Studies toward the total synthesis of the cytotoxic sponge alkaloid pyrinodemin A.

Article abstract of DOI:10.1021/ol015646q

[structure: see text]. The syntheses of the proposed structure of pyrinodemin A (1) and its cis double bond positional isomer (C15'-C16') in racemic form are described. The key reaction involved an intramolecular nitrone/double bond cycloaddition. Our res

Full text of DOI:10.1021/ol015646q

ORGANIC
LETTERS
2001
Vol. 3, No. 8
1145-1148
Studies toward the Total Synthesis of
the Cytotoxic Sponge Alkaloid
Pyrinodemin A
Jack E. Baldwin,* Stuart P. Romeril, Victor Lee, and Timothy D. W. Claridge
The Dyson Perrins Laboratory, UniVersity of Oxford, South Parks Road,
Oxford, OX1 3QY, UK
jack.baldwin@chem.ox.ac.uk
Received February 1, 2001
ABSTRACT
The syntheses of the proposed structure of pyrinodemin A (1) and its cis double bond positional isomer (C15′−C16′) in racemic form are
described. The key reaction involved an intramolecular nitrone/double bond cycloaddition. Our results suggest that neither 1 nor its double
positional isomer is the correct structure of pyrinodemin A
In 1999, Kobayashi et al. reported the isolation of a
structurally novel alkaloid, pyrinodemin A, from the marine
sponge Amphimedon sp. collected off Nakijin, Okinawa.1 On
the basis of a combination of NMR and mass spectrometry
studies, Kobayashi et al. proposed the structure of pyrino-
demin A as 1 with the relative stereochemistry as shown.
Pyrinodemin A is chiral and possesses a unique cis-
cyclopent[c]isoxazolidine moiety. Subsequently, Kobayashi
et al. reported a family of compounds similar to pyrinodemin
A obtained from the same sponge.2 Pyrinodemin A is
cytotoxic toward murine leukaemia L1210 and KB epider-
moid carcinoma cells. Our continued interest3 in sponge
alkaloid chemistry coupled with the novel structure of
pyrinodemin A prompted us to investigate its synthesis.
Herein we report the synthesis of racemic structures 1 and
19 and our finding that the spectroscopic data of synthetic 1
and 19 do not correspond to that of pyrinodemin A.4
Biosynthetically, structure 1 can be formed from an
intramolecular 1,3-dipolar cycloaddition of nitrone 2.5 Ni-
trone 2, in turn, can be derived from aldehyde 3 and
hydroxylamine 4. This biosynthetic proposal forms the basis
of our biomimetic synthesis (Figure 1).
The synthesis of aldehyde 3 commenced with the protec-
tion of 5-hexyn-1-ol 5 as its tert-butyldiphenylsilyl ether 6
in 96% yield with tert-butyldiphenylsilyl chloride and
imidazole in THF.6 Deprotonation of the terminal acetylene
in 6 with n-butyllithium followed by addition of the lithiated
acetylene to excess 1,7-dibromoheptane in 1,3-dimethyl-
3,4,5,6-tetrahydro-2-(1H)-pyrimidinone (DMPU) gave com-
pound 7 in 68% yield.7 Semi-hydrogenation of acetylene 7
in benzene with Lindlar catalyst and quinoline delivered
alkene 8 in 94% yield.8 Alkene 8 was treated with lithiated
(1) Tsuda, M.; Hirano, K.; Kubota, T.; Kobayashi, J. Tetrahedron Lett.
1999, 40, 4819-4820.
(2) Hirano, K.; Kubota, T.; Tsuda, M.; Mikami, Y.; Kobayashi, J. Chem.
Pharm. Bull. 2000, 48, 974-977.
(3) (a) Baldwin, J. E.; Claridge, T. D. W.; Culshaw, A. J.; Heupel, F.
A.; Lee, V.; Spring, D. R.; Whitehead, R. C.; Boughflower, R. J.; Mutton,
I. A.; Upton, R. J. Angew. Chem., Int. Ed. 1998, 37, 2661-2663. (b)
Baldwin, J. E.; Spring, D. R.; Atkinson, C. E.; Lee, V. Tetrahedron 1998,
45, 13655-13680. (c) Baldwin, J. E.; Vollmer, H. R.; Lee, V. Tetrahedron
Lett. 1999, 40, 5401-5404. (d) Baldwin, J. E.; Claridge, T. D. W.; Culshaw,
A. J.; Heupel, F. A.; Lee, V.; Spring, D. R.; Whitehead, R. C. Chem. Eur.
J. 1999, 5, 3154-3161. (e) Baldwin, J. E.; James, D.; Lee, V. Tetrahedron
Lett. 2000, 41, 733-736.
(4) Taken in part from Romeril, S. P. Part II Thesis, June 2000, University
of Oxford.
(5) After the synthesis of compound 1 was completed, a similar proposal
appeared in print. See ref 2.
(6) Hall, D. G.; Deslongchamps, P. J. Org. Chem. 1995, 60, 7796-7814.
(7) Poulain, S.; Noiret, N.; Nugier-Chavin, C.; Patin, H. Liepigs Ann.
1997, 35-40.
(8) Lindlar, H.; Dubuis, R. Organic Syntheses; Wiley: New York, 1973;
Collect. Vol. V, pp 880-883.
10.1021/ol015646q CCC: $20.00 © 2001 American Chemical Society
Published on Web 03/20/2001

Scheme 1a
a (a) TBDPSCI, imidazole, THF, 96%. (b) nBuLi, THF, and then
Br(CH2)7Br, DMPU, 68%. (c) Lindlar catalyst, quinoline, H2, PhH,
94%. (d) 3-Picoline, LDA, THF, DMPU, 63%. (e) NH4F, MeOH,
97%. (f) IBX, DMSO, THF, 92%.
Figure 1. The proposed structure of pyrinodemin A and its
retrosynthetic biosynthetic scheme.
sulfate12 to afford nitrone 2 in 89% (over two steps). Thermal
cyclization of nitrone 2 in anhydrous benzene under high
dilution condition afford the desired product 1 in 41% yield
after chromatographic purification (Scheme 3).
3-picoline (prepared from 3-picoline and LDA in THF/
DMPU) to deliver alkylpyridine 9 in 63% yield.9 The
deprotection of 9 was effected with ammonium fluoride in
methanol to afford alcohol 10 in 97% yield.10 Aldehyde 3
was obtained in 92% yield by oxidation of alcohol 10 with
2-iodoxybenzoic acid (IBX) in DMSO and THF11 (Scheme
1).
To prepare hydroxylamine 4, 4-pentyn-1-ol 11 was
protected as its tert-butyldiphenylsilyl ether 12 in 93% yield.6
Alkylation of 1,8-dibromooctane with the acetylide anion
generated from 12 gave compound 13 in 77% yield.7 Lindlar
hydrogenation8 of 13 gave alkene 14 in 97% yield which
was treated with lithiated 3-picoline9 to afford the protected
pyridine 15 in 59% yield. Alcohol 16 was obtained in 97%
yield from fluoride deprotection10 of 15. 16 was subjected
to IBX oxidation to give aldehyde 17 in 92% yield.11
Treatment of aldehyde 17 with hydroxylamine hydrochloride
and sodium acetate in methanol delivered oxime 18 as a
mixture of cis and trans isomers in 93% yield.12 Oxime 18
was reduced with sodium cyanoborohydride in methanol at
pH 3 to give hydroxylamine 4 (Scheme 2).
The structure of 1 was unambiguously established by
HSQC, HSQC-TOCSY13 (τm ) 80 ms), DQF-COSY, and
1D DPFGSE-NOESY14 (τm ) 400 ms) spectroscopy.15 In
the HSQC-TOCSY experiment, C-20′ in 1 correlates with
H19′, H18′, and H17′; therefore the double bond is between
C17′ and C16′.16 When the spectroscopic data of 1 were
compared with those of the natural pyrinodemin A, subtle
differences were noticed. Kobayashi assigned the position
of the double bond in natural pyrinodemin A between C16′-
C17′. This conclusion was based on the fragments m/z 204
(interpreted as cleavage of C15′-C16′ bond) and m/z 231
(interpreted as cleavage of C17′-C18′ bond plus gaining a
hydrogen atom) observed in electron impact mass spectrom-
etry (EIMS) of the natural product.
Kobayashi also reported the 13C chemical shift of the
olefinic carbons (in CDCl3) as a singlet at 129.3 ppm. In
the 13C NMR (in CDCl3) of synthetic product 1, the olefinic
carbons (C16′ and C17′) appeared as two separated signals
at 129.2 and 130.3 ppm (∆δ 1.1). Small differences were
Unpurified 4 was immediately condensed with aldehyde
3 in dichloromethane in the presence of anhydrous sodium
1
also observed in the H NMR and EIMS fragmentation
(9) Davies-Coleman, M. T.; Faulkner, D. J.; Dubouwchik, G. M.; Roth,
G. P.; Polson, C.; Fairchild, C. J. Org. Chem. 1993, 58, 5925-5930.
(10) Zhang, W.; Robins, M. J. Tetrahedron Lett. 1992, 33, 1177-1180.
(11) (a) Frigerio, M.; Santagostino, M.; Tetrahedron Lett. 1994, 35,
8019-8022. (b) Frigerio, M.; Santagostino, M.; Sputore, S.; Palmisano, G.
J. Org. Chem. 1995, 60, 7272-7276. (c) Frigerio, M.; Santagostino, M.;
Sputore, S. J. Org. Chem. 1999, 64, 4537-4538.
(13) John, B. K.; Plant, D.; Heald, S. L.; Hurd, R. E. J. Magn. Reson.
1991, 94, 664-669.
(14) Stott, K.; Keeler, J.; Van, Q. N.; Shaka, A. J. J. Magn. Reson. 1997,
125, 302-324.
(15) The HSQC, HSQC-TOCSY, DQF-COSY, and 1D DPFGSE-
NOESY experiments were conducted in CD3OD.
(16) Kobayashi’s system of numbering is adopted throughout this Letter,
see ref 1.
(12) Holmes, A. B.; Smith, A. L.; Williams, S. F.; Hughes, L. R.; Lidert,
Z.; Switenbank, C. J. Org. Chem. 1991, 56, 1393-1405.
1146
Org. Lett., Vol. 3, No. 8, 2001

Scheme 2a
Scheme 3a
a (a) 3, anhydrous Na2SO4, CH2Cl2, 89%. (b) PhH, heat, 41%.
that 19 could be derived from nitrone 20. Nitrone 20 could
be derived form aldehyde 3 and hydroxylamine 21. In
principle, hydroxylamine 21 could itself originate from
aldehyde 3. Therefore, 19 could be biosynthesized from the
common precursor 3. We were further encouraged by the
subsequent isolation of oxime 22 (see Scheme 4) as a natural
product from the same sponge together with other pyrino-
demins.2 We speculated that oxime 21 might be the biosyn-
thetic precursor of pyrinodemin A.
Scheme 4a
a (a) TBDPSCI, imidazole, THF, 93%. (b) nBuLi, THF, and then
Br(CH2)8Br, DMPU, 77% (c) Lindlar catalyst, quinoline, H2, PhH,
97%. (d) 3-Picoline, LDA, THF, DMPU, 59%. (e) NH4F, MeOH,
97%. (f) IBX, DMSO, THF, 92%. (g) NH2OH‚HCl, MeOH,
NaOAc, 93%. (h) NaCNBH3, MeOH, HCl.
pattern between synthetic product 1 and that reported for
pyrinodemin A. If 1 was the correct structure of pyrinodemin
A, it would be unlikely that the 13C resonance of C16′ and
C17′ would converge into one signal due to the unsym-
metrical nature of the double bond. In addition, Kobayashi
also conducted an HSQC-TOCSY experiment on natural
pyrinodemin A but did not report any correlation observed
between C-20′ and the olefinic H17′ and H16′.
Further inspection of Kobayashi’s electron impact mass
spectrometry data led us to propose 19, in which the double
bond is located between C15′ and C16′, as the alternative
structure of pyrinodemin A.4 It is possible that 19, upon
electron impact ionization, may undergo a McLafferty
rearrangement to give a fragment of m/z 231 via rearrange-
ment of the alkene ion (cleavage of C17′-C18′ bond with
hydrogen atom transfer from C19′ to C15′).
In addition, the local environment of the double bond in
19 is relatively more symmetrical than that of 1 because it
is further away from the isoxazolidine moiety. We envisaged
a (a) NH2OH‚HCl, MeOH, NaOAc, 84%. (b) NaCNBH3, MeOH,
HCl. (c) 3, anhydrous Na2SO4, CH2Cl2, 71%. (d) PhH, heat, 63%.
Org. Lett., Vol. 3, No. 8, 2001
1147

This proposal was reduced to practice. Oxime 22 was
prepared in 84% yield from aldehyde 3 and hydroxylamine
hydrochloride.12 Reduction of oxime 22 with sodium cy-
anoborohydride gave hydroxyamine 21, which was con-
densed immediately with aldehyde 3 to afford nitrone 20 in
71% yield over two steps. Thermal cyclization of 20 gave
19 as the desired product in 63% yield (Scheme 4).
The structure of 19 was established by NMR spectros-
copy17 and mass spectrometry. When the spectral data of 19
were compared with those of pyrinodemin A, once again
subtle differences were observed. In the 13C NMR of 19 (in
CDCl3), the olefinic signals appeared at 129.5 and 129.9 ppm
(∆δ 0.4). Therefore our data suggested that 19 is not the
correct structure of pyrinodemin A. We stress that it is
possibile that the small discrepancies between ours and
Kobayashi’s NMR data may be due to the effects of
concentration, temperature, and purity of the samples.
However, the small differences in the EIMS fragmentation
pattern between 19 and pyrinodemin A suggest that they are
different compounds.
In conclusion, we report the synthesis of the proposed
structure of pyrinodemin A and its double bond positional
isomer in racemic form based on a biosynthetic hypothesis.
We are confident that the core cis-cyclopent[c]isoxazolidine
structure of pyrinodemin A was correctly proposed by
Kobayashi. However, we believe that the EIMS method used
by Kobayashi to locate the position of the double bond in
pyrinodemin A is unreliable. It is known that alkene ions
show a strong tendency to isomerize through the migration
of double bond in EIMS.18 The normal practice to locate a
double bond in an aliphatic compound by mass spectrometry
is to chemically modify the double bond and analyze the
derivative with soft ionization technique.19 Alternatively,
collision activation decomposition in tandem mass spec-
trometry had been applied directly to natural products for
the same purpose without the need of derivation.20 These
facts therefore cast doubt on Kobayashi’s structural assign-
ment.21 Our results clearly show that the difference in
chemical shifts between the two olefinic carbons diminishes
as the double bond is moved farther away from the central
bicyclic core. Further work in establishing the true identity
of pyrinodemin A is currently underway and will be reported
in due course.
Acknowledgment. We thank the States of Jersy Educa-
tion Committee for a studentship (S.P.R.), Prof. Jun’ichi
Kobayashi for spectral data of natural pyrinodemin A, and
Dr. Robin Aplin for helpful discussion.
Note added in proof: During the process of publishing
this Letter, Snider and Shi reported the syntheses of racemic
1 and 19.22 The spectral data (1H and 13C) of 1 and 19
presented in this Letter matched those for the corresponding
structures reported by Snider and Shi. Although they too
observed the same inconsistencies between the spectral data
of 19 and natural pyrinodemin A, Snider and Shi concluded
that 19 is probably the correct structure of pyrinodemin A.
Supporting Information Available: Experimental pro-
cedures, spectral data, and full spectroscopic data of natural
pyrinodemin A, 1, and 19. This material is available free of
charge via the Internet at http://pubs.acs.org.
OL015646Q
(19) Schmitz, B.; Klein, R. A. Chem. Phys. Lipids 1986, 39, 285-311.
For a recent example, see: Devijver, C.; Salmoun, M.; Daloze, D.;
Braekman, J. C.; De Weerdt, W. H.; De Kluijver, M. J.; Gomez, R. J. Nat.
Prod. 2000, 63, 978-980.
(20) Tomer, K. B.; Crow, F. W.; Gross, M. L. J. Am. Chem. Soc. 1983,
105, 5487-5488. For a recent example, see: Watanabe, K.; Tsuda, Y.;
Yamane, Y.; Takahashi, H.; Iguchi, K.; Naoki, H.; Fujita, T.; Van Soest,
R. W. M. Tetrahedron Lett. 2000, 41, 9271-9276.
(21) Kobayashi applied the same method to determine the position of
the double bond in pyrinodemin C and natural oxime 21, see ref 2.
(22) Snider, B. B.; Shi, B. Tetrahedron Lett. 2001, 42, 1639-1642.
(17) All NMR experiments for 18 were conducted in CDCl3.
(18) McLafferty, F. W.; Turecek, F. Interpretation of Mass Spectra, 4th
ed.; University Science Books: Mill Valley, CA, 1993; p 230.
1148
Org. Lett., Vol. 3, No. 8, 2001