Synthesis of the lsoxazolo[4,3,2-de]phenanthridinone moiety of the parnafungins

Article abstract of DOI:10.1021/ol901054r

A practical route to the labile tetracyclic lsoxazolo[4,3,2-de] phenanthrldlnone moiety of the antifungal parnafunglns has been developed. Zinc reduction of a methyl 2′-hydroxymethyl-2-nltro-3-blphenylcarboxylate, which was prepared by a Suzuki coupling,

Full text of DOI:10.1021/ol901054r

ORGANIC
LETTERS
2009
Vol. 11, No. 13
2936-2939
Synthesis of the
Isoxazolo[4,3,2-de]phenanthridinone
Moiety of the Parnafungins
Quan Zhou and Barry B. Snider*
Department of Chemistry, MS 015, Brandeis UniVersity, Waltham, Massachusetts 02454-9110
snider@brandeis.edu
Received May 13, 2009
ABSTRACT
A practical route to the labile tetracyclic isoxazolo[4,3,2-de]phenanthridinone moiety of the antifungal parnafungins has been developed. Zinc
reduction of a methyl 2′-hydroxymethyl-2-nitro-3-biphenylcarboxylate, which was prepared by a Suzuki coupling, afforded a benzisoxazolone
that was treated with MsCl and base to generate the labile tetracyclic ring system in 37-47% yield. This compound decomposes to the
phenanthridine in CDCl3 and the phenanthridine N-oxide in aqueous base.
A Merck group recently reported the isolation of parnafun-
gins A1 (1a), A2 (1b), B1 (2a), and B2 (2b) that interconvert
Scheme 1. Parnafungin Structures and Rearrangement Products
readily by an elimination that opens the xanthone ring to
form an enone and a phenol and then a conjugate addition
from either phenol OH group to either face of the enone to
reform the xanthone (see Scheme 1).1 The parnafungins
demonstrate broad spectrum antifungal activity with no
antibacterial activity by inhibiting fungal polyadenosine
polymerase (PAP) and show in vivo efficacy against Candida
(1) (a) Parish, C. A.; Smith, S. K.; Calati, K.; Zink, D.; Wilson, K.;
Roemer, T.; Jiang, B.; Xu, D.; Bills, G.; Platas, G.; Pela´ez, F.; D´ıez, M. T.;
Tsou, N.; McKeown, A. E.; Ball, R. G.; Powles, M. A.; Yeung, L.; Liberator,
P.; Harris, G. J. Am. Chem. Soc. 2008, 130, 7060–7066. (b) Jiang, B.; Xu,
D.; Allocco, J.; Parish, C.; Davison, J.; Veillette, K.; Sillaots, S.; Hu, W.;
Rodriguez-Suarez, R.; Trosok, S.; Zhang, L.; Li, Y.; Rahkhoodaee, F.;
Ransom, T.; Martel, N.; Wang, H.; Gauvin, D.; Wiltsie, J.; Wisniewski,
D.; Salowe, S.; Kahn, J. N.; Hsu, M.-J.; Giacobbe, R.; Abruzzo, G.; Flattery,
A.; Gill, C.; Youngman, P.; Wilson, K.; Bills, G.; Platas, G.; Pelaez, F.;
Diez, M. T.; Kauffman, S.; Becker, J.; Harris, G.; Liberator, P.; Roemer,
T. Chem. Biol. 2008, 15, 363–374. (c) Adam, G. C.; Parish, C. A.;
Wisniewski, D.; Meng, J.; Liu, M.; Calati, K.; Stein, B. D.; Athanasopoulos,
J.; Liberator, P.; Roemer, T.; Harris, G.; Chapman, K. T. J. Am. Chem.
Soc. 2008, 130, 16704–16710. (d) Overy, D.; Calati, K.; Kahn, J. N.; Hsu,
M.-J.; Mart´ın, J.; Collado, J.; Roemer, T.; Harris, G.; Parish, C. A. Bioorg.
Med. Chem. Lett. 2009, 19, 1224–1227.
albicans in a mouse model.1b Affinity selection/mass spec-
trometry demonstrated that the linear parnafungin A binds
10.1021/ol901054r CCC: $40.75
Published on Web 06/04/2009
 2009 American Chemical Society

preferentially to PAP.1c The O-methylated parnafungins C
and D were recently isolated.1d
Scheme 4. Retrosynthesis of Tetracycle 10
The isoxazolone ring of 1 and 2 is very labile. Treatment
of 1 and 2 at neutral or basic pH generated phenanthridines
3 and 4, respectively, in less than 1 h. The same compounds
were generated over 10-20 h at pH 3. Unfortunately,
phenanthridines 3 and 4 are biologically inactive.1a
The parnafungins might be biosynthesized by oxidative
coupling of 5 with anthranilic acid to give 6 (see Scheme
2). Further oxidation of the benzylic methyl group and aniline
Scheme 2. Possible Biosynthesis of the Parnafungins
by Wierenga’s procedure from biphenylcarboxylate 12,
which has been prepared by Liu from methyl 3-chloro-2-
nitrobenzoate (16) by Suzuki coupling with phenylboronic
acid (15, R ) H).8 However, the Friedel-Crafts reaction
may not work well with an unactivated aromatic ring, and
this route differs from the proposed biosynthesis in which
the ring is closed by C-N rather than C-C bond formation.
We therefore considered an alternate approach in which the
C-N bond of 10 is formed from 13 by an intramolecular
SN2 reaction. Suzuki coupling of 16 with the appropriate
boronic acid 15, R ) CH2OH, should provide 14, which
should form 13 by zinc reduction without formaldehyde.
Suzuki coupling of 16 and phenylboronic acid (17a) as
described by Liu afforded 12 in 65% yield (see Scheme 5).
could give parnafungins A1 (1a) and A2 (1b). Blenolide C
(5b) was recently isolated2 and numerous natural products
are known that are derived from blenolide C by oxidative
dimerization or coupling at the carbon marked by an asterisk.
Bra¨se3 and Nicolaou4 have recently reported syntheses of
blenolide C (5b).
We therefore decided to start our synthetic studies by
developing a route to the tetracyclic isoxazolo[4,3,2-
de]phenanthridinone moiety 10 that could then be extended
to the synthesis of the parnafungins by starting with blenolide
C or an analogue. We were guided by the studies of
Wierenga5 that were based on earlier work of Bamberger6
and Cohen.7 Wierenga reported that reduction of methyl
o-nitrobenzoate (7) with zinc, NH4Cl, and Na2CO3 in MeOH
containing formaldehyde afforded hydroxymethylbenzisox-
azolone (9) in 44% yield (see Scheme 3). A similar reduction
Scheme 5. Synthesis of Tetracycle 22b
Scheme 3. Wierenga Benzisoxazolone Synthesis
without formaldehyde gave 8 in low yield, which could be
converted to 9 by reaction with basic formaldehyde.
Our retrosynthesis of 10 is shown in Scheme 4. An
intramolecular Friedel-Crafts alkylation of 11 could give
10. Hydroxymethylbenzisoxazolone 11 should be accessible
Org. Lett., Vol. 11, No. 13, 2009
2937

The yield was improved to 99% using SPhos, Pd(OAc)2, and
K3PO4 in wet THF.9 Reduction of 12 with Zn and NH4Cl in
THF/MeOH/H2O with sonication for 30 min afforded a 3:1
mixture of the desired benzisoxazolone 19a and amino ester
20a. Benzisoxazolone 19a decomposed on chromatography
but could be obtained in 60% yield by washing the mixture
with 9:1 hexanes/Et2O to remove 20a. Reaction of 19a with
formaldehyde and Na2CO3 in aqueous THF afforded the
desired hydroxymethylbenzisoxazolone 11 in 61% yield. This
two-step sequence gave pure 11; impure 11 was obtained in
one step by reduction of 12 with Zn and formaldehyde.
Unfortunately, all attempts to form 10 by intramolecular
Friedel-Crafts alkylation on the phenyl ring of 11 were
unsuccessful.
We then turned to a more electron-rich aromatic ring to
see if the Friedel-Crafts alkylation would work with an
optimized substrate. The analogous Suzuki coupling of 16
with 3,5-dimethoxyphenylboronic acid (17b) gave 18b in
99% yield. Zinc reduction gave 19b in 66% yield, which
was treated with formaldehyde to give 21b in 76% yield.
We were delighted to find that treatment of 21b with 10%
TFA in CH2Cl2 generated an iminium cation that added to
the aromatic ring to give 22b in 27% yield. Although this
route to 22b provided the first synthesis of an isox-
azolo[4,3,2-de]phenanthridinone, it is not applicable to the
synthesis of the parnafungins because the Friedel-Crafts
reaction would have to occur meta to the oxygen substituents
and para to the carbonyl group. We therefore turned our
attention to the alternate route using C-N bond formation.
The analogous Suzuki coupling of 16 with 2-hydroxym-
ethylphenylboronic acid (23a) afforded biphenyl 24a in 95%
yield (see Scheme 6). Zinc reduction afforded a mixture of
desired tetracyclic isoxazolo[4,3,2-de]phenanthridinone moi-
ety 10 in 37% yield from 24a.
Application of this route to a coupling product of blenolide
C (5b) would require functionalization of a benzylic methyl
group. We therefore prepared 24b in 95% yield by Suzuki
coupling of 16 with o-tolylboronic acid (23b). Bromination
of 24b with NBS and (BzO)2 in CCl4 at reflux gave 24d, R
) H, X ) Br. Reaction of 24d with KOAc in DMF afforded
24e, R ) H, X ) OAc, which was hydrolyzed with K2CO3
in MeOH to give 24a in 79% overall yield from 24b. Other
approaches were unsuccessful. Reduction of 24d with Zn
gave 25b resulting from reduction of both the nitro and
bromomethyl groups. Functionalization of the methyl group
must be carried out before formation of the benzisoxazolone
ring. Zn reduction of 24b gave 25b in 62% yield, which
decomposed on treatment with NBS.
Suzuki coupling of 16 with boronic acid 23c10 provided
biphenyl 24c (83%) with the oxygen substituents on the same
carbons as in the parnafungins. Zinc reduction gave ben-
zisoxazolone 25c, which was treated with MsCl and Et3N
to give the mesylate. Treatment of the mesylate with Na2CO3
in 1:1 THF/H2O provided 26c in 47% overall yield from
24c. This sequence provides a very short and high yield route
to the labile tetracyclic isoxazolo[4,3,2-de]phenanthridinone
moiety of the parnafungins that should be applicable to the
synthesis of the natural product.
The Merck group suggested that the rearrangement of
parnafungins 1 and 2 to 3 and 4, respectively, could occur
by either an E2 reaction or by hydrolysis of the isoxazolone
followed by loss of water. We explored the stability of 10,
22b, and 26c in CDCl3. All three compounds rearranged
cleanly to the respective phenanthridine 27a,11 27b, and 27c
without any evidence for the formation of an intermediate
(see Scheme 7). This indicates either that the reaction
Scheme 6. Synthesis of Tetracycles 10 and 26c
Scheme 7. Rearrangement of 10, 22b, and 26c in CDCl3
proceeds by an E2 elimination or that loss of water is much
faster than hydrolysis of the isoxazolone. Tetracycle 26c with
(2) Zhang, W.; Krohn, K.; Zia-Ullah; Flo¨rke, U.; Pescitelli, G.; Di Bari,
L.; Antus, S.; Kurta´n, T.; Rheinheimer, J.; Draeger, S.; Schulz, B.
Chem.sEur. J. 2008, 14, 4913–4923.
benzisoxazolone 25a and the amino ester analogous to 20.
Treatment of this mixture with MsCl and Et3N in CH2Cl2
for 15 min at 0 °C afforded the mesylate of 25a, which was
treated with Na2CO3 in 1:1 THF/H2O for 40 min to give the
(3) (a) Nising, C. F.; Ohnemu¨ller (ne´e Schmid), U. K.; Bra¨se, S. Angew.
Chem., Int. Ed. 2006, 45, 307–309. (b) Ge´rard, E. M. C.; Bra¨se, S.
Chem.sEur. J. 2008, 14, 8086–8089.
(4) Nicolaou, K. C.; Li, A. Angew. Chem., Int. Ed. 2008, 47, 6579–
6582.
2938
Org. Lett., Vol. 11, No. 13, 2009

the oxygen substituents in the same position as the parnafun-
gins rearranges fastest with a half-life of 6 days. Tetracycle
10 rearranges with a half-life of 10 days and tetracycle 22b
rearranges slowest with a half-life of 27 days. The C-methyl
group of m-methoxytoluene is deprotonated twice as fast as
that of toluene by lithium amide bases, whereas the C-methyl
group of o-methoxytoluene is deprotonated 10 times slower
than that of toluene.12 Deprotonation occurs in the rate-
determining step of an E2 elimination even if protonation
of the carbonyl group by adventitious HCl is the initial step.
Therefore, the two methoxy groups meta to the methylene
group of 26c should accelerate deprotonation and E2
elimination, whereas the methoxy groups ortho and para to
the methylene group of 22b should retard deprotonation and
elimination. The parnafungins have oxygen substituents meta
to the methylene group as in 26c and a carbonyl group para
to the methylene group, which should further increase its
acidity and accelerate the E2 elimination.
presumably by air. The autoxidation of hydroxylamine in
aqueous base has been extensively studied13 and the oxida-
tion of alkylhydroxylamines to oximes in MeOH has been
reported.14 The formation of 29 by this decomposition
pathway is noteworthy because we were unable to prepare
29 directly by oxidation of 27a using procedures that work
well on phenanthridine iteself. Both electron-withdrawing
groups and peri substituents are known to retard the
N-oxidation of heterocycles.15
In conclusion, a practical route to the labile tetracyclic
isoxazolo[4,3,2-de]phenanthridinone moiety (10 and 26c) of
the antifungal parnafungins has been developed. Zinc reduc-
tion of methyl 2′-hydroxymethyl-2-nitro-3-biphenylcarboxy-
lates 24a and 24c, which were prepared by Suzuki couplings,
afforded benzisoxazolones 25a and 25c that were treated with
MsCl and then base to generate the labile tetracyclic ring
systems 10 (37%) and 26c (47%). These compounds rear-
range to phenanthridines 27a and 27c in CDCl3 and 10
decomposes to phenanthridine N-oxide 29 in aqueous base.
We also explored the decomposition of 10 under basic
condition with NaOD in CD3CN/D2O. Under these condi-
tions, we detected an intermediate with the CH2 group shifted
upfield to δ 4.32 from δ 4.72 in 10. We tentatively assigned
structure 28 to this intermediate (see Scheme 8). To our
Acknowledgment. We are grateful to the National Insti-
tutes of Health (GM-50151) for support of this work. We
thank Professor Stephen Buchwald, Massachusetts Institute
of Technology for advice on Suzuki couplings.
Supporting Information Available: Complete experi-
mental procedures, copies of 1H and 13C NMR spectral data.
This material is available free of charge via the Internet at
http://pubs.acs.org.
Scheme 8. Decomposition of 10 in Base
OL901054R
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27a and the N-oxide 29 resulting from oxidation of 28,
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