Assignment of the liposidomycin diazepanone stereochemistry

Article abstract of DOI:10.1021/jo010355g

The liposidomycins comprise a family of complex nucleoside antibiotics that inhibit bacterial peptidoglycan synthesis. Their structures (1, 2) feature nucleoside, ribofuranoside, diazepanone, and lipid regions. Several stereogenic centers remain unassigned, including three within the diazepanone region: C-6′, C-2?, and C-3?. An intramolecular reductive amination reaction has been used to prepare model diazepanones. Analysis of 40 and two of its diastereomers by NMR spectroscopy, X-ray crystallography, and molecular modeling indicates a close relative configurational and conformational match between 40 and the liposidomycin diazepanone degradation product 43 and allows the assignment of stereochemistry of the natural products as either [C-6′(R), C-2?(R), C-3?(R)] or [C-6′(S), C-2?(S), C-3?(S)].

Full text of DOI:10.1021/jo010355g

5822
J . Org. Chem. 2001, 66, 5822-5831
Assign m en t of th e Lip osid om ycin Dia zep a n on e Ster eoch em istr y
Spencer Knapp,* Gregori J . Morriello, Santosh R. Nandan, Thomas J . Emge, George A. Doss,§
Ralph T. Mosley,§ and Lijian Chen
Department of Chemistry & Chemical Biology, Rutgers - The State University of New J ersey,
P.O. Box 939, Piscataway, New J ersey 08854-8087, and Merck & Co., P.O. Box 2000,
Rahway, New J ersey 07065-0900
knapp@rutchem.rutgers.edu
Received April 4, 2001
The liposidomycins comprise a family of complex nucleoside antibiotics that inhibit bacterial
peptidoglycan synthesis. Their structures (1, 2) feature nucleoside, ribofuranoside, diazepanone,
and lipid regions. Several stereogenic centers remain unassigned, including three within the
diazepanone region: C-6′, C-2′′′, and C-3′′′. An intramolecular reductive amination reaction has
been used to prepare model diazepanones. Analysis of 40 and two of its diastereomers by NMR
spectroscopy, X-ray crystallography, and molecular modeling indicates a close relative configura-
tional and conformational match between 40 and the liposidomycin diazepanone degradation product
43 and allows the assignment of stereochemistry of the natural products as either [C-6′(R),
C-2′′′(R), C-3′′′(R)] or [C-6′(S), C-2′′′(S), C-3′′′(S)].
In tr od u ction
Ubukata and co-workers,8 Kim and co-workers,9 Dini and
co-workers,10 Le Merrer and co-workers,11 and our-
selves,12,13 resulting in the respective syntheses of a ri-
bosyl-substituted diazepanone, a diazepanone nucleoside,
a nucleoside disaccharide, an isoascorbic acid-derived
diazepanone, and the 1,4-dimethyl-1,4-diazepan-2-ones
(3) elaborated with carbethoxy and benzoyloxy substit-
uents at C-2′′′ and C-3′′′. To this point, the cumulative
analytical data have been insufficient to permit assign-
ment of the diazepanone stereocenters. In this paper we
present the experimental details of our earlier studies
on 3, the synthesis and characterization of three diaster-
eomers of the valine-derived trisubstituted diazepanones
4, and finally the comparison of the isomers of 4 with
the degradation product of 2 and the evidence for
assigning the stereochemistry of 1 and 2 as either [C-6′-
(R), C-2′′′(R), C-3′′′(R)] or [C-6′(S), C-2′′′(S), C-3′′′(S)].
The liposidomycins are complex lipid-bearing nucleo-
side antibiotics (e. g., 1 and 2) that selectively inhibit
bacterial peptidoglycan synthesis.1-4 They have been
shown to specifically inhibit in vitro the Escherichia
coli phospho-N-acetylmuramyl-pentapeptide translocase
(translocase I), which catalyzes the first step in the
peptidoglycan lipid cycle.5,6 Additionally, 1 inhibits in rat
liver microsomes the formation of polyprenyl (pyro)-
phosphate N-acetylglucosamine, an intermediate in gly-
coprotein and teichoic/teichuronic acid biosynthesis.7
Structures were ascribed to 1 and 2 based on degradation
studies, and NMR and mass spectral evidence, but the
relative and absolute stereochemistry at four carbons on
or near the diazepanone ring (5′, 6′, 2′′′, and 3′′′) has not
yet been established. Because of the flexibility of the
seven-membered ring and the unusual nature of the ring
atoms and substituents, vicinal 1H-1H coupling con-
stants may not give a reliable indication of the relative
disposition of the vicinal substituents found on C-2′′′ and
C-3′′′, and NOE studies have likewise not yet afforded a
complete picture of more distant stereochemical relation-
ships. With the limited supply of material for further
degradation or crystallographic studies, the synthesis of
model compounds and degradation products becomes an
important tool for structural assignment. Several re-
search groups have engaged in this activity, among them
Resu lts a n d Discu ssion
Th e Red u ctive Am in a tion Ap p r oa ch to 1,4-Di-
a zep a n -2-on es. Searching in 1988 for literature prece-
dent for the synthesis of 1,4-diazepan-2-ones, we were
unable to find a method suitable for making the liposi-
domycin diazepanone itself. The reaction of 1,3-diamines
with glyoxal to give simple diazepanones had been
(8) Spada, M. R.; Ubukata, M.; Isono, K. Heterocycles 1992, 34,
1147-1167.
(9) Kim, K. S.; Ahn, Y. H. Tetrahedron: Asymmetry 1998, 9, 3601-
§ Merck & Co.
3605.
(1) Isono, K.; Uramoto, M.; Kusakabe, H.; Kimura, K.; Izaki, K.;
Nelson, C. C.; McCloskey, J . A. J . Antibiot. 1985, 38, 1617-1621.
(2) Ubukata, M.; Isono, K.; Kimura, K.; Nelson, C. C.; McCloskey,
J . A. J . Am. Chem. Soc. 1988, 110, 4416-4417.
(3) Ubukata, M.; Kimura, K.; Isono, K.; Nelson, C. C.; Gregson, J .
M.; McCloskey, J . A. J . Org. Chem. 1992, 57, 6392-6403.
(4) Esumi, Y.; Suzuki, Y.; Kimura, K.; Yoshihama, M.; Ichikawa,
T.; Uramoto, M. J . Antibiot. 1999, 52, 281-287.
(5) Kimura, K.; Miyata, N.; Kawanishi, G.; Kamio, Y.; Izaki, K.;
Isono, K. Agric. Biol. Chem. 1989, 53, 1811-1815.
(6) Brandish, P. E.; Kimura, K.; Inukai, M.; Southgate, R.; Lonsdale,
J . T.; Bugg, T. D. H. Antimicrob. Agents Chemother. 1996, 40, 1640-
1644.
(7) Muroi, M.; Kimura, K.; Osada, H.; Unukai, M.; Takatsuki, A. J .
Antibiot. 1997, 50, 103-104.
(10) Dini, C.; Collette, P.; Drochon, N.; Guillot, J . C.; Lemoine, G.;
Mauvais, P.; Aszodi, J . Bioorg. Med. Chem. Lett. 2000, 10, 1839-1843.
(11) Gravier-Pelletier, C.; Charvet, I.; Le Merrer, Y.; Depezay, J .-
C. J . Carbohydr. Chem. 1997, 16, 129-141.
(12) Knapp, S.; Nandan, S.; Resnick, L. Tetrahedron Lett. 1992, 33,
5485-5486.
(13) Additional synthetic studies: Kim, K. S.; Cho, I. H.; Ahn, Y.
H.; Park, J . I. J . Chem. Soc., Perk. I 1995, 1783-1785. Kim, K. S.;
Ahn, Y. H.; Hurh, E. Y.; Kim, K. T.; J oo, Y. H. Bull. Korean Chem.
Soc. 1994, 15, 829. Kim, K. S.; Lim, J . W.; J oo, Y. H.; Kim, K. T.; Cho,
I. H.; Ahn, Y. H. J . Carbohydr. Chem. 1995, 14, 439. Kim, K. S.;
Cheong, C. S.; Hahn, J . S.; Park, J . I. Bull. Korean Chem. Soc. 1997,
18, 465. Moore, W. J .; Luzzio, F. A. Tetrahedron Lett. 1995 36, 6599-
6602, Kalyanakumar, R.; Chadha, M. S.; Chattopadhyay, A., Mam-
dapur, V. R. Indian J . Chem. B 1996, 35, 356-358.
10.1021/jo010355g CCC: $20.00 © 2001 American Chemical Society
Published on Web 07/26/2001

Liposidomycin Diazepanone Stereochemistry
J . Org. Chem., Vol. 66, No. 17, 2001 5823
Sch em e 1. On e-P ot Dia zep a n on e Syn th esis
Sch em e 2. r-Keto Ester Ap p r oa ch es
demonstrated,14 and when the symmetrical partners
N,N′-dimethylpropane-1,3-diamine (5) and the sodium
bisulfite adduct of glyoxal (6) were heated together at
reflux, the expected 1,4-dimethyl-1,4-diazepan-2-one 7
was formed in modest yield (Scheme 1). The harsh
conditions of this reaction, however, might well cause
epimerization or elimination of the substituents of more
elaborated diazepanones such as 3 and 4, even if ap-
propriate asymmetrical partner reagents could be identi-
fied. Because of these anticipated difficulties, this ap-
proach was not pursued further.
Sch em e 3. Red u ctive Am in a tion Ap p r oa ch
Two attempts were made to develop a diazepanone
synthesis based on R-keto ester amination as a method
for closing the N-1/C-7 bond (Scheme 2). Reaction of
vinylmagnesium bromide (9) with ethyl glyoxylate (8)
was presumed to give rise initially to the unsaturated
keto ester 10, but attempts to trap 10 in situ with
sarcosine N-methylamide (11) were unsuccessful. The
tris-thio-ortho ester 14 was then prepared in the hope
that it would behave as a nonpolymerizing version of 10.
Thus tris(methylthio)methyllithium15 was added to ac-
rolein (13), and the resulting alcohol was oxidized to
provide vinyl ketone 14. Michael addition of 11 to 14
proceeded smoothly to afford the protected keto ester 15,
but several attempts to close the diazepanone ring by way
of a carbinol amide were unsuccessful, and this approach
was also abandoned. Vinyl ketone 14 is an interesting
variant on methyl vinyl ketone and may have applica-
tions in other contexts as a Michael acceptor or dieno-
phile.
smoothly with allyl iodide. The resulting protected N-
BOC amino ester 17 was N-deprotected, and the amine
was coupled with N-(carbobenzyloxy)sarcosine to afford
the dipeptide 19. Oxidative cleavage of the vinyl group
led to an aldehyde, which was not isolated, but rather
was cyclized under reductive amination conditions (which
also removed the N-Cbz) to afford the diazepanone 20
directly. The overall efficiency and mild conditions as-
sociated with this synthesis of 20 indicate that more
complex diazepanones might also be made by using
reductive amination as the key cyclization step.
Ring closure of the alternative N-4/C-5 bond by reduc-
tive amination proved to be more successful (Scheme 3).
N-(tert-Butoxylcarbonyl)sarcosine ethyl ester (16) was
converted to its lithium enolate, which was alkylated
The next level of diazepanone complexity was indeed
reached by using an aldol condensation to assemble the
initial amino ester (Scheme 4). The lithium enolate of
16 was condensed with acrolein to give a mixture of
(14) McDougall, R. H.; Malik, S. H. J . Chem. Soc. 1969, 2044-2051,
(15) Seebach, D. Angew. Chem., Int. Ed. Eng. 1967, 6, 442-443.

5824 J . Org. Chem., Vol. 66, No. 17, 2001
Knapp et al.
Sch em e 4. Sa r cosin e Ald ol Rea ction a n d P r oof of
Ster eoch em istr y
Sch em e 5. Dia zep a n on e Dia ster eom er s P r ep a r ed
by Red u ctive Am in a tion
ethoxycarbonyl substituent for pseudoaxial orientation
may reflect its avoidance of the nearly eclipsing steric
interaction with the amide N-methyl that would result
if the two groups were to lie in the equatorial plane. A
four bond “W” coupling (J 3,5 ) 2.0 Hz) between H-3eq
and H-5eq provides further evidence for the conformation
shown for 30. The cis diazepanone 31 shows an H-6/H-7
coupling constant of 2.2 Hz, which is consistent with a
conformation analogous to that of 30, but with the
benzoyloxy substituent at C-6 pseudoequatorial. The
pseudoaxial ethoxycarbonyl group in 31 likewise main-
tains its distance from the vicinal amide N-methyl. The
chairlike diazepanone conformations with pseudoaxial
ethoxycarbonyls shown as 30 and 31 are reproduced as
minima by calculations using the MacroModel program
(Amber subroutine),17 which also predicts18 H-6/H-7
coupling constants of 5.1 and 1.8 Hz, respectively.
Either diazepanone diastereomer, 30 or 31, could be
converted to the unsaturated ester 32 by treatment with
DBU in toluene solution (Scheme 6). Hydrolysis of the
ester gave the lithium salt 33, and further treatment with
anhydrous hydrochloric acid led to the ammonium salt
34. The ability to generate this tetrahydro-1,4-diazepin-
2-one carboxylate ring system from any of several ste-
reoisomeric diazepanone esters should prove useful for
eventual synthesis of the analogous liposidomycin deg-
radation product 35 (Scheme 6, in box), and thus assign-
ment of the stereochemistry at C-5′ and C-6′ of 1 and 2.
Syn th esis a n d Ch a r a cter iza tion of Va lin e-De-
r ived Dia zep a n on es. The diazepanones 30 and 31 lack
a substituent at C-3 that would correspond to the car-
bohydrate chain attached at C-6′ of the liposidomycins,
and thus they are inadequate as models for determining
the stereochemistry and conformation of the diazepanone
ring of the natural products. A bulky alkyl substituent
at C-3 might, however, provide conformational bias
similar to that in the natural system. Therefore, a route
that was expected to lead to the four diastereomers of
unstable aldol adducts 21, which were immediately
benzoylated in situ to provide two allylic benzoates 22/
23 (4:1). Although the diastereomers could not be sepa-
rated at this point, the mixture was reduced to the diols
24 and 25, which were separated and independently
converted to their acetonide derivatives (26 and 27,
respectively). The major aldol adduct could be assigned
the anti stereochemistry based on the H-4/H-5 coupling
constant of 8.4 Hz, indicating a trans-diaxial disposition
of these protons in 26. The corresponding J for the cis
H-4/H-5 protons of 27 is 3.2 Hz. Preference for the anti
stereoisomer in the lithium aldol condensation is the
expected result.16
The aldol benzoate mixture 22/23 was taken on to the
dipeptides 28 and 29 by removal of the N-BOC protecting
group, neutralization, and then peptide coupling with
N-(carbobenzyloxy)sarcosine 18 (Scheme 5). The dipep-
tides were separable at this point and were isolated in
80% overall combined yield [70% 28 (anti) and 10% 29
(syn)]. The change in isomer ratio from 4:1 to 7:1 is
possibly due to selective destruction of the syn amino
benzoate by O-to-N benzoyl migration. Ozonolysis of the
vinyl group, reduction of the ozonide, and then cyclization
by reductive amination as before gave the diazepanone
30 exclusively from anti dipeptide 28, and diazepanone
31 exclusively from syn dipeptide 29. No crossover
products were observed, demonstrating that the respec-
tive stereochemical relationships in 28 and 29 are
maintained in 30 and 31.
The structures of isomeric diazepanones 30 and 31
were established by 1H NMR spectroscopic analysis.
Trans isomer 30 exhibited an H-6/H-7 coupling constant
of 5.2 Hz, which is consistent with pseudoequatorial H's,
and pseudoaxial benzoyloxy and ethoxycarbonyl substit-
uents at C-6 and C-7, respectively. Pseudoaxial H's at
C-6 and C-7 would have been expected to show larger
coupling, perhaps 8 Hz or greater. The preference of the
(17) Mohamadi, F.; Richards, N. G. J .; Guida, W. C.; Liskamp, R.;
Lipton, M.; Caufield, C.; Chang, G.; Hendrickson, T.; Still, W. C. J .
Comput. Chem. 1990, 11, 440-467.
(18) Haasnoot, C. A. G.; de Leeuw, F. A. A. M.; Altona, C.
Tetrahedron 1980, 26, 2783-2792.
(16) Heathcock, C. H. The Aldol Addition Reaction. In Asymmetric
Synthesis, Vol. 3, Stereodifferentiating Addition Reactions, Part B;
Morrison, J . D., Ed.; Academic Press: Orlando, 1984; pp 111-212.

Liposidomycin Diazepanone Stereochemistry
J . Org. Chem., Vol. 66, No. 17, 2001 5825
Sch em e 6. Elim in a tion a n d Hyd r olysis Rea ction s
of Dia zep a n on es
The structures, stereochemistry, and conformations of
37-42 were established by 1H and 13C NMR spectroscopy
(Tables 1 and 2), by NOE studies (Figure 1) and by X-ray
crystallography (Supporting Information). The major
diazepanone and its N-methylated derivative (37 and 40,
respectively) have the configurations 3(S), 6(S), and 7(S).
In chloroform-d or acetonitrile-d3 solution they have very
similar average conformations resembling a chair flat-
tened at N-1/C-2 (“1,2C5”, see also 30), with pseudoaxial
ethoxycarbonyl and benzoyloxy groups at C-7 and C-6,
respectively, and a pseudoequatorial isopropyl group at
C-3. In the major N-methylated diazepanone 40 the N-4
methyl substituent is pseudoaxial, as suggested by three
NOE cross-peaks between this methyl and H-5eq, the
benzoyl o-H's, and the isopropyl CH, respectively. This
is also the N-methyl orientation that minimizes steric
repulsion with the vicinal and pseuodoequatorial isopro-
pyl group. The isopropyl CH and H-3 are anti (J ) 10.1
Hz, no NOE) in 40 but not in 37 (J ) 6.2 Hz, strong
NOE). An NOE cross-peak confirms the cis diaxial
relationship of H-3 and H-5ax in both compounds. An
important indication of ring conformation is the four-bond
“W” coupling (e1 Hz) observed in both 37 and 40 for the
pseudoequatorial protons H-5eq and H-7. For 40, this
coupling is somewhat more prominent in acetonitrile-d3
solution (J ) 2 Hz). Irradiation of H-7 sharpened the
broad signal for H-5eq to a clean dd (J ) 15.8, 2.2 Hz),
reflecting only the geminal coupling to H-5ax and the
vicinal coupling to H-6. As in the case of the simpler
diazepanone model 30, the ethoxycarbonyl substituent
at C-7 of 37 and 40 probably occupies a pseudoaxial
position to avoid an eclipsing steric interaction with the
pseudoequatorial N-1 methyl.
Sch em e 7. Syn th esis of Va lin e-d er ived
Dia zep a n on es
One minor diazepanone 38 and its N-methylated
derivative 41 have configurations 3(S), 6(R), and 7(R).
In other words, they are epimeric with 37/40 at C-6 and
C-7, but nevertheless also have trans pseudodiaxial
benzoyloxy and ethoxycarbonyl substituents at C-6 and
C-7, respectively (J 6,7 ) 4.8-5.3 Hz for the diazepanones
37, 38, 40, and 41). Unlike 37, the minor diazepanone
38 exists in a completely different average conformation
from its N-methylated derivative 41 in deuteriochloro-
form (or acetonitrile-d3, where the absorbances are
sharper) solution (Figure 1). The seven-membered ring
appears to accommodate the cis 3(S),7(R) relationship
(which would otherwise bring the isopropyl and ethoxy-
carbonyl substituents into close transannular contact;
compare 37), by deforming into a pseudoboat. Thus the
H-3/H-5ax NOE cross-peak observed for 37/40 is retained
in 38, but the H-5ax/H-6 cross-peak is lost. No transan-
nular cross-peaks involving the isopropyl group are
observed. Upon N-methylation to 41, the diazepanone
ring returns to a pseudochair reminiscent of 37/40, but
with a pseudoaxial isopropyl group as evidenced by a new
isopropyl CH/H-5ax NOE cross-peak. The pseudoaxial
isopropyl comes into close transannular contact with the
ethoxycarbonyl substituent at C-7. The change in con-
formation from pseudoboat (38) to pseudochair (41) upon
N-methylation can be attributed to avoidance of the
vicinal methyl/isopropyl steric interaction that would
have developed at C-3/N-4 had the ring maintained the
pseudoboat shape.
the 3-isopropyl-1,4-diazepan-2-ones 4 was devised and
implemented (Scheme 7).
The benzoylated acrolein adducts 22/23 (approximate
1:1 mixture) were N-deprotected, and the resulting
amines were neutralized and then immediately coupled
with the activated ester prepared from N-(tert-butoxy-
carbonyl)-valine and 2-chloro-4,6-dimethoxy-1,3,5-tri-
azine.19 Attempted coupling with the corresponding N-
methylvaline derivative was unsuccessful, so N-methy-
lation was postponed until after the cyclization. The di-
peptides 36 were then taken through a four-step se-
quence consisting of ozonolysis of the vinyl group, reduc-
tion of the ozonide, removal of the N-BOC with anhy-
drous HCl, and finally reductive cyclization of the amino
aldehyde under the action of sodium triacetoxyborohy-
dride. Only three of the four possible diastereomeric
cyclization products were detected: a “major” isomer 37,
isolated in pure form, and two “minor” isomers, 38 and
39, isolated and analyzed as an inseparable 4:1 mixture.
N-Methylation produced the desired pentasubstituted
1,4-diazepan-2-one models 40 (from 37) and pure 41 and
42 (from the mixture 38/39; compare 4).
Overall, the conformations of these 1,4-diazepan-2-ones
seem to be largely determined by the vicinal interactions,
sometimes with the result that possibly unfavorable
transannular interactions arise. The vicinal interactions
(19) Kaminski, Z. J . Synthesis 1987, 917-920.

5826 J . Org. Chem., Vol. 66, No. 17, 2001
Ta ble 1. 1H NMR Sp ectr a of Dia zep a n on es
Knapp et al.
position
major N-H diazepanone 37 (in CDCl3)
minor N-H diazepanone 38 (in CDCl3)
CHMe2
H-3
H-7
2.21
2.66
4.56
5.55
3.45
3.22
3.05
sept, 6.6 Hz
d, 6.2 Hz
2.67-2.75
3.25
4.32
6.01
3.72
m
d, 3.4 Hz
d, 5.3 Hz
ddd, 7.8, 5.6, 2.1
dd, 13.8, 8.0
dd, 13.8, 2.1
s
dd, 5.3, 1.0 Hz
dt, 2.5, 5.3 Hz
ddd, 15.3, 2.9, 1.0 Hz
dd, 15.4, 2.2 Hz
s
H-6
H-5eq
H-5ax
CONCH3
2.75
3.07
position
major N-Me diazepanone 40 (in CDCl3)
minor N-Me diazepanone 41 (in CDCl3)
CHMe2
H-3
H-7
2.22-2.32
2.64
4.51
5.63
3.50
3.32
2.42
3.11
m
1.68-1.79
m
3, 10.1 Hz
d, 4.8 Hz
td , 2.3, 4.9 Hz
dd, 15.8, 2.5 Hz
br d d , 15.8, 2.2 Hz
s
s
3.19
4.52
5.76
3.47
3.14
2.72
3.16
d, 11.2 Hz
d, 5.0 Hz
d d d , 5.6, 3.5, 2.4 Hz
dd, 16.0, 2.5 Hz
d d , 16.0, 3.5 Hz
br s
H-6
H-5ax
H-5eq
N-CH3
CONCH3
s
position
all-cis N-Me diazepanone 42 (in CDCl3)
CHMe2
H-3
H-7
2.22
3.19
4.94
5.62
3.48
3.30
2.38
3.15
m
d, 10 Hz
br s
br t, 2.7 Hz
d, 14.5 Hz
br m
H-6
H-5ax
H-5eq
N-CH3
CONCH3
br s
s
position
lipo degradation product 43 (in D2O)
H-5′
H-6′
H-2′′′
H-3′′′
H-4b′′′
H-4a′′′
N-CH3
CONCH3
4.18
3.68
4.22
4.46
3.16
3.21
2.41
3.09
dd, 10, 1.6 Hz
d, 10 Hz
d, 4.8 Hz
td , 2.7, 4.8 Hz
dd, 15.3, 2.7
br d d , 15.3, 2.7
s
s
Ta ble 2. 13C NMR Da ta for Dia zep a n on esa
(Figure 1) despite the greater possibilities for H-bonding
and other interactions in the solid state. This is further
evidence that vicinal steric interactions are the main
determinants of conformation in these isomers. The close
transannular approach of the isopropyl CH and the
ethoxycarbonyl in the minor diazepanone 41 can now be
directly evaluated: the distances Me2CH‚‚‚CO2 and
Me2CH‚‚‚CO2 are 3.25 and 2.4 Å, respectively. The N-4
methyl and the benzoyloxy oxygen are in close transan-
nular contact in both isomers (NCH3‚‚‚OCOPh for 40/41
is 2.98 and 2.95 Å, respectively). In 41, the chloride atom
H-bonds to the (N-4) pseudoequatorial NH+.
position
37
38
40
41
42
43 (D2O)
CHMe2
C-3
30.8
66.6
71.2
64.8
51.6
-
32.0
71.3
76.6
64.1
50.7
-
26.7
70.6
72.5
64.8
58.5
36.6
38.2
26.4
77.8
72.5
64.9
50.2
38.8
43.5
26.4
70.2
70.2
61.5
59.1
37.2
33.8
76.8
62.9
68.3
59.5
58.4
38.5
36.5
C-6
C-7
C-5
N-CH3
CONCH3
38.7
40.0
a
Chemical shifts and assignments by HETCOR in CD3CN
solution (this work) except for 43 (data for the corresponding C’s
from ref 3).
1
Mass spectral and H NMR analysis of 42 indicate it
in 40 and 41 appear to be comparable; however, 41 has
a more destabilizing transannular interaction (isopropyl/
ethoxycarbonyl). The upfield position of the isopropyl CH
of 41 (-0.5 ppm relative to 40) may be due to transan-
nular anisotropic shielding by the nearby ethoxycarbonyl.
The H-5eq/H-7 “W” coupling observed for 37/40 is not
seen in 38/41. As with 37/40, the isopropyl CH and H-3
are anti (J ) 11.2 Hz, no NOE) in 41 but not in 38 (J )
3.4 Hz, strong NOE).
X-ray crystal structures were obtained for the major
and minor N-methylated diazepanones 40 and 41, re-
spectively, although the latter formed suitable crystals
only as the hydrochloride salt. In addition, the absolute
stereochemistry 3(S),6(R),7(R) was confirmed for 41‚HCl.
Details of the crystallographic analysis including ORTEP
drawings can be found in Supporting Information. The
crystal structures fully confirm the NOE data obtained
in CD3CN solution; in fact, the conformations of 40/41‚
HCl essentially match the solution phase conformations
to be a diastereomer of 40 and 41, which must therefore
have either the 3(S),6(R),7(S), or the 3(S),6(S),7(R) ster-
eochemistry. Deuteriochloroform and acetonitrile solu-
tions of 42 showed broadened signals for H-7, H-6, the
H-5's, the isopropyl CH, and the N-4 methyl because of
conformational changes, possibly including nitrogen in-
version at N-4, that are slow on the NMR time scale. A
deuteriobenzene solution of 42 at 60 °C, however, showed
sharp and well-resolved signals for all protons, and NOE
analysis could be carried out (Figure 1). Two transan-
nular NOE cross-peaks (H-3/H-7, H-7/H-5ax), taken with
an H-5ax/H-6 cross-peak, indicate that 42 is the all-cis
isomer: 3(S),6(S),7(R). The average conformation under
these conditions has apparent pseudoequatorial isopropyl
and ethoxycarbonyl groups, and pseudoaxial N(4)-CH3
and benzoyloxy groups. Little or no coupling (J < 1 Hz)
is observed between H-6 and H-7 in any of the three
solvents.

Liposidomycin Diazepanone Stereochemistry
J . Org. Chem., Vol. 66, No. 17, 2001 5827
F igu r e 2. Most stable calculated conformation of diazepanone
model 44, showing pseudoaxial (ethoxycarbonyl and N(4)-
methyl) and pseudoequatorial (benzoyloxy and isopropyl)
substituents. The pseudoequatorial H-7 and pseudoaxial H-6
are indicated by arrows.
(Figure 1), and its H-6/H-7 coupling in several solvents
is less than 1 Hz (compare J for H-2′′′/H-3′′′ at 4.8 Hz for
43). The diazepanones 40 and 41 retain their pseudochair
solution conformations in the solid state, even (for 41)
when the amino group is protonated. For diazepanones
40, 41, and 43, H-3 () H-6′ for 43) is a wide doublet (J ∼
10 Hz), indicating an H-C-C-H dihedral angle close
to 180°. These observations provide circumstantial evi-
dence that the steric interactions that govern the con-
formation of 40 and 41 in CD3CN or CDCl3 solution might
also apply to the diazepanone carboxylic acid 43 in D2O.
F igu r e 1. Key NOESY cross-peaks for model diazepanones
(this work) and liposidomycin degradation product 43 (data
from refs 8 and 20).
1
Where there are differences in the H NMR spectra of
these two isomers (Table 1), the signals for 40 match
more closely those of the liposidomycin degradation
product 43. In particular, the respective signals for H-6,
shown in bold in Table 1, are (for 40) td, J ) 2.3, 4.9,
and (for 41) ddd, J ) 2.4, 3.5, 5.6, compared with (for
H-3′′′ of 43) td, J ) 2.7, 4.8). Additionally, isomer 40
shows the H-3/H-5 NOE cross-peak observed for 43
(Figure 1), whereas 41 does not. Interestingly, the four
bond W-coupling observed for the pseudoequatorial pro-
tons H-5eq and H-7 of 40, which is manifest as a
broadening of the H-5eq dd signal, is mimicked by
broadening of the signal for H-4a′′′ of 43,3 suggesting the
possibility that 43 has the same configuration and
pseudochair conformation. There is no corresponding
broadening in 41 (H-5eq’s are shown in bold in Table 1).
The fourth possible diazepanone isomer, 44 [3(S),-
6(R),7(S), Figure 2], was not isolated from the reduction
amination reaction described herein, and several other
routes to 44 were unsuccessful, including attempts to
invert the stereochemistry at C-6 of 40. In the absence
of a sample of 44, the 3(S*),6(R*),7(S*) stereochemistry
cannot be categorically ruled out as the relative stereo-
chemistry of the liposidomycin degradation product 43.
However, it is possible to make a reasonable guess as to
the most probable conformation of 44. This would be a
pseudochair closely resembling diazepanone 31, but with
the additional pseudoequatorial isopropyl group, situated
as in 40. The transannular steric repulsion present in
40 between the benzoyloxy and the N-methyl groups
would thereby be relieved, but a possible cost would be
Com p a r ison of Dia zep a n on e Mod els 40-42 w ith
1
th e Lip osid om ycin Dia zep a n on e. H and 13C NMR
data reported3 for the liposidomycin degradation product
43 (note liposidomycin numbering) in D2O solution are
shown in Table 1 along with those for the diazepanone
models 40 and 41 (note diazepine numbering) in CDCl3
solution, and the NOE cross-peaks reported8,20 for 43 are
shown in Figure 1. Comparisons of the configuration and
conformation of the diazepanone models with 43 (and
thus 2) can be made only with the understanding that
the solvent and certain substituents of the models are
specifically different from those of 43.21 Clearly, the
conformational preferences may also be different. Nev-
ertheless, important observations have be made that in
our view establish the relative configuration of 43 and 2
as [C-6′(R), C-2′′′(R), C-3′′′(R)], or, equivalently, [C-6′(S),
C-2′′′(S), C-3′′′(S)], matching 40.
1
The H and 13C NMR spectra of the models 40 and 41
(Tables 1 and 2) are qualitatively similar to the spectra
of 43 in many respects, despite the differences in solvents
and substitutents. The all-cis isomer 42, however, can
be declared stereochemically different from 43 in that it
lacks the H-3/H-5 NOE cross-peak observed for 43
(20) An NOE cross-peak for H-6′/N-CH3 is not observed in an
original DIFNOE analysis of 43. Prof. M. Ubukata, personal com-
munication.
(21) Attempts to hydrolyze the ethyl ester of the diazepanone models
led instead to elimination of the benzoate.

5828 J . Org. Chem., Vol. 66, No. 17, 2001
Knapp et al.
(s, 2 H), 2.84 (d, J ) 4.0, 3 H, 2.71 (t, J ) 6.6, 2 H), 2.30 (s, 3
H), 2.0 (s, 9 H); IR 3378, 1672.
increased steric repulsion between the gauche benzoyloxy
and ethoxycarbonyl groups at C-6/7. Molecular modeling
of possible conformations of 44 [energy minimization by
using MMFF94 (s)22] led to the calculated lowest-energy
conformation shown in Figure 2, matching the conforma-
tion observed for 31. The H-C-C-H dihedral angle
calculated for H-6/H-7 is 66.9°, corresponding to a
calculated coupling constant of 2.88 Hz,23 which is
significantly less than the 4.8 Hz observed for 43.
Although this is an incomplete argument against the
possibility of 3(S*),6R*),7(S*) relative stereochemistry for
43, the case against 44 is bolstered by the strong
circumstantial evidence in favor of the 3(S*),6(S*),7(S*)
relative stereochemical and conformational match with
40, examined in the context of five relevant diazepanone
models 30, 31, 40, 41, and 42.
E t h yl 2-(N-ter t-Bu t oxyca r b on yl-N-m et h yla m in o)-4-
p en ten oa te (17). A solution of 532 mg (2.46 mmol) of N-(tert-
butoxycarbonyl)sarcosine ethyl ester (16) in 2 mL of tetrahy-
drofuran was added to a solution of lithium diisopropylamide
[prepared from 0.375 mL (2.9 mmol) of diisopropylamine and
1.9 mL of 1.6 M n-butyllithium in hexanes] at -78 °C. This
solution was stirred at -78 °C for 3 h, and then 0.340 mL (3.7
mmol) of allyl iodide was added. The reaction mixture was
stirred for 5 h at -20 °C, quenched with saturated aqueous
ammonium chloride, and then extracted with ethyl acetate.
The organic layer was dried, concentrated, and then chro-
matographed by using 9:1 petroleum ether/ether as the eluant
to give 530 mg (84%) of the allyl sarcosine 17 as an oil: 1H
NMR (1:1 mixture of rotamers) 5.68-5.82 (m, 1 H), 5.05-5.15
(m, 2 H), 4.81 and 4.42 (2 dd, J ) 10.4, 5.0, 1 H), 4.17 (q,
J ) 7.0, 2 H), 2.80 and 2.85 (3 H each), 2.65-2.75 (m, 1 H),
2.42-2.65 (m, 1 H) 1.45 (s, 9 H), 1.35 (t, J ) 7.0, 3 H); IR
1741, 1697. Anal. Calcd for C13H23NO4: C, 60.67; H, 9.00; N,
5.44. Found: C, 60.57; H, 9.14; N, 5.23.
Exp er im en ta l Section
Eth yl 2-[N-(N′-Car boben zyloxysar cosyl)-N-m eth yl]am i-
n o-4-p en ten oa te (19). A solution of 470 mg (1.83 mmol) of
17 in 5 mL of dichloromethane was treated with 2 mL of
trifluoroacetic acid. The reaction mixture was stirred at room
temperature for 30 min and then concentrated to a yellow oil.
The crude deprotected amine was twice dissolved in 10 mL of
toluene and concentrated to help remove traces of trifluoro-
acetic acid. The crude product was dissolved in 5 mL of
dichloromethane, and then 450 mg (2 mmol) of N-carboben-
zyloxysarcosine (18) was added. The solution was cooled to -10
°C, and 375 mg (2.8 mmol) of N-hydroxybenzotriazole and
0.345 mL (2.2 mmol) of N,N′-diisopropylcarbodiimide was
added. The reaction mixture was stirred at -10 °C for 30 min
and at room temperature for 16 h. Dichloromethane was
added, and the organic solution was washed sequentially with
saturated aqueous sodium bicarbonate, water, 10% aqueous
hydrochloric acid, water, and finally brine. The organic layer
was dried, concentrated, and chromatographed with 2:3 pe-
troleum ether/ether as the eluant to afford 517 mg (78%) of
the dipeptide 19 as a colorless oil: 1H NMR (1:1 mixture of
rotamers) 5.6-5.8 (m, 1 H), 5.0-5.3 (m, 2 H), 3.9-4.2 (m, 3
H), 2.80-2.85 (four s, 6 H), 2.7-2.8 (m, 1 H), 2.45-2.55 (m, 1
H), 1.50 and 1.55 (two s, 9 H), 1.2-1.3 (m, 3 H); IR 1739, 1703,
1666. Anal. Calcd for C19H26N2O5: C, 62.96; H, 7.23; N, 7.73.
Found: C, 62.84; H, 7.36; N, 7.71.
7-Ca r beth oxy-1,4-d im eth yl-h exa h yd r o-1,4-d ia zep a n -2-
on e (20). A solution of 500 mg (1.38 mmol) of dipeptide 19 in
5 mL of 4:1 tetrahydrofuran/water was treated with osmium
tetraoxide (0.1 mL of a 0.01 M aqueous solution) and stirred
for 10 min. Sodium metaperiodate (650 mg, 3 mmol) was
added, and the reaction mixture was stirred for 3 h. The
reaction was concentrated and treated with ether and water,
and the aqueous layer was separated and washed with two
additional portions of ether. The combined organic extract was
dried, concentrated, and passed through a short pad of silica
gel (10 g) while washing with ether. The filtrate was concen-
trated, dissolved in 10 mL of methanol and then treated with
100 mg of 10% palladium-on-carbon and stirred under an
atmosphere of hydrogen for 24 h. The reaction was filtered
through Celite, and the cake was washed with ethyl acetate.
The filtrate was concentrated and then chromatographed with
9:1 ether/petroleum ether as the eluant to give 198 mg (67%)
of the diazepanone 20 as a yellow oil: 1H NMR 4.2 (q, J ) 8,
2 H), 3.9 (dd, J ) 6.4, 5.2, 1 H), 3.30 and 3.45 (Abq, J ) 17, 2
H), 3.1 (s, 3 H), 2.7-2.9 (m, 1 H), 2.4-2.6 (m, 2 H), 2.32 (s, 3
H), 2.05-2.25 (m, 1 H), 1.3 (t J ) 8, 3 H); 13C NMR 172.46,
170.32, 62.68, 61.00 (2 C's), 52.90, 44.40, 38.01, 29.13, 14.28;
IR (CHCl3) 1736, 1643; CI-MS m/z 215 (MH+).
Gen er a l. NMR spectra, reported in ppm downfield from
TMS, were taken on CDCl3 solutions at 200 (1H) or 50 (13C)
MHz unless otherwise indicated. Coupling constants (J ) are
reported in Hz. FT-IR spectra were taken on thin films unless
otherwise indicated; selected absorbances are reported in cm-1
.
Organic solutions were dried over sodium sulfate unless
otherwise indicated.
1,4-Dim eth yl-h exa h yd r o-1,4-d ia zep a n -2-on e (7). A solu-
tion of 1 mL (8 mmol) of N,N′-dimethyl-1,3-propanediamine
(5) and 2.5 g (8.8 mmol) of glyoxal bisulfate adduct 6 in 30
mL of toluene was heated at reflux for 16 h. The reaction
mixture was concentrated and then chromatographed with
19:1 dichloromethane/methanol as the eluant to give 0.467 g
(40%) of the diazepanone 7 as an oil: 1H NMR 3.44 (s, 2 H),
3.42 (t, J ) 5, 2 H), 3.00 (s, 3 H), 2.85, (t, J ) 5.2, 2 H), 2.42
(s, 3 H), 1.75-1.88 (m, 2 H); 13C NMR 172.2, 61.5, 58.2, 50.1,
43.3, 35.4, 25.4; IR 1652. Anal. Calcd for C7H14N2O: C, 59.12;
H, 9.92; N, 19.70. Found: C, 59.47; H, 10.08; N, 19.42.
4,4,4-Tr is(m eth ylth io)-1-bu ten -2-on e (14). n-Butyllithi-
um (6 mL of a 1.3 M solution in hexanes, 7.8 mmol) was added
to a solution of 1.16 g (7.52 mmol) of tris(methylthio)methane
in 20 mL of tetrahydrofuran at -78 °C. The resulting white
suspension was stirred at -78 °C for 1 h, and then 1.1 mL
(16.4 mmol) of acrolein was added dropwise. The reaction
mixture was warmed to room temperature and then stirred
for an additional 2 h. The reaction was quenched with
saturated ammonium chloride and extracted with ethyl ac-
etate. The organic extract was dried, concentrated, and then
chromatographed with 9:1 hexanes/ethyl acetate as the eluant
to give 1.28 g (80%) of the carbinol adduct as a clear oil: 1H
NMR 6.17 (ddd, J ) 16.8, 10.4, 5.8, 1 H), 5.47 (d, J ) 16.9, 1
H), 5.33 (d, J ) 10.4, 1 H), 4.23 (app t, J ) 5.4, 1 H), 2.94 (d,
J ) 4.9, 1 H), 2.19 (s, 9 H); IR 3451, 1720, 1641, 1419.
A suspension of 1 g (5.5 mmol) of the carbinol adduct, 5.2 g
(59.5) mmol of manganese dioxide, and 3 g of powdered
activated 3 Å molecular sieves in 25 mL of ether was stirred
at room temperature for 6 h, filtered, concentrated, and then
chromatographed with 19:1 hexanes/ethyl acetate as the
eluant to give 1.11 g (90%) of the enone 14 as a yellow oil: 1H
NMR 7.41 (dd, J ) 17, 10.5, 1 H), 6.47 (dd, J ) 17, 2.0, 1 H),
5.75 (dd, J ) 10.2, 2.0, 1 H), 1.99 (s, 9 H); IR 1686.
N-Met h yl-N-[2-(N′-m et h yla cet a m id o)]-4-a m in o-1,1,1-
tr is(m eth ylth io)-2-bu ta n on e (15). A solution of 675 mg (3.24
mmol) of enone 14 in 5 mL of methanol was treated with a
solution of 331 mg (3.24 mmol) of sarcosine N-methylamide
(11) in 5 mL of methanol. The reaction mixture was stirred
for 16 h, concentrated, and then chromatographed with ether
as the eluant to afford 650 mg (65%) of the Michael adduct 15
as an oil: 1H NMR 7.20 (br s, 1 H), 3.16 t, J ) 6.6, 2 H), 3.01
E t h yl 2-(N-ter t-Bu t oxyca r b on yl-N-m et h yl)a m in o-3-
ben zoyloxy-4-p en ten oa te (22). A solution of 435 mg (2
mmol) of ethyl N-(tert-butoxycarbonyl)sarcosinate (16) in 2 mL
of tetrahydrofuran was added to a solution of LDA [prepared
from 0.275 mL (2.18 mmol) of diisopropylamine and 1.3 mL
of 1.6 M n-butyllithium in hexanes] in 20 mL of tetrahydro-
(22) Halgren, T. A. J . Comput. Chem. 1996, 17, 490-519.
(23) Ramachandran, G. N.; Chandrasekaran, R. Biopolymers 1971,
10, 935-939. Ramachandran, G. N.; Chandrasekaran, R.; Kopple, K.
D. Biopolymers 1971, 10, 2113-2131.

Liposidomycin Diazepanone Stereochemistry
J . Org. Chem., Vol. 66, No. 17, 2001 5829
furan at -78 °C. The resulting solution was stirred at -78 °C
for 3 h, 1 mL (15 mmol) of acrolein was added, and the reaction
mixture, which presumably contained the adduct 21, was
allowed to warm to -50 °C over a period of 45 min. Benzoyl
chloride (0.44 mL, 3.8 mmol) was then added, and the reaction
mixture was stirred for 3 h at -50 °C. Saturated aqueous
sodium bisulfite was added and then ethyl acetate. The
aqueous layer was washed with additional ethyl acetate, and
the combined organic extract was dried, concentrated, and
chromatographed by using 19:1 petroleum ether/ether as the
eluant to give 605 mg (81%) of the benzoates 22 and 23 as an
oily mixture of diasteriomers (1:4 syn:anti): 1H NMR (1:1
mixture of rotamers) 5.90-6.20 (m, 2 H), 5.52-5.28 (m, 2 H),
4.81 and 4.42 (two d’s, J ) 7.0, 1 H), 4.12-4.22 (m, 2 H), 2.85,
2.95, 2.98, and 3.04 (four s, 3 H), 1.43 and 1.45 [two s, 9 H),
trifluoroacetic acid as for 19. The crude amine was dissolved
in 5 mL of dry dichloromethane, and then 530 mg (2.37 mmol)
of N-(carbobenzyloxy)sarcosine (18) was added. The coupling
reagent 2-isobutoxy-1-isobutoxycarbonyl-1,2-dihydroquinoline
(0.89 mL, 3 mmol) was added, and then the reaction mixture
was stirred for 48 h. Dichloromethane was added, and the
solution was washed sequentially with saturated aqueous
soldium carbonate, water, 10% aqueous hydrochloric acid,
water, and finally brine. The organic extract was dried,
concentrated, and chromatographed with 1:1 petroleum ether/
ether as the eluant to give respectively 77 mg (10%) of the
syn dipeptide 29 and 538 mg (70%) of the anti dipeptide 28 as
oils: 1H NMR (29, 1:1 mixture of rotamers) 8.05 (d, J ) 8, 2
H), 7.60 (br t, 1 H), 7.45 (t, J ) 8, two 2 H), 7.3-7.4 (m, 5 H),
5.75-6.05 (m, 2 H), 5.20-5.50 (m, 3 H), 5.12 and 5.18 (two s,
2 H), 4.07-4.25 (m, 4 H), 2.95-3.05 (three s, 3 H), 1.15 (t, J
1.13-1.22 (m, 3 H); IR 1731, 1694. Anal. Calcd for C20H27
-
1
NO6: C, 63.64; H, 7.21; N, 3.71. Found: C, 63.72; H, 7.34; N,
3.76.
) 7, 3 H); H NMR (28, 1:1 mixture of rotamers) 7.95 (d, J )
8, 2 H), 7.56 (br t, 1 H), 7.42 (t, J ) 8, 2 H), 7.25-7.4 (m, 5 H),
6.20 (br s, 1 H), 5.70-5.92 (m, 2 H), 5.20-5.40 (m, 2 H), 5.04-
5.15 (m, 2 H), 4.05-4.25 (m, 4 H), 2.90, 2.95, 3.12 and 3.18
(four s, 3 H), 1.15 (t, J ) 7, 3 H); IR 1731, 1711, 1673. Anal.
(28) Calcd for C26H30N2O7: C, 64.72; H, 6.27; N, 5.80. Found:
C, 64.50; H, 6.19; N, 5.76.
6-(R *)-Be n zoyloxy-7-(R *)-ca r b e t h oxy-1,4-d im e t h yl-
h exa h yd r o-1,4-d ia zep a n -2-on e (30). Anti-dipeptide 28 (64
mg, 0.13 mmol) was ozonized, cyclized, and chromatographed
as for 31 (below), giving 21 mg (48%) of the anti-diazepanone
30 as an oil: 1H NMR (400 MHz, assignments by decoupling)
8.00 (d, J ) 7, two o-Bz-H), 7.60 (t, J ) 7.3, p-Bz-H), 7.45 (t,
J ) 7.6, two m-Bz-H), 5.95 (ddd, J ) 7.2, 5.1, 4.1, H-6), 4.30
(d, J ) 5.2, H-7), 4.26 (q, J ) 7, OCH2CH3), 3.56 (dd, J ) 16,
2, H-3eq), 3.45 (d, J ) 17, H-3ax), 3.32 (ddd, J ) 14, 7.2, 2.0,
H-5eq), 3.08 (s, CONCH3), 2.60 (dd, J ) 14, 4.1, H-5ax), 2.35 (s,
CH2NCH3), 1.30 (t, J ) 7, OCH2CH3); IR 1718, 1653; CI-MS
m/z 335 (MH+).
6-(S *)-Be n zoyloxy-7-(R *)-ca r b e t h oxy-1,4-d im e t h yl-
h exa h yd r o-1,4-d ia zep a n -2-on e (31). Syn-dipeptide 29 (200
mg, 0.41 mmol) was ozonized and cyclized as for 19. Chroma-
tography with 3:7 petroleum ether/ether as the eluant gave
96 mg (70%) of the syn-diazepanone 31 as an oil: 1H NMR
(400 MHz, assignments by decoupling) 8.10 (d, J ) 7.4, two
o-Bz-H's), 7.60 (t, J ) 7.2, p-Bz-H), 7.49 (t, J ) 7.4, two
m-Bz-H's), 5.45 (ddd, J ) 8.8, 6.6, 2.1, H-6), 4.26 and 4.27 (two
q, J ) 7.1, OCH2CH3), 4.15 (d, J ) 2.2, H-7), 3.53 and 3.35
(two d, J ) 17, H-3), 3.15 (s, CONCH3), 3.11 (dd, J ) 12.3,
8.8, H-5), 2.95 (dd, J ) 12.3, 6.6, H-5), 2.42 (s, CH2NCH3), 1.30
(t, J ) 7.1, OCH2CH3); IR 1718, 1653; CI-MS m/z 335 (MH+).
Anal. Calcd for C17H22N2O5: C, 61.06; H, 6.63; N, 8.38.
Found: C, 60.92; H, 6.59; N, 8.07.
7-Ca r beth oxy-1,4-d im eth yl-1(H)-2,3,4,5-tetr a h yd r o-1,4-
d ia zep in -2-on e (32). A solution of 60 mg (0.18 mmol) of
diazepanone 31 (a mixture of 30 and 31 worked equally well)
and 0.032 mL (0.21 mmol) of 1,8-diazabicyclo[5.4.0]undec-7-
ene in 4 mL of toluene was stirred at room temperature for
12 h. The reaction mixture was concentrated and then
partitioned between ethyl acetate and water. The organic layer
was dried, concentrated, and chromatographed by using ether
as the eluant to give 34 mg (90%) of 32 as an oil: 1H NMR
6.84 (t, J ) 7.3, 1 H), 4.29 (q, J ) 7.2, 2 H), 3.30 (s, 2 H), 3.13
(d, J ) 7.3, 2 H), 3.08 (s, 3 H), 2.50 (s, 3 H), 1.34 (t, J ) 7.2,
3 H); 13C NMR 170.0, 162.6, 138.6, 127.4, 61.8, 58.4, 51.2, 45.5,
33.0, 41.1; IR 1725, 1673, 1638; CI-MS m/z 213 (MH+). Anal.
Calcd for C10H16N2O3: C, 56.58; H, 7.60; N, 13.20. Found: C,
56.22; H, 7.46; N, 12.86.
4-(N-ter t-Bu toxyca r bon yl-N-m eth yla m in o)bu ten e-3,5-
d iol (24 a n d 25). Sarcosine derivative 16 (650 mg, 3 mmol)
was converted to its enolate and condensed with acrolein as
described above. The reaction was quenched with 10 mL of
ammonium hydroxide/ammonium chloride buffer (pH ) 8), and
the aqueous layer was extracted with ether (2 × 20 mL). The
combined organic extract was dried and concentrated to afford
510 mg (62%) of the unstable adduct 21 as a dark orange oil.
This product was dissolved in 25 mL of dry ether, cooled to 0
°C, and then 77 mg (3.5 mmol) of lithium borohydride was
added. The reaction was warmed to room temperature and
stirred for 2 h. The reaction was quenched with saturated
aqueous sodium bicarbonate, and the aqueous layer was
further extracted with ether. The combined organic extract was
dried, concentrated, and chromatographed by using 2:3 petro-
leum ether/ether as the eluant to afford 77 mg (18%) of a
higher Rf syn diol 25 and 311 mg (72%) of a lower Rf anti diol
24: 1H NMR (25, 1:1 mixture of rotamers) 5.88 (ddd, J ) 17,
10.5, 5.7, 1 H), 5.35 (dt, J ) 17, 1.4, 1 H), 5.20 (d, J ) 10.5, 1
H), 4.34-4.44 (m, 1 H of one rotamer), 4.12-4.19 (m, 1 H),
3.88-3.98 (m, 1 H), 3.76-3.85 (m, 1 H), 3.54-3.65 (m, 1 H),
1
2.90 (s, 3 H), 2.72 and 1.93 (two br s, 1 H), 1.45 (s, 9 H); H
NMR (24, 1:1 mixture of rotamers) 5.92 (ddd, J ) 17, 10.4,
6.8, 1 H), 5.32 (dt, J ) 17, 1.1, 1 H), 5.19 (d, J ) 10.4, 1 H),
4.50-4.58 (br m, 1 H of one rotamer), 3.98 (br s, 2 H), 3.48-
3.54 (m, 1 H), 3.23-3.37 (m, 1 H), 2.85 (s, 3 H), 1.73 (br s, 1
H), 1.45 (s, 9 H); IR (CHCl3) 3398, 1669. Anal. (24) Calcd for
C
11H21NO4: C, 57.12; H, 9.15; N, 6.05. Found: C, 57.32; H,
9.37; N, 5.92.
2,2-Dim et h yl-5-(R*)-(N-ter t-b u t oxyca r b on yl-N-m et h -
yla m in o)-4-(S*)-vin yl-1,3-d ioxa n e (26). A solution of 57 mg
(0.25 mmol) of anti diol 24 and a catalytic amount (5 mg) of
pyridinium p-toluenesulfonate in 3 mL of 2,2-dimethoxypro-
pane was heated at reflux for 1 h. The reaction mixtue was
concentrated and then chromatographed by using 1:1 petro-
leum ether/ether as the eluant to afford 56 mg (82%) of the
trans acetal 26 as an oil: 1H NMR (1:1 mixture of rotamers,
assignments by decoupling) 5.75-5.85 (m, H-4a), 5.28-5.38
(m, two H-4b), 5.23 [d, J ) 8.3 (at 400 MHz, J 4,5 ) 8.4), H-4],
4.50-4.70 (m, 1 H), 4.0 and 4.1 (two t, J ) 9.8, 1 H), 3.82 (dd,
J ) 9.3, 6.3, 1 H), 3.7-3.8 (m, 1 H), 2.7 and 2.8 (two s, 3 H),
1.4-1.6 (three overlapping app s, 15 H); IR 1701. Anal. Calcd
for C14H25NO4: C, 61.96; H, 9.29; N, 5.16. Found: C, 62.05;
H, 9.43; N, 5.18.
2,2-Dim et h yl-5-(R*)-(N-ter t-b u t oxyca r b on yl-N-m et h -
yla m in o)-4-(R*)-vin yl-1,3-d ioxa n e (27). The syn diol 25 was
converted to its acetonide 27 by following the procedure
described for 24, above: 1H NMR (1:1 mixture of rotamers,
assignments by decoupling) 5.76-5.86 (m, H-4a), 5.37 (d, J )
18, H-4b-Z), 5.20 (app t, J ) 10, H-4b-E), 4.60 and 4.65 [two
br s, H-4 (at 400 MHz with decoupling of H-4a, J ) 3.2], 4.25
(dt, J ) 12.5, 4.0, H-5), 4.13-4.20 (overlapping m, H-6ax), 3.9
(dd, J ) 12.5, 3.5, H-6eq), 3.1 and 3.5 (two s, 3 H), 1.4-1.5
(three overlapping app s, 15 H); IR 1701.
7-Ca r boxy-1,4-d im eth yl-1(H)-2,3,4,5-tetr a h yd r o-1,4-d i-
a zep in -2-on e Hyd r och lor id e (34). A solution of 35 mg (0.165
mmol) of the diazepanone ester 32 in 3 mL of aqueous
tetrahydrofuran (1:4) was treated with 10 mg (0.24 mmol) of
lithium hydroxide. The reaction mixture was stirred at room
temperature for 30 min and then concentrated to give 35 mg
of crude lithium salt 33. Purification was achieved by adding
a solution of ethereal hydrochloric acid to a methanolic solution
of 33. The hydrochloride salt 34 precipitated upon cooling to
-25 °C and was collected by filtration and pumped to dryness
to afford 31 mg (86%) of a colorless powder: 1H NMR (33, D2O)
Eth yl 2-[N-(Car boben zyloxy)sar cosyl-N-m eth ylam in o]-
3-ben zoyloxy-4-p en ten oa te (28/29). A 4:1 mixture (600 mg,
1.59 mol) of benzoates 22 and 23 was N-deprotected with

5830 J . Org. Chem., Vol. 66, No. 17, 2001
Knapp et al.
6.58 (t, J ) 7.8, 1 H), 3.30 (s, 2 H), 3.15 (d, J ) 7.8, 2 H), 3.05
(s, 3 H), 2.69 (s, 3 H); 13C NMR (33, D2O) 175.4, 172.5, 146.5,
126.0, 59.8, 52.7, 46.7, 35.7; 1H NMR (34, D2O) 6.9 (t, J ) 7.6,
1 H), 4.02 (br s, 4 H), 3.29 and 3.19 (two s, 3 H each); 13C NMR
(34, D2O) 167.9, 167.2, 145.0, 124.4, 57.9, 51.9, 45.5, 36.4.
Eth yl 2-[N-(2-ter t-Bu toxyca r bon yla m in o-3-m eth ylbu -
ta n oyl)-N-m eth yla m in o]-3-ben zoyloxy-4-p en ten oa te (36).
A mixture of 20 mL of 4 N HCl in dioxane and a solution of
(2.50 g 6.07 mmol) of acrolein adducts 22/23 (approximate 3:2
mixture of anti/syn isomers) in 5 mL of dichloromethane was
stirred for 1 h at room temperature and then concentrated.
The residue was shaken with dichloromethane and saturated
aqueous sodium bicarbonate. The aqueous layer was washed
with dichloromethane. The combined organic layer was dried,
filtered, and concentrated to give the crude deprotected amine,
which was used immediately in the next reaction.
A solution of 1.34 mL (12.14 mmol) of N-methylmorpholine,
2.64 g (12.14 mmol) of N-tert-butyloxycarbonyl-L-valine, and
2.14 g (12.14 mmol) of 2-chloro-4,6-dimethoxy-1,3,5-triazine
in 40 mL of dichloromethane was stirred at 0 °C for 2 h. A
solution of the crude deprotected amine (∼6.07 mmol) in 10
mL of dichloromethane was added, and the resulting solution
was allowed to stir at room temperature overnight. The
reaction was washed sequentially with 1 N HCl, saturated
aqueous sodium bicarbonate, and brine. The organic layer was
dried over magnesium sulfate and then concentrated. The oily
residue was chromatographed by using a stepwise gradient
of 1:9 to 1:1 ethyl acetate/hexane as the eluant to give 2.65 g
(72%) of the dipeptide 36 as a yellow oil: 1H NMR (500 MHz,
approximate 3:2 mixture of diastereoisomers) 8.01 (overlapping
dd, J ) 1.6, 7.2; 2 H), 7.56 (app dt, J ) 1.6, 7.4, 1 H), 7.44 (br
t, J ) 7.5, 2 H) 6.05 (t, J ) 7.6, 0.4 H), 6.01 (t, J ) 7.6, 0.6 H),
5.92-5.82 (m, 1 H), 5.52 (dd, J ) 9.6, 17.2, 1 H) 5.47 (dd, J )
8.5, 17.2, 1 H), 5.29 (app dd, J ) 6.3, 10.2, 1 H), 4.48 (dd, J )
5.3, 9.2; 0.4 H), 4.44 (dd, J ) 6.7, 0.6 H), 4.24-4.09 (m, 2 H),
3.15 (s, 1.2 H), 3.12 (s, 1.8 H), 1.98 (app sext, J ) 6.6, 0.4 H),
1.93 (app sext J ) 6.6, 0.6 H), 1.44 (br s, 9 H), 1.17 (t, J ) 7.1,
3 H), 0.99 (d, J ) 5.0, 1.8 H), 0.97 (d, J ) 5.0, 1.8 H), 0.93 (d,
J ) 6.6, 1.2 H), 0.86 (d, J ) 6.6, 1.2 H).
6-Ben zoyloxy-7-eth oxyca r bon yl-3-isop r op yl-1-m eth yl-
1,4-d ia zep a n -2-on es (37, 38/39). A solution of 750 mg (1.53
mmol) of the allylic benzoates 36 in 20 mL of dichloromethane
was cooled to -78 °C and then treated with ozone until a slight
blue color persisted. Nitrogen was bubbled through the solu-
tion to remove excess ozone, and then 2 mL of methyl sulfide
was added to reduce the ozonide to the aldehyde. The mixture
was warmed to room temperature and stirred for 2 h. The
solution was concentrated, and the residue was taken up in
ethyl acetate, which was washed with water and then brine.
The organic layer was dried, filtered, and then concentrated
to a viscous oil, 722 mg, 96% crude yield, which consisted
mostly of two aldehyde isomers (approximately 3:2) according
to 1H NMR analysis. The crude aldehyde was used without
further purification. 1H NMR (500 MHz) 9.69 (d, J ) 5.3, 1
H), 8.09 (br d, J ) 7.3, 0.8 H), 8.06 (dd, J ) 8.2, 1.1, 1.2 H),
7.64 (app tt, J ) 1.1, 7.3, 1 H), 7.49 (app t, J ) 7.6, 2 H), 5.80
(d, J ) 4.3, 0.6 H), 5.76 (d, J ) 3.8, 0.4 H), 5.53 (d, J ) 3.8, 0.4
H), 5.25 (d, J ) 4.3, 0.6 H), 5.19 (d, J ) 9.4, 0.6 H), 5.15 (d, J
) 9.4, 0.4 H), 4.50 (app q, J ) 7.8, 1 H), 4.36-4.24 (m, 2 H),
3.27 (s, 1.2 H), 3.25 (s, 1.8 H), 2.03-1.92 (m, 1 H), 1.44 (s, 5.4
H), 1.42 (s, 3.6 H), 1.33-1.25 (m, 3 H), 1.01 (d, J ) 6.9, 1.8 H)
0.99 (d, J ) 7.1, 1.2 H), 0.93 (d, J ) 6.9, 1.8 H), 0.91 (d, J )
7.1, 1.2 H).
A solution of the crude aldehyde in 2 mL of dichloromethane
and 8 mL of a 4 N solution of HCl in dioxane was stirred for
1 h at 0 °C and then concentrated. The residue, which
presumably contained the dipeptide amino aldehyde, was
dissolved in 100 mL of dichloroethane and then treated with
400 mg of crushed activated 4 Å molecular sieves, 1.55 g (7.33
mmol) of sodium triacetoxyborohydride, and 279 µL (1.61
mmol) of diisopropylethylamine. The solution was stirred
overnight at room temperature, then washed sequentially with
1 N sodium hydroxide and brine. The organic layer was dried
and concentrated. Preparative TLC (1500 µm plates) of the
residue with 2:3 ethyl acetate/hexane as the eluant gave 130
mg (23.5% overall yield) of a major diazepanone 37 and 100
mg (18.2% overall yield) of a 4:1 inseparable mixture of two
additional diazepanone isomers, 38/39, as oils: 1H NMR of 37
(500 MHz, assignments by decoupling) 8.11 (dd, J ) 8, 1, two
o-Ar-H), 7.60 (t, J ) 7.4, p-Ar-H), 7.47 (t, J ) 8, two m-Ar-H),
5.55 (td, J ) 5.3, 2.5, H-6), 4.56 (dd, J ) 5.3, 1.0, H-7), 4.31
(app qd, J ) 7.1, 1.8, OCH2CH3), 3.45 (ddd, J ) 15.6, 2.4, 1.0,
H-5eq), 3.22 (dd, J ) 15.6, 2.2, H-5ax) 3.05 (s, CONCH3), 2.66
(d, J ) 6.2, H-3), 2.21 (hept, J ) 6.6, CHMe2), 1.33 (t, J ) 7.1,
OCH2CH3), 0.99 (d, J ) 6.7, isopropyl CH3), 0.96 (d, J ) 6.6,
isopropyl CH3); 1H NMR of 38/39 (500 MHz, signals of 38
unless indicated, assignments by decoupling) 8.09 (d, J ) 7.1,
two o-Ar-H of 39), 7.97 (dd, J ) 8.1, two o-Ar-H), 7.58 (t, J )
7.3, p-Ar-H), 7.44 (t, J ) 8, two m-Ar-H), 6.01 (ddd, J ) 7.8,
5.6, 2.1, H-6), 5.53 (app t, J ) 6.6, H-6 of 39), 4.32 (d, J ) 5.3,
H-7), 4.29 (dq, J ) 10.8, 7.1, one OCH2CH3), 4.15 (dq, J ) 10.8,
7.1, one OCH2CH3), 3.72 (dd, J ) 13.8, 8, H-5eq), 3.35 (dd, J
) 12.3, 8.0, H-5eq of 39), 3.25 (d, J ) 3.4, H-3), 3.23
(overlapped dd, J ) 12.3, 6.5, H-5ax of 39), 3.13 (d, J ) 3.0,
H-3a of 39), 3.15 (s, CONCH3 of 39), 3.07 (s, CONCH3), 2.75
(dd, J ) 13.8, 2.1, H-5ax), 2.73-2.65 (m, CHMe2), 1.30 (t, J )
7.1, OCH2CH3), 1.00 (d, J ) 7.1, isopropyl CH3), 0.85 (d, J )
7.0, isopropyl CH3), the remaining peaks of 39 are buried
1
beneath those of 38; H-13C HETCOR of 37 (500 MHz, CD3-
CN) 135.5 (p-Ph), 129.8 (m-Ph), 130.3 (o-Ph), 71.2 (C-6), 66.6
(C-3), 64.8 (C-7), 63.5 (OCH2CH3), 51.6 (C-5), 38.7 (CONCH3),
30.8 (CHMe2), 22.4 (isopropyl CH3), 18.8 (isopropyl CH3), 16.0
(OCH2CH3); 1H-13C HETCOR of 38 (500 MHz, CD3CN) 134.8
(p-Ph), 129.7 (m-Ph), 130.3 (o-Ph), 76.6 (C-6), 71.3 (C-3), 64.1
(C-7), 61.6 (OCH2CH3), 50.7 (C-5), 40.0 (CONCH3), 32.0
(CHMe2), 22.0 (isopropyl CH3), 18.6 (isopropyl CH3), 14.5
(OCH2CH3); LC-FAB-MS of 37 m/z 385 (MNa+), 363 (MH+);
LC-FAB-MS of 38/39 m/z 385 (MNa+), 363 (MH+).
6-Ben zoyloxy-1,4-d im eth yl-7-eth oxyca r bon yl-3-isop r o-
p yl-1,4-d ia zep a n -2-on es (40, 41, 42). A solution of 40 mg
(0.107 mmol) of the major diazepanone 37 in 1 mL of methanol
and 0.2 mL of formalin was stirred for 30 min at room
temperature. 10% palladium on carbon (12 mg) was added to
the solution, and the resulting suspension was placed under
hydrogen atmosphere. After 2 h, the catalyst was filtered and
the filtrate was concentrated. Preparative TLC (1500 µm
plates) of the residue with 35:65 ethyl acetate/hexane as the
eluant gave the 28 mg (72%) of the N-methylated product 40
as a yellow oil. Crystallization from isopropyl ether gave
colorless rods, mp 150-153 °C: 1H NMR of 40 (500 MHz,
assignments by decoupling) 8.02 (dd, J ) 8, 1, two o-Ar-H),
7.60 (t, J ) 7.3, p-Ar-H), 7.46 (br t, J ) 8, two m-Ar-H), 5.63
(td, J ) 4.9, 2.3, H-6), 4.51 (d, J ) 4.8, H-7), 4.26-4.37 (m,
OCH2CH3), 3.50 (dd, J ) 15.9, 2.3, H-5ax), 3.32 (br dd, J )
15.8, 2.2, H-5eq), 3.11 (s, CONCH3), 2.64 (d, J ) 10.1, H-3),
2.42 (s, NCH3), 2.22-2.32 (m, CHMe2), 1.33 (t, J ) 7.1,
OCH2CH3), 0.90 (d, J ) 6.4, isopropyl CH3), 0.88 (d, J ) 6.7
Hz; isopropyl CH3); LC-FAB-MS of 40 m/z 399 (MNa+), 377
(MH+). The structure of 40 was confirmed by X-ray crystal-
lographic analysis.
The minor diazepanone mixture 38/39 (50 mg) was N-
methylated as above. Preparative TLC as before gave 34 mg
(70%) of 41 and 7.3 mg of 42, both as a yellow oils. Attempts
to crystallize 41 were unsuccessful. Addition of 1 equiv of a 4
N solution of HCl in dioxane precipitated the hydrochloride
salt of 41, which was dissolved in 1 mL of isopropyl alcohol.
The solution was filtered through a Pall Gelman 0.45 µm CR
PTFE Acrodisc syringe filter and then placed into a crystal-
lization dish and set in a desiccator containing isopropyl ether.
Long rod-shaped crystals formed after 48 h, mp 124-126 °C.
The structure of 41‚HCl including absolute configuration was
1
confirmed by X-ray crystallographic analysis. H NMR of 41
(as free base, 500 MHz, assignments by decoupling) 8.03 (dd,
J ) 8, 1.1, two o-Ar-H), 7.62 (t, J ) 7.5, p-Ar-H), 7.48 (br t, J
) 8, two m-Ar-H), 5.74-5.78 (m, H-6), 4.52 (d, J ) 5.0, H-7),
4.35 (dq, J ) 10.8, 7.1, one OCH2CH3), 4.15 (dq, J ) 10.8, 7.1,
one OCH2CH3), 3.47 (dd, J ) 16.0, 2.5, H-5ax), 3.19 (d, J )
11.2, H-3), 3.16 (s, CONCH3) 3.14 (dd, J ) 16.0, 3.5, H-5eq),
2.72 (br s, NCH3), 1.67-1.75 (m, CHMe2), 1.33 (t, J ) 7.1,
OCH2CH3), 0.98 (d, J ) 7, isopropyl CH3), 0.96 (d, J ) 7,

Liposidomycin Diazepanone Stereochemistry
J . Org. Chem., Vol. 66, No. 17, 2001 5831
isopropyl CH3); 1H NMR of 42 (500 MHz, assignments by
decoupling; peaks are broadened at 23 °C compared with those
of 41) 8.02 (d, J ) 7.6, two o-Ar-H), 7.60 (t, J ) 7.6, p-Ar-H),
7.46 (t, J ) 7.6, two m-Ar-H), 5.62 (br t, J ) 2.7, H-6), 4.94 (br
s, H-7), 4.24 (app q, J ) 7.1, OCH2CH3), 3.48 (d, J ) 14.5,
H-5ax), 3.30 (br m, H-5eq), 3.19 (d, J ) 10.0, H-3), 3.15 (s,
CONCH3), 2.38 (br s, NCH3), 2.22 (br m, CHMe2), 1.27 (t, J )
7.1, OCH2CH3), 0.97 (d, J ) 6.6, isopropyl CH3), 0.95 (d, J )
6.6, isopropyl CH3); LC-FAB-MS of 41 m/z 399 (MNa+), 377
(MH+); LC-FAB-MS of 42 m/z 399 (MNa+), 377 (MH+).
Molecu la r Mod elin g. One hundred fifty conformations of
the three valine-derived diazepanones 41, 41, 42, and the
fourth possible diazepanone isomer 44, with 3(S),6(R),7(S)
absolute stereochemistry, were generated by using a distance
geometry algorithm without any constraints. These conforma-
tions were then minimized by using MMFF94 (s) with a
distance dependent dielectric of 2r. To simplify the subsequent
calculations, the conformations of each structure were sorted
by energy and then clustered on the basis of the selected
pairwise atoms (the atoms that comprise the diazepanone ring
and heavy atoms attached to them) with superposition rms of
0.25. Thus, the initial 150 conformations were reduced to 13
for 40, 15 for 44, 18 for 42, and 20 for 41. Coupling constants
were then calculated for H-6/H-7 for each of these conforma-
tions. For 44, energies ranged from 83.6 to 124.4 kcal/mol. The
lowest energy calculated conformation is shown in Figure 2.
Ack n ow led gm en t. This work was supported by
Merck & Co. and SynChem Research Inc. We are
grateful to Prof. M. Ubukata for discussion and ex-
change of spectra.
Su p p or tin g In for m a tion Ava ila ble: Details of the crys-
tallographic structure determinations and NMR spectra of
selected compounds. This material is available free of charge
via the Internet at http://pubs.acs.org.
J O010355G