Total syntheses of subereamollines A and B

Article abstract of DOI:10.1039/c0ob00636j

The first total syntheses of (+)- and (-)-subereamollines A and B are reported. The enantiomeric forms of the natural products were obtained by preparative chiral HPLC separation of the corresponding racemates.

Full text of DOI:10.1039/c0ob00636j

View Online / Journal Homepage / Table of Contents for this issue
Organic &
Biomolecular
Chemistry
Dynamic Article Links
Cite this: Org. Biomol. Chem., 2011, 9, 62
www.rsc.org/obc
COMMUNICATION
Total syntheses of subereamollines A and B†
James W. Shearman,a Rebecca M. Myers,a James D. Brentonb and Steven V. Ley*a
Received 27th August 2010, Accepted 27th September 2010
DOI: 10.1039/c0ob00636j
The first total syntheses of (+)- and (-)-subereamollines A and
B are reported. The enantiomeric forms of the natural prod-
ucts were obtained by preparative chiral HPLC separation of
the corresponding racemates.
Marine sponges of the order Verongida are a rich source of
bromotyrosine-derived natural products. Many of these com-
pounds are biologically active and exhibit a range of activities
including antiviral,1 antimicrobial,2 antifungal,3 Na+/K+ ATPase
inhibition,4 anti-HIV,5 HDAC inhibition,6 antifouling,7 histamine
H3 antagonism,8 mycothiol S-conjugate amidase inhibition,9 and
isoprenylcysteine carboxy methyltransferase (Icmt) inhibition.10
Anticancer activity has also been reported, for example, the
spirocyclohexadienylisoxazolines purealidin P and Q are cytotoxic
against murine lymphoma K1210 (IC50 2.8 and 0.95 mg mL-1
respectively) and human epidermal carcinoma KB (nasopharynx)
(IC50 7.6 and 1.2 mg mL-1 respectively) cell lines.11,12 Other
Scheme 1 Retrosynthetic analysis of subereamollines A (1) and B (2).
structurally related compounds have also exhibited cytotoxic
properties.13
correlated with that reported for (+)-aerothionin {[a]D +210
(c 1.7, MeOH)} whose absolute configuration was determined by
X-ray crystallography and circular dichroism (CD) spectroscopy.16
We envisaged that 1 and 2 could be synthesised by coupling
spiro acid ( )-3 with either amine 4 or 5 (Scheme 1). Interestingly,
3 has been isolated from the marine sponge Pseudoceratina sp. as
an optically pure isomer with its absolute configuration defined
as 1S,6R by CD analysis.17 However, the only reported synthesis
of ( )-3 is as an intermediate during the synthesis of purealin.18
The corresponding methyl ester 619–22 has been synthesised in
an enantioenriched form (74% ee) via an asymmetric oxidative
spirocyclisation which required a multi-step synthesis of a chiral
auxiliary.23 An alternative approach led to enantiopure 6 by
resolution of ( )-6 by derivatisation with (-)-camphanic chloride.24
Our approach to the naturally occurring enantiomers of 1 and 2
involved chiral HPLC separation of the corresponding racemates
as this would require no additional synthetic operations and would
also deliver enantiopure material required for biological screening.
Aldehyde 8 was prepared in 92% yield over two steps via
bromination of 2-hydroxy-4-methoxybenzaldehyde (7) with N-
bromosuccinimide and subsequent benzyl protection of the
phenolic oxygen (Scheme 2). Conversion of 8 into the corre-
sponding azlactone 9 was achieved by heating the aldehyde
with N-acetylglycine and sodium acetate in the presence of
acetic anhydride (Erlenmeyer conditions).25 Saponification of the
crude azlactone 9 with barium hydroxide and subsequent con-
densation with O-benzylhydroxylamine furnished the carboxylic
When the extracts of the Red Sea sponge Suberea mollis were
reinvestigated recently, two new bromotyrosine-derived secondary
metabolites containing the spirocyclohexadienylisoxazoline moi-
ety were identified; subereamollines A (1) and B (2) (Scheme 1).14
Small quantities (2–5 mg) were isolated and 1 was found to
have modest antimicrobial activity against Staphylococcus aureus,
however, their cytotoxic activity was not reported. As part
of our ongoing investigation into the anticancer properties of
bromotyrosine-derived natural products,15 the subereamollines
were attractive synthetic targets.
Both
1
and
2
contain
a
2,4-dibromo-1-hydroxy-3-
methoxyspirocyclohexadienylisoxazole moiety attached to
a
carbamate containing side chain via an amide linkage. Although
not explicitly stated in the isolation paper,14 the absolute
stereochemistries at C(1) and C(6) were determined to be (R)
and (S) based on the optical rotations of 1 {[a]D +156.5 (c 0.55,
MeOH)} and 2 {[a]D +22.9 (c 6.25, CH2Cl2)}. These data
aDepartment of Chemistry, Lensfield Road, University of Cambridge, Cam-
bridge, United Kingdom CB2 1EW. E-mail: svl1000@cam.ac.uk; Fax: +44
1223 3363968
bCancer Research UK Cambridge Research Institute, Li Ka Shing Centre,
Robinson Way, Cambridge, United Kingdom CB2 0RE
† Electronic supplementary information (ESI) available: experimental
procedures for the preparation of compounds 1–6 and 8–14 and full charac-
terisation of these compounds. 1H and 13C NMR spectra of 1 and 2. CCDC
reference numbers 787666 and 787667. For ESI and crystallographic data
in CIF or other electronic format see DOI: 10.1039/c0ob00636j
62 | Org. Biomol. Chem., 2011, 9, 62–65
This journal is
The Royal Society of Chemistry 2011
©

View Online
Scheme 2 Synthesis route to 1 and 2.
acid 10 in 49% yield. This yield did not improve when 9
recrystallised from acetone was used. Furthermore, oxime 11
was discovered to be a major by-product (22% yield), which
may have arisen from reversion of 9 to its parent aldehyde
under the basic conditions and subsequent condensation with
the O-benzylhydroxylamine. Azlactones are known to hydrolyse
efficiently under acidic conditions (e.g. 3 N HCl, reflux, 24 h)26
to give a-keto acid derivatives which can then be converted to the
corresponding oxime acids.27 However, when these conditions were
applied to 9 this unexpectedly resulted in the loss of the phenolic
benzyl protecting group and spontaneous lactonisation to produce
coumarin 12 (in 68% yield) as the only observable product.28
The cyclisation precursor, oxime methyl ester 13, was obtained
first by treatment of 10 with trimethylsilyldiazomethane to obtain
its methyl ester then removal of the benzyl groups via hydrogenol-
ysis over palladium black.18 The oxidative cyclisation to transform
13 to spiroisoxazoline ( )-14 in other syntheses has been achieved
with thallium(III) trifluoroacetate,19 electroorganic oxidation20 and
N-bromosuccinimide.18 However, we found that iodobenzene
diacetate21,22 was the most convenient reagent on this occasion
since the crude material could be taken on directly to the next
step following an aqueous work up. Diastereoselective reduction
29
of ( )-14 with Zn(BH4)2 furnished the desired trans-isomer ( )-
6a in a 2.03 : 1 dr over the undesired cis-isomer ( )-6b. X-Ray
crystallographic analysis confirmed the relative stereochemistry
of each isomer.30 The methyl ester was then hydrolysed with
lithium hydroxide to furnish ( )-3 in an overall yield of 11% from
aldehyde 7.
Putrescine (15) and cadaverine (16) were converted to the amine
coupling partners 4 and 5 in one step via dropwise addition of
ethyl phenyl carbonate31 to minimize bis-carbamylation. Spiro
acid ( )-3 was then coupled to amine 4 or 5 in the presence of
N,N¢-dicyclohexylcarbodiimide (DCC) and hydroxybenzotriazole
This journal is
The Royal Society of Chemistry 2011
Org. Biomol. Chem., 2011, 9, 62–65 | 63
©

View Online
(HOBt) to afford the natural products subereamolline A (1) and B
(2) respectively in 91% and 75% yield as a racemic mixture. Upon
reinvestigation of this coupling step, it was found that the use of
MTS assays36 were performed to evaluate the cytotoxic activity
of compounds ( )-1, ( )-2, (+)-1, (+)-2, 12 and 13 against an
ovarian cancer (SK-OV-3) cell line. Following 48 h treatment,
none of the compounds exhibited cytotoxic activities up to a
concentration of 100 mM.
In conclusion, the first total syntheses of the bromotyrosine-
derived natural products subereamolline A (1) and B (2) have
been reported. The enantiomeric forms of 1 and 2 were obtained
by chiral HPLC separation and, in addition, the stereochemistries
at C(1) and C(6) in the naturally occurring enantiomers (+)-1 and
(+)-2 were confirmed to be R and S respectively.
R
the coupling agent propylphosphonic anhydride (ꢀT3P) furnished
1 and 2 in 96% and 97% yields respectively.
1
The H and 13C NMR spectra of synthetic 1 and 2 matched
those of the natural materials.32 We have also established that
the originally reported 13C chemical shift assignments for C-2
(dC 122.9) and C-4 (dC 114.3) should be switched on the basis
of COLOC NMR experiments reported for related spirocyclised
products.33 This is also supported by the HMBC correlations in
the natural products.34
The enantiomeric forms of ( )-1 and ( )-2 were separated using
preparative chiral HPLC (CHIRALPAK AD-H column and 10%
iPr2OH in hexane at a flow rate of 5 mL min-1) (Scheme 2). The
optical rotations of (+)-1 (t = 40.3 min) and (+)-2 (t = 48.3 min)
agreed with those observed for natural 1 and 2. To confirm the
absolute stereochemistries at C(1) and C(6), (+)-6 (t = 30.9 min)
and (-)-6 (t = 32.9 min) were obtained from chiral HPLC
separation of ( )-6 (Scheme 3).35 Hydrolysis of (+)-6 and (-)-6
under the basic conditions furnished acids (+)-3 and (-)-3 that
were then coupled with amines 4 and 5 respectively. HPLC analysis
of the products showed them to be (+)-1 and (-)-2 respectively
(see Electronic Supplementary Information), thus confirming the
original stereochemical assignments at C(1) and C(6).
Acknowledgements
We acknowledge the Cambridge Cancer Research UK PhD
Training Programme in Medicinal Chemistry for funding (J.W.S.).
The authors thank Dr John E. Davies of the X-ray facility of the
Department of Chemistry for collecting the crystallographic data
and Dr Richard M. Turner for help with preparative chiral HPLC.
Notes and references
1 S. P. Gunasekera and S. S. Cross, J. Nat. Prod., 1992, 55, 509.
2 J. Kobayashi, M. Tsuda, K. Agemi, H. Shigemori, M. Ishibashi, T.
Sasaki and Y. Mikami, Tetrahedron, 1991, 47, 6617.
3 J.-H. Jang, R. W. M. van Soest, N. Fusetani and S. Matsunaga, J. Org.
Chem., 2007, 72, 1211.
4 H. Nakamura, H. Wu and J. Kobayashi, Tetrahedron Lett., 1985, 26,
4517.
5 S. A. Ross, J. D. Weete, R. F. Schinazi, S. S. Wirtz, P. Tharnish, P. J.
Scheuer and M. T. Hamann, J. Nat. Prod., 2000, 63, 501.
6 M. W. B. McCulloch, G. S. Coombs, N. Banerjee, T. S. Bugni, K. M.
Cannon, M. K. Harper, C. A. Veltri, D. M. Virshup and C. M. Ireland,
Bioorg. Med. Chem., 2009, 17, 2189.
7 I. Thironet, D. Daloze and J. C. Braekman, Nat. Prod. Lett., 1998, 12,
209.
8 R. A. K. Mierzwa, M. A. Conover, S. Tozzi, M. S. Puar, M. Patel and
S. J. Covan, J. Nat. Prod., 1994, 57, 175.
9 G. M. Nicholas, G. L. Newton, R. C. Fahey and C. A. Bewley, Org.
Lett., 2001, 3, 1543.
10 M. S. Buchanan, A. R. Carroll, G. A. Fechner, A. Boyle, M. Simpson,
R. Addepalli, V. M. Avery, J. N. A. Hooper, T. Cheung, H. Chen and
R. J. Quinn, J. Nat. Prod., 2008, 71, 1066.
11 T. Fujiwara, J.-H. Hwang, A. Kanamoto, H. Nagai, M. Takagi and
S.-Y. Kauzo, J. Antibiot (Tokyo), 2009, 62, 393; P. B. Shinde, Y. M.
Lee, H. T. Dang, J. Hong, C.-O. Lee and J. H. Jung, Bioorg. Med.
Chem. Lett., 2008, 18, 6414.
12 J. Kobayashi, K. Honma, T. Sasaki and M. Tsuda, Chem. Pharm. Bull.,
1995, 43, 403.
13 J. Peng, J. Li and M. T. Hamann, Alkaloids, 2005, 61, 59.
14 M. I. Abou-Shoer, L. A. Shaala, D. T. A. Youssef, J. M. Badr and
A.-A. M. Habib, J. Nat. Prod., 2008, 71, 1464.
15 J. W. Shearman, R. M. Myers, T. M. Beale, J. D. Brenton and S. V. Ley,
Tetrahedron Lett., 2010, 51, 4812.
16 J. A. McMillan, I. C. Paul, Y. M. Goo and K. L. Rinehart Jr,
Tetrahedron Lett., 1981, 22, 39.
17 A. Aiello, E. Fattorusso, M. Menna and M. Pansini, Biochem. Syst.
Ecol., 1995, 23, 377.
18 G. Zhu, F. Yang, R. Balachandran, P. Ho¨o¨k, R. B. Vallee, D. P. Curran
and B. W. Day, J. Med. Chem., 2006, 49, 2063.
19 S. Nishiyama and S. Yamamura, Tetrahedron Lett., 1983, 24, 3351.
20 T. Ogamino, Y. Ishikawa and S. Nishiyama, Heterocycles, 2003, 61,
73.
21 O. Hoshino, M. Masatoshi and Y. Kohei, Tetrahedron, 1996, 52, 14713.
22 T. R. Boehlow, J. J. Harburn and C. D. Spilling, J. Org. Chem., 2001,
66, 3111.
23 M. Murakata, M. Tamura and O. Hoshino, J. Org. Chem., 1997, 62,
4428.
Scheme 3
64 | Org. Biomol. Chem., 2011, 9, 62–65
This journal is
The Royal Society of Chemistry 2011
©

View Online
3
-3
˚
˚
24 T. Ogamino, R. Obata and S. Nishiyama, Tetrahedron Lett., 2006, 47,
727.
10.1567(2) A, V = 2725.4(3) A , Z = 8, Dc = 1.935 Mg m , T = 180(2)
K. CCDC 787667.
25 E. Erlenmeyer, Justus Liebigs Ann. Chem., 1893, 275, 1.
26 B. M. Chanda and R. S. Sulake, Tetrahedron Lett., 2005, 46, 6461; P.
Busca, F. Paradisi, E. Moynihan, A. R. Maguire and P. C. Engel, Org.
Biomol. Chem., 2004, 2, 2684.
31 Prepared using literature procedure. G. Bartoli, M. Bosco, A. Car-
lone, M. Locatelli, E. Marcantoni, P. Melchiorre, P. Palazzi and L.
Sambri, L., Eur. J. Org. Chem., 2006, 4429.
32 See Supplementary Information for 1H and 13C NMR spectra of natural
and synthetic 1 and 2.
27 N. Kotoku, H. Tsujita, A. Hiramatsu, C. Mori, N. Koizumi and M.
Kobayashi, Tetrahedron, 2005, 61, 7211.
33 P. Ciminiello, V. Costantino, E. Fattorusso, S. Magno, A. Mangoni and
M. Pansini, J. Nat. Prod., 1994, 57, 705.
28 Crystal data for 12: C10H6Br2O4, M = 349.97, orthorhombic, space
˚
˚
˚
group Pna2(1), a = 8.9710(2) A, b = 10.6189(2) A, c = 22.6185(6) A,
34 In the HMBC spectra of the natural and synthetic samples of 1 and 2,
the quaternary carbon (dC 114.3) couples strongly to the protons H-1
(dH 4.07 and 4.06 respectively) and H-5 (dH 6.42 and 6.40 respectively).
However, the quaternary carbon (dC 122.9) only couples to H-5.
Therefore the original assignments for C-2 (dC122.9) and C-4 (dC 114.3)
proposed by Habib and co-workers should be switched.
35 Both (+)-6a (t = 30.9 min) and (-)-6a (t = 32.9 min) were converted to
their corresponding camphanate esters and their NMR spectral data
matched that reported by Nishiyama and co-workers (see Ref. 24).
36 The (3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-
sulfophenyl)-2H-tetrazolium) (MTS) assay gives a colourimetric mea-
sure of the number of viable cells present in vitro.
3
V = 2154.69(8) A , Z = 8, Dc = 2.158 Mg m-3, T = 180(2) K. The
˚
X-ray structure of coumarin 12 has been previously reported. J. J.
Harburn, N. P. Rath and C. D. Spilling, J. Org. Chem., 2005, 70,
6398.
29 Prepared using literature procedure. W. J. Gensler, F. A. Johnson and
D. B. Sloan, J. Am. Chem. Soc., 1960, 82, 6074.
30 Crystal data for ( )-6a: C11H11Br2NO5, M = 397.03, triclinic, space
¯
˚
˚
˚
group P1, a = 6.5189(1) A, b = 10.7194(2) A, c = 11.0482(2) A,
3
-3
˚
V = 684.99(2) A , Z = 2, Dc = 1.925 Mg m , T = 180(2) K. CCDC
787666. Crystal data for( )-6b: C11H11Br2NO5, M = 397.03, monoclinic,
space group P-2(1)/c, a = 10.9772(1) A, b = 25.2605(3) A, c =
˚
˚
This journal is
The Royal Society of Chemistry 2011
Org. Biomol. Chem., 2011, 9, 62–65 | 65
©