rel intensity)
496.1 ([M + Na]+, 100).
(R)-2-((S,E)-5-(Bis(tert-butoxycarbonyl)amino)-6-methoxy-6-oxo-
hex-1-enyl)pentanedioic Acid (19). Acetonide 18 (75 mg, 155 µmol)
was dissolved in 50% AcOH (40 mL) at rt, and then the solution
was stirred at rt for 11 h. The solvent was removed under reduced
pressure. The crude reaction product (72 mg) was purified by flash
chromatography (EtOAc/hexane 2:3) to afford the corresponding
(3S,8S,E)-Trimethyl 8-Aminooct-4-ene-1,3,8-tricarboxylate (17).
The diacid 16 (39 mg, 82 µmol) was dissolved in MeOH (4 mL)
and the mixture was cooled to 0 °C, followed by dropwise addition
of Me3SiCHN2 (4.0 mL of a 2 M solution in diethyl ether) under
continuous stirring. Initially when Me3SiCHN2 was added, the
solution became yellow and then turned colorless. When all the
Me3SiCHN2 was added the solution remained yellow. Stirring was
continued at rt for 16 h, after which the reaction was quenched by
addition of a few drops of HOAc. Solvent was removed under
reduced pressure and the crude product (46 mg) was purified by
flash chromatography (EtOAc/hexane 2:3) to give the corresponding
1
vicinal diol as an oil (57 mg, 83%). Diol: H NMR (400 MHz;
CDCl3) δH 5.50-5.42 (m, 1H), 5.36-5.30 (m, 1H), 4.96-4.92 (m,
0.8H), 4.84-4.81 (m, 0.2H), 4.14-4.06 (m, 1H), 3.71 (s, 3H),
3.63-3.59 (m, 1H), 3.35 (br d, 0.8H), 2.91-2.82 (m, 0.3H), 2.61
(br s, 0.7H), 2.54-2.45 (m, 1H), 2.34-1.64 (m, 7H), 1.50 (br s,
18H), 1.39-1.19 (m, 1H). 13C NMR (100 MHz, CDCl3) δC 171.4,
152.1, 133.4, 133.3, 130.7, 130.3, 83.5, 83.4, 79.8, 78.8, 72.6, 72.6,
57.4, 57.2, 52.2, 52.2, 47.3, 42.1, 30.3, 30.3, 29.8, 29.5, 29.3, 28.0,
28.0, 27.5, 27.2, 24.4. Rf 0.19 (EtOAc/hexane 2:3). MS (ESI) m/z
(rel intensity) 466.2 ([M + Na]+, 100).
1
trimethyl ester derivative as an oil (35 mg, 85%). H NMR (500
MHz; CDCl3) δH 5.60-5.55 (m, 1H), 5.45-5.34 (m, 1H),
4.87-4.83 (d, J ) 8.5 Hz, 1H), 3.71-3.66 (m, 9H), 3.03-2.98
(m, 1H), 2.38-1.80 (m, 8H), 1.49 (s, 18H). 13C NMR (125 MHz,
CDCl3) δC 174.1, 173.4, 171.2, 152.1, 133.0, 127.6, 83.2, 83.1,
57.6, 52.2, 51.9, 51.6, 48.2, 31.4, 29.6, 29.1, 28.0, 27.2. Rf 0.66
(EtOAc/hexane 1:1). MS (ESI) m/z (rel intensity): 424.2 ([M -
Boc + Na]+, 100).
The trimethyl ester (44 mg, 87 µmol) was dissolved in THF (4
mL) and the mixture was cooled to 0 °C, followed by addition of
Et3SiH (2 mL) under stirring condition. Then CF3COOH (6 mL)
was added dropwise at 0 °C and stirring was continued at rt for
16 h. Solvent was removed under reduced pressure and the crude
product was purified by flash chromatography (EtOAc/hexane 4:1,
ca. 2% Et3N) to give the free amine derivative 17 as an oil (21 mg,
80%). 1H NMR (500 MHz; CDCl3) δH 5.59-5.53 (m, 1H),
5.46-5.35 (m, 1H), 3.72 (s, 3H), 3.66-3.64 (m, 6H), 3.45-3.43
(m, 1H), 3.04-2.99 (m, 1H), 2.33-2.29 (m, 2H), 2.17-2.02 (m,
3H), 1.88-1.78 (m, 2H), 1.65-1.59 (m, 1H). 13C NMR (125 MHz,
CDCl3) δC 176.4, 174.2, 173.4, 133.0, 133.3, 127.7, 127.5, 53.8,
52.01, 51.95, 51.91, 51.6, 48.2, 42.9, 34.2, 31.4, 31.2, 28.6, 27.4,
27.2, 23.8. Rf 0.17 (EtOAc/haxane 4:1). MS (ESI) m/z (rel intensity)
302.2 ([M + H]+, 100), 324.2 [M + Na]+.
(S,E)-Methyl 2-(Bis(tert-butoxycarbonyl)amino)-6-((3aR,4R,6aS)-
2,2-dimethyltetra-hydro-3aH-cyclopenta[d][1,3]dioxol-4-yl)hex-5-
enoate (18). Sulfone 14 (11 mg, 30 µmol) was dissolved in THF
(0.5 mL) under an atmosphere of argon and the mixture was cooled
to -78 °C under stirring condition. KHMDS solution (80 µL, 0.5
M, 40 µmol) was added dropwise to the sulfone at -78 °C and
stirring was continued for 15 min. The reaction mixture was then
warmed to -50 °C and stirring was continued for 1 h, after which
it was again cooled to -78 °C. A solution of methyl (S)-2-di-tert-
butoxycarbonylaminopentaneoate (7; 20 mg, 57 µmol) in THF (1.3
mL) was added dropwise into the reaction mixture and stirring was
The vicinal diol (96 mg, 216 µmol) was dissolved in THF/H2O
(v/v, 9/1, 3 mL) at rt and NaIO4 (232 mg, 1.08 mmol) was added
with stirring. After 1 h and 30 min, additional NaIO4 (231 mg,
1.07 mmol) was added and stirring was continued for 45 min during
which time a white precipitate was observed in the reaction mixture.
TLC analysis showed the complete consumption of the starting
material and formation of the desired product. The solvent was
removed under reduced pressure, then the crude product was
dissolved in DCM (25 mL) and washed with water (20 mL). The
aqueous layer was extracted with DCM (3 × 20 mL). The combined
organic layer was dried over Na2SO4, the solvent was removed
under reduced pressure, and the layer was dried under vacuum to
1
obtain the crude R,γ-dialdehyde as an oil (93 mg). H NMR (300
MHz; CDCl3) δH 9.77 (br t, 1H), 9.54 (d, J ) 1.5 Hz, 1H),
5.68-5.60 (m, 1H), 5.32-5.21 (m, 1H), 4.88-4.81 (m, 1H), 3.71
(s, 3H), 3.40-3.30 (m, 0.2H), 3.01-2.96 (m, 0.8H), 2.60-1.23
(m, 18H). 13C NMR (100 MHz, CDCl3) δC 201.4, 200.7, 171.2,
152.1, 135.9, 124.9, 83.2, 57.3, 55.0, 52.2, 40.8, 29.5, 29.4, 28.2,
27.9, 20.7. MS (ESI) m/z (rel intensity) 496.1 ([M + Na +
MeOH]+, 100). HRMS-ESI (m/z) [M + Na + MeOH]+ calcd for
C23H39NO9Na 496.2523, found 464.2260 [M + Na]+.
The crude R,γ-dialdehyde (93 mg, 0.21 mmol) was dissolved in
THF (4 mL) at 0 °C followed by addition of 2-methyl-2-butene (3
mL, 2 M). In a separate vial NaClO2 (196 mg, 2.16 mmol),
NaH2PO4 (52 mg, 0.43 mmol), and 2-methyl-2-butene (1.0 mL 2M)
were dissolved in t-BuOH/H2O (v/v, 1/1, 2 mL) at 0 °C. This
solution was added dropwise to the reaction mixture at 0 °C, and
the stirring was continued at 0 °C. After 2 h a solution of NaClO2
(98 mg, 1.08 mmol), NaH2PO4 (25 mg, 0.20 mmol), and 2-methyl-
2-butene (0.8 mL, 2 M) in t-BuOH/H2O (v/v, 1/1, 1 mL) was added
to the reaction mixture. Stirring was continued at 0 °C for an
additional 1 h and 15 min. Excess NaClO2 was quenched by
addition of 2-propanol (4.0 mL) and stirring the reaction for 30
J. Org. Chem. Vol. 74, No. 2, 2009 625

Majumdar et al.
min. Solvents were removed under reduced pressure and the crude
product was purified by flash chromatography (EtOAc/hexane 1:1,
ca. 2% AcOH) to give the desired olefinated glutaric acid 19 as an
oil (94 mg, 94%, two steps). 1H NMR (500 MHz; CDCl3) δH
5.63-5.58 (m, 1H), 5.53-5.44 (m, J ) 15.3 Hz, 1H), 4.86-4.83
(m, 1H), 3.71 (s, 3H), 3.43-3.39 (m, 0.2H), 3.07-3.03 (m, 0.8H),
2.51-1.82 (m, 8H), 1.53 (s, 18H). 13C NMR (100 MHz, CDCl3)
δC 179.4, 178.7, 171.3, 152.2, 133.2, 127.4, 83.5, 83.3, 57.5, 52.2,
48.1, 31.6, 29.4, 29.1, 28.0, 27.4. Rf 0.14 (EtOAC/hexane 2:3). MS
(ESI) m/z (rel intensity) 496.1 ([M + Na]+, 100). HRMS-ESI (m/
z) [M + Na]+ calcd for C22H35NO10Na 496.2159, found 496.2159.
(3R,8S,E)-Trimethyl 8-Aminooct-4-ene-1,3,8-tricarboxylate (20).
The diacid 19 (91 mg, 192 µmol) was dissolved in MeOH (4 mL)
and the mixture was cooled to 0 °C, followed by dropwise addition
of Me3SiCHN2 (5.0 mL of a 2 M solution in diethyl ether) under
continuous stirring. Initially when Me3SiCHN2 was added, the
solution became yellow and then turned colorless. When all the
Me3SiCHN2 was added the solution remained yellow. Stirring was
continued at rt for 17 h, after which the reaction was quenched by
addition of a few drops of HOAc. Solvent was removed under
reduced pressure and the crude product (107 mg) was purified by
flash chromatography (EtOAc/hexane 2:30) to give the correspond-
added to the above solution followed by dropwise addition of
(CF3CO)2O (40 µL, 290 µmol) at -70 °C. Stirring was continued
for 1.5 h after which the reaction temperature was allowed to rise
to 0 °C. When TLC analysis showed the complete consumption of
the starting material (10 h), solvent was removed and the crude
product was purified by flash chromatography (EtOAc/hexane 1:1)
1
to give pure N-Tfa derivative as an oil (20 mg, 62%). H NMR
(500 MHz; CDCl3) δH 7.96-7.87 (m, 2H), 7.35-7.34 (m, 2H),
6.72-6.71 (br d, 0.8H), 5.61-5.54 (m, 1H), 5.47-5.35 (m, 1H),
4.85-4.79 (m, 1H), 3.80-3.79 (s, 3H), 3.67-3.62 (m, 6H), 3.37
(br s, 3H), 3.04-2.99 (m, 1H), 2.32-1.80 (m, 8H). 13C NMR (125
MHz, CDCl3) δC 174.0, 173.3, 172.8, 165.7, 134.5, 132.3, 132.2,
131.9, 128.7, 128.6, 128.3, 128.2, 127.8, 127.7, 52.6, 52.4, 52.3,
52.2, 52.1, 52.0, 51.7, 48.13, 43.06, 39.6, 32.0, 31.7, 31.4, 31.2,
28.3, 27.3, 27.1, 23.5. Rf 0.44 (EtOAc/hexane 3:2). MS(ESI) m/z
(rel intensity) 553.17 ([M + Na]+, 100).
The N-Tfa-N-Me-pAB-olefin isostere, prepared as described
directly above (15 mg, 11 µmol), was dissolved in DCM (2 mL),
and the mixture was cooled to 0 °C under stirring condition. Then
NaHCO3 (7 mg, 84 µmol) was added followed by mCPBA (12
mg, 70 µmol) at 0 °C. Stirring was continued at rt for 24 h. TLC
and MS analysis showed the complete consumption of the starting
material and formation of the product. The reaction mixture was
filtered through Celite and Na2SO4 with a Pasteur pipet and the
crude product was purified by flash chromatography (EtOAc/hexane
7:3, 1% Et3N) to give the desired epoxide 23 as an oil (12 mg,
1
ing trimethyl ester derivative as an oil (80 mg, 83%). H NMR
(500 MHz; CDCl3) δH 5.62-5.53 (m, 1H), 5.45-5.35 (m, 1H),
4.88-4.83 (m, 1H), 3.71-3.66 (s, 9H), 3.39-3.34 (m, 0.2H),
3.03-2.98 (m, 0.8H), 2.38-1.80 (m, 8H), 1.50 (s, 18H). 13C NMR
(100 MHz, CDCl3) δC 174.2, 173.4, 171.3, 152.2, 133.0, 127.7,
83.2, 83.1, 57.5, 52.2, 51.9, 51.6, 48.2, 31.4, 29.6, 29.6, 29.1, 28.0,
27.2. Rf 0.66 (EtOAc/hexane 1:1). MS (ESI) (rel intensity) m/z 424.2
([M - Boc + Na]+, 100). HRMS-ESI (m/z) [M + Na]+ calcd for
C24H39NO10Na 524.2472, found 524.2485.
The trimethyl ester (44 mg, 87 µmol) was dissolved in THF (4
mL) and the mixture was cooled to 0 °C, followed by addition of
Et3SiH (2 mL) with stirring, after which excess CF3COOH was
added dropwise at 0 °C and stirring was continued at rt for 16 h.
Solvent was removed under reduced pressure and the crude product
was purified by flash chromatography (EtOAc/hexane 4:1, ca. 2%
Et3N) to give the free amine derivative 20 as an oil (22 mg, 84%).
1H NMR (500 MHz; CDCl3) δH 5.59-5.53 (m, 1H), 5.46-5.5.35
(m, 1H), 3.73-3.67 (s, 9H), 3.45-3.42 (m, 1H), 3.04-2.99 (m,
1H), 2.33-2.17 (m, 2H), 2.16-2.02 (m, 3H), 1.88-1.79 (m, 2H),
1.65-1.57 (m, 1H). 13C NMR (100 MHz, CDCl3) δC 176.4, 174.2,
173.4, 133.1, 127.7, 53.8, 52.0, 51.9, 51.6, 48.2, 42.9, 34.2, 31.4,
29.7, 28.6, 27.2, 23.8. Rf 0.17 (EtOAc/haxane 4:1). MS (ESI) m/z
(rel intensity): 302.2 ([M + H]+, 100).
1
80%). H NMR (500 MHz; CDCl3) δH 7.97-7.88 (m, 2H), 7.35
(br s, 2H), 4.91-4.82 (m, 1H), 3.81-3.62 (s, 9H), 3.37 (s, 3H),
3.18-2.83 (m, 3H), 2.48-1.83 (m, 8H). 13C NMR (125 MHz,
CDCl3) δC 172.9, 172.8, 172.4, 172.3, 165.8, 156.6, 143.6, 134.3,
128.65, 128.58, 127.6, 58.7, 58.5, 58.4, 57.6, 57.3, 57.1, 57.0, 56.1,
52.7, 52.2, 52.1, 51.77, 51.68, 47.18, 47.15, 39.6, 31.4, 31.3, 29.7,
29.3, 28.7, 27.6, 25.0, 23.9. 19F NMR (376 MHz, CDCl3) δF -67.01,
-70.04, -75.68. Rf 0.51 (EtOAc/hexane 4:1). MS (ESI) m/z 569.1
(rel intensity) ([M + Na]+, 100). HRMS-ESI (m/z) [M + Na]+
calcd for C24H29F3N2O9Na 569.1723, found 569.1723.
(3R,8S,E)-Trimethyl 8-(4-(Methylamino)benzamido)oct-4-ene-
1,3,8-tricarboxylate (24). Et3N (1.5 mL) was added to a stirred
solution of the N-methyl p-aminobenzoic acid HOBt ester (21; 59
mg, 219 µmol) and amine derivative 20 (22 mg, 73 µmol) in THF
(1.5 mL) at 0 °C in an atmosphere of argon. The reaction was
warmed to rt and stirring was continued for 48 h. The solvent was
removed under reduced pressure and the crude product was purified
by flash chromatography (EtOAc/hexane 1:1) to give pure product
24 as an oil (21 mg, 68%). 1H NMR (500 MHz; CDCl3) δH
7.72-7.66 (m, 2H), 6.58-6.56 (m, 2H), 6.50-6.49 (br d, 1H),
5.57-5.50 (m, 1H), 5.50-5.30 (m, 1H), 4.90-4.80 (m, 1H),
3.77-3.66 (s, 9H), 3.42-3.40 (m, 0.2H), 3.02-2.98 (m, 0.9H),
2.88 (s, 3H), 2.33-1.80 (m, 8H). 13C NMR (100 MHz, CDCl3) δC
174.1, 173.4, 173.0, 166.8, 152.1, 132.7, 132.6, 128.9, 128.8, 127.9,
127.8, 121.7, 111.4, 111.3, 52.44, 52.39, 51.97, 51.93, 51.90, 51.70,
51.63, 48.2, 32.3, 31.4, 31.1, 30.3, 30.0, 29.7, 28.3, 27.24, 27.15.
Rf 0.61 (EtOAc/hexane 4:1). MS (ESI) m/z (rel intensity) 457.2
([M + Na]+, 100). HRMS-ESI m/z [M + Na]+ calcd for
C22H30N2O7Na 457.1951, found 457.1942.
(3S,8S,E)-Trimethyl 8-(4-(Methylamino)benzamido)oct-4-ene-
1,3,8-tricarboxylate (22). Et3N (0.12 mL) was added to a stirred
solution of the N-methyl p-aminobenzoic acid HOBt ester (21; 24
mg, 86 µmol) and amine derivative 17 (26 mg, 86 µmol) in THF
(4 mL) at 0 °C in an atmosphere of argon. The reaction was warmed
to rt and stirring was continued for 26 h, after which time additional
21 (24 mg) and Et3N (0.2 mL) were added and stirring continued
for 24 h. The solvent was removed under reduced pressure and the
crude product was purified by flash chromatography (EtOAc/hexane
1
1:1) to give pure product 22 as an oil (27 mg, 73%). H NMR
(400 MHz; CDCl3) δH 7.71-7.65 (m, 2H), 6.64-6.51 (m, 3H),
5.61-5.52 (m, 1H), 5.45-5.33 (m, 1H), 4.84-4.79 (m, 1H), 4.16
(br s, 1H), 3.77-3.63 (s, 9H), 3.40-3.34 (m, 0.2H), 3.03-2.97
(m, 0.8H), 2.87 (s, 3H), 2.32-1.80 (m, 8H). 13C NMR (100 MHz,
CDCl3) δC 174.1, 173.4, 173.3, 166.8, 152.1, 132.7, 132.6, 128.9,
128.8, 127.8, 127.6, 121.7, 111.3, 52.4, 52.4, 52.0, 52.0, 51.9, 51.6,
48.2, 43.0, 32.3, 32.2, 31.4, 31.2, 30.2, 28.3, 27.3, 27.1. Rf 0.61
(EtOAc/hexane 4:1). MS (ESI) m/z (rel intensity) 457.2 ([M + Na]+,
100).
(2S)-Dimethyl 2-(3-((S)-4-Methoxy-4-oxo-3-(4-(2,2,2-trifluoro-
N-methylacetamido)benzamido)butyl)oxiran-2-yl)pentanedioate (23).
The N-Me-pAB-olefinic isostere 22 (27 mg, 62 µmol) was dissolved
in THF (2 mL) in an atmosphere of argon, and the mixture was
cooled to -70 °C and stirred. Then Et3N (500 µL, 3.6 mmol) was
(2R)-Dimethyl 2-(3-((S)-4-methoxy-4-oxo-3-(4-(2,2,2-trifluoro-
N-methylacetamido)benzamido)butyl)oxiran-2-yl)pentanedioate (25).
The N-Me-pAB-olefinic isostere 24 (21 mg, 47 µmol) was dissolved
in THF (3.5 mL) in an atmosphere of argon, and the mixture was
cooled to 0 °C and stirred. Then Et3N (68 µL, 470 µmol) was added
to the above solution followed by dropwise addition of (CF3CO)2O
(67 µL, 470 µmol) at 0 °C. Stirring was continued for 10 h. TLC
analysis showed the complete consumption of the starting material.
Solvent was removed and the crude product was purified by flash
chromatography (EtOAc/hexane 1:1) to give pure N-Tfa derivative
1
as an oil (19 mg, 76%). H NMR (500 MHz; CDCl3) δH 7.97 (d,
J ) 8.5 Hz, 0.4H), 7.90-7.88 (m, 1.7H), 7.35 (d, J ) 7.5 Hz, 2H),
7.10 (br d, 0.2H), 6.73 (br d, 0.9H), 5.64-5.54 (m, 1H), 5.48-5.36
(m, 1H), 4.85-4.75 (m, 1H), 3.80-3.63 (s, 9H), 3.37 (br s, 3H),
626 J. Org. Chem. Vol. 74, No. 2, 2009

Synthesis of Isopeptide Epoxide Peptidomimetics
3.04-2.99 (m, 1H), 2.35-1.80 (m, 8H). 13C NMR (100 MHz,
CDCl3) δC 174.1, 173.3, 172.79, 172.76, 165.7, 132.3, 132.2, 132.1,
128.8, 128.6, 128.3, 128.2, 127.1, 127.6, 64.2, 52.7, 52.6, 52.3,
52.2, 52.05, 51.95, 51.73, 51.67, 48.1, 42.7, 39.6, 32.0, 31.7, 31.4,
31.0, 28.3, 27.24, 27.15, 23.8. Rf 0.44 (EtOAc/hexane 3:2). MS
(ESI) m/z (rel intensity): 553.1 ([M + Na]+, 100). HRMS-ESI (m/
z) [M + Na]+ calcd for C24H29F3N2O8Na 553.1774, found 553.1774.
The N-Tfa-N-Me-pAB-olefin isostere, prepared as described
directly above (18 mg, 33 µmol), was dissolved in DCM (3 mL),
and the mixture was cooled to 0 °C under stirring condition. Then
NaHCO3 (18 mg, 132 µmol) was added followed by mCPBA (19
mg, 85 µmol) at 0 °C. The reaction was allowed to warm to rt.
Stirring was continued at rt for 48 h. TLC and MS analysis showed
the complete consumption of the starting material and formation
of the product. The solvent was removed under reduced pressure.
The residue was dissolved in EtOAc (20 mL) and washed with
NaHCO3 solution (3 × 10 mL) and water (2 × 10 mL). The organic
layer was dried over Na2SO4 and the crude product was purified
by flash chromatography (EtOAc/hexane 3:2, 1% Et3N) to give the
7.69-7.66 (m, 2H), 6.59-6.57 (m, 2H), 4.47-4.40 (m, 1H), 2.80
(s, 3H), 2.65-1.59 (m, 8H). 13C NMR (100 MHz, CDCl3) δC 182.8,
182.7, 179.64, 179.60, 176.0, 169.7, 169.6, 161.7, 154.5, 141.1,
134.0, 130.1, 122.4, 122.2, 119.4, 117.4, 115.5, 112.2, 112.0, 69.2,
56.8, 56.7, 56.5, 52.7, 50.5, 50.02, 49.98, 45.1, 39.0, 37.4, 31.0,
30.3, 30.1. HRMS-ESI (m/z) [M - H]- calcd for C19H23N2O8
407.1460, found 407.1469).
(2R)-2-(3-((S)-3-Carboxy-3-(4-(methylamino)benzamido)prop-
yl)oxiran-2-yl)pentanetrioic Acid (5). The epoxide derivative 25
(2 mg, 3.66 µmol) was dissolved in MeOH (2 mL) under stirring
condition and LiOH ·H2O (0.14 mL of a 0.1 M solution, 14 µmol)
was added dropwise at rt. The reaction mixture was continuously
stirred at rt, during which time additional LiOH was added in
portions while the reaction progress was monitored carefully by
MS analysis, which confirmed formation of the desired tricarboxy-
late derivative 5. After 7 days, solvent was removed under reduced
pressure and high vacuum to afford the desired product as a solid
(3 mg, quantitative yield). 1H NMR (600 MHz; CD3OD) δH
7.70-7.66 (m, 2H), 6.58 (d, J ) 8.4 Hz, 2H), 4.48-4.40 (m, 1H),
2.80 (s, 3H), 2.63-1.59 (m, 8 H). 13C NMR (100 MHz, CDCl3)
δC 182.8, 182.7, 176.9, 169.7, 161.7, 154.5, 141.1, 138.0, 130.1,
122.4, 112.2, 112.0, 69.2, 56.7, 52.8, 50.5, 50.02, 49.97, 39.0, 37.4,
34.8, 31.0, 30.3, 30.1. HRMS-ESI (m/z) [M - H]- calcd for
C19H23N2O8 407.1460, found 407.1454.
1
desired epoxide 25 as an oil (18 mg, 97%). H NMR (500 MHz;
CDCl3) δH 7.96-7.89 (m, 2H), 7.35-7.30 (m, 2H), 7.08-7.05 (m,
0.4H), 6.87 (br d, 0.7H), 6.59 (br d, 0.2H), 4.90-4.83 (m, 1H),
3.81-3.58 (s, 9H), 3.37 (s, 3H), 3.19-2.83 (m, 3H), 2.47-1.44
(m, 8H). 13C NMR (100 MHz, CDCl3) δC 173.0, 172.9, 172.5,
172.4, 165.8, 128.8, 128.6, 127.7, 59.0, 58.7, 58.4, 57.7, 57.34,
57.26, 57.1, 57.0, 52.8, 52.2, 51.8, 51.7, 47.19, 47.16, 43.5, 39.6,
31.4, 31.3, 30.6, 28.8, 28.6, 27.6, 25.0, 24.3, 23.9. 19F NMR (376
MHz, CDCl3) δF -67.02, -70.05. Rf 0.51 (EtOAc/hexane 4:1).
MS (ESI) m/z (rel intensity): 569.1 ([M + Na]+, 100). HRMS-ESI
(m/z) [M + Na]+ calcd for C24H29F3N2O9Na 569.1723, found
569.1711.
(2S)-2-(3-((S)-3-Carboxy-3-(4-(methylamino)benzamido)prop-
yl)oxiran-2-yl)pentanetrioic Acid (4). The epoxide derivative 23
(3 mg, 5.49 µmol) was dissolved in MeOH (2 mL) under stirring
condition and LiOH ·H2O (0.21 mL of a 0.1 M solution, 22 µmol)
was added dropwise at rt. The reaction mixture was continuously
stirred at rt, during which time additional LiOH was added in
portions while the reaction progress was monitored carefully by
MS analysis, which confirmed formation of the desired tricarboxy-
late derivative 4. After 7 days, solvent was removed under reduced
pressure and high vacuum to provide the desired product as a solid
(5 mg, quantitative yield). 1H NMR (600 MHz; CD3OD) δH
Acknowledgment. This research was supported by a grant
from the National Cancer Institute (CA28097). We thank Mr.
Jeff Kampf for determining the crystal structure of cis-14.
Supporting Information Available: General Experimental
Procedures, synthesis of 5-(((3aS,4S,6aR)-2,2-dimethyltetrahy-
dro-3aH-cyclopenta[d][1,3]dioxol-4-yl)methylsulfonyl)-1-phen-
yl-1H-tetrazole (cis-14), ORTEP plot of compound cis-14 and
X-ray crystallographic data, synthesis of the HOBt ester of 4-(N-
methyl)aminobenzoic acid (21), synthesis of N-Tfa, methyl ester
precursors, S3 and S6, and of terminal epoxide derivatives, 2b
and 1b, and 1H NMR and 13C NMR spectra for all compounds
described in the Experimental Section. This material is available
free of charge via the Internet at http://pubs.acs.org.
JO801907P
J. Org. Chem. Vol. 74, No. 2, 2009 627