Azadepsipeptides: Synthesis and evaluation of a novel class of peptidomimetics

Article abstract of DOI:10.1021/jo001749v

A general route to azadepsipeptides, a new class of pseudopeptides, has been established. The methodology was applied to the synthesis of a bis-aza analogue of the antiparasitic cyclooctadepsipeptide PF1022A. Comparison of the X-ray crystal structures of natural PF1022A (8) and the chimeric aza analogue 9 revealed that the introduction of nitrogen in the backbone of PF1022A results in almost complete conservation of the 3D structure with only minor deviations at the new nitrogen positions.

Full text of DOI:10.1021/jo001749v

3760
J . Org. Chem. 2001, 66, 3760-3766
Aza d ep sip ep tid es: Syn th esis a n d Eva lu a tion of a Novel Cla ss of
P ep tid om im etics
Hubert Dyker,† J u¨rgen Scherkenbeck,*,† Daniel Gondol,† Axel Goehrt,† and Achim Harder‡
Central Research and Business Group Animal Health, Bayer AG, D-51368 Leverkusen, Germany
J uergen.Scherkenbeck.J S@bayer-ag.de
Received December 15, 2000
A general route to azadepsipeptides, a new class of pseudopeptides, has been established. The
methodology was applied to the synthesis of a bis-aza analogue of the antiparasitic cyclooctadep-
sipeptide PF1022A. Comparison of the X-ray crystal structures of natural PF1022A (8) and the
chimeric aza analogue 9 revealed that the introduction of nitrogen in the backbone of PF1022A
results in almost complete conservation of the 3D structure with only minor deviations at the new
nitrogen positions.
In tr od u ction
the structure and conformation of the original peptide
should be largely preserved in the resulting biomimetic
polymer.
A wealth of peptidic natural products with manifold
biological activities have been published within the past
decade.1Additionally, peptide-protein or protein-protein
interactions, playing key roles in most of the signal-
transduction processes, have been recognized as promis-
ing targets for novel pharmaceuticals with possible
applications, for instance, in cancer, asthma, gastrointes-
tinal, and antihypertensive therapy.2 As a consequence,
immense efforts3 have been directed at improving the
pharmacological properties of biologically active peptides
by incorporating amino acid and peptide mimetics. These
peptide analogues are usually characterized by improved
enzymatic stability, bioavailability, and duration of ac-
tion.4 The alteration of peptides to peptidomimetics
encompasses side-chain manipulation, turn-mimics,5
amino acid extension,6 and backbone modifications.7 The
latter approach seems to be particularly promising since
The most common manipulations involving the R-car-
bon atoms of peptides include inversion of configuration
(D-amino acids),8 substitution of the R-hydrogen (by alkyl
or other groups),1a and replacement of the R-carbon atom
isoelectronically by a trivalent nitrogen, yielding azapep-
tides (Figure 1). This class of pseudopeptides lacks
chirality in the R-position and can be considered inter-
mediate in configuration between D- and L-amino acids.9
Azapeptides provide improved resistance against enzy-
matic cleavage, adopt conformations similar to their
natural counterparts, and display increased biological
activity in many cases.10 Although the alteration of the
backbone by replacement of R-carbons by nitrogen is a
common manipulation in peptide chemistry, only little
attention has been devoted to the synthesis of aza
analogues of depsipeptides (Figure 1).11 This is surprising
since many depsipeptides, such as the enniatins and the
cyclooctadepsipeptide PF1022A (8), have been found to
* To whom correspondence should be addressed. Fax: +49-214-
3021031.
† Central Research.
‡ Business Group Animal Health.
(1) (a) Wipf, P. Chem. Rev. 1995, 95, 2115. (b) Tan, L. T.; Williamson,
R. T.; Gerwick, W. H.; Watts, K. S.; McGough, K.; J acobs, R. J . Org.
Chem. 2000, 65, 419. (c) Robl, J . A.; Simpkins, L. M.; Asaad, M. M.
Bioorg. Med. Chem. Lett. 2000, 10, 257. (d) Umezawa, K.; Ikeda, Y.;
Uchihata, Y.; Naganawa, H.; Kondo, S. J . Org. Chem. 2000, 65, 459.
(e) Morita, H.; Shimbo, K.; Shigemori, H.; Kobayashi, J . Bioorg. Med.
Chem. Lett. 2000, 10, 469. (f) Nakayama, K.; Kawato, H. C.; Inagaki,
H.; Nakajima, R.; Kitamura, A.; Someya, K.; Ohta, T. Org. Lett. 2000,
2, 977.
(2) (a) Bikker, J . A.; Trumpp-Kallmeyer, S.; Humblet, C. J . Med.
Chem. 1998, 41, 2911. (b) Smith, W. W.; Abdel-Meguid, S. S. Exp. Opin.
Ther. Patents 1999, 9, 683. (c) Fields, B. F. Exp. Opin. Ther. Pat. 1998,
8, 633. (d) Mu¨ller, G. Top. Curr. Chem. 2000, 211, 17.
(3) (a) Hirschmann, R. Angew. Chem., Int. Ed. Engl. 1991, 30, 1278.
(b) Gante, J . Angew. Chem., Int. Ed. Engl. 1994, 33, 1699. (c) Adang,
E. P.; Hermkens, P. H. H.; Linders, J . T. M.; Ottenheijm, H. C. J .; van
Staveren, C. J . Recl. Trav. Chim. Pays-Bas 1994, 113, 63. (d) Giannis,
A.; Kolter, T. Angew. Chem., Int. Ed. Engl. 1993, 32, 1244-67.
(4) (a) Hruby, V. J .; Kazmierski, A. M.; Kawasaki, T. O. Pept. Pharm.
1991, 135-184. (b) Bladon, C. M. Amino Acids Pept. 1993, 24, 133-
200.
(7) (a) Hagihara, M.; Anthony, N. J .; Stout, T. J .; Clardy, J .;
Schreiber, S. L. J . Am. Chem. Soc. 1992, 114, 6568. (b) Cho, C. Y.;
Moran, E. J .; Cherry, S. R.; Stephans, J . C.; Fodor, S. P. A.; Adams, C.
L.; Sundaram, A.; J acobs, J . W.; Schultz, P. G. Science 1993, 261, 1303.
(c) Liskamp, R. M. J . Angew. Chem., Int. Ed. Engl. 1994, 33, 633. (d)
Burgess, K.; Linthicum, D. S.; Shin, H. Angew. Chem., Int. Ed. Engl.
1995, 34, 907. (e) Han, H.; J anda, K. D. J . Am. Chem. Soc. 1996, 118,
2539. (f) Han, H.; Yoon, J .; J anda, K. D. Methods Mol. Med. 1999, 23,
87.
(8) (a) Spatola, A. F. In Chemistry and Biochemistry of Amino Acids,
Peptides, and Proteins; Weinstein, B., Ed.; Marcel Dekker: New York,
1983; pp 267-357. (b) Mosberg, H. I.; Hurst, R.; Hruby, V. J .; Gee, K.;
Yamamura, H. I.; Galligan, J . J .; Burks, T. F. Proc. Natl. Acad. Sci.
U.S.A. 1983, 80, 5871. (c) Dooley, C. T.; Chung, N. N.; Wilkes, B. C.;
Schiller, P. W.; Bidlack, J . M.; Pasternak, G. W.; Houghten, R. A.
Science 1994, 266, 2019.
(9) (a) Gante, J . Synthesis 1989, 405. (b) Gray, C. J .; Ireson, J . C.;
Parker, R. C. Tetrahedron 1977, 33, 739.
(10) (a) Gante, J .; Krug, M.; Lauterbach, G.; Weitzel, R.; Hiller, W.
J . Pept. Sci. 1995, 2, 201. (b) Graybill, T. L.; Ross, M. J .; Gauvin, B.
R.; Gregory, J . S.; Harris, A. L.; Ator, M. A.; Rinker, J . M.; Dolle, R. E.
Bioorg. Med. Chem. Lett. 1992, 2, 1375. (c) Magrath, J .; Abeles, R. H.
J . Med. Chem. 1992, 35, 4279.
(5) (a) Goodman, M.; Zhang, J . Chemtracts 1997, 10, 629-645. (b)
Hanessian, S.; McNaughton-Smith, G.; Lombart, H.-G.; Lubell, W. D.
Tetrahedron 1997, 53, 12789.
(6) (a) Gademann, K.; Ernst, M.; Hoyer, D.; Seebach, D. Angew.
Chem., Int. Ed. Engl. 1999, 38, 1223. (b) Stachowiak, K.; Khosla, M.
C.; Plucinska, K.; Khairallah, P. A.; Bumpus, F. M. J . Med. Chem.
1979, 22, 1128.
(11) (a) Marchand-Brynaert, J . Farmaco 1995, 50, 455. (b) Dorn,
C. P.; Zimmermann, M.; Yang, S. S.; Yurewicz, E. C.; Ashe, B. M.;
Frankshun, R.; J ones, H. J . Med. Chem. 1977, 20, 1464. (c) Dorn, C.
P.; Yang, S. S. US patent P2658254.7, 1975.
10.1021/jo001749v CCC: $20.00 © 2001 American Chemical Society
Published on Web 05/03/2001

Azadepsipeptides
J . Org. Chem., Vol. 66, No. 11, 2001 3761
Sch em e 1. P r ep a r a tion of Boc-P r otected
Un sym m etr ica l N,N′-Dia lk yl Hyd r a zin es
Ta ble 1. N,N′-Dia lk ylh yd r a zin es
R
yield (%)
3a
3b
3c
3d
3e
3f
Me
i-Pr
Pr
i-Bu
Bu
Bn
83
92
89
78
93
98
75
F igu r e 1. Comparison of peptide/azapeptide and depsipep-
tide/azadepsipeptide structures.
be biologically highly active and are currently subject of
extensive studies in organic and medicinal chemistry.12
3g
4-Br-Bn
We became interested in the evaluation of azadep-
sipeptide structures in connection with our ongoing work
on the antiparasitic cyclooctadepsipeptide PF1022A (8).13
The primary goal in developing a synthesis of aza
analogues was to elucidate the functional role of the
depsipeptide backbone and the importance of the chiral
centers. In this paper, we outline the general synthesis
of azadepsipeptide building blocks as a new class of
peptidomimetics and give a full account of the synthesis
and conformational studies (X-ray and NMR) of the
chimeric cyclic depsipeptide 9 (bis-aza-PF1022A), as the
first example of an aza analogue of a natural depsipep-
tide.
Monoalkyl hydrazines 1 are either commercially avail-
able or may be easily prepared following standard
literature procedures.15 The more nucleophilic secondary
nitrogen of the hydrazine is selectively protected with the
Boc-group under carefully controlled conditions to give
2.15 The side chains are best introduced in a two-step
sequence via hydrazone formation and reduction giving
excellent yields of the unsymmetrical Boc-protected di-
alkyl hydrazines 3 (Table 1). Compared to the direct one-
step reductive alkylation of 2, hydrazone formation prior
to reduction resulted in much cleaner reactions. In the
case of hydrazines 3a -e, reduction was accomplished
with NaCNBH3 in methanol under acidic conditions
whereas for hydrazines 3f and 3g hydrogenation of the
intermediate hydrazones proved to be the method of
choice. The compounds were obtained practically pure
and in excellent yields after simple hydrolytic workup
and filtration through silica gel. If required, the carba-
zates 3 can be further purified by column chromatogra-
phy or distillation under reduced pressure. However,
distillation resulted in lower yields due to decomposition.
The second constitutive structural element of azadep-
sipeptides, R-hydroxycarboxylic esters, were obtained in
high optical purity and good to excellent yields from the
corresponding (R)- or (S)-amino acids following a desa-
mination/hydroxylation protocol.16 The cesium salts of the
resulting hydroxy acids were subsequently benzylated
with benzyl bromide in ethanol.
The formation of carbazates from dialkylhydrazines
and R-hydroxy carboxylic esters can in principle be
achieved either by converting the dialkylhydrazines into
an acylating agent or vice versa by activating the
hydroxycarboxylic acids (Scheme 2). The first route
turned out to be difficult since the alkyl hydrazines are
poorer nucleophiles than simple amines or amino acids
and thus require a highly activated carbonyl synthon.
Our initial attempts to couple the two building blocks
using bis-pentafluorophenyl carbonate,17 triphosgene, or
p-nitrophenyl chloroformate suffered from unsatisfying
yields and side reactions, mainly caused by the formation
of carbonates and azatides.
Resu lts a n d Discu ssion
Aza d ep sip ep tid e Bu ild in g Block s. The azadep-
sipeptide building blocks required for our purposes
structurally represent carbazates between an N,N′-
substituted hydrazine as the aza-amino acid portion and
an R-hydroxycarboxylic acid (Figure 1). For the genera-
tion of a set of these building blocks a number of suitably
protected N-alkylaza-amino acids had to be prepared. The
synthesis of protected alkyl hydrazines can be achieved
in general via two different routes, either by alkylation
of hydrazines and subsequent introduction of the protect-
ing group or vice versa by reductive alkylation of pro-
tected hydrazines.14 In contrast, the synthesis of pro-
tected unsymmetrical N,N′-dialkyl hydrazines (3) has not
been investigated systematically. To be able to introduce
a broad variety of natural and unnatural amino acid side
chains, we developed a three-step protection/reductive
alkylation protocol for alkyl hydrazines 3 as outlined in
Scheme 1.
(12) (a) Nakajyo, S.; Shimizu, K.; Kometani, A.; Suzuki, A.; Ozaki,
H.; Urakawa, N. J pn. J . Pharmacol. 1983, 33, 573. (b) Murakoshi, S.;
Ichinoe, M.; Tamura, S.; Suzuki, A.; Kanaoka, M.; Isogai, A. Tetrahe-
dron Lett. 1977, 25, 2167. (c) Tomoda, H.; Nishida, H.; Huang, X.-H.;
Masuma, R.; Kim, Y. K.; Omura, S. J . Antibiot. 1992, 45, 1207. (d)
Quitt, P.; Studer, R. O.; Vogler, K. Helv. Chim. Acta 1963, 46, 1715.
(e) Shemyakin, M. M.; Ovchinnikov, Yu. A.; Ivanov, V. T.; Kiryushkin,
A. A.; Ivanov, V. T. Tetrahedron Lett. 1963, 885. (f) Sasaki, T.; Takagi,
M.; Yaguchi, T.; Miyadoh, S.; Okada, T.; Koyama, M. J . Antibiot. 1992,
45, 692. (g) Kodama, Y.; Takeuchi, Y.; Suzuki, A. Sci. Rep Meiji Seika
Kaisha 1992, 31, 1.
To overcome these problems, we decided to use phos-
gene as carbonyl reagent to activate the R-hydroxycar-
(13) (a) Scherkenbeck, J .; Plant, A.; Harder, A.; Mencke, N. Tetra-
hedron 1995, 51, 8459. (b) Scherkenbeck, J .; Harder, A.; Plant, A.;
Dyker, H. Bioorg. Med. Chem. Lett. 1998, 8, 1035. (c) Scherkenbeck,
J .; Dyker, H.; Gondol, D.; Harder, A.; Plant, A.; Reichel, F. Pestic. Sci.
1999, 55, 457.
(14) Dutta, A. S.; Morley, J . S. J . Chem. Soc., Perkin Trans. 1 1975,
1712.
(15) Biel, J . H.; Drukker, A. E.; Mitchell, T. F.; Sprengeler, E. P.;
Nuhfer, P. A.; Conway, A. C.; Horita, A. J . Am. Chem. Soc. 1959, 81,
2805.
(16) (a) Kunz, H.; Lerchen, H.-G. Tetrahedron Lett. 1987, 28, 1873.
(b) Lerchen, H.-G.; Kunz, H. Tetrahedron Lett. 1985, 26, 5257.
(17) Efimov, V. A.; Kalinkina, A. L.; Chakhmakhcheva, O. G. Nucleic
Acids Res. 1993, 21, 5337.

3762 J . Org. Chem., Vol. 66, No. 11, 2001
Dyker et al.
Sch em e 2. Syn th esis of Aza d ep sip ep tid e Bu ild in g
Block s
depsipeptide chain building either by fragment conden-
sation of didepsipeptide units or by iterative coupling of
hydrazines and chloroformates.
Ch im er ic Bis-a za -P F 1022A. To evaluate the proper-
ties of azadepsipeptides as peptidomimetics, we devised
the synthesis of an aza analogue of PF1022A (9, Figure
2). Although the structural change compared to the
original peptide is minimal, the conformation of the aza
analoguesdue to the loss of chiral centerssmay be quite
different from the original structure. Since the binding
affinity and selectivity of a peptidic ligand to its receptor
is largely determined by the overall conformation and
certain key structural features of the sequence, we
decided to replace only one-half of the C2-symmetric
PF1022A structure by an azatetradepsipeptide in order
to largely preserve the native conformation (Figure 2).
The synthetic strategy is a convergent one in which
both parts of the molecule, the tetradepsipeptide frag-
ment 1519 bearing the natural part of the molecule and
fragment 16 which contains the bis-aza portion, are
preassembled separately and then connected to give the
linear precursor which was finally cyclized. On the basis
of our experience with the synthesis of PF1022A and
several derivatives, BOP-Cl was employed as simple and
efficient coupling reagent to form all N-methylamide
linkages including the final cyclization.19 Other coupling
reagents such as HATU, TBTU, or PyBroP did not give
better yields.20
The preparation of fragment 15 was started by first
constructing its two didepsipeptide subunits 9 and 10
through coupling of N-methyl-(S)-leucine with 3-phenyl-
(R)-lactate and (R)-lactate, following established proce-
dures (Scheme 3).13 After deprotection of 9 by catalytic
hydrogenolysis and 10 by acidic cleavage with TFA, the
resulting didepsipeptide units MeLeu-PheLac (11) and
MeLeu-Lac (12) were coupled with BOP-Cl to give 15 in
80% yield. In a similar way, the appropriately protected
azatetradepsipeptide fragment 16 was formed by cou-
pling the azadidepsipeptide building blocks 6h and 6i.
These were synthesized as described above by first
reacting benzyl 3-phenyl-(R)-lactate or benzyl (R)-lactate
with phosgene as carbonyl equivalent and subsequent
coupling with Boc-protected N-methyl-N′-isobutylhydra-
zine (3d ). After standard protecting group manipulation,
the condensation of the resulting two crude products 13
and 14 with BOP-Cl as coupling reagent furnished 16 in
a reasonable yield of 45%. With the two fragments in
hand, the final steps of the synthesis proved to be
straightforward. Deprotection of 15 and 16 and coupling
of the resulting products 1719 and 18 furnished 19, the
fully protected chimeric azadepsipeptide, as the linear
precursor of 9. Acidic hydrolysis of the N-terminal Boc
group and hydrogenolytic cleavage of the C-terminal
benzyl ester followed by subsequent macrocyclization
under high dilution conditions afforded the bis-aza
PF1022A analogue 9 as a stable pale yellow solid after
flash chromatography and additional HPLC purification.
Ta ble 2. Aza d id ep sip ep tid es
R1
R2
yield (%)
method
6a
6b
6c
6d
6e
6f
6g
6h
6i
Me
H
H
H
H
H
H
H
Bn
Me
96
87
67
84
76
97
85
76
59
2
2
2
2
2
2
2
1
1
i-Pr
Pr
i-Bu
Bu
Bn
4-Br-Bn
i-Bu
i-Bu
boxylic ester. In a stepwise procedure, depicted in Scheme
2, the hydroxycarboxylic esters were first converted to
the corresponding chloroformates using an excess of
phosgene. The intermediate chloroformates were isolated
and immediately reacted with the hydrazines 3 in the
presence of NEt3. Using this coupling method, 6h and 6i
were obtained in good yields of 76% and 59%. The results
are summarized in Table 2.
During the course of our work, we also attempted to
find an alternative for the use of toxic phosgene. The use
of carbon dioxide as a phosgene equivalent seemed to be
promising, since the formation of carbamate esters has
been reported in the literature.18 As shown in Scheme 2,
the reaction of hydrazines 3 with carbon dioxide and
R-bromo acetate in the presence of cesium carbonate
afforded the expected azadidepsipeptides 6 in excellent
yields. This alternative procedure for the synthesis of 6
is based on alkylation of carbamic acid intermediates,
formed through the reaction of hydrazines 3 with carbon
dioxide. The reaction, however, turned out to be limited
to primary alkyl halides as alkylating agents. When
R-chloropropionate esters were subjected to these reaction
conditions, only small amounts of the desired products
6h and 6i were obtained. Instead R,â-unsaturated esters
were formed through the elimination of HCl under basic
conditions. In summary, phosgene remains the most
general applicable reagent for the synthesis of azadidep-
sipeptides 6. For the synthesis of building blocks unsub-
stituted in the R-hydroxy ester moiety, carbon dioxide/
cesium carbonate is an excellent and simple to use
alternative. The techniques outlined above allow aza-
(19) (a) Van Der Auwera, C.; Anteunis, M. J . O. Int. J . Peptide Res.
1987, 29, 574. (b) Angell, Y. M.; Garc´ıa-Echeverr´ıa, C.; Rich, D. H.
Tetrahedron Lett. 1994, 35, 5981. (c) Tung, R. D.; Rich, D. H. J . Am.
Chem. Soc. 1985, 107, 4342.
(20) (a) Coste, J .; Fre´rot, E.; J ouin, P. Tetrahedron Lett. 1991, 32,
1967. (b) Sapia, A. C.; Slomczynska, U.; Marshall, G. R. Lett. Pept.
Sci. 1995, 1, 283. (c) Reid, G. E.; Simpson, R. J . Pept.: Chem. Biol.,
Proc. Am. Pept. Symp., 12th 1992, 1991, p 647. (d) Sueiras-Diaz, J .;
Horton, J . Tetrahedron Lett. 1992, 33, 2721-4. (e) Hoffmann, E.; Beck-
Sickinger, A. G.; J ung, G. Liebigs Ann. Chem. 1991, 585-90.
(18) Butcher, K. J . Synlett 1994, 825. McGhee, W.; Riley, D.; Christ,
K.; Pan, Y.; Parnas, B. J . Org. Chem. 1995, 60, 2820.

Azadepsipeptides
J . Org. Chem., Vol. 66, No. 11, 2001 3763
F igu r e 2. PF1022A and bis-aza PF1022A.
Sch em e 3. Syn th esis of Bis-a za P F 1022A (8)
Both X-ray crystal structure analysis21 and NMR
studies (COSY, TOCSY, ROESY) of 9 revealed that the
introduction of nitrogen in the backbone of PF1022A (8)
results in almost complete conservation of the 3D struc-
ture of the natural product with only minor changes at
the new nitrogen positions. Like the native PF1022A (8),
the chimeric aza analogue (9) also consists of two
conformers in solution, a symmetrical one with all amide
bonds in the trans configuration and an asymmetrical
conformation with one trans-amide bond. The trans-
amide bond in 9 is found between azaleucine and the
lactic acid moiety, stabilizing the asymmetrical conformer
significantly (100:7 in 9 compared to 3:1 in 8). However,
when tested in vivo in sheep infected with the two
nematodes Haemonchus contortus and Trichostrongylus
colubriformis a decrease in biological activity by a factor
of 5-10 was observed compared with the natural
product.13b It seems to be most reasonable that the
reduced anthelmintic activity can be attributed either to
the loss of chirality and an increased flexibility at the
R-carbons or to a reduced metabolic stability.
(21) X-ray structure of 8: Kodama, Y.; Takeuchi, Y.; Suzuki, A. Sci
Rep. of Meji Seika Kaisha 1992, 31, 1.

3764 J . Org. Chem., Vol. 66, No. 11, 2001
Dyker et al.
δ 1.02 (d, 6H, J ) 8 Hz), 1.48 (s, 9H), 3.03 (s, 3H); MS (CI) m/e
133 (8), 89 (100).
In summary, we have established a general synthesis
for azadepsipeptides of variable length and applied this
methodology to the synthesis of a bis-aza analogue of the
antiparasitic cyclooctadepsipeptide PF1022A (8). Al-
though the substitution of two R-carbons by nitrogens
caused only minor alterations in the 3D structure, the
anthelmintic activity of 9 was significantly reduced. By
subtly changing the synthetic route and by choosing
differently substituted hydrazines the substitution pat-
tern can be modified in a flexible manner and novel
heterocyclic ring systems of different size are accessible.
The strategy described above should be generally ap-
plicable to the construction of azadepsipeptide libraries
either by solution or by solid-phase chemistry.
1-Boc-1-m eth yl-2-p r op ylh yd r a zin e 3c: yield 89% (HPLC
purity 100%); colorless liquid; Rf ) 0.46 (cyclohexanes-ethyl
acetate 3:1); IR 1694 cm-1; 1H NMR (500 MHz, CDCl3) δ 0.94
(t, 3H, J ) 8 Hz), 1.48 (s, 9H + m, 2H), 2.77 (t, 2H, J ) 8 Hz),
3.0 (s, 3H); MS (CI) m/e 132 (38), 103 (25), 87 (30); HRMS (ESI)
m/e 211.1415 (calcd for C9H20N2O2Na 211.1417).
1-Boc-1-m eth yl-2-isobu tylh ydr azin e 3d: yield 78% (HPLC
purity 100%); colorless liquid; Rf ) 0.86 (cyclohexanes-ethyl
acetate 1:1); IR 1700 cm-1 (CO); 1H NMR (500 MHz, CDCl3) δ
1.12 (d, 6H, J ) 8 Hz), 1.54 (s, 9H), 2.63 (m, 1H), 3.17 (s, 3H),
6.88 (d, 1H, J ) 8 Hz); MS (CI) m/e 202 (M+, 4), 146 (43), 103
(77); HRMS (ESI) m/e 225.1572 (calcd for C10H22N2O2Na
225.1573).
1-Boc-1-m eth yl-2-bu tylh yd r a zin e 3e: yield 93% (HPLC
purity 97%); colorless liquid; IR 1694 cm-1; 1H NMR (500 MHz,
CDCl3) δ 0.91 (t, 3H, J ) 8 Hz), 1.48 (s, 9H), 1.3-1.6 (m, 4H),
2.80 (t, 2H, J ) 8 Hz), 3.01 (s, 3H); MS (ESI) m/e 203 ([M +
H]+, 100); HRMS (ESI) m/e 203.1752 (calcd for C10H22N2O2Na
203.1754).
1-Boc-1-m eth yl-2-ben zylh yd r a zin e 3f: yield 98% (HPLC
purity 97%); pale yellow liquid; Rf ) 0.62 (cyclohexanes-ethyl
acetate 2:1); IR 1702 cm-1; 1H NMR (500 MHz, CDCl3) δ 1.48
(s, 9H), 2.93 (s, 3H), 3.91 (s, 2H), 7.3 (m, 5H); MS (CI) m/e 181
(8), 137 (100); HRMS (ESI) m/e 259.1417 (calcd for C13H20N2O2-
Na 259.1417).
Exp er im en ta l Section
All reactions were performed in oven-dried glassware under
a positive pressure of argon. Solvents were dried by filtration
through basic alumina (ICN alimina, act. I). Solvent evapora-
tion was performed under reduced pressure at 40 °C using a
rotary evaporator. The instrumentation used was as follows:
1H NMR AMX 500 (Bruker); FT-IR FTS-60 (BIORAD); MS
(Finnigan); CI-MS MAT 8340; FAB-MS MAT 900; EI-MS MAT
212. LC: preparative low-pressure chromatography was per-
formed using silica gel 60 µm (230-400 mesh, E. Merck),
Ismatec pump, UV detector Uvicord SII (Pharmacia LKB).
HPLC analyses were performed on analytical columns (25 cm,
RP 18) using an acetonitrile-water gradient and flow-rates
of 1.0-1.5 mL/min. UV absorption was measured at 214 nm.
Abbreviations: Bop-Cl, N,N′-bis(2-oxo-3-oxazolidinyl)phos-
phinic chloride; HATU, 2-(1H-azabenzotriazol-1-yl)-1,1,3,3-
tetramethyluronium hexafluorophosphate; TBTU, 2-(1H-
Benzotriazol-1-yl)-1,1,3,3-tetramethyluronium tetrafluoroborate;
PyBroP, bromotripyrrolidinophosphonium hexafluorophos-
phate.
Gen er a l P r oced u r e for th e Syn th esis of 1-Boc-1-m eth -
yl-2-a lk ylh yd r a zin es (3). (a ) P r ep a r a tion of th e h yd r a -
zon es. Boc-methylhydrazine was reacted for 1-2 h with the
aldehyde or ketone as a 1:1 mixture in i-PrOH under reflux.
The solvent was removed under vacuo, and the crude products
were filtered over a pad of silica gel to give the hydrazones
virtually pure in quantitative yield. (b ) R ed u ct ion of t h e
Hyd r a zon es. To a solution of the above-prepared crude
hydrazones (68.4 mmol) in MeOH (150 mL), NaCNBH3 (88
mmol), and acetic acid (pH 3-4) were added at 0 °C. The
reaction mixture was stirred for 3-5 h at 0 °C. During this
time, the pH of the reaction mixture was kept at pH 3-4 by
addition of acetic acid. The reaction mixture was evaporated,
the residue dissolved in ethyl acetate (350 mL), and the
solution washed with water (20 mL). After drying with MgSO4,
the solvent was evaporated and the crude products 3a -e were
purified by filtration over silica gel (ethyl acetate/hexane). The
hydrazines 3 were obtained in 78-93% yield and were used
as crude products. They can be further purified either by
distillation or column chromatography. (b) Hyd r ogen a tion
of th e Hyd r a zon es. A solution of the hydrazone (90 mmol)
in 300 mL of ethanol was hydrogenated at atmospheric
hydrogen pressure in the presence of 260 mg of Pd/C (10%)
until the reaction was complete. The catalyst was removed by
filtration through Celite, and the solvent was evaporated to
give the crude hydrazines 3f and 3g in 75-98% yield. The
hydrazines 3 were used as crude products.
1-Boc-1-m eth yl-2-(4-br om oben zyl)h yd r a zin e 3g: yield
75% (HPLC purity 97%); pale yellow liquid; Rf ) 0.66 (cyclo-
hexanes-ethyl acetate 2:1); 1680 cm-1 1H NMR (500 MHz,
;
CDCl3) δ 1.47 (s, 9H), 2.91 (s, 3H), 3.90 (s, 2H), 7.35 (m, 4H);
MS (CI) m/e 259 (10), 214 (10), 171 (38), 169 (40); HRMS (ESI)
m/e 337.0520 (calcd for C13H19BrN2O2Na 337.0522).
Gen er a l P r oced u r e for th e Syn th esis of Boc-P r otected
Aza d id ep sip ep tid es A (6). 1. P h osgen e P r oced u r e (P r o-
ced u r e 1). A solution of the R-hydroxycarboxylic ester (15.0
mmol) and triethylamine (16 mmol) in THF (10 mL) was added
slowly (1 h) to a 1.93 M solution of phosgene (18.0 mmol) in
toluene at 0 °C. The reaction mixture was stirred for 30 min
at 0 °C, warmed to room temperature, stirred for another 1 h,
filtered, and concentrated in vacuo. The residue was dissolved
in THF (8 mL) and cooled to 0 °C. Within 1 h, a solution of
the hydrazine 3 (16.0 mmol) and triethylamine (16.0 mmol)
in THF (10 mL) was added dropwise. After the solution was
stirred for 12 h at room temperature, the solvent was evapo-
rated. The residue was dissolved in ethyl acetate, and the
solution was washed three times with brine, dried over Na2-
SO4, and evaporated. Chromatographic purification afforded
the didepsipeptides 6 in yields ranging from 59 to 76%.
2. Ca r bon Dioxid e P r oced u r e (P r oced u r e 2). Carbon
dioxide was passed through a suspension of cesium carbonate
(70.0 mmol) and hydrazine 3 (35.0 mmol) in DMF (140 mL)
for 30-60 min. tert-Butyl bromoacetate (35.0 mmol) was added
slowly, and carbon dioxide was bubbled through the mixture
for another 45 min. After being stirred for 12 h at room
temperature, the mixture was poured into water and extracted
with ethyl acetate (three times). The organic layers were
combined, dried over Na2SO4, filtered, and concentrated.
Chromatographic purification afforded the didepsipeptides 6
in yields ranging from 67 to 97%.
Ben zyl N-Boc-N-m eth yla za a la n ylh yd r oxya ecta te 6a :
procedure 2; yield 96% (HPLC purity 96%); yellow viscous oil;
IR 1712, 1758 cm-1; 1H NMR (500 MHz, CDCl3) δ 1.45 (s, 9H),
3.1 (s, 6H), 4.40-5.30 (m, 4H), 7.35 (5H); MS (FAB) m/e 353
([M + H]+, 100), 295 ([M - t-Bu]+, 5), 253 ([M + H - Boc]+,
40), 91 (100); HRMS (ESI) m/e 375.1523 (calcd for C17H24N2O6-
Na 375.1526).
Ben zyl N-Boc-N-m et h yla za va lylh yd r oxya cet a t e 6b :
procedure 2; yield 87% (HPLC purity 97%); yellow viscous oil;
IR 1715, 1764 cm-1; 1H NMR (500 MHz, CDCl3) δ 1.20 (d, 6H,
J ) 8 Hz), 1.45 (s, 9H), 3.08 (m, 3H), 4.3-5.25 (m, 5H), 7.35
(m, 5H); MS (FAB) m/e 325 (5), 281 ([M + H - Boc]+); HRMS
(ESI) m/e 403.1836 (calcd for C19H28N2O6Na 403.1839).
Ben zyl N-Boc-N-m et h yla za n or va lylh yd r oxya cet a t e
6c: procedure 2; yield 67% (HPLC purity 95%); yellow viscous
1-Boc-1-m eth yl-2-m eth ylh ydr azin e 3a: yield 83% (HPLC
purity 100%); colorless liquid; Rf ) 0.36 (cyclohexanes-ethyl
acetate 2:1); IR 1672 cm-1; 1H NMR (500 MHz, CDCl3) δ 1.49
(s, 9H), 2.55 (s, 3H), 3.0 (s, 3H); MS (CI) m/e 161 ([M + H]+,
3), 105 (100). HRMS (ESI) m/e 183.1104 (calcd for C7H16N2O2-
Na 183.1104).
1-Boc-1-m et h yl-2-isop r op ylh yd r a zin e 3b : yield 92%
(HPLC purity 100%); colorless liquid; Rf ) 0.57 (cyclohexane-
1
ethyl acetate 2:1); IR 1694 cm-1; H NMR (500 MHz, CDCl3)

Azadepsipeptides
J . Org. Chem., Vol. 66, No. 11, 2001 3765
1
oil; IR 1715, 1729, 1762 cm-1; H NMR (500 MHz, CDCl3) δ
0.93 (t, 3H, J ) 9 Hz); 1.47 (s, 9H), 1.63 (m, 2H), 2.90-3.55
(m, 5H), 4.4-5.25 (m, 4H), 7.36 (m, 5H); MS (FAB) m/e 381
([M + H]+, 5), 281 ([M + H - Boc]+, 50), 280 ([M - Boc]+, 53);
HRMS (ESI) m/e 403.1834 (calcd for C19H28N2O6Na 403.1839).
Ben zyl N-Boc-N-m eth yla za leu cylh yd r oxya ceta te 6d :
procedure 2; yield 84% (HPLC purity 92%); yellow viscous oil;
IR 1716, 1731 cm-1;1H NMR (500 MHz, CDCl3) δ 0.95 (d, 6H,
J ) 8 Hz), 1.45 (s, 9H), 1.95 (m, 1H), 3.0-3.4 (m, 5H), 4.4-
5.25 (m, 4H), 7.35 (m, 5H); MS (FAB) m/e 395 ([M + H]+, 20),
340 (25), 339 (100), 295 ([M + H - Boc]+, 18); HRMS (ESI)
m/e 417.1992 (calcd for C20H30N2O6Na 417.1996).
3H), 3.27 (d, 2H, J ) 8 Hz), 5.18 (AB system + m, 3H), 7.35
(m, 5H); MS (FAB) m/e 309 ([M + H]+, 100), 308 (M+, 89).
Ben zyl N-Boc-N-m eth yla za leu cyl-(R)-p h en ylla ctyl-N-
m eth yla za leu cyl-(R)-la cta te 16. Ethyl diisopropylamine
(9.69 g, 75.0 mmol) and BOP-Cl (9.16 g, 36 mmol) were added
at 0 °C to a solution of 14 (11.9 g, 30.0 mmol) and 13 (9.22 g,
30.0 mmol) in CH2Cl2 (120 mL). After 2 h at 0 °C, the reaction
mixture was allowed to warm to room temperature and stirred
for 12 h. The solution was washed three times with water,
dried over Na2SO4, and concentrated. Column chromatography
(cyclohexanes-ethyl acetate 6:1) of the residue gave 16 (9.25
g, 45%, HPLC purity 94%) as a pale yellow foam: [R]20
)
D
-24.196 (c ) 0.84, CHCl3); IR 1691, 1720, 1753 cm-1; 1H NMR
(500 MHz, CDCl3, mixture of conformers) δ 0.8-1.1 (m), 1.3-
1.7 (m), 2.7-3.3 (m), 5.0-5.3 (m), 7.1-7.5 (m); HRMS (FAB)
m/e 707.3628 (calcd for C36H52N4O9Na 707.3627).
Ben zyl N-Boc-N-m et h yla za n or leu cylh yd r oxya cet a t e
6e: procedure 2; yield 76% (HPLC purity 100%); yellow viscous
1
oil; IR 1714, 1731, 1764 cm-1; H NMR (500 MHz, CDCl3) δ
0.95 (m, 3H), 1.30-1.60 (m, 13H), 3.05-3.65 (m, 5H), 4.4-5.3
(m, 4H), 7.35 (m, 5H); MS (FAB) m/e 395 ([M + H]+, 5), 339
([M + H - t-Bu]+, 80), 295 ([M + 1 - Boc]+, 100); HRMS (ESI)
m/e 417.1999 (calcd for C20H30N2O6Na 417.1996).
Ben zyl N-Meth yla za leu cyl-(R)-p h en ylla ctyl-N-m eth y-
la za leu cyl-(R)-la ct a t e 18. To a solution of 16 (3.08 g, 4.5
mmol) in CH2Cl2 (60 mL) was added TFA (5.14 g, 45 mmol)
dropwise at 0 °C. The solution was allowed to warm to room
temperature and stirred for 15 h. Afterward, the solvent was
evaporated. The residue was dissolved in CH2Cl2, washed with
saturated aqueous NaHCO3 solution, and dried over Na2SO4.
Evaporation of the solvent afforded crude 18 (2.67 g, quant)
as a pale yellow foam that was used for subsequent reaction
without further purification: MS (FAB) m/e 585 ([M + H]+,
35), 584 (M+, 20).
Ben zyl N-Boc-N-m eth ylazaph en ylalan ylh ydr oxyacetate
6f: procedure 2; yield 97% (HPLC purity 97%); yellow viscous
oil; IR 1714 cm-1; H NMR (500 MHz, CDCl3) δ 1.45 (s, 9H),
1
2.80 (s, 3H), 4.3-5.3 (m, 6H), 7.35 (m, 10H); MS (FAB) m/e
429 ([M + H]+, 5), 371 (75), 329 ([M + H - Boc]+, 100), 328
(80); HRMS (ESI) m/e 451.1832 (calcd for C23H28N2O6Na
451.1839).
Ben zyl N-Boc-N-m eth yla za -4-br om op h en yla la n ylh y-
d r oxya ceta te 6g: procedure 2; yield 85% (HPLC purity 95%);
Ben zyl N-Boc-N-m eth yl-(S)-leu cyl-(R)-p h en ylla ctyl-N-
m eth yl-(S)-leu cyl-(R)-la ctyl-N-m eth yla za leu cyl-(R)-p h e-
n ylla ctyl-N-m eth yla za leu cyl-(R)-la cta te 19. To a solution
of 17 (2.67 g, 4.5 mmol), BOP-Cl (1.40 g, 5.50 mmol), and
N-methylmorpholine (0.59 g, 5.8 mmol) in CH2Cl2 (45 mL)-
was added 18 (2.62 g, 4.5 mmol), dissolved in CH2Cl2 (10 mL),
dropwise at 0 °C. Another 0.59 g of N-methylmorpholine was
added, and the reaction was stirred for 12 h at room temper-
ature. Subsequently, the solvent vas evaporated. The residue
was dissolved in ethyl acetate, washed with brine, dried over
Na2SO4, and concentrated. Column chromatography (cyclo-
hexanes-ethyl acetate 3:1) furnished 19 (2.40 g, 46%, HPLC
yellow viscous oil; IR 1720 cm-1; H NMR (500 MHz, CDCl3)
1
δ 1.45 (ms, 9H), 2.85 (ms, 3H), 4.25-5.35 (m, 6H), 7.45 (m,
9H); MS (FAB) m/e 507 (M+, 10), 453 (70), 451 (80), 409 (85),
407 (100); HRMS (ESI) m/e 529.0950 (calcd for C23H27BrN2O6-
Na 529.0945).
Ben zyl N-Boc-N-m eth yla za leu cyl-(R)-3-p h en ylla cta te
6h : procedure 1; yield 76% (HPLC purity 100%); yellow
viscous oil; Rf ) 0.82 (cyclohexanes-ethyl acetate 2:1); [R]20
D
) +4.676 (c ) 0.38, CHCl3); IR 1744 cm-1; 1H NMR (500 MHz,
CDCl3) δ 0.94 (d, 6H, J ) 8 Hz), 1,47 (s, 9H), 1.85 (m, 1H),
2,62 (d, 2H, J ) 8 Hz), 3.0 (s, 3h), 3,15 (m, 2H), 5.1 (m, 3H),
7.2 (m, 10H); MS (FAB) m/e 484 (M+, 1), 428 (10), 385 (40),
91(100); HRMS (ESI) m/e 507.2467 (calcd for C27H36N2O6Na
507.2466).
purity 96%): Rf ) 0.50 (cyclohexanes-ethyl acetate 1:1); [R]20
D
-54.816 (c ) 0.39, CHCl3); IR 1690, 1729 cm-1; 1H NMR (500
MHz, CDCl3, mixture of conformers) δ 0.8-1.0 (m), 1.35-1.60
(m), 2.7-3.7 (m), 4.6-5.5 (m), 7.1-7.4 (m); MS (FAB) m/e 1059
([M + H - Boc]+, 100), 1181 ([M + Na]+, 58); HRMS (FAB)
m/e 1181.6362 (calcd for C62H90N6O15Na 1181.6349).
Ben zyl N-Boc-N-m eth yla za leu cyl-(R)-la cta te 6i: proce-
dure 1; yield 59% (HPLC purity 94%); yellow viscous oil; Rf )
0.61 (cyclohexanes-ethyl acetate 3:1); [R]20 ) +3.871 (c )
D
Bis-a za P F 1022A 9. TFA (1.34 g, 11.73 mmol) was added
dropwise to a solution of 19 (1.36 g, 1.173 mmol) in CH2Cl2
(10 mL) at 0 °C. The reaction mixture was stirred for 12 h at
0 °C followed by evaporation of the solvent and dissolution of
the residue in ethyl acetate. The solution was washed with
saturated NaHCO3 solution and brine, dried over Na2SO4, and
concentrated. The crude material (0.95 g, 76%, MS (ESI) m/e
1059 (M+, 100)) was dissolved again in ethyl acetate (30 mL)
and hydrogenated (Pd/C 10%, 500 mg) at room temperature
until the hydrogen consumption ceased. The catalyst was
removed by filtration through Celite, followed by evaporation
of the solvent. The crude product (800.6 mg, 0.825 mmol, MS
(ESI) m/e 969 (M+, 100)) was dissolved in CH2Cl2 (1000 mL),
ethyl diisopropylamine (266.3 mg, 2.06 mmol) and BOP-Cl (252
mg, 0.99 mmol) were added at 0 °C, and the reaction was
allowed to warm to room temperature where it was stirred
for 24 h. Subsequently, the solution was washed with satu-
rated aqueous NaHCO3 solution and water. After drying over
Na2SO4 and evaporation of the solvent, the product was
purified by column chromatography (toluene-2-propanol 10:
1) and preparative HPLC (CH3CN-H2O 75:25) to give 9 (321
mg, 41%, HPLC purity 94%) as a white solid: mp 90-91 °C;
0.46, CHCl3); IR 1709, 1754 cm-1; 1H NMR (500 MHz, CDCl3)
δ 0.94 (d, 6H, J ) 8 Hz), 1.48 (s, 9H), 1.53 (d, 3H, J ) 8.5 Hz),
1.70 (m, 1H), 2.62 (d, 2H, J ) 7.5 Hz), 3.02 (s, 3H), 5.06 (m,
1H), 5.20 (AB-system, 2H), 7.34 (m, 5H); MS (FAB) m/e 409
([M + H]+, 2), 353 (100), 309 ([M + H - Boc]+, 45); HRMS
(ESI) m/e 431.2154 (calcd for C21H32N2O6Na 431.2153).
N-Boc-N-m eth yla za leu cyl-(R)-3-p h en ylla ctic Acid 13.
Compound 6g (15.54 g, 30.0 mmol) was dissolved in ethyl
acetate (150 mL) and hydrogenated (Pd/C, 10%, 400 mg) for 4
h at room temperature. After filtration of the catalyst and
evaporation of the solvent, the crude product was obtained as
a yellow viscous oil in quantitative yield (11.9 g): Rf ) 0.08
(cyclohexanes-ethyl acetate 2:1); IR 1750, 2900-3400 cm-1
;
1H NMR (500 MHz, CDCl3) δ 0.95 (d, 6H, J ) 8 Hz), 1.35-
1.55 (m, 10H), 1.85 (m, 1H), 2.75 (s, 3H), 2.95-3.30 (m, 8H);
5.1 (m, 1H), 7.30 (m, 5H); MS (FAB) m/e 417 ([M + Na]+, 20),
395 ([M + H]+, 10), 394 (M+, 5), 339 (70), 321 (40), 295 (100).
Ben zyl N-Meth yla za leu cyl-(R)-la cta te 14. TFA (27.36 g,
240 mmol) was added dropwise at 0 °C to a solution of 6h
(12.25 g, 30.0 mmol) in CH2Cl2 (100 mL). The reaction mixture
was stirred for 24 h at room temperature. Then the solvent
was evaporated, and the residue was dissolved in ethyl acetate
and washed with aqueous NaHCO3 solution and brine. After
the solution was washed with Na2SO4, the solvent was
removed and the residue purified by column chromatography
(cyclohexanes-ethyl acetate 7:1), furnishing 14 (7.21 g, 78%,
HPLC purity 98%) as a yellow viscous oil: Rf ) 0.50 (cyclo-
hexanes-ethyl acetate 2:1); 1H NMR (500 MHz, CDCl3) δ 0.90
(d, 6H, J ) 8 Hz), 1.53 (d, 3H, J ) 8 Hz), 2.0 (m, 1H), 2.70 (s,
[R]20 ) -80.613 (c ) 0.34, CHCl3); IR 1668, 1728 cm-1 1H
;
D
NMR (500 MHz, CDCl3, assignment for the asymmetrical
conformer) 0.82 and 0.93 (d, 6H, Hδ-MeLeu2, J ) 8.0 Hz), 0.86
and 0.97 (d, 6H, Hδ-aza-MeLeu4, J ) 8.0 Hz), 0.88 (d, 6H, Hδ-
aza-MeLeu6, J ) 8.0 Hz), 0.92 (d, 6H, Hδ-MeLeu8, J ) 8.0 Hz),
1.08 (d, 3H, Hâ-Lac3, J ) 6.8 Hz), 1.32 (m, 1H, Hγ-MeLeu8),
1.39 (d, 3H, Hâ-Lac,7 J ) 6.9 Hz), 1.53 (m, 2H, Hâ-MeLeu2),
1.54 (m, 2H, Hγ-aza-MeLeu6 + Hγ-MeLeu2), 1.55 (m, 1H, Hâ-

3766 J . Org. Chem., Vol. 66, No. 11, 2001
Dyker et al.
MeLeu8), 1.65 (ddd, 2H, Hâ-MeLeu2, J ) 4.2, 10.5, 14.7 Hz),
1.72 (ddd, 1H, Hâ-MeLeu8, J ) 4.2, 10.8, 14.7 Hz), 2.06 (m,
1H, Hγ-aza-MeLeu4), 2.88 (s, 3H, NCH3-MeLeu8), 2.94 (dd, 1H,
Hâ-aza-MeLeu6, J ) 5.1, 14.7 Hz), 2.97 (s, 3H, NCH3-MeLeu2),
3.13 and 3.14 (m, 2H, Hâ-PheLac5), 3.13 and 3.18 (m, 2H, Hâ-
PheLac1), 3.14 (s, 3H, NCH3-aza-MeLeu4), 3.16, (s, 3H, NCH3-
aza-MeLeu6), 3.54 (dd, 1H, Hâ-aza-MeLeu4, J ) 5.9, 14.4 Hz),
3.66 (dd, 1H, Hâ-aza-MeLeu4, J ) 5.9, 14.4 Hz), 3.67 (dd, 1H,
Hâ-aza-MeLeu6, J ) 6.7, 14.7 Hz), 4.95 (q, 1H, HR-Lac3, J )
6.8 Hz), 5.39 (dd, 1H, HR-MeLeu2, J ) 4.2, 11.8 Hz), 5.42 (t,
1H, HR-PheLac,5 7.3 Hz), 5.55 (dd, 1H, HR-MeLeu8, J ) 4.2,
11.9 Hz), 5.58 (t, 1H, HR-PheLac1, J ) 7.7 Hz), 5.59 (q, 1H,
HR-Lac,7 J ) 6.9 Hz); MS (FAB) m/e 973 ([M + Na]+, 100),
951 ([M + H]+, 36); HR-MS (ESI) m/e 973.5264 (calcd for
C
50H74N6O12Na 973.5262).
Ack n ow led gm en t . We thank Meiji Seika Kaisha
Ltd. for providing us with an authentic sample of
PF1022A.
1
Su p p or tin g In for m a tion Ava ila ble: Copies of H NMR
of 3c,d , 6, 16, 19, and 9 and 13C NMR spectra of 3, 6, 16, 19,
and 9 as well as ORTEP figures for 8 and 9. This material is
available free of charge via the Internet at http://pubs.acs.org.
J O001749V