A synthesis of crambescidin 359

Article abstract of DOI:10.1016/S0040-4039(02)02531-5

A potentially biomimetic synthesis of the guanidine-containing marine natural product crambescidin 359 via a double Michael addition of guanidine to a suitably functionalised bis-enone is reported.

Full text of DOI:10.1016/S0040-4039(02)02531-5

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 251–254
A synthesis of crambescidin 359
Christopher G. Moore,a Patrick J. Murphy,a,* Harri L. Williams,a Alan T. McGownb and
Nigel K. Smithb
aDepartment of Chemistry, University of Wales, Bangor, Gwynedd LL57 2UW, UK
bChristie CRC Research Centre, Paterson Institute for Cancer Research, Christie NHS Trust, Wilmslow Road,
Manchester M20 4BX, UK
Received 16 October 2002; revised 8 November 2002; accepted 11 November 2002
Abstract—A potentially biomimetic synthesis of the guanidine-containing marine natural product crambescidin 359 via a double
Michael addition of guanidine to a suitably functionalised bis-enone is reported. © 2002 Elsevier Science Ltd. All rights reserved.
Crambescidin 359 1 was recently isolated from the
marine sponge Monanchora unguiculata by Braekman
and co-workers.1 The structure of 1 consists of a penta-
cyclic guanidine unit similar to that present in the
related natural product, ptilomycalin A 2.2 It differs
from 2 in that it lacks a pendent spermidine group
attached to the pentacyclic core via an v-hydroxy fatty
acid chain and an ester linkage (Fig. 1).
similar to the pentacyclic moiety in ptilomycalin A
(Scheme 1).
We are prompted to report our synthesis4 of
crambescidin 359 1 by a publication by Nagasawa and
co-workers who prepared 1 using a nitrone cycloaddi-
tion approach.5
Our approach towards 1 relies upon the preparation of
a suitable bis-enone precursor for the guanidine addi-
tion step. This initially involved the preparation of ylid
7, which was obtained in five steps from ethyl-(R)-3-
hydroxybutyrate 5. Thus 5 was silyl protected, follow-
ing which the ester function was reduced to the alcohol
and then converted to iodide 6 via the corresponding
Our general synthetic approach towards the synthesis
of 2 and related natural products which encompasses a
biomimetic hypothesis has already been reported.3 For
example, addition of guanidine to bis-a,b-unsaturated
ketone 3 followed by deprotection and spirocyclisation
gave the pentacyclic guanidine 4, which is structurally
Figure 1. X−=unspecified.
Keywords: crambescidin; ptilomycalin A; guanidine; Michael addition.
* Corresponding author.
0040-4039/03/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(02)02531-5

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C. G. Moore et al. / Tetrahedron Letters 44 (2003) 251–254
generated from 3-carboxypropyltriphenylphosphonium
bromide 10, followed by esterification with dia-
zomethane gave the cis-alkene 11 together with the
trans-isomer (minor) in 45% yield and in a 4:1 ratio.7
After separation from the trans-isomer, cis-11 was
treated with 2 equiv. of methylenetriphenylphospho-
rane to give the stabilised phosphorane 12 which on
reaction with an excess of freshly prepared succinalde-
hyde gave the desired aldehyde 13a in 42% yield. Whilst
this route was convenient in terms of the number of
steps, the low yields in several of the reactions together
with the separation of the E/Z isomers in the prepara-
tion of 11 were problematical. We also prepared the
known compound 13b via an alternative route8 as part
of a study on the Knoevenagel condensation4 and it
was this compound which proved to be the best precur-
sor for the final transformations (Scheme 3).
Scheme 1. Reagents and conditions: (a) (i) guanidine, DMF, 3
h, (ii) MeOH, HCl, 0°C–rt, 24 h, (iii) NaBF4 (satd, aq.), 25%
overall.
tosylate. This was then treated with the lithium anion
of acetylmethylene triphenylphosphorane to afford 7 in
51% yield (Scheme 2).
The synthesis of the required bis-enone 14 was accom-
plished in 73% yield by reaction of ylid 7 with aldehyde
13b in dichloromethane. Addition of a DMF solution
of guanidine to 14, also in DMF, at 0°C followed by
stirring at this temperature for 6 h resulted in the
complete consumption of the starting bis-enone. After
dilution with water and addition of a solution of
methanolic HCl we obtained, after work-up, a mixture
of compounds in which the TBS protecting group had
been removed, but the TPS group remained; NMR
evidence strongly suggested the presence of the tetra-
cycle 15.9 This mixture was therefore treated with 5
The preparation of aldehyde 13a proved to be a more
significant challenge. Starting from (S)-2-aminobutyric
acid 8, aldehyde 9 was prepared in five steps. Thus 8
was diazotised in the presence of sodium nitrite and
1 M H2SO4 to give the corresponding a-hydroxy acid6
which was then esterified using HCl in methanol (23%
over two steps). Silyl protection of the alcohol function
(47%) was followed by DIBAL-H reduction (72%) and
Swern oxidation of the resulting alcohol to give alde-
hyde 9 in 93% yield. Wittig reaction of 9 with the ylid
Scheme 2. Reagents and conditions: (a) TBSCl, DMF, imid., 95%; (b) DIBAL-H, hexane, −78–0°C, 8 h, 79%; (c) TosCl, py.,
0°C–rt, 16 h; 78%; (d) NaI, acetone, reflux, 4 h, 95%; (e) CH3COCHPPh3, nBuLi, −78°C, then 6, warm to rt, 51%.
Scheme 3. Reagents and conditions: (a) NaNO2, H2SO4 0°C–rt, 24 h; (b) MeOH, HCl, 48 h (23%, two steps); (c) TBSCl, imid.,
DMF, 0°C–rt, 48 h, 47%; (d) DIBAL-H, −78°C–rt, 5 h, 72%; (e) Swern ox., 93%; (f) (i) MeO2CCH2CH2CH2PPh3+Br− 10,
NaHMDS, THF, reflux, 30 min, (ii) 9, rt, 30 min, (iii) CH2N2, Et2O (45%, 4:1, Z:E); (g) (i) 2 equiv. CH2ꢀPPh3, THF, −78°C–rt,
2 h, (ii) succinaldehyde, THF, 42 h (42%, two steps).

C. G. Moore et al. / Tetrahedron Letters 44 (2003) 251–254
253
Scheme 4. Reagents and conditions: (a) DCM, 0°C–rt, 36 h, 73%; (b) (i) guanidine, DMF, 0°C, 6 h, (ii) H2O, MeOH, HCl, 0°C–rt,
16 h; (c) (i) THF, TBAF, 25–30°C, 3 h, then rt, 64 h, (ii) MeOH, HCl, 0°C, 4 h, then NaBF4 (satd, aq.), CH2Cl2, 18% overall.
equiv. of TBAF in THF at 25–30°C for 3 h and then at
ambient temperature for a further 64 h. Finally, acid-
mediated cyclisation followed by counterion exchange
and purification by column chromatography on silica
gel gave the desired pentacycle 1·BF4 in 18% yield from
14† (Scheme 4).
In conclusion, we have reported a convergent and
potentially biomimetic synthesis of crambescidin 359 1
and have shown that it displays lower levels of biologi-
cal activity than ptilomycalin A, another member of
this group of natural products.
The structure and stereochemistry of 1·BF4 were confi-
rmed by detailed 2D NMR and NOE spectroscopy.
The analytical data11 for 1·BF4 was found to be consis-
tent with that reported.1,5 In particular the optical
rotation of −8.2 (c 0.2, CH2Cl2) compares favourably
with that of the synthetic material5 (−8.0, c 0.2,
CH2Cl2) and the natural product1 (−9.0, c 0.2, CH2Cl2)
isolated as the chloride salt. We also investigated the
biological activity of synthetic crambescidin 359 1·BF4,
which was tested against four cancer cell lines in com-
parison with ptilomycalin A12 (Table 1). As can be seen
1·BF4 is active against all the cell lines tested, however,
the levels of activity are all significantly lower than
those reported for ptilomycalin A. This is in line with
our previous studies in this area which suggest that the
fatty acid chain and spermidine residue are essential for
high levels of activity in this series of compounds.12
Acknowledgements
Thanks are given to the EPSRC for quota awards to
HLW and CGM and to the EPSRC Mass Spectrometry
Centre at Swansea.
References
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Table 1.
Compound
K562a
A2780a
H-460a
P388a
1·BF4
Ptilomycalin A 2
12.30
0.35
24.44
0.27
10.36
0.35
2.93
0.11b
a Cytotoxic activity (IC50/mg ml−1); K562, human chronic myeloge-
nous leukaemia; A2780, human, ovarian carcinoma; H-460, human
large cell carcinoma, lung. High DT-Diaphorase; P388: mouse,
lymphoid neoplasm.
b Data obtained from Ref. 2a.
† This yield is comparable with our previous work in this area, and as
the guanidine addition is not diastereoselective, half of the tricyclic
bis-silylated guanidine adduct does not undergo cyclisation to 1.

254
C. G. Moore et al. / Tetrahedron Letters 44 (2003) 251–254
5. Nagasawa, K.; Georgieva, A.; Koshino, H.; Nakata, T.;
CH-19), 2.59 (1H, dd, J=12.7, 5.7 Hz, CH-9b), 2.55 (1H,
br t, J=13.7 Hz, CH-7a), 2.32 (2H, m, CH-6b, CH-12b)
2.28 (1H, m, CH-14b), 2.19 (1H, dd, J=14.0, 5.7 Hz,
CH-11b), 2.16 (1H, m, CH-6a), 2.06 (1H, m, CH-17a),
1.87 (1H, dd, J=14.3, 5.7 Hz, CH-7b), 1.72 (3H, m,
CH-16a, CH-16b, CH-17b), 1.70 (1H, m, CH-12a), 1.69
(1H, m, CH-11a), 1.62 (1H, m, CH-18a), 1.52 (1H, m,
CH-2a), 1.49 (1H, t, J=12.7 Hz, CH-14a), 1.42 (1H, m,
CH-2b), 1.36 (1H, t, J=12.4 Hz, CH-9a), 1.20 (1H, m,
CH-18b), 1.06 (3H, d, J=6.4 Hz, CH3-20), 0.83 (3H, t,
J=7.2 Hz, CH3-1) ppm. 13C NMR: l 147.68 (C-21),
133.61 (CH-4), 129.77 (CH-5), 84.06 (C-8), 80.46 (C-15),
70.94 (C-3), 67.17 (C-19), 53.50 (CH-10), 51.97 (CH-13),
39.80 (CH2-14), 37.08 (CH2-9), 36.46 (CH2-7), 33.56
(CH2-16), 32.19 (CH2-18), 30.04 (CH2-11), 29.84 (CH2-
12), 29.21 (CH2-2), 23.72 (CH2-6), 21.65 (CH3-20), 17.91
(CH2-17), 10.21 (CH3-1) ppm. MS (CI) m/z: 360 (5%
[M+H]+). HRMS (EI) m/z: found: 359.2566, C21H33N3O2
([M]+) requires: 359.2573. [h]D26=−8.2 (c 0.2, CH2Cl2).
12. Black, G. P.; Coles, S. J.; Hizi, A.; Howard-Jones, A. G.;
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9. Overman10 and Nagasawa5 have reported difficulties with
the acid-mediated cyclisation of the allylic alcohol por-
tion of precursors similar in structure to 14 and 15.
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11. Selected data for 1·BF4: IR wmax: 3224 (w), 2975 (m), 2933
(s), 2868 (w), 1656 (s), 1604 (s), 1445 (w), 1342 (w), 1236
(w), 1066 (s), 1023 (s) cm−1. 1H NMR (CDCl3, 500 MHz):
l 7.89 (1H, br s, NH), 7.83 (1H, br s, NH), 5.66 (1H, ddt,
J=11.4, 7.7, 2.4 Hz, CH-5), 5.49 (1H, dt, J=11.0, 2.8,
CH-4), 4.47 (1H, br d, J=9.8 Hz, CH-3), 4.06 (2H, m,
CH-10, CH-13), 3.81 (1H, ddq, J=12.6, 6.5, 2.8 Hz,