Synthesis of the spiroiminal moiety of marineosins A and B

Article abstract of DOI:10.1021/ol100333d

(Figure Presented) A model for the spiroiminal moiety of marineosins A and B was prepared starting from methylvalerolactone. Addition of vinylmagnesium bromide, protection of the alcohol, and reaction of the vinyl ketone with a protected pyrrole-2-carbonl

Full text of DOI:10.1021/ol100333d

ORGANIC
LETTERS
2010
Vol. 12, No. 7
1600-1603
Synthesis of the Spiroiminal Moiety of
Marineosins A and B
Xiao-Chuan Cai, Xiaoxing Wu, and Barry B. Snider*
Department of Chemistry MS 015, Brandeis UniVersity,
Waltham, Massachusetts 02454-9110
snider@brandeis.edu
Received February 8, 2010
ABSTRACT
A model for the spiroiminal moiety of marineosins A and B was prepared starting from methylvalerolactone. Addition of vinylmagnesium
bromide, protection of the alcohol, and reaction of the vinyl ketone with a protected pyrrole-2-carbonitrile N-oxide gave an isoxazoline.
Hydrogenolysis of the N-O bond with Raney nickel gave a keto imine that cyclized to a hemi-iminal. O-Methylation, acid-catalyzed cleavage
of the TES group and spiroiminal formation, and deprotection completed a seven-step synthesis.
Fenical and co-workers recently reported the isolation of the
cytotoxic spiroiminals marineosins A (1) and B (2) from a
Scheme 1
.
Structures of Marineosins A and B and Biosynthesis
Suggested by Fenical
marine-derived Streptomyces-related actinomycete (see Scheme
1).1 The structures were assigned by analysis of the NMR
spectra, and the stereochemistry was assigned from the
NOESY spectra. These compounds are clearly members of
the prodigiosin family of bacterial pigments.2 It is noteworthy
that marineosins A (1) and B (2) differ in stereochemistry
at both C-7 and C-8. Analysis of models and molecular
mechanics calculations suggest that the isomers of the
marineosins at the spiroiminal center (C-8) are much less
stable than marineosins A and B because of steric interactions
between the methoxy group and the adjacent pyrrole that is
part of the macrocycle in the two isomers that were not
isolated. The major isomer, marineosin A (1), inhibits human
colon carcinoma HCT-116 with an IC50 of 0.5 µM. Testing
in the NCI 60 cell line panel showed considerable selectivity
against melanoma and leukemia cell lines.
Fenical suggested that the marineosins might be biosyn-
thesized by an intramolecular hetero Diels-Alder reaction
of 3 to give 4, which would then be reduced to give
marineosins A and B. However, this biosynthetic pathway
seemed unlikely to us since it would require a six-electron
oxidation of a likely precursor, undecylprodigiosin (5) (see
(1) Boonlarppradab, C.; Kauffman, C. A.; Jensen, P. R.; Fenical, W.
Org. Lett. 2008, 10, 5505–5508.
(2) (a) Fu¨rstner, A. Angew. Chem., Int. Ed. 2003, 42, 3582–3603. (b)
Williamson, N. R.; Fineran, P. C.; Gristwood, T.; Chawrai, S. R.; Leeper,
F. J.; Salmond, G. P. C. Future Microbiol. 2007, 2, 605–618.
10.1021/ol100333d  2010 American Chemical Society
Published on Web 03/10/2010

Scheme 2), to form 3, followed by a four-electron reduction
of 4 to give marineosins A and B. Lindsley recently reported
that the conversion of 3 (prepared by an eight-step route) to
4 could not be achieved in the laboratory.3
Radical 8 could undergo a facile 1,5-hydride shift to give
macrocycle 9 with the radical at the side chain carbon
adjacent to the methyl group. This radical is close to the
enzyme active site that generated radical 7 and could
therefore be easily oxidized to form macrocyclic alcohol 10.
2H-Pyrrole 10 could undergo a facile 1,5-sigmatropic hydride
shift to form 1H-pyrrole 11, which could cyclize to form
marineosins A and B. However, cyclization of 11 to 1 and
2 would require the loss of aromaticity of pyrrole 11.
Alternatively, a 1,5-sigmatropic hydride shift from 10 could
also give 3H-pyrrole 12, which should rapidly cyclize to form
spiroaminal 13. Tautomerization of the enamine to an imine
would give marineosins A (1) and B (2). This scheme is
appealing because only a single enzyme would be needed
to convert undecylprodigiosin (5) to the marineosins.
There are numerous examples of spiroaminals and many
examples of iminals, but the spiroiminal moiety of the
marineosins appears to be unprecedented. We therefore chose
to start our synthetic planning for the marineosins by
exploring routes to the spiroiminal moiety. We were par-
ticularly interested in routes in which the dihydropyrrole ring
of the spiroiminal was formed before the tetrahydropyran
because this approach is related to the biosynthesis proposed
in Scheme 2. Our retrosynthesis is shown in Scheme 3. The
marineosins should be available by spiroiminal formation
from 14, which should be readily available from keto
isoxazoline 15 by hydrogenolysis of the N-O bond over
Raney nickel with spontaneous formation of the hemi-iminal5
and then O-methylation. Isoxazoline 15 will be prepared by
nitrile N-oxide cycloaddition to the vinyl ketone prepared
by addition of vinylmagnesium bromide to lactone 16 and
protection of the secondary alcohol.
Scheme 2. Alternative Biosynthesis of the Marineosins
Scheme 3. Retrosynthesis of the Marineosins
The gene cluster responsible for the biosynthesis of
undecylprodigiosin (5) and butyl-meta-cycloheptylprodigi-
osin (6) in Streptomyces coelicolor has been sequenced (see
Scheme 2).4 A single enzyme RedG, which is probably a
nonheme iron-dependent dioxygenase, appears to convert 5
to 6 by oxidation of the CH2 group to a radical or cation
that undergoes an intramolecular Friedel-Crafts reaction to
the pyrrole to form 6. A related enzyme could easily oxidize
5 at the adjacent carbon to give radical 7, which could add
to the exocyclic double bond to give macrocyclic radical 8.
We chose to investigate this sequence with the readily
available model lactone 17, which lacks the macrocyclic ring
of 16 (see Scheme 4). Addition of vinylmagnesium bromide
to lactone 17 afforded the known hydroxy ketone 18 in 85%
yield,6 which was protected as its triethylsilyl ether to give
enone 19 in 96% yield. The pyrrole ring is relatively
unstable7 and susceptible to methylation, so we initially
(3) Aldrich, L. N.; Dawson, E. W.; Lindsley, C. W. Org. Lett. 2010,
12, 1048–1051.
(4) (a) Cerdeno˜, A. M.; Bibb, M. J.; Challis, G. L. Chem. Biol. 2001, 8,
817–829. (b) Williamson, N. R.; Fineran, P. C.; Leeper, F. J.; Salmond,
G. P. C. Nat. ReV. Microbiol. 2006, 4, 887–899. (c) Mo, S.; Sydor, P. K.;
Corre, C.; Alhamadsheh, M. M.; Stanley, A. E.; Haynes, S. W.; Song, L.;
Reynolds, K. A.; Challis, G. L. Chem. Biol. 2008, 15, 137–148.
Org. Lett., Vol. 12, No. 7, 2010
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Scheme 4
.
Preparation of Iminal 22
Scheme 5. Preparation of Spiroiminals 24a,c, 25a,c, and 27a,c
in either no reaction or extensive decomposition. This
suggests that methoxypyrrole 11 may not be an intermediate
in the biosynthesis of the marineosins (see Scheme 2).
The presence of the imine double bond makes the
formation of spiroiminals from 22 quite different from that
of spiroketals and spiroaminals. Protonation of the methoxy
group and loss of MeOH would give a cumulene (CdN+dC)
in a five-membered ring. Spiroiminal formation may proceed
by tautomerization to an enamine analogous to 13, which
can easily lose MeOH. Other pathways are presented in the
Supporting Information. Equilibration of the spiroiminals can
also occur by formation of enamines or by protonation on
the imine nitrogen and ring-opening of the dihydropyrrole
ring to give a six-membered oxocarbenium ion. Remarkably,
very little pyrrole 23a was formed from 22a or during the
equilibration of the spiroiminals.
The structure of 27a was established by an NOE between
proton H-4 adjacent to the methoxy group and proton H-7
adjacent to the methyl group, as shown. These protons are
too far apart in the other three isomers for an NOE to be
observed. This NOE also allowed us to assign the structure
of 25a, which differs from 27a only in the stereochemistry
at the spiroiminal center. The structure of 25a was confirmed
by NOEs between proton H-4 adjacent to the methoxy group
and the adjacent CH2 group on the tetrahydropyran group
as shown. The axial nitrogen deshields proton H-7 adjacent
to the methyl group in 24a (δ 4.41) and 25a (δ 4.45), which
absorbs much further downfield than H-7 in 26a (δ 3.78)
and 27a (δ 3.79) with an axial carbon.8 As expected, the
major isomer 24a has no NOEs between the protons on the
tetrahydropyran ring and those on the dihydropyrrole ring.
Having developed a route to phenyl-substituted spiroimi-
nals, we turned our attention to the problem of a pyrrole
substituent. Treatment of 19 with the oxime of pyrrole-2-
carboxaldehyde in THF with NCS and Et3N at -78 °C
afforded isoxazoline 20b in 62% yield. Hydrogenolysis of
investigated the reaction of 19 with benzonitrile N-oxide.
Reaction of 19, benzaldehyde oxime, NCS, and Et3N in THF
at -78 to +25 °C provided isoxazoline 20a in 78% yield.
Hydrogenolysis (1 atm) over Raney nickel 2800 in MeOH
for 45 min afforded hemi-iminal 21a as a mixture of isomers.
Methylation with NaH and MeI in THF at 25 °C gave methyl
ether iminal 22a in 58% yield from 20a.
Treatment of 22a with 2 M aqueous hydrochloric acid in
1:1 THF/CH3CN hydrolyzed the triethylsilyl ether and
effected loss of methanol to give the desired spiroiminals
24a (41%), 25a (13%), and 27a (12%) and the undesired
pyrrole 23a (8%) (see Scheme 5). Solutions of either pure
25a or 27a equilibrated to an identical 3:1 mixture of 25a
and 27a in CDCl3 (containing adventitious HCl) over 2-3
weeks, establishing that these two compounds have the
identical relative stereochemistry at C-4 and C-7 and differ
only at the iminal center C-5. The major isomer 24a
equilibrated in 2 weeks to give a 19:1 mixture of 24a and
26a, establishing that these two compounds have the identical
relative stereochemistry at C-4 and C-7. We were unable to
convert methoxypyrrole 23a to the desired spiroiminals
24a-27a under a variety of acidic conditions, which resulted
(5) Hydrogenolysis of a keto isoxaoline afforded a compound whose
spectral data are in agreement with those of a hemi-iminal. See compound
12 in: Andersen, S. H.; Sharma, K. K.; Torssell, K. B. G. Tetrahedron
1983, 39, 2241–2245.
(6) Cohen, N.; Banner, B. L.; Blount, J. F.; Weber, G.; Tsai, M.; Saucy,
G. J. Org. Chem. 1974, 39, 1824–1833.
(7) Cycloadditions with the nitrile N-oxide derived from benzaldehyde
oxime can be carried out at room temperature, whereas those with the nitrile
N-oxide derived from the oxime of pyrrole-2-carboxaldehyde must be carried
out at -70 °C. See: Ghabrial, S. S.; Thomsen, I.; Torssell, K. B. G. Acta
Chem. Scand. B 1987, B41, 426–434.
(8) For a similar effect by an axial oxygen in spiroketals, see: Doubsky´,
ˇ
J.; Saman, D.; Zedn´ık, J.; Vasˇ´ıcˇkova´, S.; Koutek, B. Tetrahedron Lett. 2005,
46, 7923–7926.
1602
Org. Lett., Vol. 12, No. 7, 2010

the N-O bond over Raney Ni proceeded uneventfully to
give 21b, but methylation with NaH and MeI methylated
the pyrrole NH in addition to the two hydroxy groups to
give 22d in 41% yield from 20b. Therefore, the pyrrole NH
needed to be protected.
yield. The stereochemistry of 24b to 27b was again assigned
by analogy to 24a to 27a.
Comparison of the structures of model spiroiminals 24-27
with those of marineosins A (1) and B (2) is complicated
by the trans-fused macrocyclic ring of the marineosins that
forces the methyl group into an axial conformation, whereas
it is equatorial in the models lacking the fused macrocyclic
ring. Spiroiminal 25b has the same relative stereochemistry
as marineosin B (2). Although 27b is only slightly less stable
than 25b, examination of models and molecular mechanics
calculations suggest that steric interactions between the
methoxy group and macrocyclic ring will make the stereo-
isomer of marineosin that corresponds to 27b much less
stable than marineosin B (2). Marineosin A (1) has the same
relative stereochemistry as 26a, which was only observed
as a minor isomer in equilibrium with 24a. However, steric
interactions with the axial methyl group should strongly favor
the spiroiminal isomer with an axial nitrogen (marineosin
A) over that with the stereochemistry corresponding to 24a,
which must exist in the conformation with a large axial
carbon and also has steric interactions between the methoxy
group and the macrocyclic ring. Therefore, treatment of 14
with acid should afford mainly marineosins A (1) and B (2)
rather than the other two, presumably less stable, isomers.
In conclusion, we have developed a seven-step route to
spiroiminals 24b, 25b, and 27b from lactone 17 that should
be amenable to the conversion of lactone 16 to marineosins
A (1) and B (2). A plausible biosynthesis from undecylpro-
digiosin (5) to the marineosins is proposed that only requires
a single two-electron oxidation.
A SEM-protected pyrrole appeared to be compatible with
the reagents in this sequence.9 N-SEM-Pyrrole-2-carboxal-
dehyde10 was converted to the oxime in 84% yield with
NH2OH·HCl and NaOAc in aqueous MeOH. However,
treatment of this oxime with NCS and enone 19 in THF at
-78 °C gave 20c in <30% yield. Fortunately, reaction of
this oxime in CH2Cl2 at 25 °C with 5% aqueous NaOCl11
generated the nitrile N-oxide, which added cleanly to enone
19 to give 20c in 73% yield. Both hydrogenolysis over Raney
Ni and methylation now proceeded uneventfully to give 22c
in 42% yield from 20c. Treatment of 22c with 2 M aqueous
hydrochloric acid in 1:3 THF/CH3CN hydrolyzed the trieth-
ylsilyl ether and effected loss of methanol to give the
protected spiroiminal 24c (34%), an inseparable equilibrium
3:2 mixture of spiroiminals 25c and 27c (34%), and the
undesired pyrrole 23c (<2%). The stereochemistry of these
spiroiminals was assigned by analogy to 24a to 27a, which
1
have virtually identical H and 13C NMR spectra in the
aliphatic region.
Deprotection of 24c with TBAF and molecular sieves in
THF at 60 °C for 3 h provided spiroiminal 24b in 54% yield
(see Scheme 6). A similar sequence on the 3:2 mixture of
25c and 27c afforded a 7:3 mixture of 25b and 27b in 56%
Scheme 6. Preparation of Spiroiminals 24b, 25b, and 27b
Acknowledgment. We are grateful to the National Insti-
tutes of Health (GM-50151) for support of this work.
Supporting Information Available: Complete experi-
mental procedures, spectral assignment of 24a-27a, discus-
sion of spiroiminal formation and equilibration, and copies
of 1H and 13C NMR spectral data. This material is available
free of charge via the Internet at http://pubs.acs.org.
OL100333D
(9) Jolicoeur, B.; Chapman, E. E.; Thompson, A.; Lubell, W. D.
Tetrahedron 2006, 62, 11531–11563.
(10) Muchowski, J. M.; Solas, D. R. J. Org. Chem. 1984, 49, 203–205.
(11) Lee, G. A. Synthesis 1982, 508–509.
Org. Lett., Vol. 12, No. 7, 2010
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