Synthesis of a natural product-inspired eight-membered ring lactam library via ring-closing metathesis

Article abstract of DOI:10.1016/j.bmcl.2008.07.074

We have prepared a novel speculative eight-membered lactam demonstration library based on the skeletal structure of the potent antitumor marine natural product octalactin A. The basic scaffold was readily constructed in a convergent fashion via ring-closing metathesis chemistry from the corresponding diene amides. A cursory examination of the biological properties of the library validates the relevance and significance of these structures.

Full text of DOI:10.1016/j.bmcl.2008.07.074

Bioorganic & Medicinal Chemistry Letters 18 (2008) 4876–4879
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Synthesis of a natural product-inspired eight-membered ring lactam library
via ring-closing metathesis
Neil Brown a,b, Baohan Xie a, Nataliya Markina a, David VanderVelde b, Jean-Pierre H. Perchellet c,
Elisabeth M. Perchellet a,c, Kyle R. Crow c, Keith R. Buszek a,b,
*
a Department of Chemistry, University of Missouri-Kansas City, 205 Spencer Chemical Laboratories, 5100 Rockhill Road, Kansas City, MO 64110, USA
b Center for Chemical Methodologies and Library Development, University of Kansas, 1501 Wakarusa Drive, Lawrence, KS 66047, USA
c Anti-Cancer Drug Laboratory, Kansas State University, Division of Biology, Ackert Hall, Manhattan, KS 66506, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
We have prepared a novel speculative eight-membered lactam demonstration library based on the skel-
etal structure of the potent antitumor marine natural product octalactin A. The basic scaffold was readily
constructed in a convergent fashion via ring-closing metathesis chemistry from the corresponding diene
amides. A cursory examination of the biological properties of the library validates the relevance and sig-
nificance of these structures.
Received 13 June 2008
Revised 16 July 2008
Accepted 17 July 2008
Available online 24 July 2008
Ó 2008 Elsevier Ltd. All rights reserved.
Keywords:
Eight-membered
Natural product
Lactam
Library
Ring-closing metathesis
Medium-ring
Octalactin
Parallel synthesis
Libraries of compounds based on natural products have gained
in importance in recent years.1,2 Although the structures contained
in these libraries tend to be fairly diverse, one class of compound
remains to be fully exploited, namely, the monocyclic medium-
ring.3,4 An interesting natural product from the standpoint of struc-
tural architecture and biological profile is the powerful antitumor
marine natural product octalactin A, 1 (Fig. 1).5,6 This fascinating
natural product which features a rare saturated eight-membered
lactone core continues to receive considerable attention in total
synthesis efforts from many laboratories.7
We have previously constructed this lactone by means of an
especially facile and direct lactonization from the corresponding
seco acid (‘zip-up’ approach),7a,8 and by an equally facile ring-clos-
ing metathesis (RCM) route (‘zip-down’ approach) to afford the
unsaturated oxocene.9 These enabling technologies prompted us
to investigate the feasibility of generating a speculative eight-
membered ring library inspired by octalactin A.
retained the eight-membered core feature of the octalactins. The
strategy is depicted in Scheme 1.
O
HO
O
OH
O
Me
O
Me Me
1
Figure 1. Octalactin A.
R2
R2
O
O
R3HN
FmocHN
2
N
N
7
4
In order to simplify the synthesis, we elected in the present
work to construct a simpler lactam scaffold that at a minimum
R1
R1
2
3
NHFmoc
CO2H
N
R1
R2
ArCHO + ArCH2NH2
+
H
6
7
4
5
* Corresponding author. Tel.: +1 816 235 2292; fax: +1 816 235 5502.
E-mail address: buszekk@umkc.edu (K.R. Buszek).
Scheme 1. Retrosynthetic analysis of the lactam scaffold.
0960-894X/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2008.07.074

N. Brown et al. / Bioorg. Med. Chem. Lett. 18 (2008) 4876–4879
4877
1. MgSO4, Et2O
rt, 24 h
2. filter, evaporate
1. PhOCOCl
O
N
NaOMe
MeOH
rt, 2 h
OMe
PhMe, -42 °C to -78 °C
CHO
+
30 min
NH2
R1
R2
3. vinylmagnesium chloride
THF, BF3 OEt2
2. allylmagnesium chloride
THF, -78 °C, 3.5 h
3. 10% HCl, rt, 20 min
90%
86%
N
6a-h
7a-f
O
OPh
-78 °C to rt, 12 h
6a R1 = o-Br
6b R1 = m-Br
6c R1 = p-Br
6d R1 = o-F
6e R1 = m-F
6f R1 = p-F
6g R1 = p-Me
6h R1 = p-OMe
7a R2 = H
67-87%
8
9
7b R2 = o-F
7c R2 = m-F
7d R2 = p-F
7e R2 = p-Me
1. NaIO4, H2O
rt, 2 h
2. NaOH, rt, 20 h
3. neutralize w/ HCl
O
N
5a-m
R1
R2
NHFmoc
CO2H
H
7f R2 = p-OMe
4. Fmoc-Cl, NaHCO3
dioxane, 7 h
45%
N
H
4
10
Scheme 2. Synthesis of amine sublibrary 5a–m.
Scheme 3. Synthesis of Fmoc-protected 3-amino-2-hexenoic acid 4.
The unsaturated lactam scaffold 2 would be constructed by
means of RCM. The diene amides 3 would result from the
condensation of the racemic Fmoc-protected 3-amino-5-hexenoic
acid 4 and the sublibrary of various racemic secondary allylic
amines 5. The allylic amines in turn would be derived from vinyl
Grignard addition to the Schiff bases. By using readily prepared
racemic components, a mixture of diastereomers 2 would be ob-
tained resulting in a larger library with a relative minimum of ef-
fort and cost. Separation of the diastereomeric scaffolds would
precede derivitization of the primary amines, thus generating the
desired lactam library.
R2
O
FmocHN
1. (COCl)2, cat. DMF
PhMe/CH2Cl2, rt, 2 h
N
NHFmoc
CO2H
R1
3a-m
2. 5a-m, DMAP, CH2Cl2
4
rt, 2 h
3a (81%) 3e (96%) 3i (75%)
3b (87%) 3f (96%) 3j (70%)
3c (84%) 3g (61%) 3k (94%)
3d (64%) 3h (63%) 3l (94%)
3m(88%)
61-96%
Scheme 4. Diene amide formation via the acid chloride.
The allylic amine sublibrary was prepared as shown in Scheme
2. Reaction of a series of commercially available benzaldehydes 6a-
h and benzyl amines 7a-f in ether at room temperature in the pres-
ence of magnesium sulfate10 gave after filtration the Schiff bases in
nearly quantitative yield in all cases.
Addition of vinyl Grignard with BF3ÁOEt2 at À78 °C for 12 h gave
a representative 13-member sublibrary in 67–87% yield (Fig. 2).
Synthesis of the racemic Fmoc-protected aminohexenoic acid
component 4 was achieved using a modified literature procedure
(Scheme 3).11,12 Addition of allylmagnesium chloride to the acylpy-
ridinium salt formed between 4-methoxypyridine and phenylchlo-
roformate gave the allyl-substituted dihydropyridone carbamate 9
in 90% isolated yield.
Methanolysis of the carbamate to give 10 was achieved in 86%
yield. Oxidative cleavage with sodium periodate, followed by basic
hydrolysis of the intermediate formamide, and finally Fmoc protec-
tion of the resulting primary amine gave 4 in 45% overall yield from
10 on a 5-g scale.
Several methods for the condensation of
4 and 5 were
attempted, including DCC coupling,13 HATU-mediated coupling,14
activation of the carboxylic acid as its pentafluorophenyl (Pfp)
ester,15 and the in situ generated acid chloride with oxalyl chloride
and catalytic DMF.16 Of these attempts, only the last method
(Scheme 4) gave consistent and reliable yields (61–96%) of the
amides with our components.
Construction of the eight-membered lactam scaffolds via RCM
was investigated next. Although we previously showed that the
monocyclic oxocenes in the octalactin series could be obtained
via RCM at only 40 °C,9 no ring closure with the amides 3a–m
was observed at temperatures up to 80 °C. We attribute this obser-
vation to an unfavorable amide rotamer population present at
these temperatures. Gratifyingly, RCM was eventually effected
with 15 mol% of the Grubbs’ second-generation catalyst in reflux-
ing toluene for 5–12 h to give a diastereomeric 1:1 separable mix-
ture of lactams 2 in yields ranging from 55% to 93% (Scheme 5).
There was no observed cyclization preference for either amide dia-
stereomer, except in the case of 3a which failed to cyclize, presum-
ably due to the sterically hindered o-bromobenzaldehyde moiety.
In this manner 24 diastereomerically pure eight-membered lactam
scaffolds were obtained on a 20–50 mg scale.
N
N
H
N
H
H
Br
F
Br
F
5c (84%)
5a (70%)
5b (80%)
Br
F
N
H
N
H
N
H
5f (67%)
5e (83%)
5d (81%)
F
F
N
N
H
H
MeO
MeO
MeO
MeO
Me
5h (87%)
5g (85%)
N
H
N
H
F
MeO
Me
OMe
5j (85%)
5i (85%)
R2
O
Grubbs 2nd
N
H
N
H
Generation catalyst FmocHN
N
Me
(15 mol%)
5k (83%)
3a-m
2b-m
5l (85%)
PhMe, 110 °C, 5-12 h
R1
55-93%
2b (87%) 2f (75%) 2j (89%)
2c (84%) 2g (93%) 2k (77%)
2d (92%) 2h (55%) 2l (89%)
2e (84%) 2i (63%) 2m(88%)
N
H
Me
5m (76%)
Figure 2. Allylic amines 5a–m.
Scheme 5. RCM reaction to give lactam library 2b–m.

4878
N. Brown et al. / Bioorg. Med. Chem. Lett. 18 (2008) 4876–4879
O
O
O
1. 12a-j
DIEA, CH2Cl2
TBAF
THF/MeOH
FmocHN
F
H2N
F
ZHN
F
N
N
N
rt, 20 min
rt, 1 h
11i-trans
2. PS-trisamine
rt, 1 h
100%
11i
2i-trans
OMe
OMe
OMe
13a-e Z = COR (51-100%)
13f-j Z = SO2R (32-96%)
32-100%
O
O
N
TBAF
THF/MeOH
rt, 20 min
H2N
FmocHN
O
N
1. 12a-o
DIEA, CH2Cl2
ZHN
N
100%
11l
2l-cis
rt, 1 h
Me
Me
11l-cis
2. PS-trisamine
rt, 1 h
Me
Scheme 6. Fmoc deprotection with TBAF.
14a-e Z = COR (62-74%)
14f-j Z = SO2R (27-59%)
27-100%
14k-o Z = CONHR (38-100%)
Scheme 7. Generation of the eight-membered lactam library.
O
O
O
O
O
Cl
Cl
Cl
Cl
12a
Cl
Table 1
Yields of primary amine derivativesshown in Scheme 7a
N
MeO
12d
12i
12c
12h
12e
12b
Electrophile
Primary amine 11i
Primary amine 11l
SO2Cl
SO2Cl
SO2Cl
N
CH3SO2Cl
SO2Cl
12a
12b
12c
12d
12e
12f
12g
12h
12i
12j
12k
12l
12m
12n
12o
13a 0%
13b 100%
13c 98%
13d 51%
13e 0%
13f 0%
13g 32%
13h 96%
13i 54%
13j 71%
13k —
14a 0%
14b 74%
14c 62%
14d 62%
14e 0%
14f 59%
14g 27%
14h 51%
14i 52%
14j 54%
14k 42%
14l 64%
14m 100%
14n 0%
12f
12g
12j
NCO
NCO
NCO
NCO
Ph
NCO
12k
F3C
12l
12n
12m
12o
Figure 3. Representative electrophiles for derivitization.
13l —
13m —
13n —
The stereochemical assignments for the lactam diastereomers
were made on the basis of NOE experiments. It was noted that
for the high Rf diastereomers, an NOE was observed between the
methine proton at C7 and one of the benzylic protons attached
to the amide nitrogen which led to an assignment of the cis isomer.
This effect was absent in all of the low Rf compounds; however, in
these structures, an NOE between the methine proton at C7 and
one of the methylene protons at C2 led to the assignment of the
trans isomer. This remarkable correlation between Rf and the rela-
tive stereochemical configuration was seen throughout the entire
series.
Two scaffolds, namely, 2i-trans and 2l-cis, were selected for
preparation on a larger scale (about 1 gram) for use in derivitiza-
tion of the unmasked primary amine. Removal of the Fmoc group17
was accomplished with TBAF in THF/MeOH (1:1)18 (Scheme 6) to
afford the corresponding amines 11i and 11l in quantitative yield.
These amines were then reacted with a series of 15 reagents
12a–o (Fig. 3), such as acyl chlorides, sulfonyl chlorides, and isocy-
anates, to give a library of amides, sulfonamides, and ureas.
Reaction of the amines with the reagents 12a–o with DIEA fol-
lowed by scavenging the excess reagents with PS-trisamine19
(Scheme 7) gave 19 of 30 possible products, which were further
purified by chromatography.
The isolated yields ranged from 27% to 100% (Table 1). These
derivatives combined with the original scaffolds and afforded a
43-component unsaturated eight-membered lactam library.
To our knowledge this represents the first such library based on
a monocyclic medium-ring scaffold.
Finally, of the 19-member derivitized library members synthe-
sized, seven were randomly selected for an initial antiproliferative
biological screen with murine L1210 lymphocytic leukemia, hu-
man HL-60 promyelocytic leukemia, and murine Pan02 pancreatic
ductal adenocarcinoma cell lines. The results are summarized in
Table 2.
13o —
14o 38%
aIsolated yields after chromatographic purification.
daunorubicin (DAU) and mitoxantrone (MITOX), which, under sim-
ilar experimental conditions, consistently inhibit the growth of
various tumor cell lines in the low nanomolar range, they never-
theless validate the success and relevance of these medium-ring li-
braries. Additionally, the entire demonstration library has already
been submitted to the NIH MLSCN screening network for more
expansive biological evaluation. The outcome of these studies will
be reported as the results become available and posted on Pub-
Chem (http://pubchem.ncbi.nlm.nih.gov/).
In summary we have prepared a novel, biologically relevant
natural product-inspired eight-membered lactam library using a
Table 2
Antiproliferative activity of novel eight-membered lactams inspired by octalactin A in
various tumor cell lines in vitroa
Compound L1210 cells (day 4)
HL-60 cells (day 4)
IC50 M)
Pan02 cells (day 4)
IC50 M)
b
IC50
(l
M)
(l
(l
13b
13c
13d
13g
13h
13i
27.1 ( 2.4)
25.2 ( 1.9)
40.1 ( 3.2)
37.9 ( 2.2)
11.0 ( 0.6)
8.5 ( 0.8)
54.7 ( 7.2)
40.1 ( 0.8)
40.6 ( 3.8)
34.9 ( 3.7)
nac
na
na
na
na
—
—
—
—
—
13j
15.4 ( 1.4)
a
Concentrations of compound required to inhibit by 50% (IC50 values) the met-
abolic activity of L1210, HL-60, and Pan02 tumor cells, using MTS:PMS assay at day
4 in vitro. IC50 values were calculated from linear regression of the slopes of the log-
transformed concentration survival curves.
b
Means SD (n = 3).
c
Although the antiproliferative activities of these compounds are
far from matching those of established anticancer drugs such as
Value not available because the compound is unable to inhibit below 50% when
tested at 62.5–156.25 lM.

N. Brown et al. / Bioorg. Med. Chem. Lett. 18 (2008) 4876–4879
4879
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We acknowledge support of this work by the National Institutes
of Health (The University of Kansas Chemical Methodology and Li-
brary Development Center of Excellence), P50 GM069663. We
thank Mr. Benjamin Neuenswander from the KU-CMLD Library De-
sign and Analysis Core for performing HRMS and LC–MS analysis.
Additional support from the Howard Hughes Medical Institute
(Biological Sciences Education Grant, KSU) and the Terry C. Johnson
Center for Basic Cancer Research, KSU) is hereby acknowledged.
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