An expedient synthesis of the proposed biosynthetic precursor of the oxepine natural product, janoxepin

Article abstract of DOI:10.1016/j.tetlet.2012.03.025

An efficient synthetic route to the putative biosynthetic intermediate of the anti-plasmodial natural product janoxepin is described. This novel enamine-containing pyrazino[2,1-b]quinazoline-3,6-dione, and its synthetic precursors, should be of value in s

Full text of DOI:10.1016/j.tetlet.2012.03.025

Tetrahedron Letters 53 (2012) 2533–2536
Contents lists available at SciVerse ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
An expedient synthesis of the proposed biosynthetic precursor
of the oxepine natural product, janoxepin
⇑
Richard G. Doveston, Richard J.K. Taylor
Department of Chemistry, University of York, Heslington, York YO10 5DD, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
An efficient synthetic route to the putative biosynthetic intermediate of the anti-plasmodial natural
product janoxepin is described. This novel enamine-containing pyrazino[2,1-b]quinazoline-3,6-dione,
and its synthetic precursors, should be of value in studies to elucidate the biosynthetic pathway leading
to the oxepine family of natural products. The cornerstones of the synthesis are amide coupling,
pyrazino[2,1-b]quinazoline-3,6-dione construction and aldol introduction of the enamine.
Ó 2012 Elsevier Ltd. All rights reserved.
Received 6 February 2012
Revised 24 February 2012
Accepted 7 March 2012
Available online 13 March 2012
Keywords:
Janoxepin
Oxepine
Biosynthesis
Pyrazino[2,1-b]quinazoline-3,6-dione
Me
O
Me
Me
Me
There is increasing fascination with the growing family of oxe-
pine-containing natural products, particularly in terms of their
structural determination, biological activity and biogenesis. We
have become interested in this area from a synthetic viewpoint
and recently reported the first total synthesis of the anti-plasmo-
dial, oxepine-pyrimidinone-containing natural product janoxepin
(1) (Fig. 1).1
Ph
H
N
H
N
H
N
O
O
O
Me
HO
N
HO
Ph
Me
N
N
N
N
N
Me
O
O
O
O
O
In terms of the biosynthesis of oxepine-containing natural prod-
ucts such as janoxepin (1) and related compounds, oxepinamide D
(2)2 and brevianamide O (3),3 a common proposal is that the oxe-
pine ring is generated via enzymatic arene-epoxidation of a pyraz-
ino[2,1-b]quinazoline-3,6-dione 4 followed by a rearrangement
and ring-opening sequence (Scheme 1).2–4
The pyrazino[2,1-b]quinazoline-3,6-dione skeleton seen in 4
can be found in numerous natural products with considerable bio-
logical potential. Many of this family, including acetylardeemin,5
fiscalin B6 and serantrypinone7 (amongst others8) have been the
subject of elegant syntheses. However, these compounds do not
bear the enamine moiety at C-4 present in 4 and the only natural
product examples with this structural feature are aurantiomide C
(6),9 verrucine F (7)10 and quinadoline A (8)11 (Fig. 2), none of
which have succumbed to total synthesis to date. We therefore
decided to develop a synthetic route to prepare the putative
janoxepin precursor 4, both to aid biosynthetic studies in the
oxepine natural product area, and to provide an entry into the
pyrazino[2,1-b]quinazoline-3,6-dione natural products shown in
Figure 2.
janoxepin 1
brevianamide O 3
oxepinamide D 2
Figure 1. Representative oxepine natural products.
The retrosynthetic analysis of 4 (Scheme 2) relied upon intro-
duction of the aliphatic enamine moiety via an aldol addition to
an imidate-protected version of the known12 pyrazino[2,1-b]qui-
nazoline-3,6-dione 9, followed by a selective elimination to furnish
the Z-enamine. The tricyclic intermediate 9 would itself be con-
structed by a double cyclisation of the novel amine 10, whilst an
amide coupling sequence starting from the inexpensive and readily
available isatoic anhydride (11), Fmoc-Gly-Cl 12 and
methyl ester (13) would ultimately be employed to synthesise this
key intermediate.
D-leucine
The study therefore commenced with a modified coupling of
the commercially-available isatoic anhydride (11) and D-leucine
methyl ester (13) (Scheme 3) to furnish the known amide 14 in a
much improved yield to that previously reported (87% vs 32%13).
This was followed by a second coupling with the Fmoc-protected
glycine chloride 12 to furnish the novel intermediate 15. The Fmoc
protecting group was then removed by treatment of 15 with piper-
idine to provide the key novel amine intermediate 10 in three
⇑
Corresponding author. Tel.: +44 (0)1904 322606; fax: +44 (0)1904 324523.
E-mail address: richard.taylor@york.ac.uk (R.J.K. Taylor).
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.03.025

2534
R. G. Doveston, R. J. K. Taylor / Tetrahedron Letters 53 (2012) 2533–2536
Me
Me
4
Me
Me
Me
Me
H
N
H
N
H
N
O
O
O
Me
Me
Me
1
N
N
Me
N
N
Me
N
N
Me
O
O
O
O
O
4
5
janoxepin 1
Scheme 1. Proposed biosynthesis of janoxepin (1).
Me
Me
Ph
Me
H
H
H
N
N
O
N
O
O
Me
HO
N
N
CONH2
N
N
CONH2
N
N
N
O
O
O
HN
O
Me
Me
quinadoline A 8
aurantiomide C 6
verrucine F 7
Figure 2. Structures of aurantiomide C (6), verrucine F (7) and quinadoline A (8).
Me
Me
H
N
H
N
O
O
Me
Me
imidate
protection
cyclisation
N
N
Me
N
N
Me
aldol
O
O
condensation
9
4
FmocHN
MeO
H2N
O
H2N
MeO
O
Me
Me
O
Cl
amide
12
coupling
Me
O
NH HN
Me
O
13
O
HN
O
10
O
11
Scheme 2. Retrosynthetic analysis.
FmocHN
12
O
MeO
O
O
Cl
Me
MeO
H2N
O
Et3N, EtOAc
77 °C, 18 h
HN
O
CH2Cl2, aq. Na2CO3 (1.0 M)
rt, 1.5 h
Me
Me
NH2 HN
+
O
Me
87%
O
79%
13
11
14
H
FmocHN
O
MeO
O
H2N
O
MeO
O
N
O
Me
Me
Me
piperidine, CH2Cl2
rt, 30 min
Sc(OTf)3 (1.0 eq), DMF
Me
NH HN
-W, 140 °C, 50 W
Me
µ
NH HN
N
N
Me
O
68%
95% ee
O
62%
O
15
10
9
Scheme 3. Synthesis of pyrazino[2,1-b]quinazoline-3,6-dione 9.

R. G. Doveston, R. J. K. Taylor / Tetrahedron Letters 53 (2012) 2533–2536
2535
Me
N
N
O
H
N
O
Me
i) LiHMDS, THF, −78 °C, 10 min
ii) iso-butyraldehyde, THF
−78 °C, 10 min
Me
Me
BF4OEt3, K2CO3
CH2Cl2, 45 °C, 1 h
N
N
N
Me
67%
O
66%
O
16
9
Me
HO
Me
Me
Me
Me
Me
Me
N
O
N
O
Me
MsCl, pyridine
50 °C, 2 h
N
N
Me
N
N
Me
77%
O
O
18
17
nOe
H
Me
Me
Me
Me
H
N
H
N
O
O
Me
Me
aq. AcOH
50 °C, 1.5 h
N
N
Me
N
N
Me
84%
O
O
4
4
Scheme 4. Synthesis of pyrazino[2,1-b]quinazoline-3,6-dione 4.
steps, 47% overall yield and in 95% ee (as shown by chiral HPLC
analysis).14
‘discrete’ synthesis in 53% overall yield from
L
-leucine-Wang re-
sin, anthranilic acid and Fmoc-Gly-Cl; no isolated mass reported,
optical purity unspecified.12 The long reaction times and large
excesses of anthranilic acid, Fmoc-Gly-Cl and iodine needed did
not lend this procedure to the gram scale preparation of 9
required in this context].
In order to effect the double cyclisation to convert amine 10
into pyrazino[2,1-b]quinazoline-3,6-dione 9 we utilised the Lewis
acid mediated cyclisation procedure reported by Chu and co-
workers.15 Thus, a solution of amine 10 in DMF was treated with
1 equiv of scandium triflate and irradiated in a microwave reac-
tor (CEM Discover) at 140 °C, 50 W for 2 min as reported,15 but
only a trace of the desired tricycle 9 was observed along with
unreacted amine 10. However, microwave irradiation at the same
temperature for an extended period (10 min) gave pyrazino[2,1-
b]quinazoline-3,6-dione 9 in 62% yield (Scheme 3). This result
demonstrates that the Chu procedure15 is compatible with a sub-
stituent at C-1 on the ketopiperazine ring. Disappointingly
however, due either to the long reaction times, or simply the
base-sensitivity of the compounds, complete racemisation was
observed (9, 51:49 er) under the cyclisation conditions employed
here.16 Future work will investigate the effect of different side
chains at the C-1 position and screen alternative Lewis acids in
order to demonstrate the broader scope of this procedure. [Com-
pound 9 has previously been prepared by a four-step solid phase
The key step now remaining was to install the enamine moiety.
Amide 9 was first converted into the novel imidate 16 using trieth-
yloxonium tetrafluoroborate. This choice of amide protection has
the virtue of facile subsequent removal under mild conditions,
whilst being stable to the basic conditions required for ketopiper-
azine deprotonation and further manipulations. Aldol addition
was effected by treatment of compound 16 with LiHMDS in THF
at À78 °C followed by the addition of iso-butyraldehyde to afford
alcohol 17 as a single diastereomer and in good yield. Treatment
of 17 with methanesulfonyl chloride in pyridine, with heating to
50 °C, gave enamine 18 directly (as a single isomer) via a one-pot
mesylation–elimination sequence. The preparation was completed
by imidate hydrolysis using aqueous acetic acid with heating to
50 °C to furnish the target compound 4 in 43% overall yield over
the three steps in racemic form (Scheme 4).
Figure 3. X-ray crystallography of ( )-4. ORTEP representations shown with ellipsoids at 50% probability. A single enantiomer (R) is shown for clarity. CCDC 862145.18

2536
R. G. Doveston, R. J. K. Taylor / Tetrahedron Letters 53 (2012) 2533–2536
Table 1
Epoxidation conditions screened
Entry
Oxidant
Conditions
1
2
3
m-CPBA
NaOCl
DMDO
CH2Cl2/aq NaHCO3 (sat.), rt, 18 h
n-Bu4NHSO4, CH2Cl2, rt, 18 h
Acetone, 0 °C–rt, 18 h
The structure of 4 was confirmed by 1H and 13C NMR spectros-
copy in conjunction with COSY and HSQC experiments.17 NOE cor-
relations (Scheme 4) confirmed that the enamine was in the
desired Z-configuration, and further proof of this was provided
by X-ray crystallographic analysis (Fig. 3).18
studies and to Dr. K. Heaton for invaluable assistance with mass
spectrometry.
References and notes
1. Doveston, R. G.; Steendam, R.; Jones, S.; Taylor, R. J. K. Org. Lett. 2012, 14, 1122–
1125.
2. Lu, X.-H.; Shi, Q.-W.; Zheng, Z.-H.; Ke, A.-B.; Zhang, H.; Huo, C.-H.; Ma, Y.; Ren,
X.; Li, Y.-Y.; Lin, J.; Jiang, Q.; Gu, Y.-C.; Kiyota, H. Eur. J. Org. Chem. 2011, 802–
807.
3. Li, G.-Y.; Li, L.-M.; Yang, T.; Chen, X.-Z.; Fang, D.-M.; Zhang, G.-L. Helv. Chim.
Acta 2010, 93, 2075–2080.
4. Sprogøe, K.; Manniche, S.; Larsen, T. O.; Christophersen, C. Tetrahedron 2005, 61,
8718–8721.
Preliminary, if speculative, experiments were carried out to
investigate whether chemical oxidation could be employed to ef-
fect the proposed biomimetic oxepine formation (Table 1). Pyrazi-
no[2,1-b]quinazoline-3,6-dione 9 was chosen as the substrate for
this study to preclude any problems with competing epoxidisation
of an enamine double bond. Unfortunately, there was no sign of the
hoped-for oxepine 19 (nor the intermediate epoxide) and only an
inseparable mixture of N-oxide compounds, starting material and
degradation products was observed (oxidation at the ketopiper-
azine C-4 position cannot be ruled out as this process has been re-
ported previously19). In order to remove the possibility of N-oxide
formation, the corresponding N-Boc derivative 20 was subjected to
the same reagents with similarly disappointing results. We plan to
investigate the chemical oxidation of the putative biosynthetic
intermediate 4 and its imidate precursor 18 in due course; how-
ever these preliminary results suggest that arene oxidation will
be difficult to achieve synthetically.
5. Takiguchi, S.; Iizuka, T.; Kumakura, Y.-S.; Murasaki, K.; Ban, N.; Higuchi, K.;
Kawasaki, T. J. Org. Chem. 2010, 75, 1126–1131.
6. (a) Liu, J.-F.; Ye, P.; Zhang, B. L.; Bi, G.; Sargent, K.; Yu, L.; Yohannes, D.; Baldino,
C. M. J. Org. Chem. 2005, 70, 6339–6345; (b) Hernández, F.; Buenadicha, F. L.;
Avendaño, C.; Söllhuber, M. Tetrahedron: Asymmetry 2001, 12, 3387–3398.
7. Hart, D. J.; Oba, G. Tetrahedron Lett. 2007, 48, 7069–7071.
8. See also: (a) Snider, B. B.; Zeng, H. J. Org. Chem. 2003, 68, 545–563; (b)
Hernández, F.; Lumetzberger, A.; Avendaño, C.; Söllhuber, M. Synlett 2001,
1387–1390; (c) Kende, A. S.; Fan, J.; Chen, Z. Org. Lett. 2003, 5, 3205–3208; (d)
Hart, D. J.; Magomedov, N. A. J. Am. Chem. Soc. 2001, 123, 5892–5899; (e)
Cledera, P.; Avendaño, C.; Menéndez, J. C. J. Org. Chem. 2000, 65, 1743–1749; (f)
Wang, H.; Sim, M. M. J. Nat. Prod. 2001, 64, 1497–1501.
9. Xin, Z. H.; Fang, Y.; Du, L.; Zhu, T.; Duan, L.; Chen, J.; Gu, Q.-Q.; Zhu, W.-M. J. Nat.
Prod. 2007, 70, 853–855.
In summary, the putative biosynthetic precursor to janoxepin
(1), pyrazino[2,1-b]quinazoline-3,6-dione 4, has been prepared in
an efficient eight-step synthesis starting from readily available
and inexpensive starting materials. The methodology for installa-
tion of the enamine moiety, as well as that for pyrazino[2,1-b]qui-
nazoline-3,6-dione construction, not only has application in the
preparation of other oxepine biosynthetic precursors, but also in
the synthesis of pyrazino[2,1-b]quinazoline-3,6-dione natural
products 6–8 (Fig. 2). Strategies for the asymmetric synthesis of
compounds of this type are currently being investigated. With a
view to developing a biomimetic strategy for the synthesis of oxe-
pine natural products, a preliminary investigation of chemical
arene-epoxidation procedures was unsuccessful. However, an effi-
cient route to these key biosynthetic precursors is now available to
10. Leong, S.-l.; Schnürer, J.; Broberg, A. J. Nat. Prod 2008, 71, 1455–1457.
11. Koyama, N.; Inoue, Y.; Sekine, M.; Hayakawa, Y.; Homma, H.; Omura, S.;
Tomoda, H. Org. Lett. 2008, 10, 5273–5276.
12. Wang, H.; Ganesan, A. J. Comb. Chem. 2000, 2, 186–194.
¯
13. Flores-López, L. Z.; Parra-Hake, M.; Somanathan, R.; Ortega, F.; Aguirre, G.
Synth. Commun. 2000, 30, 147–155. for the procedure followed, see Ref. 15.
14. HPLC: Chiralpak AD-H (80:20 n-hexane/i-PrOH, 1.0 mL minÀ1
) 7.25 min
(94.91%), 9.09 min (5.09%).
15. Tseng, M.-C.; Yang, H.-Y.; Chu, Y.-H. Org. Biomol. Chem. 2010, 8, 419–427.
16. HPLC: Chiralpak OD (80:20 n-hexane/i-PrOH, 1.0 mL minÀ1) 9.71 min (50.59%),
18.83 min (49.41%).
17. ( )-4: mp 229–231 °C (MeOH/n-hexane); Rf 0.55 (1:1 PE/EtOAc); Found: C,
69.89; H, 7.09; N, 12.81; C19H23N3O2 requires: C, 70.13; H, 7.12; N, 12.91%;
t
max/cmÀ1 3187, 3078, 2960, 1686, 1582, 1562; dH (400 MHz, CDCl3) 8.59 (1 H,
br s), 8.27 (1 H, dd, J 8.0, 1.5), 7.76 (1 H, ddd, J 8.5, 7.0, 1.5), 7.69 (1 H, dd, J 8.5,
1.5), 7.47 (1 H, ddd, J 8.0, 7.0, 1.5), 6.45 (1 H, d, J 10.0), 5.58 (1 H, ddd, J 8.5, 5.5,
1.0), 2.75 (1 H, d sept, J 10.0, 6.5), 1.83–1.71 (2 H, m), 1.65 (1 H, ddd, J 13.5, 8.5,
5.5), 1.21 (3 H, d, J 6.5), 1.18 (3 H, d, J 6.5), 1.08 (3 H, d, J 6.5), 0.94 (3 H, d, J 6.5);
dC (100 MHz, CDCl3) 167.4, 160.4, 147.4, 145.1, 134.6, 127.8, 127.4, 126.8,
126.8, 124.9, 120.0, 53.7, 42.6, 26.1, 24.9, 23.1, 22.4, 22.3, 21.5; m/z (ESI) 326
[MH]+. Calcd for C19H24N3O2: 326.1863. Found: [M+H]+, 326.1859 (0.9 ppm
error); HPLC: Chiralcel OD (95:5 n-hexane/i-PrOH, 1.0 mL minÀ1) 9.74 min
(50.10%), 20.02 min (49.90%).
underpin
future
studies
using
enzymatic
epoxidative
biotransformations.
Acknowledgments
18. X-Ray crystallography: CCDC 862145 contains the supplementary
crystallographic data for this compound. Crystals were grown by slow
diffusion (Et2O/CH2Cl2).
We would like to thank the EPSRC (R.G.D.) for PhD funding. We
are also grateful to Dr. A. C. Whitwood for X-ray crystallography
19. He, F.; Snider, B. B. Synlett 1997, 483–484.