Synthesis of a hydroxyethylene isostere of the tripeptide Arg-Gly-Leu via a convergent acyl-like radical addition strategy

Article abstract of DOI:10.1021/jo0505775

A hydroxyethylene isostere of the tripeptide Arg-Gly-Leu, representing an important fragment of a novel cyclic-peptide-based uPA inhibitor, was synthesized in few steps employing as the key step a samarium diiodide promoted coupling of either the 4-thiopy

Full text of DOI:10.1021/jo0505775

Synthesis of a Hydroxyethylene Isostere of the Tripeptide
Arg-Gly-Leu via a Convergent Acyl-like Radical Addition Strategy
Christina M. Jensen,† Karl B. Lindsay,† Peter Andreasen,‡ and Troels Skrydstrup*,†
Department of Chemistry, University of Aarhus, Langelandsgade 140, 8000 Aarhus C, Denmark, and
Department of Molecular Biology, University of Aarhus, Gustav Wied’s Vej 10C, 8000 Aarhus C, Denmark
ts@chem.au.dk
Received March 22, 2005
A hydroxyethylene isostere of the tripeptide Arg-Gly-Leu, representing an important fragment of
a novel cyclic-peptide-based uPA inhibitor, was synthesized in few steps employing as the key step
a samarium diiodide promoted coupling of either the 4-thiopyridyl ester of NR-Fmoc- or NR-Cbz-
protected L-ornithine with the N-acryloyl derivative of L-leucine methyl ester. Epimerization under
the coupling conditions at the chiral center in the R-position to the ketone was demonstrated not
to take place. A stereoselective reduction of the Cbz-protected aminoketone obtained from this radical
reaction was promoted by the same single-electron reducing agent in the presence of methanol
providing the syn-amino alcohol with a diastereoselectivity of 85:15. With the use of lithium tri-
tert-butoxyaluminum hydride in methanol, the corresponding anti-isomer was obtained almost
exclusively. Subsequent elaboration of the ornithine moiety in the anti-isomer by introduction of
the guanidine group followed by hydrolysis of the C-terminal ester bond and protection of the alcohol
as its tert-butyldimethylsilyl ether provided the desired tripeptide mimic. The long reaction times
required for the radical addition reactions with Nδ-Boc-L-ornithine (up to 5 days) led to a short
study where a series of 4-thiopyridyl esters of Cbz-protected amino acids were reacted with two
acrylates. Whereas Nδ-Boc-L-ornithine, alanine, phenylalanine, proline, and leucine all provided
the aminoketone in 43-79% yield, valine only afforded traces of the coupling product.
Introduction
Because of the pathophysiological functions of uPA,
there has been extensive interest in generating specific
uPA inhibitors. Several classes of synthetic low molecular
weight inhibitors of uPA are known.4 The binding modes
for many of these have been characterized by X-ray
crystal structure analysis. In general, such inhibitors
have as a main constituent an Arg analogue inserting
into the S1 pocket of the active site of uPA. With low
molecular weight inhibitors, the challenge is to achieve
selectivity for uPA over other serine proteases with P1
Arg specificity, by utilizing small variations between the
various proteases within the S1 pocket and its immediate
surroundings.4a,b,5-8 With the most promising of such
inhibitors, a series of 6-halo-5-amidinoindole and 6-halo-
Urokinase-type plasminogen activator (uPA) is a serine
protease which catalyses the conversion of the serine
protease zymogen, plasminogen, into the active protease
plasmin through cleavage of an Arg-Val bond in plas-
minogen. uPA-catalyzed plasmin generation has been
implicated in the turn-over of extracellular matrix pro-
teins in tissue remodeling.1 Abnormal expression of uPA
is believed to be responsible for tissue damage in several
pathological conditions. In particular, abnormal expres-
sion of uPA is a key to a malignant tumor’s capacity for
invasion through basement membranes.2,3
* Corresponding author.
† Department of Chemistry.
(2) Andreasen, P. A.; Egelund, R.; Petersen, H. H. Cell. Mol. Life
Sci. 2000, 57, 25.
(3) (a) Duffy, M. J. Blood Coagul. Fibrinolysis 1990, 1, 681. (b) Duffy,
M. J. Clin. Cancer Res. 1996, 2, 613.
‡ Department of Molecular Biology.
(1) Andreasen, P. A.; Kjøller, L.; Christensen, L.; Duffy, M. J. Int.
J. Cancer 1997, 72, 1.
10.1021/jo0505775 CCC: $30.25 © 2005 American Chemical Society
Published on Web 08/20/2005
7512
J. Org. Chem. 2005, 70, 7512-7519

Hydroxyethylene Isostere of Arg-Gly-Leu
SCHEME 1
5-amidinobenzimidazole compounds, Ki values in the
nanomolar range and 100-2000-fold selectivities for uPA
over plasmin, trypsin, thrombin, tPA, and fXa were
achieved.9
To find new principles for inhibiting the enzymatic
activity of serine proteases, one of us (P.A.) has screened
phage-displayed random peptide repertoires with uPA as
bait. The most frequent of the isolated phage clones
contained the disulfide bridge-constrained dodecapeptide
sequence Cys-Ser-Trp-Arg-Gly-Leu-Glu-Asn-His-Arg-
Met-Cys, which we have designated upain-1. A chemi-
cally synthesized peptide with this sequence inhibited
uPA with a Ki of 30 µM and, importantly, exhibited a
selectivity by factors of 50-500 over a number of other
serine proteases, including trypsin, tPA, plasmin, throm-
bin, activated protein C, fXa, fVIIa, and plasma kal-
likrein.10 This selectivity is comparable or superior to the
most selective of the previously published small weight
uPA inhibitors.4 The fact that a reasonable affinity and
an excellent selectivity were achieved in only a single
selection step suggests that affinity maturation by a
combined use of molecular biological methods and organic
synthesis may eventually lead to highly specific and
efficient uPA inhibitors and may give mechanistic infor-
mation about serine protease catalysis and inhibition.
One of the possibilities in the chemical modification of
peptide-based enzyme inhibitors is a stabilization of the
scissile peptide bond toward nontarget proteases by
replacing it with a nonamide bond. This will also allow
studies of the importance of the nature of that bond for
the mechanism of inhibition. For this reason, we set out
to examine stable synthetic analogues of upain-1 with
the incorporation of a hydroxyethylene isostere of the
sequence Arg-Gly-Leu. In this report, we present our
preliminary synthetic work on the construction of such
a modified cyclic inhibitor dealing with the rapid prepa-
ration of the hydroxyethylene isostere of this tripeptide
sequence.11 Our synthetic approach exploits our newly
discovered acyl-like radical addition reaction as the key
carbon-carbon bond-forming step between two amino
acid precursors, which provides a short and effective
synthesis of the tripeptide analogue.
Results and Discussion
We have previously reported the samarium diiodide12
promoted coupling of thioester derivatives of Cbz-
protected amino acids to acrylamides and acrylates as a
viable approach to 4-ketoamides and -esters via a formal
acyl radical addition reaction in yields up to 90% (Scheme
1).13 The mechanism for this reaction is believed to
involve a SmI2-promoted reduction of the thioester 1 to
a ketyl radical type intermediate 2 followed by its radical
addition to the R,â-unsaturated amide or ester. A second
electron transfer from SmI2 then affords a samarium-
(III) enolate which eventually undergoes protonation
affording the aminoketone 3.
Selective reduction of the keto-functionality to either
of the two epimeric alcohols was also shown to be
feasible,14 revealing this two-step radical addition, ketone
reduction sequence as an interesting alternative for the
synthesis of peptides containing a hydroxyethylene isos-
(4) For some examples, see: (a) Wendt, M. D.; Rockway, T. W.;
Geyer, A.; McClellan, W.; Weitzberg, M.; Zhao, X.; Mantei, R.;
Nienaber, V. L.; Stewart, K.; Klinghofer, V.; Giranda, V. L. J. Med.
Chem. 2004, 47, 303. (b) Schweinitz, A.; Steinmetzer, T.; Banke, I. J.;
Arlt, M. J.; Stu¨rzebecher, A.; Schuster, O.; Geissler, A.; Giersiefen, H.;
Zeslawska, E.; Jacob, U.; Kru¨ger, A.; Stu¨rzebecher, J. J. Biol. Chem.
2004, 279, 33613. (c) Mackman, R. L.; Hui, H. C.; Breitenbucher, J.
G.; Katz, B. A.; Luong, C.; Martelli, A.; McGee, D.; Radika, K.; Sendzik,
M.; Spencer, J. R.; Sprengeler, P. A.; Tario, J.; Verner, E.; Wang, J.
Bioorg. Med. Chem. Lett. 2002, 12, 2019. (d) Tamura, S. Y.; Weinhouse,
M. I.; Roberts, C. A.; Goldman, E. A.; Masukawa, K.; Anderson, S. M.;
Cohen, C. R.; Bradbury, A. E.; Bernardino, V. T.; Dixon, S. A.; Ma, M.
G.; Nolan, T. G.; Brunck, T. K. Bioorg. Med. Chem. Lett. 2000, 10, 983.
(5) Mackman, R. L.; Katz, B. A.; Breitenbucher, J. G.; Hui, H. C.;
Verner, E.; Luong, C.; Liu, L.; Sprengeler, P. A. J. Med. Chem. 2001,
44, 3856.
(11) For examples on the application of hydroxyethylene isosteres
for the inhibition of a variety of proteases see ref 1 and: (a) Holladay,
M. W.; Rich, D. H. Tetrahedron Lett. 1983, 24, 4401. (b) Greenlee, W.
J. Med. Res. Rev. 1990, 10, 173. (c) Huff, J. R. J. Med. Chem. 1991,
34, 2305. (d) Vacca, J. P.; Dorsey, B. D.; Schleif, W. A.; Levin, R. B.;
McDaniel, S. L.; Darke, P. L.; Zugay, J.; Quintero, J. C.; Blahy, O. M.;
Roth, E.; Sardana, V. V.; Schlabach, A. J.; Graham, P. I.; Condra, J.
H.; Gotlib, L.; Holloway, M. K.; Lin, J.; Chen, I.-W.; Vastag, K.; Ostovic,
D.; Anderson, P. S.; Emini, E. A.; Huff, J. R. Proc. Natl. Acad. Sci.
U.S.A. 1994, 91, 4096. (e) Ghosh, A. K.; Shin, D.; Downs, D.; Koelsch,
G.; Lin, X.; Ermolieff, J.; Tang, J. J. Am. Chem. Soc. 2000, 122, 3522.
(f) Schearman, M. S.; Beher, D.; Clarke, E. E.; Lewis, H. D.; Harrison,
T.; Hunt, P.; Nadin, A.; Smith, A. L.; Stevenson, G.; Castro, J. L.
Biochemistry 2000, 39, 8698. (g) Hong, L.; Koelsch, G.; Lin, X.; Wu,
S.; Terzyan, S.; Ghosh, A. K.; Zhang, X. C.; Tang, J. Science 2000, 290,
150.
(12) (a) Edmonds, D. J.; Johnston, D.; Procter, D. J. Chem. Rev.
2004, 104, 3371. (b) Kagan, H. B. Tetrahedron 2003, 59, 10351. (c)
Steel, P. G. J. Chem. Soc., Perkin Trans. 1 2001, 2727. (d) Krief, A.;
Laval, A.-M. Chem. Rev. 1999, 99, 745. (e) Molander, G. A.; Harris, C.
R. Tetrahedron 1998, 54, 3321. (f) Molander, G. A.; Harris, C. R. Chem.
Rev. 1996, 96, 307.
(13) (a) Blakskjær, P.; Høj, B.; Riber, D.; Skrydstrup, T. J. Am.
Chem. Soc. 2003, 125, 4030.
(14) Mikkelsen, L. M.; Jensen, C. M.; Høj, B.; Blakskjær, P.;
Skrydstrup, T. Tetrahedron 2003, 59, 10541.
(6) Sperl, S.; Jacob, U.; Arroyo de Prada, N.; Sturzebecher, J.;
Wilhelm, O. G.; Bode, W.; Magdolen, V.; Huber, R.; Moroder, L. Proc.
Natl. Acad. Sci. U.S.A. 2000, 97, 5113.
(7) Spencer, J. R.; McGee, D.; Allen, D.; Katz, B. A.; Luong, C.;
Sendzik, M.; Squires, N.; Mackman, R. L. Bioorg. Med. Chem. Lett.
2002, 12, 2023.
(8) Zeslawska, E.; Jacob, U.; Schweinitz, A.; Coombs, G.; Bode, W.;
Madison, E. J. Mol, Biol. 2003, 328, 109.
(9) (a) Katz, B. A.; Sprengeler, P. A.; Luong, C.; Verner, E.; Elrod,
K.; Kirtley, M.; Janc, J.; Spencer, J. R.; Breitenbucher, J. G.; Hui, H.;
McGee, D.; Allen, D.; Martelli, A.; Mackman, R. L. Chem. Biol. 2001,
8, 1107. (b) Mackman, R. L.; Hui, H. C.; Breitenbucher, J. G.; Katz, B.
A.; Luong, C.; Martelli, A.; McGee, D.; Radika, K.; Sendzik, M.; Spencer,
J. R.; Sprengeler, P. A.; Tario, J.; Verner, E.; Wang, J. Bioorg. Med.
Chem. Lett. 2002, 12, 2019. (c) Katz, B. A.; Elrod, K.; Verner, E.;
Mackman, R. L.; Luong, C.; Shrader, W. D.; Sendzik, M.; Spencer, J.
R.; Sprengeler, P. A.; Kolesnikov, A.; Tai, V. W.-F.; Hui, H. C.;
Breitenbucher, J. G.; Allen, D.; Janc, J. W. J. Mol. Biol. 2003, 329, 93.
(d) Katz, B. A.; Luong, C.; Ho, J. D.; Somoza, J. R.; Gjerstad, E.; Tang,
J.; Williams, S. T.; Verner, E.; Mackman, R. L.; Young, W. B.;
Sprengeler, P. A.; Chan, H.; Mortara, K.; Janc, J. W.; McGrath, M. E.
J. Mol. Biol. 2004, 344, 527.
(10) Hansen, M.; Andreasen, P. A. Manuscript in preparation.
J. Org. Chem, Vol. 70, No. 19, 2005 7513

Jensen et al.
SCHEME 2
SCHEME 3
FIGURE 1. Retrosynthetic analysis for the synthesis of the
tripeptide analogue of the Arg-Gly-Leu sequence.
tere,15 including the target tripeptide analogue of Arg-
Gly-Leu. With these results in mind, the synthesis of the
desired tripeptide mimic 4 or 5 would then simply require
the coupling of an acyl-like radical equivalent of a
suitably functionalized arginine 6 to the N-acryloyl
derivative of leucine 7, as depicted in Figure 1, followed
by a stereoselective reduction.
Preparation of the two amino acid coupling partners
for the radical addition reaction is illustrated in Scheme
2. To avoid a potential intervention of arginine’s guani-
dine group in the SmI2-mediated radical addition step,
we commenced the synthesis with an arginine precursor
represented by L-ornithine, introducing the guanidine
functionality at a later stage of the synthesis. Hence, the
thioester 9 was synthesized on a 7 mmol scale from NR-
Cbz-Nδ-Boc-L-ornithine (8) and 4-mercaptopyridine via
an EDC coupling affording the desired product in an
improved yield of 67% yield compared to our earlier
report on a smaller reaction scale (1.3 mmol).14 Reaction
of acryloyl chloride with the methyl ester of L-leucine 10
in the presence of triethylamine in dichloromethane
provided in a nondramatic fashion the acrylamide 7 in
84% yield.16
Additionally, in our earlier publication, we reported
that the coupling of the thioester 9 with the acrylamide
7 required a reaction time of 80 h in order to provide the
ketone 11 in a 67% yield (Scheme 3).14 This was some-
what surprising since similar coupling reactions per-
formed with the thioesters of Cbz-protected phenylala-
nine or leucine afforded coupling products in similar
yields but with greatly reduced reaction times (24 h).13
When this reaction time was employed for the coupling
of 7 and 9, the product of radical addition 11 was only
isolated in a 30% yield along with unreacted starting
material. For this study, we extended the reaction time
considerably to 5 days, which furnished 11 in a good 86%
yield on a 0.3 mmol scale (79% on a 0.8 mmol scale). A
supplementary point regarding the workup procedure for
this C-C bond-forming reaction should be made. In our
original procedure, a sodium hypochlorite solution was
used to oxidize the thiol and disulfide byproducts ob-
tained upon quenching of the reaction mixture with
oxygen and ammonium chloride, thus facilitating their
separation from the desired ketone.13 Application of this
procedure to the workup of 11 led to considerable
amounts of selective N-chlorination of the ornithine side
chain.17 Omission of this oxidative workup provided the
pure ketone after careful column chromatography.
(15) For representative examples of approaches to the synthesis of
hydroxyethylene isosters, see: (a) Brewer M.; James, C. A.; Rich, D.
H. Org. Lett. 2004, 6, 4779. (b) Haug, B. E.; Rich, D. H. Org. Lett. 2004,
6, 4783. (c) Brewer M.; Rich, D. H. Org. Lett. 2001, 3, 945. (d) Benedetti,
F.; Berti, F.; Garau, G.; Martinuzzi, I.; Norbedo, S. Eur. J. Org. Chem.
2003, 1973. (e) Hom, R. K.; Gailunas, A. F.; Mamo, S.; Fang, L. Y.;
Tung, J. S.; Walker, D. E.; Davis, D.; Thorsett, E. D.; Jewett, N. E.;
Moon, J. B.; John, V. J. Med. Chem. 2004, 47, 158. (f) Våbenø, J.; Lejon,
T.; Nielsen, C. U.; Steffansen, B.; Chen, W.; Ouyang, H.; Borchardt,
R. T.; Luthman, K. J. Med. Chem. 2004, 47, 1060. (g) Tossi, A.;
Benedetti, F.; Norbedo, S.; Skrbec, D.; Berti, F.; Romeo, D. Bioorg. Med.
Chem. 2003, 11, 4719.
(16) The synthesis of an N-Fmoc-arginine(Pbf) thioester was at-
tempted, but the desired product turned out to be unstable. Similar
compounds are reported to cyclize, see: (a) Hojo, K.; Maeda, M.; Iguchi,
S.; Smith, T.; Okamoto, H.; Kawaski, K. Chem. Pharm. Bull. 2000,
48, 1740. (b) Tamura, S. Y.; Goldman, E. A.; Brunck, T. K.; Ripka, W.
C.; Semple, J. E. Bioorg. Med. Chem. Lett. 1997, 7, 331.
7514 J. Org. Chem., Vol. 70, No. 19, 2005

Hydroxyethylene Isostere of Arg-Gly-Leu
SCHEME 4
FIGURE 2. Section of 1H NMR spectrum of (a) 11, (b) 13,
and (c) an approximately 1:1 mixture of 11 and 13.
To determine whether 11 had undergone any epimer-
ization at the sensitive chiral center adjacent to the
ketone under the reaction conditions or workup, a sample
of the diastereomer 13 was prepared in a similar fashion
from the coupling of the thioester 9 with the N-acryloyl
derivative 12 of D-leucine (Scheme 3). The two diaster-
eomeric ketones 11 and 13 were compared on the basis
of their 1H NMR spectra, and Figure 2 displays the region
containing the proton signals from the ethylene spacer
for (a) 11, (b) 13, and (c) as a mixture of the two. Analysis
of the spectra recorded after the coupling of 9 with 7
revealed that if epimerization had occurred, the presence
of the C3-epimer was less than 5%, representing the limit
of detection. Hence, it can be concluded that these SmI2-
induced coupling reactions proceed under sufficiently
mild conditions without significant levels of epimerization
R to the ketone functionality.
The reactivity discrepancy observed between orni-
thine and phenylalanine or leucine was unanticipated
and is undoubtedly related to the intervention of the
ornithine side chain. A small study was initiated to
examine in more detail the influence of the amino acid
side chains on these radical addition reactions. Hence,
the coupling reaction between six amino acid thioesters
9 and 14-18 and simple radical acceptors, such as butyl
and methyl acrylate was investigated, the results of
which are depicted in Scheme 4. All reactions were
performed with excess samarium diiodide (3 equiv), and
acrylate (3 equiv) with reaction times of 48 h. As
indicated, the coupling yields for the thioesters 9 and 14-
17 of Cbz-protected Nδ-Boc-L-ornithine, alanine, pheny-
lalanine, proline, and leucine, interestingly all provided
the coupling products 19-23 in acceptable to good yields.
However, the thiopyridyl ester of Cbz-protected valine
proved ineffective for the transformation of 18 to 24 with
isolation of both recovered starting material (approxi-
mately 50%) and the aldehyde 25 (10%). These results
suggests that R-substitution in the side chain of the
amino acid thioester leads to considerable steric cones-
tion in the transition state of the radical addition step
where ultimately either hydrogen abstraction or reduc-
tion of the intermediate ketyl-like radical precedes
the radical addition step, allowing for the formation of
the aldehyde 25 upon workup. However, importantly
no reaction discrepancy was noted in the coupling
step between ornithine and the other reactive amino
acids, implying that the slow reactivity between 9 and 7
may be a result of the combination of these two deriva-
tives.
The next step requires a stereoselective reduction of
the ketone function. Various protocols exist for achieving
this reduction step of R-aminoketones with high diaster-
eomeric excess.18,19 Luthman and co-workers have previ-
ously demonstrated the ability of (S)-alpine-hydride
and (S)-enantride to be excellent reagents for achiev-
ing Felkin-Anh control20 with essentially exclusive for-
mation of the syn-amino alcohol.18 We have also applied
this protocol successfully with other di- and tripeptide
mimics,14,21 whereas application of the more common
reducing agents was either low-yielding or displayed little
facial selectivity. However, the aminoketone 11 proved
quickly to deviate considerably from the results obtained
from these earlier reduction studies, as illustrated in
Table 1. Subjecting 11 to (S)-alpine-hydride, provided
essentially a 1:1 mixture of diastereomeric amino alcohols
26 and 27 as determined by HPLC analysis of the crude
reaction mixtures (entries 1 and 2). Undoubtedly, the
ornithine side chain must again be intervening with this
bulky borohydride reagent, and a potential solution
would be to examine the influence of the nitrogen-
protecting group on diastereoselectivities with this reduc-
ing agent. Whereas as this study would require us to
modify the starting materials for this synthesis, instead
a small study was initiated in an attempt to identify a
more selective reducing reagent for this particular sub-
strate 11.
(18) Våbenø, J.; Brisander, M.; Lejon, T.; Luthman, K. J. Org. Chem.
2002, 67, 9186.
(19) Hoffman, R. V.; Maslouh, N.; Cervantes-Lee, F. J. Org. Chem.
2002, 67, 1045.
(20) (a) Che´rest, M.; Felkin, H.; Prudent, N. Tetrahedron Lett. 1968,
18, 2199. (b) Che´rest, M.; Felkin, H. Tetrahedron Lett. 1968, 18, 2205.
(c) Anh, N. T.; Eisenstein, O. Nouv. J. Chem. 1977, 1, 61.
(21) Lindsay, K. B.; Skrydstrup, T. Unpublished results with the
tripeptide examples.
(17) Curini, M.; Epifano, F.; Marcotullio, M. C.; Rosati, O.; Tsadjout,
A. Synlett 2000, 813.
J. Org. Chem, Vol. 70, No. 19, 2005 7515

Jensen et al.
TABLE 1. Survey of Reducing Agents for the
SCHEME 5
Stereoselective Reduction of the r-Amino Ketone 11
reducing
reagent
temp
°C
entry
solvent
time
yield anti/syna
1
2
3
4
5
(S)-alpine-hydride THF
(S)-alpine-hydride THF
-78 5 h
-78 16 h
-78 24 h
-78 20 h
ndb
42:58
44:56
36:64
80:20
15:85
ndb
DIBALH
THF
THF
THF
30%c
71%c
61%d
LS-selectride
SmI2 (3 equiv)
MeOH (1.2 equiv)
SmI2 (3 equiv)
MeOH (2 equiv)
SmI2 (3 equiv)
MeOH (10 equiv)
NaBH4
20
20
20
5 h
6
7
THF
THF
6.5 h
ndb
19:81
33:67
10 min ndb
8
9
10
11
12
MeOH -78 3.5 h
EtOH -78 3.5 h
93%c
66%c
57:43
65:35
69:31
86:14
NaBH4
NaBH4, CeCl3
NaBH4, CeCl3
LiAlH(O-t-Bu)3
(6 equiv)
MeOH -78 45 min 98%c
EtOH -78 45 min 97%c
MeOH -78 16 h
60%d >95:5
a The anti/syn ratio was determined by HPLC. b Not deter-
mined. c Yield obtained from 1H NMR spectra with the remaining
material being unreacted ketone. d Isolated yield.
trate the importance of the size of the reactive alkoxy-
borohydride species on the selectivity of the reduction.
Finally, completion of the tripeptide mimic required a
selective deprotection of the nitrogen-containing side
chain of the ornithine unit followed by introduction of
the guanidine group, the transformations of which were
attempted with the anti-isomer 26. Hence, removal of the
Boc group with trifluoroacetic acid and treatment of the
primary amine with N,N′-di-Boc-N′′-triflylguanidine as
reported by Goodman and co-workers afforded the argi-
nine derivative 28 in a yield of 74% (Scheme 5).25 To
provide a more appropriately functionalized mimic, 28
was submitted to basic hydrolysis followed by an alcohol
protection step to afford the silyl ether 29 in 53% yield.
We next examined the possibility of exploiting other
nitrogen-protecting groups other than CBz for the radi-
cal-coupling step. To this end, the coupling efficiencies
of three phenylalanine thioesters possessing three dif-
ferent nitrogen-protecting groups with acrylamide 7 were
compared in a model study as shown in Scheme 6.
Whereas the Cbz-protected derivative 16 afforded a high
yield of the aminoketone 32 in the SmI2-promoted radical
addition reaction as previously reported,13 the corre-
sponding Boc-protected amino acid 30 essentially and
unexpectedly led to no conversion to the corresponding
Boc-protected aminoketone 33. Fortunately, a coupling
reaction performed with the Fmoc-protected phenylala-
nine 31 led to the desired coupling product 34 in a
satisfactory yield of 76%, demonstrating the compatibility
of this protecting group with these radical addition
reactions. Finally, in light of these results, a coupling
DIBALH reductions usually furnish the Felkin-Anh
product, but with the ketone 11 the selectivity was only
moderate (entry 3). Selectrides have previously been
reported to predominantly yield the syn-isomer,18 or show
no selectivity,19 but in the case of 11, employing LS-
selectride (entry 4), the major product was the anti-
isomer. On the other hand, reduction with the single-
electron transferring reagent, samarium diiodide, in the
presence of a proton source provided an alternative route
for accessing the syn-isomer 27. In particular, it was
found that subjecting 11 to SmI2 with 1.2 equiv of
methanol at 20 °C (entry 5) afforded the syn-amino
alcohol 27 in a yield of 61% where the diastereomeric
ratio attained a value of 85:15.22 It is important that the
reaction time does not exceed the 5 h used for this
reduction as prolonged subjection to these conditions led
to extensive decomposition and hence reduced yields of
27. Increasing the number of equivalents of MeOH had
an adverse effect on the diastereoselectivity of the reduc-
tion (entries 6 and 7), yet with 20 equiv the reaction time
was reduced to only 10 min.
Of the various hydride-based reducing agents tested,
only the lithium tri-tert-butoxyaluminum hydride proved
to be sufficiently selective, though in favor of the anti-
product 26 with an anti/syn ratio greater than 95:5 (entry
12). The stereochemical assignments were tentatively
made based on earlier reduction studies of aminoketones
with this reagent by the Hoffman group.19 In the NaBH4
reductions (entries 8-11) an increase in the anti-selectiv-
ity in favor of the Cram chelate product 2623 is observed
on going from MeOH to EtOH as the solvent and also
upon the addition of CeCl3.24 These results nicely illus-
attempt with the NR-Fmoc-Nδ-Boc-L-ornithine derivative
35 was made. Quite pleasingly, the tripeptide analogue
36 was generated in a 43% yield after a reaction time of
4 days.
To conclude these studies, we examined the possibility
of incorporating a fourth amino acid directly into the
(22) Keck, G. E.; Wager, C. A.; Sell, T.; Wager, T. T. J. Org. Chem.
1999, 64, 2172.
(23) Cram, D. J.; Elhafez, F. A. A. J. Am. Chem. Soc. 1952, 74, 5828.
(24) Gemal, A. L.; Luche, J.-L. J. Am. Chem. Soc. 1981, 103, 5454.
(25) (a) Feichtinger, K.; Zapf, C.; Sings, H. L.; Goodman, M. J. Org.
Chem. 1998, 63, 3804. (b) Baker, T. J.; Luedtke, N. W.; Tor, Y.;
Goodman, M. J. Org. Chem. 2000, 65, 9054.
7516 J. Org. Chem., Vol. 70, No. 19, 2005

Hydroxyethylene Isostere of Arg-Gly-Leu
SCHEME 6
SCHEME 7
involved a samarium diiodide promoted carbon-carbon
bond formation followed by a stereoselective ketone
reduction mediated by the same single-electron reducing
agent for the obtention of the syn-amino alcohol. Alter-
natively, the anti-isomer was prepared by reduction with
lithium tri-tert-butoxyaluminum hydride. An investiga-
tion was undertaken to examine appropriate protecting
groups on the amino acid precursors such that the
eventual tripeptide fragment could be made adaptable
to solid-phase peptide synthesis. The Fmoc nitrogen-
protecting group was shown to be compatible with the
SmI2-induced radical addition reaction. Further work is
now underway where the results of this work will be
exploited to prepare a suitably protected tripeptide mimic
for the completion of the synthesis of a potential inhibitor
of the protease, uPA. This work will be reported in due
course.
peptide mimic before further elaboration on the solid-
phase synthesis. As illustrated in the Introduction, the
leucine residue is flanked by a glutamate unit at the
C-terminal end. We have commenced studies for prepar-
ing a similar hydroxyethylene mimic of this extended
peptide structure and have already examined the radical
coupling step between the model, phenyl alanine thioester
16, and the acryloyl derivative of the protected dipeptide
Leu-Glu 37 (Scheme 7). Under similar coupling condi-
tions as for the preparation of 11, only a 19% yield was
obtained for the tetrapeptide analogue 38. Repeated
attempts with longer reaction times did not improve the
coupling efficiency. Again, it is difficult to provide a
plausible explanation for this loss of reactivity, taking
into consideration our previous successful formation of
other tetrapeptide analogues using this radical addition
chemistry.13 It is becoming increasingly apparent that
this C-C bond-forming reaction is particularly sensitive
to the amino acid structures of both the radical donor
and acceptor.
Experimental Section
(2S)-2-Acryloylamino-4-methylpentanoic Acid Methyl
Ester (7): General Procedure for the Formation of
Acrylamides. The hydrochloride salt of L-Leu-OMe (2.20 g,
15.2 mmol) was transferred to a flame-dried flask, and the
flask was flushed with nitrogen. Dry dichloromethane (100
mL) and triethylamine (4.3 mL, 30.8 mmol) were added, and
the solution was cooled to 0 °C before distilled acryloyl chloride
(1.25 mL, 15.2 mmol) was added in a dropwise manner. The
mixture was allowed to warm to room temperature and left
stirring for 2 h. The reaction was quenched by addition of an
aqueous solution of NaHCO3, and the solution was extracted
with dichloromethane (3 times). The combined organic phases
were washed with saturated NaHCO3 and dried over MgSO4.
After concentration in vacuo, 2.59 g was obtained of the crude
product 7 (84% yield) as a yellow powder which was of
sufficient purity to be used in the subsequent transformations.
1H NMR (400 MHz, CDCl3) δ (ppm): 6.32 (dd, 1H, J ) 16.8,
1.2 Hz), 6.15 (dd, 1H, J ) 16.8, 10.0 Hz), 6.05 (br d, 1H, J )
6.8 Hz), 5.68 (dd, 1H, J ) 10.0, 1.2 Hz), 4.75 (dt, 1H, J ) 8.4,
4.8 Hz), 3.75 (s, 3H), 1.72-1.64 (m, 2H), 1.63-1.53 (m, 1H),
0.96 (d, 3H, J ) 6.0 Hz), 0.95 (d, 3H, J ) 6.4 Hz). 13C NMR
(100 MHz, CDCl3) δ (ppm): 174.0, 165.5, 130.6, 127.3, 52.5,
Conclusions
We have successfully applied our acyl-like radical
addition chemistry to a short synthesis of a hydroxyeth-
ylene isostere of the tripeptide sequence Arg-Gly-Leu,
with the eventual goal of incorporating this fragment into
the cyclic peptide based inhibitor of the urokinase plas-
minogen activator. The key steps for this synthesis
J. Org. Chem, Vol. 70, No. 19, 2005 7517

Jensen et al.
50.9, 41.8, 25.0, 23.0, 22.1. HRMS C10H17NO3 [M + Na+]:
product was obtained by column chromatography on silica gel
(20-50% EtOAc in pentane) giving 76% yield as a colorless
oil. 1H NMR (400 MHz, CDCl3) δ (ppm): 7.26 (m, 5H), 5.60 (d,
J ) 6.8 Hz, 1H), 5.08 (s, 2H), 4.64-4.73 (m, 1H), 3.38-4.46
(m, 1H), 3.64 (s, 3H), 3.05-3.17 (m, 2H), 2.86 (dt, J ) 18.4,
6.4 Hz, 1H), 2.51-2.78 (m, 3H), 1.89-1.99 (m, 1H), 1.40-1.65
(m, 3H), 1.42 (s, 9H). 13C NMR (100 MHz, CDCl3) δ (ppm):
207.3, 173.0, 156.2 (2C), 136.3, 128.6 (2C), 128.3 (2C), 79.3,
67.1, 59.4, 52.0, 40.0, 34.3, 28.7, 28.5, 27.6, 25.7. HRMS
C22H32N2O7 [M + Na+]: calculated, 459.2107; found, 459.2107.
(2S)-2-[(4R),(5S)-5-Benzyloxycarbonylamino-8-tert-bu-
toxycarbonyl-amino-4-hydroxy-octanoylamino]-4-meth-
ylpentanoic Acid Methyl Ester (26). Dry MeOH (2 mL) was
cooled to -78 °C before adding LiAlH(O-t-Bu)3 (118 mg, 0.46
mmol). A solution of ketone 11 (63 mg, 0.11 mmol) in MeOH
(1 mL) was added dropwise to the solution, and the reaction
mixture was stirred at -78 °C for 16 h. The reaction was
quenched with 10% citric acid, warmed to 20 °C, and extracted
3 times with EtOAc. The combined organic phases were
washed with water and brine, dried over Na2SO4, and the
solvent was removed in vacuo. Reverse phase HPLC analysis
of the crude mixture showed the desired alcohol 26 was
obtained in a >95:5 anti/syn ratio, and flash chromatography
on silica gel using pentane/ethyl acetate (2:3 to 1:5) afforded
37.9 mg (60% yield) of the product 26. 1H NMR (400 MHz,
CDCl3) δ (ppm): 7.35-7.32 (m, 5H), 6.22 (br d, 1H, J ) 6.8
Hz), 5.11 (br t, 1H, J ) 7.2 Hz), 5.08 (s, 2H), 4.66-4.56 (m,
2H), 3.89 (br s, 1H), 3.71 (s, 3H), 3.66-3.59 (m, 2H), 3.16-
3.07 (m, 2H), 2.48-2.36 (m, 2H), 1.81-1.49 (m, 9H), 1.43 (s,
9H), 0.92 (d, 6H, J ) 6.0 Hz). 13C NMR (100 MHz, CDCl3) δ
(ppm): 173.8, 173.7, 157.0, 156.2, 136.6, 128.6 (2C), 128.2 (3C),
79.2, 74.0, 66.9, 55.7, 52.5, 51.0, 41.5, 40.4, 33.1, 28.5 (5C),
26.8, 25.0, 22.9, 22.0. HRMS C28H45N3O8 [M +N a+]: calcu-
lated, 574.3104; found, 574.3107.
(2S)-2-[(4S),(5S)-5-Benzyloxycarbonylamino-8-tert-bu-
toxycarbonyl-amino-4-hydroxy-octanoylamino]-4-meth-
ylpentanoic Acid Methyl Ester (27). Ketone 11 (22 mg,
0.040 mmol) was flushed with argon before adding THF (2 mL)
and MeOH (2 µL, 0.05 mmol). SmI2 (1.7 mL, 0.1 M, 0.17 mmol)
was added dropwise to the solution, and the mixture was left
stirring at room temperature for 5 h. The reaction was
quenched by adding oxygen until the blue color disappeared
and the reaction mixture was yellow. Diethyl ether (5 mL) and
10% citric acid (0.5 mL) were added, and the mixture was
allowed to stir for 10 min. The solution was extracted with
Et2O (3 times), and the combined organic phases were washed
with Na2S2O3(aq) and brine, dried over Na2SO4, and concen-
trated in vacuo. Reverse phase HPLC showed the desired
alcohol 27 was obtained in a 85:15 syn/anti ratio, and flash
chromatography using pentane/ethyl acetate (3:1 to 5:1)
yielded 14.3 mg (65%) of 27. 1H NMR (400 MHz, CDCl3) δ
(ppm): 7.35-7.29 (m, 5H), 6.15 (br s, 1H), 5.16 (br d, 1H, J )
9.6 Hz), 5.10 (d, 1H, J ) 12.2 Hz), 5.06 (d, 1H, J ) 12.2 Hz),
4.66-4.56 (m, 2H), 4.10 (br s, 1H), 3.72 (s, 3H), 3.67-3.54 (m,
2H), 3.15-3.07 (m, 2H), 2.48-2.14 (m, 2H), 1.88-1.47 (m, 9H),
1.43 (s, 9H), 0.93 (d, 6H, J ) 6.0 Hz). 13C NMR (100 MHz,
CDCl3) δ (ppm): 173.8, 173.6, 157.0, 156.2, 136.7, 128.6 (2C),
128.2, 128.1 (2C), 79.2, 72.9, 66.8, 55.2, 52.5, 51.0, 41.6, 40.4,
33.5, 30.1, 30.0, 28.6 (3C), 26.8, 25.0, 22.9, 22.1. HRMS
C28H45N3O8 [M + Na+]: calculated, 574.3104; found, 574.3106.
(2S)-2-[(4R),(5S)-8-(2,3-bis(tert-Butoxycarbonyl)guani-
dino)-5-benzyloxycar bonylamino-4-hydroxy-octanoy-
lamino]-4-methylpentanoic Acid Methyl Ester (28). Amino
alcohol 26 (27 mg, 0.048 mmol) was treated with trifluoroacetic
acid (0.5 mL) and dichloromethane (2 mL) for 30 min before
evaporating the solvent off. To a solution of the crude product
and triethylamine (0.030 mL, 0.22 mmol) in dichloromethane
(2 mL) was added N,N′-di-Boc-N′′-triflylguanidine (21 mg,
0.053 mmol). The reaction mixture was left stirring for 2 h
and 15 min before adding dichloromethane. The organic phase
was washed with 2 M NaHSO4(aq), saturated NaHCO3(aq),
and brine and dried over Na2SO4. After filtration, the organic
calculated, 222.1106; found, 222.1098.
(2S)-2-Benzyloxycarbonylamino-5-tert-butoxycarbon-
ylaminothiopentanoic Acid S-Pyridin-4-yl Ester (9): Gen-
eral Procedure for the Formation of 4-Pyridyl Thioesters.
EDC (1.71 g, 8.90 mmol) was added to a stirred solution of
NR-Cbz-Nδ-Boc-L-ornithine (8) (2.72 g, 7.40 mmol) and 4-mer-
captopyridine (0.824 g, 7.40 mmol) in dichloromethane (80 mL)
at 0 °C under a nitrogen atmosphere. After 10 min the solution
was warmed to 20 °C and left stirring for an additional 1.2 h.
The reaction was quenched with water, and the organic phase
was washed with water (4 times) and brine. The organic phase
was dried over Na2SO4, filtered, and concentrated in vacuo to
yield the crude thioester. The crude product was purified by
flash chromatography using pentane/ethyl acetate (2:3) as the
eluant, affording 2.28 g of the thioester (67% yield) as a slightly
1
yellow powder. H NMR (400 MHz, CDCl3) δ (ppm): 8.62 (d,
2H, J ) 5.6 Hz), 7.37-7.34 (m, 7H), 5.77 (br s, 1H), 5.17 (s,
2H), 4.63 (br t, 1H, J ) 5.8 Hz), 4.55-4.50 (m, 1H), 3.22-3.08
(m, 2H), 2.02-1.92 (m, 1H), 1.77-1.68 (m, 1H), 1.64-1.56 (m,
2H), 1.43 (s, 9H). 13C NMR (100 MHz, CDCl3) δ (ppm): 197.3,
156.4, 156.3, 150.1 (2C), 138.8, 136.2, 128.8 (2C), 128.6, 128.4
(2C), 128.4 (2C), 79.7, 67.7, 61.5, 40.0, 29.5, 28.6 (3C), 26.6.
HRMS C23H29N3O5S [M+Na+]: calculated, 482.1726; found,
482.1736.
(2S)-2-[(5S)-5-Benzyloxycarbonylamino-8-tert-butoxy-
carbonyl-amino-4-ox o-octanoylamino]-4-methylpentano-
ic Acid Methyl Ester (11): General Procedure for SmI2-
Promoted Coupling of a Thioester and an Acrylamide.
The thioester 9 (201 mg, 0.43 mmol) and acrylamide 7 (58 mg,
0.29 mmol) were flushed with argon before THF (3 mL) was
added. The solution was added dropwise over 15 min via a
syringe pump to a solution of 0.1 M SmI2 in THF (19 mL, 1.9
mmol) cooled to -78 °C under an argon atmosphere, and the
solution was left stirring at this temperature for 5 days. The
excess SmI2 was oxidized by flushing the mixture with oxygen
from a balloon. To the now yellow solution was added
saturated NH4Cl(aq) at -78 °C followed by warming the
solution to room temperature. The THF was evaporated off,
and 10% citric acid or 0.5 M HCl was added followed by
extraction (3 times) with EtOAc. The combined organic phases
were washed with brine, dried over Na2SO4, and concentrated
in vacuo. The coupling product was purified by flash chroma-
tography on silica gel using pentane/ethyl acetate (1:1 to 1:3)
as the eluant, affording 137 mg of 11 (86%) as a yellow oil. 1H
NMR (400 MHz, CDCl3) δ (ppm): 7.35 (s, 5H), 6.02 (br d, 1H,
J ) 7.6 Hz), 5.59 (br d, 1H, J ) 7.2 Hz), 5.01 (s, 2H), 4.76 (br
s, 1H), 4.62-4.57 (m, 1H), 4.45-4.41 (m, 1H), 3.71 (s, 3H),
3.13 (d, 2H, J ) 10 Hz), 2.98-2.90 (m, 1H), 2.77-2.70 (m, 1H),
2.59-2.46 (m, 2H), 2.00-1.90 (m, 1H), 1.72-1.59 (m, 6H), 1.43
(s, 9H), 0.93 (d, 3H, J ) 5.2 Hz), 0.92 (d, 3H, J ) 4.8 Hz). 13
C
NMR (100 MHz, CDCl3) δ (ppm): 208.1, 173.8, 171.4, 156.3
(2C), 136.5, 128.7 (2C), 128.4, 128.3 (2C), 79.3, 67.2, 59.5, 52.5,
51.0, 41.8, 40.2, 34.7, 29.6, 28.8, 28.6 (3C), 25.6, 25.0, 23.0,
22.1. IR (KBr, cm-1): 2958, 1744, 1682, 1650, 1526. HRMS
C28H43N3O8 [M + Na+]: calculated, 572.2948; found, 572.2932.
Methyl 8-[[(1,1-dimethylethyl)carbonyl]amino]-4-oxo-
(5S)-5-[[(phenylmethoxy)carbo nyl]-amino]octanoate
(19): General Method for the Coupling of Thioesters
with Acrylates. The thioester 9 (215 mg, 0.469 mmol) was
dissolved in anhydrous THF (7.5 mL), and the reaction vessel
was flushed with argon for 10 min. Methyl acrylate (111 µL,
1.50 mmol) and tert-butyl alcohol (135 µL, 1.50 mmol) were
added, after which the solution was cooled to -78 °C, before a
0.1 M solution of SmI2 (15 mL, 1.50 mmol) was added slowly
via syringe over 10 min. The reaction mixture was stirred at
-78 °C for 48 h, and then the flask was flushed with O2 before
saturated NH4Cl(aq) (2 mL) was added and the mixture
warmed to 20 °C. The mixture was poured into 0.5 M HCl (40
mL) and extracted with EtOAc (3 × 20 mL). The combined
organic phases were washed with saturated Na2S2O3 (30 mL),
dried (MgSO4), filtered, and evaporated in vacuo. The pure
7518 J. Org. Chem., Vol. 70, No. 19, 2005

Hydroxyethylene Isostere of Arg-Gly-Leu
phase was concentrated in vacuo. Flash chromatography on
silica gel using dichloromethane/methanol (60:1) as eluant
(2C), 128.2, 128.0 (2C), 83.4, 79.6, 74.1, 66.9, 53.6, 51.2, 41.3,
40.8, 31.5, 29.2, 28.3 (3C), 28.2 (3C), 25.9 (4C), 25.5, 25.0, 23.0,
22.0, 18.1, -4.2, -4.7. ES-MS C39H67N5O10Si [M + Na+]: 816.2.
1
afforded 25 mg of 28 (74%) as a colorless film. H NMR (400
MHz, CDCl3) δ (ppm): 11.45 (br s, 1H), 8.34 (br s, 1H), 7.35-
7.32 (m, 5H), 6.10 (br d, 1H, J ) 7.6 Hz), 5.66 (br d, 1H, J )
8.4 Hz), 5.10 (s, 2H), 4.63-4.57 (m, 1H), 3.88 (br s, 1H), 3.72
(s, 3H), 3.68-3.64 (m, 2H), 3.54-3.48 (m, 2H), 3.36-3.29 (m,
2H), 2.48-36 (m, 2H), 1.81-53 (m, 7H), 1.49 (s, 9H), 1.46 (s,
9H), 0.93 (d, 6H, J ) 6.0 Hz). 13C NMR (100 MHz, CDCl3) δ
(ppm): 174.0, 173.8, 163.9, 157.4, 156.8, 153.7, 137.0, 128.9
(2C), 128.5 (3C), 83.3, 79.4, 74.2, 66.9, 56.3, 52.5, 51.0, 41.7,
40.7, 33.3, 28.4 (4C), 28.2 (3C), 26.6, 25.8, 25.0, 22.9, 22.1.
HRMS C34H55N5O10 [M + Na+]: calculated, 716.3046; found,
716.3937.
(2S)-2-[(4R,5S)-8-(2,3-bis(tert-Butoxycarbonyl)guani-
dino)-5-benzyloxycarb onylamino-4-tert-butyldimethyl-
silyloxy-octanoylamino]-4-methylpentanoic Acid (29).
Methyl ester 28 (186 mg, 0.27 mmol) was dissolved in
methanol (5.5 mL) and treated with LiOH (23 mg, 0.54 mmol)
in water (1.8 mL) for 6 h. Et2O and 10% citric acid were added
to the reaction mixture, and the aqueous phase was extracted
with Et2O (3 times). The combined organic phases were
washed with water and brine and then dried over Na2SO4.
After evaporation in vacuo the resulting film was redissolved
in DMF (2 mL). Imidazole (393 mg, 5.77 mmol) and TBDMS-
Cl (394 mg, 2.61 mmol) were added to the solution followed
by stirring for 66 h. MeOH (16.5 mL) was added, and the
solution was stirred for another 3.5 h. The reaction mixture
was evaporated, and EtOAc was added. The organic phase was
washed with 10% citric acid, H2O (4 times), and brine before
drying over Na2SO4. The organic phase was filtered and
evaporated to dryness. Flash chromatography on silica gel
using dichoromethane/methanol (40:1) with 0.1% HCOOH
afforded 112 mg of 29 (53%) as a colorless film. 1H NMR (400
MHz, CDCl3) δ (ppm): 8.39 (br s, 1H), 7.36-7.28 (m, 5H), 6.68
(br d, 1H, J ) 8.0 Hz), 5.10 (br s, 1H), 5.08 (s, 2H), 4.52 (m,
1H), 3.71-3.60 (m, 2H), 3.49-3.41 (m, 1H), 3.34-3.25 (m, 1H),
2.37-2.21 (m, 2H), 1.89-1.78 (m, 2H), 1.71-1.53 (m, 7H), 1.48
(s, 9H), 1.46 (s, 9H), 0.92 (d, 6H, J ) 6.0 Hz), 0.86 (s, 9H),
0.05 (s, 3H), 0.01 (s, 3H). 13C NMR (100 MHz, CDCl3) δ
(ppm): 175.6, 173.2, 163.2, 156.5, 156.4, 153.3, 136.5, 128.6
(2S)-2-[(5S)-5-(9H-Fluoren-9-ylmethoxycarbonylamino)-
8-tert-butoxycarbonylamino-4-oxo-octanoylamino]-4-
methylpentanoic Acid Methyl Ester (36). Thioester 35 (103
mg, 0.19 mmol) and acrylamide 7 (26 mg, 0.13 mmol) were
reacted according to the general procedure for the preparation
of compound 11 with a reaction time of 4 days. The coupling
product was purified by flash chromatography on silica gel
using pentane/ethyl acetate (2:1 to 1:4) as the eluant, affording
1
35 mg of 35 (43% yield) as a yellow oil. H NMR (400 MHz,
CDCl3) δ (ppm): 7.76 (d, 2H, J ) 8.8 Hz), 7.59 (d, 2H, J ) 6.8
Hz), 7.39 (dt, 2H, J ) 7.6, 0.6 Hz), 7.31 (dt, 2H, J ) 7.6, 0.6
Hz), 6.06 (br d, 1H, J ) 8.4 Hz), 5.67 (br d, 1H, J ) 6.4 Hz),
4.78 (br s, 1H), 4.62-4.57 (m, 2H), 4.40 (d, 2H, J ) 6.8 Hz),
4.20 (t, 1H, J ) 6.8 Hz), 3.71 (s, 3H), 3.13 (br d, 2H, J ) 5.6
Hz), 2.95-2.87 (m, 1H), 2.77-2.70 (m, 1H), 2.60-2.47 (m, 2H),
1.99-1.91 (m, 1H), 1.67-1.48 (m, 6H), 1.44 (s, 9H), 0.93 (d,
3H, J ) 4.0 Hz), 0.92 (d, 3H, J ) 4.0 Hz). 13C NMR (100 MHz,
CDCl3) δ (ppm): 208.0, 173.7, 171.3, 156.3, 156.2, 144.0, 143.9,
141.5 (2C), 127.8 (2C), 127.2 (2C), 125.2 (2C), 120.1 (2C), 79.3,
67.0, 59.5, 52.4, 50.9, 47.4, 41.8, 40.1, 34.6, 29.6, 28.5 (4C),
25.6, 24.9, 22.9, 22.0. HRMS C35H47N3O8 [M + Na+]: calcu-
lated, 660.3261; found, 660.3282.
Acknowledgment. We are indebted to the Danish
National Science Research Council, the Lundbeck Foun-
dation, and the University of Aarhus for generous
financial support.
Supporting Information Available: Experimental pro-
tocol for the preparation of compounds 13-18, 20-23, 31, 34,
37, and 38; copies of 1H NMR and 13C NMR spectra for
compounds 7, 9, 11, 13, 19-23, 26-29, 31, and 34-38; HPLC
chromatograms in the reduction studies of the aminoketone
11. This material is available free of charge via the Internet
at http:// pubs.acs.org.
JO0505775
J. Org. Chem, Vol. 70, No. 19, 2005 7519